DiagnosisClinical DiagnosisPrimary hyperoxaluria type 1 (PH1) is suspected in an individual with any of the following [Milliner 2005, Bobrowski & Langman 2008]:Frequent recurrent nephrolithiasis (deposition of calcium oxalate stones in the renal pelvis/urinary tract). Renal ultrasound examination often reveals bilateral and multiple radiopaque calculi [Jamieson et al 2000, Hoppe et al 2009].
Note: (1) Computed tomography (CT) and kidney ureter bladder (KUB) x-ray may demonstrate similar findings. (2) CT scan is more sensitive than KUB in the detection of stones [Barrett & Danpure 1999]; however, one must consider the radiation risk associated with CT. Nephrocalcinosis (deposition of calcium oxalate in the renal parenchyma). In older children or adults, the strongest echoes are from the corticomedullary regions, whereas in infants the pattern is more likely to be that of diffuse nephrocalcinosis with few if any observable discrete stones.
Note: Computed tomography (CT) and kidney ureter bladder (KUB) x-ray may demonstrate similar findings.End-stage renal disease with a history of renal stones or calcinosisStone composition of pure calcium oxalate monohydrate (whewellite). PH1 calculi also demonstrate peculiar morphologic characteristics including a whitish or pale yellow surface and a distinct crystalline structure, not found in stones formed as a result of other hyperoxaluric states [Daudon et al 2008]. PH1 may be suspected in a child younger than age six to 12 months with failure to thrive. PH1 is often associated with nephrocalcinosis, anemia, and metabolic acidosis [Cochat et al 1999]. Most affected individuals are symptomatic before age ten years [Sikora et al 2008]. In rare cases, diagnosis may be complicated by co-occurrence of more common unrelated kidney disorders [Devriendt et al 2011]. See Testing Strategy for algorithms to aid in diagnosis and treatment.TestingUrinary oxalate-to-creatinine molar ratio in a random urine sample. Urinary excretion of creatinine on a per-kg basis differs between males and females and does not stabilize until ages 14 to 18 years [Remer et al 2002]. Table 1 shows age-related normal values.Table 1. Normal Age-Related Values for Urinary Oxalate, Glycolate, and l-Glycerate Excretion View in own windowParameter AgeNormal ValuesOxalate ^{1Glycolatel-GlycerateValues in Spot Urine Samples 20-6 months 314-2057-24 months 314-2052-5 years14-2055-14 years23-138>16 yearsValues in 24-Hour Urine Samples>16 years2 per day2 per dayAdapted by permission from Macmillan Publishers Ltd: Kidney International. Hoppe et al 2009, copyright 2009.1. To prevent alkaline conversion of ascorbate to oxalate in urine, the sample must be strongly acidified to stabilize ascorbate and minimize formation of calcium crystals [Marangella & Petrarulo 1995].2. Expressed as molar creatinine ratios (mmol/mol)3. In children younger than age 1.5-2.0 years, rapidly changing glomerular filtration rates make the interpretation of oxalate to creatinine ratio of little practical value [Applegarth, personal communication]. Normal newborns and young infants/children can excrete ≤3-5 times the amount of oxalate excreted by adults; this amount slowly decreases into the normal adult range in the older child [Leumann et al 1990, von Schnakenburg et al 1994, Marangella & Petrarulo 1995].Concentration of glycolic acid (glycolate). Measurement of glycolate in urine depends on either reversed phase high-pressure liquid chromatography or ion chromatography. Normal ranges (Table 1) are defined for each separate assay [Petrarulo et al 1998]:Approximately 66% of individuals with PH1 have hyperoxaluria and high concentration of glycolic acids in the urine (hyperglycolic aciduria) [Milliner 2005].Approximately 25% of individuals with PH1 do not have hyperglycolic aciduria [Danpure 2001].Thus, hyperglycolic aciduria suggests but does not confirm the diagnosis of PH1 in an individual with hyperoxaluria.Plasma concentration of oxalate. Plasma oxalate concentration exceeding the upper limit of the normal range when corrected for renal function is consistent with but not diagnostic of primary hyperoxaluria. Individuals with primary hyperoxaluria type 1 usually have plasma oxalate levels two to five times the upper limit of normal. Pediatric normal ranges are defined for the specific assay used [Petrarulo et al 1998] and are affected by the degree of renal dysfunction and/or need for dialysis (see Table 2).Interpretation of results warrants caution: in affected individuals with adequate renal function, elevation of plasma oxalate concentration may be modest; in individuals with end-stage renal disease, absolute excretion of oxalate and glycolic acid is lower even with a very high plasma oxalate concentration [Danpure 2001, Hoppe et al 2009].In plasma, the spontaneous occurrence of oxalogenesis requires stringent handling of the sample, including handling in a cold environment, immediate deproteinization, and freezing until assayed.Table 2. Plasma Oxalate Concentrations in Individuals with PH1 by Renal FunctionView in own windowIndividual with PH1 by Renal FunctionIndividual w/out PH1GFR <30 mL/min/1.73 m 1GFR <20 mL/min/1.73 m 1 or ESRD 2Maintenance HD 1Maintenance HD 1Plasma oxalateconcentration 3>20 µmol/L2 µmol/L50 - >100 µmol/L10 - 40 µmol/LGFR = glomerular filtration rateHD = hemodialysis1. Sample drawn pre-dialysis2. End-stage renal disease but not on dialysis3. Individual without PH1 normal value <2.5 µmol/L based on oxalate oxidase assayAssay of AGT catalytic activity. Although liver biopsy and AGT enzymatic assay were used to confirm the diagnosis of PH1 in the past, this has been largely replaced by molecular genetic testing.Assay of AGT catalytic activity requires at least 2 mg of liver taken by percutaneous needle biopsy or at autopsy. Several methods in use:AGT assay. A radiochemical method using 14C-alanine with and without aminooxyacetic acid to inhibit AGT specifically in the presence of other enzymatic activities Automated assay of AGT enzyme activity [Horvath & Wanders 1994, Horvath & Wanders 1995, Rumsby et al 1997]Note: (1) Approximately 50%-70% of affected individuals have undetectable levels of AGT catalytic activity, and approximately 30%-50% of affected individuals have substantial residual AGT catalytic activity (2%-48% of mean normal activity) [Danpure 2001]. In many human metabolic disorders, this level of residual activity is sufficient for normal function. Most individuals with classic PH1 who have residual AGT enzymatic activity have a unique protein-targeting (or trafficking) defect in which functional AGT enzyme is synthesized in adequate amounts but approximately 90% of the enzyme produced is mislocalized to mitochondria and only approximately 10% is properly localized in the peroxisomes (where it catalyzes the glyoxylate substrate). Individuals with such a protein-trafficking defect have classic PH1 despite the presence of residual AGT activity. (2) The detection of a mistargeting mutation clarifies the significance of high residual AGT enzyme activity (when the diagnosis of PH1 has been established by liver biopsy and AGT enzyme assay), helping to distinguish between mislocalized AGT enzyme (i.e., classic PH1) and true partial AGT enzyme activity (no known associated phenotype).Molecular Genetic TestingGene. AGXT is the only gene in which mutations are known to cause primary hyperoxaluria type 1, defined as a deficiency of AGT enzyme activity.Clinical testingTargeted mutation analysis. Targeted mutation analysis refers to testing for specific common AGXT mutation(s):Disease-causing mutations. Testing panels vary but generally include the two most common disease-causing mutations, p.Gly170Arg and c.33dupC (p.Lys12Glnfs*156). At least 50% of individuals with PH1 have at least one copy of p.Gly170Arg [Rumsby et al 2004], which is specifically associated with mistargeting of AGT to the mitochondria. The selection of less common mutations for testing may be influenced by the ethnic background of the affected individual.
The mutation detection frequency depends on which mutations are included in the panel. Molecular genetic testing for the four most common mutations identifies both alleles in approximately 34% of individuals with PH1 and one allele in approximately 28% [Rumsby et al 2004]. A screening panel including the four common mutations plus an additional group of recurrent mutations detected at least one mutation in 83% of individuals with PH1 and both mutations in 51% [Coulter-Mackie et al 2008]. Sequencing of selected exons detected at least one mutation in 70% and both in 50% [Williams & Rumsby 2007]."AGXT minor allele." This term refers to a haplotype of three polymorphisms: p.Pro11Leu, p.Ile340Met, and the 74-bp duplication c.165+14_+88del74 [Purdue et al 1991] in intron 1:Disease-causing alleles that occur in cis configuration with the minor allele include p.Gly170Arg (the most common PH1-causing mutation) and the remaining two common mutations, p.Ile244Thr and p.Phe152Ile. Thus, approximately half of the individuals affected with PH1 have at least one copy of the AGXT minor allele in addition to AGXT disease-causing mutations.The disease-causing allele that occurs in cis configuration with the major allele is c.33dupC, the second most common mutation [Pirulli et al 1999, Coulter-Mackie et al 2004].Sequence analysis. Sequence analysis can detect the more common AGXT mutations that are included in the panels used for targeted mutation analysis as well as rarer mutations. Deletion/duplication analysis is available clinically. Out of more than 150 known mutations in AGXT, only five large deletions have been reported [Monico et al 2007, Williams et al 2009]. The sensitivity of this approach is expected to be low.Linkage analysis. Linkage analysis has largely been replaced by testing for specific mutations identified by gene sequencing. It may remain an option for affected individuals and families in which only one disease-causing mutation is identified by sequencing. Under optimal conditions with a correct clinical diagnosis, family structure, and informative genetic markers, accuracy can be as high as 98%-99%.Table 3. Summary of Molecular Genetic Testing Used in Primary Hyperoxaluria Type 1View in own windowGene SymbolTest MethodMutations DetectedMutation Detection Frequency for at Least One AGXT Mutation in a Proband by Test Method 1Test AvailabilityAGXTTargeted mutation analysisPanel of more common mutations 2, 350%-70%ClinicalSequence analysisSequence variants 3, 4100% 5Deletion / duplication analysis 6Exonic or whole-gene deletions1. The ability of the test method used to detect a mutation that is present in the indicated gene2. Mutation panels and detection frequencies may vary by laboratory.3. Analysis generally includes testing for the minor allele status. 4. Examples of mutations detected by sequence analysis may include small intragenic deletions/insertions and missense, nonsense, and splice site mutations; typically, exonic or whole-gene deletions/duplications are not detected.5. Enzymatically confirmed PH1 or one common mutation already identified [Coulter-Mackie et al 2008] 6. Testing that identifies deletions/duplications not readily detectable by sequence analysis of the coding and flanking intronic regions of genomic DNA; a variety of methods including quantitative PCR, long-range PCR, multiplex ligation-dependent probe amplification (MLPA), or targeted chromosomal microarray analysis (gene/segment-specific) may be used. A full chromosomal microarray analysis that detects deletions/duplications across the genome may also include this gene/segment. Interpretation of test results. For issues to consider in interpretation of sequence analysis results, click here.Information on specific allelic variants may be available in Molecular Genetics (see Table A. Genes and Databases and/or Pathologic allelic variants).Testing StrategyTo confirm/establish the diagnosis in a proband. An evidence-based guideline for diagnosis that incorporates aspects of clinical and laboratory approaches to the diagnosis of primary hyperoxaluria has been developed [Harambat et al 2011] (see Figure 1). The intent of this algorithm is to facilitate recognition and diagnosis of affected individuals and to enable earlier treatment. The algorithm also assists the reader in differentiating between PH types 1, 2, and 3 and provides guidance as to best-evidence conservative management of the conditions.FigureFigure 1. Proposed algorithm for the diagnosis and conservative treatment of primary hyperoxalurias
PH = primary hyperoxaluria
GFR = glomerular filtration rate
Uox = urinary oxalate
Pox = plasma oxalate
CaOx (more...)Measurement of urinary and plasma metabolites is a screening test.Finding homozygosity, or compound heterozygosity, for AGXT mutations known to be disease-causing confirms the diagnosis of PH1. Finding one AGXT mutation in a symptomatic individual strongly supports the diagnosis.
Molecular genetic testing may begin with EITHER:Whole-gene sequencing [Monico et al 2007]
ORTargeted mutation analysis or sequencing of selected exonsIf only one mutation is identified, follow with whole-gene sequencing [Williams & Rumsby 2007, Coulter-Mackie et al 2008]. Deletion/duplication analysis may be useful if only one mutation has been identified or if hemizygosity is suspected. However, so far, only five of more than 150 known mutations are of the type that would be detectable by this approach.Failure to detect at least one common, known, or otherwise proven AGXT mutation requires that a liver biopsy for the purpose of assaying AGT enzyme be considered. In practice, the request for the assay depends on the strength of the initial clinical diagnosis and the status of the affected individual. Carrier testing for at-risk relatives requires prior identification of the disease-causing mutations in the family.Note: Carriers are heterozygotes for this autosomal recessive disorder and are not at risk of developing the disorder.Prognostic testing for affected individuals. Testing for the presence of the specific mutations p.Gly170Arg and p.Phe152Ile, if not already determined through molecular genetic analysis, may have predictive value as these mutations are associated with a positive response to pyridoxine supplementation especially for homozygotes [van Woerden et al 2004, Monico et al 2005a, Monico et al 2005b].Prenatal diagnosis and preimplantation genetic diagnosis (PGD) for at-risk pregnancies require prior identification of the disease-causing mutations in the family.Genetically Related (Allelic) DisordersSome have speculated that homozygosity for the minor allele may lead to mild hyperoxaluria associated with idiopathic calcium oxalate stone disease [Lumb & Danpure 2000, Danpure 2001]. Studies of the frequency of the AGXT minor allele in various populations have suggested a high frequency of the p.Pro11Leu allele in Western populations [Caldwell et al 2004]. Although no definitive data on the incidence of idiopathic stones in all population groups studied are available, Western cultures tend to represent the high allele frequency group and also tend to have a high incidence of stones.}

Natural HistoryIn primary hyperoxaluria type 1, supersaturation of the urine with oxalate leads to nephrolithiasis/nephrocalcinosis, renal tubular damage, and renal failure with eventual development of systemic manifestations (oxalosis) [Marangella et al 2001]. The presentation of PH1 is variable. Age at onset of symptoms ranges from one to 57 years [Milliner et al 1998, Marangella et al 2001, van Woerden et al 2003]; exceptions occur.Renal manifestations. Overall the renal manifestations of PH1 can be quite variable, with individuals generally falling into one of five groups [Cochat et al 2010]: Early nephrocalcinosis and renal failure leading to a diagnosis in infancy or early childhood (infantile form)Recurrent urolithiasis and progressive renal failure with diagnosis in childhood or adolescenceLate onset form with occasional stones diagnosed in adulthoodDiagnosis secondary to recurrence following renal transplantation Diagnosis in a presymptomatic individual because of a positive family history The median age for end-stage renal disease (ESRD) is 25 to 40 years [Lieske et al 2005b], but it can appear as early as age six months and is present in 50% of children at the time of diagnosis [Cochat et al 1999]. No validated tools are available to predict the renal course in PH1; however, Diallo et al [2004] described two patterns on renal ultrasound examination in individuals known to have PH1 and determined that all five individuals with cortical nephrocalcinosis developed ESRD, in contrast to 2/8 with medullary nephrocalcinosis. From the authors’ experience 19% of affected individuals present before age four to six months with severe early-onset (infantile) disease. In this group, presenting signs and symptoms include nephrocalcinosis (91%) with or without nephrolithiasis (21%), failure to thrive (22%), urinary tract infection (21%), and uremia (14%). Early death is common; 50% have ESRD at diagnosis and 80% develop ESRD by age three years [Cochat et al 1999, Millan et al 2003]. The severity of infantile-onset PH1 is illustrated in case reports [Mayordomo-Colunga et al 2011].Some affected infants are pyridoxine sensitive and show improvement in renal function with high-dose vitamin B₆ therapy [van Woerden et al 2003]. Case reports emphasize the importance of early diagnosis and prompt initiation of appropriate therapy [Patwardhan & Higgins 2005, Khoo et al 2006, Chand & Kaskel 2009, Fargue et al 2009, Orazi et al 2009].Approximately 54% of individuals present in late childhood or early adolescence. Although hematuria, dysuria, and urinary tract infections occur, nephrolithiasis is the most common presentation. In contrast, the presenting finding in the younger child is more likely to be recurrent urinary tract infections or enuresis [Watts 1998, Milliner 2005].The remainder of affected individuals present in adulthood with recurrent renal stones [Amoroso et al 2001]. For many of these adults, the diagnosis was either previously missed or delayed. Those individuals who present at a later age often have urinary oxalate excretion at the lower range of hyperoxaluria values [Watts 1998]. Some individuals, mainly adults, may present with acute renal failure secondary to bilateral renal obstruction caused by oxalate stones. A higher prevalence of ESRD without a prior history of renal stones was observed in a national study from the Netherlands [van Woerden et al 2003]. The natural history of untreated PH1 is one of inexorable decline in renal function as a result of progressive nephrolithiasis/nephrocalcinosis, with eventual progression to oxalosis and certain death from ESRD and/or complications of oxalosis in the absence of treatment [Watts 1998, Cochat & Rolland 2003, Bobrowski & Langman 2008, Hoppe et al 2009]. Oxalosis. When the glomerular filtration rate (GFR) is less than 25 mL/min/1.73m², the daily production of oxalate far outstrips renal oxalate clearance, resulting in a rapid decline in residual renal function with a concurrent increase in body oxalate stores (oxalosis).Oxalosis is the deposition of oxalate in a variety of tissues, including the kidneys, retina, myocardium, and bone marrow (see Table 4). Oxalosis tends to occur in the presence of substantial preexisting renal impairment [Watts 1998, Milliner 2005, Cochat et al 2006]. Calcium oxalate deposition occurs in the blood vessels, retina, heart, peripheral nerves, bone and bone marrow, subcutaneous tissue, and synovia [Watts 1998].Bone is the largest repository for excess oxalate. Deposition of oxalate in bone, commonly identified as dense suprametaphyseal bands on x-ray, may lead to pain, erythropoietin-resistant anemia, and spontaneous fracture [Cochat et al 2006]. Bone mineral density measurements, as opposed to the gold standard of bone biopsy, allow for noninvasive assessment of oxalate burden [Behnke et al 2001].Atherosclerotic oxalosis. A distinct form of cardiovascular involvement in oxalosis is the deposition of calcium oxalate crystals in atherosclerotic plaques of the coronary artery and other sites in the absence of renal insufficiency [Fishbein et al 2008]. Table 4. Some Tissues Involved in Systemic Calcium Oxalate Deposition in PH1View in own windowTissueClinical Symptom and PathologyKidney and urinary tractUrolithiasis
Nephrocalcinosis
Renal failure
Hematuria
Pyelonephritis
HydronephrosisBoneBone pain
Multiple fractures
OsteosclerosisEyeRetinopathy
Optic atrophyTeeth, mouth, and associated structuresRoot resorption
Pulp exposure
Tooth mobility
Dental painNervesPeripheral neuropathy (axonal degeneration and segmental demyelination)Brain and meningesUnclear as to brain depositionHeartHeart block
Myocarditis
Cardioembolic strokeDeep vasculatureVasospasmPeripheral vasculature and skinLivedo reticularis
Peripheral gangrene
Calcinosis cutis metastaticaBone marrowPancytopenia
HepatosplenomegalyCartilage, ligaments, and synovial tissueArthropathyMiscellaneous: thymus, adipose tissue, skeletal muscle and liver blood vesselsHypothyroidism
Calcification of vesselsFrom Danpure [2001]Pregnancy. Pregnancy does not appear to be an important risk factor for the development of ESRD in the majority of women with PH1 [Norby & Milliner 2004]; however, women in whom renal function deteriorated during the pregnancy and remained abnormal post-delivery have been reported [Cimino et al 2005]. Following pregnancy women have an increased risk for stone formation.Women with PH1 have had successful pregnancies post-liver/kidney transplantation. In one woman liver function was apparently preserved, but renal graft function declined transiently after the birth of her first child and permanently after the birth of her second child [Pruvot et al 1997].Generally, the offspring of women with PH1 have done well [Norby & Milliner 2004].

Genotype-Phenotype CorrelationsIndividuals with mutations that result in mistargeting of AGT enzyme activity (e.g., p.Gly170Arg) are the most likely to respond to B₆ (pyridoxine) therapy [Danpure 2001]. The response to B₆ therapy is relative to the number of copies of the p.Gly170Arg mutation present [Monico et al 2005a, Monico et al 2005b].Another disease-causing mutation, p.Phe152Ile, is also associated with mistargeting; however, the residual AGT enzyme activity is expected to be lower than that observed with the p.Gly170Arg mutation. The p.Phe152Ile mutation is apparently responsive to B₆ as well; data are insufficient to determine if this is also a graded response [Monico et al 2005a]. Although it may be possible to establish a relationship between genotype and AGT function in vitro, in most cases it is difficult to correlate enzyme activity with clinical severity [Danpure 2001, Pirulli et al 2003, Danpure & Rumsby 2004], possibly because of numerous non-genetic factors influencing the clinical course. In a study of 33 individuals with PH1 that compared genotype and biochemical phenotype with clinical outcome, a spectrum of clinical heterogeneity was observed [van Woerden et al 2004]. However, homozygosity for either p.Gly170Arg or p.Phe152Ile correlated with B₆ responsiveness and such individuals benefited from early treatment.A large retrospective study of individuals with PH1 suggested that the p.Gly170Arg mutation is associated with longer preservation of renal function with conservative treatment compared to other mutations [Harambat et al 2010]. It is clear that in PH1 other genetic and environmental factors play a role in determining the course of the disease [Hoppe 2010].AGXT mutations may not provide clues to the clinical differences in the course of the disease. Approximately 19% of affected individuals present with a severe, very early-onset form of PH1 in the first few months of life. At the other end of the spectrum of clinical severity seen in PH1, some individuals remain apparently asymptomatic for more than 40-50 years [Danpure 2001].Although the clinical course of PH1 in affected sibs is usually similar, families with members having identical mutations but different disease manifestations have been described. In one family, six sibs were homozygous for the common c.33dupC mutation; one was affected prenatally, four were diagnosed between ages two and seven months, and one was asymptomatic at age 20 years [Frishberg et al 2005]. In another family, two adult males were affected but their two younger (adult) sisters who had the same genotype remain asymptomatic. This sex-related difference fits with the preponderance of males over females observed in the Italian hyperoxaluria registry [Mandrile et al 2008]. A cohort of adults with the same mutation (p.Ile244Thr) showed no consistent clinical phenotype [Beck & Hoppe 2006, Lorenzo et al 2006].Possible causes for this variation include differences in activity level of other enzymes important in oxalate synthesis, modifier genes, the quantity of oxalate precursors in the diet, renal oxalate handling, absorption of dietary oxalate, hydration status, infections, and urinary crystallization factors (reviewed in Danpure [2001]). A recent study of an oxalate transporter (SLC26A6) variant suggested that heterozygosity had no effect on plasma and urine oxalate in persons with PH1 [Monico et al 2008, Rumsby 2008]. Studies of oxalate absorption in children with primary hyperoxaluria indicated low intestinal oxalate absorption compared to normal controls [Sikora et al 2008].The reason for a poorer outcome in infants with the same mutations as older individuals is not clear. Possibilities include [Luyckx & Brenner 2005]:The low glomerular filtration rate (GFR) (both absolute and corrected) in children under age six to 12 months, which predisposes to earlier oxalate deposition; The normal variation in number of nephrons present at birth.

Differential DiagnosisPrimary hyperoxaluria type 2 (PH2) is caused by deficiency of the cytosolic enzyme glyoxylate reductase (GR), which catalyzes the reduction of glyoxylate and hydroxypyruvate. GR is encoded by GRHPR. PH2 is rarer than PH1. In PH2 glyoxylate removal is impaired, resulting in the metabolism of glyoxylate to oxalate and L-glycerate. The diagnosis of PH2 can be established by assay of GR enzymatic activity in liver.From a small cohort of individuals with PH1 and PH2 from one center, PH1 as a group appears to differ from PH2 in the following respects:PH2 is considered a less aggressive disease than PH1, even when onset is early.PH1 has statistically higher urine oxalate excretions and more stone-forming activity and thus requires more frequent stone removal.Individuals with PH1 have statistically lower urine osmolalities and lower urine concentration of calcium, citrate, and magnesium [Milliner et al 2001]. (For a single individual with hyperoxaluria, the differences observed CANNOT reliably distinguish PH1 from PH2.)In PH1, urinary glycolate and oxalate are elevated.In PH2, urinary L-glycerate and oxalate are elevated [Danpure 2001]; however, exceptions exist.Primary hyperoxaluria type 3 (PH3). Elevated oxalate and glycolate have been seen in about 5% of affected individuals in what appears to be a primary hyperoxaluria with normal AGT and GR enzymatic activity [Monico et al 2002, Hoppe et al 2009], now designated PH3. Mutations in HOGA1 which encodes 2-keto-4-hydroxy-glutarate aldonlase have been proposed as causative of PH3 [Belostotsky et al 2010]. The role of this gene remains the subject of investigation as mutations would have to activate the enzyme rather than inactivate it given our present understanding of the metabolic pathway. Enteric hyperoxaluria. Diseases affecting the small bowel, including celiac disease [Ciacci et al 2008], Crohn's disease, pancreatitis, and short bowel syndrome can be associated with hyperoxaluria. The precipitation of enteric calcium by non-absorbed free fatty acids leads to loss of the normal inhibition in oxalate reabsorption from the gut, increasing plasma oxalate concentration by increasing paracellular and transcellular transport. Delivery of excess fatty acids and bile salts to the colon also injures the mucosa and increases oxalate absorption [Milliner 2005, Hoppe et al 2009]. Individuals with PH1 show low to normal levels of oxalate absorption [Sikora et al 2008]. Gastric bypass procedures used in the treatment of obesity have been associated with increased oxalate absorption, high levels of hyperoxaluria, and increased risk of kidney stone formation [Asplin & Coe 2007, Kleinman 2007, Duffey et al 2008, Lieske et al 2008]. Urinary risk factors for stones such as hyperoxaluria occur more commonly in individuals with Roux-en-Y gastric bypass than gastric banding [Semins et al 2010, Kumar et al 2011, Tasca 2011].Dietary hyperoxaluria. Excess intake of foods high in oxalate including chocolate, cocoa, leafy greens (especially rhubarb and spinach), black tea, nuts, peanut butter, or starfruit [Holmes & Kennedy 2000, Monk & Bushinsky 2000] may lead to elevated plasma concentration of oxalate and hence increased urinary concentration of oxalate.It was previously thought that dietary oxalate accounted for little of the urinary oxalate levels (<10%), but Holmes et al [2001] showed that between 24% and 53% of urinary oxalate is attributable to oxalate from the diet [Holmes & Assimos 2004]. Therapy consists of dietary oxalate restriction and use of calcium carbonate or calcium citrate at meal times to bind dietary oxalate [Penniston & Nakada 2009]. Idiopathic calcium oxalate urolithiasis is associated with "mild metabolic hyperoxaluria." Features that often differentiate this from PH1:Lower urinary oxalate excretion (see Table 5)Less severe stone diseaseLess common development of ESRDTendency to hypercalciuria as opposed to hypocalciuria in PH1 (or PH2)Day-to-day variability in the levels of urinary oxalate excretion in contrast to PH1, in which levels are persistently elevated in the urine [Milliner 2005]Table 5. Urinary Oxalate Excretion Rates in Disorders Considered in the Differential Diagnosis of PH1View in own windowCustomary Urinary Oxalate Excretion Rates ^{1NormalIn Individuals with:PH1PH2 2Enteric hyperoxaluriaDietary hyperoxaluriaIdiopathic calcium oxalate urolithiasis>1(2)>0.46 3>1
(fluctuates with diet)Based on data from algorithm in Milliner [2005], with permission from S Karger AG, publisher1. Calculated as mmol/1.73 m2/day2. PH2 information based on Kemper et al [1997]3. The presence of urinary L-glycerate levels >28 µmol/mmol of creatinine distinguishes PH2 from PH1 [Kemper et al 1997].Dent's disease. The clinical features of Dent's disease may overlap those of PH1. Both are associated with nephrocalcinosis and urolithiasis in childhood and progress to renal failure (see Table 6) [Milliner 2006].Table 6. Clinical and Diagnostic Features of Dent's Disease and Primary Hyperoxaluria Type 1View in own windowDent's DiseasePH1Clinical FeaturesNephrocalcinosis3+2+Urolithiasis3+4+Osteodystrophy1+1+ 1Renal failure2+2+Differentiating FindingsHypercalciuria1+—Hyperoxaluria—3+Low-molecular-weight proteinuria1+— 2Gene symbolCLCN-5 AGXTInheritanceX-linked recessiveAutosomal recessiveFrom Milliner [2006]1. After renal failure established2. May be observed following renal damage but not an early or characteristic findingOther. Acute renal failure secondary to oxalate deposition in the kidneys has occurred in persons taking large doses ("megadoses") of ascorbic acid (vitamin C) [Petrarulo et al 1998, Mashour et al 2000].Ingestion of ethylene glycol, an oxalate precursor, can lead to excess production and increased concentrations of oxalate in both the plasma and urine [Milliner 2005].Hyperoxaluria in association with total parenteral nutrition (TPN) has been described in premature infants [Sikora et al 2003] and adults [Buchman et al 1995].Hyperoxaluria has been documented in peroxisomal biogenesis disorders, Zellweger spectrum despite the apparent cytoplasmic stability of AGT [van Woerden et al 2006]. The presence of hyperoxaluria was statistically correlated with the degree of neurologic involvement.Note to clinicians: For a patient-specific ‘simultaneous consult’ related to this disorder, go to , an interactive diagnostic decision support software tool that provides differential diagnoses based on patient findings (registration or institutional access required).}

ManagementEvaluations Following Initial DiagnosisTo establish the extent of disease in an individual diagnosed with primary hyperoxaluria type 1 (PH1), the following evaluations are recommended:Determination of genotype if not already done Measurement of plasma/urinary oxalate and glomerular filtration rate (GFR)Given the systemic nature of the oxalate deposition, it is reasonable to evaluate the following organ systems for oxalate deposition/burden, in addition to assessing the renal function [Hoppe & Langman 2003]:Bone x-rays to evaluate for radiodense metaphyseal bands and diffuse demineralizationOphthalmologic evaluation for evidence of retinal oxalate depositionThyroid function testingElectrocardiogram to evaluate for an associated atrioventricular blockHemoglobin to evaluate for anemia associated with either renal dysfunction or marrow deposition of oxalateHistory and physical examination to assess the risk of arterial insufficiency or ischemia based on vessel wall depositionTreatment of ManifestationsReduction of Calcium Oxalate SupersaturationThe general therapies for nephrolithiasis benefit all individuals with PH1. Early diagnosis and initiation of conservative therapy are critical in preserving adequate renal function for as long as possible [Fargue et al 2009]:Drinking large volumes of fluid (2-3 L/m²/24 hours) at regular intervals over the entire day/night prevents calcium oxalate supersaturation [Cochat et al 2006]. This has been shown to be effective regardless of genotype. Small children may require gastrostomy or nasogastric tube insertion for both feeds and fluid supplementation. Extreme care should be taken during any illness that could lead to hypovolemia or decreased oral fluid intake.Supplementation of dietary calcium (300 mg) or provision of supplemental calcium (300-500 mg) at each meal significantly decreased urinary calcium oxalate without altering calcium excretion [Penniston & Nakada 2009]. While promising, this strategy remains unproven in individuals with PH1.Drugs such as the thiazides and potassium citrate (potassium magnesium citrate) or neutral orthophosphates can decrease urinary calcium excretion and inhibit stone formation, respectively [Leumann & Hoppe 2001, Hoppe et al 2009]. Any significant intake of vitamin C or D is to be avoided as both may promote stone formation.Reduction of Oxalate BiosynthesisAGT is a pyridoxal phosphate (PLP)-dependent enzyme. Approximately 10%-30% of individuals with PH1 respond to treatment with pyridoxine (vitamin B₆, precursor to LPL) as defined by a greater than 30% reduction in plasma oxalate concentration or normalization of urinary oxalate excretion after a minimum of three months of maximal therapy [Watts et al 1985, Leumann & Hoppe 2001]. Of this group, only 40% show normalization and the other 60% a partial reduction in the concentration of plasma and urine oxalate [Toussaint 1998].Monico et al [2005b] showed that the presence of the p.Gly170Arg mutation was associated with pyridoxine responsiveness — homozygotes showing normalization (urine oxalate concentration <0.5-0.7 mmol/1.73m²/day) and compound heterozygotes demonstrating a partial reduction in plasma oxalate concentration by at least 30% of prior documented levels.At present, two different approaches are used to titrate the pyridoxine dose: A stepwise increase from initial low levels (1-2 mg/kg/day) [Bobrowski & Langman 2008]Initial high doses to maximize oxalate removal, with subsequent reduction to establish the minimal effective doseWith either approach, doses of pyridoxine in the range of 5 mg/kg/day appear adequate in the treatment of those likely to respond, with no additional benefit expected at doses higher than 10 mg/kg/day [Monico et al 2005b, Bobrowski & Langman 2008, Hoppe et al 2009]. In adults, a dose of 500 mg is felt to remain below the toxic range. Paresthesias, a known complication of large doses of pyridoxine [Toussaint 1998], have only developed in one individual on a dose of 2.1 mg/kg/day and resolved following discontinuation of the drug. Pyridoxal phosphate levels may be followed in individuals to ensure that adequate absorption is occurring [Harambat et al 2011].Individuals responsive to pyridoxine should continue this therapy after a kidney transplantation to decrease the burden of oxalate on the kidney.DialysisThe maximal elimination of oxalate is 950-1400 mmol/day using standard maintenance hemodialysis (HD) or peritoneal dialysis (PD), whereas the oxalate production in an individual with PH1 requiring dialysis is between 3,500 and 7,500 mmol/day [Cochat 1999]. In general, HD is much better than PD but the two are often used together. To date, there has been limited success in stabilizing and even reversing an affected individual's oxalate burden by use of long (6- to 8-hour) daily hemodialysis. Intensified dialysis protocols can be more effective but this intensity of care is difficult to maintain long term [Cochat 1999, Diaz et al 2004, Bobrowski & Langman 2008].Despite the limitations of dialysis, Cochat et al [2010] have suggested six situations where dialysis may be indicated: When PH1 is not yet diagnosed in an individual requiring dialysis for other reasonsIn a small child/infant with oxalosis awaiting transplantation As a strategy to deplete body oxalate burden preceding or after liver transplantation As an adjunct therapy to decrease oxalate burden in the presence of delayed or poor renal function after transplantationIn older individuals if transplantation is not deemed an option In countries with no access to organ transplantation [Cochat et al 2006]Organ TransplantationAs neither maintenance hemodialysis (HD), peritoneal dialysis (PD), nor a combination of the two clears oxalate quickly enough to prevent systemic oxalosis in an individual with a glomerular filtration rate (GFR) lower than 25-30 mL/min/1.73m² [Thamilselvan & Khan 1998], transplantation is an acceptable option for disease therapy or perhaps cure [Marangella et al 2001].Much discussion has occurred regarding the best transplantation strategy for an individual with PH1. The three current organ transplantation strategies are: (1) isolated kidney transplantation (restores oxalate excretion to "normal"); (2) "preemptive" liver transplantation before end-stage renal disease (restores enzyme activity, decreases ongoing oxalate synthesis); or (3) combined liver-kidney transplantation, either concurrent or sequential (reduces oxalate synthesis and increases oxalate excretion) [Jamieson 1998, Kemper et al 1998, Scheinman 1998, Marangella 1999, Saborio & Scheinman 1999, Nolkemper et al 2000, Marangella et al 2001, Kemper 2005a, Kemper 2005b, Eytan Mor & Weismann 2009, Jamieson & Jamieson 2009, Cochat et al 2010, Scheinman 2010]. The importance of establishing the diagnosis of PH1 before transplantation is illustrated by two recent reports of individuals with undiagnosed late-onset disease who received isolated kidney transplants and subsequently experienced unexpected rapid recurrence of stones and systemic oxalosis. In both cases a diagnosis of PH1 was made post-transplant [Kim et al 2005, Madiwale et al 2008].It is important to note that in all forms of transplantation, in particular isolated kidney, the individual must:Be monitored closely and even dialyzed following surgery to prevent further calcium oxalate deposition in the kidney graft from mobilization of the body burden of oxalate;Continue pyridoxine supplementation to promote excretion of the total body store of oxalate if responsiveness has been documented prior to transplantation [Marangella 1999].Isolated kidney transplantation has generally been supplanted by combined liver-kidney or preemptive liver transplantation (see following section). Even in an era of much-improved survival post-renal transplantation and with optimized conservative therapies, the recurrence of disease in the graft (i.e., nephrocalcinosis/urolithiasis) and the ongoing burden of disease remain problematic [Saborio & Scheinman 1999, Lorenzo et al 2006, Cochat et al 2009]. Other than in individuals with total pyridoxine responsiveness or very slow progression of their disease, isolated kidney transplantation is not currently recommended in individuals with PH1 [Leumann & Hoppe 2001, Kemper 2005a, Bobrowski & Langman 2008, Hoppe et al 2009], although it may play a role in the treatment of individuals with PH2 [Harambat et al 2011].Preemptive liver transplantation is an attractive approach for individuals with PH1, as replacing the liver halts the production of oxalate and may arrest any further damage to the kidneys (and/or other organs) [Bobrowski & Langman 2008]. Optimal timing of transplantation is controversial, varying from a GFR greater than 80 mL/min/1.73 m² to one between 40 and 60 mL/min/1.73 m². Some have used this strategy in individuals with GFRs as low as 30 mL/min/1.73 m² in hopes of stabilizing the renal function once the hyperoxaluria is repaired [Nolkemper et al 2000].In his review of the 11 published cases of preemptive liver transplantation in individuals with PH1, Kemper [2005a] is critical of the strategy calling for isolated liver transplantation at either normal or significantly reduced GFR, and suggests (as have others) that evaluation for potential preemptive liver transplant occur at a GFR of between 40 and 60 mL/min/1.73m² and be reserved for those with progressive loss of function or severe urolithiasis despite optimal use of more conservative therapy [Kemper 2005a]. Using the above parameters, the window of opportunity for isolated liver transplantation may be narrow; in affected individuals with a GFR 60 mL/min/1.73 m², the rate of progression of disease and the effectiveness of conservative management strategies are unpredictable [Scheinman 2010]. Despite much-improved outcomes in liver transplantation even in small children [Broering et al 2004, Grabhorn et al 2004, Brinkert et al 2009, Jamieson & Jamieson 2009, Galanti & Contreras 2010], morbidity and mortality are considerations and lend pause to the decision to proceed with preemptive liver transplantation at near-normal levels of renal function. The need to replace a fully functional liver in an individual with PH1 in order to repair an isolated metabolic defect raises ethical concerns regarding surgical risk and the need for lifelong immunosuppression [Leumann & Hoppe 2000, Hoppe et al 2009]. The attendant surgical and immmunosuppression risks of preemptive liver transplantation need to be balanced with those of combined liver/kidney transplantation. The level of expertise of the center performing the transplantation should also be considered in the decision for preemptive liver transplantation. Combined liver/kidney transplantation has excellent outcome data from both the European Registry [Jamieson 1998] and the US [Millan et al 2003] with greater graft survival than in isolated kidney transplantation [Bobrowski & Langman 2008, Brinkert et al 2009, Jamieson & Jamieson 2009]. Kidney graft loss is much less when transplantation is combined with a liver (95% vs 56% at 3 years) [Bergstralh et al 2010].Combined liver/kidney transplantation can be performed in one of two sequences: sequential (liver then kidney) or concurrent (simultaneous). The issues around the superiority of sequential vs. concurrent liver-kidney transplantation remain unclear with little prospective evidence in support of one over the other [Kemper 2005b]. In infants and very young children, there may be problems around obtaining size-matched organs at the optimal time for transplantation [Eytan Mor & Weismann 2009, Heffron et al 2009, Harps et al 2011]. Sequential liver then kidney transplantation may hold some advantage over concurrent liver/kidney transplantation in certain scenarios including individuals already on dialysis. Sequential transplant may also be the strategy of choice for very small children if an appropriate kidney is not initially available. In two such cases, the liver transplant allowed some clearing of the tissue oxalate burden which was thought to reduce the load on a subsequent renal graft [Rosenblatt et al 2006, Malla et al 2009]. However, data on this approach have rarely been published for PH1 [Kemper 2005b], making it difficult to assess risks and benefits; at least two individuals have died in an earlier-reported series. Advantages of the concurrent strategy include: immediate cure of the metabolic defect; replacement of poor renal function, which assists in removing the burden of body oxalate; and minimization of the risk of both morbidity and mortality as seen in individuals with renal failure post-liver transplantation [Richardson 2001]. Evidence also supports potential immunologic benefit for the highly sensitized renal graft recipient in the face of a combined liver/kidney transplant [Olausson et al 2002]. A recent report emphasizes the need for intensive hemodialysis and other measures to decrease the oxalate load prior to surgery to prevent damage to the new kidney. Likewise the immediate post-operative ICU care of a child receiving combined liver/kidney transplantation is complex and critical for long-term success of the organs. The additional risk of the dual surgery includes possible excess bleeding, which is amplified in individuals requiring heparinization for ongoing hemodialysis post-transplantation [Kemper 2005b].Cochat et al [2010] have recently outlined a staged approach to determining which transplantation strategy could be deemed appropriate based on renal function or age of diagnosis [Cochat et al 2010 (see table; registration or institutional access required)].Prevention of Primary ManifestationsMaintenance of high fluid intake to maximize calcium oxalate solubility in the urine is the most important preventive measure in an asymptomatic individual with AGXT disease-causing mutations. Recommendations vary, but aiming for a minimum of 2.5 L/m² is likely to achieve the desired urine concentration of oxalate lower than 0.5 mmol/L. This fluid intake must continue throughout the 24-hour day; in small children this may require use of a nasogastric or gastrostomy tube [Leumann & Hoppe 2001].Pyridoxine supplements are indicated in those individuals identified to be pyridoxine responsive (see Treatment of Manifestations).Addition of potassium or sodium citrate (100-150 mg/kg/day in 3-4 divided doses) or neutral orthophosphate (20-60 mg/kg/day) is indicated for primary prevention [Leumann & Hoppe 2001, Bobrowski & Langman 2008].Prevention of Secondary ComplicationsSecondary complications may arise as a result of systemic oxalosis. Table 4 lists the broad range of tissues and organs that suffer consequences of oxalosis including, for example: Cardiac manifestations [Quan & Biblo 2003, Van Driessche et al 2007]; Vascular occlusions [Bogle et al 2003];Hypothyroidism, which requires thyroid hormone replacement therapy [Frishberg et al 2000]. Regular dental care should be part of the patient management. SurveillanceThe following are appropriate:Annual surveillance for renal dysfunction, hypertension, anemia, and involvement of the skin, bone, eye, heart, or thyroid (see Note below).This should include regular monitoring of plasma/urinary oxalate concentrations as well as glomerular filtration rate (GFR).Regular renal ultrasound examinations to assess renal stone burden and to rule out a secondary renal obstruction. In individuals with symptoms of stones or obstruction, this may be recommended every 12-24 months (see Note).For pregnant women with PH1, close monitoring during pregnancy by both an obstetrician and nephrologist because of the increased risk of developing nephrolithiasis after deliveryNote: Investigations should likely occur more often in newly diagnosed symptomatic individuals or in children younger than age two to three years.Agents/Circumstances to AvoidAvoid the following:Intravascular volume depletion (The importance of maintaining dilute urine CANNOT be overemphasized.)Foods high in oxalate (chocolate, rhubarb, and starfruit in particular)Megadoses of vitamin C (ascorbate) or vitamin DLoop diuretics to maintain dilute urine, which can lead to hypercalciuria and increase the production of calcium oxalate stonesOne infant with PH1 developed hepatitis after exposure to the anesthetic sevoflurane; this was felt to be an idiosyncratic reaction [Reich et al 2004].Evaluation of Relatives at RiskConsideration should be given to testing asymptomatic at-risk family members in order to plan early treatment, monitoring, and preventive intervention. If the family-specific PH1-causing mutations are known, the affected/carrier status of at-risk family members can be confirmed by molecular genetic testing. The benefits of early initiation of conservative measures cannot be ignored [Chand & Kaskel 2009, Fargue et al 2009, Martin et al 2011].Asymptomatic individuals:Can be monitored periodically for renal function and urinary oxalate; Should maintain adequate hydration and avoid high-oxalate foods.In addition: Potassium citrate administration may also be considered as an aid to reducing calcium oxalate excretion [Leumann & Hoppe 2001]. For those with the mutation p.Gly170Arg or p.Phe152Ile, pyridoxine should be supplemented. See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.Pregnancy ManagementPregnancy does not appear to be an important risk factor for the development of end-stage renal disease (ESRD) in the majority of women with PH1 [Norby & Milliner 2004]; however, women in whom renal function deteriorated during the pregnancy and remained abnormal post-delivery have been reported [Cimino et al 2005]. Therapies Under InvestigationSeveral novel therapies are under investigation. Oxalate-degrading bacteria. Approaches involving oral administration of bacteria such as Oxalobacter formigenes or lactic acid bacteria to degrade oxalate [Sidhu et al 2001] and reduce the amount of oxalate available for intestinal absorption [Campieri et al 2001, Lieske et al 2005a, Azcarate-Peril et al 2006] are being investigated. O formigenes shows the most promise as a potential therapy for the hyperoxalurias. It is a normal component of the intestinal flora although many individuals do not maintain colonization. O formigenes is also thought to stimulate secretion of endogenous oxalate into the intestine for its own metabolic use [Hatch et al 2006, Hatch & Freel 2008]. A controlled diet was compared to two probiotic preparations to evaluate urinary oxalate levels and calcium oxalate supersaturation in individuals with hyperoxaluria and calcium oxalate stones of unknown etiology [Lieske et al 2010]. The probiotic Oxadrop contains L acidophilus, L brevis, S thermophilus, and Bifidobacterium infantis. AKSB is a formulation designed by the Mayo Clinic which contains Enterococcus faecium, Saccharomyces cerevisiae subspecies boulardi, LEVUCELL SB (yeast), S cerevisiae and fructooligosaccharide. When administered to stone formers, neither formulation reduced the amount of urinary oxalate excreted by affected individuals or the urinary calcium-oxalate supersaturation levels. However, an oxalate-restricted diet alone for one week significantly reduced the calcium oxalate supersaturation and the urinary levels of oxalate.A probiotic, VSL#3 (Sigma-Tau Pharmaceuticals, Inc., Gaithersburg, MD, USA), which contains freeze-dried live lactic acid bacteria made up of Streptococcus thermophilus, Bifidobacterium breve, B longum, B infantis, Lactobactillus acidophlus, L Plantarum, L paracasei and L delbrueckii subspecies bulgaricus was administered to 13 healthy volunteers who were then challenged to an oral load of 80 mg oxalate [Okombo & Liebman 2010]. Four of the subjects who started with higher baseline oxalate levels showed the largest reduction in urinary oxalate levels compared to the other subjects. The authors suggest that individuals who are hyperabsorbers of oxalate may benefit most from the probiotic VSL#3. This study must be validated in individuals with PH1. Similar studies have been performed in mice genetically altered to mimic PH1. AGXT-deficient mice were both hyperoxaluric and hyperoxalemic [Hatch et al 2011]. In AGXT-deficient mice colonized with Oxalobacter formigenes, urinary and plasma levels of oxalate decreased by 50%. Advances have been made to circumvent the colonization of O formigenes by expressing bacterial oxalyl-CoA decarboxylase and formyl-CoA transferase in human embryo kidney (HEK) 293 cells [Ye et al 2007]. Further experiments have shown that the enzymes are expressed in the cytosol of cells and transfected cells were able to degrade oxalate to some degree. Although still in the experimental stages, the transfer of genes encoding oxalate-degrading enzymes may be a potential candidate for gene therapy of hyperoxalurias. In a different approach, a crystalline-stabilized oxalate-degrading enzyme has been used successfully in a mouse model system and may avoid the colonization issue [Grujic et al 2009].Hepatocyte transplantation and gene therapy. Repopulation of the liver of an individual with PH1 with normal or genetically corrected hepatocytes is less invasive than liver transplantation. However, host cells must be ablated as they would continue to produce oxalate and the donor hepatocytes would then require a growth advantage to achieve repopulation. The effectiveness of this approach has been demonstrated in a mouse model of PH1 [Guha et al 2005, Jiang et al 2008]. Koul et al [2005] transfected AGXT genetically engineered for selective peroxisomal delivery into cultured human hepatocytes by amplifying the cDNA and using liposomal transfection techniques. They demonstrated high efficiency of transfection and appropriate intracellular localization to peroxisomes [Koul et al 2005]. More recently Salido et al have demonstrated successful replacement of AGT enzyme activity in the livers of a knockout mouse model of PH1 utilizing a somatic gene transfer via two adeno-associated viral vectors. That they were able to do so in the absence of either hepatic toxicity or immunogenicity for at least the first 50 days is very encouraging [Salido et al 2011]. Pyridoxamine. This approach aims to reduce oxalate by targeting precursors in the metabolic pathway and preventing their eventual metabolism to oxalate. Pyridoxamine, a drug touted as therapy for human diabetic nephropathy, is used to trap glycoaldehyde and glyoxylate. Animal studies have shown 50% reduction of urinary oxalate excretion [Chetyrkin et al 2005, Scheinman et al 2005]; current evidence from preclinical and phase II trials in humans seems to demonstrate a favorable toxicity profile of pyridoxamine. Chemical chaperones. These small molecules facilitate folding of new proteins offering protection from cellular quality-control degradative processes. Stabilization of missense AGT may permit the protein to achieve a folded state with some degree of enzymatic activity [Danpure 2005a, Danpure 2005b]. In PH1 specifically, this type of effect has been demonstrated in vitro for both mistargeting and aggregation/accelerated degradation polymorphism-mutation combinations [Lumb et al 2003, Coulter-Mackie & Lian 2008, Hopper et al 2008]. Chemical chaperones may have general stabilizing functions or they may be designed to target specific mutations. Missense mutations in which the mode of action is not through loss of stability are not suitable candidates for this pharmacogenetic approach; nor are mutations including insertions, deletions, nonsense mutations, or splice junction changes that usually do not produce a protein product.Manipulation of the metabolic pathway. The concept of substrate depletion is aimed at reducing the amount of available glyoxylate, the immediate precursor of oxalate, thereby reducing the oxalate concentration [Coulter-Mackie 2006]. An Agxt knockout mouse model has been developed to explore the effects of substrate depletion and to clarify the various adjustments in the metabolic pathway that result from absence of AGT [Hernandez-Fernaud & Salido 2010]. A model system developed in CHO cells uses stable transfection with all combinations of GO, GA, GR, and AGT, allowing investigation of the interaction of these enzymes and the effects of deficiencies of one or more [Behnam et al 2006]. Another study examined the effect on urinary oxalate of dietary hydroxyproline from collagen, which enters the pathway farther upstream. Results suggested that hydroxyproline metabolism may be a significant contributor to glyoxylate and oxalate [Knight et al 2006]. A study of eight recombinant cytosolic aminotransferases suggested that phosphoserine aminotransferase and alanine transaminase were able to transaminate glyoxylate to glycine efficiently. These reactions may compete with conversion of glyoxylate to oxalate [Donini et al 2009].Severe dietary restriction of either oxalate [Hatch & Freel 1995] or glycine, a glyoxylate precursor, is of little utility in decreasing the accumulation of oxalate in individuals with PH1 [Danpure 2001].Trapping glycine in the liver utilizing benzoate or inhibiting the conversion of glycine to glyoxylate is of little or no clinical utility [Danpure 2001].Search ClinicalTrials.gov for access to information on clinical studies for a wide range of diseases and conditions.

Molecular GeneticsInformation in the Molecular Genetics and OMIM tables may differ from that elsewhere in the GeneReview: tables may contain more recent information. —ED.Table A. Primary Hyperoxaluria Type 1: Genes and DatabasesView in own windowGene SymbolChromosomal LocusProtein NameLocus SpecificHGMDAGXT2q37​.3Serine--pyruvate aminotransferaseAGXT mutation database
AGXT homepage - Mendelian genesAGXTData are compiled from the following standard references: gene symbol from HGNC; chromosomal locus, locus name, critical region, complementation group from OMIM; protein name from UniProt. For a description of databases (Locus Specific, HGMD) to which links are provided, click here.Table B. OMIM Entries for Primary Hyperoxaluria Type 1 (View All in OMIM) View in own window 259900HYPEROXALURIA, PRIMARY, TYPE I; HP1 604285ALANINE-GLYOXYLATE AMINOTRANSFERASE; AGXTMolecular Genetic PathogenesisWhen alanine:glyoxylate aminotransferase (AGT) enzymatic activity is deficient, the substrate glyoxylate accumulates and is converted to oxalate by glycolate oxidase in peroxisomes or in the cytosol by lactate dehydrogenase [Holmes & Assimos 1998, Danpure 2001]. Oxalate forms insoluble calcium oxalate salts that the body cannot readily eliminate. In the most common primary hyperoxaluria type 1 (PH1) genotype, the AGT enzyme is mistargeted to the mitochondria rather than to the peroxisomes, where the substrate remains. The mistargeted AGT enzyme retains substantial enzymatic activity but has no contact with its substrate, and thus the consequences are the same as for those mutations resulting in no enzymatic activity. Mistargeting and high residual activity are seen in heterozygotes and homozygotes for the mistargeting mutation [Danpure 1998, Danpure 2001].Normal allelic variants. Normal AGXT consists of 11 exons. At present, two normal alleles of AGXT are known: the major allele (80% frequency in individuals of European origin) and the minor allele (20% frequency in individuals of European origin, 2% in Japanese, 3% in South African Blacks) [Danpure et al 1994b, Coulter-Mackie et al 2003].The minor allele is a haplotype of at least three intragenic variants, p.Pro11Leu, p.Ile340Met, and a 74-bp duplication in intron 1 (c.165+14_+88del74). The only normal allelic variant of functional significance is the p.Pro11Leu substitution, which leads to an altered amino acid sequence at the N-terminal of the AGT protein thought to permit formation of an α-helical conformation similar to that found in proteins normally targeted to the mitochondria [Purdue et al 1991]. In the absence of a disease-causing mutation, this mitochondrial targeting caused by the minor allele is of no clinical significance.In vitro studies have suggested that p.Ile340Met is associated with a decreased affinity for the PLP cofactor whereas p.Pro11Leu is associated with decreased enzymatic activity [Coulter-Mackie et al 2005b]. Results of denaturation studies of AGT from the major and minor alleles are consistent with a destabilization of the minor allele protein relative to the major allele protein due to p.Pro11Leu [Cellini et al 2010a].High frequencies of p.Pro11Leu were speculated to be linked to populations with an ancestral diet high in meat whereas lower frequencies were associated with populations with lower meat consumption or vegetarian diet [Caldwell et al 2004]. A subsequent study suggests that these differences are attributable more to demographic history than to local dietary adaptation [Ségurel et al 2010].Three other normal variant single-base substitutions documented are c.X41C>A(3'UTR) and two silent changes, c.264C>T and c.654A>G, which do not change the amino acid encoded at the site [Purdue et al 1990, von Schnakenburg & Rumsby 1997]. Exceptions to the major and minor allele haplotypes: A third variant, an African minor allele, has a 74-bp duplication in intron 1 (c.165+14_+88del74) but lacks p.Pro11Leu and p.Ile340Met. It is found in 12% of Blacks from southern Africa [Coulter-Mackie et al 2003].In rare cases, the 74-bp duplication is absent [Amoroso et al 2001] or the p.Ile340Met change is absent [Tarn et al 1997].A VNTR (variable number tandem repeat) located in intron 4 has four allelic variants: c.645+722_754[12]; c.645+722_754[17]; c.645+722_754[32]; c.645+722_754[38]. The allele frequencies vary with ethnicity. In individuals of European origin, the minor allele haplotype is always associated with a c.645+722_754[38] [Danpure et al 1994a]. These normal variants are useful intragenic markers for linkage analysis [Tarn et al 1997] and for determination of phase of mutations. See Table 7.Pathologic allelic variants. More than 140 AGXT mutations have been documented [Williams et al 2009]. A database of AGXT mutations is available on Dr. Gill Rumsby’s Web site.Most AGXT mutations are private (i.e., they have not been documented in more than one family). Missense mutations make up close to 50% of PH1-causing mutations. The four common mutations p.Gly170Arg, p.Phe152Ile, p.Ile244Thr, and c.33dupC together account for more than 50% of PH1 alleles. p.Gly170Arg alone accounts for approximately 25%-40% of PH1-causing alleles:In association with the p.Pro11Leu polymorphism of the minor allele, p.Gly170Arg greatly increases the efficiency of (abnormal) mitochondrial targeting. In conjunction with the p.Pro11Leu variant of the minor allele, p.Gly170Arg has a specific effect on the rate of dimerization of AGT monomers, causing the peroxisome-to-mitochondrion mistargeting [Lumb et al 1999, Lumb & Danpure 2000]. Denaturation studies support a destabilizing effect of p.Gly170Arg [Cellini et al 2010a]. Analysis of the crystal structure of AGT with p.Gly170Arg indicates significant local structural changes that may be associated with decreased protein stability [Djordjevic et al 2010].The therapeutic response of individuals with the p.Gly174Arg to pyridoxine is likely attributable at least in part to enhancement of the dimerization process by increased PLP [Cellini et al 2010b].p.Phe152Ile is also associated with mistargeting. In the absence of saturating PLP, p.Phe152Ile is thought to monomerize and be susceptible to mistargeting [Cellini et al 2009, Cellini et al 2010b]. This is consistent with the positive response to pyridoxine in patients with the p.Phe152Ile variant.p.Ile244Thr appears to be the result of a founder effect within the Canary Islands population [Santana et al 2003]. AGT with the p.Ile244Thr mutation apparently has an altered conformation [Santana et al 2003].A few other mutations have had limited recurrences within specific population groups while the common c.33dupC occurs in a variety of ethnic groups [Coulter-Mackie 2005].Mutations documented in more than one family include p.Gly82Glu, p.Arg233Cys, p.Gly41Arg, and p.Gly156Arg.Most missense mutations have not had specific biochemical phenotypes associated with them other than degradation and loss of enzymatic activity [Coulter-Mackie & Lian 2006]. The pathologic mechanism of a few of the rarer missense mutations is known:p.Gly82Glu apparently prevents binding of the essential cofactor pyridoxine (vitamin B₆) [Lumb & Danpure 2000, Cellini et al 2007]. Rather than an intrinsic inability to bind PLP, this is now thought to be due to an altered binding state of PLP and the AGT-PMP intermediate [Cellini et al 2010b].p.Gly41Arg results in intraperoxisomal aggregation of AGT protein [Danpure et al 1993]. In vitro studies of p.G41 mutations demonstrate a propensity for aggregation particularly in the absence of bound PLP [Cellini et al 2010b, Cellini et al 2010c].p.Ser205Pro results in an unstable protein that is rapidly degraded [Nishiyama et al 1993, Coulter-Mackie & Lian 2008].In addition to the missense mutations, splicing mutations, nonsense mutations, and several small insertions and deletions are known (reviewed by Coulter-Mackie & Rumsby [2004], Williams et al [2009]). Large documented deletions include:Two deletions of the entire 5' one third to one half of the gene and contiguous upstream regions [Nogueira et al 2000, Coulter-Mackie et al 2001]; Two other large intragenic deletions and one extending into the 3’ untranslated region [Coulter-Mackie et al 2005a, Monico et al 2007, Williams et al 2009].See Table 7.Table 7. Selected AGXT Allelic VariantsView in own windowClass of
Variant AlleleDNA Nucleotide Change
(Alias ¹)Protein Amino
Acid ChangeReference
SequencesNormalc.32C>Tp.Pro11LeuNM_000030​.2
NP_000021​.1c.165+14_+88del74
g.241457134_241457207del74
(76-bp dup)NoneNM_000030​.2
NC_000002​.10c.264C>T
(c.386C>T)p.=NM_000030​.2
NP_000021​.1c.645+722_754[12]--c.645+722_754[17]--c.645+722_754[32]--c.645+722_754[38]--c.654A>G
(c.776G>A)p.=c.1020A>Gp.Ile340Metc.X41C>A
(1341C>A)--Pathologicc.33dupC
(33_34insC)p.Lys12Glnfs*156c.121G>Ap.Gly41Argc.245G>Ap.Gly82Gluc.454T>Ap.Phe152Ilec.466G>Ap.Gly156Argc.508G>Ap.Gly170Argc.560C>Tp.Ser187Phec.613T>Cp.Ser205Proc.697C>Tp.Arg233Cysc.731T>Cp.Ile244Thrc.738G>Ap.Trp246XSee Quick Reference for an explanation of nomenclature. GeneReviews follows the standard naming conventions of the Human Genome Variation Society (www​.hgvs.org).1. Variant designation that does not conform to current naming conventionsNormal gene product. The normal alanine:glyoxylate aminotransferase (AGT) protein comprises 392 amino acids and has a molecular mass of 43 kd. In humans, AGT is synthesized mainly in the liver and is normally located exclusively in the peroxisomes [Danpure 2001]. The enzyme is translated in the cytosol and transported into the peroxisomes. AGT is a key enzyme in the detoxification of glyoxylate, converting glyoxylate to glycine [Holmes & Assimos 1998, Danpure 2001]. In humans, glyoxylate is produced in the peroxisomes. PLP is an essential cofactor for AGT activity. The PLP site in AGT lies in a highly conserved amino acid sequence and is critical in the catalytic activity of the enzyme. The crystal structure of the normal AGT protein has been determined [Zhang et al 2003], allowing a delineation of the active site and the dimerization interface.In some literature alanine:glyoxylate aminotransferase (AGT) may be referred to as serine-pyruvate aminotransferase (SPT) (e.g., as in Table A). AGT and SPT are two separate enzymatic activities on the same protein coded by AGXT. AGT is the major activity and the one associated with PH1.Abnormal gene product. Approximately 50% of all individuals with PH1 show no AGT enzymatic activity and produce no immunologically detectable AGT protein.Mutations resulting in nonsense codons, frameshifts, or splice junction mutations are usually expected to result in little or no functional protein.Approximately one third of affected individuals display a high level of residual AGT activity. Most of these individuals exhibit the mistargeting defect in which an otherwise functional AGT enzyme is synthesized in adequate amounts but is mislocalized to mitochondria instead of peroxisomes, where it is normally found and where the substrate glycolate remains. These individuals have classic PH1 despite the residual AGT enzymatic activity.Mutations that cause true partial enzymatic activity appear to be rare and may be associated with late-onset or mild disease.With many genetic diseases, it is now clear that a common consequence of a missense mutation is protein misfolding and subsequent elimination by intracellular quality-control processes [Waters 2001]. This biologic instability of protein carrying a missense change has been documented in p.Ser205Pro [Nishiyama et al 1993] and with a variety of other missense mutations in AGT [Coulter-Mackie & Lian 2006, Coulter-Mackie & Lian 2008, Hopper et al 2008]. Interestingly, some missense mutant proteins remain quite stable to both endogenous proteasome degradation and exogenous protease [Coulter-Mackie & Lian 2008]. One such mutant is p.Gly82Glu, which has been demonstrated to have a reduced affinity for the pyridoxal phosphate cofactor [Cellini et al 2007]. A minor allele background may exacerbate the effect of a given missense mutation. In vitro studies have shown increased stability and enzymatic activity for some mutations when expressed on a major allele background compared to a minor allele [Williams & Rumsby 2007, Coulter-Mackie & Lian 2008, Williams et al 2009]. It has been speculated that some missense mutations found on the minor allele in association with PH1 might not cause disease if they occurred on the major allele. However, some missense mutations (e.g., p.Gly41Arg) found on both major and minor alleles cause disease in both cases.The recent determination of a crystal structure for AGT [Zhang et al 2003] has permitted the rationalization of the effects of selected missense mutations, in particular the mistargeting mutation p.Gly170Arg, p.Gly82Glu (prevents cofactor binding), p.Gly41Arg (aggregation) [Danpure 2004, Danpure & Rumsby 2004, Danpure 2006], p.Gly47Arg (affects dimerization), and p.Ser81Leu (no effect on dimerization) [Robbiano et al 2010]. See Pathologic allelic variants for additional descriptions of abnormal proteins.