Disorder of urea cycle metabolism and ammonia detoxification
-Rare genetic disease
Comment:
Hyperornithinemia-hyperammonemia-homocitrullinuria belongs to the class of urea cycle disorders and is caused by mutations in the gene SLC25A15 (ORNT1) encoding the ornthine/citrulline transporter.
HHH syndrome is characterized by high clinical variability (PMID:25874378). Most of the patients with ORNT1-deficiencies have a later onset and milder clinical course when compared to patients with ornithine transcarbamylase deficiency since additional transporters with overlapping function exist (PMID:19287344).
Chadefaux et al. (1989) suggested that the first-trimester diagnosis of HHHS can be achieved by study of the incorporation of (14)C-ornithine into proteins of chorionic villi. They referred to a case of untreated ... - Prenatal Diagnosis Chadefaux et al. (1989) suggested that the first-trimester diagnosis of HHHS can be achieved by study of the incorporation of (14)C-ornithine into proteins of chorionic villi. They referred to a case of untreated HHH syndrome in the mother being associated with a mentally retarded offspring. Shih et al. (1992) described neonatal death in the HHH syndrome and successful prenatal diagnosis of the disorder in a subsequent pregnancy in this family. Thus, the severity ranges from minimal neurologic dysfunction in adulthood (Tuchman et al., 1990) to neonatal death. Diagnostic of the condition in amniotic cells was the combination of normal OAT activity and the inability of the cells to utilize ornithine.
Shih et al. (1969) reported a child with mental retardation and myoclonic seizures associated with hyperornithinemia, hyperammonemia, and homocitrullinemia. The findings were consistent with an inherited disorder of amino acid metabolism.
Rodes et al. (1987) reported ... Shih et al. (1969) reported a child with mental retardation and myoclonic seizures associated with hyperornithinemia, hyperammonemia, and homocitrullinemia. The findings were consistent with an inherited disorder of amino acid metabolism. Rodes et al. (1987) reported a family in which 2 brothers and a sister were affected with HHH syndrome. One patient had progressive spastic paraparesis. At least 2 of the individuals voluntarily avoided a high protein diet. Dionisi Vici et al. (1987) found reports of 23 patients with HHH syndrome in the literature, only 1 of whom showed symptoms as a neonate. Koike et al. (1987) reported Japanese brothers, aged 13 and 19 years, with HHH syndrome. The clinical picture included protein intolerance, mental retardation, seizures, and stuporous episodes. One patient had cerebellar ataxia, myoclonus, convulsive seizure, and muscular weakness in both legs. The older brother had refused to eat fish and meat, and had episodes of vomiting when fed a high-protein formula. Both brothers also had myoclonus epilepsy. Nakajima et al. (1988) reported a Japanese child, born of healthy first-cousin parents, with HHH syndrome. Development was delayed in late infancy, and spastic paraplegia was noted at the age of 3 years. The patient always avoided the intake of meat, milk, and eggs. At age 10, he had an episode of lethargy and hyperammonemia. Brain CT showed diffuse white matter low density and atrophy of the cerebellar vermis. Tuchman et al. (1990) described this disorder in a 39-year-old man and his 42-year-old sister, both vegetarians, who had had episodic confusion for many years, but normal mental function between these episodes. Hyperammonemia was documented during an episode of confusion in the male sib but not in his sister. During therapy with citrulline and phenylbutyrate sodium, plasma ornithine levels increased in both patients, while plasma levels of glutamine and alanine decreased to normal. With therapy, their clinical conditions improved and no recurrent neurologic dysfunction was observed over a follow-up period of 20 months. Miyamoto et al. (2001) reported a 52-year-old woman who had spastic gait and cerebellar signs, including dysmetria, dysdiadochokinesis, and scanning speech, since adolescence, but did not have mental retardation. She had spastic paraparesis of the arms since age 27. Hyperammonemia was noted at the age of 36 years, and a protein-restricted diet and kanamycin were prescribed. At age 37, she had surgery to elongate Achilles tendons in both legs. The diagnosis of HHH syndrome was made at the age of 52, on the basis of hyperornithinemia and homocitrullinuria. Salvi et al. (2001) reported a follow-up on 8 Italian patients who had been diagnosed with HHH syndrome. Age at onset ranged from infancy to 18 years. The predominant neurologic finding was spastic paraparesis, seen in 5 patients. The remaining 3 patients showed signs of pyramidal dysfunction, which the authors suggested may progress to spastic paraparesis later in life. Mental retardation and clonic movements were variably present. Debray et al. (2008) reviewed the medical records of 16 French Canadian patients with HHH syndrome, 15 of whom were homozygous for the common F188del founder mutation in the SLC25A15 gene (603861.0001). Six of the patients had previously been reported by Lemay et al. (1992). The median age at presentation was 2.7 years (range, 3 months to 16 years). Common features included failure to thrive, developmental delay, liver dysfunction with secondary coagulation defects, hyperammonemia, hyperornithinemia, and abnormally increased liver enzymes.
Among 11 patients with the HHH syndrome, Camacho et al. (1999) identified 2 mutations in the ORNT1 gene (see, e.g., F188del; 603861.0001 and E180K; 603861.0002), and a larger deletion. The F188del mutation accounted for 19 of 20 possible ... Among 11 patients with the HHH syndrome, Camacho et al. (1999) identified 2 mutations in the ORNT1 gene (see, e.g., F188del; 603861.0001 and E180K; 603861.0002), and a larger deletion. The F188del mutation accounted for 19 of 20 possible mutant ORNT1 alleles among French Canadian patients, consistent with a founder effect in that population. In 2 unrelated Japanese patients with HHH syndrome, Miyamoto et al. (2001) identified a homozygous mutation in the SLC25A15 gene (R179X; 603861.0003) substitution. One of the patients had previously been reported by Nakajima et al. (1988). In 8 Italian patients with HHH syndrome, Salvi et al. (2001) identified 9 different mutations in the SLC25A15 gene, 7 of which were novel (see, e.g., 603861.0004). Debray et al. (2008) noted that 22 different mutations of the SLC25A15 gene had been described in 49 patients from 31 unrelated families with HHH syndrome to date. In 16 patients from 13 unrelated families with HHH syndrome, Tessa et al. (2009) identified 13 different mutations in the SLC25A15 gene, including 11 novel mutations (see, e.g., 603861.0003; 603861.0006-603861.0008). In vitro functional expression assays showed mutant proteins with decreased transport activity between 4 and 19% of control values. There were no apparent genotype/phenotype correlations. - Genetic Modifiers Camacho et al. (2003) identified ORNT2 (SLC25A2; 608157), an intronless gene encoding a protein 88% identical to ORNT1. ORNT2 targets to mitochondria and is expressed in human liver, pancreas, kidney, and cultured fibroblasts from control and HHH patients. When ORNT2 was overexpressed transiently in cultured fibroblasts from HHH patients, it rescued the deficient ornithine metabolism in those cells. Camacho et al. (2003) suggested that expression of ORNT2 may in part be responsible for the milder phenotype in HHH patients secondary to a gene redundancy effect. Camacho et al. (2006) identified a homozygous mutation in the SLC25A15 gene (T32R; 603861.0009) in 5 affected members of 2 related families of Mexican descent with HHH syndrome. Overexpression studies showed that the mutant protein targeted normally to the mitochondrial and retained some residual activity. However, basal ornithine transport of primary untransfected patient fibroblasts showed loss of function; the observations were important, since they showed a discordance between the clinical and cellular phenotype in patients with HHH syndrome. The patients showed phenotypic variability, with 1 patient in particular having neurologic involvement, including poor school performance, low IQ (55), dysarthria, hyperreflexia, and cortical atrophy on MRI. This patient died from complications of hyperammonemic encephalopathy. The other patients had mild learning disabilities, but no neurologic deficits. Two patients with the mildest defects were found to be carriers for a gain of function val181-to-gly (V181G) polymorphism in the ORNT2 gene, whereas the members of the family who had the patient with the more severe phenotype had the wildtype val181 ORNT2 variant. The mitochondrial haplotypes of the 2 families also differed. Camacho et al. (2006) suggested that the genotype of HHH patients cannot predict the clinical course of the disease, and that other modifying factors, such as gene redundancy or mitochondrial background may further influence the phenotype. Camacho and Rioseco-Camacho (2009) found that mouse and human SLC25A29 (615064), a mitochondrial carnitine/acylcarnitine transporter, rescued defective ornithine metabolism in skin fibroblasts cultured from patients with HHH syndrome.
Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is caused by mutations in SLC25A15, the gene that encodes ORNT1 (mitochondrial ornithine transporter 1), which is involved in the urea cycle and the ornithine degradation pathway. ...
DiagnosisClinical DiagnosisHyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is caused by mutations in SLC25A15, the gene that encodes ORNT1 (mitochondrial ornithine transporter 1), which is involved in the urea cycle and the ornithine degradation pathway. Clinical PresentationNeonatal onset (~12% of individuals). Findings that result from hyperammonemia usually begin 24-48 hours after the start of feeding and can include lethargy, somnolence, refusal to feed, vomiting, tachypnea with respiratory alkalosis, and/or seizures.Infantile, childhood, and adult onset (~88% of individuals). Individuals with HHH syndrome may present with any of the following:Chronic neurocognitive deficits including developmental delay, ataxia, spasticity, learning disabilities, cognitive deficits, and/or unexplained seizuresAcute encephalopathy secondary to hyperammonemic crisis, which can be precipitated by infection, fasting, or injury (or occur for no apparent reason) and can manifest as lethargy, decreased appetite, nausea, vomiting, increased respiratory rate and seizuresChronic liver dysfunction characterized by unexplained elevation of liver enzymes (AST and ALT) with or without mild coagulopathy and with or without mild hyperammonemia and protein intolerance.TestingThe metabolic triad of episodic or postprandial hyperammonemia, persistent hyperornithinemia, and urinary excretion of homocitrulline establishes the diagnosis of HHH syndrome. Note: An incomplete metabolic triad may be observed because: (1) individuals whose protein intake was restricted during early childhood may never have experienced hyperammonemia; (2) affected individuals who come to medical attention because of learning disabilities or school difficulties may only have isolated persistent hyperornithinemia at the time of evaluation; or (3) a low-protein diet can be associated with little to no homocitrulline in the urine. Episodic or postprandial mild to moderate hyperammonemia. The plasma ammonia concentrations in 42 individuals with HHH syndrome at the time of diagnosis (between 2006 and 2011) are summarized in Table 1 [Camacho et al 2006, Fecarotta et al 2006, Al-Hassnan et al 2008, Debray et al 2008, Mhanni et al 2008, Tessa et al 2009, Tezcan et al 2011].In HHH syndrome the degree of hyperammonemia is usually significantly less than in other urea cycle disorders such as OTC, ASS, or CPS-I deficiency (see Urea Cycle Disorders).Note: Once an affected individual is placed on a protein-restricted diet and treated with sodium phenylbutyrate (see Management), plasma ammonia concentrations return to normal.Table 1. Plasma Ammonia Concentrations Observed in HHH Syndrome by Age of OnsetView in own windowStudyPlasma Ammonia Concentration in µmol/L by Age of Onset 1Neonatal (n=4)1st 3 years of life (n=21)Childhood (n=9)Adolescence to Adulthood (n=8)Fecarotta et al [2006]141Camacho et al [2006] 45 43 55 100 40Debray et al [2008]173 49 54 58 100 109 120 217 315119 139 216 325 64 125 250Mhanni et al [2008] 98 186Tessa et al [2009]100 400 700 62 75 96 125 137 200 321 370235222 306Al-Hassnan et al [2008]532 54 77Tezcan et al [2011]1401. The upper limit of normal for plasma ammonia can vary among laboratories, but values of 50 μmol/L or less are usually considered normal for most neonates, infants, children, and adults. However, higher normal upper limits of plasma ammonia concentration for neonates (100 μmol/L) have been reported (see Argininosuccinate Lyase Deficiency). Hyperornithinemia (increased plasma concentration of ornithine). At the time of initial diagnosis, plasma concentration of ornithine can range from 200 to 1100 μmol/L (normal: 30-110 μmol/L).Note: While plasma concentration of ornithine decreases significantly with a protein-restricted diet, it very rarely normalizes. Homocitrullinuria (urinary excretion of homocitrulline). In persons with HHH syndrome homocitrullinuria is a key feature of the disease; however, exceptions exist: some infants with neonatal-onset HHH syndrome do not excrete homocitrulline in significant amounts and individuals with HHH syndrome who self-restrict protein intake may excrete minimal or no homocitrulline in the urine [Korman et al 2004, Valle & Simell 2001]. In unaffected individuals, homocitrulline is not detected in the urine.Note: (1) Homocitrulline may be found in infant formulas due to the carbamylation of lysine during manufacture and, thus, may cause a false positive result. (2) For laboratories that do not measure homocitrulline directly, an increase in urinary excretion of methionine (with normal methionine plasma concentrations) may indicate homocitrullinuria, because the peaks of homocitrulline and methionine overlap [Camacho et al 2006].Additional clinical biochemical abnormalities that may be observed include: Elevations in plasma glutamine concentration. During periods of hyperammonemia, significant elevations in plasma glutamine concentrations are expected. However, as plasma ammonia concentrations return to normal, plasma glutamine concentrations may remain mildly elevated (1.5- to twofold the upper limits of control values). Low normal or low plasma concentrations of lysineIncreased urinary excretion of:Ornithine. Tthe degree of urinary ornithine excretion does not seem to correlate with the plasma concentration of ornithine [Valle & Simell 2001].Orotic acid. Increased urinary excretion of orotic acid (orotic aciduria) may vary from person to person independent of the level of hyperammonemia and metabolic control. Organic acids. An increase in the urinary excretion of components of the Krebs cycle (succinate, citrate, fumaric, α-ketoglutaric) and lactate has been documented in a few reports [Korman et al 2004, Fecarotta et al 2006]. Cellular mitochondrial transport of radiolabelled 14C-ornithine in cultured skin fibroblasts. Cultured fibroblasts from persons with null alleles demonstrate an approximately 75%-80% reduction in ornithine transport, thus suggesting that residual transport is present and most likely mediated by redundant transporters [Camacho et al 1999, Camacho et al 2003]. No correlation exists between ornithine transport capacity, genotype, and phenotype [Camacho et al 1999, Camacho et al 2006]. The diagnosis of HHH syndrome may be confirmed by studying the cellular mitochondrial transport of radiolabelled 14C-ornithine in cultured skin fibroblasts. Molecular Genetic Testing Gene. SLC25A15 (previously known as ORNT1) is the only gene in which mutations are known to cause the hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome [Camacho et al 1999]. Clinical testingTable 2. Summary of Molecular Genetic Testing Used in Hyperornithinemia-Hyperammonemia-Homocitrullinuria SyndromeView in own windowGene SymbolTest MethodMutations DetectedMutation Detection Frequency by Test Method 1Test AvailabilitySLC25A15Sequence analysisSequence variants 299%ClinicalTargeted mutation analysisc.562_564delTTC 3100% for target variantDeletion / duplication analysis 4Exonic or whole-gene deletionsSee footnote 5 1. The ability of the test method used to detect a mutation that is present in the indicated gene2. Examples of mutations detected by sequence analysis may include small intragenic deletions/insertions and missense, nonsense, and splice site mutations.3. p.Phe188del (c.562_564delTTC) is the predominant mutant allele found in ~50% of affected individuals, most of whom are of French Canadian descent. 4. Testing that identifies deletions/duplications not readily detectable by sequence analysis of the coding and flanking intronic regions of genomic DNA; included in the variety of methods that may be used are: quantitative PCR, long-range PCR, multiplex ligation-dependent probe amplification (MLPA), and chromosomal microarray (CMA) that includes this gene/chromosome segment.5. One exonic-intronic microdeletion (~4.5 kb) has been reported [Camacho et al 1999].Interpretation of test results. For issues to consider in interpretation of sequence analysis results, click here.Testing Strategy To confirm/establish the diagnosis in a symptomatic proband, the initial work-up should include plasma ammonia concentration and plasma amino acid analysis; urine amino acid analysis; urine organic acid analysis; and urine orotic acid analysis.The finding of the classic metabolic triad of episodic or postprandial hyperammonemia, persistent hyperornithinemia, and urinary excretion of homocitrulline establishes the diagnosis of HHH syndrome.When biochemical findings are equivocal, SLC25A15 molecular genetic testing can be used to confirm the diagnosis: sequence analysis is performed first, followed by deletion/duplication analysis if only one or no mutant SLC25A15 alleles are identified.Newborn screening (NBS). In the US, testing for HHH syndrome is included in some newborn screening programs (California, Oregon, Massachusetts, Mississippi, Nebraska, New York, North Dakota, Pennsylvania, South Dakota, and Tennessee). However, tandem mass spectrometry (MS/MS), the standard newborn screening methodology, is probably not reliable in detecting newborns with HHH syndrome. In a recent study of newborns in Canada in an isolated population in northern Saskatchewan which is a mixture of French-Canadian and Aboriginal descendants at high risk for the p.Phe188del (c.562_564delTTC) SLC25A15 mutation, the newborn screening samples of infants with HHH syndrome identified by molecular genetic testing did not demonstrate elevated plasma ornithine levels by MS/MS [Sokoro et al 2010]. This finding suggests that the rise of plasma ornithine levels occurs after the first few days of life when NBS blood samples are typically obtained.Note: It is likely that HHH syndrome has been included in some NBS programs because measurement of plasma concentration of ornithine (like arginine) is technically easy. Until publication of the study of Sokoro et al [2010], the ineffectiveness of NBS in screening for HHH syndrome was unknown. Future retrospective studies of the plasma ornithine concentration of NBS samples of persons with confirmed HHH syndrome are needed.Carrier testing for at-risk relatives requires prior identification of the disease-causing mutations in the family.Heterozygotes (e.g., parents and carrier sibs) do not exhibit biochemical abnormalities in plasma or urine; therefore, molecular genetic testing is the only reliable method of carrier detection.Note: Carriers are heterozygotes for this autosomal recessive disorder and are not at risk of developing the disorder.Prenatal diagnosis for at-risk pregnancies can be performed by molecular genetic testing if the disease-causing mutations have been identified in the family.Preimplantation genetic diagnosis (PGD) for at-risk pregnancies requires prior identification of the disease-causing mutations in the family.Genetically Related (Allelic) Disorders No clinical or biochemical phenotypes other than those discussed in this GeneReview are known to be associated with mutations in SLC25A15.
In general, the age of onset and clinical presentation of individuals with hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome vary....
Natural History In general, the age of onset and clinical presentation of individuals with hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome vary.Onset can be divided into four different age periods: neonatal, first three years of life, childhood, and adolescence to adulthood.Neonatal onset accounts for about 12% of affected individuals. Usually the prenatal and perinatal courses are uncomplicated. The neonatal-onset presentation is indistinguishable from that of other neonatal-onset urea cycle disorders: the infant is asymptomatic for the first 24-48 hours and, thereafter, has episodes of poor feeding, vomiting, lethargy, low temperature, and/or rapid breathing related to hyperammonemia (see Table 1).Little is known about the long-term outcome of individuals with the neonatal onset form of HHH syndrome:One child with neonatal-onset who had an initial plasma ammonia concentration of 317 μmol/L had normal growth, development, and neuroimaging studies at age 18 months. Follow-up brain imaging at age six was normal [Salvi et al 2001].By age six years female twins who appeared to have had lethargy and coma during the neonatal period had developed pyramidal signs [Tessa et al 2009]. The twin with the higher plasma ammonia concentration (700 μmol/L) had seizures and significant intellectual disability, whereas the twin with the lower plasma concentration of ammonia (100 μmol/L) had only mild cognitive impairment.Two other neonatal-onset cases evaluated in their late teens had pyramidal signs of the lower limbs (hyperreflexia, clonus, tip-toe gait, and/or spastic ataxia) and moderate cortical atrophy on neuroimaging [Salvi et al 2001].Infancy, childhood, and adult presentation account for approximately 88% of affected individuals. Onset at or before age three years occurs in about 40%, childhood onset in about 29%, and adolescent to adult onset in about 19% [Salvi et al 2001, Korman et al 2004, Fecarotta et al 2006, Al-Hassnan et al 2008, Debray et al 2008, Mhanni et al 2008, Tessa et al 2009, Tezcan et al 2011].Affected individuals in this group come to medical attention for findings related to a mild degree of hyperammonemia with or without liver dysfunction or for evaluation of developmental delay, intellectual disability, learning disabilities, recurring vomiting, school difficulties, ataxia, and/or seizure activity.Even when ammonia levels are normal, a history of protein intolerance or neurologic symptoms suggestive of hyperammonemia (periods of lethargy, nausea, vomiting, decreased appetite, headaches, changes in mood, or altered behavior) may sometimes be elicited during the initial evaluation of a patient. A college-educated male age 35 years with adult onset disease who had no history of learning disabilities, liver disease, psychiatric illness, or neurologic deficits was diagnosed with HHH syndrome after deviating from a vegetarian diet [Tezcan et al 2011]. Two previous accounts of sibs with adult-onset HHH syndrome attributed the mildness of the phenotype, in part, to adherence to a vegetarian diet [Tuchman et al 1990].The cognitive development of persons with HHH syndrome ranges from normal to severe impairment, with the majority having mild neurocognitive impairment. In some reports persons with adolescent-onset and adult-onset disease have significant neurologic deficits such as spasticity and ataxia without cognitive impairment. Of note, pyramidal signs of the lower extremities (hyperreflexia, clonus, tip-toe gait, and/or spastic ataxia) may develop years after the initial diagnosis [Salvi et al 2001, Debray et al 2008].Despite early detection and adequate metabolic control (i.e., absence of hyperammonemia), some individuals with HHH syndrome continue to worsen neurologically with progressive pyramidal tract disease and cognitive deterioration [Debray et al 2008]. In some individuals with early childhood onset, gait abnormalities and spasticity are the predominant findings. Liver dysfunction, present in 20%-25% of affected individuals, generally manifests as mild coagulopathy and elevated liver enzymes (AST and ALT) with or without hyperammonemia. In a few reports acute liver failure prompted consideration of liver transplantation [Fecarotta et al 2006, Mhanni et al 2008]. However, the liver dysfunction that may occur during the initial clinical presentation does not appear to cause long-term complications. Once the hyperammonemia is treated with standard intravenous infusion of dextrose and arginine and protein intake is restricted, the liver dysfunction subsides [Korman et al 2004, Camacho et al 2006, Fecarotta et al 2006, Debray et al 2008, Mhanni et al 2008, Tessa et al 2009]. Neuroimaging in HHH syndrome has revealed evidence of cortical or subtentorial atrophy, demyelinization, stroke-like lesions, and/ or calcifications of the basal ganglia [Salvi et al 2001, Camacho et al 2006, Al-Hassnan et al 2008].
The SLC25A15 (ORNT1) genotype does not correlate with the clinical or biochemical phenotype of HHH syndrome. Functional studies of SLC25A15 mutations using in vitro cell culture and liposome reconstitution studies revealed no genotype-phenotype correlation [Fiermonte et al 2003, Camacho et al 2006]: some SLC25A15 missense and nonsense mutations (p.Phe188del, p.Thr32Arg, and p.Gly190Asp) had mild residual function and others (p.Gly220Arg, p.Arg179*, p.Gly27Arg, p.Arg275Gln, and p.Arg275*) had no function. Individuals with completely nonfunctional SLC25A15 mutations did not have neonatal hyperammonemia....
Genotype-Phenotype Correlations The SLC25A15 (ORNT1) genotype does not correlate with the clinical or biochemical phenotype of HHH syndrome. Functional studies of SLC25A15 mutations using in vitro cell culture and liposome reconstitution studies revealed no genotype-phenotype correlation [Fiermonte et al 2003, Camacho et al 2006]: some SLC25A15 missense and nonsense mutations (p.Phe188del, p.Thr32Arg, and p.Gly190Asp) had mild residual function and others (p.Gly220Arg, p.Arg179*, p.Gly27Arg, p.Arg275Gln, and p.Arg275*) had no function. Individuals with completely nonfunctional SLC25A15 mutations did not have neonatal hyperammonemia.
Hyperammonemia. Most commonly, neonates with hyperammonemia and neonatal onset HHH syndrome are initially suspected of having sepsis....
Differential DiagnosisHyperammonemia. Most commonly, neonates with hyperammonemia and neonatal onset HHH syndrome are initially suspected of having sepsis.Like urea cycle disorders, HHH syndrome should be included in the differential diagnosis of any individual with hyperammonemia, including women who experience hyperammonemia during or following pregnancy. The onset and severity of findings in HHH syndrome are more variable and less severe when compared to urea cycle disorders like ornithine transcarbamylase (OTC) deficiency or carbamyl phosphate synthase (CPS-I) deficiency. Urea cycle disorders usually present with isolated elevation in plasma ammonia concentration and metabolic alkalosis. Plasma amino acid analysis, urine amino acid analysis, organic acid analysis, and urine orotic acid measurements allow diagnosis of the specific urea cycle disorder (see Urea Cycle Disorders) or HHH syndrome. A complete chemistry panel, lactate/pyruvate determination, arterial blood gases, and urinalysis should always be included in the evaluation of any person with an elevated plasma ammonia concentration to evaluate for conditions including the following:Organic acidemias (presents with acidosis). See Organic Acidemias.Lysinuric protein intolerance (low plasma ornithine concentration)Fatty-acid oxidation defects (associated with hypoglycemia). See MCAD Deficiency.Pyruvate carboxylase deficiency (presents with lactic acidosis and hypoglycemia)Hyperornithinemia. The only other condition that causes chronic elevations in plasma ornithine concentration is deficiency of ornithine amino transferase (OAT), a mitochondrial matrix enzyme involved in the ornithine degradation pathway. However, OAT deficiency never presents with the neurologic and clinical biochemical features of HHH syndrome (e.g., elevation in plasma ammonia concentration and glutamine, urinary excretion of homocitrulline and/or orotic acid). OAT deficiency presents mostly with ophthalmologic findings known as hyperornithinemia with gyrate atrophy of the choroid and retina that manifest as chorioretinal degeneration with loss of peripheral vision, night blindness, and often posterior subcapsular cataracts [Valle & Simell 2001]. Given that the neurologic non-acute presentation for HHH syndrome may be indistinguishable from primary mitochondrial disease, urine organic acid analysis should also be ordered. In some cases of HHH syndrome, urinary excretion of Krebs cycle components (succinate, fumarate, citrate and α-ketoglutarate) and lactate have been reported [Korman et al 2004]. This pattern of excretion of organic acids, which is commonly seen in children and adults with defects in the mitochondrial complex I or III, may create the impression that persons with HHH syndrome have a primary rather than a secondary mitochondrial defect.Homocitrullinuria. Other conditions in which homocitrullinuria can be observed should be included in the differential diagnosis of HHH syndrome: Homocitrulline is a by-product of canned milk production that arises from the reaction of cyanate and the terminal ε-amino group of lysine. In canned formulas, cyanate is produced from heat-induced urea breakdown. When homocitrulline is consumed in the diet from sources such as these, it is absorbed in the small intestine via a transport system similar to that of cationic amino acids and excreted in the urine [Valle & Simell 2001]. In contrast, homocitrullinuria detected in neonates given IV glucose only (and no dietary source of protein) indicates the presence of a metabolic disorder. Some individuals with lysinuric protein intolerance (LPI) have been shown to excrete homocitrulline [Palacin et al 2004]. Although these individuals may also have hyperammonemia, their clinical biochemical profile demonstrates low concentrations of plasma ornithine, lysine, and arginine and persistent urinary excretion of lysine, ornithine, and arginine. Homocitrullinuria has also been observed in arginase deficiency; however, in this disorder, the plasma concentration of arginine is increased and homoarginine is excreted in the urine [Valle & Simell 2001].Neurologic findings. In those individuals with early childhood onset in whom gait abnormalities and spasticity predominate, the differential diagnosis includes cerebral palsy and early-onset inherited spastic paraplegia.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).
To establish the extent of disease and needs of an individual diagnosed with the hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome, the following evaluations are recommended:...
ManagementEvaluations Following Initial Diagnosis To establish the extent of disease and needs of an individual diagnosed with the hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome, the following evaluations are recommended:Neurocognitive evaluation of the affected individual including brain imaging Evaluation of school performance with attention to possible learning disabilitiesTreatment of ManifestationsOf primary importance in the management of individuals with HHH syndrome is rapid control of the hyperammonemic episodes that may result from changes in diet (e.g., protein intake), infection, fasting, or injury, or have no apparent cause. Long-term management of affected individuals focuses on the prevention of episodes of hyperammonemia and attempts to lower the level of plasma ornithine concentration.It is critical that the acute and long-term management of individuals with HHH syndrome be performed in conjunction with a metabolic specialist.Treatment of acute hyperammonemic episodes. Assess plasma ammonia concentration, complete chemistry panel, arterial blood gases (ABGs), CBC and differential (to evaluate for an infectious process), urinalysis, urine and plasma amino acids, and urine orotic acid. Plasma ammonia concentrations ≥100-125 μmol/L (~2 times control values) should be treated immediately. Discontinue all oral intake until the patient’s condition is stabilized because vomiting may be induced by hyperammonemia and/or some of the drugs given intravenously to promote ammonia removal (e.g., as part of nitrogen scavenging therapy). This approach also stops all protein intake. Initial intravenous infusion should be 10% dextrose (with 1/4 normal saline and 20mEq/liter KCl) at twice maintenance; ammonia and glucose/Na/K/Cl/CO2 concentrations should be monitored every two hours or when neurologic status changes. It is important to note that patients respond neurologically differently to rising ammonia concentrations: an individual with HHH syndrome is more likely to respond to the initial IV infusion of dextrose and to normalize his/her plasma ammonia concentration when compared to an individual with a urea cycle disorder such as OTC deficiency or ASS deficiency. If clinical status does not improve, infusion of supplemental arginine and ammonia removal drugs is added to the regimen. Follow published protocols for treatment of acute hyperammonemic episodes similar to those instituted for OTC deficiency [Brusilow & Horwich 2001]. These protocols consist of arginine supplementation and use of intravenous bolus and maintenance infusions of the ammonia removal drugs sodium benzoate and sodium phenylacetate. The New England Consortium of Metabolic Programs has a complete set of treatment protocols and algorithms that are freely available and easily accessible. An initial priming dose of arginine, benzoate, and phenylacetate is given (see Table 3).Table 3. Initial Priming Dose of Arginine, Benzoate, and Phenylacetate by Age GroupView in own windowInfusion 1Infants and ChildrenAdolescents and Adults10% arginine HCl210 mg/kg/day4.0 g/m2Sodium benzoate250 mg/kg/day5.5 g/m2Sodium phenylacetate250 mg/kg/day5.5 g/m21. Mix solutions of arginine, benzoate, and phenylacetate in a 10% dextrose solution at a dose of 25 mL of 10% dextrose/kg and infuse over 90 min. The solutions containing arginine, benzoate, and phenylacetate should be given in conjunction with 10% dextrose + one quarter normal saline + 20 mEq/L KCL solution.If ammonia levels stabilize, the same arginine, benzoate, and phenylacetate solution is infused over 24 hrs.Note: All preparations of arginine and ammonia removal drugs should be double- or triple-checked given the potential for drug intoxication if high doses are given or continued CNS ammonia toxicity if low doses are given. If sodium benzoate or sodium phenylacetate solutions are not available, infusion of arginine should be started.Table 4. Mechanisms of Drug Action in Treatment of HyperammonemiaView in own windowDrugActionGlucose• Raises insulin levels • Induces anabolic state • Causes protein sparing effect from skeletal muscle amino acidsArginine 1 • Needs to be supplemented in those with a urea cycle disorder • Stimulates secretion of insulin • Plays a role in the first step of the synthesis of creatine 2Sodium benzoate 3• Forms benzoate-glycine (hippurate) via the benzoylCoA:glycine acyltransferase reaction 4• Eliminates one amino group in the urineSodium phenylacetate 3• Forms a phenylacetate-glutamine compound via the phenylacetateCoA:glutamine acetyl-transferase reaction 4• Eliminates two amino groups in the urine1. A non-essential amino acid in humans2. Interruption in the synthesis of brain creatine secondary to hyperammonemia has been proposed as a contributing factor to the neurologic findings in affected individuals.3. Initially esterified to its CoA-ester via the medium chain fatty acid enzyme, acyl-CoA ligase4. Reaction takes place in the mitochondrial matrix (liver and kidney) [Brusilow & Horwich 2001]Hemodialysis. If the patient fails to respond to the above treatment of hyperammonemia, if the plasma ammonia concentration increases, and/or if the neurologic status deteriorates, hemodialysis should be started promptly to remove ammonia from the circulation. Infusion of arginine, benzoate, and phenylacetate should continue during hemodialysis. Dialysis may be prolonged if the catabolic state persists.Nutrition. Twenty-four to 36 hours after initial admission the patient should start receiving intravenous alimentation including daily doses of only essential amino acids, carnitine, vitamins, and lipids to help avoid a catabolic state which will prolong hyperammonemia. Note: Non-essential amino acids (i.e., glutamine, proline, and glycine) should be avoided since they increase the nitrogen load to an already compromised urea cycle. Prevention of Primary ManifestationsDepending on their age, individuals with HHH syndrome should be maintained on a protein-restricted diet. For infants and children the dietary protein needs to be restricted to control hyperammonemia, but sufficient for normal growth and development. Dietary supplementation with Cyclinex®-1 (infants and children) or Cyclinex®-2 (adult) formulas that provide only essential amino acids and other nutritional supplements have been helpful for some affected individuals.Citrulline supplementation at 0.17 g/kg/day or 3.8 g/m2/day is preferred to arginine because citrulline accepts an aspartate (via the arginosuccinate synthase reaction) and therefore eliminates two amino groups per cycle. Moreover, there may be a possible association between arginine supplementation and progression of lower limb spasticity [HHH International Round Table, Roma 2006, unpublished]. Sodium phenylbutyrate (Buphenyl®) is given at 450-600 mg/kg/day in three divided doses. Sodium phenylbutyrate initially is imported into the mitochondria where it undergoes β-oxidation to produce phenylacetate.Lysine supplementation is indicated when plasma lysine concentrations are low. Low plasma lysine concentrations have been associated with delayed growth and development.Plasma concentrations of ammonia, glutamine, arginine, and essential amino acids should be maintained within the normal range. Note: (1) Although elevated plasma ornithine concentrations may decrease significantly if dietary management is followed, complete normalization of plasma ornithine concentration is rarely observed. (2) Even in the absence of hyperammonemic episodes, affected individuals may continue to develop neurologic complications such as spasticity or learning disabilities. Maintaining as low a level of plasma ornithine concentration as possible by restricting protein intake could help prevent some of the progressive neurologic complications seen in these individuals.Liver transplantation is not indicated for persons with HHH syndrome. Because SLC25A15 and the ornithine degradation pathway are expressed in all tissues (i.e., brain, kidney) and most cell types (i.e. astrocytes, fibroblasts), liver transplantation may correct the hyperammonemia, but it will not correct tissue-specific metabolic abnormalities that also contribute to the neurologic pathology. Three individuals with HHH syndrome who had acute fulminant hepatic failure and coagulopathy rapidly stabilized after protein restriction and arginine or citrulline supplementation [Fecarotta et al 2006, Mhanni et al 2008].SurveillanceAll surveillance of patients with HHH syndrome should be a combined effort of the general pediatrician or adult practitioner and a metabolic specialist.Height, weight, and head circumference should be assessed routinely in children from the time of diagnosis to adolescence.Plasma ammonia concentration, plasma and urine amino acid concentrations, urine organic acids, and urine orotic acid need to be monitored routinely, based on age and history of compliance and metabolic decompensation.Low plasma concentrations of essential amino acids could trigger a catabolic state, requiring readjustment of the diet/formulas. Low plasma concentrations of lysine may lead to delays in growth and development in infants. Parents of infants and small children should be alert to subtle changes in mood, behavior, and eating and/or the onset of vomiting, which may suggest that plasma concentrations of glutamine and ammonia are increasing.School performance should be monitored since poor school performance may lead to low self-esteem and/or behavioral problems that could influence compliance with a protein-restricted diet.Periodic neurologic evaluation is warranted to monitor for neurologic deterioration even when metabolic control is optimal. Agents/Circumstances to AvoidAvoid the following:Excess dietary protein intakeNon-prescribed protein supplements such as those used to increase size of skeletal muscle during exercise regimensProlonged fasting during an illness or weight lossUse of intravenous steroidsValproic acid, which exacerbates hyperammonemia in urea cycle disordersExposure to communicable diseasesEvaluation of Relatives at RiskIf the disease-causing mutations in a family are known, use molecular genetic testing to clarify the genetic status of at-risk relatives to allow for early diagnosis and treatment, perhaps even before symptoms occur. Note: Plasma concentrations of ornithine and urinary excretion of orotic acid and homocitrulline may be unreliable in asymptomatic persons. See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.Pregnancy ManagementPregnancy in a woman with HHH syndrome has not been published. Rebecca Mardach, MD (Kaiser Permanente, Los Angeles) provided information to the authors about a healthy boy born after an uneventful pregnancy to a clinically asymptomatic woman in her early 20s who had been diagnosed with HHH syndrome at age 13 years in the course of evaluation of her affected five-year-old sib [Camacho et al 2006]. During her pregnancy the mother was maintained on a protein and citrulline regimen that allowed normal fetal development and maternal health; plasma and urine amino acids, orotic acid, plasma ammonia, and urine organic acids were monitored.There are no well-controlled epidemiologic studies of the fetal effects of sodium benzoate, phenylacetate, or phenylbutyrate during human pregnancy. However, sodium benzoate has been reported to lead to malformations and neurotoxicity/nephrotoxicity in zebrafish larvae [Tsay et al 2007] and to possible teratogenicity in rats [Minor & Becker 1971, Onodera et al 1978]. As a known differentiating agent, sodium phenylbutyrate also functions as a histone deacetylase (HDAC) inhibitor with potential teratogenicity given its ability to alter gene expression in fetal mice [Di Renzo et al 2007]. The FDA has assigned sodium benzoate and sodium phenylacetate to pregnancy category C (“potential benefits may warrant use of the drug in pregnant women despite potential risks”). Theoretically, the use of benzoate/phenylacetate and in particular sodium phenylbutyrate should be avoided during pregnancy, especially during the first trimester. The use of these medications should be carefully evaluated for each individual (benefit/risk ratio) in consultation with a metabolic genetics specialist. Despite the report of successful administration of sodium phenybutyrate to an OTC-deficient woman throughout her 11-33 week gestation period, caution is advised and the alternate use of sodium benzoate, if deemed necessary, is recommended [Redonnet-Vernhet et al 2000]. Mendez-Figueroa et al [2010] reported a small number of females with OTC deficiency who successfully completed their pregnancies without need of ammonia removal medications. This study also reported the use of sodium benzoate to manage a pregnant woman with OTC deficiency who developed mild hyperammonemia during induction of labor.No specialized care for a fetus known to have HHH syndrome is warranted. Therapies Under InvestigationSearch ClinicalTrials.gov for access to information on clinical studies for a wide range of diseases and conditions. Note: There may not be clinical trials for this disorder.
Information in the Molecular Genetics and OMIM tables may differ from that elsewhere in the GeneReview: tables may contain more recent information. —ED....
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. Hyperornithinemia-Hyperammonemia-Homocitrullinuria Syndrome: Genes and DatabasesView in own windowGene SymbolChromosomal LocusProtein NameLocus SpecificHGMDSLC25A1513q14.11Mitochondrial ornithine transporter 1SLC25A15 @ LOVDSLC25A15Data 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 Hyperornithinemia-Hyperammonemia-Homocitrullinuria Syndrome (View All in OMIM) View in own window 238970HYPERORNITHINEMIA-HYPERAMMONEMIA-HOMOCITRULLINURIA SYNDROME 603861SOLUTE CARRIER FAMILY 25 (MITOCHONDRIAL CARRIER, ORNITHINE TRANSPORTER), MEMBER 15; SLC25A15Molecular Genetic Pathogenesis Decreased ornithine transport across the inner mitochondrial membrane in the periportal region of the liver (hepatocytes in the area of the portal vein, artery, and bile duct) disrupts the function of the urea cycle at the level of the ornithine transcarbamylase (OTC) reaction thus causing episodic or postprandial hyperammonemia (Figure 1). In spite of the impermeability of the inner mitochondrial membrane to solutes, persons with HHH syndrome come to medical attention mostly during infancy and childhood and tend to exhibit mild or no elevations in plasma ammonia concentration when initially diagnosed [Brusilow & Horwich 2001, Valle & Simell 2001, Palmieri 2008].FigureFigure 1. Compartmentalization of the biochemical pathways involved in HHH syndrome as a result of deficiency of the mitochondrial ornithine transporter (ORNT1; encoded by SLC25A15), leading to abnormal accumulation of the metabolites marked in black (more...)The chronic elevation in plasma ornithine concentration (150-1,200 μmol/L) occurs because SLC25A15 is also expressed in the pericentral region of the liver (hepatocytes surrounding the central vein) and most peripheral tissues (i.e., brain, kidney, testis) and cells (i.e., skin fibroblasts, astrocytes) where ornithine is normally catabolized into proline and glutamate via the intramitochondrial enzyme, ornithine amino transferase (OAT). Increased cytoplasmic ornithine leads to hyperornithinemia and increased polyamine biosynthesis (Figure 1). The elevated concentration of plasma ornithine on occasion may be similar to that seen in individuals with gyrate atrophy (OAT deficiency), a genetic disorder with mostly ophthalmologic symptoms [Valle & Simell 2001].The homocitrullinuria occurs when intramitochondrial carbamyl phosphate is underused and accumulates in the presence of deficient ornithine transport into the mitochondrial matrix. Excess carbamyl phosphate either condenses with intramitochondrial lysine to form homocitrulline or is shunted through the cytosolic pyrimidine biosynthetic pathway leading to increased urinary excretion of orotic acid and uracil (Figure 1) [Valle & Simell 2001]. Given that the inner mitochondrial membrane is impermeable to solutes, it is thought that additional mitochondrial carrier proteins with redundant function to ORNT1 (mitochondrial ornithine transporter 1) are, in part, responsible for the late onset and variable clinical phenotype of HHH syndrome. The residual ornithine transport in cultured fibroblasts and liver of affected individuals reinforces the notion of gene redundancy in HHH syndrome. SLC25A2 (ORNT2) and SLC25A29 (ORNT3), two additional mitochondrial ornithine transporters, are thought to mediate the residual ornithine transport in HHH syndrome and may serve as modifying genes [Camacho et al 2003, Camacho & Rioseco-Camacho 2009]. These ornithine transporters have additional functions such as the transport of D- and L-histidine, L-homoarginine, D- and L-amino acids (ORNT2) [Fiermonte et al 2003], and carnitine/acylcarnitine (ORNT3) [Camacho & Rioseco-Camacho 2009]. Given that HHH syndrome is in part a disorder of the urea cycle, the central nervous system (CNS) cellular pathophysiology has been attributed to the toxic effects of elevated ammonia and glutamine on the astrocyte, including osmotic swelling, abnormalities in creatine metabolism, changes in cellular bioenergetics, mitochondrial dysfunction, and alterations in glutamine-glutamate cycling [Braissant 2010, Sofroniew & Vinters 2010]. These cellular pathophysiologic changes may lead to CNS alterations including atrophy, demyelinization, or stroke-like lesions [Enns 2008, Gropman 2010]. However, it is unlikely that hyperammonemia is solely responsible for the pathophysiology of HHH syndrome since affected individuals who are diagnosed early and maintain good metabolic control nonetheless develop progressive neurologic dysfunction (e.g., progressive spastic paraparesis) years after their initial diagnosis. Of note, this progressive pyramidal tract involvement is reminiscent of arginase deficiency [Rodes et al 1987, Debray et al 2008].Since ORNT1 also functions as a cationic amino acid transporter, it is reasonable to propose that the pathophysiology of HHH syndrome may also be dependent on the interruption of the physiologic functions of ORNT1, including its role in mitochondrial protein synthesis, metabolism of arginine and lysine, and synthesis of polyamines [Palmieri 2008]. Of note, recent work has demonstrated that excessive ornithine and homocitrulline can cause protein and lipid oxidation, as well as negatively interfere in cellular bioenergetics, oxidative phosphorylation, and Krebs cycle function of the rat brain [Viegas et al 2011]. Another factor that may contribute to the mechanisms of disease in HHH syndrome is the synthesis of creatine: it has been observed that ornithine negatively regulates the activity of L-arginine:glycine amidinotransferase (AGAT), the enzyme that catalyzes the first step in the synthesis of creatine [Valle & Simell 2001]. Though low creatine excretion was previously reported in two persons with HHH syndrome, this negative effect of ornithine on AGAT function may not be as significant in HHH syndrome as in OAT deficiency given the differences in the localization and activity of the mitochondrial and cytoplasmic isoforms of AGAT [Humm et al 1997]. (See also Creatine Deficiency Syndromes.)Normal allelic variants. SLC25A15 (previously ORNT1) comprises eight exons (NM_014252.3). The open reading frame (ORF) is encoded by exons 2 through 7 [Tsujino et al 2000, Camacho et al 2006]. Exon 1 encodes part of the 5’UTR and exon 8 encodes only part of the 3’UTR. At least eight non-processed SLC25A15 pseudogenes are located on different chromosomes: chromosome 3 (AC073022.12), 10 (AC027723.2), 13 (AC018739.4 & AL356259.11), 16 (AC141274.1), 21 (AF254982.4), 22 (NW_001838735.2), and Y (AC019099). Although these non-processed SLC25A15 pseudogenes have truncations of several exons, they consistently have more than 90% conserved regions of exons 6 and 7.Pathologic allelic variants. Approximately 20 SLC25A15 pathogenic allelic variants that produce missense and nonsense mutations, plus an in-frame deletion, have been reported. Ten other allelic variants that cause splicing errors, microdeletions, and insertions have been described [Debray et al 2008, Tessa et al 2009]. Almost all mutations have occurred in exons 2 through 7, which correspond to the SLC25A15 coding region. Exon 5 has the highest mutational frequency. Importantly, exon 5 encodes for the fourth transmembrane domain, a region that forms part of the solute (ornithine, lysine, and arginine) recognition site of the SLC25A15 transporter [Palmieri 2008]. The two most common mutations occur in this region [Camacho & Rioseco-Camacho 2009]:c.562_564delTTC (p.Phe188del), found predominately in persons of French Canadian descent, accounts for the majority of individuals (~50%) reported with HHH syndrome. c.535C>T (p.Arg179*), appears to be prevalent in individuals of Japanese heritage and Middle Eastern heritage [Miyamoto et al 2001, Tessa et al 2009]. See Table 5 (pdf) for additional variants.Table 6. Selected Pathologic SLC25A15 Allelic Variants =View in own windowDNA Nucleotide ChangeProtein Amino Acid ChangeReference Sequencesc.95C>Gp.Thr32Arg 1, 2NM_014252.3 NP_055067.1c.535C>Tp.Arg179* 3c.562_564delTTCp.Phe188del 1, 3c.569G>Ap.Gly190Asp 1, 3 c.658G>Ap.Gly220Arg 3, 4c.824G>Ap.Arg275Gln 4See Quick Reference for an explanation of nomenclature. GeneReviews follows the standard naming conventions of the Human Genome Variation Society (www.hgvs.org). 1. Residual function 2. Targets normally to mitochondria 3. SLC25A15 (ORNT1) mutations associated with neonatal onset 4. No residual functionNormal gene product. Mitochondrial ornithine transporter 1 (ORNT1) is a member of the mitochondrial carrier family (MCF) of proteins that transports solutes across the inner mitochondrial membrane and has at least 45 members including CITRIN (aspartate/glutamate transporter), carnitine/acylcarnitine translocator (CACT), and the ADP/ATP-1 and citrate transporters. ORNT1 belongs to a subfamily that transports charged amino compounds. ORNT1 subfamily members include ORNT2, ORNT3 (also transports carnitine/acylcarnitine), CACT, and SLC25A45. Structurally, ORNT1 is a 301-amino acid, six-transmembrane domain carrier protein inserted in the inner mitochondrial membrane with its amino and carboxy terminal domains facing the cytoplasm [Camacho et al 2006]. Biochemical studies have demonstrated that in pericentral hepatocytes and peripheral tissues, ORNT1 transports ornithine, lysine, and arginine into the mitochondrial matrix in exchange for an intramitochondrial hydrogen ion (H+). In periportal hepatocytes that express components of the urea cycle, ORNT1 exchanges intramitochondrial citrulline and H+ for cytoplasmic ornithine. Given ORNT1's role in the catabolism of ornithine via the OAT reaction and its transport of arginine and lysine into the mitochondrion, it is anticipated that ORNT1 plays a complex biochemical role in tissues (liver, brain, pancreas, and kidney) and cells (astrocytes, fibroblasts) where it is expressed. Abnormal gene productSeveral SLC25A15 missense and nonsense mutations and the in-frame deletion have been analyzed by in vitro cell culture and lipid reconstitution studies to determine if the observed amino acid change alters basic transporter function; many exhibited residual ability to transport ornithine. The ORNT1 protein with the most common mutation, p.Phe188del, produces an unstable protein of 300 amino acids with 10%-15% residual function when compared to controls [Camacho et al 1999, Fiermonte et al 2003, Morizono et al 2005]. Thus, it is possible that in some affected individuals other factors involved in regulating protein stability may allow a significant level of expression that influences the neurologic phenotype. The protein with the nonsense mutation p.Arg179* produces a truncated and non-functional ORNT1 protein [Tsujino et al 2000, Fiermonte et al 2003] that is associated with both neonatal- and late-onset disease. A protein with the substitution p.Gly190Asp had approximately 33% residual activity and was associated with a neonatal-onset phenotype. The p.Thr32Arg SLC25A15 amino acid substitution produces a transporter protein that targets normally to the mitochondria and has approximately 50% residual function. This mutant transporter protein was found in five related individuals with HHH syndrome with late-onset disease. The non-functional protein with the p.Gly220Arg substitution targets normally to the mitochondria and was observed in a family in which the proband had stroke-like lesions of the frontal lobe and two other affected sibs had a mild phenotype consisting of learning disabilities.