DiagnosisClinical DiagnosisAlkaptonuria is caused by deficiency of homogentisate 1,2-dioxygenase, an enzyme that converts homogentisic acid (HGA) to maleylacetoacetic acid in the tyrosine degradation pathway (Figure 1). FigureFigure 1. The tyrosine degradation pathway. Alkaptonuria is characterized by deficiency of homogentisate 1,2-dioxygenase, which converts homogentisic acid (HGA) to maleylacetoacetic acid. Alkaptonuria has three major features:HGA in the urine. Oxidation of the HGA excreted in the urine produces a melanin-like product and causes the urine to turn dark upon standing. Individuals with alkaptonuria usually have dark urine or urine that turns dark on standing or exposure to an alkaline agent. However, darkening may not occur for several hours after voiding and many individuals never observe any abnormal color to their urine. Ochronosis (bluish-black pigmentation of connective tissue). Accumulation of HGA and its oxidation products (e.g., benzoquinone acetic acid) in connective tissue leads to ochronosis. Brown pigmentation of the sclera is observed midway between the cornea and the outer and inner canthi at the insertion of the recti muscles. Pigment deposition may also be seen in the conjunctiva and cornea. The pigmentation does not affect vision [Chevez Barrios & Font 2004]. Ear cartilage pigmentation is first seen in the concha and antihelix, and later in the tragus. The cartilage is slate blue or gray and feels irregular or thickened. Calcification of the ear cartilage may be observed on radiographs. Pigment also appears in cerumen and in perspiration, causing discoloration of clothing. A deep purple discoloration may be seen on the skin of the hands, corresponding to the underlying tendons, or in the web between the thumb and index finger. Arthritis. Arthritis often begins in the spine and resembles ankylosing spondylitis in its large-joint distribution. Radiographs of the spine showing flattened and calcified intervertebral disks are pathognomonic. Findings include degeneration of the intervertebral disks followed by disk calcification and eventually fusion of the vertebral bodies. Osteophyte formation and calcification of the intervertebral ligaments are minimal. Radiographs of the large joints may show joint space narrowing, subchondral cysts, and infrequent osteophyte formation. Enthesopathy can be seen at the muscle insertions [Mannoni et al 2004]. TestingBiochemical testing. The diagnosis of alkaptonuria is based on the detection of a significant amount of HGA in the urine by gas chromatography-mass spectrometry analysis. The amount of HGA excreted per day in individuals with alkaptonuria is usually between one and eight grams [La Du 2001, Phornphutkul et al 2002]. A normal 24-hour urine sample contains 20-30 mg of HGA.Carriers. Biochemical testing cannot detect the carrier state. Molecular Genetic TestingGene. HGD, the gene encoding homogentisate 1,2-dioxygenase, is the only gene in which mutations are known to cause alkaptonuria [Pollak et al 1993, Janocha et al 1994, Fernandez-Canon et al 1996]. Clinical testing Sequence analysis. Sequence analysis detects the eight HGD mutations found in individuals of Slovak descent (see Targeted mutation analysis) as well as other sequence variants that may occur in any population.Targeted mutation analysis identifies the eight mutations found in individuals of Slovak descent.
Four of the mutations – c.481G>A (p.Gly161Arg), c.457dup (p.Asp153Glyfs*26), c.808G>A (p.Gly270Arg), and c.1111dup (p.His371Profs*4) – represent Slovak founder mutations, accounting for 80% of all mutations found in the Slovak population.
The six mutations c.688C>T (p.Pro230Ser), c.899T>G (p.Val300Gly), c.174delA (p.Ser59Alafs*52), c.16-1G>A (p.Tyr6_Gln29del), c.342+1G>A (p.Leu95_Ser114del), and c.140C>T (p.Ser47Leu) are common in other populations, but rare in the Slovak population; mutation frequency in other populations is unknown. Table 1. Summary of Molecular Genetic Testing Used in AlkaptonuriaView in own windowGeneTest MethodMutations DetectedMutation Detection Frequency by Test Method ^{1Test AvailabilityHGDTargeted mutation analysisMutations include those common in the Slovak population>80%Clinical Sequence analysisSequence variants 2 90% 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.Interpretation of test results. For issues to consider in interpretation of sequence analysis results, click here.Testing StrategyTo confirm/establish the diagnosis in a probandAnalysis by gas chromatography-mass spectrometry detects gram quantities of HGA in a daily urine sample and confirms the diagnosis of alkaptonuria. HGD molecular genetic testing: If the proband is of Slovak descent, targeted mutation analysis for the identified mutations in that population should be performed first, followed by sequence analysis for individuals in whom only one or neither mutation is identified. For affected individuals of other populations, HGD sequence analysis should be performed.Carrier testing for at-risk relatives requires prior identification of the disease-causing mutations in the family. Note: Carriers are heterozygotes for an autosomal recessive disorder and are not at risk of developing the disorder.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) DisordersNo other phenotypes are associated with mutations in HGD.}

Natural HistoryThe clinical findings of alkaptonuria include darkening of urine upon standing as a result of the presence of HGA and its oxidation products, connective tissue ochronosis, and arthritis of the spine and larger joints. HGA excretion and disease severity can vary significantly within the same family. In some cases, the diagnosis of alkaptonuria is made only after the individual seeks medical attention because of chronic joint pain or after black articular cartilage is noted during orthopedic surgery. Alkaptonuria does not cause developmental delay or cognitive impairment and does not generally reduce the life span of affected individuals.Urinary changes. Individuals with alkaptonuria usually have dark urine or urine that turns dark on standing or exposure to an alkaline agent. However, darkening may not occur for several hours after voiding and many individuals never observe any abnormal color to their urine. Connective tissue. In general, pigmentary changes are observed after age 30 years. Tendon-related findings, including a thickened Achilles tendon, tendonitis, and rupture, have also been observed clinically [Phornphutkul et al 2002] and are demonstrable by MRI. Joints. Ochronotic arthritis is a regular manifestation of longstanding alkaptonuria. Joint symptoms involving the spine usually appear in the third decade. In one large series, low back pain was observed prior to age 30 years in 49% of individuals and prior to age 40 years in 94% [Phornphutkul et al 2002]. Lumbar and thoracic spine symptoms precede cervical spine symptoms. The sacroiliac region is usually spared. Limitation of spine flexion directly correlates with degree of disability. Individuals with decreased forward flexion demonstrate impaired function and increased fatigue [Perry et al 2006].Joint disease appears to start earlier and progress more rapidly in males than in females. Knees, hips, and shoulders are frequently affected. Fifty percent of individuals require at least one joint replacement by age 55 years [Phornphutkul et al 2002]. Small joint involvement is not significant. Because the kidneys are responsible for secreting massive quantities of HGA, impaired renal function can accelerate the development of ochronosis and joint destruction [Introne et al 2002].Other organ involvement Heart. Pigment deposition in the heart and blood vessels leads to aortic or mitral valve calcification or regurgitation and occasionally aortic dilatation. Aortic valve thickening causes insufficiency and may necessitate aortic valve replacement. Aortic stenosis has also been described [Cercek et al 2002]. Coronary artery calcification has been demonstrated on chest CT. Cardiac findings usually appear in the sixth decade [Phornphutkul et al 2002]. Renal stones. By age 64 years, 50% of individuals with alkaptonuria have a history of renal stones. Prostate stones. Black prostate stones occur relatively frequently in individuals with alkaptonuria. In one series, eight of 27 men age 31-60 years had prostate stones. The passage of these stones is extremely painful and has occasionally prompted consideration of prophylactic surgery [Phornphutkul et al 2002].

Genotype-Phenotype CorrelationsNo correlation is observed between the type of HGD mutation and amount of HGA excreted or severity of the disease.

Differential DiagnosisOchronosis. Ochronosis resulting from alkaptonuria may be confused with acquired, reversible pigmentary changes following prolonged use of carbolic acid dressings for chronic cutaneous ulcers [La Du 2001]. Chemically induced ochronosis has also been described following long-term use of either the antimalarial agent Atabrine^® [Ludwig et al 1963], the skin-lightening agent hydroquinone, or the antibiotic minocycline [Suwannarat et al 2004]. In one case of alkaptonuria, the ochronotic pigment in the eye was misdiagnosed as melanosarcoma, resulting in enucleation of the eye [Skinsnes 1948]. A thorough history combined with failure to demonstrate the excretion of HGA in the urine should eliminate false positive diagnoses.Arthritis. The arthritis of alkaptonuria resembles ankylosing spondylitis in its damage to the spine and large joints, although it differs in sparing the sacroiliac joint and in its radiographic appearance. Radiographic findings of the spine also differentiate alkaptonuria from rheumatoid arthritis and osteoarthritis. 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 alkaptonuria, the following evaluations are recommended: Complete history and physical examination with particular attention to range of motion in the spine and large joints Ophthalmologic evaluation Physical medicine and rehabilitation evaluation if limited range of motion or joint pain occurs Twenty-four hour urine collection for HGA quantification, performed by organic acid analysis Electrocardiogram and echocardiogram in individuals older than age 40 years Renal ultrasound examination or helical abdominal CT to evaluate for the presence of renal calculi Treatment of ManifestationsJoint pain is substantial in individuals with alkaptonuria, and close attention to pain control is necessary. Optimal pain management should be tailored to the individual with close follow-up and long-term management.Physical and occupational therapy are important to promote optimal muscle strength and flexibility. Knee, hip, and shoulder replacement surgeries are options for managing of significant arthritis. In general, the goal of joint replacement is pain relief rather than increased range of motion. Joint replacement in individuals with alkaptonuria is associated with prosthetic survival comparable to that found in individuals with osteoarthritis [Spencer et al 2004].Treatment of prostate stones and renal stones may include surgical intervention. Prevention of Primary ManifestationsAlthough several therapeutic modalities have been investigated, no preventive or curative treatment is available. See Therapies Under Investigation. A clinical trial investigating the potential benefit of nitisinone in alkaptonuria is currently underway. Nitisinone is approved for use in tyrosinemia type I; its registration does not include alkaptonuria as an indication. Prevention of Secondary ManifestationsMaintaining joint range of motion through moderate non-weight-bearing exercise such as swimming may have beneficial effects. Younger individuals with alkaptonuria should be directed toward non-contact and lower-impact sports.SurveillanceSurveillance for cardiac complications every one to two years is advisable after age 40 years and should include the following:Echocardiography to detect aortic dilation and aortic or mitral valve calcification and regurgitation CT to detect coronary artery calcification Urologic complications become more prevalent after age 40 years. Routine surveillance is not recommended, but awareness of this potential complication is advised. Ochronotic prostate stones appear on radiography and kidney stones can be identified by ultrasonography and helical abdominal CT.Agents/Circumstances to AvoidAvoiding physical stress to the spine and large joints, including heavy manual labor or high-impact sports, may reduce the progression of severe arthritis. Evaluation of Relatives at RiskAlkaptonuria can be detected in sibs of affected individuals by testing for the presence of elevated urinary HGA. Young individuals should be counseled to avoid high-impact and contact sports. Career considerations include avoidance of occupations involving heavy physical labor. Instruction on joint strengthening and flexibility exercises, in conjunction with appropriate physical activity, can help preserve overall joint mobility and function. See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.Therapies Under InvestigationPharmacologic treatment of alkaptonuria with oral administration of 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC) or nitisinone has been proposed [Anikster et al 1998]. Nitisinone is a triketone herbicide that inhibits 4-hydroxyphenylpyruvate dioxygenase, the enzyme that produces HGA. Nitisinone is approved for the treatment of tyrosinemia type I.Nitisinone reduced urinary HGA excretion by at least 69% in two individuals, but at the expense of an elevated plasma tyrosine concentration [Phornphutkul et al 2002]. The only known side effects are elevated plasma tyrosine concentration resulting in photophobia and, rarely, corneal crystals. Theoretically, neurologic complications associated with tyrosinemia type III may develop. More recently, low-dose nitisinone reduced urinary HGA by up to 95% in nine individuals with alkaptonuria. In the same study, seven individuals were treated for up to 15 weeks with nitisinone while receiving normal protein intake; all had elevated plasma tyrosine concentrations. No ophthalmic, neurologic, or severe dermatologic complications were observed. Two individuals had transient elevations in liver transaminase levels that returned to normal after stopping nitisinone [Suwannarat et al 2005].Further investigations to determine the benefits of nitisinone in slowing the progression of joint disease are in progress.Search ClinicalTrials.gov for access to information on clinical studies for a wide range of diseases and conditions.OtherNo therapy is proven to prevent or correct the pigmentary changes of ochronosis.Dietary restriction of phenylalanine and tyrosine has been proposed to reduce the production of HGA, but severe restriction of these amino acids is not practical in the long term and may be dangerous. High-dose vitamin C decreases urinary benzoquinone acetic acid, a derivative of HGA, but has no effect on HGA excretion [Wolff et al 1989]. It has been hypothesized that high-dose ascorbic acid may prevent the deposition of ochronotic pigment, although it does not alter the basic metabolic defect [Wolff et al 1989]. No credible studies have demonstrated the clinical efficacy of ascorbic acid [La Du 2001]. Oral bisphosphonate therapy has been suggested to halt the progressive bone loss; however, a prospective study of four affected individuals failed to demonstrate benefit [Aliberti et al 2007].

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. Alkaptonuria: Genes and DatabasesView in own windowGene SymbolChromosomal LocusProtein NameLocus SpecificHGMDHGD3q13​.33Homogentisate 1,2-dioxygenaseAKU database
HGD mutation database
HGD homepage - Mendelian genesHGDData 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 Alkaptonuria (View All in OMIM) View in own window 203500ALKAPTONURIA 607474HOMOGENTISATE 1,2-DIOXYGENASE; HGDNormal allelic variants. HGD is 54.3 kb in length and has 14 exons coding for a 1715-bp transcript [Granadino et al 1997]. Pathologic allelic variantsAt least 67 mutations in HGD have been reported and are found in different allelic combinations [Phornphutkul et al 2002]. The mutations are distributed throughout the HGD sequence. The majority of mutations are missense, but nonsense, frame shift, and splice-site mutations do occur. In the Dominican Republic, p.Cys120Trp is a founder mutation [Goicoechea De Jorge et al 2002]. In the Slovak population, evidence exists for mutational hot spots, such as c.342+1G>A [Zatkova et al 2000], and a founder effect, as seen with the frequent p.Gly161Arg mutation [Srsen et al 2002]. In Europe, excluding the Slovak population, the most prevalent mutation is c.1102A>G (p.Met368Val). In the US, no mutational hot spot or founder effect has been identified [Phornphutkul et al 2002]. Table 2. Selected Pathologic HGD Allelic VariantsView in own windowDNA Nucleotide Change
(Alias ¹)Protein Amino Acid Change
(Alias ¹)Reference
Sequencesc.140C>Tp.Ser47LeuNM_000187​.3
NP_000178​.2c.16-1G>A
(IVS1-1G>A)p.Tyr6_Gln29delc.174delAp.Ser59Alafs*52
(Ser59Alafs*31) (R58fs)c.360T>Gp.Cys120Trpc.342+1G>A
(IVS5+1G>A)p.Leu95_Ser114delc.457dupp.Asp153Glyfs*26
(G152fs)c.481G>Ap.Gly161Argc.688C>Tp.Pro230Serc.808G>Ap.Gly270Argc.899T>Gp.Val300Glyc.1102A>Gp.Met368Val c.1111dup
(111_1112insC)p.His371Profs*4
(P370fs)See 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 protein product of HGD is homogentisate 1,2-dioxygenase, an enzyme in the phenylalanine and tyrosine degradation pathway. The enzyme is composed of 445 amino acids and is expressed predominantly in the liver and kidney, with some expression in the small intestine, colon, and prostate [Fernandez-Canon et al 1996]. Homogentisate 1,2-dioxygenase functions in the metabolism of HGA by catalyzing an oxidative cleavage of the benzene ring to yield maleylacetoacetic acid. It requires oxygen, ferrous iron, and sulfhydryl groups. Abnormal gene product. Most mutant alleles of HGD are predicted to result in complete loss of enzymatic activity.