Figure 1. The tyrosine degradation pathway. Alkaptonuria is characterized by deficiency of homogentisate 1,2-dioxygenase, which converts homogentisic acid (HGA) to maleylacetoacetic acid.
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GeneReviews designates a molecular genetic test as clinically available only if the test is listed in the GeneTests Laboratory Directory by either a US CLIA-licensed laboratory or a non-US clinical laboratory. GeneTests does not verify laboratory-submitted information or warrant any aspect of a laboratory's licensure or performance. Clinicians must communicate directly with the laboratories to verify information.—ED.
Genetics clinics, staffed by genetics professionals, provide information for individuals and families regarding the natural history, treatment, mode of inheritance, and genetic risks to other family members as well as information about available consumer-oriented resources. See the GeneTests Clinic Directory.
For current information on availability of genetic testing for disorders included in this section, see GeneTests Laboratory Directory. —ED.
Genetic counseling is the process of providing individuals and families with information on the nature, inheritance, and implications of genetic disorders to help them make informed medical and personal decisions. The following section deals with genetic risk assessment and the use of family history and genetic testing to clarify genetic status for family members. This section is not meant to address all personal, cultural, or ethical issues that individuals may face or to substitute for consultation with a genetics professional. To find a genetics or prenatal diagnosis clinic, see the GeneTests Clinic Directory.
Diagnosis/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. Alkaptonuria results from mutations in the HGD gene encoding homogentisate 1,2-dioxygenase. Molecular genetic testing is available on a clinical basis.
Management. Treatment of manifestations: management of joint pain tailored to the individual; physical and occupational therapy to help maintain muscle strength and flexibility; knee, hip, and shoulder replacements when needed; surgical intervention for prostate stones and renal stones as needed. Surveillance: in individuals over age 40 years, echocardiography to detect aortic dilation, aortic or mitral valve calcification, and regurgitation; CT to detect coronary artery calcification. Agents/circumstances to avoid: physical stress to the spine and large joints, including heavy manual labor or high-impact sports, to try to reduce progression of severe arthritis. Testing of relatives at risk: testing for the presence of elevated urinary HGA in sibs of affected individuals.
Genetic counseling. Alkaptonuria is inherited in an autosomal recessive manner. At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Once an at-risk sib is known to be unaffected, the chance of his/her being a carrier is 2/3. No laboratories offering prenatal testing are listed in the GeneTests Laboratory Directory; however, for families in which the disease-causing mutations have been identified, prenatal diagnosis using molecular genetic testing may be available through laboratories offering custom prenatal testing.
Figure 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 adiographs.
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].
Biochemical 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. For laboratories offering biochemical testing, see
.
Carriers. Biochemical testing available on a clinical basis cannot detect the carrier state.
GeneReviews designates a molecular genetic test as clinically available only if the test is listed in the GeneTests Laboratory Directory by either a US CLIA-licensed laboratory or a non-US clinical laboratory. GeneTests does not verify laboratory-submitted information or warrant any aspect of a laboratory's licensure or performance. Clinicians must communicate directly with the laboratories to verify information.—ED.
Gene. HGD, the gene encoding homogentisate 1,2-dioxygenase, is the only gene known to be associated with alkaptonuria [Pollak et al 1993, Janocha et al 1994, Fernandez-Canon et al 1996].
Clinical testing
Sequence analysis. Sequence analysis will detect the panel of eight HGD mutations found in individuals of Slovak descent as well as other mutations that may occur in any population.
Targeted mutation analysis. A panel of eight mutations found in individuals of Slovak descent is available for clinical testing.
The four mutations p.Gly161Arg, p.Asp153GlyfsX25 (c.457_458insG) (also known as G152fs), p.Gly270Arg, and p.His371ProfsX3 (c.1111_1112insC) (also known as P370fs) represent Slovak founder mutations, accounting for 80% of all mutations found in the Slovak population.
The four mutations p.Pro230Ser, p.Val300Gly, p.Ser59AlafsX31 (c.174delA) (also known as R58fs), and p.Met368Val are common in other populations, but rare in the Slovak population; mutation frequency in other populations is unknown.
Mutation designations are those used in Genbank sequence NM_000187.2, following the naming conventions of the Human Genome Variation Society (www.hgvs.org).
| Gene | Test Method | Mutations Detected | Mutation Detection Frequency by Test Method | Test Availability |
|---|---|---|---|---|
| HGD | Targeted mutation analysis | p.Gly161Arg, p.Asp153GlyfsX25 1, p.Gly270Arg, p.His371ProfsX3 2, c.16-1G>A 3, c.712+1G>A 4, p.Ser47Leu, p.Pro230Ser in HGD | >80% | Clinical
|
| Sequence analysis | Sequence variants | 90% |
1. Also known as G152fs
2. Also known as P370fs
3. Also known as IVS1-1G>A
4. Also known as IVS5+1G>A
Confirmation of the diagnosis in a proband
Analysis by gas chromatography-mass spectrometry detects gram quantities of HGA in a daily urine sample and confirms the diagnosis of alkaptonuria.
Molecular genetic testing of the HGD gene by sequence analysis. If the proband is of Slovak descent, targeted mutation analysis for the eight identified mutations in that population should be done first with sequence analysis to follow for individuals in whom both mutations are not 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.
No other phenotypes are associated with mutations in HGD.
The 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 require 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 tones.
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].
No correlation is observed between the type of HGD mutation and amount of HGA excreted or severity of the disease.
Elevated urinary HGA and ochronotic arthritis occur in all individuals who are homozygous or compound heterozygous for mutations in HGD.
Occasionally alkaptonuria is referred to collectively, and incorrectly, as ochronosis.
At least 1000 cases of alkaptonuria have been described; this is likely an underestimate [La Du 2001]. The incidence of alkaptonuria in the US is estimated to be 1:250,000 to 1:1,000,000 live births.
Alkaptonuria occurs worldwide; a high prevalence has been observed in the Dominican Republic [Milch 1960] and near the Slovakian-Bohemian border, likely as the result of a founder effect [Srsen 1983].The prevalence of alkaptonuria is estimated to be 1:19,000 in Slovakia [Zatkova et al 2003].
For current information on availability of genetic testing for disorders included in this section, see GeneTests Laboratory Directory. —ED.
Ochronosis. 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.
To establish the extent of disease in an individual diagnosed with alkaptonuria, the following evaluations are recommended:
Complete history and physical examination 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
Joint 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.
Although 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.
Maintaining 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.
Surveillance for cardiac complications every one to two years is advisable after age 40 years. It 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.
Avoiding physical stress to the spine and large joints, including heavy manual labor or high-impact sports, may reduce the progression of severe arthritis.
Alkaptonuria 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.
Pharmacologic 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.
No 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].
Genetics clinics, staffed by genetics professionals, provide information for individuals and families regarding the natural history, treatment, mode of inheritance, and genetic risks to other family members as well as information about available consumer-oriented resources. See the GeneTests Clinic Directory.
See Consumer Resources for disease-specific and/or umbrella support organizations for this disorder. These organizations have been established for individuals and families to provide information, support, and contact with other affected individuals.
Genetic counseling is the process of providing individuals and families with information on the nature, inheritance, and implications of genetic disorders to help them make informed medical and personal decisions. The following section deals with genetic risk assessment and the use of family history and genetic testing to clarify genetic status for family members. This section is not meant to address all personal, cultural, or ethical issues that individuals may face or to substitute for consultation with a genetics professional. To find a genetics or prenatal diagnosis clinic, see the GeneTests Clinic Directory.
Alkaptonuria is inherited in an autosomal recessive manner.
Parents of a proband
The parents of an affected child are obligate heterozygotes and therefore carry one mutant allele.
Heterozygotes (carriers) are asymptomatic.
Sibs of a proband
At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier.
Once an at-risk sib is known to be unaffected, the chance of his/her being a carrier is 2/3.
Heterozygotes are asymptomatic.
Offspring of a proband. The offspring of an individual with alkaptonuria are obligate heterozygotes (carriers) for a mutant allele causing alkaptonuria.
Other family members of a proband. Each sib of the proband's parents is at a 50% risk of being a carrier.
Molecular genetic testing. Carrier testing for at-risk family members is available on a clinical basis once the mutations have been identified in the family.
Biochemical testing. Biochemical genetic testing is not reliable as a method of carrier detection.
Family planning
The optimal time for determination of genetic risk is before pregnancy.
It is appropriate to offer genetic counseling (including discussion of potential risks to offspring and reproductive options) to young adults who are affected, are carriers, or are at risk of being carriers.
DNA banking. DNA banking is the storage of DNA (typically extracted from white blood cells) for possible future use. Because it is likely that testing methodology and our understanding of genes, mutations, and diseases will improve in the future, consideration should be given to banking DNA of affected individuals. DNA banking is particularly relevant when the sensitivity of currently available testing is less than 100%. See
for a list of laboratories offering DNA banking.
No laboratories listed in the GeneTests Laboratory Directory offer molecular genetic testing for prenatal diagnosis of alkaptonuria. However, prenatal testing may be available for families in which the disease-causing mutations have been identified. For laboratories offering custom prenatal testing, see
.
Requests for prenatal testing for conditions such as alkaptonuria that do not affect intellect or life span and have some treatment available are not common. Differences in perspective may exist among medical professionals and within families regarding the use of prenatal testing, particularly if the testing is being considered for the purpose of pregnancy termination rather than early diagnosis. Although most centers would consider decisions about prenatal testing to be the choice of the parents, careful discussion of these issues is appropriate.
Preimplantation genetic diagnosis (PGD). Preimplantation genetic diagnosis may be available for families in which the disease-causing mutations have been. For laboratories offering PGD, see
.
Information in the Molecular Genetics and OMIM tables may differ from that elsewhere in the GeneReview: tables may contain more recent information. —ED.
| Gene Symbol | Chromosomal Locus | Protein Name | Locus Specific | HGMD |
|---|---|---|---|---|
| HGD | 3q21-q23 | Homogentisate 1,2-dioxygenase | AKU database | HGD |
Normal allelic variants. The normal HGD gene is 54.3 kb in length and has 14 exons coding for a 1715-bp transcript [Granadino et al 1997].
Pathologic allelic variants
At least 67 mutations in HGD have been reported and are found in different combinations [Phornphutkul et al 2002]. The mutations are distributed throughout the HGD gene 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.712+1G>A (also known as IVS5+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 p.Met368Val.
In the US, no mutational hot spot or founder effect has been identified [Phornphutkul et al 2002].
Note: Mutation designations as in Genbank sequence NM_000187.2
Normal 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.
See Consumer Resources for disease-specific and/or umbrella support organizations for this disorder. These organizations have been established for individuals and families to provide information, support, and contact with other affected individuals. GeneTests provides information about selected organizations and resources for the benefit of the reader; GeneTests is not responsible for information provided by other organizations.—ED.
Medical Genetic Searches: A specialized PubMed search designed for clinicians that is located on the PubMed Clinical Queries page
No specific guidelines regarding genetic testing for this disorder have been developed.
Dr. Introne is a pediatrician, clinical geneticist, and biochemical geneticist.
Dr. Kayser is a pediatrician, clinical geneticist, and biochemical geneticist.
Dr. Gahl is a pediatrician, clinical geneticist, and biochemical geneticist who performs clinical and basic research into rare diseases.
William A Gahl, MD, PhD (2003-present)
Wendy J Introne, MD (2003-present)
Michael A Kayser, DO (2007-present)
Chanika Phornphutkul, MD; Brown University (2003-2007)
Pim Suwannarat, MD; Mahidol University (2003-2007)
2 July 2009 (cd) Revision: sequence analysis available clinically
4 December 2007 (me) Comprehensive update posted to live Web site
26 June 2006 (ca) Revision: targeted mutation analysis available for eight mutations
24 May 2005 (me) Comprehensive update posted to live Web site
9 May 2003 (me) Review posted to live Web site
4 March 2003 (ps) Original submission
