Alkaptonuria: A Case Report With Diagnostic Challenge.

Alkaptonuria is a rare autosomal recessive metabolic disorder caused by deficiency of homogentisic acid (HGA) oxidase, the only enzyme capable of catabolizing HGA. Deficiency of this enzyme leads to excess HGA which deposits in the connective tissue. We present a case of a 64-year-old woman who was referred to the dermatology clinic for a full body mole check and skin cancer screening. Clinically she had blue/gray pigmentation of the external ear and sclera. Also she had a domed papule on the left cheek with punctate gray pigmentation which was biopsied. Histopathological examination showed a benign dermal nevus and nonpolarizable, yellow-brown, irregular shaped fibers. Subsequent organic acid screen showed markedly elevated urinary HGA, diagnostic of alkaptonuria. On specific inquiry, the patient revealed she had a history of bilateral Achilles tendon rupture, black urine, arthritis, and external ear discoloration for many years. The pigmented material was then considered to be HGA deposition within the dermal collagen fibers. However, without the appropriate clinical data and confirmatory lab findings, the pigmented fragments on skin biopsy represent a diagnostic challenge. Measures like low protein diet and ascorbic acid supplementation will slow down the disease progression and potential complications later in life; however, there is no definitive treatment for the disease. We emphasize the prompt recognition of the clinical signs and symptoms as well as the importance of the microscopic findings.

An update on molecular genetics of Alkaptonuria (AKU).

Alkaptonuria (AKU) is an autosomal recessive disorder caused by a deficiency of homogentisate 1,2 dioxygenase (HGD) and characterized by homogentisic aciduria, ochronosis, and ochronotic arthritis. The defect is caused by mutations in the HGD gene, which maps to the human chromosome 3q21-q23. AKU shows a very low prevalence (1:100,000-250,000) in most ethnic groups, but there are countries such as Slovakia and the Dominican Republic in which the incidence of this disorder rises to as much as 1:19,000. In this work, we summarize the genetic aspects of AKU in general and the distribution of all known disease-causing mutations reported so far. We focus on special features of AKU in Slovakia, which is one of the countries with an increased incidence of this rare metabolic disorder.

Mutation spectrum of homogentisic acid oxidase (HGD) in alkaptonuria.

Alkaptonuria (AKU) is a rare autosomal recessive metabolic disorder, characterized by accumulation of homogentisic acid, leading to darkened urine, pigmentation of connective tissue (ochronosis), joint and spine arthritis, and destruction of cardiac valves. AKU is due to mutations in the homogentisate dioxygenase gene (HGD) that converts homogentisic acid to maleylacetoacetic acid in the tyrosine catabolic pathway. Here we report a comprehensive mutation analysis of 93 patients enrolled in our study, as well as an extensive update of all previously published HGD mutations associated with AKU. Within our patient cohort, we identified 52 HGD variants, of which 22 were novel. This yields a total of 91 identified HGD variations associated with AKU to date, including 62 missense, 13 splice site, 10 frameshift, 5 nonsense, and 1 no-stop mutation. Most HGD variants reside in exons 3, 6, 8, and 13. We assessed the potential effect of all missense variations on protein function, using five bioinformatic tools specifically designed for interpretation of missense variants (SIFT, POLYPHEN, PANTHER, PMUT, and SNAP). We also analyzed the potential effect of splice-site variants using two different tools (BDGP and NetGene2). This study provides valuable resources for molecular analysis of alkaptonuria and expands our knowledge of the molecular basis of this disease.