Mutation spectrum of homogentisic acid oxidase (HGD) in alkaptonuria

Abstract

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.

FIGURE 1

Clinical features of AKU. Detection of excess of homogentisic acid in urine on standing or artificially by adding a base. A: Left panel: Control urine. Left without base added, right with base added. Right panel: AKU urine. Left without base added, right with base added. Three AKU patients with increasing severity (left to right) of clinical symptoms. B: Ochronotic pigmentation of sclerae. C: Auricular cartilages with ochronosis deposition. D: Radiography of dorsolumbar spines: reduction and calcifications of the intervertebral discs (Arrows).

FIGURE 2

Distribution of HGD variants over the 14 exons of the homogentisic acid oxidase gene. The symbols above the gene represent the 52 variants found in this study, each symbol representing one of the 157 variants. Previously reported variants are filled in black or grey (if a different mutation occurred at the same position) and novel ones are filled in red. The positions of other previously reported variants (not found in the current study) are indicated by unfilled symbols under the gene.