First Report of a Deletion Encompassing an Entire Exon in the Homogentisate 1,2-Dioxygenase Gene Causing Alkaptonuria

Alkaptonuria is often diagnosed clinically with episodes of dark urine, biochemically by the accumulation of peripheral homogentisic acid and molecularly by the presence of mutations in the homogentisate 1,2-dioxygenase gene (HGD). Alkaptonuria is invariably associated with HGD mutations, which consist of single nucleotide variants and small insertions/deletions. Surprisingly, the presence of deletions beyond a few nucleotides among over 150 reported deleterious mutations has not been described, raising the suspicion that this gene might be protected against the detrimental mechanisms of gene rearrangements. The quest for an HGD mutation in a proband with AKU revealed with a SNP array five large regions of homozygosity (5-16 Mb), one of which includes the HGD gene. A homozygous deletion of 649 bp deletion that encompasses the 72 nucleotides of exon 2 and surrounding DNA sequences in flanking introns of the HGD gene was unveiled in a proband with AKU. The nature of this deletion suggests that this in-frame deletion could generate a protein without exon 2. Thus, we modeled the tertiary structure of the mutant protein structure to determine the effect of exon 2 deletion. While the two beta-pleated sheets encoded by exon 2 were missing in the mutant structure, other beta-pleated sheets are largely unaffected by the deletion. However, nine novel alpha-helical coils substituted the eight coils present in the native HGD crystal structure. Thus, this deletion results in a deleterious enzyme, which is consistent with the proband's phenotype. Screening for mutations in the HGD gene, particularly in the Middle East, ought to include this exon 2 deletion in order to determine its frequency and uncover its origin.