Gene expression & biochemical analysis in alkaptonuria caused by a founder pathogenic variant across different age groups from India

Background & objectives Alkaptonuria (AKU) is an autosomal recessive disease wherein biallelic pathogenic variants in the homogentisate 1,2- dioxygenase (HGD) gene encoding the enzyme homogentisate 1,2 dioxygenase cause high levels of homogentisic acid (HGA) to circulate within the body leading to its deposition in connective tissues and excretion in urine. A homozygous splice donor variant (c.87+1G>A) has been identified to be the founder variant causing alkaptonuria among Narikuravars, a group of gypsies settled in Tamil Nadu. Methods Blood and urine samples of 30 homozygous splice site donor variant individuals (2 groups aged 7-20 and 21-83 yr, with 9 and 21 individuals, respectively), carriers and 30 wild-type individuals from the Narikuravars were collected during field visits after obtaining informed consent. Clinical evaluation and genetic counselling were done. The plasma and urine HGA levels were estimated by high-performance liquid chromatography. RNA was extracted from the peripheral blood and reverse transcribed. Sanger sequencing was done to check the consequence of the splice donor variant. Relative quantification of the cDNA in the three groups was done by real-time qPCR (RT-qPCR) studies using reference genes followed by Pearson's correlation analysis. Results In our cohort, among the affected alkaptonuria individuals, the minimum age for eye pigmentation detected was 23 yr. Similarly, the minimum age for back pain and any joint pain was 30 yr and 38 yr, respectively. Sequencing of the cDNA confirmed exon 2 skipping in affected individuals. In comparison to the normal individuals, the affected individuals showed reduced HGD expression. HGD relative expression showed a significant correlation (P<0.05) with mean plasma HGA levels in the younger (≤22 yr) age group but not in the older one. There was also a significant correlation (P<0.05) of reduced HGD expression with back pain in the 21-37 yr age group. Increasing age showed a positive correlation with circulating mean plasma HGA levels and a negative correlation with excreted HGA. Interpretation & conclusions As per the authors' knowledge, this is the first study to confirm the functional effect by RT-PCR of this highly prevalent founder HGD variant causing alkaptonuria in the Narikuravar community. Both plasma and urinary HGA levels correlated well with the gene expression of this variant and could serve as potential markers of AKU severity for those with this variant.