Homogentisate 1,2 dioxygenase is expressed in brain: implications in alkaptonuria

Alkaptonuria is an ultra-rare autosomal recessive disease developed from the lack of homogentisate 1,2-dioxygenase (HGD) activity, causing an accumulation in connective tissues of homogentisic acid (HGA) and its oxidized derivatives in polymerized form. The deposition of ochronotic pigment has been so far attributed to homogentisic acid produced by the liver, circulating in the blood, and accumulating locally. In the present paper, we report the expression of HGD in the brain. Mouse and human brain tissues were positively tested for HGD gene expression by western blotting. Furthermore, HGD expression was confirmed in human neuronal cells that also revealed the presence of six HGD molecular species. Moreover, once cultured in HGA excess, human neuronal cells produced ochronotic pigment and amyloid. Our findings indicate that alkaptonuric brain cells produce the ochronotic pigment in loco and this may contribute to induction of neurological complications.