Pathobiological Insights into Neurological Involvement in Cobalamin C Deficiency

Cobalamin C (Cbl-C) defects are inherited autosomal recessive disorders of vitamin B-12 (or cyanocobalamin [CNCbl]) metabolism. These defects are caused bymutations in the methylmalonic aciduria and homocystinuria Cbl-C type (MMACHC; MIM # 609831) gene located on chromosome 1p34.1, which catalyzes the reductive decyanation of CNCbl, thus impairing the biosynthesis of 5'-deoxyadenosylcobalamin, adenosylcobalamin, and methylcobalamin. This impairment results in methylmalonic acidemia [MMA; MIM # 277400] combined with hyperhomocysteinemia and hypomethioninemia. Clinically, Cbl-C defects are characterized by a constellation of systemic signs and symptoms, including neurological, cognitive, psychiatric, and thromboembolic events. Retinal phenotypes, including maculopathy, pigmentary retinopathy, and optic atrophy, are common in the early-onset form of the disease, but are rare in its adult-onset counterpart. Administration of hydroxocobalamin (OHCbl), betaine, and folinic acid represents main therapeutic approaches. No proven efficacy has been demonstrated for carnitine and dietary protein restrictions. Although early introduction of OHCbl is crucial, no standardized protocols regarding dose adjustment exist. Despite these measures, the long-term outcome is unsatisfactory especially in patients with early onset, who experience frequent progression of their neurological and ocular impairment. The unfavorable outcome suggests that a better understanding of the pathophysiology of the disease is needed to improve treatment protocols and to develop new therapeutic approaches.