AB173. Heterozygous carriers of classical homocystinuria tend to have higher fasting serum homocysteine concentrations than non-carriers in a folate deficiency area where has inordinately high homocystinuria prevalence

Background: The newborn screening of homocystinuria in Taiwan has started since 1984. Out of 5 million newborns screened, only 3 newborns (Han Taiwanese) suffering from homocystinuria were detected in this newborn screening program. The prevalence is less than 1 in 1 million. However we recently found 8 patients presenting with homocystinuria in an Austronesian Taiwanese Tao tribe. All the Tao patients are homozygous for a novel mutation (p.D47E, c.141T > A). Among the 428 adult islanders screened for the D47E mutation, approximately 1 in 7.78 is a carrier of the mutation, and an estimated 1 in 240 islanders suffered from homocystinuria. This is the highest known prevalence of homocystinuria worldwide. The expression study revealed that this p.D47E mutation interferes not only with the function but also with the stability of the CBS protein in vivo. We evaluated if the CBS carriers tend to have higher fasting serum tHcy concentrations than non-carriers in presence of folate deficiency.

Methods: The serum tHcy and folate levels before and after folate replacement was measured in 48 adult Tao carriers, 40 age-matched Tao non-carriers and 40 age-matched Han Taiwanese controls.

Results: We found that serum tHcy level of the Tao CBS carriers (17.9±3.8 µmol/L) was significantly higher than in Tao non-carriers (15.7±3.5 µmol/L; P<0.008) in the presence of folate deficiency. Of note, the difference in tHcy levels between the carriers and non-carriers was eliminated by folate supplementation (carriers: 13.65±2.13 µmol/L; non-carriers: 12.39±3.25 µmol/L, P=0.321). This finding implies that CBS carriers tend to have the risk of cardiovascular disease in presence of folate deficiency.

Conclusions: CBS carriers tend to have a higher tHcy level in the presence of folate deficiency than non-carriers. Although many reports have indicated that CBS carriers are not associated with cardiovascular disease, the risk for CBS carriers with folate deficiency has not been well studied. Owing to a significantly elevated level of fasting tHcy without methionine loading, this finding implies that CBS carriers tend to have the risk of cardiovascular disease in presence of folate deficiency.

Keywords: Folate; homocysteine; homocystinuria; hyperhomocysteinemia; Vitamin B₁₂