Glutamate carboxypeptidase II: a polymorphism associated with lower levelsof serum folate and hyperhomocysteinemia

Low blood folate levels result in hyperhomocysteinemia, which has been asso ciated with increased risk for cardiovascular disease, neural tube defects and cognitive deficits. Intake of dietary folates is the chief determinant of blood folate levels, Molecular defects in the intestinal absorption of d ietary folates that precipitate low blood folate levels and hyperhomocystei nemia have not been investigated previously, Dietary folates are a mixture of poly-glutamylated lolates which are digested to mono-glutamyl folates by the action of polylpoly-gamma -glutamate carboxypeptidase (FGCP), an enzym e that is anchored to the intestinal brush border membrane and is expressed by the glutamate carboxypepidase I (GCPII) gene, We cloned GCPII cDNA from human intestine and identified both a full-length transcript and a 93 bp s horter transcript lacking exon 18, consistent with the presence of a splice variant. In addition, we identified an H475Y polymorphism in GCPII in DNA samples from a healthy Caucasian population (n = 75), We found that membran es of transfected COS-7 cells expressing the H475Y variant GCPII cDNA had 5 3% less FGCP activity than did cells expressing wild-type GCPII. The presen ce of the H475Y GCPII allele was significantly associated with lower folate and higher homocysteine levels in this population. These data suggest that the presence of the H475Y GCPII allele impairs the intestinal absorption o f dietary folates, resulting in relatively low blood folate levels and cons equent hyperhomocysteinemia.