Folate deficiency, a prevalent vitamin deficiency in America, can stem from environmental and/or genetic causes. The most common inborn error of folate metabolism is deficiency of methylenetetrahydrofolate reductase (MTHFR), which catalyzes the reduction of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate. Severe MTHFR deficiency results in hyperhomocysteinemia and homocystinuria; patients present with developmental delay, and various neurological and vascular disorders. This thesis describes three mutations identified in the MTHFR locus in patients with severe deficiency: 1025T→C (M→T), 1027T→G (W→G), and 1768G→A (E→K). Genotype-phenotype correlations are described, along with biochemical characterization of three mutations (983A→G (N→S), 1025T→C, 1027T→G). All three mutations exert their effect by decreasing Vmax without changing the enzyme's affinity for its substrate, 5-methyltetrahydrofolate. The 983A→G variant also conferred decreased affinity for FAD, a cofactor. / The more common and mild deficiency observed in the general healthy population is probably due in part to insufficient dietary intake of folate. Folate deficiency has been associated with increased risk for colon cancer. In a pilot study presented here, the impact of altered folate intake on tumor multiplicity in the Min mouse, a model for multiple intestinal neoplasia, was assessed. Folate deficient diets did not produce a consistent change in tumor numbers. However, a linear correlation between S-adenosylmethionine and S-adenosylhomocysteine content of preneoplastic tissue and tumor multiplicity was identified. / This thesis contributes to our understanding of the impact of genetic- and/or dietary-induced folate deficiency on cellular and organismal functions.

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