Folate derivatives are essential for DNA synthesis and methylation. A large proportion of the Caucasian population is heterozygous for a common substitution, 677C-->T (alanine-->valine), in methylenetetrahydrofolate reductase (MTHFR), an enzyme of folate interconversion. Homozygous mutant individuals, approximately 10-15% of North Americans, have been reported to have a reduced risk of colorectal cancer. We examined lymphocyte and tumor tissue DNA from colorectal carcinoma patients from two different populations to assess loss of heterozygosity (LOH) of MTHFR. We observed LOH in approximately 16% of colorectal tumors; in 8 of the 11 tumors with LOH, the mutant valine allele was lost. Additional studies are required to determine if preferential loss of the mutant allele is a common finding that could contribute to colorectal tumorigenesis.

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