Folate status: Effects on pathways of colorectal carcinogenesis

Many epidemiologic, animal and human studies suggest that folate status modulates carcinogenesis. Although these observations have been made in a number of tissues, the data are clearly most compelling for the colorectum. The mechanism(s) by which this modulation is mediated remains ill defined. Alterations in either genome-wide or gene-specific DNA methylation and/or alterations in DNA stability, resulting from DNA strand breaks or uracil misincorporation, are leading candidates in this regard. Folate has a central role in biological methylation and nucleotide synthesis, and therefore it is not surprising that folate depletion has been observed to alter DNA methylation and diminish DNA stability. The hypothesis that these two pathways are the means by which folate modulates cancer risk is also supported by the epidemiological observation that a common polymorphism in the methylenetetrahydrofolate reductase (MTHFR; EC 1.5.1.20) gene differentially affects the relative risk of colon cancer depending on folate status, because MTHFR catalyzes the reaction that determines whether cellular folate is diverted into biological methylation or nucleotide synthesis. This phenomenon suggests that it is an imbalance between biological methylation and nucleotide synthesis that is responsible for folate-related carcinogenesis. The control of cell proliferation, which also is related to DNA methylation, is another candidate mechanism by which folate status modulates carcinogenesis. In cell culture studies, folate supplementation has been observed to suppress excessive cell proliferation. Understanding the mechanisms by which folate status modulates carcinogenesis is important for advancing insight into cancer biology and for facilitating those efforts to translate research in folate and carcinogenesis into effective and safe public health initiatives.