Folate depletion in human lymphocytes up-regulates p53 expression despite marked induction of strand breaks in exons 5 – 8 of the gene

Authors: CROTT, JIMMY - HNRCA AT TUFTS; LIU, ZHENHUA - HNRCA AT TUFTS; Choi, Sang-Woon; Mason, Joel

Submitted to: Mutation Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/3/2006
Publication Date: 11/13/2006
Citation: Crott, J.W., Liu, Z., Choi, S., Mason, J.B. 2007. Folate depletion in human lymphocytes up-regulates p53 expression despite marked induction of strand breaks in exons 5 – 8 of the gene. Mutation Research. 626:171-179.

Interpretive Summary: Low dietary folate intake is associated with an elevated risk for carcinogenesis. One possible mechanism by which folate depletion promotes carcinogenesis is breaks in DNA, the code from which proteins are made. We studied the effect of DNA breaks within the p53 gene. The p53 gene is commonly mutated in cancer. Using 2 newly developed methods, we show that low folate concentrations result in an increased amount of breaks in the p53 gene. Despite this increased damage to the DNA code, cells maintained an elevated expression of the p53 protein under these conditions. The induction of p53 strand breaks by folate depletion does not impair p53 expression or action within all human cell lines.

Technical Abstract: Low dietary folate intake is associated with an elevated risk for carcinogenesis. One putative mechanism by which folate depletion promotes carcinogenesis is by inducing gene-specific strand breakage and impaired expression of affected genes. Primary human lymphocytes were cultured in media containing 15, 30 or 120nM folic acid. p53 strand breaks, gene and protein expression, and p21 transcript were determined. Cells grown in 15 nM folate developed significant levels of p53 strand breaks, reflected by reductions in amplifiable DNA from p53 exons 5–8 (~40% loss, P<0.0001) and exons 7–8 (~26% loss, P< 0.0001) compared to 30 and 120 nM. Nevertheless, steady-state p53 transcript was elevated 2-fold in 15 and 30 compared to 120 nM (P< 0.001). p53 protein abundance increased with decreasing media folate, as did p21 transcript. The cytokinesis-block micronucleus assay demonstrated a 3-fold increase in chromosomal damage at the two lower folate concentrations (P< 0.01). In primary human lymphocytes, folate depletion induces a marked increase in p53 exon 5 – 8 breaks, but does not reduce steady state levels of p53 mRNA, protein, or impair downstream signaling. The induction of p53 strand breaks by folate depletion does not impair p53 expression or action within all human cell lines.