Folic acid plays a key role in the maintenance of
genomic stability, providing methyl groups for the conversion of
uracil to
thymine and for DNA methylation. Besides dietary habits,
folic acid metabolism is influenced by genetic polymorphism. The C677T polymorphism of the
methylene-tetrahydrofolate reductase (MTHFR) gene is associated with a reduction of catalytic activity and is suggested to modify
cancer risk differently depending on
folate status. In this work the effect of
folic acid deficiency on
genome stability and radiosensitivity has been investigated in cultured lymphocytes of 12 subjects with different MTHFR genotype (four for each genotype). Cells were grown for 9 days with 12, 24 and 120 nM
folic acid and analyzed in a comprehensive micronucleus test coupled with centromere characterization by CREST immunostaining. In other experiments, cells were grown with various
folic acid concentrations, irradiated with 0.5 Gy of gamma rays and analyzed in the micronucleus test. The results obtained indicate that
folic acid deficiency induces to a comparable extent chromosome loss and breakage, irrespective of the MTHFR genotype. The effect of
folic acid was highly significant (P < 0.001) and explained >50% of variance of both types of micronuclei. Also nucleoplasmic bridges and buds were significantly increased under low
folate supply; the increase in bridges was mainly observed in TT cells, highlighting a significant effect of the MTHFR genotype (P = 0.006) on this
biomarker.
Folic acid concentration significantly affected radiation-induced micronuclei (P < 0.001): the increased incidence of radiation-induced micronuclei with low
folic acid was mainly accounted for by carriers of the variant MTHFR allele (both homozygotes and heterozygotes), but the overall effect of genotype did not attain statistical significance. Treatment with ionizing radiations also increased the frequency of nucleoplasmic bridges. The effect of
folic acid level on this end-point was modulated by the MTHFR genotype (P for interaction = 0.02), with TT cells grown at low
folic acid concentration apparently resistant to the induction of radiation-induced bridges. Finally, the effect of in vitro
folate deprivation on global DNA methylation was evaluated in lymphocytes of six homozygous subjects (three CC and three TT). The results obtained suggest that, under the conditions of this work,
folic acid deprivation is associated with global
DNA hypermethylation.