Dr. Susan J. Duthie, Rowett Research Institute, Greenburn Rd., Bucks-
burn, Aberdeen AB21 9SB, UK. Phone: +44 1224 712751 ext. 2324.
FAX: +44 1224 716629. E-mail: sd@rri.sari.ac.uk.
tumorigenesis is induced (33,34). In addition, DNA hypo-
methylation is frequently found in human colonic neoplasms
(35,36). In this experiment, DNA extracted from folate-
deficient human colon cells was hypomethylated.
Submitted 5 January 2000; accepted in final form 30 March 2000.
In addition to negatively affecting DNA synthesis, folate
deficiency-mediated alterations in dNTP pools may inhibit
DNA repair systems, which likewise require a precise bal-
ance of precursors (37). Oxidative damage to DNA, caused
by reactive oxygen species, has been implicated in the devel-
opment of cancer (38). Similarly, DNA alkylation after ex-
posure to ionizing radiation or endogenous/exogenous
alkylating agents may induce mutagenesis (39). In this
study, human colonocytes grown in folate-free media were
unable to effectively repair oxidative- or alkylation-induced
DNA damage. This type of damage is removed by the exci-
sion repair pathway, which replaces damaged bases with ap-
propriate DNA bases by use of the complementary strand as
template. Perturbations in the dNTP pool could result in a
breakdown in DNA repair. Folate deficiency significantly
reduces DNA excision repair in rodent colonocytes and hu-
man lymphocytes exposed to H2O2 ex vivo (26,40) and in-
hibits DNA repair in vitro in response to radiation in Chinese
hamster ovary cells (41). Folate status may also be associ-
ated with efficiency of DNA mismatch repair (42). Errors
during DNA replication or recombination are corrected by
the DNA mismatch repair system. An inverse (but non-
significant) association between colonic folate concentra-
tions in patients with inflammatory bowel disease and DNA
microsatellite instability as an indicator of efficient mis-
match repair has recently been reported (42). Microsatellites
are repetitive DNA sequences dispersed throughout the ge-
nome. Instability within these sequences is regarded as a
biomarker for altered DNA repair (42). Ulcerative colitis pa-
tients were shown to have an increased rate of microsatellite
instability (13%) and 30–50% lower serum, whole blood,
and colonic folate concentrations. Moreover, folate supple-
mentation reduced to normal microsatellite instability in one
of the volunteers (42).
A diet poor in folic acid will probably also be deficient in
other protective phytochemicals, including vitamin E, carot-
enoids, and vitamin C, which safeguard cellular components,
such as DNA and membranes, from attack from oxidizing and
alkylating agents. Compromised antioxidant status, together
with folate deficiency-mediated DNA instability and reduced
DNA repair, may contribute to the increased risk of colorectal
cancer associated with diets low in vegetables and fruit.
In conclusion, folate deficiency in vitro induces DNA in-
stability and hypomethylation in immortalized human
colonocytes. These data show that poor folate status in these
cells can influence uracil misincorporation and DNA
methylation status, both of which have been implicated as
risk factors for colorectal cancer.
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Acknowledgments and Notes
This work is supported by the World Cancer Research Fund and the
Scottish Executive Rural Affairs Department. Address correspondence to
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Nutrition and Cancer 2000