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Title: The influence of the MTHFR C677T genotype and folate status on genomic DNA methylation and uracil misincorporation in the colon of subjects without colorectal neoplasia
Author: Hanks, Joanna
ISNI:       0000 0004 6498 1450
Awarding Body: King's College London
Current Institution: King's College London (University of London)
Date of Award: 2017
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Background: Colorectal cancer (CRC) is a leading cause of cancer-related mortality in the UK. Folate deficiency and the MTHFR C677T polymorphism may increase colorectal cancer risk by altering normal patterns of DNA methylation and by causing uracil misincorporation into DNA which can in turn induce deletions and double-strand breaks. The methylenetetrahydrofolate reductase (MTHFR) enzyme is pivotal in maintaining the balance between these two critical folate-mediated pathways (DNA methylation and uracil misincorporation) and the commonly occurring MTHFR C667T polymorphism has been associated with a reduction in risk of colorectal cancer under conditions of adequate folate status, but increased risk when folate status is low. Design: In this PhD project, a large cross-sectional study of neoplasia-free volunteers (n=336) recruited at clinically indicated colonoscopy, was designed to investigate the influence of folate status and the MTHFR genotype on genomic DNA methylation and uracil misincorporation in DNA. The relationship between serum and red cell folate, plasma homocysteine and colonic folate concentration was also investigated. A second study, designed as a randomised controlled trial (RCT) in a subset of participants from the cross-sectional study, was conducted to determine whether folate supplementation influences genomic DNA methylation and folate levels in the colon. The relationship between demographic and lifestyle factors was also explored in this small dataset (n=15). Methods: 336 volunteers without colorectal neoplasia were recruited into the cross-sectional study. A health questionnaire was used to collect data on age, weight, height, ethnicity, and smoking, drinking, and nutritional supplement intake. A previously validated food frequency questionnaire was used to collect data on habitual dietary folate intake. Blood samples were taken to determine serum and red cell folate, plasma homocysteine, vitamin B12 and MTHFR C677T genotype, and colonic tissue biopsies were collected to determine colonic tissue folate, genomic DNA methylation and uracil misincorporation in DNA. Additionally, a small number of participants from the cross-sectional study were recruited into a small RCT taking short-term folate supplementation (400g/d) for 12 weeks. Serum and red cell folate, serum vitamin B12, MTHFR C677T genotype, colonic folate and DNA methylation were compared before and after supplementation. Results: The MTHFR C677T genotype did not influence systemic or colonic tissue folate biomarkers, genomic DNA methylation or uracil content in the colonic mucosa. DNA methylation was not influenced by biomarkers of folate status or homocysteine. Uracil content in DNA was positively associated with serum folate and nutritional supplement use after adjustment for age, gender, ethnicity, smoking and systemic folate biomarkers. Colonic tissue folate correlated positively with serum folate and and negatively with smoking. For the 15 subjects who were recruited into the RCT on folic acid supplementation, an increase in serum folate and a non-significant increase in colonic tissue folate was observed in comparison with subjects on placebo. Folate supplementation did not influence genomic DNA methylation, red cell folate plasma homocysteine or serum vitamin B12 levels. Conclusion: The MTHFR C667T genotype and folate status were not associated with DNA methylation or uracil misincorporation into DNA. Uracil misincorporation and colonic tissue folate were both influenced by serum folate, and tissue folate was affected by smoking in this population of neoplasia-free subjects with adequate folate status. Short term supplementation increased serum but conferred no influence on genomic DNA methylation in this population.
Supervisor: Emery, Peter William ; Pufulete, Maria Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available