Carcinogenesis, Vol. 24, No. 1, 39-45,
January 2003
© 2003 Oxford University Press
CANCER BIOLOGY |
Impact of Dnmt1 deficiency, with and without low folate diets, on tumor numbers and DNA methylation in Min mice
1 Departments of Pediatrics and Human Genetics, McGill University-Montreal Children's Hospital, 4060 Ste. Catherine St. West, Montreal, Quebec H3Z 2Z3, Canada,
2 Department of Pharmacology and Therapeutics, McGill University, Montreal, Quebec, Canada,
3 Division of Biochemical Toxicology, National Center for Toxicological Research, Jefferson, Arizona, USA and
4 Massachussetts General Hospital, Charlestown and Harvard Medical School, Boston, Massachussetts, USA
Although a number of studies have suggested that diets with low intake of folate, an important methyl donor, are associated with increased risks of colon cancer and its precursor the adenomatous polyp, the underlying mechanisms are poorly understood. Dysregulation and instability of DNA methylation and alterations in the levels of the predominant DNA methylating enzyme, DNA (cytosine-5)-methyltransferase 1 (Dnmt1), have also been linked to tumorigenesis. We have used a combination of genetic and dietary manipulation to assess the effects of reduced Dnmt1 expression with and without folate deficiency on tumor induction in the ApcMin mouse. ApcMin mice with a reduction in Dnmt1 expression (ApcMin/+/Dnmt1C/+) had significantly lower tumor numbers than ApcMin mice with normal Dnmt1 (ApcMin/+/Dnmt1+/+). Dietary folate deficiency from weaning to 13 weeks of age did not affect tumor number or size in ApcMin/+/Dnmt+/+ mice. However, in ApcMin/+/Dnmt1C/+ mice with high baseline tumor numbers (41 ± 4), folate deficiency was associated with a decreased absolute number of tumors (27 ± 3), but a higher proportion of larger tumors as compared with mice on the control diet. In the repeat experiment, ApcMin/+/Dnmt1C/+ mice had low baseline tumor numbers (20 ± 2) and folate deficiency did not affect tumor number (23 ± 4) or size as compared with the same mice on the control diet. These results suggest that, in the presence of Dnmt1 deficiency, the effects of folate deficiency on tumor number and size may depend on the stage of adenoma development when folate deficiency is initiated. We also show that folate deficiency with or without reductions in Dnmt1 did not affect overall genomic DNA methylation or the methylation levels of two candidate genes, E-cadherin or p53, in normal or neoplastic intestinal tissue. In conclusion, genetic deficiency in Dnmt1 with or without folate deficiency decreases tumor number in the ApcMin mouse model, but this effect may not be mediated by changes in SAM or SAH levels, nor by alterations in global methylation in the pre-neoplastic intestinal tissue.
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