Methylation in the p21^WAF1/cip1 promoter of Apc^+/-, p21^+/- mice and lack of response to sulindac

p21^WAF1/cip1 plays a critical role in regulating intestinal cell proliferation, maturation and tumorigenesis. Our previous work demonstrated that inactivation of a single p21 allele in Apc1638^+/- mice was sufficient to accelerate Ape-initiated tumor formation, and that inactivation of only one p21 allele was also sufficient to abrogate duodenal tumor inhibition by sulindac, a nonsteroidal anti-inflammatory drug. To dissect the role of p21 in sulindac inhibition of intestinal tumor development in Apc1638^+/- mice, we quantified p21 expression from Apc^+/-, p21 ^{+/+, +/- or -/-} mice fed sulindac. In Apc^+/-, p21 wild-type mice fed the sulindac supplemental diet, both p21 mRNA and protein were significantly increased in the flat mucosa and tumors of the duodenum. However, p21 was not induced by sulindac in the duodenal flat mucosa and tumors of Apc^+/- mice in which one or both p21 alleles had been inactivated. Further investigation revealed that the cytosine residues in a CpG cluster in the promoter region of the mouse p21 gene displayed hypermethylation in the Apc^+/-, p21^+/- mice. This suggested that although the p21^+/- mice retained a wild-type allele, this allele was functionally modulated by hypermethylation, and that the inability of sulindac to inhibit tumor formation in Apc^+/-, p21^+/- mice is likely due to the inability to induce expression of the wild-type, but differentially methylated, p21 allele.