Age at exposure and Apc status influence the levels of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-DNA adducts in mouse intestine and liver

Truncation mutations abolish chromatin-associated activities of adenomatous polyposis coli

The adenomatous polyposis coli (APC) is a tumor suppressor whose loss of function leads to colon cancer. APC shuttles between the nucleus and cytoplasm, however its role in the nucleus remains elusive. We have found that nuclear APC specifically associates with transcriptionally active chromatin through structural elements located downstream to the region of frequent truncation mutations found in colorectal tumors. We show that a recombinant APC fragment comprising such elements associates in vivo with euchromatin and preferentially binds in vitro to acetylated histone H3. Induction of DNA double-strand breaks (DSB) stimulates accumulation of APC at the damaged DNA chromatin marked by histone H2AX and S139-phosphorylated histone H2AX. A nuclear complex containing the DNA-dependent protein kinase catalytic subunit (DNAPKcs) and APC associates with chromatin in response to DNA DSB. APC knockdown with siRNA decreased the rate of DNA DSB-induced S139 histone H2AX phosphorylation in cells expressing endogenous full-length APC, but not in colon cancer cells with its truncation mutants, whereas ectopic APC expression stimulated the H2AX phosphorylation regardless of the type of endogenous APC. Our data suggest that APC involves in the DSB DNA repair and that truncation mutations impair chromatin-associated functions of APC.

Role of nucleotide excision repair deficiency in intestinal tumorigenesis in multiple intestinal neoplasia (Min) mice

Mice deficient in the Xeroderma pigmentosum group A (Xpa) gene are defective in nucleotide excision repair (NER) and highly susceptible to skin carcinogenesis after dermal exposure to UV light or chemicals. Min (multiple intestinal neoplasia) mice, heterozygous for a germline nonsense mutation in the tumor suppressor gene adenomatous polyposis coli (Apc), develop intestinal tumors spontaneously and show additional intestinal tumors after exposure to the food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). In this study, we investigated the impact of loss of XPA function on PhIP-induced intestinal tumorigenesis in F1 offspring of Min/+ (Apc(+/-)) mice crossed with Xpa gene-deficient mice. Apc(+/-) mice lacking both alleles of Xpa had higher susceptibility towards toxicity of PhIP, higher levels of PhIP-DNA adducts in the middle and distal small intestines, as well as in liver, and a higher number of small intestinal tumors at 11 weeks, compared with Apc(+/-) mice with one or two intact Xpa alleles. Localization of tumors was not affected, being highest in middle and distal small intestines in all genotypes. At 11 weeks of age, the number of spontaneous intestinal tumors was not significantly increased by homozygous loss of Xpa, but untreated Apc(+/-)/Xpa(-/-) mice had significantly shorter life-spans than their XPA-proficient littermates. Heterozygous loss of Xpa did not affect any of the measured end points. In conclusion, the Xpa gene and the NER pathway are involved in repair of bulky PhIP-DNA adducts in the intestines and the liver, and most probably of DNA lesions leading to spontaneous intestinal tumors. These results confirm a role of the NER pathway also in protection against cancer in internal organs, additional to its well-known importance in protection against skin cancer. An effect of Apc(+/-) on adduct levels, additional to that of Xpa(-/-), indicates that the truncated APC protein may affect a repair pathway other than NER.

Impact of genetic background on spontaneous or 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-induced intestinal tumorigenesis in Min/+ mice

We have studied the impact of genetic background on susceptibility to spontaneous or 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-induced intestinal tumorigenesis. The increase in small intestinal tumor number after PhIP exposure was 3.8- and 3.7-fold above the spontaneous levels in multiple intestinal neoplasia (Min)/+ F1 mice with AKR/J and A/J backgrounds, respectively, compared with only 3-fold in C57BL/6J mice. In the colon, PhIP increased the number of tumors slightly more in C57BL/6J mice (3.3-fold) than in A/J mice (3.0-fold). AKR/J mice had no colonic tumors. Most of the tumors were located in the distal two-thirds of the small intestine in all three strains.