Epistatic interactions govern chemically‐induced lung tumor susceptibility and Kras mutation site in murine C57BL/6J‐ChrA/J chromosome substitution strains

Abstract

Cancer susceptibility results from interactions between sensitivity and resistance alleles. We employed murine chromosome substitution strains to study how resistance alleles affected sensitive alleles during chemically-induced lung carcinogenesis. The C57BL/6J-Chr# A/J strains, constructed by selectively breeding sensitive A/J and resistant C57BL/6J (B6) mice, each contain one pair of A/J chromosomes within an otherwise B6 genome. Pas1, the major locus responsible for this differential strain response to urethane carcinogenesis, resides on Chr 6, but C57BL/6J-Chr6 A/J mice (hereafter CSS-6) developed few tumors following a single urethane injection, which demonstrates epistatic interactions with other B6 alleles. CSS6 mice developed dozens of lung tumors after chronic urethane exposure, however, indicating that these epistatic interactions could be overcome by repeated carcinogen administration. Unlike A/J, but similar to B6 mice, CSS6 mice were resistant to lung carcinogenesis induced by 3-methylcholanthrene (MCA). Tumor multiplicity increased if BHT administration followed urethane exposure, showing that a Chr 6 gene(s) regulates sensitivity to chemically-induced tumor promotion. Unlike A/J tumors (predominantly codon 61 AfiT transversions), Kras mutations in tumors induced by urethane in CSS-6 mice were similar to B6 tumors (codon 61 AfiG transitions). DNA repair genes not located on Chr 6 may determine the nature of Kras mutations. CSS-6 mice are a valuable resource for testing the ability of candidate genes to modulate lung carcinogenesis.Lung cancer is the leading cause of cancer death in both men and women in the USA, with 200,000 new cases of lung cancer estimated to be diagnosed this year. The dismal 15% survival rate over 5 years indicates that only 30,000 of these patients will be alive in 2014. 1 Smoking contributes to 85-90% of all lung cancers, but non-smoking-related lung cancer, which is more common in women than men, is the 7 th leading cause of cancer death.2 Since only 15% of smokers develop lung cancer, genetic factors presumably contribute to disease susceptibility.Recent lung cancer familial linkage studies delineated regions on human Chromosomes 6 and 12 that contributed to the risk of lung cancer, chronic obstructive pulmonary disease (COPD), and diminished lung function.3,4 Chronic pulmonary inflammation predisposes toward lung cancer, and several chromosomal sites in mice that determine responsiveness to pro-inflammatory pneumotoxicants correspond to loci containing lung cancer susceptibility genes. 5Mice develop pulmonary adenocarcinoma (AC) that is histologically and genetically similar to human AC within a relatively short time span (6-42 wks) in response to chemical carcinogens 6 and within 2-36 wks in response to genetic induction with oncogenessuch as Kras.7-9 A/J mice are particularly sensitive to carcinogen-induced lung tumorigenesis, and develop spontaneous lung tumors during their lifespan.

Loss of the SWI/SNF ATPase subunits BRM and BRG1 drives lung cancer development

“…Previous attempts to inactivate Brg1 in normal lung tissue resulted in apoptosis, while in contrast, we observed that Brg1 could be successfully inactivated when lung cells had progressed to adenomas. This is thought to occur because of the frequent Kras mutations induced by ethyl carbamate administration [21, 22] that not only cause the development of adenomas but also the suppression of apoptosis [22, 23]. Hence, we then selectively inactivated (i.e., knocked out) Brg1 in the lungs of the mice through the induction of Cre recombinase via tetracycline administration (Figure 1B).…”

Activation of liver X receptor inhibits the development of pulmonary carcinomas induced by 3-methylcholanthrene and butylated hydroxytoluene in BALB/c mice

“…In this model, MCA causes K-ras mutation, while BHT can promote inflammation which enhances formation of lung tumors89. The sensitivity to MCA/BHT-induced lung tumors are different among the mouse strains with an order of A/J > BABL/c > C57BL/61011. Interestingly, more MCA/BHT-induced tumors can be found in mice lacking IFN-γ expression than wild type mice which implies that the endogenous IFN-γ production may protect the animals against MCA/BHT-induced tumorigenesis23.…”

The Kras Mutational Spectra of Chemically Induced Lung Tumors in Different Inbred Mice Mimics the Spectra of KRAS Mutations in Adenocarcinomas in Smokers versus Nonsmokers

“…A/J and cBy mice have the same sensitive Kras haplotype, so other loci presumably are responsible for generating their distinct arrays of Kras mutations. We recently showed using chromosome substitution strains that genes mapping to chromosomes other than Chr 6 influence the nature of Kras codon 61 mutations in lung tumors induced by urethane (7). …”

“…In 9 A/J and 12 cBy mice, MCA treatment was followed by 6 weekly i.p. BHT injections (150 mg/gm, week 1; 200 mg/gm thereafter), as described (6, 7). DNA was isolated from 68 A/J and 67 cBy tumors (8), an average of 4 tumors/mouse, harvested 20 wks after MCA initiation.…”

“…Metabolites of 3-methylcholanthrene (MCA), a polycyclic aromatic hydrocarbon formed by carbon pyrolysis and found in tobacco smoke and tar, mutate Kras at codon 12 (5). MCA treatment of A/J mice induces 5.4 tumors/mouse with 100% incidence, while MCA-treated BALB/cBy (hereafter cBy) mice only develop 0.4 tumors/mouse, with 44% incidence (6, 7). When MCA treatment is followed by repeated exposure to the non-carcinogenic pneuomotoxin, butylated hydroxytoluene (BHT), A/J and cBy develop 23 and 25 tumors/mouse (respectively), with 100% incidence in both strains.…”