Susceptibility to Alkylator-Induced Cancer Is a Heritable Trait.

Abstract

Secondary malignancies are a serious adverse consequence of alkylator chemotherapy treatment. The incidence of secondary hematopoietic malignancies (t-MDS/AML) exceeds 11% in several recent reports. Secondary solid tumors (lung, breast) also occur after alkylator exposure. While patient age, primary diagnosis, and chemotherapy dose are established risk factors, there is increasing evidence that germline genetic factors are also important in determining risk. The genetic basis of susceptibility to alkylator-induced secondary neoplasms is poorly understood. Most studies have focused on genes in drug metabolic pathways. A limited number of genetic polymorphisms with modest impact on risk for alkylator-induced cancer have been reported. We hypothesize that other genetic contributors to alkylator-induced cancer susceptibility have not yet been identified. To screen genome-wide for novel heritable susceptibility factors, we established a mouse model of alkylator-induced malignancy. In collaboration with the Mouse Phenome Project, we exposed mice from 20 inbred strains to the prototypical alkylating agent, N-nitroso-N-ethylurea (ENU). Mice (n=12 per strain) received two doses of ENU (100mg/kg, IP) or no treatment (n=12 per strain) at 9 and 10 weeks of age. ENU was a potent carcinogen in many of the strains tested, inducing 140 tumors in 240 ENU-treated mice (66% incidence of at least one tumor in evaluable mice), compared to a background incidence of 8% spontaneous tumors in 240 strain, age, and sex-matched control mice (relative risk = 8.4, p< 0.0001). A wide variety of tumor histologies were noted, including epithelial carcinomas, soft tissue sarcomas, and hematopoietic tumors. Cancer susceptibility was a heritable trait for the most common tumor types, lung adenocarcinoma (H2 = 0.25, implying that 25% of cancer risk is heritable), T-cell lymphoma (H2 = 0.19), and myeloid malignancies (H2 = 0.10). The development of hematopoietic tumors (either lymphoid or myeloid) was negatively associated with the likelihood of developing gastrointestinal or lung adenocarcinoma. In silico quantitative trait locus (QTL) mapping was used to identify genomic intervals associated with cancer susceptibility. Strains were segregated by haplotype using a panel of 10,900 single nucleotide polymorphisms (SNPs). Differences in cancer susceptibility between haplotypes were assessed by ANOVA. After correcting for false detection, genomic intervals significantly associated with specific cancer phenotypes were identified. Preliminary analysis linked lung cancer susceptibility to four intervals on chromosomes 1,2,6, and 12. The latter interval contains Ahr, a highly polymorphic gene encoding the aryl hydrocarbon receptor previously implicated in carcinogenicity of environmental toxins. Lymphoma susceptibility was linked to two regions on chromosomes 3 and 18. The latter contains a member of the frizzled family of Wnt receptors. This novel mouse model recapitulates many features of human alkylator-associated cancer and supports the hypothesis that susceptibility to this syndrome is influenced by inherited polymorphisms that could be used to inform clinical treatment decisions.