Cancer modifier alleles inhibiting lung tumorigenesis are common in inbred mouse strains

Mapping of novel loci involved in lung and colon tumor susceptibility by the use of genetically selected mouse strains

Two non-inbred mouse lines, phenotypically selected for maximal (AIRmin) and minimal (AIRmax) acute inflammatory response, show differential susceptibility/resistance to the development of several chemically-induced tumor types. An intercross pedigree of these mice was generated and treated with the chemical carcinogen dimethylhydrazine, which induces lung and intestinal tumors. Genome wide high-density genotyping with the Restriction Site-Associated DNA genotyping (2B-RAD) technique was used to map genetic loci modulating individual genetic susceptibility to both lung and intestinal cancer. Our results evidence new common quantitative trait loci (QTL) for those phenotypes and provide an improved understanding of the relationship between genomic variation and individual genetic predisposition to tumorigenesis in different organs.

Complex genetic control of lung tumorigenesis in resistant mice strains

The SM/J mouse strain is resistant to chemically‐induced lung tumorigenesis despite having a haplotype, in the pulmonary adenoma susceptibility locus (Pas1) locus, that confers tumor susceptibility in other strains. To clarify this inconsistent genotype‐phenotype correlation, we crossed SM/J mice with another resistant strain and conducted genome‐wide linkage analysis in the (C57BL/6J × SM/J)F2 progeny exposed to urethane to induce lung tumors. Overall, >80% of F2 mice of both sexes developed from 1 to 20 lung tumors. Genotyping of 372 F2 mice for 744 informative non‐redundant SNPs dispersed over all autosomal chromosomes revealed four quantitative trait loci (QTLs) affecting lung tumor multiplicity, on chromosomes 3 (near rs13477379), 15 (rs6285067), 17 (rs33373629) and 18 (rs3706601), all with logarithm of the odds (LOD) scores >5. Four QTLs modulated total lung tumor volume, on chromosome 3 (rs13477379), 10 (rs13480702), 15 (rs6285067) and 17 (rs3682923), all with LOD scores >4. No QTL modulating lung tumor multiplicity or total volume was detected in Pas1 on chromosome 6. The present study demonstrates that the SM/J strain carries, at the Pas1 locus, the resistance allele: a finding that will facilitate identification of the Pas1 causal element. More generally, it demonstrates that lung tumorigenesis is under complex polygenic control even in a pedigree with low susceptibility to this neoplasia, suggesting that the genetics of lung tumorigenesis is much more complex than evidenced by the pulmonary adenoma susceptibility and resistance loci that have, so far, been mapped in a small number of crosses between a few inbred strains.

A specific nuclear DNA background is required for high frequency lymphoma development in transmitochondrial mice with G13997A mtDNA

We previously found that mouse mitochondrial DNA (mtDNA) with a G13997A mutation (G13997A mtDNA) controls not only the transformation of cultured lung carcinoma cells from poorly metastatic into highly metastatic cells, but also the transformation of lymphocytes into lymphomas in living C57BL/6 (B6) mice. Because the nuclear genetic background of the B6 strain makes the strain prone to develop lymphomas, here we examined whether G13997A mtDNA independently induces lymphoma development even in mice with the nuclear genetic background of the A/J strain, which is not prone to develop lymphomas. Our results showed that the B6 nuclear genetic background is required for frequent lymphoma development in mice with G13997A mtDNA. Moreover, G13997A mtDNA in mice did not enhance the malignant transformation of lung adenomas into adenocarcinomas or that of hepatocellular carcinomas from poorly metastatic into highly metastatic carcinomas. Therefore, G13997A mtDNA enhances the frequency of lymphoma development under the abnormalities in the B6 nuclear genome, and does not independently control tumor development and tumor progression.

Genetic linkage analysis identifies Pas1 as the common locus modulating lung tumorigenesis and acute inflammatory response in mice

Selective breeding for the acute inflammatory response (AIR) generated two mouse lines characterized by maximum (AIRmax) and minimum (AIRmin) responses, explained by the additive effect of alleles differentially fixed in quantitative trait loci (QTLs). These mice also differ in their susceptibility to lung tumorigenesis, raising the possibility that the same loci are involved in the control of both phenotypes. To map the QTLs responsible for the different phenotypes, we carried out a genome-wide linkage analysis using single-nucleotide polymorphism arrays in a pedigree consisting of 802 mice, including 693 (AIRmax × AIRmin)F2 intercross mice treated with urethane and phenotyped for AIR and lung tumor multiplicity. We mapped five loci on chromosomes 4, 6, 7, 11 and 13 linked to AIR (logarithm of odds (LOD)=3.56, 3.52, 15.74, 7.74 and 3.34, respectively) and two loci linked to lung tumor multiplicity, on chromosomes 6 and 18 (LOD=12.18 and 4.69, respectively). The known pulmonary adenoma susceptibility 1 (Pas1) locus on chromosome 6 was the only locus linked to both phenotypes, suggesting that alleles of this locus were differentially fixed during breeding and selection of AIR mice. These results represent a step toward understanding the link between inflammation and cancer.

A polygenic model with common variants may predict lung adenocarcinoma risk in humans

Genome-wide screening for genetic loci associated with risk of lung adenocarcinoma (ADCA) was carried out in pooled DNA using the Illumina 300K single-nucleotide polymorphism (SNP) array, in a joint analysis of 2 Italian case-control series matched by age, gender and smoking habit. The rare allele carrier status of 8 SNPs was associated with a decreased lung ADCA risk [odds ratios (OR): 0.6-0.8]. In a polygenic model characterized by additive and interchangeable effects, individuals carrying 2 to 6 rare alleles at these 8 SNPs showed a significant trend toward a decreased risk of lung ADCA (up to OR of 0.3). These results suggest the relevance of a polygenic model in the modulation of individual risk of lung ADCA in the general population.

Loss of Nkx2.8 deregulates progenitor cells in the large airways and leads to dysplasia

Nkx2.8, a homeodomain transcription factor, has been characterized in liver cancer and in the developing central nervous system. We now show that this factor is also expressed in the lung, where it localizes in adults to a discrete population of tracheobronchial basal cells. To target the mouse gene, the first exon was replaced by a LacZ marker gene joined to the intact 5'-untranslated region. Marker expression was observed throughout the lower respiratory tract, beginning on E11 in a few cells of the distal lung buds. The region of expression then spread upward. By neonatal day 1, expression was greatest in the large airways and the Nkx2.8-/- mice exhibited generalized tracheobronchial hyperplasia. Bromodeoxyuridine (BrdUrd) labeling studies showed that a higher rate of bronchial cell proliferation persisted at 6 to 8 months. In adults, Nkx2.8 marker expression decreased with progressive differentiation into ciliated and secretory cells. The cell localizations and patterns of coexpression with BrdUrd and differentiation markers suggest a progenitor relationship: the cells that most strongly express Nkx2.8 seem to function as tracheobronchial stem cells. Moreover, Nkx2.8 acts to limit the number of these progenitor cells because the marker-expressing population was greatly expanded in Nkx2.8-/- mice. Increased proliferation and an altered progenitor relationship caused progressive bronchial pathology, which manifested as widespread dysplasia in the large airways of 1-year-old Nkx2.8-/- mice.

Mapping cis-acting regulatory variation in recombinant congenic strains

We present an integrated approach for the enriched detection of genes subject to cis-acting variation in the mouse genome. Gene expression profiling was performed with lung tissue from a panel of recombinant congenic strains (RCS) derived from A/J and C57BL/6J inbred mouse strains. A multiple-regression model measuring the association between gene expression level, donor strain of origin (DSO), and predominant strain background identified over 1,500 genes (P < 0.05) whose expression profiles differed according to the DSO. This model also identified over 1,200 genes whose expression showed dependence on background (P < 0.05), indicating the influence of background genetic context on transcription levels. Sequences obtained from 1-kb segments of 3'-untranslated regions identified single nucleotide polymorphisms in 64% of genes whose expression levels correlated with DSO status, compared with 29% of genes that displayed no association (P < 0.01, Fisher exact test). Allelic imbalance was identified in 50% of genes positive for expression-DSO association, compared with 22% of negative genes (P < 0.05, Fisher exact test). Together, these results demonstrate the utility of RCS mice for identifying the roles of proximal genetic determinants and background genetic context in determining gene expression levels. We propose the use of this integrated experimental approach in multiple tissues from this and other RCS panels as a means for genome-wide cataloging of genetic regulatory mechanisms in laboratory strains of mice.

Genetic linkage between Pol iota deficiency and increased susceptibility to lung tumors in mice

Pol iota is a member of the Y-family DNA polymerases, characterized by their capacity for translesion DNA synthesis and low fidelity base incorporation, and has therefore been assumed to play important roles in mutagenesis and carcinogenesis. In fact, the mouse Pol iota gene is located within the Par2 (pulmonary adenoma resistance 2) locus on distal chromosome 18, which we have identified as a major susceptibility locus regarding urethane induction of pulmonary adenomas. Indeed, Pol iota has been suggested to be a candidate for Par2 from both the genetic and biological standpoints. Taking advantage of 129X1/SvJ mice naturally deficient in Pol iota due to a nonsense mutation within the coding region of the gene, we here analyzed urethane-treated (A/J x 129X1/SvJ)F(1) x A/J backcross and (A/J x 129X1/SvJ)F(2) intercross mice and observed the defective 129X1/SvJ Pol iota allele to be genetically linked with an increased susceptibility to lung tumors relative to the A/J allele. Thus, among the already known mouse Pol iota alleles, the defective 129X1/SvJ allele is associated exclusively with the highest susceptibility to lung tumors. The result indicates a possibility that the Pol iota gene may participate in error-free repair of damaged DNA and prevention of lung tumor development.

Met proto-oncogene juxtamembrane rare variations in mouse and humans: differential effects of Arg and Cys alleles on mouse lung tumorigenesis

Analysis of seven candidate genes mapping in the 1-Mb region of the mouse pulmonary adenoma resistance 4 (Par4) locus revealed a single amino-acid change, consisting in a nonconservative Arg968Cys variation in the juxtamembrane domain of the Met proto-oncogene-encoded protein. The BALB/c strain (resistant allele) carried the Arg allele, whereas the SWR/J mouse strain (Par4-susceptible allele) carried the Cys variation, recently proven to functionally modulate tumorigenesis. Seven genetic linkage crosses herein analysed and six crosses reported in the literature pointed to the candidacy of the Met gene for Par4. Analysis of genomic DNA of 126 lung adenocarcinoma patients for the Met juxtamembrane domain revealed the same Arg/Cys variation at the mouse homologous position in one patient; two other patients carried additional variants in the same domain, suggesting a potential role for rare MET juxtamembrane variants in human lung cancer.

Inherited predisposition to early onset lung cancer according to histological type

The role of hereditary factors in lung cancer is less well understood than in many other human neoplastic diseases. We used a nation-wide family dataset to search for evidence for a genetic predisposition in lung cancer. The Swedish Family-Cancer Database includes all Swedes born in 1932 and later (0- to 68-year-old offspring) with their parents, totaling over 10.2 million individuals. Cancer cases were retrieved from the Swedish Cancer Registry up to year 2000. Standardized incidence ratios (SIR) and 95% confidence limits (CI) were calculated for age-specific familial risks in offspring by parental or sibling proband, separately. A Kappa test was used to examine the association between familial risk and histology. Compared to the rate of lung cancers among persons without family history, a high risk by parental family history in adenocarcinoma (2.03) and large cell carcinoma (2.14) was found, and only a slightly lower risk was found among patients with squamous cell carcinoma (1.63) and small cell carcinoma (1.55). Among siblings, an increased risk was shown for concordant adenocarcinoma and small cell carcinoma at all ages and for all histological types when cancer was diagnosed before age 50. At young age, risks between siblings were higher than those between offspring and parents. The present data suggest that a large proportion of lung cancers before age 50 years appears to be heritable and probably due to a high-penetrant recessive gene or genes that predispose to tobacco carcinogens; however, this hypothesis needs to be tested in segregation analysis with a large number of pedigrees.

Pas1 haplotype-dependent genetic predisposition to lung tumorigenesis in rodents: a meta-analysis

Rodent species and strains show wide variations in susceptibility to lung tumorigenesis. In mice, hierarchical clustering of 29 inbred laboratory strains by pulmonary adenoma susceptibility 1 (Pas1) locus polymorphisms separated the strains into either an A/J- or a C57BL/6J-type Pas1 haplotype. A pooled analysis (including >8500 mice) of studies on spontaneous and chemically induced lung tumorigenesis in these strains revealed a significantly higher risk of spontaneous lung tumors [odds ratio (OR) 12.17; 95% confidence interval (CI) 9.00-16.45] as well as of chemically induced lung tumors (OR 15.14; 95% CI 12.51-18.31) in the A/J-type haplotype. Strain differences were observed with six different carcinogens, suggesting that Pas1 locus activity is carcinogen-independent. Thus, the present meta-analysis indicates a link between the genetic control of spontaneous and chemically induced lung tumor susceptibility in mice. The Pas1 susceptibility allele is frequent in the population of inbred mouse strains, whereas a counterpart appears to be absent or rare in rat and hamster strains. These findings might help in the interpretation of results of rodent carcinogenicity bioassays and assessing the risk of lung carcinogenesis from chemicals.

Haplotype sharing suggests that a genomic segment containing six genes accounts for the pulmonary adenoma susceptibility 1 (Pas1) locus activity in mice

The pulmonary adenoma susceptibility 1 (Pas1) locus affects inherited predisposition and resistance to chemically induced lung tumorigenesis in mice. The A/J and C57BL/6J mouse strains carry the susceptibility and resistance allele, respectively. We identified and genotyped 65 polymorphisms in the Pas1 locus region in 29 mouse inbred strains, and delimited the Pas1 locus to a minimal region of 468 kb containing six genes. That region defined a core Pas1 haplotype with 42 tightly linked markers, including intragenic polymorphisms in five genes (Bcat1, Lrmp, Las1, Ghiso, and Kras2) and amino-acid changes in three genes (Lrmp, Las1, Lmna-rs1). In (A/J x C57BL/6J)F1 mouse lung tumors, the Lmna-rs1 gene was completely downregulated, whereas allele-specific downregulation of the C57BL/6J-derived allele was observed at the Las1 gene, suggesting the potential role of these genes in tumor suppression. These results indicate a complex multigenic nature of the Pas1 locus, and point to a functional role for both intronic and exonic polymorphisms of the six genes of the Pas1 haplotype in lung tumor susceptibility.