Linkage Disequilibrium and Physical Mapping of Pas1 in Mice

Experimental Models to Define the Genetic Predisposition to Liver Cancer

Hepatocellular carcinoma (HCC) is a frequent human cancer and the most frequent liver tumor. The study of genetic mechanisms of the inherited predisposition to HCC, implicating gene-gene and gene-environment interaction, led to the discovery of multiple gene loci regulating the growth and multiplicity of liver preneoplastic and neoplastic lesions, thus uncovering the action of multiple genes and epistatic interactions in the regulation of the individual susceptibility to HCC. The comparative evaluation of the molecular pathways involved in HCC development in mouse and rat strains differently predisposed to HCC indicates that the genes responsible for HCC susceptibility control the amplification and/or overexpression of c-Myc, the expression of cell cycle regulatory genes, and the activity of Ras/Erk, AKT/mTOR, and of the pro-apoptotic Rassf1A/Nore1A and Dab2IP/Ask1 pathways, the methionine cycle, and DNA repair pathways in mice and rats. Comparative functional genetic studies, in rats and mice differently susceptible to HCC, showed that preneoplastic and neoplastic lesions of resistant mouse and rat strains cluster with human HCC with better prognosis, while the lesions of susceptible mouse and rats cluster with HCC with poorer prognosis, confirming the validity of the studies on the influence of the genetic predisposition to hepatocarinogenesis on HCC prognosis in mouse and rat models. Recently, the hydrodynamic gene transfection in mice provided new opportunities for the recognition of genes implicated in the molecular mechanisms involved in HCC pathogenesis and prognosis. This method appears to be highly promising to further study the genetic background of the predisposition to this cancer.

Germline Variants Impact Somatic Events during Tumorigenesis

Cancer is characterized by diverse genetic alterations in both germline and somatic genomes that disrupt normal biology and provide a selective advantage to cells during tumorigenesis. Germline and somatic genomes have been extensively studied independently, leading to numerous biological insights. Analyses integrating data from both genomes have identified genetic variants impacting somatic events in tumors, including hotspot driver mutations. Interactions among specific germline variants and somatic events influence cancer subtypes, treatment response, and clinical outcomes. Investigation of these complex interactions is increasing our understanding of aberrant pathways in tumors that may uncover novel therapeutic targets. Here, we review the literature describing the role of germline genetic variants in promoting the selection and generation of specific mutations during tumorigenesis.

K-rasG12V mediated lung tumor models identified three new quantitative trait loci modifying events post-K-ras mutation

A high incidence of oncogenic K-ras mutations is observed in lung adenocarcinoma of human cases and carcinogen-induced animal models. The process of oncogenic K-ras-mediated lung adenocarcinogenesis can be dissected into two parts: pre- and post-K-ras mutation. Adoption of transgenic lines containing a flox-K-rasG12V transgene eliminates the use of chemical carcinogens and enables us to study directly crucial events post-K-ras mutation without considering the cellular events involved with oncogenic K-ras mutation, e.g., distribution and metabolism of chemical carcinogens, DNA repair, and somatic recombination by host factors. We generated two mouse strains C57BL/6J-Ryr2(tm1Nobs) and A/J-Ryr2(tm1Nobs) in which K-rasG12V can be transcribed from the cytomegalovirus early enhancer/chicken beta actin promoter in virtually any tissue. Upon K-rasG12V induction in lung epithelial cells by an adenovirus expressing the Cre recombinase, the number of tumors in the C57BL/6J-Ryr2(tm1Nobs/+) mouse line was 12.5 times that in the A/J-Ryr2(tm1Nobs/+) mouse line. Quantitative trait locus (QTL) analysis revealed that new three modifier loci, D3Mit19, D3Mit45 and D11Mit20, were involved in the differential susceptibility between the two lines. In addition, we found that differential expression of the wild-type K-ras gene, which was genetically turn out to be anti-oncogenic activity on K-rasG12V, could not account for the different susceptibility in our two K-rasG12V-mediated lung tumor models. Thus, we provide a genetic system that enables us to explore new downstream modifiers post-K-ras mutation.

Welding fume exposure and associated inflammatory and hyperplastic changes in the lungs of tumor susceptible a/j mice

It has been suggested that welding fume (WF) exposure increases lung cancer risk in welders. Epidemiology studies have failed to conclude that WF alone causes lung cancer and animal studies are lacking. We examined the course of inflammation, damage, and repair in the lungs of A/J mice, a lung tumor susceptible strain, caused by stainless steel WF. Mice were exposed by pharyngeal aspiration to 40 mg/kg of WF, silica, or saline. Bronchoalveolar lavage (BAL) was performed 24 hours, 1 and 16 weeks to assess lung injury and inflammation and histopathology was done 1, 8, 16, 24, and 48 weeks postexposure. Both exposures increased inflammatory cells, lactate dehydrogenase and albumin at 24 hr and 1 week. At 16 weeks, these parameters remained elevated in silica-exposed but not WF-exposed mice. Histopathologic evaluation at 1 week indicated that WF induced bronchiolar epithelial hyperplasia with associated cellular atypia, alveolar bronchiolo-alveolar hyperplasia (BAH) in peribronchiolar alveoli, and peribronchiolar lymphogranulomatous inflammation. Persistent changes included foci of histiocytic inflammation, fibrosis, atypical bronchiolar epithelial cells, and bronchiolar BAH. The principle changes in silica-exposed mice were histiocytic and suppurative inflammation, fibrosis, and alveolar BAH. Our findings that WF causes persistent bronchiolar and peribronchiolar epithelial changes, suggest a need for studies of bronchiolar changes after WF exposure.

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.

Pulmonary adenoma susceptibility 1 (Pas1) locus affects inflammatory response

Two outbred mouse lines, phenotypically selected for differential subcutaneous (s.c.) acute inflammatory response (AIR), were analysed for urethane-induced lung inflammatory response and susceptibility to lung tumorigenesis. AIR(min) mice, which show a low response to s.c. acute inflammation, developed a persistent subacute lung inflammatory response and a 40-fold higher lung tumor multiplicity than did AIR(max) mice, which are selected for high response to s.c. acute inflammation and showed a transient lung inflammatory response. A highly significant linkage disequilibrium pattern was observed in AIR(max) and AIR(min) mice at marker alleles located within a 452-kb pulmonary adenoma susceptibility 1 (Pas1) locus region, thus defining the location of gene candidacy for inflammatory response and for the biological effects of Pas1 in this region. AIR(min) and AIR(max) mice segregated by descent the Pas1(s) and Pas1(r) alleles, respectively, providing evidence for the involvement of the Pas1 locus in the inflammatory response. The 452-kb region contains Kras2 and four additional genes, including the lymphoid-restricted membrane protein (Lrmp) gene, whose Pro-->Leu nonconservative variation was linked with inflammatory response and Pas1 allelotype. These results provide a model to explore the mechanism underlying inherited predisposition to lung cancer in the context of a link to inflammation.

Loss of heterozygosity analysis of mouse pulmonary adenomas induced by coal tar

Manufactured gas plant (MGP) residues, commonly known as coal tars, were generated several decades ago as a byproduct of residential and industrial gas production from the distillation of coal. Previous short-term exposure studies have shown MGP residues to be tumorigenic in mouse liver and lung. In order to gain further insight into carcinogenesis by complex mixtures of environmental chemicals containing known carcinogenic polycyclic aromatic hydrocarbons, we investigated mouse pulmonary tumors for loss of heterozygosity (LOH) as a result of multiple exposure to MGP residues. Twenty mouse lung adenomas produced in (C57BL/6 x C3H)F1 hybrid mice and manually microdissected were selected to examine genome-wide allelic losses at 58 microsatellite loci. Regions of chromosomes 1, 4, 5, 8, and 11 were affected in 30-40% of tumors. The elevated rates of allelic imbalance in these chromosomes may indicate the location of unknown tumor suppressor genes significant to neoplastic transformation in mouse lung tissues. Laser capture microdissection-based LOH analysis of pulmonary adenomas showed that contamination of nonneoplastic tissues was not masking the allelic losses in the manually microdissected tumor analysis. The low frequency of chromosome instability in these tumors, measured by means of inter-simple sequence repeat PCR, suggests the presence of discrete regions of LOH instead of extensive structural rearrangements.

Linkage disequilibrium mapping of novel lung tumor susceptibility quantitative trait loci in mice

Linkage disequilibrium (LD) has been used to map chromosomal regions regulating quantitative traits, also called quantitative trait loci (QTLs). With the increasing number of available mouse polymorphic genetic markers, LD can be estimated for the purpose of fine-mapping a given QTL or in the identification of novel QTLs. A whole-genome LD analysis was conducted for mapping mouse lung tumor susceptibility QTLs in 25 strains of mice with known susceptibility to lung cancer using 5638 genetic markers. A total of 63 markers were found to be significantly associated with lung tumor susceptibility, many of which were novel QTLs. This study demonstrates the feasibility of using LD to map QTLs on a whole genome level. Further characterization of the newly identified lung tumor susceptibility QTLs may lead to the identification of genes whose human homologue may predispose some individuals to lung cancer.

Fine chromosomal localization of the mouse Par2 gene that confers resistance against urethane-induction of pulmonary adenomas

BALB/cByJ mice are 14 times more resistant to urethane-induction of pulmonary adenomas than the susceptible A/J strain. Our previous linkage analysis of (A/J x BALB/cByJ)F1 x A/J backcross mice provided statistical evidence that a major resistance locus of BALB/cByJ with a dominant effect, designated Par2 (Pulmonary adenoma resistance 2), exists within an approximately 25 cM section of distal chromosome 18. To facilitate molecular identification of the Par2 locus, the present study was conducted to finely localize its chromosomal position utilizing Par2-congenic mice. Male BALB/cByJ mice were mated with female C57BL/6J mice carrying recessive Par2 alleles and their male F1 progeny were backcrossed to female BALB/cByJ mice. A male backcross mouse heterozygous within the Par2 interval of 25 cM was randomly selected and again backcrossed to female BALB/cByJ mice. This backcross-selection cycle was simply repeated to produce semi-congenic mice with a general BALB/cByJ genetic background except for the Par2 interval, where the mice were heterozygous with paternal C57BL/6J alleles and maternal BALB/cByJ alleles. After the 6th or 7th backcross, nine male mice possessing a recombination within the paternal Par2 interval were retained and crossed to female A/J mice. Resultant progeny were treated with urethane and examined for lung tumor development in order to deduce the Par2 genotypes of the recombinants through linkage analysis. By comparing the deduced Par2 genotype of each recombinant with its recombinational breakpoint, the Par2 locus was confined to an approximately 0.5 cM region flanked by D18Mit103 and D18Mit188 loci. Our results indicate that fully congenic mice conventionally established by at least nine simple backcrosses or by the speed congenic method are not necessarily required for fine mapping of quantitative trait loci. In the case of the Par2 locus, we found that semi-congenic mice after as few as four simple backcrosses were useful for this purpose. The map information obtained in this study should enable subsequent positional cloning of the Par2 gene.

A cancer modifier role for parathyroid hormone-related protein

The parathyroid hormone-related protein (PTHrP) gene (Pthlh) maps in the distal region of mouse chromosome 6 that contains a quantitative trait locus associated with genetic predisposition to skin tumorigenesis. Here, we report a genetic polymorphism located in the osteostatin encoding region of the Pthlh gene and that produces Thr/ Pro PTHrP variants. PthlhThr and PthlhPro alleles were significantly linked with resistance and susceptibility to skin carcinogenesis in phenotypically selected Car-R and Car-S outbred mice. Transfection of human NCI-H520 squamous cell carcinoma cells with the PthlhPro allele resulted in cells growing in clusters, tending to pile up, and growing at a significantly faster rate in nude mice than non-transfected and PthlhThr-transfected cells. These results point to the role of the Pthlh gene as a cancer modifier gene in skin tumorigenesis.