Isolation of two regions on rat chromosomes 5 and 18 affecting mammary cancer susceptibility

Induced mammary cancer in rat models: pathogenesis, genetics, and relevance to female breast cancer

Mammary cancer, or breast cancer in women, is a polygenic disease with a complex etiopathogenesis. While much remains elusive regarding its origin, it is well established that chemical carcinogens and endogenous estrogens contribute significantly to the initiation and progression of this disease. Rats have been useful models to study induced mammary cancer. They develop mammary tumors with comparable histopathology to humans and exhibit differences in resistance or susceptibility to mammary cancer depending on strain. While some rat strains (e.g., Sprague-Dawley) readily form mammary tumors following treatment with the chemical carcinogen, 7,12-dimethylbenz[a]-anthracene (DMBA), other strains (e.g., Copenhagen) are resistant to DMBA-induced mammary carcinogenesis. Genetic linkage in inbred strains has identified strain-specific quantitative trait loci (QTLs) affecting mammary tumors, via mechanisms that act together to promote or attenuate, and include 24 QTLs controlling the outcome of chemical induction, 10 QTLs controlling the outcome of estrogen induction, and 4 QTLs controlling the outcome of irradiation induction. Moreover, and based on shared factors affecting mammary cancer etiopathogenesis between rats and humans, including orthologous risk regions between both species, rats have served as useful models for identifying methods for breast cancer prediction and treatment. These studies in rats, combined with alternative animal models that more closely mimic advanced stages of breast cancer and/or human lifestyles, will further improve our understanding of this complex disease.

Genome-Wide Analysis Identifies Germ-Line Risk Factors Associated with Canine Mammary Tumours

Canine mammary tumours (CMT) are the most common neoplasia in unspayed female dogs. CMTs are suitable naturally occurring models for human breast cancer and share many characteristics, indicating that the genetic causes could also be shared. We have performed a genome-wide association study (GWAS) in English Springer Spaniel dogs and identified a genome-wide significant locus on chromosome 11 (p_raw = 5.6x10^-7, p_perm = 0.019). The most associated haplotype spans a 446 kb region overlapping the CDK5RAP2 gene. The CDK5RAP2 protein has a function in cell cycle regulation and could potentially have an impact on response to chemotherapy treatment. Two additional loci, both on chromosome 27, were nominally associated (p_raw = 1.97x10^-5 and p_raw = 8.30x10^-6). The three loci explain 28.1±10.0% of the phenotypic variation seen in the cohort, whereas the top ten associated regions account for 38.2±10.8% of the risk. Furthermore, the ten GWAS loci and regions with reduced genetic variability are significantly enriched for snoRNAs and tumour-associated antigen genes, suggesting a role for these genes in CMT development. We have identified several candidate genes associated with canine mammary tumours, including CDK5RAP2. Our findings enable further comparative studies to investigate the genes and pathways in human breast cancer patients.

Dogs provide an excellent model system for several human diseases, including cancer. Heavy breeding for certain behavioural or phenotypic traits has created genetic isolates–breeds–characterised by low levels of genetic variation and a limited number of genetic disease variants within each breed. Cancer is the most common cause of death in dogs today, and canine mammary tumours (CMT) are the most prevalent tumour type in unspayed female dogs. These tumours are very similar to human breast cancer and could therefore be used as a naturally occurring model for the human disease. We have investigated genetic variants associated with CMT in English Springer Spaniels pointing to a gene involved in cell cycle regulation (CDK5RAP2). The CDK5RAP2 could therefore have a key role in the development of mammary tumours and we suggest that further studies should be performed in both dogs and women to investigate CDK5RAP2 and its possible effect on disease and treatment response.

Significant overlap between human genome-wide association-study nominated breast cancer risk alleles and rat mammary cancer susceptibility loci

Introduction

Human population-based genome-wide association (GWA) studies identify low penetrance breast cancer risk alleles; however, GWA studies alone do not definitively determine causative genes or mechanisms. Stringent genome- wide statistical significance level requirements, set to avoid false-positive associations, yield many false-negative associations. Laboratory rats (Rattus norvegicus) are useful to study many aspects of breast cancer, including genetic susceptibility. Several rat mammary cancer associated loci have been identified using genetic linkage and congenic strain based-approaches. Here, we sought to determine the amount of overlap between GWA study nominated human breast and rat mammary cancer susceptibility loci.

Methods

We queried published GWA studies to identify two groups of SNPs, one that reached genome-wide significance and one comprised of SNPs failing a validation step and not reaching genome- wide significance. Human genome locations of these SNPs were compared to known rat mammary carcinoma susceptibility loci to determine if risk alleles existed in both species. Rat genome regions not known to associate with mammary cancer risk were randomly selected as control regions.

Results

Significantly more human breast cancer risk GWA study nominated SNPs mapped at orthologs of rat mammary cancer loci than to regions not known to contain rat mammary cancer loci. The rat genome was useful to predict associations that had met human genome-wide significance criteria and weaker associations that had not.

Conclusions

Integration of human and rat comparative genomics may be useful to parse out false-negative associations in GWA studies of breast cancer risk.

Mapping of three genetic determinants of susceptibility to estrogen-induced mammary cancer within the Emca8 locus on rat chromosome 5

The ACI rat model of 17β-estradiol (E2)-induced mammary cancer has gained wide use in the study of breast cancer etiology, prevention, and genetics. Emca8, a QTL that determines susceptibility to E2-induced mammary cancer, was previously mapped to rat chromosome 5 (RNO5) in an intercross between resistant Brown Norway (BN) and susceptible ACI rats. In this study, a panel of congenic rat strains, each of which carries BN alleles across a defined segment of RNO5 on the ACI genetic background, was generated and used to map more precisely the Emca8 determinants of mammary cancer susceptibility. Three distinct genetic determinants were localized within Emca8, and two of these were mapped to intervals of less than 15 megabases. Emca8.1 harbors Cdkn2a, Cdkn2b, and other genes and is orthologous to the 9p21 breast cancer locus identified in genome-wide and candidate gene association studies. Emca8.2 harbors Cdkn2c and other genes and is orthologous to the 1p32 locus in humans that is frequently deleted in breast cancers. Both Emca8.1 and Emca8.2 harbor copy number variants that are orthologous to copy number variant regions in humans. Gene expression profiles were defined for mammary tissues from E2-treated ACI and ACI.BN-Emca8 rats to define the impact of Emca8 on gene expression and identify differentially expressed genes residing within Emca8.1 and Emca8.2. This study further illustrates the relevance of the ACI rat model of E2-induced mammary cancer for identifying novel genetic determinants of breast cancer susceptibility and defining the mechanisms through which estrogens contribute to breast cancer development. Cancer Prev Res; 6(1); 59-69. ©2012 AACR.

BAC CGH-array identified specific small-scale genomic imbalances in diploid DMBA-induced rat mammary tumors

BACKGROUND

Development of breast cancer is a multistage process influenced by hormonal and environmental factors as well as by genetic background. The search for genes underlying this malignancy has recently been highly productive, but the etiology behind this complex disease is still not understood. In studies using animal cancer models, heterogeneity of the genetic background and environmental factors is reduced and thus analysis and identification of genetic aberrations in tumors may become easier. To identify chromosomal regions potentially involved in the initiation and progression of mammary cancer, in the present work we subjected a subset of experimental mammary tumors to cytogenetic and molecular genetic analysis.

METHODS

Mammary tumors were induced with DMBA (7,12-dimethylbenz[a]anthrazene) in female rats from the susceptible SPRD-Cu3 strain and from crosses and backcrosses between this strain and the resistant WKY strain. We first produced a general overview of chromosomal aberrations in the tumors using conventional kartyotyping (G-banding) and Comparative Genome Hybridization (CGH) analyses. Particular chromosomal changes were then analyzed in more details using an in-house developed BAC (bacterial artificial chromosome) CGH-array platform.

RESULTS

Tumors appeared to be diploid by conventional karyotyping, however several sub-microscopic chromosome gains or losses in the tumor material were identified by BAC CGH-array analysis. An oncogenetic tree analysis based on the BAC CGH-array data suggested gain of rat chromosome (RNO) band 12q11, loss of RNO5q32 or RNO6q21 as the earliest events in the development of these mammary tumors.

CONCLUSIONS

Some of the identified changes appear to be more specific for DMBA-induced mammary tumors and some are similar to those previously reported in ACI rat model for estradiol-induced mammary tumors. The later group of changes is more interesting, since they may represent anomalies that involve genes with a critical role in mammary tumor development. Genetic changes identified in this work are at very small scales and thus may provide a more feasible basis for the identification of the target gene(s). Identification of the genes underlying these chromosome changes can provide new insights to the mechanisms of mammary carcinogenesis.

Contrasting epistatic interactions between rat quantitative trait loci controlling mammary cancer development

We previously defined quantitative trait loci (QTLs) that control susceptibility to 7,12-dimethylbenz(alpha)anthracene-induced mammary carcinoma in SPRD-Cu3 (susceptible) and WKY (resistant) rats. Two of these QTLs, assigned to chromosomes (Chr) 10 and 18, control tumor growth rate and invasiveness. In this study we characterized a congenic strain in which a large segment of WKY Chr 10 was introduced in the SPRD-Cu3 genetic background and demonstrated that this chromosome segment controls this tumor trait. The WKY allele at this QTL (Mcsta1) reduces the growth rate of the fastest growing tumors by 26%. We also previously showed that two SPRD-Cu3-WKY congenic strains containing a WKY chromosome segment derived either from Chr 5 or from Chr 18 exhibit a reduction in tumor multiplicity (QTLs Msctm1 and Mcstm2, respectively) (with no reduction in tumor growth rate in the Chr 18 congenic). In this study we generated a double congenic strain, which contains the two WKY differential segments from Chr 5 and 18, to determine how these two segments interact with one another. Interestingly, two types of epistatic interactions were found: no additive effect was seen with respect to tumor multiplicity, while a reduction in tumor growth rate was observed. It thus appears that WKY alleles located on Chr 5 and Chr 8 interact epistatically in a contrasting manner to modulate tumor multiplicity (in a nonadditive manner) and growth rate (in a synergic manner). Tumor growth rate is thus influenced by two QTLs, on Chr 10 (Mcsta1) and on Chr 18 (Mcsta2), the action of the latter being dependent on the presence of the Chr5 QTL (Mcstm1). The expression level of positional and functional candidate genes was also analyzed. On Chr 5, Pla2g2a is subject to a syntenic control while expression of the Tp53 (Chr 10) and Pmai1/Noxa (Chr 18) genes appears to be controlled by several mammary cancer resistance QTLs.

Mammary cancer resistance and precocious mammary differentiation in the WKY rat: Identification of 2 quantitative trait loci

The COP and WKY rat strains are resistant to mammary cancer. It has shown previously that upon chemical carcinogen treatment, COP females exhibit mammary preneoplastic lesions which disappear within a few weeks. We show here that in similar conditions, WKY females do not exhibit any visible preneoplastic lesions. WKY females are characterized by precocious mammary tissue differentiation, including active expression of the beta-casein gene in young virgin females. This trait might be critical in resistance to mammary carcinogenesis of WKY rats. To test this hypothesis, we took advantage of 2 congenic strains that contain a limited chromosome segment of WKY origin, derived either from chromosome 5 or from chromosome 18, introgressed in the susceptible genetic background (SPRD-Cu3). Each of these congenic strains has been shown to be partially resistant to chemically induced mammary carcinogenesis (reduction in tumour multiplicity with respect to the susceptible SPRD-Cu3 rats). We show here that these 2 congenic strains also exhibit precocious mammary differentiation, though to a lower extent than the WKY females. The conclusion of this study is thus 2-fold: (i) eradication of preneoplastic lesions is not a general phenomenon in mammary cancer resistance; (ii) the same segment of rat chromosomes 5 or 18 that controls mammary cancer resistance also contains a quantitative trait locus imposing precocious mammary differentiation. These 2 traits are thus associated, supporting the hypothesis that there might be a cause-effect relationship between precocious mammary differentiation and cancer resistance.