The non-protein coding breast cancer susceptibility locus Mcs5a acts in a non-mammary cell-autonomous fashion through the immune system and modulates T-cell homeostasis and functions

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.

Prospects of Immunotherapy for Triple-Negative Breast Cancer

In the classification and typing of breast cancer, triple-negative breast cancer (TNBC) is one type of refractory breast cancer, while chemotherapy stays in the traditional treatment methods. However, the impact of chemotherapy is short-lived and may lead to recurrence due to incomplete killing of tumor cells. The occurrence, development, and relapse of breast cancer are relevant to T cell dysfunction, multiplied expression of related immune checkpoint molecules (ICIs) such as programmed death receptor 1 (PD-1), programmed cell death 1 ligand 1 (PD-L1), and cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) produce immunosuppressive effect. Immunotherapy (namely, immune checkpoint inhibitors, adoptive cellular immunotherapy, CAR-T immunotherapy and some potential treatments) provides new hope in TNBC. This review focuses on the new immune strategies of TNBC patients.

Tumor microenvironment: Challenges and opportunities in targeting metastasis of triple negative breast cancer

Triple negative breast cancer (TNBC) is most aggressive subtype of breast cancers with high probability of metastasis as well as lack of specific targets and targeted therapeutics. TNBC is characterized with unique tumor microenvironment (TME), which differs from other subtypes. TME is associated with induction of proliferation, angiogenesis, inhibition of apoptosis and immune system suppression, and drug resistance. Exosomes are promising nanovesicles, which orchestrate the TME by communicating with different cells within TME. The components of TME including transformed ECM, soluble factors, immune suppressive cells, epigenetic modifications and re-programmed fibroblasts together hamper antitumor response and helps progression and metastasis of TNBCs. Therefore, TME could be a therapeutic target of TNBC. The current review presents latest updates on the role of exosomes in modulation of TME, approaches for targeting TME and combination of immune checkpoint inhibitors and target chemotherapeutics. Finally, we also discussed various phytochemicals that alter genetic, transcriptomic and proteomic profiles of TME along with current challenges and future implications. Thus, as TME is associated with the hallmarks of TNBC, the understanding of the impact of different components can improve the clinical benefits of TNBC patients.

Mapping Mammary Tumor Traits in the Rat

For nearly a century, the rat has served as a key model for studying the pathophysiology and genetic risk modifiers of breast cancer. Rat mammary tumors that initiate after exposure to carcinogens or estrogens closely resemble the etiological, histopathological, and genomic features of human breast cancer. Recent developments in genome-editing techniques in the rat have also enabled the development of sophisticated models for identifying the genetic modifiers of the nonmalignant tumor microenvironment that contribute to the formation, progression, and outcome of breast cancer. In this protocol review, we discuss the current methodologies for the three genetic mapping techniques in the rat that are widely used for identifying and testing the heritable genetic modifiers of breast cancer.

Genetic Modifiers of the Breast Tumor Microenvironment

Multiple nonmalignant cell types in the tumor microenvironment (TME) impact breast cancer risk, metastasis, and response to therapy, yet most heritable mechanisms that influence TME cell function and breast cancer outcomes are largely unknown. Breast cancer risk is ∼30% heritable and >170 genetic loci have been associated with breast cancer traits. However, the majority of candidate genes have poorly defined mechanistic roles in breast cancer biology. Research indicates that breast cancer risk modifiers directly impact cancer cells, yet it is equally plausible that some modifier alleles impact the nonmalignant TME. The objective of this review is to examine the list of current breast cancer candidate genes that may modify breast cancer risk and outcome through the TME.

Analysis of Immune Cells from Human Mammary Ductal Epithelial Organoids Reveals Vδ2+ T Cells That Efficiently Target Breast Carcinoma Cells in the Presence of Bisphosphonate

Developing strategies to enhance cancer prevention is a paramount goal, particularly given recent concerns about surgical treatment of preinvasive states such as ductal carcinoma in situ. Promoting effective immunosurveillance by leukocytes that scan for nascent neoplastic transformations represents a potential means to achieve this goal. Because most breast cancers arise within the ductal epithelium, enhancing protective immunosurveillance will likely necessitate targeting one or more of the distinctive lymphocyte types found in these sites under normal conditions. Here, we have characterized the intraepithelial lymphocyte compartment of non-cancerous human breast tissue and identified a subset of T lymphocytes that can be pharmacologically targeted to enhance their responses to breast cancer cells. Specifically, Vδ2(+) γδ T cells were consistently present in preparations of mammary ductal epithelial organoids and they proliferated in response to zoledronic acid, an aminobisphosphonate drug. Vδ2(+) T cells from breast ductal organoids produced the antitumor cytokine IFNγ and efficiently killed bisphosphonate-pulsed breast carcinoma cells. These findings demonstrate the potential for exploiting the ability of Vδ2(+) γδ T cells to respond to FDA-approved bisphosphonate drugs as a novel immunotherapeutic approach to inhibit the outgrowth of breast cancers.

Chemically Induced Oncogenesis in the Peripheral Nervous System Is Suppressed in Congenic BDIX.BDIV-Mss1 and -Mss7 Rats

Human malignant peripheral nerve sheath tumors (MPNSTs) are highly aggressive soft-tissue sarcomas with a poor prognosis that arise either in the context of neurofibromatosis 1 or sporadically. Inbred BDIX and BDIV rat strains highly susceptible and resistant, respectively, to the development of ethylnitrosourea-induced MPNST enable us to identify, by using methods not applicable in humans, variant alleles involved in the pathways underlying individual MPNST risk. On the basis of a genome-wide association analysis using reciprocal intercrosses of BDIX and BDIV, BDIV alleles of two loci on chromosome 10, Mss1 and Mss7, were predicted to lower the risk of MPNST, the latter locus with a female bias. In this study we confirm the two nonoverlapping loci by exposing two congenic strains, BDIX.BDIV-Mss1 (Mss1) and BDIX.BDIV-Mss7 (Mss7), each carrying a BDIV genomic segment spanning the respective locus, to ethylnitrosourea. Compared with BDIX rats, the rate of MPNST is reduced 6.2-fold and 2.0-fold for Mss1 and Mss7 rats of both sexes, respectively. Although a moderate gain of survival time (30−50 days) is seen in Mss1 rats of both sexes and Mss7 males, Mss7 females survive 134 days longer than BDIX females. BDIV alleles at Mss7 obviously cause a markedly increased intrastrain sex difference regarding survival time in Mss7 compared with BDIX rats. Fine mapping will lead to the identification of allelic variants modulating rat MPNST risk and subsequently to their human counterparts. This is of particular relevance, because so far neither gene nor anonymous sequence variants have been identified that influence the risk of human sporadic Schwann cell malignancy.

Differential 12-O-Tetradecanoylphorbol-13-acetate-induced activation of rat mammary carcinoma susceptibility Fbxo10 variant promoters via a PKC-AP1 pathway

Rat mammary carcinoma susceptibility 5a1 (Mcs5a1), which is concordant to human MCS5A1 breast cancer risk locus, mediates susceptibility by a non-mammary cell-autonomous mechanism associated with T cell differential expression of F-box protein 10 (Fbxo10). Human FBXO10, an evolutionarily conserved ubiquitin ligase gene, was shown to have a potential role in regulating cell death by controlling the degradation of Bcl-2, a key protein involved in apoptosis. Breast cancer susceptibility is controlled by interactions between environmental and genetic factors; therefore, we sought to determine if breast cancer risk-associated environmental chemicals interact with Mcs5a1 variants using luciferase reporter constructs containing 4.2 kb Fbxo10 promoters based on alleles of mammary cancer susceptible Wistar Furth (WF) and resistant Wistar Kyoto (WKY) rat strains. 12-O-Tetradecanoylphorbol-13-acetate (TPA) induced activation of a 4.2 kb WF Fbxo10 promoter region, but lower levels of activation of the homologous WKY Fbxo10 promoter region. Using general and specific protein kinase inhibitors, we identified a protein kinase C (PKC) pathway that mediated TPA activation. We narrowed the possible PKCs to a member of the atypical PKC isoforms, namely PKCµ. We also determined that activator protein 1 (AP1) family member c-Fos mediated TPA activation of the 4.2 kb WF Fbxo10 promoter. TPA was shown to induce endogenous FBXO10 mRNA and FBXO10 protein in Jurkat cells, a human T cell line, with a maximal level of expression from 1.5 to 2.5 h after exposure. These results indicate that FBXO10/Fbxo10 expression is regulated by a PKC-dependent pathway acting through c-Fos, which binds AP1-specific DNA elements in Mcs5a1.

The role of genetics in estrogen responses: a critical piece of an intricate puzzle

The estrogens are female sex hormones that are involved in a variety of physiological processes, including reproductive development and function, wound healing, and bone growth. They are mainly known for their roles in reproductive tissues--specifically, 17β-estradiol (E2), the primary estrogen, which is secreted by the ovaries and induces cellular proliferation and growth of the uterus and mammary glands. In addition to the role of estrogens in promoting tissue growth and development during normal physiological states, they have a well-established role in determining susceptibility to disease, particularly cancer, in reproductive tissues. The responsiveness of various tissues to estrogen is genetically controlled, with marked quantitative variation observed across multiple species, including humans. This variation presents both researchers and clinicians with a veritable physiological puzzle, the pieces of which--many of them unknown--are complex and difficult to fit together. Although genetics is known to play a major role in determining sensitivity to estrogens, there are other factors, including parent of origin and the maternal environment, that are intimately linked to heritable phenotypes but do not represent genotype, per se. The objectives of this review article were to summarize the current knowledge of the role of genotype, and uterine and neonatal environments, in phenotypic variation in the response to estrogens; to discuss recent findings and the potential mechanisms involved; and to highlight exciting research opportunities for the future.

A strategy to identify dominant point mutant modifiers of a quantitative trait

A central goal in the analysis of complex traits is to identify genes that modify a phenotype. Modifiers of a cancer phenotype may act either intrinsically or extrinsically on the salient cell lineage. Germline point mutagenesis by ethylnitrosourea can provide alleles for a gene of interest that include loss-, gain-, or alteration-of-function. Unlike strain polymorphisms, point mutations with heterozygous quantitative phenotypes are detectable in both essential and nonessential genes and are unlinked from other variants that might confound their identification and analysis. This report analyzes strategies seeking quantitative mutational modifiers of Apc^Min in the mouse. To identify a quantitative modifier of a phenotype of interest, a cluster of test progeny is needed. The cluster size can be increased as necessary for statistical significance if the founder is a male whose sperm is cryopreserved. A second critical element in this identification is a mapping panel free of polymorphic modifiers of the phenotype, to enable low-resolution mapping followed by targeted resequencing to identify the causative mutation. Here, we describe the development of a panel of six “isogenic mapping partner lines” for C57BL/6J, carrying single-nucleotide markers introduced by mutagenesis. One such derivative, B6.SNVg, shown to be phenotypically neutral in combination with Apc^Min, is an appropriate mapping partner to locate induced mutant modifiers of the Apc^Min phenotype. The evolved strategy can complement four current major initiatives in the genetic analysis of complex systems: the Genome-wide Association Study; the Collaborative Cross; the Knockout Mouse Project; and The Cancer Genome Atlas.

Human MCS5A1 candidate breast cancer susceptibility gene FBXO10 is induced by cellular stress and correlated with lens epithelium-derived growth factor (LEDGF)

Genetic variation and candidate genes associated with breast cancer susceptibility have been identified. Identifying molecular interactions between associated genetic variation and cellular proteins may help to better understand environmental risk. Human MCS5A1 breast cancer susceptibility associated SNP rs7042509 is located in F-box protein 10 (FBXO10). An orthologous Rattus norvegicus DNA-sequence that contains SNV ss262858675 is located in rat Mcs5a1, which is part of a mammary carcinoma susceptibility locus controlling tumor development in a non-mammary cell-autonomous manner via an immune cell-mediated mechanism. Higher Fbxo10 expression in T cells is associated with Mcs5a increased susceptibility alleles. A common DNA-protein complex bound human and rat sequences containing MCS5A1/Mcs5a1 rs7042509/ss262858675 in electrophoretic mobility shift assays (EMSAs). Lens epithelium-derived growth factor (LEDGF), a stress-response protein, was identified as a candidate to bind both human and rat sequences using DNA-pulldown and mass spectrometry. LEDGF binding was confirmed by LEDGF-antibody EMSA and chromatin immunoprecipitation (ChIP). Ectopic expression of LEDGF/p75 increased luciferase activities of co-transfected reporters containing both human and rat orthologs. Over-expressed LEDGF/p75 increased endogenous FBXO10 mRNA levels in Jurkat cells, a human T-cell line, implying LEDGF may be involved in increasing FBXO10 transcript levels. Oxidative and thermal stress of Jurkat cells increased FBXO10 and LEDGF expression, further supporting a hypothesis that LEDGF binds to a regulatory region of FBXO10 and increases expression during conditions favoring carcinogenesis. We conclude that FBXO10, a candidate breast cancer susceptibility associated gene, is induced by cellular stress and LEDGF may play a role in expression of this gene.

Mapping of Mcs30, a new mammary carcinoma susceptibility quantitative trait locus (QTL30) on rat chromosome 12: identification of fry as a candidate Mcs gene

Rat strains differ dramatically in their susceptibility to mammary carcinogenesis. On the assumption that susceptibility genes are conserved across mammalian species and hence inform human carcinogenesis, numerous investigators have used genetic linkage studies in rats to identify genes responsible for differential susceptibility to carcinogenesis. Using a genetic backcross between the resistant Copenhagen (Cop) and susceptible Fischer 344 (F344) strains, we mapped a novel mammary carcinoma susceptibility (Mcs30) locus to the centromeric region on chromosome 12 (LOD score of ∼8.6 at the D12Rat59 marker). The Mcs30 locus comprises approximately 12 Mbp on the long arm of rat RNO12 whose synteny is conserved on human chromosome 13q12 to 13q13. After analyzing numerous genes comprising this locus, we identified Fry, the rat ortholog of the furry gene of Drosophila melanogaster, as a candidate Mcs gene. We cloned and determined the complete nucleotide sequence of the 13 kbp Fry mRNA. Sequence analysis indicated that the Fry gene was highly conserved across evolution, with 90% similarity of the predicted amino acid sequence among eutherian mammals. Comparison of the Fry sequence in the Cop and F344 strains identified two non-synonymous single nucleotide polymorphisms (SNPs), one of which creates a putative, de novo phosphorylation site. Further analysis showed that the expression of the Fry gene is reduced in a majority of rat mammary tumors. Our results also suggested that FRY activity was reduced in human breast carcinoma cell lines as a result of reduced levels or mutation. This study is the first to identify the Fry gene as a candidate Mcs gene. Our data suggest that the SNPs within the Fry gene contribute to the genetic susceptibility of the F344 rat strain to mammary carcinogenesis. These results provide the foundation for analyzing the role of the human FRY gene in cancer susceptibility and progression.

The gene desert mammary carcinoma susceptibility locus Mcs1a regulates Nr2f1 modifying mammary epithelial cell differentiation and proliferation

Genome-wide association studies have revealed that many low-penetrance breast cancer susceptibility loci are located in non-protein coding genomic regions; however, few have been characterized. In a comparative genetics approach to model such loci in a rat breast cancer model, we previously identified the mammary carcinoma susceptibility locus Mcs1a. We now localize Mcs1a to a critical interval (277 Kb) within a gene desert. Mcs1a reduces mammary carcinoma multiplicity by 50% and acts in a mammary cell-autonomous manner. We developed a megadeletion mouse model, which lacks 535 Kb of sequence containing the Mcs1a ortholog. Global gene expression analysis by RNA-seq revealed that in the mouse mammary gland, the orphan nuclear receptor gene Nr2f1/Coup-tf1 is regulated by Mcs1a. In resistant Mcs1a congenic rats, as compared with susceptible congenic control rats, we found Nr2f1 transcript levels to be elevated in mammary gland, epithelial cells, and carcinoma samples. Chromatin looping over ∼820 Kb of sequence from the Nr2f1 promoter to a strongly conserved element within the Mcs1a critical interval was identified. This element contains a 14 bp indel polymorphism that affects a human-rat-mouse conserved COUP-TF binding motif and is a functional Mcs1a candidate. In both the rat and mouse models, higher Nr2f1 transcript levels are associated with higher abundance of luminal mammary epithelial cells. In both the mouse mammary gland and a human breast cancer global gene expression data set, we found Nr2f1 transcript levels to be strongly anti-correlated to a gene cluster enriched in cell cycle-related genes. We queried 12 large publicly available human breast cancer gene expression studies and found that the median NR2F1 transcript level is consistently lower in 'triple-negative' (ER-PR-HER2-) breast cancers as compared with 'receptor-positive' breast cancers. Our data suggest that the non-protein coding locus Mcs1a regulates Nr2f1, which is a candidate modifier of differentiation, proliferation, and mammary cancer risk.

Effective flow cytometric phenotyping of cells using minimal amounts of antibody

Here we introduce a modified antibody staining method that uses up to 80% less antibody for flow cytometry. We demonstrate this method for the detection of antigens expressed at high, moderate, or low levels in mouse and rat lymphocytes as well as mouse mammary epithelial cells. We obtained reproducibly accurate results for the detection of up to seven parameters for activation induced-proliferation, cell cycle analysis, and phenotyping of cell-surface and intracellular antigens.

Long-range transcriptional regulation of breast cancer genes

Breast cancer is a major health problem and understanding the genetic basis of this disease is crucial for predicting risk and developing effective targeted therapeutics. Several breast cancer predisposing genes have been identified, but mutations in the coding regions of these genes only accounts for a small proportion of risk. Research now suggests that combinations of multiple non-coding changes in breast cancer susceptibility genes, which cause moderate alterations in gene expression, will be responsible for the remaining inherited risk. These non-coding changes will include variants in proximal and distal transcriptional and post-transcriptional regulatory elements and may affect the levels and function of trans-acting factors, including proteins and RNAs, which act on these elements. Somatic changes in such elements and factors have also been associated with breast cancer progression. With the recent advent of techniques allowing the detection of long-range DNA interactions spanning the human genome, it has become increasingly clear that long-range regulatory elements constitute an important mechanism for gene regulation. Recent studies have identified several such elements that are important for regulating genes involved in breast cancer, raising the possibility that defects in these sequences may contribute to breast cancer predisposition and progression. In this review, we discuss the emerging functions of cis-regulatory elements and a subset of trans-acting factors in breast tumorigenesis. We also discuss some recent progress in our understanding of how dysregulation in these transcriptional components may contribute to breast cancer, and the potential implications for molecular diagnosis, prognosis prediction, and the treatment of this disease.

Rat Mcs1b is concordant to the genome-wide association-identified breast cancer risk locus at human 5q11.2 and MIER3 is a candidate cancer susceptibility gene

Low-penetrance alleles associated with breast cancer risk have been identified in population-based studies. Most risk loci contain either no or multiple potential candidate genes. Rat mammary carcinoma susceptibility 1b (Mcs1b) is a quantitative trait locus on RN02 that confers decreased susceptibility when Copenhagen (COP)-resistant alleles are introgressed into a Wistar Furth (WF)-susceptible genome. Five WF.COP congenic lines containing COP RN02 segments were compared. One line developed an average of 3.4 ± 2.0 and 5.5 ± 3.6 mammary carcinomas per rat ± SD when females were Mcs1b-resistant homozygous and Mcs1b heterozygous, respectively. These phenotypes were significantly different from susceptible genotype littermates (7.8 ± 3.1 mean mammary carcinomas per rat ± SD, P = 0.0001 and P = 0.0413, respectively). All other congenic lines tested were susceptible. Thus, Mcs1b was narrowed to 1.8 Mb of RN02 between genetic markers ENSRNOSNP2740854 and g2UL2-27. Mammary gland-graft carcinoma susceptibility assays were used to determine that donor (P = 0.0019), but not recipient Mcs1b genotype (P = 0.9381), was associated with ectopic mammary carcinoma outcome. Rat Mcs1b contains sequence orthologous to human 5q11.2, a breast cancer susceptibility locus identified in multiple genome-wide association studies. Human/rat MAP3K1/Map3k1 and mesoderm induction early response (MIER; MIER3)/MIER3 are within these orthologous segments. We identified MIER3 as a candidate Mcs1b gene based on 4.5-fold higher mammary gland levels of MIER3 transcripts in susceptible compared with Mcs1b-resistant females. These data suggest that the human 5q11.2 breast cancer risk allele marked by rs889312 is mammary gland autonomous, and MIER3 is a candidate breast cancer susceptibility gene.

Genetically determined inflammatory-response related cytokine and chemokine transcript profiles between mammary carcinoma resistant and susceptible rat strains

Multiple human breast and rat mammary carcinoma susceptibility (Mcs) alleles have been identified. Wistar Kyoto (WKY) rats are resistant to developing mammary carcinomas, while Wistar Furth (WF) females are susceptible. Gene transcripts at Mcs5a1, Mcs5a2, and Mcs5c are differentially expressed between resistant WKY and susceptible WF alleles in immune-system tissues. We hypothesized that immune-related gene transcript profiles are genetically determined in mammary carcinoma resistant and susceptible mammary glands. Low-density QPCR arrays were used to compare inflammation related genes between mammary carcinoma resistant WKY and susceptible WF females. Mammary gland gene transcript levels predicted to be different based on arrays were tested in independent samples. In total, 20 females per strain were exposed to 7,12-dimethylbenz(a)anthracene (DMBA) to induce mammary carcinogenesis. Twelve age-matched controls per strain without DMBA were included to determine main effects of DMBA-exposure. Significant (ANOVA P ≤ 0.01) effects of strain on mammary gland transcript level were observed for Cx3cl1, Il11ra, Il4, C3, Ccl20, Ccl11, Itgb2, Cxcl12, and Cxcr7. Significant effects of DMBA-exposure were observed for Cx3cl1, Il11ra, Cxcr4, Il4ra, and Il4. Strain and DMBA-exposure interaction effects were significant for Cx3cl1. Transcript levels of Cxcr7 relative to Cxcr4 were modified differently by DMBA in mammary carcinoma resistant and susceptible strains. In conclusion, several genetically-determined differences in cytokine, chemokine, and receptor gene transcript levels were identified between mammary carcinoma susceptible and resistant mammary glands, which may be indicative of cell populations and activities that suppress mammary carcinogenesis in resistant genotypes.

An insulator loop resides between the synthetically interacting elements of the human/rat conserved breast cancer susceptibility locus MCS5A/Mcs5a

Many low-penetrance breast cancer susceptibility loci are found to be located in non-protein-coding regions, suggesting their involvement in gene expression regulation. We identified the human/rat-conserved breast cancer susceptibility locus MCS5A/Mcs5a. This locus has been shown to act in a non-mammary cell-autonomous fashion through the immune system. The resistant Mcs5a allele from the Wistar-Kyoto (WKy) rat strain consists of two non-protein-coding genetic elements that must be located on the same chromosome to elicit the phenotype. In this study, we show the presence of a conserved higher order chromatin structure in MCS5A/Mcs5a located in between the synthetically interacting genetic elements. The looped elements are shown to be bound by CTCF and cohesin. We identify the downregulation of Fbxo10 expression in T cells as a strong candidate mechanism through which the interacting genetic elements of the resistant Mcs5a allele modulate mammary carcinoma susceptibility. Finally, we show that the human MCS5A polymorphisms associated with breast cancer risk are located at both sides of the looped structure and functionally interact to downregulate transcriptional activity, similar to rat Mcs5a. We propose a mechanistic model for MCS5a/Mcs5a in which a CTCF-mediated insulator loop encompassing the TOMM5/Tomm5 gene, resides in between and brings into closer physical proximity the synthetically and functionally interacting resistant genetic variants.

Quantification of epithelial cell differentiation in mammary glands and carcinomas from DMBA- and MNU-exposed rats

Rat mammary carcinogenesis models have been used extensively to study breast cancer initiation, progression, prevention, and intervention. Nevertheless, quantitative molecular data on epithelial cell differentiation in mammary glands of untreated and carcinogen-exposed rats is limited. Here, we describe the characterization of rat mammary epithelial cells (RMECs) by multicolor flow cytometry using antibodies against cell surface proteins CD24, CD29, CD31, CD45, CD49f, CD61, Peanut Lectin, and Thy-1, intracellular proteins CK14, CK19, and FAK, along with phalloidin and Hoechst staining. We identified the luminal and basal/myoepithelial populations and actively dividing RMECs. In inbred rats susceptible to mammary carcinoma development, we quantified the changes in differentiation of the RMEC populations at 1, 2, and 4 weeks after exposure to mammary carcinogens DMBA and MNU. DMBA exposure did not alter the percentage of basal or luminal cells, but upregulated CD49f (Integrin α6) expression and increased cell cycle activity. MNU exposure resulted in a temporary disruption of the luminal/basal ratio and no CD49f upregulation. When comparing DMBA- or MNU-induced mammary carcinomas, the RMEC differentiation profiles are indistinguishable. The carcinomas compared with mammary glands from untreated rats, showed upregulation of CD29 (Integrin β1) and CD49f expression, increased FAK (focal adhesion kinase) activation especially in the CD29hi population, and decreased CD61 (Integrin β3) expression. This study provides quantitative insight into the protein expression phenotypes underlying RMEC differentiation. The results highlight distinct RMEC differentiation etiologies of DMBA and MNU exposure, while the resulting carcinomas have similar RMEC differentiation profiles. The methodology and data will enhance rat mammary carcinogenesis models in the study of the role of epithelial cell differentiation in breast cancer.

Rats: gnawing through the barriers to understanding genetic susceptibility and breast cancer

Advances in genotyping technology have provided us with a large number of genetic loci associated with cancer susceptibility; however, our ability to understand the functional effects of the genetic variants of these loci remains limited. In the previous issue, Smits and colleagues demonstrate the use of congenic rat strains to discover that the Mcs5a breast cancer susceptibility locus is most likely acting through the immune system, via novel transcriptional regulatory mechanisms. This challenges our conventional thinking of cancer susceptibility and gene regulation pathways, and illustrates the potential for rodent models to help us functionally characterize polymorphisms of cancer-associated loci.