A delayed chemically induced tumorigenesis in Brca2 mutant mice

Large-scale generation and phenotypic characterization of zebrafish CRISPR mutants of DNA repair genes

A systematic knowledge of the roles of DNA repair genes at the level of the organism has been limited due to the lack of appropriate experimental approaches using animal model systems. Zebrafish has become a powerful vertebrate genetic model system with availability due to the ease of genome editing and large-scale phenotype screening. Here, we generated zebrafish mutants for 32 DNA repair and replication genes through multiplexed CRISPR/Cas9-mediated mutagenesis. Large-scale phenotypic characterization of our mutant collection revealed that three genes (atad5a, ddb1, pcna) are essential for proper embryonic development and hematopoiesis; seven genes (apex1, atrip, ino80, mre11a, shfm1, telo2, wrn) are required for growth and development during juvenile stage and six genes (blm, brca2, fanci, rad51, rad54l, rtel1) play critical roles in sex development. Furthermore, mutation in six genes (atad5a, brca2, polk, rad51, shfm1, xrcc1) displayed hypersensitivity to DNA damage agents. Our zebrafish mutant collection provides a unique resource for understanding of the roles of DNA repair genes at the organismal level.

Mouse Models for Deciphering the Impact of Homologous Recombination on Tumorigenesis

Homologous recombination (HR) is a fundamental evolutionarily conserved process that plays prime role(s) in genome stability maintenance through DNA repair and through the protection and resumption of arrested replication forks. Many HR genes are deregulated in cancer cells. Notably, the breast cancer genes BRCA1 and BRCA2, two important HR players, are the most frequently mutated genes in familial breast and ovarian cancer. Transgenic mice constitute powerful tools to unravel the intricate mechanisms controlling tumorigenesis in vivo. However, the genes central to HR are essential in mammals, and their knockout leads to early embryonic lethality in mice. Elaborated strategies have been developed to overcome this difficulty, enabling one to analyze the consequences of HR disruption in vivo. In this review, we first briefly present the molecular mechanisms of HR in mammalian cells to introduce each factor in the HR process. Then, we present the different mouse models of HR invalidation and the consequences of HR inactivation on tumorigenesis. Finally, we discuss the use of mouse models for the development of targeted cancer therapies as well as perspectives on the future potential for understanding the mechanisms of HR inactivation-driven tumorigenesis in vivo.

MPA/DMBA-driven mammary carcinomas

The polycyclic aromatic hydrocarbon 7,12-dimethylbenz[a]anthracene (DMBA, D) administered per os to wild-type female mice bearing slow-release medroxyprogesterone (MPA, M) pellets s.c. drives the formation of mammary carcinomas that recapitulate numerous immunobiological features of human luminal B breast cancer. In particular, M/D-driven mammary carcinomas established in immunocompetent C57BL/6 female mice (1) express hormone receptors, (2) emerge by evading natural immunosurveillance and hence display a scarce immune infiltrate largely polarized toward immunosuppression, (3) exhibit exquisite sensitivity to CDK4/CDK6 inhibitors, and (4) are largely resistant to immunotherapy with immune checkpoint blockers targeting PD-1. Thus, M/D-driven mammary carcinomas evolving in immunocompetent female mice stand out as a privileged preclinical platform for the study of luminal B breast cancer. Here, we provide a detailed protocol for the establishment of M/D-driven mammary carcinomas in wild-type C57BL/6 female mice. This protocol can be easily adapted to generate M/D-driven mammary carcinomas in female mice with most genetic backgrounds (including genetically-engineered mice).

Brca2 deficiency leads to T cell loss and immune dysfunction

Germline mutations in the breast cancer type 2 susceptibility gene (BRCA2) are linked to familial breast cancer and the progressive bone marrow failure syndrome Fanconi anaemia. Established Brca2 mouse knockout models show embryonic lethality, but those with a truncating mutation at the C-terminus survive to birth and develop thymic lymphoma at an early age. To overcome early lethality and investigate the function of BRCA2, we used T cell-specific conditional Brca2 knockout mice, which were previously shown to develop thymic lymphoma at a low penetrance. In the current study we showed that the number of peripheral T cells, particularly naïve pools, drastically declined with age. This decline was primarily ascribed to improper peripheral maintenance. Furthermore, heterozygous mice with one wild-type Brca2 allele manifested reduced T cell numbers, suggesting that Brca2 haploinsufficiency might also result in T cell loss. Our study reveals molecular events occurring in Brca2-deficient T cells and suggests that both heterozygous and homozygous Brca2 mutation may lead to dysfunction in T cell populations.

Chromosome instability and carcinogenesis: insights from murine models of human pancreatic cancer associated with BRCA2 inactivation

Chromosomal instability is a hallmark of human cancer cells, but its role in carcinogenesis remains poorly resolved. Insights into this role have emerged from studies on the tumour suppressor BRCA2, whose inactivation in human cancers causes chromosomal instability through the loss of essential functions of the BRCA2 protein in the normal mechanisms responsible for the replication, repair and segregation of DNA during cell division. Humans who carry heterozygous germline mutations in the BRCA2 gene are highly predisposed to cancers of the breast, ovary, pancreas, prostate and other tissues. Here, we review recent studies that describe genetically engineered mouse models (GEMMs) for pancreatic cancer associated with BRCA2 mutations. These studies not only surprisingly show that BRCA2 does not follow the classical Knudson "two hit" paradigm for tumour suppression, but also highlight features of the interplay between TP53 inactivation and carcinogenesis in the context of BRCA2 deficiency. Thus, the models reveal novel aspects of cancer evolution in carriers of germline BRCA2 mutations, provide new insights into the tumour suppressive role of BRCA2, and establish valuable new preclinical settings for testing approaches to pancreatic cancer therapy; together, these features emphasize the value of GEMMs in cancer research.

Deletion of cyclooxygenase 2 in mouse mammary epithelial cells delays breast cancer onset through augmentation of type 1 immune responses in tumors

Inhibition of cyclooxygenase (COX) 2, which is associated with >40% of breast cancers, decreases the risk of tumorigenesis and breast cancer recurrence. To study the role of COX-2 in breast cancer, we engineered mice that lack selectively mammary epithelial cell (MEC) COX-2 (COX-2 KO(MEC)). Compared with wild type (WT), MEC from COX-2 KO(MEC) mice expressed >90% less COX-2 messenger RNA (mRNA) and protein and produced 90% less of the dominant pro-oncogenic COX-2 product, prostaglandin (PG) E(2). We confirmed COX-2 as the principle source of PGE(2) in MEC treated with selective COX-2 and COX-1 inhibitors. Tumors were induced in mice using medroxyprogesterone acetate and 7,12-dimethylbenz[a]anthracene. Breast cancer onset was significantly delayed in COX-2 KO(MEC) compared with WT (P = 0.03), equivalent to the delay following systemic COX-2 inhibition with rofecoxib. Compared with WT, COX-2 KO(MEC) tumors showed increased mRNA for Caspase-3, Ki-67 and common markers for leukocytes (CD45) and macrophages (F4/80). Analysis of multiple markers/cytokines, namely CD86, inducible nitric oxide synthase (iNOS), interleukin-6, tumor necrosis factor α (TNFα) and Tim-3 indicated a shift toward antitumorigenic type 1 immune responses in COX-2 KO(MEC) tumors. Immunohistochemical analysis confirmed elevated expression of CD45, F4/80 and CD86 in COX-2 KO(MEC) tumors. Concordant with a role for COX-2 in restraining M1 macrophage polarization, CD86 and TNFα expression were offset by exogenous PGE(2) in bone marrow-derived macrophages polarized in vitro to the M1 phenotype. Our data reveal the importance of epithelial COX-2 in tumor promotion and indicate that deletion of epithelial COX-2 may skew tumor immunity toward type 1 responses, coincident with delayed tumor development.

Mouse models of BRCA1 and BRCA2 deficiency: past lessons, current understanding and future prospects

Germline mutations in BRCA1 and BRCA2 are responsible for a large proportion of hereditary breast and ovarian cancers. Soon after the identification of both genes in the mid-1990s, investigators set out to develop mouse models for the associated disease. Whereas conventional Brca1 and Brca2 mouse mutants did not reveal a strong phenotype in a heterozygous setting, most homozygous mutations caused embryonic lethality. Consequently, development of mouse models for BRCA-associated tumorigenesis required the generation of tissue-specific conditional knockout animals. In this review, we give an overview of the conventional and the conditional mouse models of BRCA1 and BRCA2 deficiency generated over the last decade, as well as the contribution of these models to our understanding of the biological and molecular functions of BRCA1 and BRCA2. The most advanced mouse models for BRCA1- and BRCA2-associated tumorigenesis mimic human disease to the extent that they can be used in studies addressing clinically relevant questions. These models will help to resolve yet unanswered questions and to translate our increasing knowledge of BRCA1 and BRCA2 biology into clinical practice.

Mutant Brca2/p53 mice exhibit altered radiation responses in the developing mammary gland

Appropriate balance between proliferation and apoptosis is critical for mammary gland development and is often altered during tumorigenesis. Carcinogens like radiation induce DNA damage and activate protective responses such as cell cycle arrest and apoptosis. We used mice carrying Brca2(-/-) and/or p53(-/-) mutations to evaluate the individual and combined effects of these genes on cell proliferation and apoptosis in the developing mammary gland. Mice were exposed to 5Gy of radiation or chamber exposure (controls) followed by injection with BrdU. Mammary glands were collected 6 h post-radiation exposure and evaluated for proliferation (BrdU) and apoptosis (TUNEL) in terminal end buds (TEB) and ducts. Under control conditions, the Brca2 mutation reduced proliferation and apoptosis in TEB but not ducts, whereas the p53 mutation reduced apoptosis in TEB and ducts but did not influence proliferation. Despite these alterations in proliferation and/or apoptosis, neither mutation, either individually or combined, significantly altered the overall balance between the two as measured by the proliferation to apoptosis ratio (growth index). Following irradiation, the Brca2 mutation had no significant effect on proliferation or apoptosis, whereas the p53 mutation resulted in reduced apoptosis in TEB and ducts but did not significantly influence proliferation. Neither mutation by itself altered the growth index in the TEB after irradiation although combined Brca2/p53 mutation caused significantly increased proliferation, reduced apoptosis, and an elevated growth index in TEB and ducts. These results reveal both independent and collaborative growth regulatory roles for Brca2 and p53 under normal and adverse environmental conditions. Additionally, we demonstrate the importance of gene-environment interactions by showing that Brca2- and p53-deficient mice can compensate for their genetic deficiencies under control conditions but not after exposure to radiation. We also demonstrate distinct spatial differences in the cellular functions of Brca2 and p53 and show that combined mutation of both genes is more detrimental than loss of either gene alone.