CD26-negative and CD26-positive tissue-resident fibroblasts contribute to functionally distinct CAF subpopulations in breast cancer.

Cancer-associated fibroblasts (CAFs) are abundantly present in the microenvironment of virtually all tumors and strongly impact tumor progression. Despite increasing insight into their function and heterogeneity, little is known regarding the origin of CAFs. Understanding the origin of CAF heterogeneity is needed to develop successful CAF-based targeted therapies. Through various transplantation studies in mice, we show that CAFs in both invasive lobular breast cancer and triple-negative breast cancer originate from mammary tissue-resident normal fibroblasts (NFs). Single-cell transcriptomics, in vivo and in vitro studies reveal the transition of CD26+ and CD26- NF populations into inflammatory CAFs (iCAFs) and myofibroblastic CAFs (myCAFs), respectively. Functional co-culture experiments show that CD26+ NFs transition into pro-tumorigenic iCAFs which recruit myeloid cells in a CXCL12-dependent manner and enhance tumor cell invasion via matrix-metalloproteinase (MMP) activity. Together, our data suggest that CD26+ and CD26- NFs transform into distinct CAF subpopulations in mouse models of breast cancer.

Truncated FGFR2 is a clinically actionable oncogene in multiple cancers.

Somatic hotspot mutations and structural amplifications and fusions that affect fibroblast growth factor receptor 2 (encoded by FGFR2) occur in multiple types of cancer1. However, clinical responses to FGFR inhibitors have remained variable1-9, emphasizing the need to better understand which FGFR2 alterations are oncogenic and therapeutically targetable. Here we apply transposon-based screening10,11 and tumour modelling in mice12,13, and find that the truncation of exon 18 (E18) of Fgfr2 is a potent driver mutation. Human oncogenomic datasets revealed a diverse set of FGFR2 alterations, including rearrangements, E1-E17 partial amplifications, and E18 nonsense and frameshift mutations, each causing the transcription of E18-truncated FGFR2 (FGFR2ΔE18). Functional in vitro and in vivo examination of a compendium of FGFR2ΔE18 and full-length variants pinpointed FGFR2-E18 truncation as single-driver alteration in cancer. By contrast, the oncogenic competence of FGFR2 full-length amplifications depended on a distinct landscape of cooperating driver genes. This suggests that genomic alterations that generate stable FGFR2ΔE18 variants are actionable therapeutic targets, which we confirmed in preclinical mouse and human tumour models, and in a clinical trial. We propose that cancers containing any FGFR2 variant with a truncated E18 should be considered for FGFR-targeted therapies.

Transcriptomics and Transposon Mutagenesis Identify Multiple Mechanisms of Resistance to the FGFR Inhibitor AZD4547.

In human cancers, FGFR signaling is frequently hyperactivated by deregulation of FGF ligands or by activating mutations in the FGFR receptors such as gene amplifications, point mutations, and gene fusions. As such, FGFR inhibitors are considered an attractive therapeutic strategy for patients with mutations in FGFR family members. We previously identified Fgfr2 as a key driver of invasive lobular carcinoma (ILC) in an in vivo insertional mutagenesis screen using the Sleeping Beauty transposon system. Here we explore whether these FGFR-driven ILCs are sensitive to the FGFR inhibitor AZD4547 and use transposon mutagenesis in these tumors to identify potential mechanisms of resistance to therapy. Combined with RNA sequencing-based analyses of AZD4547-resistant tumors, our in vivo approach identified several known and novel potential resistance mechanisms to FGFR inhibition, most of which converged on reactivation of the canonical MAPK-ERK signaling cascade. Observed resistance mechanisms included mutations in the tyrosine kinase domain of FGFR2, overexpression of MET, inactivation of RASA1, and activation of the drug-efflux transporter ABCG2. ABCG2 and RASA1 were identified only from de novo transposon insertions acquired during AZD4547 treatment, demonstrating that insertional mutagenesis in mice is an effective tool for identifying potential mechanisms of resistance to targeted cancer therapies.Significance: These findings demonstrate that a combined approach of transcriptomics and insertional mutagenesis in vivo is an effective method for identifying potential targets to overcome resistance to therapy in the clinic. Cancer Res; 78(19); 5668-79. ©2018 AACR.

PTEN Loss in E-Cadherin-Deficient Mouse Mammary Epithelial Cells Rescues Apoptosis and Results in Development of Classical Invasive Lobular Carcinoma.

Invasive lobular carcinoma (ILC) is an aggressive breast cancer subtype with poor response to chemotherapy. Besides loss of E-cadherin, a hallmark of ILC, genetic inactivation of PTEN is frequently observed in patients. Through concomitant Cre-mediated inactivation of E-cadherin and PTEN in mammary epithelium, we generated a mouse model of classical ILC (CLC), the main histological ILC subtype. While loss of E-cadherin induced cell dissemination and apoptosis, additional PTEN inactivation promoted cell survival and rapid formation of invasive mammary tumors that recapitulate the histological and molecular features, estrogen receptor (ER) status, growth kinetics, metastatic behavior, and tumor microenvironment of human CLC. Combined inactivation of E-cadherin and PTEN is sufficient to cause CLC development. These CLCs showed significant tumor regression upon BEZ235-mediated inhibition of PI3K signaling. In summary, this mouse model provides important insights into CLC development and suggests inhibition of phosphatidylinositol 3-kinase (PI3K) signaling as a potential therapeutic strategy for targeting CLC.

BRCA1185delAG tumors may acquire therapy resistance through expression of RING-less BRCA1.

Heterozygous germline mutations in breast cancer 1 (BRCA1) strongly predispose women to breast cancer. BRCA1 plays an important role in DNA double-strand break (DSB) repair via homologous recombination (HR), which is important for tumor suppression. Although BRCA1-deficient cells are highly sensitive to treatment with DSB-inducing agents through their HR deficiency (HRD), BRCA1-associated tumors display heterogeneous responses to platinum drugs and poly(ADP-ribose) polymerase (PARP) inhibitors in clinical trials. It is unclear whether all pathogenic BRCA1 mutations have similar effects on the response to therapy. Here, we have investigated mammary tumorigenesis and therapy sensitivity in mice carrying the Brca1185stop and Brca15382stop alleles, which respectively mimic the 2 most common BRCA1 founder mutations, BRCA1185delAG and BRCA15382insC. Both the Brca1185stop and Brca15382stop mutations predisposed animals to mammary tumors, but Brca1185stop tumors responded markedly worse to HRD-targeted therapy than did Brca15382stop tumors. Mice expressing Brca1185stop mutations also developed therapy resistance more rapidly than did mice expressing Brca15382stop. We determined that both murine Brca1185stop tumors and human BRCA1185delAG breast cancer cells expressed a really interesting new gene domain-less (RING-less) BRCA1 protein that mediated resistance to HRD-targeted therapies. Together, these results suggest that expression of RING-less BRCA1 may serve as a marker to predict poor response to DSB-inducing therapy in human cancer patients.

A high-throughput functional complementation assay for classification of BRCA1 missense variants.

UNLABELLED: Mutations in BRCA1 and BRCA2 account for the majority of hereditary breast and ovarian cancers, and therefore sequence analysis of both genes is routinely conducted in patients with early-onset breast cancer. Besides mutations that clearly abolish protein function or are known to increase cancer risk, a large number of sequence variants of uncertain significance (VUS) have been identified. Although several functional assays for BRCA1 VUSs have been described, thus far it has not been possible to conduct a high-throughput analysis in the context of the full-length protein. We have developed a relatively fast and easy cDNA-based functional assay to classify BRCA1 VUSs based on their ability to functionally complement BRCA1-deficient mouse embryonic stem cells. Using this assay, we have analyzed 74 unclassified BRCA1 missense mutants for which all predicted pathogenic variants are confined to the BRCA1 RING and BRCT domains. SIGNIFICANCE: BRCA1 VUSs are frequently found in patients with hereditary breast or ovarian cancer and present a serious problem for clinical geneticists. This article describes the generation, validation, and application of a reliable high-throughput assay for the functional classification of BRCA1 sequence variants of uncertain significance.

Polycomb group gene Ezh2 regulates mammary gland morphogenesis and maintains the luminal progenitor pool.

Specification of the cellular hierarchy in the mammary gland involves complex signaling that remains poorly defined. Polycomb group proteins are known to contribute to the maintenance of stem cell identity through epigenetic modifications, leading to stable alterations in gene expression. The polycomb protein family member EZH2 is known to be important for stem cell maintenance in multiple tissues, but its role in mammary gland development and differentiation remains unknown. Our analyses show that EZH2 is predominantly expressed in luminal cells of the mouse mammary epithelium. As mammary gland development occurs mostly after birth, the analysis of EZH2 gene function in postnatal development is precluded by embryonic lethality of conventional EZH2 knockout mice. To investigate the role of EZH2 in normal mammary gland epithelium, we have generated novel transgenic mice that express doxycycline-regulatable short hairpin (sh) RNAs directed against Ezh2. Knockdown of EZH2 results in delayed outgrowth of the mammary epithelium during puberty, due to impaired terminal end bud formation and ductal elongation. Furthermore, our results demonstrate that EZH2 is required to maintain the luminal cell pool and may limit differentiation of luminal progenitors into CD61(+) differentiated luminal cells, suggesting a role for EZH2 in mammary luminal cell fate determination. Consistent with this, EZH2 knockdown reduced lobuloalveolar expansion during pregnancy, suggesting EZH2 is required for the differentiation of luminal progenitors to alveolar cells.

Loss of 53BP1 causes PARP inhibitor resistance in Brca1-mutated mouse mammary tumors.

UNLABELLED: Inhibition of PARP is a promising therapeutic strategy for homologous recombination-deficient tumors, such as BRCA1-associated cancers. We previously reported that BRCA1-deficient mouse mammary tumors may acquire resistance to the clinical PARP inhibitor (PARPi) olaparib through activation of the P-glycoprotein drug efflux transporter. Here, we show that tumor-specific genetic inactivation of P-glycoprotein increases the long-term response of BRCA1-deficient mouse mammary tumors to olaparib, but these tumors eventually developed PARPi resistance. In a fraction of cases, this resistance is caused by partial restoration of homologous recombination due to somatic loss of 53BP1. Importantly, PARPi resistance was minimized by long-term treatment with the novel PARP inhibitor AZD2461, which is a poor P-glycoprotein substrate. Together, our data suggest that restoration of homologous recombination is an important mechanism for PARPi resistance in BRCA1-deficient mammary tumors and that the risk of relapse of BRCA1-deficient tumors can be effectively minimized by using optimized PARP inhibitors. SIGNIFICANCE: In this study, we show that loss of 53BP1 causes resistance to PARP inhibition in mouse mammary tumors that are deficient in BRCA1. We hypothesize that low expression or absence of 53BP1 also reduces the response of patients with BRCA1-deficient tumors to PARP inhibitors.

BRCA1 RING function is essential for tumor suppression but dispensable for therapy resistance.

Hereditary breast cancers are frequently caused by germline BRCA1 mutations. The BRCA1(C61G) mutation in the BRCA1 RING domain is a common pathogenic missense variant, which reduces BRCA1/BARD1 heterodimerization and abrogates its ubiquitin ligase activity. To investigate the role of BRCA1 RING function in tumor suppression and therapy response, we introduced the Brca1(C61G) mutation in a conditional mouse model for BRCA1-associated breast cancer. In contrast to BRCA1-deficient mammary carcinomas, tumors carrying the Brca1(C61G) mutation responded poorly to platinum drugs and PARP inhibition and rapidly developed resistance while retaining the Brca1(C61G) mutation. These findings point to hypomorphic activity of the BRCA1-C61G protein that, although unable to prevent tumor development, affects response to therapy.

Genome-wide retroviral insertional tagging of genes involved in cancer in Cdkn2a-deficient mice.

We have used large-scale insertional mutagenesis to identify functional landmarks relevant to cancer in the recently completed mouse genome sequence. We infected Cdkn2a(-/-) mice with Moloney murine leukemia virus (MoMuLV) to screen for loci that can participate in tumorigenesis in collaboration with loss of the Cdkn2a-encoded tumor suppressors p16INK4a and p19ARF. Insertional mutagenesis by the latent retrovirus was synergistic with loss of Cdkn2a expression, as indicated by a marked acceleration in the development of both myeloid and lymphoid tumors. We isolated 747 unique sequences flanking retroviral integration sites and mapped them against the mouse genome sequence databases from Celera and Ensembl. In addition to 17 insertions targeting gene loci known to be cancer-related, we identified a total of 37 new common insertion sites (CISs), of which 8 encode components of signaling pathways that are involved in cancer. The effectiveness of large-scale insertional mutagenesis in a sensitized genetic background is demonstrated by the preference for activation of MAP kinase signaling, collaborating with Cdkn2a loss in generating the lymphoid and myeloid tumors. Collectively, our results show that large-scale retroviral insertional mutagenesis in genetically predisposed mice is useful both as a system for identifying genes underlying cancer and as a genetic framework for the assignment of such genes to specific oncogenic pathways.