PRMT1, the major protein arginine methyltransferase in mammals, catalyzes monomethylation and asymmetric dimethylation of arginine side chains in proteins. Initially described as a regulator of chromatin dynamics through the methylation of histone H4 at arginine 3 (H4R3), numerous non-histone substrates have since been identified. The variety of these substrates underlines the essential role played by PRMT1 in a large number of biological processes such as transcriptional regulation, signal transduction or DNA repair. This review will provide an overview of the structural, biochemical and cellular features of PRMT1. After a description of the genomic organization and protein structure of PRMT1, special consideration was given to the regulation of PRMT1 enzymatic activity. Finally, we discuss the involvement of PRMT1 in embryonic development, DNA damage repair, as well as its participation in the initiation and progression of several types of cancers.

Estrogen receptor alpha 36 (ERα36) is a variant of the canonical estrogen receptor alpha (ERα66), widely expressed in hormone sensitive cancer cells and whose high expression level correlates with a poor survival prognosis for breast cancer patients. While ERα36 activity have been related to breast cancer progression or acquired resistance to treatment, expression level and location of ERα36 are poorly documented in the normal mammary gland. Therefore, we explored the consequences of a ERα36 overexpression in vitro in MCF-10A normal mammary epithelial cells and in vivo in a unique model of MMTV-ERα36 transgenic mouse strain wherein ERα36 mRNA was specifically expressed in the mammary gland. By a combination of bioinformatics and computational analyses of microarray data, we identified hierarchical gene networks, downstream of ERα36 and modulated by the JAK2/STAT3 signaling pathway. Concomitantly, ERα36 overexpression lowered proliferation rate but enhanced migration potential and resistance to staurosporin-induced apoptosis of the MCF-10A cell line. In vivo, ERα36 expression led to duct epithelium thinning and disruption in adult but not in prepubescent mouse mammary gland. These phenotypes correlated with a loss of E-cadherin expression. Here, we show that an enhanced expression of ERα36 is sufficient, by itself, to disrupt normal breast epithelial phenotype in vivo and in vitro through a dominant-positive effect on nongenomic estrogen signaling pathways. These results also suggest that, in the presence of adult endogenous steroid levels, ERα36 overexpression in vivo contributes to alter mammary gland architecture which may support pre-neoplastic lesion and augment breast cancer risk.

Estrogen nuclear receptors, represented by the canonical forms ERα66 and ERβ1, are the main mediators of the estrogen-dependent pathophysiology in mammals. However, numerous isoforms have been identified, stimulating unconventional estrogen response pathways leading to complex cellular and tissue responses. The estrogen receptor variant, ERα36, was cloned in 2005 and is mainly described in the literature to be involved in the progression of mammary tumors and in the acquired resistance to anti-estrogen drugs, such as tamoxifen. In this review, we will first specify the place that ERα36 currently occupies within the diversity of nuclear and membrane estrogen receptors. We will then report recent data on the impact of ERα36 expression and/or activity in normal breast and testicular cells, but also in different types of tumors including mammary tumors, highlighting why ERα36 can now be considered as a marker of malignancy. Finally, we will explain how studying the regulation of ERα36 expression could provide new clues to counteract resistance to cancer treatments in hormone-sensitive tumors.

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Fetal and neonatal exposure to long-chain alkylphenols has been suspected to promote breast developmental disorders and consequently to increase breast cancer risk. However, disease predisposition from developmental exposures remains unclear. In this work, human MCF-10A mammary epithelial cells were exposed in vitro to a low dose of a realistic (4-nonylphenol + 4-tert-octylphenol) mixture. Transcriptome and cell-phenotype analyses combined to functional and signaling network modeling indicated that long-chain alkylphenols triggered enhanced proliferation, migration ability, and apoptosis resistance and shed light on the underlying molecular mechanisms which involved the human estrogen receptor alpha 36 (ERα36) variant. A male mouse-inherited transgenerational model of exposure to three environmentally relevant doses of the alkylphenol mix was set up in order to determine whether and how it would impact on mammary gland architecture. Mammary glands from F3 progeny obtained after intrabuccal chronic exposure of C57BL/6J P0 pregnant mice followed by F1–F3 male inheritance displayed an altered histology which correlated with the phenotypes observed in vitro in human mammary epithelial cells. Since cellular phenotypes are similar in vivo and in vitro and involve the unique ERα36 human variant, such consequences of alkylphenol exposure could be extrapolated from mouse model to human. However, transient alkylphenol treatments combined to ERα36 overexpression in mammary epithelial cells were not sufficient to trigger tumorigenesis in xenografted Nude mice. Therefore, it remains to be determined if low-dose alkylphenol transgenerational exposure and subsequent abnormal mammary gland development could account for an increased breast cancer susceptibility.

Breast cancer remains the major cause of cancer-induced morbidity and mortality in women. Among the different molecular subtypes, luminal tumors yet considered of good prognosis often develop acquired resistance to endocrine therapy. Recently, misregulation of ERα36 was reported to play a crucial role in this process. High expression of this ERα isoform was associated to preneoplastic phenotype in mammary epithelial cells, disease progression, and enhanced resistance to therapeutic agents in breast tumors. In this study, we identified two mechanisms that could together contribute to ERα36 expression regulation. We first focused on hsa-miR-136-5p, an ERα36 3’UTR-targeting microRNA, the expression of which inversely correlated to the ERα36 one in breast cancer cells. Transfection of hsa-miR136-5p mimic in MCF-7 cells resulted in downregulation of ERα36. Moreover, the demethylating agent decitabine was able to stimulate hsa-miR-136-5p endogenous expression, thus indirectly decreasing ERα36 expression and counteracting tamoxifen-dependent stimulation. The methylation status of ERα36 promoter also directly modulated its expression level, as demonstrated after decitabine treatment of breast cancer cell and confirmed in a set of tumor samples. Taken together, these results open the way to a direct and an indirect ERα36 epigenetic modulation by decitabine as a promising clinical strategy to counteract acquired resistance to treatment and prevent relapse.

We propose a new methodology for selecting and ranking covariates associated with a variable of interest in a context of high-dimensional data under dependence but few observations. The methodology successively intertwines the clustering of covariates, decorrelation of covariates using Factor Latent Analysis, selection using aggregation of adapted methods and finally ranking. A simulation study shows the interest of the decorrelation inside the different clusters of covariates. We first apply our method to transcriptomic data of 37 patients with advanced nonsmall-cell lung cancer who have received chemotherapy, to select the transcriptomic covariates that explain the survival outcome of the treatment. Secondly, we apply our method to 79 breast tumor samples to define patient profiles for a new metastatic biomarker and associated gene network in order to personalize the treatments.

Background: Onapristone (ONA) is a progesterone receptor (PR) antagonist that prevents PR-mediated DNA transcription. ONA is currently being evaluated in metastatic breast cancers (MBC), as well as other hormone-dependent cancers. Recently, therapeutic options for metastatic estrogen receptor (ER) positive breast cancers have been expanded by the introduction of cyclin-dependent kinase 4/6 inhibitors (CDK4/6i), administered in combination with endocrine treatments, as first line therapy. Moreover, the selective PI3K inhibitor alpelisib (ALP) has been approved for the treatment of PIK3CA-mutated endocrine resistant MBC. The objective of this study was to evaluate the efficacy of ONA in combination with fulvestrant (FUL) and palbociclib (PAL) or ALP in different PDX models established from ER and PR positive breast cancers. Methods: PDX models were established from primary tumors or biopsies from bone metastases from endocrine therapy patients with progressing tumors. ER and PR expression were analyzed by immunohistochemistry (IHC) and western blot (WB) analysis in 17 PDX models. The anti-tumor activity of onapristone alone or combined with FUL and PAL was tested in 2 ER+/PR+ PDX studies, while the combination with ALP+FUL was tested in one PIK3CA-mutated PDX study. Phosphorylation of both PR and ER-PR interactions were analyzed in treated tumors by WB and by the in situ Proximity Ligation Assay (PLA), respectively. Results: PR expression was found in 1/9 PDX established from primary breast tumors and 4/8 PDX established from bone metastases. ONA in vivo activity was tested in 2 PDX of bone metastases: PDX BC1101 (PR low) and BC1117 (PR high). BC1101 showed amplification of FGFR1 and CCND1 genes, while BC1117 has an activating mutation of PIK3CA gene. In the low PR PDX BC1101, treatment with ONA was ineffective in both the monotherapy and combination setting. Conversely, in the PDX with high expression of PR, BC1117, ONA treatment, given as monotherapy, decreased tumor growth. The anti-tumor activity of ONA+FUL+PAL combination was significantly increased as compared to FUL+PAL, with the majority of xenografts showing tumor regression. Expression of PR and phospho-PR were inhibited in the tumors treated by FUL, ONA+FUL, and ONA+FUL+PAL. Interaction between ER and PR, analyzed by the in situ PLA assay, was inhibited in the ONA+FUL treated xenografts ±PAL. Finally, treatment with the triple combination of ONA+FUL+ALP was also highly effective and significantly greater than the combination of FUL+ALP. RNAseq analysis of treated xenografts is ongoing to identify transcriptomic changes in the different treatment arms. In conclusion, our study demonstrates that ONA improved the response to endocrine treatment using CDK4/6 or PI3K inhibitors in a MBC-derived PDX model with high PR expression. Citation Format: Martin Lehr, Ahmed Dahmani, Léa Huguet, Rania El-Botty, Charlène Thiebaut, Muriel Romancer, Paul Cottu, Elisabetta Marangoni. Targeting progesterone receptor (PR) with the antiprogestin onapristone in patient-derived xenograft (PDX) models of estrogen receptor positive (ER+), PR positive (PR+) bone metastasis of breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 649.