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Repression of endogenous retroviruses is required for mammary gland development and homeostasis [microarray]

ABSTRACT: We demonstrate that loss of the histone H3K9 methyltransferase G9a in the developing mammary epithelium results in de novo chromatin opening which leads to severely impaired development of the mammary ductal tree, concomitant with impaired stem cell potential and disrupted intraductal polarity. Intriguingly, these phenotypes are not attributed to alterations in lineage specification and fidelity of the basal and luminal cell fates. Instead, increased chromatin opening in G9a-ablated mammary epithelium results in derepression of Long Terminal Repeat (LTR) sequences, most prominently of the ERVK family. When reverse-transcribed, these endogenous retroviruses generate double-stranded DNA (dsDNA) that accumulates in the cytosol of G9acKO mammary epithelial cells and trigger a pyroptotic antiviral innate immune response within mammary fat pads. Importantly, this altered G9acKO mammary milieu precludes functional outgrowth of even wild-type mammary stem cells upon transplantation. Our results show that tight repression of endogenous retroviruses is required for proper mammary gland development and maintenance. We demonstrate that altering chromatin accessibility of retroviral elements, as occurs during aging and cancer, severely disrupts functional mammary gland development and mammary stem cell activity through both cell autonomous and cell non-autonomous mechanisms.

ORGANISM(S): Mus musculus

PROVIDER: GSE168624 | GEO | 2021/05/18

REPOSITORIES: GEO

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Project description:CCAAT/enhancer binding protein beta (C/EBPb) is a member of a family of highly conserved transcription factors that regulates numerous genes involved in proliferation and differentiation in a variety of tissues. C/EBPb is deregulated in human breast cancer and germline deletion of this gene results in multiple defects in mammary gland development. We hypothesized that C/EBPb regulates mammary stem cell self-renewal, maintenance and/or differentiation through the regulation of multiple target genes that coordinate mammary gland development. Utilizing both a germline knockout mouse model and a conditional knockout strategy, we demonstrated that mammosphere formation was significantly decreased in C/EBPb-deficient mammary epithelial cells (MECs). The ability of C/EBPb-deleted MECs to regenerate the mammary gland in vivo was severely impaired when transplanted at limiting dilution. Furthermore, serial transplantation of C/EBPb-null mammary tissue resulted in decreased outgrowth potential when compared to wildtype, and an early senescence phenotype. Flow cytometric analysis revealed that C/EBPb-null MECs contain a lower frequency of repopulating stem cells accompanied by an increase in committed, differentiated luminal cells as compared to wildtype. Microarray analysis of stem/progenitor cell populations was performed and revealed an alteration in cell fate specification in C/EBPb-null glands, exemplified by the aberrant expression of basal markers in the luminal cell compartment. Collectively, our studies demonstrate that C/EBPb is a critical regulator of mammary stem cell differentiation, and an important determinant of luminal cell fate specification. Experiment Overall Design: To identify potential signaling pathways regulated by C/EBPb in stem/progenitor cells, microarray analysis was performed on two stem/progenitor cell subpopulations. For this analysis, subpopulations defined by LIN-CD24+CD29hi and LIN-CD24hiCD29lo were FACS sorted from wildtype and germline C/EBPb-/- glands, and RNA was isolated from each group.