Two forms of XCI ensure the selective silencing of female sex chromosomes during mouse embryogenesis. Imprinted XCI begins with the detection of Xist RNA expression on the paternal X chromosome (Xp) around the four cell stage of embryonic development. In the embryonic tissues of the inner cell mass (ICM), a random form of XCI occurs in blastocysts which inactivates either the Xp or the maternal X chromosome (Xm) 1,2. Both forms of XCI require the non-coding Xist RNA which coats the inactive X chromosome (Xi) from which it is expressed. Xist plays crucial functions for the silencing of X-linked genes including Rnf12 3,4 encoding the ubiquitin ligase RLIM. Targeting a conditional knockout (KO) of Rnf12 to oocytes where RLIM accumulates to high levels, we find that the maternal transmission of the mutant X chromosome (Δm) leads to embryonic lethality due to defective imprinted XCI. We show that in Δm female embryos the initial formation of Xist clouds and Xp silencing is inhibited. In contrast, ES cells lacking RLIM are able to form Xist clouds and silence at least some X-linked genes during random XCI. These results assign crucial roles to the maternal deposit of Rnf12/RLIM for the initiation of imprinted XCI.

MicroRNA-mediated post-transcriptional regulation plays key roles in stem cell self-renewal and tumorigenesis. However, the in vivo functions of specific microRNAs in controlling mammary stem cell (MaSC) activity and breast cancer formation remain poorly understood. Here we show that miR-31 is highly expressed in MaSC-enriched mammary basal cell population and in mammary tumors, and is regulated by NF-κB signaling. We demonstrate that miR-31 promotes mammary epithelial proliferation and MaSC expansion at the expense of differentiation in vivo. Loss of miR-31 compromises mammary tumor growth, reduces the number of cancer stem cells, as well as decreases tumor-initiating ability and metastasis to the lung, supporting its pro-oncogenic function. MiR-31 modulates multiple signaling pathways, including Prlr/Stat5, TGFβ and Wnt/β-catenin. Particularly, it activates Wnt/β-catenin signaling by directly targeting Wnt antagonists, including Dkk1. Importantly, Dkk1 overexpression partially rescues miR31-induced mammary defects. Together, these findings identify miR-31 as the key regulator of MaSC activity and breast tumorigenesis.

Triple-negative breast cancer (TNBC) exhibits poor prognosis, with high metastasis and low survival. Long non-coding RNAs (lncRNAs) play critical roles in tumor progression. Here, we identified lncRNA MIR100HG as a pro-oncogene for TNBC progression. Knockdown of MIR100HG decreased cell proliferation and induced cell arrest in the G1 phase, whereas overexpression of MIR100HG significantly increased cell proliferation. Furthermore, MIR100HG regulated the p27 gene to control the cell cycle, and subsequently impacted the progression of TNBC. In analyzing its underlying mechanism, bioinformatics prediction and experimental data demonstrated that MIR100HG participated in the formation of RNA–DNA triplex structures. MIR100HG in The Cancer Genome Atlas (TCGA) and breast cancer cell lines showed higher expression in TNBC than in other tumor types with poor prognosis. In conclusion, our data indicated a novel working pattern of lncRNA in TNBC progression, which may be a potential therapeutic target in such cancers.