C9orf142 transcriptionally activates MTBP to drive progression and resistance to CDK4/6 inhibitor in triple‐negative breast cancer

Abstract Background Triple‐negative breast cancer (TNBC) presents the most challenging subtype of all breast cancers because of its aggressive clinical phenotypes and absence of viable therapy targets. In order to identify effective molecular targets for treating patients with TNBC, we conducted an integration analysis of our recently published TNBC dataset of quantitative proteomics and RNA‐Sequencing, and found the abnormal upregulation of chromosome 9 open reading frame 142 (C9orf142) in TNBC. However, the functional roles of C9orf142 in TNBC are unclear. Methods In vitro and in vivo functional experiments were performed to assess potential roles of C9orf142 in TNBC. Immunoblotting, real‐time quantitative polymerase chain reaction (RT‐qPCR), and immunofluorescent staining were used to investigate the expression levels of C9orf142 and its downstream molecules. The molecular mechanisms underlying C9orf142‐regulated mouse double minute 2 (MDM2)‐binding protein (MTBP) were determined by chromatin immunoprecipitation (ChIP) and dual‐luciferase reporter assays. Results In TNBC tissues and metastatic lymph nodes, we observed that C9orf142 exhibited abnormal up‐regulation, and its elevated expression was indicative of unfavorable prognosis for TNBC patients. Both in vitro and in vivo functional experiments demonstrated that C9orf142 accelerated TNBC growth and metastasis. Further mechanism exploration revealed that C9orf142 transcriptionally activated MTBP, thereby regulating its downstream MDM2/p53/p21 signaling axis and the transition of cell cycle from G1 to S phase. Functional rescue experiment demonstrated that knockdown of MTBP attenuated C9orf142‐mediated tumour growth and metastasis. Furthermore, depletion of C9orf142 remarkably increased the responsiveness of TNBC cells to CDK4/6 inhibitor abemaciclib. Conclusions Together, these findings unveil a previously unrecognized effect of C9orf142 in TNBC progression and responsiveness to CDK4/6 inhibitor, and emphasize C9orf142 as a promising intervention target for TNBC treatment.