Excessive transcription-replication conflicts are a vulnerability of BRCA1-mutant cancers

BRCA1 mutations are associated with increased breast and ovarian cancer risk. BRCA1-mutant tumors are high-grade, recurrent, and often become resistant to standard therapies. Herein, we performed a targeted CRISPR-Cas9 screen and identified MEPCE, a methylphosphate capping enzyme, as a synthetic lethal interactor of BRCA1. Mechanistically, we demonstrate that depletion of MEPCE in a BRCA1-deficient setting led to dysregulated RNA polymerase II (RNAPII) promoter-proximal pausing, R-loop accumulation, and replication stress, contributing to transcription-replication collisions. These collisions compromise genomic integrity resulting in loss of viability of BRCA1-deficient cells. We also extend these findings to another RNAPII-regulating factor, PAF1. This study identifies a new class of synthetic lethal partners of BRCA1 that exploit the RNAPII pausing regulation and highlight the untapped potential of transcription-replication collision-inducing factors as unique potential therapeutic targets for treating cancers associated with BRCA1 mutations

Roles of phosphatidylinositol 3-kinase and p38 mitogen-activated protein kinase in the regulation of protein kinase C-α activation in interferon-γ-stimulated macrophages

Members of the protein kinase C (PKC) family are activated by interferon-γ (IFN-γ) and modulate IFN-γ-induced cellular responses by regulating the activity of transcription factors. We previously reported that PKC-α enhances the ability of IFN regulatory factor-1 to transactivate the class II transactivator (CIITA) promoter IV in IFN-γ-stimulated macrophages. In addition, we showed that IFN-γ induces the nuclear translocation of PKC-α but the mechanisms for this remain to be elucidated. In this study, we sought to identify signalling pathways involved in IFN-γ-induced activation of PKC-α and to characterize their potential roles in modulating IFN-γ-induced responses in macrophages. IFN-γ-mediated nuclear translocation of PKC-α was a Janus activated kinase 2 (JAK2)-independent process, which required phosphatidylinositol 3-kinase (PI3K) and p38 mitogen-activated protein kinase (MAPK). However, PKC-α phosphorylation was independent of PI3K and p38 MAPK, indicating that IFN-γ-induced phosphorylation and nuclear translocation of PKC-α are mediated by distinct mechanisms. In addition, inhibition of PI3K, but not of p38 MAPK, strongly impaired IFN-γ-induced CIITA and MHC II gene expression. Finally, PKC-α associated with signal transducer and activator of transcription 1 (STAT1) and was required for the phosphorylation of STAT1 on serine 727 in IFN-γ-stimulated macrophages. Taken together, our data indicate that PI3K and p38 MAPK modulate IFN-γ-stimulated PKC-α nuclear translocation independently of JAK2 activity and that both PI3K and PKC-α are required for type IV CIITA and MHC II gene expression in IFN-γ-stimulated macrophages