Epigenetic Regulation of Fanconi Anemia Genes Implicates PRMT5 Blockage as a Strategy for Tumor Chemosensitization.

Strengthened DNA repair pathways in tumor cells contribute to the development of resistance to DNA-damaging agents. Consequently, targeting proteins in these pathways is a promising strategy for tumor chemosensitization. Here, we show that the expression of a subset of Fanconi anemia (FA) genes is attenuated in glioblastoma tumor cells deficient in methylthioadenosine phosphorylase (MTAP), a common genetic alteration in a variety of cancers. Subsequent experiments in cell line models of different cancer types illustrate that this reduced transcription of FA genes can be recapitulated by blockage of Protein Arginine Methyltransferase 5 (PRMT5), a promising therapeutically targetable epigenetic regulator whose enzymatic activity is compromised in MTAP-deficient cells. Further analyses provide evidence to support that PRMT5 can function as an epigenetic regulator that contributes to the increased expression of FA genes in cancer cells. Most notably and consistent with the essential roles of FA proteins in resolving DNA damage elicited by interstrand crosslinking (ICL) agents, PRMT5 blockage, as well as MTAP loss, sensitizes tumor cells to ICL agents both in vitro and in xenografts. Collectively, these findings reveal a novel epigenetic mechanism underlying the upregulated expression of FA genes in cancer cells and suggest that therapeutically targeting PRMT5 can have an additional benefit of chemosensitizing tumor cells to ICL agents. IMPLICATIONS: PRMT5 positively regulates the expression of FA genes. Inhibition of PRMT5 attenuates FA-dependent DNA repair pathway and sensitizes tumor cells to ICL agents.

Role of INSL4 Signaling in Sustaining the Growth and Viability of LKB1-Inactivated Lung Cancer.

BACKGROUND: The LKB1 tumor suppressor gene is commonly inactivated in non-small cell lung carcinomas (NSCLC), a major form of lung cancer. Targeted therapies for LKB1-inactivated lung cancer are currently unavailable. Identification of critical signaling components downstream of LKB1 inactivation has the potential to uncover rational therapeutic targets. Here we investigated the role of INSL4, a member of the insulin/IGF/relaxin superfamily, in LKB1-inactivated NSCLCs. METHODS: INSL4 expression was analyzed using global transcriptome profiling, quantitative reverse transcription PCR, western blotting, enzyme-linked immunosorbent assay, and RNA in situ hybridization in human NSCLC cell lines and tumor specimens. INSL4 gene expression and clinical data from The Cancer Genome Atlas lung adenocarcinomas (n = 515) were analyzed using log-rank and Fisher exact tests. INSL4 functions were studied using short hairpin RNA (shRNA) knockdown, overexpression, transcriptome profiling, cell growth, and survival assays in vitro and in vivo. All statistical tests were two-sided. RESULTS: INSL4 was identified as a novel downstream target of LKB1 deficiency and its expression was induced through aberrant CRTC-CREB activation. INSL4 was highly induced in LKB1-deficient NSCLC cells (up to 543-fold) and 9 of 41 primary tumors, although undetectable in all normal tissues except the placenta. Lung adenocarcinomas from The Cancer Genome Atlas with high and low INSL4 expression (with the top 10th percentile as cutoff) showed statistically significant differences for advanced tumor stage (P < .001), lymph node metastasis (P = .001), and tumor size (P = .01). The INSL4-high group showed worse survival than the INSL4-low group (P < .001). Sustained INSL4 expression was required for the growth and viability of LKB1-inactivated NSCLC cells in vitro and in a mouse xenograft model (n = 5 mice per group). Expression profiling revealed INSL4 as a critical regulator of cell cycle, growth, and survival. CONCLUSIONS: LKB1 deficiency induces an autocrine INSL4 signaling that critically supports the growth and survival of lung cancer cells. Therefore, aberrant INSL4 signaling is a promising therapeutic target for LKB1-deficient lung cancers.