Cancer is a disease consisting of both genetic and epigenetic changes. While increasing evidence demonstrates that tumour progression entails chromatin-mediated changes such as DNA methylation, the role of histone variants in cancer initiation and progression currently remains unclear. Here, we report that the histone variant macroH2A (mH2A) suppresses tumour progression of malignant melanoma. Loss of mH2A isoforms, histone variants generally associated with condensed chromatin and fine-tuning of developmental gene expression programs1-4, is positively correlated with increasing malignant phenotype of melanoma cells in culture and human tissue samples. Knockdown of mH2A isoforms in melanoma cells of low malignancy results in significantly increased proliferation and migration in vitro and growth and metastasis in vivo. Restored expression of mH2A isoforms rescues these malignant phenotypes in vitro and in vivo. We demonstrate that the tumour promoting function of mH2A loss is mediated, at least in part, through direct transcriptional up-regulation of CDK8. Suppression of CDK8, a colorectal cancer oncogene5, 6, inhibits proliferation of melanoma cells, and knockdown of CDK8 in cells depleted of mH2A suppresses the proliferative advantage induced by mH2A loss. Moreover, a significant inverse correlation between mH2A and CDK8 expression levels exists in melanoma patient samples. Taken together, our results demonstrate that mH2A is a critical component of chromatin that suppresses the development of malignant melanoma, a highly intractable cutaneous neoplasm.

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This paper presents an algorithm for generating correlated vectors of random numbers. The user need not fully specify the joint distribution function; instead, the user "partially specifies" only the marginal distributions and the correlation matrix. The algorithm may be applied to any set of continuous, strictly increasing distribution functions; the marginal distributions need not all be of the same functional form. The correlation matrix is first checked for mathematical consistency (positive semi-definiteness), and adjusted if necessary. Then the correlated random vectors are generated using a combination of Cholesky decomposition and Gauss-Newton iteration. Applications are made to cost analysis, where correlations are often present between cost elements in a work breakdown structure.Simulation, Random Number Generation, Correlation, Gauss-Newton Method

While multiple mechanisms of BRAFV600-mutant melanoma resistance to targeted MAPK signaling inhibitors (MAPKi) have been reported, the epigenetic regulation of this process remains undetermined. Here, using a CRISPR–Cas9 screen targeting chromatin regulators, we discover that haploinsufficiency of the histone deacetylase SIRT6 allows melanoma cell persistence in the presence of MAPKi. Haploinsufficiency, but not complete loss of SIRT6 promotes IGFBP2 expression via increased chromatin accessibility, H3K56 acetylation at the IGFBP2 locus, and consequent activation of the IGF-1 receptor (IGF-1R) and downstream AKT signaling. Combining a clinically applicable IGF-1Ri with BRAFi overcomes resistance of SIRT6 haploinsufficient melanoma cells in vitro and in vivo. Using matched melanoma samples derived from patients receiving dabrafenib + trametinib, we identify IGFBP2 as a potential biomarker for MAPKi resistance. Our study has not only identified an epigenetic mechanism of drug resistance, but also provides insights into a combinatorial therapy that may overcome resistance to standard-of-care therapy for BRAFV600-mutant melanoma patients.