Melanocyte reprogramming requires chromatin andtranscription remodelling.

Abstract

Microphthalmia-associated transcription factor (MITF) plays a fundamental role in melanocyte development and contributes to melanomagenesis by regulating the expression of genes involved in differentiation, proliferation and survival. Loss of MITF expression is a hallmark of quiescent melanocyte stem cells, which, when stimulated, transiently reactivate MITF expression to proliferate and restore the pool of differentiated pigment-producing melanocytes. The dual activities of MITF are also evident in melanoma; low MITF melanoma cells tend to be slow cycling, invasive and display tumour-initiating properties, whereas MITF gene amplification and overexpression is oncogenic in a subset of melanomas (Garraway et al., 2005). Recent studies have suggested that MITF may regulate gene expression by modulating the structure of chromatin, specifically by binding to the BRG1- and PBAF-SWI/SNF (De La Serna et al., 2006; Laurette et al., 2015) and NURF-ISWI families of remodelling enzymes. MITF has been shown to activate pigment-related genes by recruiting the SWI/SNF enzymes to melanocyte-specific promoters, and the report of Koludrovic et al. (2015) extends these studies by investigating the influence of nucleosome remodelling factor (NURF) on MITF activity and melanocyte development. The NURF enzyme catalyses nucleosome sliding on DNA and is necessary for the chromatin remodelling required for transcription. The core mammalian NURF complex comprises the SMARCA1 ATPase subunit, the RBBP4 histone binding polypeptide and the 450kD BPTF bromodomain PHD finger transcription factor. BPTF binds active gene promoters and shows oncogenic activity in melanoma, the gene is frequently amplified in primary melanomas, protein overexpression in melanoma is associated with poor patient prognosis, and ectopic BPTF expression diminishes melanoma cell sensitivity to the BRAF inhibitor vemurafenib (Dar et al., 2015). Koludrovic et al. (2015) also confirmed the proliferative role of BPTF, showing that silencing of BPTF expression inhibited the proliferation of melanoma cells and melanocytes in vitro and promoted senescent-like changes, including enlarged and flattened cell morphology and increased senescence-associated β-galactosidase activity. Significantly, these phenotypic changes resembled the changes associated with MITF silencing, and predictably MITF and BPTF regulate a common set of genes. In particular, 39% of genes down-regulated by BPTF silencing and 41% of genes induced by BPTF knockdown were regulated in an analogous manner by MITF suppression. Thus, BPTF and MITF positively co-regulate genes required for cell cycle progression and negatively regulate genes involved in adhesion, morphology and senescence. Importantly, BPTF suppression also induced senescence in MITF-negative melanoma cells, but not in non-melanoma cell models. These data indicate that BPTF has MITF-dependent and MITF-independent functions that are specific to the melanocytic lineage. The melanocyte-specific inactivation of Bptf in vivo (Tyr-Cre/o::Bptflox/lox) produced animals with premature greying that involved the progressive loss of pigmented melanocytes in the hair bulb. Critically, in the absence of BPTF, the adult melanocyte stem cells, which are located in the bulge region of the hair follicle, were established and maintained as late as nine months after the mice went completely white. Thus, BPTF promotes the differentiation or survival of mature melanocytes from the stem cell population. In contrast, hair greying in mice expressing a hypomorphic mutant MITF allele appears to result from the loss of stem cells possibly as a result of their premature differentiation, which ultimately leads to the progressive loss of pigmented mature melanocytes (Nishimura et al., 2005). “transcriptionally repressed melanocyte stem cells require BPTF/NURF-mediated chromatin remodelling for activation”