Probes for INS

The deubiquitinating enzyme BAP1 is mutated in a hereditary cancer syndrome with a high risk for mesothelioma and melanocytic tumors. Here, we show that Bap1 deletion in melanocytes cooperates with the constitutively active, oncogenic form of Braf (BrafV600E ) and UV to cause melanoma in mice, albeit at very low frequency. In addition, Bap1 null melanoma cells derived from mouse tumors are more aggressive and colonize and grow at distant sites more than their wild-type counterparts. Molecularly, Bap1 null melanoma cell lines have increased DNA damage measured by γH2aX and hyperubiquitination of histone H2a. Therapeutically, these Bap1 null tumors are completely responsive to BRAF and MEK-targeted therapies. Therefore, BAP1 functions as a tumor suppressor and limits tumor progression in melanoma.

Abstract

PURPOSE:

Understanding tumour heterogeneity is an important challenge in current cancer research. Transcription and epigenetic profiling of cultured melanoma cells have defined at least two distinct cell phenotypes characterised by distinctive gene expression signatures associated with high or low/absent expression of Microphthalmia-associated transcription factor (MITF). Nevertheless, heterogeneity of cellpopulations and gene expression in primary human tumours is much less well characterised.

EXPERIMENTAL DESIGN:

We performed single cell gene expression analyses on 472 cells isolated from needle biopsies of 5 primary human melanomas, 4 superficial spreading and one acral melanoma. The expression of MITF-high and MITF-low signature genes was assessed and compared to investigate intra and inter-tumoural heterogeneity and correlated gene expression profiles.

RESULTS:

Single cell gene expression analyses revealed varying degrees of intra and inter-tumour heterogeneity conferred by the variable expression of distinct sets of genes in different tumours. Expression of MITF partially correlated with that of its known target genes while SOX10 expression correlated best with PAX3 and ZEB2. Nevertheless, cells simultaneously expressing MITF-high and MITF-low signature genes were observed both by single cell analyses and RNAscope.

CONCLUSIONS:

Single cell analyses can be performed on limiting numbers of cells from primary human melanomas revealing their heterogeneity. While tumours comprised variable proportions of cells with the MITF-high and MITF-low gene expression signatures characteristic of melanoma cultures, primary tumours also comprised cells expressing markers of both signatures defining a novel cell state in tumours in vivo.