Differential impact of BRAFV600E isoforms on tumorigenesis in a zebrafish model of melanoma.

BRAFV600E comes as two main splicing variants. The well-studied ref isoform and the recently discovered X1 isoform are co-expressed in cancer cells and differ in terms of 3'UTR length and sequence, as well as C-term protein sequence. Here, we use a melanoma model in zebrafish to study the role played by each isoform in larval pigmentation, nevi formation, and their progression into melanoma tumours. We show that both BRAFV600E-ref and BRAFV600E-X1 proteins promote larval pigmentation and nevi formation, while melanoma-free survival curves performed in adult fish indicate that BRAFV600E-ref protein is a much stronger melanoma driver that BRAFV600E-X1 protein. Crucially, we also show that the presence of the 3'UTR suppresses the effect of ref protein. Our data highlight the necessity to undertake a systematic study of BRAFV600E isoforms, in order to uncover the full spectrum of their kinase-(in)dependent and coding-(in)dependent functions, hence to develop more informed strategies for therapeutic targeting.

PARP1 negatively regulates MAPK signaling by impairing BRAF-X1 translation.

In human cells BRAF oncogene is invariably expressed as a mix of two coding transcripts: BRAF-ref and BRAF-X1. These two mRNA isoforms, remarkably different in the sequence and length of their 3'UTRs, are potentially involved in distinct post-transcriptional regulatory circuits. Herein, we identify PARP1 among the mRNA Binding Proteins that specifically target the X1 3'UTR in melanoma cells. Mechanistically, PARP1 Zinc Finger domain down-regulates BRAF expression at the translational level. As a consequence, it exerts a negative impact on MAPK pathway, and sensitizes melanoma cells to BRAF and MEK inhibitors, both in vitro and in vivo. In summary, our study unveils PARP1 as a negative regulator of the highly oncogenic MAPK pathway in melanoma, through the modulation of BRAF-X1 expression.

p38 MAPK-dependent phosphorylation of transcription factor SOX2 promotes an adaptive response to BRAF inhibitors in melanoma cells.

Despite recent advances in the development of BRAF kinase inhibitors (BRAFi) for BRAF-mutant melanomas, development of resistance remains a major clinical problem. In addition to genetic alterations associated with intrinsic resistance, several adaptive response mechanisms are known to be rapidly activated to allow cell survival in response to treatment, limiting efficacy. A better understanding of the mechanisms driving resistance is urgently needed to improve the success of BRAF-targeted therapies and to make therapeutic intervention more durable. In this study, we identify the mitogen-activated protein kinase (MAPK) p38 as a novel mediator of the adaptive response of melanoma cells to BRAF-targeted therapy. Our findings demonstrate that BRAFi leads to an early increase in p38 activation, which promotes phosphorylation of the transcription factor SOX2 at Ser251, enhancing SOX2 stability, nuclear localization, and transcriptional activity. Furthermore, functional studies show that SOX2 depletion increases sensitivity of melanoma cells to BRAFi, whereas overexpression of a phosphomimetic SOX2-S251E mutant is sufficient to drive resistance and desensitize melanoma cells to BRAFi in vitro and in a zebrafish xenograft model. We also found that SOX2 phosphorylation at Ser251 confers resistance to BRAFi by binding to the promoter and increasing transcriptional activation of the ATP-binding cassette drug efflux transporter ABCG2. In summary, we unveil a p38/SOX2-mediated mechanism of adaptive response to BRAFi, which provides prosurvival signals to melanoma cells against the cytotoxic effects of BRAFi prior to acquiring resistance.

Inducible modulation of miR-204 levels in a zebrafish melanoma model.

Here, we present miniCoopR-I, an inducible upgrade of the constitutive miniCoopR vector. We developed miniCoopR-I-sponge-204 and miniCoopR-I-pre-miR-204 vectors and we successfully tested them for their ability to achieve time- (embryo/juvenile/adult) and space- (melanocytic lineage) restricted inhibition/overexpression of miR-204, a positive modulator of pigmentation previously discovered by us. Furthermore, melanoma-free survival curves performed on induced fish at the adult stage indicate that miR-204 overexpression accelerates the development of BRAFV600E-driven melanoma. miniCoopR-I allows study of the impact that coding and non-coding modulators of pigmentation exert on melanomagenesis in adult zebrafish, uncoupling it from the impact that they exert on melanogenesis during embryonic development.This article has an associated First Person interview with the first author of the paper.

Characterization of paralogous uncx transcription factor encoding genes in zebrafish.

The paired-type homeodomain transcription factor Uncx is involved in multiple processes of embryogenesis in vertebrates. Reasoning that zebrafish genes uncx4.1 and uncx are orthologs of mouse Uncx, we studied their genomic environment and developmental expression. Evolutionary analyses indicate the zebrafish uncx genes as being paralogs deriving from teleost-specific whole-genome duplication. Whole-mount in situ mRNA hybridization of uncx transcripts in zebrafish embryos reveals novel expression domains, confirms those previously known, and suggests sub-functionalization of paralogs. Using genetic mutants and pharmacological inhibitors, we investigate the role of signaling pathways on the expression of zebrafish uncx genes in developing somites. In identifying putative functional role(s) of zebrafish uncx genes, we hypothesized that they encode transcription factors that coordinate growth and innervation of somitic muscles.

Characterization of paralogous uncx transcription factor encoding genes in zebrafish.

The paired-type homeodomain transcription factor Uncx is involved in multiple processes of embryogenesis in vertebrates. Reasoning that zebrafish genes uncx4.1 and uncx are orthologs of mouse Uncx, we studied their genomic environment and developmental expression. Evolutionary analyses indicate the zebrafish uncx genes as being paralogs deriving from teleost-specific whole-genome duplication. Whole-mount in situ mRNA hybridization of uncx transcripts in zebrafish embryos reveals novel expression domains, confirms those previously known, and suggests sub-functionalization of paralogs. Using genetic mutants and pharmacological inhibitors, we investigate the role of signaling pathways on the expression of zebrafish uncx genes in developing somites. In identifying putative functional role(s) of zebrafish uncx genes, we hypothesized that they encode transcription factors that coordinate growth and innervation of somitic muscles.