To examine the association between level and patterns of baseline intra-tumoural BRAFV600E protein expression and clinical outcome of BRAFV600E melanoma patients treated with selective BRAF inhibitors.,Fifty-eight BRAFV600E metastatic melanoma patients treated with dabrafenib or vemurafenib on clinical trials had pre-treatment tumour BRAFV600E protein expression immunohistochemically (IHC) assessed using the BRAF V600E mutant-specific antibody VE1.,Sections were examined for staining intensity (score 1-3) and percentage of immunoreactive tumour cells, and from this an immunoreactive score (IRS) was derived (intensity × per cent positive/10).,The presence of intra-tumoural heterogeneity for BRAFV600E protein expression was also assessed.,BRAFV600E expression was correlated with RECIST response, time to best response (TTBR), progression-free survival (PFS) and overall survival (OS).,Expression was generally high (median IRS 28 (range 5-30)) and homogeneous (78%).,Expression of mutated protein BRAFV600E as measured by intensity, per cent immunoreactive cells, or IRS did not correlate with RECIST response, TTBR, PFS or OS, including on multivariate analysis.,Heterogeneity of staining was seen in 22% of cases and did not correlate with outcome.,In the current study population, IHC-measured pre-treatment BRAFV600E protein expression does not predict response or outcome to BRAF inhibitor therapy in BRAFV600E metastatic melanoma patients.
We characterized the mutational landscape of melanoma, the form of skin cancer with the highest mortality rate, by sequencing the exomes of 147 melanomas.,Sun-exposed melanomas had markedly more ultraviolet (UV)-like C>T somatic mutations compared to sun-shielded acral, mucosal and uveal melanomas.,Among the newly identified cancer genes was PPP6C, encoding a serine/threonine phosphatase, which harbored mutations that clustered in the active site in 12% of sun-exposed melanomas, exclusively in tumors with mutations in BRAF or NRAS.,Notably, we identified a recurrent UV-signature, an activating mutation in RAC1 in 9.2% of sun-exposed melanomas.,This activating mutation, the third most frequent in our cohort of sun-exposed melanoma after those of BRAF and NRAS, changes Pro29 to serine (RAC1P29S) in the highly conserved switch I domain.,Crystal structures, and biochemical and functional studies of RAC1P29S showed that the alteration releases the conformational restraint conferred by the conserved proline, causes an increased binding of the protein to downstream effectors, and promotes melanocyte proliferation and migration.,These findings raise the possibility that pharmacological inhibition of downstream effectors of RAC1 signaling could be of therapeutic benefit.