Abstract 5602: BRAF V600 mutated allelic specific imbalance (MASI) impact on sensitivity to vemurafenib
Authors: Boespflug Amelie , Thomas Luc , Rousset Pascal , Dumontet Charles , Caramel Julie , Ferraro-Peyret Carole , Bringuier Pierre Paul , Balme Brigitte , Dalle Stephane 
Published: May 2015

Abstract
V600 mutations of BRAF are present in 40% of cutaneous melanoma and are currently targeted for therapy by specific inhibitors (vemurafenib, dabrafenib). While BRAFV600 mutations are mostly heterozygous, the impact of BRAF mutated allelic specific imbalance (MASI) on the sensitivity to BRAF inhibitors and on clinical outcome has not been evaluated so far. Here we compared cell proliferation, tumor growth and vemurafenib sensitivity of heterozygous versus homozygous BRAF V600E mutated melanoma cell lines. We also performed a retrospective study evaluating the impact on overall survival and drug response of BRAF V600E mutated melanoma patients with or without MASI.
Methods:
We conducted proliferation and cytotoxity assays to compare cell growth and sensitivity to vemurafenib and trametinib in heterozygous, versus homozygous BRAF V600 mutated human melanoma cell lines. Their in vivo tumorigenesis and sensitivity to vemurafenib was also evaluated upon engraftment in SCID mice. A retrospective series of 179 patients with metastatic melanoma harboring a BRAF V600 mutations were analyzed and early first line treatment response was compared between patients with or without MASI.
Results:
Our results showed that heterozygous BRAF V600 mutated melanoma cell lines proliferate less rapidly than homozygous cell lines in vitro (cell proliferation counts after 7 days range from 3.6 x106 to 7.7x106 cells for homozygote cells, and from 4.8x104 to 5.5x105 for heterozygous cell lines) and in vivo (palpable tumors appeared on average 7 days after inoculation for homozygous cells but after 39 days for heterozygous cells). Heterozygous cell lines are more resistant to BRAF inhibitors either in vitro or in vivo (IC50 of vemurafenib ranged from 0,1 to 0,198 µM for homozygous cells and from 8,62 to 22,65 µM for heterozygous cell lines). We have also shown that patients with BRAF V600 mutated tumors with MASI may have initially a poorer prognosis (median overall survival 733 days IC95% [381; 1762] versus 1011 days IC95%[879;2969] p=0.236) but tend to have better responses to systemic drugs than patients without MASI (percentage of primary resistance 23% versus 34%; p=0.56).
In conclusion:
This translational study shows that a MASI of BRAF in melanoma is associated with a higher cell proliferation and tumorigenic potential in vitro and in vivo. This mutational status is also associated with a better response to BRAF inhibitor in vitro and in mice xenografts. Clinical relevance of these data should be more thoroughly evaluated on larger and more homogeneous series of patients however our preliminary data show a trend toward a clinical-experimental correlation with similar yet still statistically insignificant results. If confirmed, our data suggest that study of the MASI of BRAF mutation could serve as a predictive marker of patient response to vemurafenib. The role of paradoxical activation of the MAPK signaling pathway in cells without MASI of BRAF needs to be further explored.
Citation Format: Amelie Boespflug, Luc Thomas, Pascal Rousset, Charles Dumontet, Julie Caramel, Carole Ferraro-Peyret, Pierre Paul Bringuier, Brigitte Balme, Stephane Dalle. BRAF V600 mutated allelic specific imbalance (MASI) impact on sensitivity to vemurafenib. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5602. doi:10.1158/1538-7445.AM2014-5602