Tumorigenesis Driven by BRAFV600E Requires Secondary Mutations that Overcome it's Feedback Inhibition of RAC1 and Migration.

Abstract

BRAFV600E mutations occur in 46% of melanomas and drive high levels of ERK activity and ERK-dependent proliferation. However, BRAFV600E is insufficient to drive melanoma in genetically engineered mouse models, and 82% of human benign nevi harbor BRAFV600E mutations. We found that BRAFV600E inhibited mesenchymal migration by causing feedback inhibition of RAC1 activity. ERK pathway inhibition induced RAC1 activation and restored migration and invasion. In cells with BRAFV600E, mutant RAC1 or PTEN inactivation restored RAC1 activity and cell motility. Together, these lesions occurred in 26% of melanomas with BRAFV600E mutations. Thus, although BRAFV600E activation of ERK deregulates cell proliferation, it prevents full malignant transformation by causing feedback inhibition of RAC1. Secondary mutations are, therefore, required for tumorigenesis. One mechanism underlying tumor evolution may be the selection of lesions that rescue the deleterious effects of oncogenic drivers.