RAF inhibitors such as vemurafenib and dabrafenib block BRAF–mediated cell proliferation and achieve meaningful clinical benefit in the vast majority of patients with BRAF^V600E-mutant melanoma. However, some patients do not respond to this regimen, and nearly all progress to therapeutic resistance. We used a pooled RNA interference screen targeting more than 16,500 genes to discover loss-of-function events that could drive resistance to RAF inhibition. The highest ranking gene was NF1, which encodes neurofibromin, a tumor suppressor that inhibits RAS activity. NF1 loss mediates resistance to RAF and mitogen-activated protein kinase (MAPK) kinase kinase (MEK) inhibitors through sustained MAPK pathway activation. However, cells lacking NF1 retained sensitivity to the irreversible RAF inhibitor AZ628 and an ERK inhibitor. NF1 mutations were observed in BRAF–mutant tumor cells that are intrinsically resistant to RAF inhibition and in melanoma tumors obtained from patients exhibiting resistance to vemurafenib, thus showing the clinical potential for NF1-driven resistance to RAF/MEK-targeted therapies.

Significance: This work identifies functional loss of NF1 as a mediator of resistance to RAF inhibitors in BRAF^V600E-mutant cancers. Furthermore, we nominate new therapeutic modalities to treat this mechanism of resistance. Cancer Discov; 3(3); 350–62. ©2012 AACR.

See related commentary by Gibney and Smalley, p. 260

This article is highlighted in the In This Issue feature, p. 239