Genetic disruption of the small GTPase RAC1 prevents plexiform neurofibroma formation in mice with neurofibromatosis type 17

Abstract

Neurofibromatosis type 1 (NF1) is a common cancer predisposition syndrome caused by mutations in the <i>NF1</i> tumor suppressor gene. <i>NF1</i> encodes neurofibromin, a GTPase-activating protein for RAS proto-oncogene GTPase (RAS). Plexiform neurofibromas are a hallmark of NF1 and result from loss of heterozygosity of <i>NF1</i> in Schwann cells, leading to constitutively activated p21RAS. Given the inability to target p21RAS directly, here we performed an shRNA library screen of all human kinases and Rho-GTPases in a patient-derived <i>NF1</i>^-/- Schwann cell line to identify novel therapeutic targets to disrupt PN formation and progression. Rho family members, including Rac family small GTPase 1 (RAC1), were identified as candidates. Corroborating these findings, we observed that shRNA-mediated knockdown of RAC1 reduces cell proliferation and phosphorylation of extracellular signal-regulated kinase (ERK) in <i>NF1</i>^-/- Schwann cells. Genetically engineered <i>Nf1^flox/flox;PostnCre</i>⁺ mice, which develop multiple PNs, also exhibited increased RAC1-GTP and phospho-ERK levels compared with <i>Nf1^flox/flox;PostnCre</i>^- littermates. Notably, mice in which both <i>Nf1</i> and <i>Rac1</i> loci were disrupted (<i>Nf1^flox/floxRac1^flox/flox;PostnCre</i>⁺) were completely free of tumors and had normal phospho-ERK activity compared with <i>Nf1^flox/flox</i> ;<i>PostnCre</i>⁺ mice. We conclude that the RAC1-GTPase is a key downstream node of RAS and that genetic disruption of the <i>Rac1</i> allele completely prevents PN tumor formation <i>in vivo</i> in mice.