Oncogenic RABL6A promotes NF1-associated MPNST progression in vivo

Background Malignant peripheral nerve sheath tumors (MPNSTs) are aggressive sarcomas that display complex molecular and genetic alterations. Powerful tumor suppressors CDKN2A and TP53 are commonly disrupted in these lesions along with NF1 , a gene that encodes a negative regulator of Ras. Many additional factors have been implicated in MPNST pathogenesis. A greater understanding of critical drivers of the disease is needed to guide more informed targeted therapies for patients. RABL6A is a newly identified driver of MPNST cell survival and proliferation whose in vivo role in the disease is unknown. Methods Using CRISPR-Cas9 targeting of Nf1 + Cdkn2a or Nf1 + Tp53 in the sciatic nerve to form de novo MPNSTs, we investigated the biological significance of RABL6A in MPNST development. Molecular evaluation of terminal tumors (western blot, qRT-PCR, immunohistochemistry) yielded several insights. Results Mice lacking Rabl6 displayed slower tumor growth and extended survival relative to wildtype animals in both genetic contexts. YAP oncogenic activity was selectively downregulated in RABL6A-null, Nf1 + Cdkn2a lesions but not in RABL6A-null, Nf1 + Tp53 tumors. Regardless of genetic context, loss of RABL6A caused upregulation of the CDK inhibitor, p27 in tumors. Paradoxically, both models displayed elevated Myc protein expression and Ki67 staining in terminal tumors lacking RABL6A. Conclusions These findings demonstrate RABL6A is required for optimal tumor progression of NF1 mutant MPNSTs in vivo in both Cdkn2a and p53 inactivated settings. However, sustained RABL6A loss may provide selective pressure for molecular alterations, such as Myc upregulation, that ultimately promote an unwanted, hyper-proliferative tumor phenotype akin to drug resistant lesions. Importance of the Study MPNSTs are aggressive, deadly, and challenging to treat tumors due to location around nerves and high mutational burden. Many factors implicated in MPNST genesis have yet to be fully tested for biological significance in disease formation. We establish a critical physiological role for a new oncoprotein, RABL6A, in promoting NF1-associated MPNST progression. We identify novel RABL6A-regulated pathways that likely contribute to tumor growth, specifically YAP and Myc signaling, and found that sustained RABL6A loss eventually yields more proliferative tumors. We liken RABL6A deficient tumors to those being treated with therapies targeting RABL6A effectors, such as CDKs. Therefore, those lesions should provide a powerful platform to uncover key mediators of drug resistance. Our data suggest oncogenic YAP and Myc could be such mediators of resistance. This study provides a novel system to examine one of the most pressing clinical challenges, drug resistant tumor growth and relapse, in cancer therapy. ### Competing Interest Statement The authors have declared no competing interest.