Rac1-mediated DNA damage and inflammation promote Nf2 tumorigenesis but also limit cell cycle progression by inducing p53 checkpoint and cellular senescence

Neurofibromatosis type 2 (NF2) is caused by mutations of the tumor suppressor MERLIN/NF2. Prior studies established Yap as the driver of proliferation and tumorigenesis upon Nf2 inactivation in a well defined, genetically engineered murine liver model. Here, we report that in this model system Nf2 tumorigenesis also involves DNA damage and inflammation via Rac1-mediated production of ROS. Ablation of Rac1 blocks Nf2 tumorigenesis in spite of hyperactivation of the cyclinD1-pRb-E2F1 pathway and profound increase in liver size associated with the loss of Rac1-dependent p53 checkpoint and senescence programs. Surprisingly, Erk, Akt and Stat3 signaling does not correlate with proliferation or tumorigenesis despite being activated in Nf2 deficient livers, indicating a lesser role of these pathways for Nf2 tumorigenesis. Because a senescence gene signature is associated with benign NF2 tumors but not with malignant NF2 mutant mesotheliomas, we conclude that senescence may underlie the benign nature of NF2. A total of 15 independent liver samples from 5 genetic cohorts of 3-month-old mice were included in this analysis. Three independent wild-type samples were used as controls and one was replicated so as to include a reference sample across two independent plates. Within each genetic cohort, three independent mouse samples were tested in order to achieve statistical significance from the analysis. The Alb-cre;Nf2flox/flox genetic cohort presents with frank tumors on the liver, which were separated from the tumor associated region upon dissection and separately included for analysis, denoted as Nf2KO_TA. All other samples were isolated from representative liver tissue chunks.