Abstract LB-318: A direct in vivo RNAi screen unveils a myosin as potent tumor suppressor in head & neck squamous cell carcinomas
Authors: Schramek Daniel , Sondoel Ataman , Segal Jeremy P. , Heller Evan , Beronja Slobodan , Oristian Daniel , Reva Boris , Fuchs Elaine 
Published: May 2015

Modern genomics is revealing hundreds of alterations associated with cancer. Mining this information for cancer therapies is now predicated on weeding out ‘bystander’ alterations, identifying the ‘driver’ mutations responsible for tumor initiation and/or metastasis, and elucidating how these mutations alter the fundamental molecular pathways governing tissue growth. We, therefore, devise and employ a direct in vivo RNAi methodology in mice that allows us to simultaneously test hundreds of candidate human cancer genes. We focus on head and neck squamous cell carcinomas (HNSCCs), which arise from the epithelial lining of the upper aerodigestive tract, including the oral cavity, tongue, lip, pharynx and larynx. Despite their frequency, aggressive recurrence and poor prognosis (6th most common cancer; 50% mortality) HNSCCs are relatively poorly studied.

Using non-invasive, ultrasound-guided in utero lentiviral-mediated delivery of RNAi, we selectively transduce the single-layered surface ectoderm of living E9.5 mouse embryos where stem cells stably incorporate and propagate the desired genetic alterations into adulthood in a mosaic fashion. To screen for putative driver mutations and to dissect the physiological relevance of epigenetic changes in gene expression that occur in the development of SCC tumor-initiating (stem) cells, we employed various tumor-susceptible mouse backgrounds and identified nine tumor suppressors, seven of which have not been hitherto linked to tumor development. Our top hit, Myh9, encodes the non-muscle myosin-IIa heavy chain (NMHCIIa). On tumor-susceptible backgrounds, epithelial-specific Myh9 RNAi or Myh9 knockout trigger formation of multiple invasive SCCs and even distant lung metastasis. Surprisingly, myosin-IIa’s function is manifested not only in conventional actin-related processes, but also in regulating DNA damage-induced, post-transcriptional p53 activation and p53 nuclear accumulation.

Clinically, we show that ∼20% of human HNSCCs have lost myosin-IIa protein expression, ∼5% harbor evolutionarily conserved domain-specific MYH9 mutations, and that low MYH9 expression in HNSCCs correlates with poor survival (uni-variant Kaplan-Meier analysis).

These findings establish novel SCC suppressors with potential prognostic and therapeutic relevance highlighting the utility of direct in vivo RNAi to integrate cancer genomics and mouse modeling to rapidly discover and validate potent but low penetrance cancer driver mutations. Importantly, our methodology tests function within the native environment and immune system prior to the gross perturbations in tissue architecture that invariably accompanies cancer progression.

Citation Format: Daniel Schramek, Ataman Sondoel, Jeremy P. Segal, Evan Heller, Slobodan Beronja, Daniel Oristian, Boris Reva, Elaine Fuchs. A direct in vivo RNAi screen unveils a myosin as potent tumor suppressor in head & neck squamous cell carcinomas. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr LB-318. doi:10.1158/1538-7445.AM2014-LB-318