Abstract A37: Synthetic lethality as a tool to reveal novel vulnerabilities in NF1-associated tumorigenesis
Authors: Williams Kyle B. , Williams Rory L. , Rathe Sue K. , Hawkinson Jon , Largaespada David A. 
Published: December 2017

Neurofibromatosis Type 1 (NF1) is a common (1:2500 births) genetic disorder and cancer predisposition syndrome, caused by mutations in the tumor suppressor gene NF1. One of the known functions of the protein encoded by NF1 (neurofibromin) is that of a negative regulator of Ras pathway signaling, through its Ras-GAP activity. Among other symptoms, individuals with NF1 often develop benign tumors of the peripheral nervous system (called dermal or plexiform neurofibromas), originating from the Schwann cell linage or their precursors. While these benign tumors can cause significant pain and mobility problems, some (~10%) progress further to malignant peripheral nerve sheath tumors (MPNSTs) and are a leading cause of death among NF1 patients. Both plexiform neurofibromas and MPNSTs arise following loss of the remaining WT NF1 allele within the Schwann cell linage (or their precursors). It is believed the majority of NF1-associated MPNSTs originate from preexisting plexiform neurofibromas. Treatment options for the benign plexiform neurofibromas and MPNSTs are extremely limited, mostly relying on surgical resection and broad-spectrum chemotherapy. Finding new molecular targets for therapeutics effective against both benign tumors and MPNSTs are critical for improved patient outcomes and quality of life. The genetic basis of NF1 syndrome make it a top candidate for using synthetic lethal genetic and therapeutic approaches to uncover unique variabilities in NF1 deficient cells.

We have developed a drug discovery pipeline to identify targeted therapeutics for treatment NF1-related neoplasia, including MPNSTs. Using CRISPR/Cas9, we have created an immortalized human Schwann cell line that is deficient for the NF1 gene, which when paired with its isogenic wild-type parent is an outstanding research tool for identifying synthetic lethal interactions. The NF1 deficient cells exhibit increased oncogenic phenotypes, including increased anchorage independent growth under low serum conditions, higher basal levels of Ras-GTP, and a tendency to form tumors in athymic nude mice.

These isogenic cell lines are currently being utilized for a number of synthetic lethal screens to identify therapeutics and targets specific to cells lacking NF1. These include: 1. A large-scale screen (~24,000 compounds from focused libraries) for drugs that preferentially kill the NF1 deficient cells. This could identify new targeted therapies for potential use in the clinic. 2. Synthetic lethal genetic screens using genome-wide RNAi and CRISPR/Cas9 approaches to knockdown/out expression of additional genes.

The design of our system incorporates two key components that we believe provide the best chance of success. First, our studies utilize the correct, tumor specific, human cell type (Schwann cells). Secondly, we have created isogenic pairs of these cells, differing only in their NF1 status, in which to conduct our synthetic lethal screens. NF1 dependent vulnerabilities identified from this study might also have wider reaching implications and uses, as NF1 is often somatically altered in other cancers. The convergence of genome editing technologies with high throughput drug and synthetic lethal genetic screening provide an exciting opportunity for investigation of NF1 and other genetic cancer predisposition syndromes.

Citation Format: Kyle B. Williams, Rory L. Williams, Sue K. Rathe, Jon Hawkinson, David A. Largaespada. Synthetic lethality as a tool to reveal novel vulnerabilities in NF1-associated tumorigenesis [abstract]. In: Proceedings of the AACR Precision Medicine Series: Opportunities and Challenges of Exploiting Synthetic Lethality in Cancer; Jan 4-7, 2017; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2017;16(10 Suppl):Abstract nr A37.