Abstract B010: ELUCIDATING DNA DAMAGE REPAIR SIGNALING AND THERAPEUTIC VULNERABILITIES ACTIVATED BY TRANSPOSASE-DERIVED PGBD5

Abstract

One hallmark of pediatric tumors is their low mutational burden, which contrasts with adult tumors, where cancer-causing mutations often accumulate over decades of DNA replication and environmental exposure. This raises the question of how such mutations arise in young patients. We have proposed that developmental mutators can promote somatic mutations during development which leads to a mosaic cancer predisposition in that tissue, and offers an explanation for the increase in mutation rate beyond that of normal DNA replication. We have previously nominated PiggyBac Transposable Element Derived 5 (PGBD5), an evolutionarily conserved vertebrate domesticated DNA transposase-derived gene, as one such developmental mutator. PGBD5 is expressed in the majority of human solid tumors in children and young adults, including medulloblastomas (MB) and rhabdoid tumors. We have shown that PGBD5 activity induces double strand DNA breaks and complex structural variants in rhabdoid tumor, MB and normal brain tissue. Furthermore, PGBD5 is sufficient to transform human cells and promote oncogenic DNA rearrangements. Mechanistically, PGBD5 activity requires end-joining DNA repair and activates a DNA damage response (DDR) that sensitizes cells to ATR inhibition. However, the key mediators of the PGBD5-activated DDR, including of the ATR signaling pathway or potentially other DDR kinase pathways, are unknown. Here, we investigate PGBD5-dependent signaling in MB cells to define the DDR pathways activated by PGBD5 and to identify synthetic lethal vulnerabilities dependent on DNA repair or secondary adaptive signaling. Using global quantitative phosphoproteomics of PGBD5-expressing and non-expressing cells treated with selective inhibitors of distinct DDR kinases, including ATM, ATR and DNAPK, we identify kinase signaling mediators that are activated by PGBD5-dependent DNA damage. Through parallel phosphoproteomic analysis of PGBD5-expressing versus non-expressing cells, we also identify PGBD5-dependent tumor maintenance signaling pathways, that may create therapeutic vulnerabilities for PGBD5-expressing tumors. These studies build on our recently identification of a synthetic lethal axis between EZH2-PGBD5-ATR and nominated the rational therapeutic combination strategy of EZH2 and ATR inhibition for rhabdoid tumors. By systematically mapping the DDR and tumor maintenance signaling pathways activated by PGBD5, we aim to identify additional synthetic lethal dependencies that can be targeted alone or in combination with ATR or broader DDR inhibition. Given the broad expression of PGBD5 across pediatric and adult tumors, and our recent finding of epigenetic therapy-induced expression, targeting the signaling pathways activated by PGBD5 in combination with ATR or DDR inhibition may define a therapeutic platform applicable across multiple tumor types. Together, these findings uncover mechanistic insight into how PGBD5 contributes to tumor initiation and tumor maintenance, and nominate actionable signaling dependencies for therapeutic development in PGBD5-expressing tumors.