Understanding and targeting unique vulnerabilities governing G1/S transition and replication stress in small cell lung cancer - Project Summary/Abstract: Small cell lung cancer (SCLC) is a recalcitrant cancer that will kill more than 30,000 Americans this year. Annual incidence is nearly equal to mortality rates, and even patients with earlier stage disease infrequently live for two years beyond their diagnosis. There is currently a desperate need for novel therapies for SCLC. Any condition leading to elevated levels of DNA damage will result in replication stress which in turn is a source of genomic instability and increased vulnerability to specific agents including ATR inhibitors and DNA damaging agents. SCLC is characterized by high levels of replication stress (RS) although the exact mechanism causing the increased RS in SCLC is poorly understood. Interestingly, I have demonstrated that low LMNA (a nuclear structural protein) is highly correlated with increased RS, and that SCLC cells have very low levels of LMNA. I have demonstrated that low LMNA leads to increased chromatin accessibility and increased R-loops (RNA DNA hybrids) due to promoter proximal pausing of RNA Pol-II. R-loops are highly associated with RS, and replication fork collisions with R-loops can even cause DNA damage. The first aim of this grant is to fundamentally understand whether low LMNA causes increased RS in SCLC, and if so to determine if the mechanism R-loop dependent. Secondarily, during the R00 portion of this work, I plan to target R-loops and LMNA utilizing R-loops stabilizing agents and LMNA inhibitors to augment ATR/DNA damage based therapeutic regiments we have demonstrated have greater efficacy in SCLC patients with increased RS. SCLC cancers are characterized as having strong neuroendocrine (NE) differentiation, and a high prevalence (~80%) of RB (tumor suppressor which suppresses G1/S transition) mutation in patient tumors. I have demonstrated that SCLC cells compared to other cancer types are highly sensitive to loss of G1/S transition facilitating targets (G1ST) including CDK2, E2F, Cyclin E and SKP2. This is unsurprising as many of these targets have been found to be synthetic lethal with RB mutation. However, I have demonstrated that RB proficient SCLC cells (no RB1/2 mutation or copy number loss) are still more sensitive to loss of G1ST, and this increased dependency is strongly correlated with NE differentiation. My second aim is to determine whether the increased dependence of SCLC on these G1ST is caused by RB mutation, NE differentiation, or a combination of both factors, and to determine the mechanistic underpinning of this interaction. During the R00 portion of this work I will target G1ST to augment standard of care regimens in SCLC. This work will provide deliverables as follows: 1) determine the mechanistic underpinning of increased RS in SCLC 2) a novel therapeutic approach targeting RS, LMNA and R-loops in SCLC 3) determine mechanisms behind increased SCLC on G1ST 4) a novel therapeutic approach targeting G1ST in SCLC.
