Replication Stress Sensing and Signaling Defects in SCLC

PROJECT SUMMARY The underlying mechanisms that account for initial sensitivity to of SCLC to chemotherapy followed by profound therapeutic resistance are just beginning to be elucidated. These mechanisms appear to converge on the ways in which SCLC copes with the replication stress and DNA damage induced by chemotherapy-induced

lesions on DNA. One such mechanism is the epigenetic silencing of SLFN11, a gene implicated as a broad- spectrum sensitizer to replication stress and DNA damage. Interestingly, mutations in hallmark DNA damage response and repair genes are infrequent in SCLC which instead appears to be sensitized to chemotherapy by

SLFN11 expression. The precise cellular responses to replication stress altered by SLFN11 expression are not well-understood and are remarkably understudied, despite SLFN11 expression having been more strongly associated with chemotherapy sensitivity than mutations in DNA repair genes pan-cancer. This study proposes three Specific Aims. Specific Aim 1 focuses on clarifying the functional

requirements of SLFN11 on the early stages of the replication stress response. To do this, we will express mutant SLFN11 proteins in cell lines. With these cell lines in hand, we will be able to separate the chronic effects of SLFN11 expression, which may include altering the steady state levels of many proteins in the cell,

from the acute effects that have been reported to occur at stalled replications forks. In a complementary series of experiments, Specific Aim 2 will focus on specific downstream signaling cascades impinged upon by SLFN11. Specific Aim 3 will specifically interrogate downstream targets of SLFN11 to assess their suitability

as candidate drug targets in cell line and animal models of SCLC. These experiments will be important to determine whether it is possible to develop new strategies to sensitize SCLC to existing therapies that work through DNA damage. These Specific Aims will resolve long-standing questions in the field vis-à-vis to what extent the

translation inhibition or fork poisoning effects of SLFN11 are important in sensitization to replication stress. They will also make it possible to benchmark new therapeutic strategies for their ability to mimic SLFN11 expression and chemosensitize SCLC that lose SLFN11 expression. Together, these studies will significantly

enhance our understanding of SLFN11 as a candidate biomarker of response while also enabling the development of new therapeutic strategies to restore and sustain its expression in SCLC.