PROJECT SUMMARY
Small cell lung cancer (SCLC) patients have an initial robust response to combinations of DNA damaging
agents (e.g. cisplatin, etoposide, radiotherapy), however, many patients inevitably suffer from relapse and
resistant disease. A clear understanding of these resistance mechanisms remains elusive. Consequently,
there is a critical need to: (1) understand the mechanisms of therapeutic resistance and (2) develop novel
therapeutics.
Poly-(ADP)-ribose polymerase enzymes (PARP) protein levels are upregulated in SCLC relative to other lung
cancers, and initial studies suggest that this upregulation is associated with increased sensitivity of SCLC to
PARP inhibitors (PARPi) in vitro. PARP inhibitors are synthetic lethal with BRCA1/2 mutated homologous
recombination (HR) deficient tumors and restoration of HR by BRCA reversion mutations is a known
mechanism of PARPi and cisplatin resistance. However, as BRCA1/2 mutations are exceedingly rare in
SCLC non-BRCA mechanisms must be operant.
We performed a genome-wide CRISPR knockout screen to identify novel mechanisms of PARPi resistance.
From subsequent functional validation and clinical genomic correlation, we identified deficiency in an F-box
protein coding gene as a putative biomarker of resistance to PARP inhibitors and cisplatin in SCLC that may
be present in up to ~20% of relapse patient tumors. Loss of this F-box protein abrogates the function of its
corresponding SKP1, CUL1, F-box (SCF) E3 ubiquitin ligase complex. By proximity-dependent biotin
identification (BioID) of this F-box protein, we have identified a high confidence interactor with substrate-like
behavior for SCF-mediated ubiquitin-proteasomal degradation that is important for regulation of HR and DNA
repair.
This proposal aims to: (1) determine the mechanism of this specific SCF complex with its substrate to engage
the ubiquitin-proteasome pathway; (2) elucidate the impact of this F-box protein on HR, DNA repair, and
therapeutic sensitivity to PARPi/cisplatin; and (3) identify synthetic lethal interactions with deficiencies in this
F-box protein to provide biologic insight and characterize immediately translatable approaches for relapsed
treatment resistant SCLC.
