More than 90% of small cell lung cancers (SCLCs) harbor loss-of-function mutations in the tumor suppressor gene RB1. The canonical function of the RB1 gene product, pRB, is to repress the E2F transcription factor family, but pRB also functions to regulate cellular differentiation in part through its binding to the histone demethylase KDM5A (also known as RBP2 or JARID1A). We show that KDM5A promotes SCLC proliferation and SCLC's neuroendocrine differentiation phenotype in part by sustaining expression of the neuroendocrine transcription factor ASCL1. Mechanistically, we found that KDM5A sustains ASCL1 levels and neuroendocrine differentiation by repressing NOTCH2 and NOTCH target genes. To test the role of KDM5A in SCLC tumorigenesis in vivo, we developed a CRISPR/Cas9-based mouse model of SCLC by delivering an adenovirus (or an adeno-associated virus [AAV]) that expresses Cre recombinase and sgRNAs targeting Rb1, Tp53, and Rbl2 into the lungs of Lox-Stop-Lox Cas9 mice. Coinclusion of a KDM5A sgRNA decreased SCLC tumorigenesis and metastasis, and the SCLCs that formed despite the absence of KDM5A had higher NOTCH activity compared to KDM5A +/+ SCLCs. This work establishes a role for KDM5A in SCLC tumorigenesis and suggests that KDM5 inhibitors should be explored as treatments for SCLC.

Most intracellular proteins lack hydrophobic pockets suitable for altering their function with drug-like small molecules. Recent studies indicate that some undruggable proteins can be targeted by compounds that can degrade them. For example, thalidomide-like drugs (IMiDs) degrade the critical multiple myeloma transcription factors IKZF1 and IKZF3 by recruiting them to the cereblon E3 ubiquitin ligase. Current loss of signal (“down”) assays for identifying degraders often exhibit poor signal-to-noise ratios, narrow dynamic ranges, and false positives from compounds that nonspecifically suppress transcription or translation. Here, we describe a gain of signal (“up”) assay for degraders. In arrayed chemical screens, we identified novel IMiD-like IKZF1 degraders and Spautin-1, which, unlike the IMiDs, degrades IKZF1 in a cereblon-independent manner. In a pooled CRISPR-Cas9–based screen, we found that CDK2 regulates the abundance of the ASCL1 oncogenic transcription factor. This methodology should facilitate the identification of drugs that directly or indirectly degrade undruggable proteins.