Genomic alterations were found to cause the inactivation of the tumour suppressor Adenomatous polyposis coli (APC) genes in more than 80% of colon cancers. Although there is clear evidence demonstrating the importance of APC in colon cancer pathogenesis, we still lack approaches for direct targeting of tumour suppressors. However, previous studies created a new window to identify a class of genetic vulnerabilities that are associated with DNA loss of a tumour suppressor termed CYCLOPS (Copy-number alterations Yielding Cancer Liabilities Owing to Partial loss). Most notably, SRP19, which is a component of the signal recognition particle complex, has been identified as a CYCLOPS. The primary objective of this study is to evaluate SRP19 as a new therapeutic target and to develop strategies to inhibit SRP19 to treat colon cancer.
Initially we found APCLoss cancers have lower level of the mRNA and protein of SRP19, and siRNAs targeting SRP19 were used, which showed that APCLoss cancers are highly sensitive to further suppression of SRP19. We have validated that with rescue experiments and xenograft mouse model. In this project, we are also aimed at identifying inhibitors of SRP19 activity as a component of SRP complex. As SRP19 is an RNA-binding protein, which was reported to bind the GGAG tetraloop in helix 6 of the 7SL RNA in a sequence specific manner, we could potentially use the RNA sequence containing the SRP19 binding site as an inhibitor. The specific RNA oligos can be conjugated with an E3 ligand to construct an RNA-PROTAC for SRP19 degradation. Besides, we are also testing conditional degron tags (CDTs) by expression of a validated degron tag (dTAG or IKZF3d) at the C-terminal or N-terminal of SRP19 using CRISPR-mediated locus-specific knock-in. What’s more, we are finding small molecule inhibitors targeting SRP19 using DNA-encoded library (DEL) screening. The effect of candidate SRP19 inhibitors will be evaluated in cells and also in animal models.