Abstract
The anticancer sulfonamide indisulam inhibits protumorigenic pre-mRNA splicing.
Major finding: The anticancer sulfonamide indisulam inhibits protumorigenic pre-mRNA splicing.
Mechanism: Indisulam drives the recruitment of RBM39 to the CUL4–DCAF15 ubiquitin ligase for degradation.
Impact: The drug class of splicing inhibitor sulfonamides may be potent against tumors that overexpress DCAF15.
Mutations in canonical splicing factors have been identified in cancers, most frequently in hematologic and lymphoid cancers, but it has been difficult to identify small-molecule inhibitors targeting cancer-related splicing factors. The aryl sulfonamide indisulam, which was shown to exhibit anticancer activity in a screen of human cancer cell lines, is known as a potent inhibitor of carbonic anhydrase (CA). To identify mutations that lead to indisulam resistance, Han and colleagues treated HCT116 colon cancer cells, which are mismatch repair deficient, with indisulam or chemically distinct CA inhibitors and showed that only indisulam inhibited HCT116 proliferation, suggesting that the anticancer activity of indisulam is mediated by a CA-independent mechanism. Indisulam-resistant cells harbored mutations in the second RNA recognition motif (RRM2) of the pre-mRNA splicing factor RNA binding motif 39 (RBM39), and expression of RBM39 RRM2 mutations conferred indisulam resistance to cultured HCT-116 cells and xenograft tumors derived from HCT-116 cells in vivo. Indisulam treatment of HCT116 cells resulted in the proteasomal degradation of RBM39, which caused splicing defects in a subset of genes, leading to cell death, while mutations in the RRM2 domain of RBM39 conferred indisulam resistance. Mechanistically, indisulam triggers the ubiquitination of RBM39 by recruiting it to the E3 ubiquitin ligase receptor DCAF15. In a cell-free system, RBM39 and DCAF15 formed a complex in the presence of indisulam, while RBM39 harboring RRM2 mutations did not associate with DCAF15. Further, DCAF15 expression and copy-number levels correlated with indisulam sensitivity in hematopoietic and lymphoid cancer cell lines. Together, these results identify the mechanism of action underlying the anticancer activity of aryl sulfonamides and suggest that this drug class may be a potent therapy for DCAF15-positive, RBM39 tumors.