Kazuhiro FUKUMURA

Human pre-mRNA splicing requires the removal of introns with highly variable lengths, from tens to over a million nucleotides. Therefore, mechanisms of intron recognition and splicing are likely not universal. Recently, we reported that splicing in a subset of human short introns with truncated polypyrimidine tracts depends on RBM17 (SPF45), instead of the canonical splicing factor U2 auxiliary factor (U2AF) heterodimer. Here, we demonstrate that SAP30BP, a factor previously implicated in transcriptional control, is an essential splicing cofactor for RBM17. In vitro binding and nuclear magnetic resonance analyses demonstrate that a U2AF-homology motif (UHM) in RBM17 binds directly to a newly identified UHM-ligand motif in SAP30BP. We show that this RBM17-SAP30BP interaction is required to specifically recruit RBM17 to phosphorylated SF3B1 (SF3b155), a U2 small nuclear ribonucleoprotein (U2 snRNP) component in active spliceosomes. We propose a mechanism for splicing in a subset of short introns, in which SAP30BP guides RBM17 in the assembly of active spliceosomes.

BACKGROUND/AIM: Immune checkpoint inhibitors (ICIs) are currently a standard treatment tool for non-small cell lung cancer (NSCLC). RNA-binding motif protein 17 (RBM17), a splicing factor, is frequently over-expressed in NSCLC, but little is known about the role of RBM17 in the efficacy of ICIs for NSCLC. Thus, we investigated the correlation between RBM17 expression and ICI efficacy in NSCLC. PATIENTS AND METHODS: Biopsy or surgical specimens were collected from patients with advanced or recurrent NSCLC who received ICI monotherapy or chemo-immunotherapy in a first-line setting. RBM17 expression was examined using immunohistochemistry. The correlation between the efficacy of ICI monotherapy or chemo-immunotherapy and RBM17 expression was evaluated. RESULTS: Among the 218 cases, 115 (52.8%) cases were positive for RBM17 expression. RBM17 expression was not associated with the objective response rate (ORR) or progression-free survival (PFS) in either of the ICI monotherapy or chemo-immunotherapy groups. However, among those with a low PD-L1 expression level (PD-L1 <50%; n=86), RBM17 expression was significantly associated with a better ORR (p=0.045) and a better PFS (p<0.001) in the ICI monotherapy group, and was significantly associated with a poor ORR in the chemo-immunotherapy group (p=0.041). CONCLUSION: RBM17 might be a useful predictive marker for a higher efficacy of ICI monotherapy in NSCLC patients with a low PD-L1 expression level.

The early splicing complex A occupies at least eighty nucleotides of intron, in which U2AF covers the polypyrimidine tract. SPF45 (RBM17) functionally substitutes for U2AF on a subset of short introns. Since SPF45 expression confers resistance to various anticancer drugs, SPF45-dependent splicing may play a critical role in multidrug resistance.