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Somatic mutations in the spliceosome gene U2AF1 are common in patients with myelodysplastic syndromes. U2AF1 mutations that code for the most common amino acid substitutions are always heterozygous, and the retained WT allele is expressed, suggesting that mutant hematopoietic cells may require the residual WT allele to be viable. We show that hematopoiesis and RNA splicing in U2af1 heterozygous knockout mice were similar to those in control mice, but that deletion of the WT allele in U2AF1(S34F) heterozygous mutant-expressing hematopoietic cells (i.e., hemizygous mutant) was lethal. These results confirm that U2AF1 mutant hematopoietic cells are dependent on the expression of WT U2AF1 for survival in vivo and that U2AF1 is a haplo-essential cancer gene. Mutant U2AF1(S34F)-expressing cells were also more sensitive to reduced expression of WT U2AF1 than nonmutant cells. Furthermore, mice transplanted with leukemia cells expressing mutant U2AF1 had significantly reduced tumor burden and improved survival after the WT U2af1 allele was deleted compared with when it was not deleted. These results suggest that selectively targeting the WT U2AF1 allele in heterozygous mutant cells could induce cancer cell death and be a therapeutic strategy for patients harboring U2AF1 mutations.

Nonsense-mediated RNA decay (NMD) is recognized as an RNA surveillance pathway that targets aberrant mRNAs with premature translation termination codons (PTC) for degradation, however, its molecular mechanisms and roles in health and disease remain incompletely understood. In this study, we developed a novel reporter system to accurately measure NMD activity in individual cells. A genome-wide CRISPR-Cas9 knockout screen using this reporter system identified novel NMD-promoting factors, including multiple components of the SF3B complex and other U2 spliceosome factors. Interestingly, cells with mutations in the spliceosome genes SF3B1 and U2AF1 , which are commonly found in myelodysplastic syndrome (MDS) and cancers, have overall attenuated NMD activity. Compared with wild-type (WT) cells, SF3B1- and U2AF1-mutant cells were more sensitive to NMD inhibition, a phenotype that is accompanied by elevated DNA replication obstruction, DNA damage, and chromosomal instability. Remarkably, the sensitivity of spliceosome mutant cells to NMD inhibition was rescued by overexpression of RNase H1, which removes R-loops in the genome. Together, these findings shed new light on the functional interplay between NMD and RNA splicing and suggest a novel synthetic lethal strategy for the treatment of MDS and cancers with spliceosome mutations. SIGNIFICANCE: This study has developed a novel NMD reporter system and identified a potential therapeutic approach of targeting the NMD pathway to treat cancer with spliceosome gene mutations., (©2021 American Association for Cancer Research.)

With the goal of elucidating the molecular and cellular mechanisms of chloroprene toxicity, we examined the potential DNA cross-linking of the bifunctional chloroprene metabolite, (1-chloroethenyl)oxirane (CEO). We used denaturing polyacrylamide gel electrophoresis to monitor the possible formation of interstrand cross-links by CEO within synthetic DNA duplexes. Our data suggest interstrand cross-linking at deoxyguanosine residues within 5'-GC and 5'-GGC sites, with the rate of cross-linking depending on pH (pH 5.0 > pH 6.0 > pH 7.0). A comparison of the cross-linking efficiencies of CEO and the structurally similar cross-linkers diepoxybutane (DEB) and epichlorohydrin (ECH) revealed that DEB > CEO > or = ECH. Furthermore, we found that cytotoxicity correlates with cross-linking efficiency, supporting a role for interstrand cross-links in the genotoxicology of chloroprene.