R-Loop Accumulation in Spliceosome Mutant Leukemias Confers Sensitivity to PARP1 Inhibition by Triggering Transcription-Replication Conflicts

RNA splicing factor (SF) gene mutations are commonly observed in patients with myeloid malignancies. Here we showed that SRSF2- and U2AF1-mutant leukemias are preferentially sensitive to PARP inhibitors (PARPi), despite being proficient in homologous recombination repair. Instead, SF-mutant leukemias exhibited R-loop accumulation that elicited an R-loop-associated PARP1 response, rendering cells dependent on PARP1 activity for survival. Consequently, PARPi induced DNA damage and cell death in SF-mutant leukemias in an R-loop-dependent manner. PARPi further increased aberrant R-loop levels, causing higher transcription-replication collisions and triggering ATR activation in SF-mutant leukemias. Ultimately, PARPi-induced DNA damage and cell death in SF-mutant leukemias could be enhanced by ATR inhibition. Finally, the level of PARP1 activity at R-loops correlated with PARPi sensitivity, suggesting that R-loop-associated PARP1 activity could be predictive of PARPi sensitivity in patients harboring SF gene mutations. This study highlights the potential of targeting different R-loop response pathways caused by spliceosome gene mutations as a therapeutic strategy for treating cancer.

SIGNIFICANCE

Spliceosome-mutant leukemias accumulate R-loops and require PARP1 to resolve transcription-replication conflicts and genomic instability, providing rationale to repurpose FDA-approved PARP inhibitors for patients carrying spliceosome gene mutations.

Impact of Sclerotinia Stem Rot on Yield of Canola

Sclerotinia sclerotiorum is the causal agent of Sclerotinia stem rot (SSR) of canola (Brassica napus). In North Dakota, the leading canola producer in the United States, SSR is an endemic disease. In order to estimate the impact of this disease on canola yield, field experiments were conducted from 2000 to 2004 at several locations in North Dakota and Minnesota. Experimental plots were either inoculated with laboratory-produced ascospores or infected by naturally occurring inoculum in commercial fields. Applying fungicides at different concentrations and timings during the flowering period created epiphytotics of diverse intensities. Disease incidence was measured once prior to harvesting the crop on 50 to 100 plants per plot. Results of the study indicated that 0.5% of the potential yield (equivalent to 12.75 kg/ha) was lost for every unit percentage of SSR incidence (range of 0.18 to 0.96%). Considering the current cost of fungicide applications and the market value of this commodity, a 17% SSR incidence could cause losses similar to the cost of a fungicide application. Additional efforts are required to improve current levels of tolerance of canola plants to this pathogen.

Efficacy of Fungicides for Control of Sclerotinia Stem Rot of Canola

Sclerotinia stem rot (SSR), incited by Sclerotinia sclerotiorum, causes yield reductions to canola (Brassica napus) grown in North Dakota and Minnesota. Field trials were conducted in North Dakota and Minnesota from 2000 to 2004 to evaluate the effect of foliar fungicides on SSR and canola yield. Levels of SSR varied among years and location. In general, fungicides that consistently reduced SSR incidence compared with an untreated control were azoxystrobin, benomyl, boscalid, iprodione, prothioconazole, tebuconazole, thiophanate-methyl, trifloxystrobin, and vinclozolin. Significant reductions in SSR incidence with fungicides did not always translate into differences in canola yield, however. Inconsistent results were observed with different timings of applications based on percent bloom. This indicates that application timing should be based on factors in addition to percent bloom.