Fig. 5. G4 “sequestered” PARP1 is distinct from “trapped” PARP1 and provides insight that loss of S-phase activity underlies PARPi toxicity in BRCA1 deficient cells.

a Model depicting the question: Is PARP1 trapping distinct from G4 sequestering? b Representative WB from two independent experiments to analyze of the expression levels of the indicated proteins from whole cell lysates for the indicated cells lines and knock down reagents. c Representative images and quantification of clonogenic survival assays of the indicated RPE1 cell lines and siRNA treatments with increasing doses of Olaparib from three independent experiments. Dots represents the mean percentage ± SD for a given cell line, knockdown reagent and Olaparib concentration. Significance determined by unpaired t-test (two-tailed, unequal variance) comparing XRCC1 KO NSC to siFANCJ. d Representative images and quantification of clonogenic assays for RPE1 WT and PARP1 KO cells with indicated siRNA, KO cell and Olaparib treatment (1 µM). Mean survival percentages normalized to NSC from 5 independent experiments. Dots represent each individual experiment while the top of the bar indicates the mean percentage ± SD. Significance determined by two-way ANOVA followed by Tukey’s test. e Representative images and quantification of clonogenic assays for the indicated cells. DKO double knockout. Mean percent clonogenic efficiencies (normalized to WT) from three independent experiments. Dots represent each individual experiment while the top of the bar indicates the mean percentage ± SD. Significance determined by one-way ANOVA (unequal variance) followed by Dunnett’s test. f Model of proposed findings: PARPi toxicity in BRCA1 deficient cells derives from an inability of PARP1 to function in S-phase resulting in the accumulation of replication-associated ssDNA lesions and PARP1 trapping. Loss of FANCJ in BRCA1 deficient cells sequesters PARP1 on G4s prohibiting PARP1 from efficiently functioning during replication. Source data are provided as a Source Data file.