Effects of PARP Inhibitor Therapy on p53-Deficient Hematopoietic Stem and Progenitor Cell Fitness

Poly (ADP-ribose) polymerase (PARP) inhibitors are an important new class of anti-cancer therapies. Therapy-related myeloid neoplasia (tMN) has been reported following PARPi therapy and is associated with adverse outcomes. We have previously shown, in retrospective data, that prior chemotherapy increases the incidence of clonal hematopoiesis (CH), especially in DNA damage response (DDR) pathway genes including TP53, PPM1D, and CHEK2 and is associated with progression to tMN. In particular, patients who receive PARPi therapy are more likely to have CH compared to other therapies or untreated patients. In the IMPACT study of CH in 10,156 cancer patients, exposure to PARPi were more likely to have CH (33%) compared to untreated patients (16%). This was particularly pronounced for DDR gene mutations, with 25% of PARPi treated patients with DDR CH compared to 2% of untreated patients. In multivariate analysis accounting for demographics and exposure to other chemotherapy or radiation therapy, exposure to PARPi conferred an increased risk of DDR CH (OR = 3.6, 95% CI 1.5-8.5, p = 0.004). From these data, we hypothesize that mutations in DDR pathway genes provide a fitness advantage to hematopoietic stem/progenitor cells (HSPCs) following PARPi treatment, leading to clonal hematopoiesis. A major limitation, however of our previous work in retrospective clinical samples, is the inability to completely adjust for the confounding effect of prior exposure to cytotoxic therapy (in particular platinum therapies) and germline BRCA1/2 mutations; both which have been shown or hypothesized to increase the risk of tMN.

To test whether PARPi exposure might provide a fitness advantage to HSPCs independent of prior exposure to other therapies, we first examined the response of CRISPR-gene edited TP53^-/- MOLM13 cells to the PARPi Olaparib and, as a control, Cisplatin. As expected, TP53^-/- cells had increased resistance to both agents, though the response was much more pronounced in Cisplatin-treated cells (Figure 1A,B). Next, we implemented a mouse model of TP53-mutant clonal hematopoiesis, by generating mixed bone marrow chimeras transplanted with a 1:9 ratio of wildtype (CD45.1) to TP53 ^R172H+/- (CD45.2) cells. The "baseline" contribution of TP53 ^R172H+/- (CD45.2) cells to peripheral blood leukocytes 8 weeks after transplantation was determined by flow cytometry. Mice were then randomized into the following three cohorts: 1) Cisplatin (6mg/kg on days 1, 8, and 15); 2) Olaparib (50mg/kg daily for 3 weeks); and 3) vehicle alone. Peripheral blood chimerism was assessed 3, 9, and 12 weeks after initiating treatment. In addition, the contribution of TP53 ^R172H+/- to lineage ^-Sca1 ⁺Kit ⁺ (LSK) cells in the bone marrow was determined. Cisplatin treatment resulted in a significant increase in the contribution of TP53 ^R172H+ to peripheral blood total leukocytes, granulocytes, and bone marrow LSK cells (Figure 1C-E). In contrast, Olaparib treated mice showed no change in CD45 chimerism.

From these results we conclude that p53-deficiency does not confer a strong fitness advantage to mouse HSPCs in response to PARPi treatment. This suggests that the strong association observed between prior PARPi therapy, CH and tMN in clinical cohorts may in part be due to the confounding effects of prior (often heavy) exposure to platinum-based therapy. However, the majority of patients receiving PARPi have germline heterozygous BRCA1/2 mutations that could be contributing to their hematopoietic response to PARPi therapy. Experiments are underway to test this possibility by analyzing mixed bone marrow chimeras carrying heterozygous mutations of both Brca1 and Trp53.

Figure 1

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Figure 1

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