The role of HPF1 in radiation and genotoxic cancer therapy

PROJECT SUMMARY/ABSTRACT Cancer radiation therapy generates reactive oxygen species (ROS), which cause both single and double-strand DNA breaks that rapidly recruit and activate Poly ADP-ribose polymerase 1 and 2 (PARP1&2). Dual-specificity inhibitors for PARP1 and 2 (PARPi) are promising treatments for BRCA1 or 2-deficient cancers, alone or in combination with radiation. Progressive fatigue, a common side effect of radiation therapy, is often associated with anemia. Severe anemia and the related therapy-induced MDS/AML are also the most common dose-limiting toxicity of PARPi, causing the FDA to withdraw two PARPi from maintenance therapy in 2022. But why radiation and PARPi preferentially target erythropoiesis than other genotoxic therapies (e.g., cisplatin or nucleoside analogs) remains elusive. A major clinical challenge for combination therapy is minimizing bone marrow toxicity. Active PARP1&2 uses NAD+ as a substrate to covalent link ADP-ribose (ADPr) to themselves and other proteins to generate Poly-ADPr (PAR) chains. While all negatively charged, PAR can come in many different flavors. PARP1 and 2 can ribosylate substrate proteins on different residues - Serine, Glutamine, and Aspartate. PAR chains can be short (a few ADP-riboses) or long, straight, or branched. Histone PARylation Factor 1 (HPF1) is a newly discovered PARP1&2 regulator. HPF1 is essential for initiating serine PARylation by PARP1&2 and suppresses PAR chain elongation. Alteration in the PAR chain structure and organization could affect some but not other PAR-binding proteins (readers), leading to different biological impacts. In preliminary studies, we showed that 1) PARPi-induced acute hematological toxicity requires the presence of PARP1 protein; 2) HPF1 is critical for both PARP1 and PARP2 auto-PARylation during normal replication and in DNA damage response; 3) Yet, HPF1 is dispensable for hematopoiesis and murine development. 4) Hpf1/Parp1 double knockout (DKO) mice have no measurable development defects. 5) But, Hpf1/Parp2 DKO mice with Parp1 expression died embryonically with severe anemia. Based on these and other data, we hypothesize that HPF1 might be a promising target for cancer therapy by balancing DNA damage response vs. normal hematological function. We further hypothesize that HPF1 cooperates with PARP2 to regulate PARP1 activation and dynamics during normal hematopoiesis with implications in radiation and PARPi-induced bone marrow toxicity. To test this, we propose to determine 1) how Hpf1 and Parp2 collaborate to regulate Parp1 dynamics at damage sites; 2) how HPF1 regulates the substrate and PAR chain structural specificity upon DNA damage; 3) how HPF1 deletion or inactivation (E284A) causes hematopoietic failure by increasing PARP1 protein accumulation. The results will uncover previously unrecognized structural functions of Hpf1 in radiation and PARPi-induced hematological toxicity, address the substrate and PAR chain specificity of PARP1 activation during normal replication, radiation, and cancer therapy, and eventually provide the strategy to mitigate the hematological toxicity.