Give megs a break: minor DNA stress, platelets in excess Open Access

In this issue of Blood Advances, Camacho et al¹ report that moderate DNA damage enhances megakaryopoiesis and platelet production in mice, providing the first evidence that direct DNA damage can influence megakaryocyte biology.

To induce DNA damage, Camacho et al relied on niraparib, an US Food and Drug Administration–approved poly (ADP-ribose) polymerase (PARP)1/2 inhibitor used to treat ovarian, fallopian tube, and primary peritoneal cancers. Niraparib prevents PARP-dependent recruitment of DNA repair effectors, thereby promoting the accumulation of DNA double-strand breaks.² In their study, the authors began by comparing the effects of high (50 mg/kg) and low (25 mg/kg) intraperitoneal doses of niraparib in mice. After 1 week of treatment, mice that received high doses of niraparib developed severe anemia and thrombocytopenia, an observation that mimics the adverse effects reported in some patients.³ Interestingly, low-dose treatment also led to progressive anemia, but this time resulted in mild thrombocytosis. This result was confirmed using another structurally unrelated PARP1/2 inhibitor and was reversible, with no effect on downstream platelet functions.

To understand how low doses of niraparib increased platelet counts, Camacho et al performed a detailed analysis of the hematopoietic tree in the bone marrow of treated mice using flow cytometry. This analysis revealed a significant expansion in the number of long-term hematopoietic stem cells (HSC), multipotent progenitor 2 (MPP), megakarycotytic progenitors (MkPs), and megakaryocytes. This expansion was accompanied by clear evidence of DNA damage, as shown by comet assays and the accumulation of phosphorylated H2Ax across most analyzed populations. To assess whether these changes reflected PARP1/2-specific effects or represented a general response to DNA damage, the authors exposed cells to low-dose ionizing radiation (<200 cGy). Similar results were obtained, indicating that DNA damage, rather than PARP-specific inhibition, was the underlying cause.

Qualitative shifts in megakaryocyte biology were also evident beyond quantitative changes. Bone marrow megakaryocytes from niraparib-treated mice exhibited higher ploidy levels, particularly ≥32N, which is a hallmark of advanced maturation. In vitro assays also showed that niraparib promoted the differentiation of HSC and progenitor cells into megakaryocytes, once again resulting in higher average ploidy levels. Moreover, short-term exposure of cultured megakaryocytes to low doses of niraparib also increased proplatelet formation, whereas higher doses had the opposite effect. This suggests that DNA damage not only expands the megakaryocyte pool, but also improves their maturation and platelet-producing capacity.

Put together, these results provide evidence that moderate DNA damages induced by low doses of niraparib or by ionizing radiation expand the megakaryocyte lineage at multiple levels of the hematopoietic hierarchy. The resulting megakaryocytes reach higher ploidy states, and ultimately produce more proplatelets. This coordinated set of changes could explain the observed increase in platelet counts in vivo following low-dose niraparib treatment.

The precise mechanism by which moderate DNA damage enhances megakaryopoiesis remains an open question. Previous studies reported that DNA damage can prime HSC toward the megakaryocyte lineage through upregulation of markers such as CD41.⁴ However, Camacho et al did not detect an increase in CD41⁺ HSC, suggesting that bias at the HSC level does not explain their observations. Instead, the effect may involve the MPP2 population, which is expanded following treatment and is known to contain megakaryocyte-primed cells.⁵ Interestingly, low-dose niraparib not only induced mild thrombocytosis but also mild anemia, raising the possibility that DNA damage could skew megakaryocyte-eruthroid cells (MEP) lineage output toward megakaryocytes at the expense of erythropoiesis, and thereby contributing to the increased megakaryocyte numbers observed in the bone marrow.

At the cellular level, PARP inhibition has been shown to induce replication stress, S-phase stalling, and G2 delay, ultimately resulting in mitotic arrest.⁶ These effects could promote megakaryocyte polyploidization, albeit through pathways that are distinct from the canonical endomitotic program of incomplete cytokinesis and karyokinesis. Moreover, PARPs exert functions beyond DNA repair. For example, PARPs regulate centrosome copy number, and centrosome defects can independently trigger mitotic failure and endomitosis. Interestingly, work from the same group demonstrated that centrosome clustering precedes proplatelet formation. This raises the possibility that PARP inhibition might also influence the final stages of platelet production.⁷ Additional studies will therefore be required to clarify the mechanistic relationship between PARP inhibition, DNA damage, and enhanced megakaryopoiesis.

By positioning DNA damage as a regulator of megakaryopoiesis, the study by Camacho et al opens new avenues for both basic and translational research. This connection may be relevant in pathological contexts where DNA damage accumulation has been observed, such as in myeloproliferative neoplasms, which are often associated with thrombocytosis.⁸ Conversely, the ability of moderate DNA damage to expand and mature megakaryocytes raises the intriguing possibility of harnessing this biology to transiently boost platelet production in settings of acute platelet consumption, such as immune thrombocytopenia. However, this approach will require careful titration of DNA damage and rigorous evaluation of long-term risks.

Beyond therapy, the findings of Camacho et al could also have significant implications for transfusion medicine. Despite years of progress, current in vitro platelet production platforms, which combine bioreactors with small molecules such as rho-associated protein kinase inhibitors and AhR antagonists, still yield far fewer platelets than the theoretical 1000 to 3000 per megakaryocyte. This restricts their clinical utility and makes large-scale production prohibitively expensive.^9,10 The report that DNA damage promotes both megakaryocyte maturation and platelet release suggests a new strategy to improve yields. Importantly, as platelets are anucleate and cannot transmit DNA mutations, this approach carries fewer safety concerns than might otherwise be anticipated when manipulating genomic integrity of nucleated cell.

Taken together, Camacho et al provide elegant evidence that moderate DNA damage exerts broad and unexpected consequences on the megakaryocyte lineage, from stem cells to progenitors, and ultimately mature megakaryocytes and platelets. By revealing DNA damage to be a novel regulator of megakaryopoiesis, their study paves the way for more–in-depth mechanistic research and translational applications.

Conflict-of-interest disclosure: The authors declare no competing financial interests.