Stress-triggered hematopoietic stem cell proliferation relies on PrimPol-mediated repriming.

Stem cell division is linked to tumorigenesis by yet-elusive mechanisms. The hematopoietic system reacts to stress by triggering hematopoietic stem and progenitor cell (HSPC) proliferation, which can be accompanied by chromosomal breakage in activated hematopoietic stem cells (HSCs). However, whether these lesions persist in their downstream progeny and induce a canonical DNA damage response (DDR) remains unclear. Inducing HSPC proliferation by simulated viral infection, we report that the associated DNA damage is restricted to HSCs and that proliferating HSCs rewire their DDR upon endogenous and clastogen-induced damage. Combining transcriptomics, single-cell and single-molecule assays on murine bone marrow cells, we found accelerated fork progression in stimulated HSPCs, reflecting engagement of PrimPol-dependent repriming, at the expense of replication fork reversal. Ultimately, competitive bone marrow transplantation revealed the requirement of PrimPol for efficient HSC amplification and bone marrow reconstitution. Hence, fine-tuning replication fork plasticity is essential to support stem cell functionality upon proliferation stimuli.

Targeting Chemotherapy to Decondensed H3K27me3-Marked Chromatin of AML Cells Enhances Leukemia Suppression.

Despite treatment with intensive chemotherapy, acute myelogenous leukemia (AML) remains an aggressive malignancy with a dismal outcome in most patients. We found that AML cells exhibit an unusually rapid accumulation of the repressive histone mark H3K27me3 on nascent DNA. In cell lines, primary cells and xenograft mouse models, inhibition of the H3K27 histone methyltransferase EZH2 to decondense the H3K27me3-marked chromatin of AML cells enhanced chromatin accessibility and chemotherapy-induced DNA damage, apoptosis, and leukemia suppression. These effects were further promoted when chromatin decondensation of AML cells was induced upon S-phase entry after release from a transient G1 arrest mediated by CDK4/6 inhibition. In the p53-null KG-1 and THP-1 AML cell lines, EZH2 inhibitor and doxorubicin cotreatment induced transcriptional reprogramming that was, in part, dependent on derepression of H3K27me3-marked gene promoters and led to increased expression of cell death-promoting and growth-inhibitory genes.In conclusion, decondensing H3K27me3-marked chromatin by EZH2 inhibition represents a promising approach to improve the efficacy of DNA-damaging cytotoxic agents in patients with AML. This strategy might allow for a lowering of chemotherapy doses, with a consequent reduction of treatment-related side effects in elderly patients with AML or those with significant comorbidities. SIGNIFICANCE: Pharmacological inhibition of EZH2 renders DNA of AML cells more accessible to cytotoxic agents, facilitating leukemia suppression with reduced doses of chemotherapy.See related commentary by Adema and Colla, p. 359.

Smarcal1 and Zranb3 Protect Replication Forks from Myc-Induced DNA Replication Stress.

The cellular DNA replication stress response functions to stabilize DNA replication forks and inhibits genome instability and tumorigenesis induced by oncogenes. However, the specific proteins required for resolving oncogenic stress remain poorly understood. Here we report that Smarcal1 and Zranb3, closely related replication fork-remodeling proteins, have nonredundant functions in resolving Myc-induced DNA replication stress. In Myc-overexpressing primary cells, significant differences in replication fork stalling, collapse, and DNA damage were detected between cells deficient in Smarcal1 or Zranb3, leading to changes in proliferation and apoptosis. These differences were also reflected in Myc-induced lymphoma development; haploinsufficiency of Smarcal1 resulted in accelerated lymphomagenesis, whereas haploinsufficiency of Zranb3 inhibited lymphoma development. Complete loss of either protein resulted in disparate survival outcomes. Our results reveal that endogenous replication stress from Myc in primary cells requires both alleles of Smarcal1 and Zranb3 and demonstrate the requirement of both proteins to stabilize replication forks upon Myc dysregulation in a nonredundant manner. SIGNIFICANCE: Smarcal1 and Zranb3 are essential, but nonredundant, for responding to DNA replication stress and stabilizing replication forks following Myc overexpression.See related commentary by Sotiriou and Halazonetis, p. 1297.