Highly regenerative tissues such as blood must possess effective DNA damage responses (DDR) that balance long-term regeneration with protection from leukemogenesis. Hematopoietic stem cells (HSCs) sustain life-long blood production, yet their response to DNA damage remains largely unexplored. We report that human HSCs exhibit delayed DNA double-strand break rejoining, persistent gammaH2AX foci, and enhanced p53- and ASPP1-dependent apoptosis after gamma-radiation compared to progenitors. p53 inactivation or Bcl-2 overexpression reduced radiation-induced apoptosis and preserved in vivo repopulating HSC function. Despite similar protection from irradiation-induced apoptosis, only Bcl-2-overexpressing HSCs showed higher self-renewal capacity, establishing that intact p53 positively regulates self-renewal independently from apoptosis. The reduced self-renewal of HSCs with inactivated p53 was associated with increased spontaneous gammaH2AX foci in secondary transplants of HSCs. Our data reveal distinct physiological roles of p53 that together ensure optimal HSC function: apoptosis regulation and prevention of gammaH2AX foci accumulation upon HSC self-renewal.

Cancer-associated fibroblasts (CAFs) are highly prominent in breast tumors, but their functional heterogeneity and origin are still largely unresolved. We report that bone marrow (BM)–derived mesenchymal stromal cells (MSCs) are recruited to primary breast tumors and to lung metastases and differentiate to a distinct subpopulation of CAFs. We show that BM-derived CAFs are functionally important for tumor growth and enhance angiogenesis via up-regulation of Clusterin. Using newly generated transgenic mice and adoptive BM transplantations, we demonstrate that BM-derived fibroblasts are a substantial source of CAFs in the tumor microenvironment. Unlike resident CAFs, BM-derived CAFs do not express PDGFRα, and their recruitment resulted in a decrease in the percentage of PDGFRα-expressing CAFs. Strikingly, decrease in PDGFRα in breast cancer patients was associated with worse prognosis, suggesting that BM-derived CAFs may have deleterious effects on survival. Therefore, PDGFRα expression distinguishes two functionally unique CAF populations in breast tumors and metastases and may have important implications for patient stratification and precision therapeutics.

Replicative senescence is an irreversible cell cycle arrest that limits the proliferation of damaged cells and may be an important tumor suppression mechanism in vivo. This process is regulated at critical steps by the tumor suppressor p53. To identify genes that may regulate the senescence process, we performed cDNA microarray analysis of gene expression in senescent, young proliferating, and hTERT-immortalized primary human fibroblasts. The histone methyltransferase (HMTase), EZH2, was specifically downregulated in senescent cells. Activated p53 suppressed EZH2 gene expression through repression of the EZH2 gene promoter. This activity of p53 requires intact p53 transactivation and DNA binding domains. Furthermore, the repression of EZH2 promoter by p53 is dependent on p53 transcriptional target p21 Waf1 inactivating RB/E2F pathways. In addition, the knockdown of EZH2 expression retards cell proliferation and induces G2/M arrest. We suggest that the p53-dependent suppression of EZH2 expression is a novel pathway that contributes to p53-mediated G2/M arrest. EZH2 associated complex possesses HMTase activity and is involved in epigenetic regulation. Activated p53 suppresses EZH2 expression, suggesting a further role for p53 in epigenetic regulation and in the maintenance of genetic stability. Suppression of EZH2 expression in tumors by p53 may lead to novel approaches to control cancer progression.

Myeloproliferative neoplasms (MPNs) are blood cancers characterized by excessive production of mature myeloid cells, which result from the acquisition of somatic driver mutations in hematopoietic stem cells (HSCs). Epidemiologic studies indicate a substantial disease heritability that is among the highest known for cancers 1 . However, only a limited set of genetic risk loci have been identified, and the underlying biological mechanisms leading to MPN acquisition remain unexplained. Here, we conducted a large-scale genome-wide association study (3,797 cases and 1,152,977 controls) to identify 17 MPN risk loci (p < 5.0 × 10 −8 ), seven of which have not been previously reported. We find a shared genetic architecture between MPN risk and several hematopoietic traits spanning distinct lineages, an enrichment for risk variants mapping to accessible chromatin in HSCs, and associations of increased MPN risk with longer leukocyte telomere length and other clonal hematopoietic states, collectively implicating HSC function and self-renewal. Gene mapping identifies modulators of HSC biology and targeted variant-to-function assays suggest likely roles for CHEK2 and GFI1B in altering HSC function to confer disease risk. Overall, we demonstrate the power of human genetic studies to illuminate a previously unappreciated mechanism for inherited MPN risk through modulation of HSC function.