Stem Cell-Specific Mechanisms Ensure Genomic Fidelity within HSCs and upon Aging of HSCs

Abstract

Whether aged hematopoietic stem and progenitor cells (HSPCs) have impaired DNA damage repair is controversial. Using a combination of DNA mutation indicator assays, we observe a 2-3 fold increase in the number of DNA mutations in the hematopoietic system upon aging. Young and aged HSCs and HPCs do not show an increase in mutation upon irradiation-induced DNA damage repair, and young and aged HSPCs respond very similarly to DNA damage with respect to cell cycle checkpoint activation and apoptosis. Both, young and aged HSPCs show impaired activation of the DNA-damage induced G1-S checkpoint. Induction of chronic DNA double strand breaks by zinc-finger nucleases suggest that HSPCs undergo apoptosis rather than faulty repair. These data reveal a protective mechanism in both the young and aged hematopoietic system against accumulation of mutations in response to DNA damage.

Aging of hematopoietic stem cells: DNA damage and mutations?

“…Finally, novel data from our laboratory demonstrated that the quality of the DDR in HSCs does not change upon aging. HSCs, both young and old, entering cell cycle in vivo upon DNA damage, without initiating a strong G1-S cell cycle arrest as reported for fibroblasts (Moehrle et al 2015). They also do not present with a marked difference in apoptosis in vivo upon induction of DSBs.…”

“…In general though, aged murine hematopoietic cells (either BM or PB, and independent of the type of mutation assay) show a 2–3 fold increase in mutation frequency in hematopoiesis compared to young (Dempsey, Pfeiffer, and Morley 1993; Vijg et al 2005; Moehrle et al 2015), which is also in the range of changes in mutation frequency recently reported for human hematopoietic cells, determined via deep-sequencing approaches (Cancer Genome Atlas Research Network 2013; Welch et al 2012; Genovese et al 2014; Jaiswal et al 2014; McKerrell et al 2015; Xie et al 2014; Papaemmanuil et al 2013). Interestingly, while the overall increase in mutation frequencies in blood cells was found to be again in the range of 2–3 fold, a set of genes including DNMT3A, TET2, JAK2, ASXL1, SF3B1 or SRSF2 were frequently mutated, both in aging associated leukemia but also in aging-associated changes in clonality.…”

“…Analyses of young and aged HSCs with respect to changes in the frequency of DNA damage and the frequency of DNA mutations (comet assay, γH2AX foci, DNA mutation frequency, loss of heterozygousity assays) revealed a minor increase in these parameters upon aging in steady state hematopoiesis (Rossi et al 2007; Rübe et al 2011; Beerman et al 2014; Moehrle et al 2015). Elevated levels of γH2AX foci in aged HSCs though are also associated with replication stress as well as ribosomal biogenesis stress and might therefore not be unequivocally associated with DNA damage (Flach et al 2014b).…”

Do DNA Double-Strand Breaks Drive Aging?

“…For example, when quiescent, HSCs are not affected by DSBs; however, when prompted to replicate in response to stress, HSCs from aged mice were found to divide more slowly or die (Flach et al, 2014; Moehrle et al, 2015; Rossi et al, 2007; Walter et al, 2015). Indeed, replication stress has been identified as a potent driver of functional decline of old HSCs (Flach et al, 2014).…”

Aging, Clonality, and Rejuvenation of Hematopoietic Stem Cells

“…Secondly, these conclusions were largely based on γH2AX foci formation as a DNA damage ‘marker’ when γH2AX foci are also associated with stalled replication forks and ribosomal biogenesis [44], bringing into question the idea that aged HSCs have more DNA damage than young HSCs. Finally, Moehrle et al recently demonstrated in vivo that aged HSCs repair double-strand breaks as efficiently as young HSCs without accumulating additional DNA mutations [43], data which further question the impact of aging on hematopoietic genetic instability. Consequently, additional studies are warranted to investigate to what extent DDR and impaired genomic integrity impact HSC aging.…”