Simulating Space Conditions Evokes Different DNA Damage Responses in Immature and Mature Cells of the Human Hematopoietic System

The impact of space radiation and microgravity on DNA damage responses has been discussed controversially, largely due to the variety of model systems engaged. Here, we performed side-by-side analyses of human hematopoietic stem/progenitor cells (HSPC) and peripheral blood lymphocytes (PBL) cultivated in a 2D clinostat to simulate microgravity before, during and after photon and particle irradiation. We demonstrate that simulated microgravity (SMG) accelerates the early phase of non-homologous end joining (NHEJ)-mediated repair of simple, X-ray-induced DNA double-strand breaks (DSBs) in PBL, while repair kinetics in HSPC remained unaltered. Repair acceleration was lost with increasing LET of ion exposures, which increases the complexity of DSBs, precluding NHEJ and requiring end resection for successful repair. Such cell-type specific effect of SMG on DSB repair was dependent on the NF-кB pathway pre-activated in PBL but not HSPC. Already under unperturbed growth conditions HSPC and PBL suffered from SMG-induced replication stress associated with accumulation of single-stranded DNA and DSBs, respectively. We conclude that in PBL, SMG-induced DSBs promote repair of radiation-induced damage in an adaptive-like response. HSPC feature SMG-induced single-stranded DNA and FANCD2 foci, i.e., markers of persistent replication stress and senescence that may contribute to a premature decline of the immune system in space.

p53 isoforms differentially impact on the POLι dependent DNA damage tolerance pathway

The recently discovered p53-dependent DNA damage tolerance (DDT) pathway relies on its biochemical activities in DNA-binding, oligomerization, as well as complex formation with the translesion synthesis (TLS) polymerase iota (POLι). These p53-POLι complexes slow down nascent DNA synthesis for safe, homology-directed bypass of DNA replication barriers. In this study, we demonstrate that the alternative p53-isoforms p53β, p53γ, Δ40p53α, Δ133p53α, and Δ160p53α differentially affect this p53-POLι-dependent DDT pathway originally described for canonical p53α. We show that the C-terminal isoforms p53β and p53γ, comprising a truncated oligomerization domain (OD), bind PCNA. Conversely, N-terminally truncated isoforms have a reduced capacity to engage in this interaction. Regardless of the specific loss of biochemical activities required for this DDT pathway, all alternative isoforms were impaired in promoting POLι recruitment to PCNA in the chromatin and in decelerating DNA replication under conditions of enforced replication stress after Mitomycin C (MMC) treatment. Consistent with this, all alternative p53-isoforms no longer stimulated recombination, i.e., bypass of endogenous replication barriers. Different from the other isoforms, Δ133p53α and Δ160p53α caused a severe DNA replication problem, namely fork stalling even in untreated cells. Co-expression of each alternative p53-isoform together with p53α exacerbated the DDT pathway defects, unveiling impaired POLι recruitment and replication deceleration already under unperturbed conditions. Such an inhibitory effect on p53α was particularly pronounced in cells co-expressing Δ133p53α or Δ160p53α. Notably, this effect became evident after the expression of the isoforms in tumor cells, as well as after the knockdown of endogenous isoforms in human hematopoietic stem and progenitor cells. In summary, mimicking the situation found to be associated with many cancer types and stem cells, i.e., co-expression of alternative p53-isoforms with p53α, carved out interference with p53α functions in the p53-POLι-dependent DDT pathway.

Sex-specific differences in DNA double-strand break repair of cycling human lymphocytes during aging

The gender gap in life expectancy and cancer incidence suggests differences in the aging process between the sexes. Genomic instability has been recognized as a key factor in aging, but little is known about sex-specific differences. Therefore, we analyzed DNA double-strand break (DSB) repair in cycling human peripheral blood lymphocytes (PBL) from male and female donors of different age. Reporter-based DSB repair analyses revealed differential regulation of pathway usage in PBL from male and female donors with age: Non-homologous end joining (NHEJ) was inversely regulated in men and women; the activity of pathways requiring end processing and strand annealing steps such as microhomology-mediated end joining (MMEJ) declined with age in women but not in men. Screening candidate proteins identified the NHEJ protein KU70 as well as the end resection regulatory factors ATM and BLM showing reduced expression during aging in women. Consistently, the regulatory factor BLM contributed to the MMEJ proficiency in young but not in old women as demonstrated by knockdown analysis. In conclusion, we show that DSB repair is subject to changes upon aging and age-related changes in DSB repair are distinct in men and women.