Replication Stress Response and CDKN1A Engagement Constrain Fetal Hematopoietic Stem Cell Pool Size in Fanconi Anemia

Bone marrow (BM) failure is the principal source of morbidity and mortality in Fanconi Anemia (FA) patients. Recessively inherited germline mutations in one of 25 genes lead to deficits by in a pathway central to DNA crosslink repair. Functionally, FA proteins protect adult hematopoietic stem cells (HSC) from p53 mediated apoptosis elicited by alkylating agents, a range of experimental inflammatory cues or aldehyde exposure. However, these mechanisms do not seem to account for depleted hematopoietic stem and progenitor cell pools in very young FA patients, or the spontaneous, non-apoptotic and p53-independent fetal HSC deficits observed in murine models. Building on our previous observation of a quantitatively constrained fetal HSC pool in FA mice (Fancd2^-/-), the current experiments reveal the specific developmental timeframe for the onset of stem cell deficits during HSC expansion in the fetal liver (FL). Cell cycle studies using an EdU/BrdU pulse chase protocol reveal delays in S-phase entry and progression at E13.5. Building on the role of FA proteins (FANCM, FANCI and FANCD2) in countering experimental replication stress (RS) in cell line models, we reasoned that rapid rates of proliferation required during expansion in the FL may similarly confer RS on the FA HSC pool. Experiments in E13.5 FL HSC confirmed the predicted increase in single stranded DNA and accumulation of nuclear replication associated protein (pRpa), along with activation of pChk1, a critical cell cycle checkpoint in cells under RS. For comparison, pChk1 in unperturbed adult cells was no different between Fancd2^-/- and WT. The data are also consistent with gains in RAD51 and alkaline comet assays we previously published (Yoon et al., Stem Cell Reports 2016). The cell cycle regulator Cdkn1a (p21) is considered a canonical downstream component of the p53 response in adult FA HSC, but it also performs p53 independent functions in the RS response that coincide with its role in the nucleus. Here, we observed an increase in nuclear localization of p21 in Fancd2^-/- FL HSC. TGF-β is a critical developmental morphogen that regulates p21 activity, and TGF-β inhibitors can partially reverse adult FA HSC function along with suppression of NHEJ mediated DNA repair. To test regulation of p21 in fetal HSC under RS, we first treated WT FL HSC with aphidicolin to experimentally simulate RS and found that SD208, a small molecule TGF-β-R1 inhibitor, completely rescued the p21 nuclear localization. We then went on to demonstrate that pharmacological inhibition of TGF-β signaling also reversed the nuclear p21 translocation in FA FL HSC (under physiological RS) and functionally rescued the primitive myeloid progenitor colony formation (CFU-GEMM) in vitro. Altogether, our data show that HSC deficits in FA first emerge in the fetal liver, where rapid fetal expansion causes RS that elicits pChk1 activation and nuclear p21 translocation, which restrain cell cycle progression and act as principal mechanisms limiting fetal HSC pool size in FA. Our experiments suggest a central and p53-independent role for p21 in fetal FA HSC regulation. Detailed knowledge of the physiological role of FA proteins in fetal phenotype HSC has the potential to lead to new therapies that delay or rescue hematopoietic failure in FA patients.