LNK (SH2B3) Deficiency Ameliorates Hematopoietic Stem Cell Defects in Fanconi Anemi

Fanconi Anemia (FA) is one of the most common inherited bone marrow failure (iBMF) syndromes. Although initially identified over 85 years ago, FA remains a fatal genetic disease. Nineteen FA genes cooperate in a genome stability pathway that is essential for repair of DNA damage and tolerance of replication stress. Cells derived from FA patients are hypersensitive to DNA interstrand crosslink (ICL)-inducing agents, such as Mitomycin C (MMC), and exhibit DNA damage checkpoint and mitosis defects. Mutations in FA genes result in hematopoietic stem cell (HSC) defects, bone marrow failure and cancer predisposition. Importantly, interventions to mitigate HSC defects do not exist, aside from allogeneic bone marrow transplantation (BMT).

HSCs deficient for FancD2, a central component of the FA signaling pathway, are markedly compromised in reconstituting the hematopoietic system in murine BMT models. Remarkably, we found that loss of the adaptor protein Lnk (also called Sh2b3) restores HSC function in FancD2 knockout mice without accelerating neoplastic transformation. LNK negatively regulates HSC self-renewal, in part by attenuating cytokine signaling-activated JAK2 signaling in HSCs (J Clin Invest. 2008;118(8):2832-2844). Fancd2^-/-;Lnk^-/- (DKO) mice exhibit increased phenotypic HSCs in comparison to wildtype (WT) animals, and DKOHSC function is also largely restored to WT levels in serial transplantation assays. Primary DKOHSC and progenitors (HSPCs) are still sensitive to MMC, indicating that LNK does not play an overt role in ICL repair. Instead, Lnk deficiency notably reduces spontaneous DNA damage and genome instability. This is in agreement with recent studies that reveal a requirement for FA proteins in replication stress, which is a separable function from their role in DNA repair (Cell. 2011;145(4):529-542; Cancer Cell. 2012;22(1):106-116). Strikingly, we demonstrated that Lnk deficiency mitigates replication stress by stabilizing stalled replication forks, and that this effect is dependent upon cytokine signaling.

Together, our data demonstrate that Lnk deficiency ameliorates FA HSPC defects by alleviating replication stress associated DNA damage and genome instability. To our knowledge, this is one of the first examples of in vivo genetic suppression of FA-associated HSPC defects. Our work sheds light on mechanisms underlying the origin of bone marrow failure in FA patients and has implications for new therapeutic strategies to treat FA associated bone marrow failure.