Reducing DNA Damage Associated with Endogenous Genotoxic Stress in Hematopoietic Stem Cells

Fanconi anemia (FA) is a bone marrow failure (BMF) syndrome that arises from mutations in a network of FA genes essential for DNA interstrand crosslink (ICL) repair and replication stress tolerance. Endogenous formaldehyde is ubiquitous within cells as it is produced as a byproduct during normal cellular metabolism such as histone, RNA and DNA demethylation. Previous studies show that endogenous formaldehyde is a hematopoietic stem cell (HSC) genotoxin and metabolic carcinogen. We previously showed that endogenous formaldehyde is removed by the enzyme alcohol dehydrogenase 5 (ADH5), and Adh5-deficient mice therefore accumulate formaldehyde adducts in DNA that require the FA pathway to repair. Mice lacking Adh5 together with the FA pathway central component Fancd2 exhibit bone marrow failure and early lethality. Adh5 haploinsufficiency combined with Fancd2 deficiency, or Fancd2^-/-Adh5^+/- mice, demonstrates severely compromised HSC activity. However, the contributing factors that cause DNA damage and the protective mechanisms by which cells reduce DNA damage during endogenous genotoxic stress are not fully understood. Here, we show that the genetic inactivation of Lnk/Sh2b3 in Fancd2^-/-Adh5^+/- mice restores HSC function. Adh5 haploinsufficiency exacerbated Fancd2 deficiency, while Lnk deficiency rescued HSC homeostasis and transplantability of Fancd2^-/-Adh5^+/- mice. Moreover, Lnk deficiency increased HSC quiescence and reduced endogenous DNA damage in Fancd2^-/-Adh5^+/- mice. Importantly, Lnk deficiency does not participate in repairing formaldehyde-induced DNA damage, nor does it metabolize formaldehyde or reduce genomic formaldehyde-deoxyguanine adduct in vivo. Rather, Lnk deficiency reduced single-strand DNA (ssDNA) breaks and ssDNA binding protein RPA in HSCs during the cell cycle, thereby suppressing the ATR pathway activation upon endogenous replication stress. Moreover, loss of Lnk increased S-phase progression and fork recovery in Fancd2^-/-Adh5^+/- HSPCs at the stressed replication fork upon HU treatment, thereby preventing replication fork collapse and DNA damage. These findings highlight a new mechanism by which Lnk deficiency mitigates replication stress-induced DNA damage in response to endogenous genotoxic insult from cellular metabolism during HSC regeneration.