Ligase4 is essential for hematopoietic stem cell maintenance

Hematopoietic stem cells (HSCs) maintain the blood system during steady state and in response to stress. As a result, the accumulation of DNA damage has been proposed as a principal factor that contributes to the functional decline in HSC renewal during aging and stress. However, how the HSCs are sustained in the face of increased DNA damage remains unknown. Here, we address this question by studying the role of non-homologous end joining (NHEJ) in HSC. Using a mouse model of a naturally occurring human hypomorphic Lig4 mutation, we show a definitive effect on the steady state of activated HSCs. This population is severely diminished in the Lig4 mutant mice from the loss of selfrenewing HSCs. Our data suggests that defective NHEJ resulting from this Lig4 mutation depletes only the cycling HSCs but not the dormant HSCs, demonstrating that Lig4 is required for maintaining HSC by preserving their cycling pool. Lig4mut/+ mice were intercrossed to generate Lig4mut and littermate wild-type control (WT - Lig4+/+) mice. HSC (CD150+CD48- KSL) were sorted from 6-8 week-old WT control and Lig4mut mice into Trizol. Samples were prepared for microarray analysis after RNA isolation using the Nugen kit (Ovation Pico WTA System V2). There are three biological replicates for each subtype (WT, Lig4mut).