DNA damage-induced CHK2 activation compromises germline stem cell self-renewal and lineage differentiation

Stem cells in adult tissues are constantly exposed to genotoxic stress and also accumulate DNA damage with age. DNA damage has been proposed to cause stem cell loss and cancer formation. However, it remains a mystery how DNA damage leads to both stem cell loss and cancer formation. In this study, we use germline stem cells (GSCs) in the Drosophila ovary to show that DNA damage retards stem cell self-renewal and lineage differentiation in a CHK2 kinase-dependent manner. Both heatshock-inducible endonuclease I-CreI expression and X-ray irradiation can efficiently introduce double-strand breaks in GSCs and their progeny, resulting in a rapid GSC loss and an accumulation of ill-differentiated GSC progeny. Elimination of CHK2 or its kinase activity can almost fully rescue the GSC loss and the progeny differentiation defect caused by DNA damage induced by I-CreI or X-ray. Surprisingly, checkpoint kinases ATM and ATR have distinct functions from CHK2 in GSCs in response to DNA damage. The reduction in BMP signaling and E-cadherin only makes limited contribution to DNA damage-induced GSC loss. Finally, DNA damage also decreases the expression of the master differentiation factor Bam in a CHK2-dependent manner, which helps explain the GSC progeny differentiation defect. Therefore, this study demonstrates, for the first time in vivo, that CHK2 kinase activation is required for the DNA damage-mediated disruption of adult stem cell self-renewal and lineage differentiation, and might also offer novel insight into how DNA damage causes tissue aging and cancer formation. Our study also demonstrates that inducible I-CreI is a convenient genetic system for studying DNA damage responses in stem cells.