Checkpoint kinase 1 is essential for fetal haematopoiesis and hematopoietic stem cell survival

Checkpoint kinase 1 is critical for S-phase fidelity and preventing premature mitotic entry in the presence of DNA damage. Tumour cells have developed a strong dependence on CHK1 for survival and hence this kinase has developed into a popular drug-target. Chk1-deficiency in mice results in blastocyst death due to G2/M checkpoint-failure showing that it is an essential gene and may be difficult to target therapeutically without side-effects. Here, we show that chemical inhibition of CHK1 kills murine hematopoietic stem and progenitor cells (HSPCs) as well as human CD34+ HSPCs by the induction of BCL2-regulated but p53-independent apoptosis. Moreover, Chk1 is essential for stem cell survival and definite hematopoiesis in the mouse embryo. Remarkably though, cell death inhibition in hematopoietic stem cells (HSC) cannot restore blood cell formation in utero as Chk1 loss causes severe DNA damage that ultimately prevents HSC expansion. Our findings establish a previously unrecognized role for CHK1 in establishing hematopoiesis; they also suggest adverse effects of therapeutic CHK1-inhibtion, particularly under conditions forcing stem cells out of dormancy, such as chemotherapy-induced myelosuppression.