Repair defects put stem cells in a fix

I f you can't help out during an emergency , it's better to get out of the way and let someone else take over. The same principle could apply to DNA repair; as Zhang et al. reveal, mutating DNA-dependent protein kinase (DNA-PK) blocks several repair pathways, resulting in the loss of hematopoietic stem cells (1). DNA-PK consists of a catalytic sub-unit (DNA-PKcs) and the DNA-binding proteins Ku70/80, which recruit the complex to double-stranded breaks (DSBs). The complex helps repair these breaks by initiating the nonhomologous end-joining (NHEJ) pathway. NHEJ is best known for repairing DSBs caused by ionizing radiation and completing the process of V(D)J recombination in developing lympho-cytes. Mice lacking DNA-PKcs are therefore hypersensitive to radiation and are immunodefi cient. But NHEJ may also repair DSBs generated during DNA replica-tion, alongside another repair pathway called homologous recombination (2). Benjamin Chen from UT Southwestern Medical Center in Dallas, TX, is interested in how DNA-PKcs is regulated by the two kinases ATM and ATR, which phosphorylate DNA-PKcs on a cluster of threonine residues (3, 4). Blocking this phosphorylation event by mutating the threonine residues to alanines inhibits DSB repair and increases cells' sensitivity to UV and ionizing radiation (3, 4). " We wanted to understand the physiological signifi cance of this phosphorylation, " explains Chen. " So we made a knockin mouse expressing this mutant DNA-PKcs. " Apart from being immunodefi cient and radiosensitive, mice completely lacking DNA-PKcs are fairly healthy and live a normal lifespan. Yet mice expressing the non-phosphorylatable DNA-PKcs mutant (DNA-PKcs 3A/3A) failed to thrive and died a few weeks after birth (1). " We were surprised by this phenotype, " Chen admits. Similar to DNA-PKcs–null animals, DNA-PKcs 3A/3A mice had reduced numbers of B and T lymphocytes, indicating that V(D)J recombination was inhibited in the absence of DNA-PKcs phosphorylation. But the knockin mice lacked other blood-cell lin-eages too, due to a loss of hematopoietic stem cells (HSCs) from their bone marrow. Transplanting in wild-type bone marrow restored blood cell development and extended the lives of DNA-PKcs 3A/3A mice. Defects in HSCs fi rst appeared in fetal liver, where blood progenitors normally undergo rapid expansion during embryo-genesis. HSCs from mutant mice failed to proliferate, appearing instead to accumulate DSBs and die by apop-tosis. Blocking cell death by crossing DNA-PKcs 3A/3A animals to p53-null mice rescued hematopoiesis and prolonged the survival of DNA-PKcs 3A/3A mutants. " We also see …