Artemis deficiency confers a DNA double-strand break repair defect and Artemis phosphorylation status is altered by DNA damage and cell cycle progression

Abstract

Mutations in the Artemis gene are causative in a subset of human severe combined immunodeficiencies (SCIDs) and Artemis-deficient cells exhibit radiation sensitivity and defective V(D)J recombination, implicating Artemis function in non-homologous end joining (NHEJ). Here we show that Artemis-deficient cells from Athabascan-speaking Native American SCID patients (SCIDA) display significantly elevated sensitivity to ionizing radiation (IR) but only a very subtle defect in DNA double-strand (DSB) break repair in contrast to the severe DSB repair defect of NHEJ-deficient cells. Primary human SCIDA fibroblasts accumulate and exhibit persistent arrest at both the G1/S and G2/M boundaries in response to IR, consistent with the presence of persistent DNA damage. Artemis protein is phosphorylated in a PI3-like kinase-dependent manner after either IR or a number of other DNA damaging treatments including etoposide, but SCIDA cells are not hypersensitive to treatment with etoposide. Inhibitor studies with various DNA damaging agents establish multiple phosphorylation states and suggest multiple kinases function in Artemis phosphorylation. We observe that Artemis phosphorylation occurs rapidly after irradiation like that of histone H2AX. However, unlike H2AX, Artemis de-phosphorylation is uncoupled from overall DNA repair and correlates instead with cell cycle progression to or through mitosis. Our results implicate a direct and non-redundant function of Artemis in the repair of a small subset of DNA double-strand breaks, possibly those with hairpin termini, which may account for the pronounced radiation sensitivity observed in Artemis-deficient cells.