A novel radiosensitive SCID patient with a pronounced G 2/M sensitivity

Abstract

V(D)J rearrangement in lymphoid cells involves repair of double-strand breaks (DSBs) through non-homologous end joining (NHEJ). Defects in this process lead to increased radiosensitivity and severe combined immunodeficiency (RS-SCID). Here, a SCID patient, M3, is described with a T −B +NK + phenotype but without causative mutations in CD3δ, ɛ, ζ or IL7Rα, genes specifically involved in T cell development. Clonogenic survival of M3 fibroblasts showed an increased sensitivity to the DSB-inducing agents ionizing radiation and bleomycin, as well as the crosslinking compound, mitomycin C. We did not observe inactivating mutations in known NHEJ genes and results of various DSB-repair assays in G 1 M3 cells were indistinguishable from those obtained with normal cells. However, we found increased chromosomal radiosensitivity at the G 2 phase of the cell cycle. Checkpoint analysis indicated functional G 1/S and intra-S checkpoints after irradiation but impaired activation of the “early” G 2/M checkpoint. Together these results indicate a novel class of RS-SCID patients characterized by the specific absence of T lymphocytes and associated with defects in G 2-specific DSB repair. The pronounced G 2/M radiosensitivity of the RS-SCID patient described here, suggests a defect in a putative novel and uncharacterized factor involved in cellular DNA damage responses and T cell development.