Irradiation Promotes V(D)J Joining and RAG-Dependent Neoplastic Transformation in SCID T-Cell Precursors

Abstract

Defects in the nonhomologous end-joining (NHEJ) pathway of double-stranded DNA break repair severely impair V(D)J joining and selectively predispose mice to the development of lymphoid neoplasia. This connection was first noted in mice with the severe combined immune deficient (SCID) mutation in the DNA-dependent protein kinase (DNA-PK). SCID mice spontaneously develop thymic lymphoma with low incidence and long latency. However, we and others showed that low-dose irradiation of SCID mice dramatically increases the frequency and decreases the latency of thymic lymphomagenesis, but irradiation does not promote the development of other tumors. We have used this model to explore the mechanistic basis by which defects in NHEJ confer selective and profound susceptibility to lymphoid oncogenesis. Here, we show that radiation quantitatively and qualitatively improves V(D)J joining in SCID cells, in the absence of T-cell receptor-mediated cellular selection. Furthermore, we show that the lymphocyte-specific endonuclease encoded by the recombinase-activating genes (RAG-1 and RAG-2) is required for radiation-induced thymic lymphomagenesis in SCID mice. Collectively, these data suggest that irradiation induces a DNA-PK-independent NHEJ pathway that facilitates V(D)J joining, but also promotes oncogenic misjoining of RAG-1/2-induced breaks in SCID T-cell precursors.

ACKNOWLEDGMENTS

We thank David Schatz, Ferenc Livak, and Stephen Meyn for critical reading of the manuscript. We also thank David Schatz and Ferenc Livak for the TCRδ probe, Susanna Lewis for the pWTSJΔ plasmid and advice on the use of extrachromosomal recombination substrate plasmids, Gill Wu for pDR42, Al Singer and Wendy Shores for TCRβ-transgenic mice, and Andrew Paterson for statistical calculations.

C.W. was supported by a RESTRACOMP studentship from the Hospital for Sick Children. J.D. and C.G. hold Scientist Awards from the National Cancer Institute of Canada and the Medical Research Council of Canada, respectively. This work was supported by the National Cancer Institute of Canada (with funds from the Canadian Cancer Society).