RAG1 and RAG2 mediated DNA double-strand breaks induce chromosomal rearrangements

Aberrant repair of DNA double-strand break (DSB) is thought to be important in the generation of gross chromosomal rearrangements (GCRs); these GCRs are often associated with cellular transformation. The RAG1 and RAG2 proteins are known to initiate V(D)J recombination by producing a DSB between the recombination signal sequence (RSS) and the neighboring DNA. It has been suspected for a long time that inappropriate DNA recombination mediated by RAG1 and RAG2 may induce oncogenic chromosomal alterations in lymphoid cells. To examine if and how DSBs might lead to chromosomal rearrangements, we investigated the consequences of the repair of a single DSB in the human histiocytic lymphoma cell line U937. An RSS recognition site was introduced into chromosome 4 of the recipient cells and the resultant cell line was named P16. This sequence was inserted between a constitutive promoter (EF1α) and the Herpes simplex virus thymidine kinase (HSTK) gene which confers sensitivity to gancyclovir (GCV). Transfection of RAG1, RAG2 and E2A expression vectors into P16 cells caused a single DSB at RSS site in a subset of cells. Clones that had aberrant repair and separated the EF1α promoter from the HSTK gene were resistant to GCV treatment. Southern blotting analysis revealed that mutated clones have various rearranged bands compared to the parental P16 cell line. Further analysis through inverse PCR and sequencing demonstrated the genomic DNA was rearranged at the breakpoints, consistent with the features of GCRs. Analysis of the breakpoint junctions indicated that these rearrangements were due to the large interstitial deletions, ranging from 5 kb to more than 260 kb in size. This model system provides direct evidence that RAG1 and RAG2 can be involved in the process of generation of DNA rearrangement