Orientation-specific RAG activity in chromosomal loop domain contribute to Tcrd V(D)J recombination during T cell development

T cell antigen receptor d (Tcrd) variable region exons are assembled by RAG-initiated V(D)J recombination. Here, we employ a high throughput method to map hundreds of thousands of RAG-initiated Tcrd D segment (Trdd1 and Trdd2) rearrangements in developing thymocytes. We find that Trdd2 joins directly to Trdv, Trdd1, and Trdj segments, but Trdd1 joining is ordered with joining to Trdd2 a prerequisite for further rearrangement. We also find frequent, previously unappreciated Trdd1 and Trdd2 rearrangements that inactivate Tcrd. Moreover, we find numerous RAG off-targets that are generated via unidirectional RAG tracking across the loop-domain containing Trdd1, Trdd2 and Trdj. Correspondingly, disruption of the upstream domain boundary causes spreading of on- and off-target RAG activity to the proximal Trdv domain. Overall design: RAG-initiatd Tcrd D segment rearrangements in developing thymocytes were generated by deep sequencing using illumine Miseq