Resumen de Orientation-specific joining of aid-initiated dna breaks promotes antibody class switching
Junchao Dong, Rohit A. Panchakshari, Tingting Zhang, Yu Zhang, Jiazhi Hu, Sabrina A. Volpi, Robin M. Meyers
During B-cell development, RAG endonuclease cleaves immunoglobulin heavy chain (IgH) V, D, and J gene segments and orchestrates their fusion as deletional events that assemble a V(D)J exon in the same transcriptional orientation as adjacent Cµ constant region exons1,2. In mice, six additional sets of constant region exons (CHs) lie 100–200 kilobases downstream in the same transcriptional orientation as V(D)J and Cµ exons2. Long repetitive switch (S) regions precede Cµ and downstream CHs. In mature B cells, class switch recombination (CSR) generates different antibody classes by replacing Cµ with a downstream CH (ref. 2). Activation-induced cytidine deaminase (AID) initiates CSR by promoting deamination lesions within Sµ and a downstream acceptor S region2,3; these lesions are converted into DNA double-strand breaks (DSBs) by general DNA repair factors3. Productive CSR must occur in a deletional orientation by joining the upstream end of an Sµ DSB to the downstream end of an acceptor S-region DSB. However, the relative frequency of deletional to inversional CSR junctions has not been measured. Thus, whether orientation-specific joining is a programmed mechanistic feature of CSR as it is for V(D)J recombination and, if so, how this is achieved is unknown. To address this question, we adapt high-throughput genome-wide translocation sequencing4 into a highly sensitive DSB end-joining assay and apply it to endogenous AID-initiated S-region DSBs in mouse B cells. We show that CSR is programmed to occur in a productive deletional orientation and does so via an unprecedented mechanism that involves in cis Igh organizational features in combination with frequent S-region DSBs initiated by AID. We further implicate ATM-dependent DSB-response factors in enforcing this mechanism and provide an explanation of why CSR is so reliant on the 53BP1 DSB-response factor.
