DNA damage responses from light chain recombination integrate with B cell developmental programs 2141

Description In early B cell development, pre-B cells undergo rapid clonal expansion followed by cell cycle arrest and recombination of the immunoglobulin light chain (Igl) genes. Igl rearrangement generates programmed DNA double-stranded breaks (DSBs) by the RAG endonuclease. RAG DSBs trigger canonical DNA damage responses (DDRs) for repair and unique non-canonical DDRs (ncDDRs) that integrate with developmental signaling. We have identified that ncDDRs activate SPIC/BCLAF1, a transcription factor complex, which suppresses the pro-proliferative kinase SYK. Using novel SYK- and SPIC-reporter mice, we find dynamic changes in SYK expression associated with ncDDR activation in small pre-B cells in vivo. We show DDRs are induced in SYK-high small pre-B cells with initiation of Igl rearrangement and SYK is subsequently suppressed in conjunction with resolution of DDRs. Additionally, we find increased BCLAF1 and concomitantly reduced PU.1 binding genome-wide and at the Syk promoter in pre-B cells with high ncDDR signaling. Single-cell RNA sequencing reveals BCLAF1 loss results in increased pre-B cell proliferation and disrupted developmental progression. Currently, we are using VDJ-sequencing to define the impact of BCLAF1’s role in maintaining cell cycle arrest and developmental integrity on the rate of successful Igl rearrangement and the genetic diversity of mature B cell receptors. These findings establish critical and novel roles for DNA damage responses in B cell development.