Epigenetic enhancer marks and transcription factor binding influence Vk gene rearrangement frequencies in pre-B cells and pro-B cells

Deep sequencing has the potential to provide great insight into the composition of the antibody repertoire and is now widely used. To date there has not been a study directly comparing relative rearrangement frequencies obtained with unbiased amplification from genomic DNA (gDNA) and cDNA from the same set of cells to elucidate any differences. Here we use a recently developed method for unbiased amplification of gDNA and directly compare the data to the unbiased RNA IgK repertoire from the same pre-B cell pool. We find that ~20% of Vk genes have relative rearrangement frequencies > 2-fold up or down in the RNA vs DNA libraries, including many members of the Vk4, Vk3 and Vk6 families. Regression analysis indicates that Ikaros and E2A binding are associated with strong promoters. Within the pre-B repertoire, we found that the vastly unequal Vk gene rearrangement frequencies are best predicted by epigenetic marks of enhancers. In particular, the levels of newly arising H3K4me1 peaks associated with many Vk genes in pre-B cells are most predictive of rearrangement levels. Comparison of IgK rearrangements that occur in pro-B cells and pre-B cells from the same mice reveal a pro-B cell bias towards usage of Jk-distal Vk genes, particularly Vk 10-96 and surrounding genes. Regression analysis indicates that PU.1 binding is the highest predictor of high Vk gene rearrangement frequency in pro-B cells. Lastly, repertoires of iEk -/- pre-B cells reveal that the intronic enhancer actively influences Vk gene usage and displays a germline transcriptional sphere of influence extending to Vk 6-15, overlapping with the deficit in Jk proximal Vk gene usage. These represent previously unknown roles for iEk in addition to its major function in stimulating overall IgK rearrangement. Input and immunoprecipitated DNA was given to the Next Generation Sequencing Core (TSRI, La Jolla, CA) where it was prepared for massively parallel sequencing on Illumina HiSeq2000.