YY1 Control of Mitochondrial-Related Genes does not Account for Regulation of Immunoglobulin Class Switch Recombination

Immunoglobulin class switch recombination (CSR) occurs in activated B cells with increased mitochondrial mass and membrane potential. Transcription factor YY1 is critical for CSR and for formation of the DNA loops involved in this process. We therefore sought to determine if YY1 knockout impacts mitochondrial gene expression and mitochondrial function in murine splenic B cells, providing a potential mechanism for regulating CSR. We identified numerous genes in splenic B cells differentially regulated when cells are induced to undergo CSR. YY1 conditional knockout caused differential expression of 1129 genes, with 59 being mitochondrial-related genes. ChIP-seq analyses showed YY1 directly bound to nearly half of these mitochondrial-related genes. Surprisingly, at the time when YY1 knockout dramatically reduces DNA loop formation and CSR, mitochondrial mass and membrane potential were not significantly impacted, nor was there a significant change in mitochondrial oxygen consumption, extracellular acidification rate, or mitochondrial complex I or IV activities. Our results indicate that YY1 regulates numerous mitochondrial-related genes in splenic B cells, but this does not account for the impact of YY1 on CSR, or long-distance DNA loop formation. Overall design: Follicular B cells were isolated from the spleens of 3 C57Bl/6 yy1 fl/fl mice. For each spleen, 1/3 of the cells were not treated with any reagent (naive cells), 1/3 the cells received mock treatment (mock cells) and 1/3 received TATCRE (tatcre cells). The mock and tatcre samples were then grown in RPMI medium along with LPS+IL4 and 10% FBS for 72 hours. All cells were lysed and RNA was isolated for library preparation. Expression differences between Mock and TATCRE treated cells were determined to understand the role of yy1 in B cell activation.