Role of PTIP in class switch recombination and long-range chromatin interactions at the immunoglobulin heavy chain locus.

How distal transcriptional enhancer sequences interact with proximal promoters is poorly understood within the context of chromatin. In this report, we have used the immunoglobulin heavy chain locus to address the role of the PTIP protein in transcription regulation and class switch recombination in B cells, a process that depends on regulated transcription and DNA recombination via Pax5 and distal 3' enhancer sequences. We first show that PTIP is recruited to a Pax5 binding site to promote histone H3 lysine 4 (H3K4) methylation. Using a CD19-Cre driver strain, we deleted PTIP in mature B cells. Loss of PTIP inhibited class switch recombination by suppressing transcription and histone H3K4 methylation at the germ line transcript promoters. In the absence of PTIP, Pax5 binding to the promoter regions is reduced and long-range chromatin interactions between the distal enhancer at the 3' regulatory region and the germ line transcript promoters are not detected. We propose a model whereby PTIP stabilizes the Pax5 DNA interactions that promote chromatin looping and regulate transcriptional responses needed for class switch recombination.