Characterization of an immunodeficiency-associated EZH2 variant.

Regulation of gene expression is central to the development of immune cells and their ability to respond to infection. As part of a clinical evaluation, we identified two sisters with recurrent infections, hypogammaglobulinemia, and memory B cell deficiency, diagnosed as common variable immunodeficiency. Whole exome sequencing identified a heterozygous variant (Leu50Ser, L50S) in a conserved region of EZH2, the catalytic subunit of the epigenetic gene repressor Polycomb Repressive Complex 2 (PRC2). EZH2-catalyzed histone H3 lysine 27 methylation (H3K27me) in bulk was not overall significantly disrupted by this variant, in patient samples or cell lines expressing EZH2-L50S. EZH2-L50S protein is expressed similar to wild-type and can form PRC2. However, we find that specific genomic regions that normally have high wild-type levels of H3K27me3 are deficient in the L50S context, particularly around gene promoters. EZH2-L50S is still recruited to these sites, but is not as active. Using recombinant purified PRC2, we determine that L50S affects methylation of nucleosomes and disrupts allosteric stimulation that normally amplifies H3K27me3, consistent with the location of L50 in the allosteric regulatory region of PRC2. Thus, variation of EZH2 L50, occurring at low frequency in the population may interfere with normal B cell gene expression patterns, contributing to immunodeficiency. This study has implications for genetic variation in PRC2 in the general population.