Characterization of an immunodeficiency-associated EZH2 variant

Area of Science:

• Immunology
• Epigenetics
• Molecular Biology

Background:

• Gene expression regulation is crucial for immune cell development and function.
• Common variable immunodeficiency (CVID) involves recurrent infections and B cell defects.
• EZH2 is the catalytic subunit of Polycomb Repressive Complex 2 (PRC2), an epigenetic gene silencer.

Purpose of the Study:

• To investigate the molecular basis of immunodeficiency in two sisters with a novel EZH2 variant.
• To determine the functional impact of the EZH2 Leu50Ser (L50S) variant on PRC2 activity and histone methylation.
• To explore the implications of EZH2 genetic variation in the general population.

Main Methods:

• Whole exome sequencing to identify genetic variants.
• Analysis of EZH2 protein expression and PRC2 complex formation.
• Assessment of histone H3 lysine 27 trimethylation (H3K27me3) levels at specific genomic regions.
• Biochemical assays using purified recombinant PRC2 to study the L50S variant's effect on nucleosome methylation and allosteric stimulation.

Main Results:

• A heterozygous EZH2 L50S variant was identified in patients with CVID.
• The EZH2-L50S variant did not disrupt overall bulk H3K27me3 but caused deficiencies at specific gene promoters.
• Purified PRC2 with the L50S mutation showed reduced nucleosome methylation activity and impaired allosteric stimulation.
• EZH2-L50S protein is recruited to target sites but exhibits reduced catalytic activity.

Conclusions:

• The EZH2 L50S variant impairs PRC2 function by affecting allosteric regulation, leading to aberrant B cell gene expression.
• This specific EZH2 variation contributes to hypogammaglobulinemia and memory B cell deficiency, causing CVID.
• Low-frequency EZH2 variations in the population may influence B cell development and immune responses, impacting susceptibility to immunodeficiency.