Ezh2 Mutations Found in the Weaver Overgrowth Syndrome Cause a Partial Loss of H3K27 Histone Methyltransferase Activity

Context Weaver syndrome is characterized by tall stature, advanced bone age, characteristic facies, and variable intellectual disability. It is caused by heterozygous mutations in enhancer of zeste homolog 2 (EZH2), a histone methyltransferase responsible for histone H3 at lysine 27 (H3K27) trimethylation. However, no early truncating mutations have been identified, suggesting that null mutations do not cause Weaver syndrome. Objective To test alternative hypotheses that EZH2 variants found in Weaver syndrome cause either a gain of function or a partial loss of function. Design Exome sequencing was performed in a boy with tall stature, advanced bone age, and mild dysmorphic features. Mutant or wild-type EZH2 protein was expressed in mouse growth plate chondrocytes with or without endogenous EZH2, and enzymatic activity was measured. A mouse model was generated, and histone methylation was assessed in heterozygous and homozygous embryos. Results A de novo missense EZH2 mutation [c.1876G>A (p.Val626Met)] was identified in the proband. When expressed in growth plate chondrocytes, the mutant protein showed decreased histone methyltransferase activity. A mouse model carrying this EZH2 mutation was generated using CRISPR/Cas9. Homozygotes showed perinatal lethality, whereas heterozygotes were viable, fertile, and showed mild overgrowth. Both homozygous and heterozygous embryos showed decreased H3K27 methylation. Conclusion We generated a mouse model with the same mutation as our patient, found that it recapitulates the Weaver overgrowth phenotype, and demonstrated that EZH2 mutations found in Weaver syndrome cause a partial loss of function.