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.