Ade novomissense variant inEZH1associated with developmental delay exhibits functional deficits inDrosophila melanogaster

AbstractEZH1(Enhancer of Zeste, homolog 1), a Polycomb Repressive Complex-2 (PRC2) component, is involved in a myriad of cellular processes through modifying histone 3 lysine27 (H3K27) residues.EZH1represses transcription of downstream target genes through H3K27 trimethylation (H3K27me3). Genetic mutations in histone modifiers have been associated with developmental disorders, whileEZH1has not yet been linked to any human disease. However, the paralogEZH2is associated with Weaver syndrome. Here we report a previously undiagnosed individual with a novel neurodevelopmental phenotype identified to have ade novovariant inEZH1, p.Ala678Gly, through exome sequencing. The individual presented in infancy with neurodevelopmental delay and hypotonia and was later noted to have proximal muscle weakness. The variant, p.A678G, is in the SET domain, known for its methyltransferase activity, and was the best candidate variant found in the exome. HumanEZH1/2are homologous to flyEnhancer of zeste E(z), an essential gene in flies, and the residue (A678 in humans, A691 inDrosophila) is conserved. To further study this variant, we obtainedDrosophilanull alleles and generated transgenic flies expressing wild-type(E(z)WT)and the variant(E(z)A691G). TheE(z)A691Gvariant led to hyper H3K27me3 while theE(z)WTdid not, suggesting this is as a gain-of-function allele. When expressed under the tubulin promotorin vivothe variant rescued null-lethality similar to wild-type but theE(z)A691Gflies exhibit bang sensitivity and shortened lifespan. In conclusion, here we present a novelEZH1 de novovariant associated with a neurodevelopmental disorder. Furthermore, we found that this variant has a functional impact inDrosophila. Biochemically this allele leads to increased H3K27me3 suggesting gain-of-function, but when expressed in adult flies theE(z)A691Ghas some characteristics of partial loss-of-function which may suggest it is a more complex allelein vivo.