Functional analysis of human EED variants using <i>Drosophila</i>

The Polycomb Repressive Complex 2 is an epigenetic reader/writer that methylates histone H3K27. Rare germline partial loss-of-function (pLoF) variants in core members of the complex (EZH2, EED, and SUZ12) cause overgrowth and intellectual disability syndromes, whereas somatic variants are implicated in cancer. However, up to 1% of the general population will have a rare variant in one of these genes, most of which would be classified as variants of uncertain significance (VUS). Towards screening these VUS for partial LoF alleles that may contribute to disease, here we report functional assays in Drosophila to interrogate Embryonic Ectoderm Development (EED) missense variants. We mimicked the amino acid change(s) of EED variants into its Drosophila ortholog, esc, and tested their function. Known likely benign variants functioned wildtype and known pathogenic variants were LoF. We further demonstrate the utility of this calibrated assay as a scalable approach to assist clinical interpretation of human EED VUS.