Identification of CNTN2 as a genetic modifier of PIGA-CDG in a family with incomplete penetrance and in Drosophila

Loss-of-function mutations in the X chromosome gene PIGA lead to phosphatidylinositol glycan class A congenital disorder of glycosylation (PIGA-CDG), an ultra-rare CDG typically presenting with seizures, hypotonia, and neurodevelopmental delay. We identified two brothers (probands) with PIGA-CDG, presenting with epilepsy and mild developmental delay. Both probands carry PIGA c.395C>G (p.Ser132Cys), an ultra-rare variant predicted to be damaging. Strikingly, the maternal grandfather and a great uncle also carry the same PIGA variant, but neither presents with symptoms associated with PIGA-CDG. We hypothesized that genetic modifiers might contribute to this reduced penetrance.