Variable expressivity of a transmitted pathogenic KAT6B variant

Pathogenic variants in KAT6B (Lysine acetyltransferase 6B) are associated with two clinically overlapping autosomal dominant disorders Say-Barber-Biesecker-Young-Simpson syndrome (SBBYSS) (OMIM 603736), and Genitopatellar syndrome (GPS) (OMIM 606170). More recently, the clinical spectrum of KAT6B disorders has expanded and KAT6B disorders have been suggested to consist of a spectrum of disorders with intermediate and overlapping clinical manifestations. Pathogenic variants in KAT6B mainly occur de novo, with only 3 reports of inherited variants. Here, we describe clinical and molecular findings in a three-generation Danish family with a segregating, previously unreported, pathogenic KAT6B variant. The variant is associated with a phenotype not otherwise specified (neither SBBYSS nor GPS) and with variable expressivity, adding further evidence that KAT6B disorders should be seen as a broad clinical spectrum. Furthermore, we highlight the existence of intra-familial variability and that pathogenic variants in KAT6B can be inherited from mildly affected parents.