Delineation of a Phenotype Caused by a KAT6B Missense Variant Not Resembling Say-Barber-Biesecker-Young-Simpson and Genitopatellar Syndromes.

Say-Barber-Biesecker-Young-Simpson syndrome (SBBYSS) and genitopatellar syndrome (GPS) are caused by variants of lysine acetyltransferase 6B (KAT6B). These variants tend to occur in the terminal exons of KAT6B. Here, we report a patient with global developmental delay, intellectual disability, autistic behavior, muscular hypotonia, facial dysmorphism, and seizures caused by a novel missense variant in exon 7 of KAT6B. The patient showed a phenotype differing from those of SBBYSS and GPS. We also report patients with missense variants in the proximal exons of KAT6B showing dysmorphic features and autistic behavior not resembling the characteristics of SBBYSS and GPS. Missense variants in the proximal exons of KAT6B may have a dominant negative effect or cause gain of function, leading to unique phenotypes not resembling those of SBBYSS and GPS.