Long QT syndrome type I is an autosomal dominant disorder caused by a heterozygous loss-of-function mutation in the KCNQ1 gene, located on chromosome 11p15. This chromosomal region is subject to genomic imprinting and is also implicated in Beckwith-Wiedemann syndrome. This report describes a female patient with inherited long QT syndrome type I, present in her mother and sister, concurrently with Beckwith-Wiedemann syndrome due to hypomethylation in the imprinting control region 2 (IC2). Methylation-specific multiplex ligation-dependent probe amplification (MS-MLPA) identified hypomethylation at the KvDMR/IC2 locus, while Sanger sequencing confirmed a pathogenic variant in the KCNQ1 gene. Not all individuals harboring both IC2 hypomethylation and a pathogenic KCNQ1 variant exhibit features of both syndromes, and the mechanisms behind this variability remain unclear. Timely diagnosis, coordinated multidisciplinary care, and appropriate therapeutic management are essential for optimal outcomes and optimal growth.
