A New High Penetrant Intronic Pathogenic Variant Related to Long QT Syndrome Type 2

<b>Background/Objectives</b>: Long QT Syndrome type 2 (LQT2) is a cardiac channelopathy linked to pathogenic variants in the <i>KCNH2</i> gene, which encodes the Kv11.1 potassium channel, essential for cardiac repolarization. Variants affecting splice sites disrupt potassium ion flow, prolong QT interval, and increase the risk of arrhythmias and sudden cardiac death (SCD). Understanding genotype-phenotype correlations is key, given the variability of clinical manifestations even within families sharing the same variant. We aimed to evaluate new pathogenic variants by analyzing genotype-phenotype correlations in informative families. <b>Methods</b>: Genetic and clinical assessments were performed on index cases and family members carrying <i>KCNH2</i> pathogenic variants, referred for genetic testing between 2010 and June 2023. The next-generation sequencing (NGS) of 210 cardiovascular-related genes was conducted. Clinical data, including demographic details, family history, arrhythmic events, electrocardiographic parameters, and treatments, were collected. <b>Results</b>: Among 390 patients (152 probands) tested for LQTS, only 2 <i>KCNH2</i> variants had over 5 carriers. The detailed clinical information of 22 carriers of this <i>KCNH2</i> p.Ser261fs. has already been reported by our research group. Moreover, we identified 12 carriers of the <i>KCNH2</i> c.77-2del variant, predicted to disrupt a splice site and not previously reported. Segregation analysis showed its high penetrance, supporting its classification as pathogenic. <b>Conclusions</b>: The newly identified <i>KCNH2</i> c.77-2del variant is a pathogenic, as strongly supported by the segregation analysis. Our findings underscore the importance of further research into splice site variants to enhance clinical management and genetic counseling for affected families.