Reverse genotyping: unveiling Alu element insertion as a new cause of Kabuki syndrome using DNA methylation signature.

Kabuki syndrome type 1 (KS1) is a monogenic disorder arising from pathogenic variants within KMT2D and characterized by syndromic neurodevelopmental delay. We report the retrospective identification of a causative AluY insertion within KMT2D in a genetically unsolved individual with typical KS1 features, after identification of a DNA methylation signature. This is the first documentation of Alu insertion as a molecular mechanism responsible for KS1. This study emphasizes the need for reanalyzing inconclusive sequencing data in individuals with gene-specific phenotypes and reinforces episignature as a reliable diagnostic tool when NGS approaches fail to provide conclusive results in individuals with rare diseases.