Identification of a pathogenic RNU4-2 variant in patients with mitochondrial disease: Broadening the spectrum of non-coding RNA gene variants in mitochondrial dysfunction.

Mitochondrial diseases are characterized by impaired energy production due to mitochondrial dysfunction. Despite advances in sequencing technologies, many cases remain genetically undiagnosed. We report two cases of mitochondrial disease harboring identical de novo variant in the non-coding RNA gene RNU4-2, previously associated with neurodevelopmental disorders. Re-analysis of whole genome sequencing data from 357 patients ascertained as possibly having mitochondrial disease (see Methods: Supplementary Data S1) identified two cases with a pathogenic RNU4-2 variant (GRCh38: chr.12:120291839: T > TA; NR_003137.2: n.64_65insT). Both patients exhibited decreased oxygen consumption rates and clinical features including developmental delay, microcephaly, short stature. This study provides the first evidence linking RNU4-2 variant to mitochondrial disease, expanding the phenotypic spectrum associated with this gene. Our findings highlight the importance of re-analyzing genomic data and considering non-coding RNA gene variants in mitochondrial disease diagnostics, potentially improving the diagnosis of previously unsolved cases.