Critically unwell infants and children with mitochondrial disorders diagnosed by ultra-rapid genomic sequencing.

Purpose: To characterize the diagnostic and clinical outcomes of a cohort of critically ill infants and children with suspected mitochondrial disorders (MD) undergoing ultra-rapid genomic testing as part of a national program.

Methods: Ultra-rapid genomic sequencing was performed in 454 families (genome sequencing: n=290, exome sequencing +/- mitochondrial DNA sequencing: n=164). In 91 individuals, MD was considered, prompting analysis using an MD virtual gene panel. These individuals were reviewed retrospectively and scored according to modified Nijmegen Mitochondrial Disease Criteria.

Results: A diagnosis was achieved in 47% (43/91) of individuals, 40% (17/43) of whom had an MD. Seven additional individuals in whom an MD was not suspected were diagnosed with an MD following broader analysis. Gene-agnostic analysis led to the discovery of two novel disease genes, with pathogenicity validated through targeted functional studies (CRLS1 and MRPL39). Functional studies enabled diagnosis in another four individuals. Of the 24 individuals ultimately diagnosed with an MD, 79% had a change in management, which included 53% whose care was redirected to palliation.

Conclusion: Ultra-rapid genetic diagnosis of MD in acutely unwell infants and children is critical for guiding decisions about the need for additional investigations and clinical management.