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The diagnosis of mitochondrial disorders remains a challenging and often unmet need. We sought to investigate a sibling pair with suspected mitochondrial disease and a clinical presentation notable for global developmental delay, poor growth, sensorineural hearing loss, and brain MRI abnormalities, both with early death. Following uninformative exome and genome sequencing of the family quartet, RNA sequencing was pursued as an orthogonal testing strategy. RNA sequencing of fibroblasts from the older sibling identified the presence of a cryptic exon in intron 1 of NDUFB10, that included an in-frame stop codon. NDUFB10 encodes a subunit of mitochondrial OXPHOS complex I. Differential expression analysis relative to control samples suggested significantly decreased expression. The cryptic exon was found to contain a rare intronic variant, NM_004548.3:c.131-442G>C, that was homozygous in both affected siblings and absent from population allele frequency databases. Immunoblot and quantitative proteomic analysis of fibroblasts from the older sibling revealed decreased abundance of complex I subunits associated with NDUFB10, providing evidence of isolated complex I deficiency. Biallelic variants in NDUFB10 have previously been reported in a single individual with infantile-onset mitochondrial disease. We present data implicating a deep intronic variant in NDUFB10 as the cause of mitochondrial disease in two further individuals. This variant results in loss of expression and overall destabilization of mitochondrial OXPHOS complex I and highlights the importance of RNA sequencing as a complementary diagnostic tool in patients undergoing genome-wide diagnostic evaluation.

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