Mutations in Complex I Assembly Factor TMEM126B Result in Muscle Weakness and Isolated Complex I Deficiency.

Sánchez-Caballero L, Ruzzenente B, Bianchi L, Assouline Z, Barcia G, Metodiev MD, Rio M, Funalot B, van den Brand MA, Guerrero-Castillo S, Molenaar JP, Koolen D, Brandt U, Rodenburg RJ, Nijtmans LG, Rötig A.

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Am. J. Hum. Genet.

2017 May 19

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Mitochondrial complex I deficiency results in a plethora of often severe clinical phenotypes manifesting in early childhood. Here, we report on three complex-I-deficient adult subjects with relatively mild clinical symptoms, including isolated, progressive exercise-induced myalgia and exercise intolerance but with normal later development. Exome sequencing and targeted exome sequencing revealed compound-heterozygous mutations in TMEM126B, encoding a complex I assembly factor. Further biochemical analysis of subject fibroblasts revealed a severe complex I deficiency caused by defective assembly. Lentiviral complementation with the wild-type cDNA restored the complex I deficiency, demonstrating the pathogenic nature of these mutations. Further complexome analysis of one subject indicated that the complex I assembly defect occurred during assembly of its membrane module. Our results show that TMEM126B defects can lead to complex I deficiencies and, interestingly, that symptoms can occur only after exercise.

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