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Mitochondrial complex IV deficiency caused by a novel frameshift variant in MT-CO2 associated with myopathy and perturbed acylcarnitine profile

Journal article
Authors Sara Roos
Kalliopi Sofou
Carola Oldfors Hedberg
Gittan Kollberg
Ulrika Lindgren
Christer Thomsen
Mar Tulinius
Anders Oldfors
Published in European Journal of Human Genetics
Volume 27
Issue 2
Pages 331-335
ISSN 1018-4813
Publication year 2019
Published at Institute of Biomedicine
Institute of Clinical Sciences, Department of Pediatrics
Pages 331-335
Language en
Keywords cytochrome-c-oxidase, subunit-ii, mutation, gene
Subject categories Medical Genetics


Mitochondrial myopathies are a heterogeneous group of disorders associated with a wide range of clinical phenotypes. We present a 16-year-old girl with a history of exercise intolerance since childhood. Acylcarnitine species suggestive of multiple acyl-CoA dehydrogenase deficiency were found in serum, however genetic analysis did not reveal variants in genes associated with this disorder. Biochemical analyses of skeletal muscle mitochondria revealed an isolated and extremely low activity of cytochrome c oxidase (COX). This finding was confirmed by enzyme histochemistry, which demonstrated an almost complete absence of fibers with normal COX activity. Whole-exome sequencing revealed a single base-pair deletion (m.8088delT) in MT-CO2, which encodes subunit 2 of COX, resulting in a premature stop codon. Restriction fragment length polymorphism-analysis confirmed mtDNA heteroplasmy with high mutant load in skeletal muscle, the only clinically affected tissue, but low levels in other investigated tissues. Single muscle fiber analysis showed segregation of the mutant genotype with respiratory chain dysfunction. Immuno-histochemical studies indicated that the truncating variant in COX2 has an inhibitory effect on the assembly of the COX holoenzyme.

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