Benign mitochondrial myopathy with exercise intolerance in a large multigeneration family due to a homoplasmic m.3250T>C mutation in MTTL1.
Most mitochondrial disorders with onset in early childhood are progressive and involve multiple organs. The m.3250T>C mutation in MTTL1 has previously been described in a few individuals with a possibly riboflavin-responsive myopathy and an association with sudden infant death syndrome was suspected. We describe a large family with this mutation and evaluate the effect of riboflavin treatment.



Medical data were collected with the help of a standardized data collection form. Sanger sequencing was used to screen for variants in mitochondrial DNA and the proportion of the mutation was analyzed in different tissues. Biochemical and muscle morphological investigations of muscle tissue were performed in two individuals. The effect of riboflavin treatment was evaluated in two individuals.



Thirteen family members experienced exercise intolerance with fatigue and weakness. Inheritance was maternal with 100% penetrance. The course was either static or showed improvement over time. There was no evidence of other organ involvement except for a possible mild transient cardiac enlargement in one child. Muscle investigations showed isolated complex I deficiency and mitochondrial proliferation. The level of m.3250T>C was apparently 100%, i.e. homoplasmic, in all examined tissues. Riboflavin treatment showed no effect in any treated family member and there have been no cases of sudden infant death in this family.



This study illustrates the importance of considering mitochondrial disorders in the work-up of individuals with exercise intolerance and provides a better understanding of the phenotype associated with the m.3250T>C mutation in MTTL1.
Department of Pediatrics, Sahlgrenska University Hospital, University of Gothenburg, Gothenburg.
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Darin
Department of Pathology and Genetics, Sahlgrenska University Hospital, University of Gothenburg, Gothenburg.
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Hedberg-Oldfors
Department of Pediatrics, Sahlgrenska University Hospital, University of Gothenburg, Gothenburg.
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Kroksmark
Department of Pathology and Genetics, Sahlgrenska University Hospital, University of Gothenburg, Gothenburg.
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Moslemi
Department of Clinical Chemistry and Transfusion Medicine, Sahlgrenska University Hospital, University of Gothenburg, Gothenburg, Sweden.
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Kollberg
Department of Pathology and Genetics, Sahlgrenska University Hospital, University of Gothenburg, Gothenburg.
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Oldfors
D000328:Adult; D002675:Child, Preschool; D004272:DNA, Mitochondrial / Q000235:genetics*; D017079:Exercise Tolerance / Q000235:genetics*; D005260:Female; D006801:Humans; D007223:Infant; D008297:Male; D008875:Middle Aged; D008928:Mitochondria / Q000235:genetics; D017240:Mitochondrial Myopathies / Q000188:drug therapy / Q000235:genetics*; D009154:Mutation; D010375:Pedigree; D010641:Phenotype; D012343:RNA, Transfer / Q000235:genetics*; D012256:Riboflavin / Q000627:therapeutic use; D014803:Vitamin B Complex / Q000627:therapeutic use; D055815:Young Adult
D004272:DNA, Mitochondrial; D014803:Vitamin B Complex; D012343:RNA, Transfer; D012256:Riboflavin

MTTL1
; benign; disease; exercise intolerance; m.3250T>C; mitochondrial; mitochondrial DNA; myopathy