Mitochondrial DNA Depletion and Deletions in Paediatric Patients with Neuromuscular Diseases: Novel Phenotypes
Objective: To study the clinical manifestations and occurrence of mtDNA depletion and deletions in paediatric patients with neuromuscular diseases and to identify novel clinical phenotypes associated with mtDNA depletion or deletions.
Methods: Muscle DNA samples from patients presenting with undefined encephalomyopathies or myopathies were analysed for mtDNA content by quantitative real-time PCR and for deletions by long-range PCR. Direct sequencing of mtDNA maintenance genes and whole-exome sequencing were used to study the genetic aetiologies of the diseases. Clinical and laboratory findings were collected.
Results: Muscle samples were obtained from 104 paediatric patients with neuromuscular diseases. mtDNA depletion was found in three patients with severe early-onset encephalomyopathy or myopathy. Two of these patients presented with novel types of mitochondrial DNA depletion syndromes associated with increased serum creatine kinase (CK) and multiorgan disease without mutations in any of the known mtDNA maintenance genes; one patient had pathologic endoplasmic reticulum (ER) membranes in muscle. The third patient with mtDNA depletion was diagnosed with merosine-deficient muscular dystrophy caused by a homozygous mutation in the LAMA2 gene. Two patients with an early-onset Kearns-Sayre/Pearson-like phenotype harboured a large-scale mtDNA deletion, minor multiple deletions and high mtDNA content.
Conclusions: Novel encephalomyopathic mtDNA depletion syndrome with structural alterations in muscle ER was identified. mtDNA depletion may also refer to secondary mitochondrial changes related to muscular dystrophy. We suggest that a large-scale mtDNA deletion, minor multiple deletions and high mtDNA content associated with Kearns-Sayre/Pearson syndromes may be secondary changes caused by mutations in an unknown nuclear gene.
KeywordsPigmentary Retinopathy Pearson Syndrome LAMA2 Gene Genome Quebec Innovation Increase Serum Creatine Kinase
Mitochondrial DNA depletion syndrome
Phenol-chloroform-isoamyl alcohol extraction
Real-time quantitative PCR
The authors thank Ms. Anja Heikkinen and Ms. Pirjo Keränen for their expert assistance. The work was supported by grants from the Research Council for Health of the Academy of Finland (JU, Decision number 138566; KM, Decision number 127764; RH, Decision number 266498 and 273790), the Sigrid Juselius Foundation, the Finnish Medical Foundation, the Arvo ja Lea Ylppö Foundation, the Foundation for Pediatric Research, the Alma and K.A. Snellman Foundation, the Emil Aaltonen Foundation, National Graduate School of Clinical Investigation (CLIGS), a Marie Curie International Outgoing Fellowship of the European Union’s Seventh Framework Programme under the grant agreement number 273669 (BioMit), Special State Grants for Health Research in the Department of Pediatrics and Adolescence and the Department of Neurology at Oulu University Hospital, Oulu, Finland.
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