New mutations and range of SPG 7 HSP expands
As well as expanding knowledge of the role of paraplegin mutations in damaging mitochondrial DNA, and discovering new mutations, this study also established that SPG 7 HSP should be considered when someone presents with progressive external ophthalmoplegia, especially if spasticity is present.
Spastic paraplegia 7 is an autosomal recessive disorder caused by mutations in the gene encoding paraplegin, a protein located at the inner mitochondrial membrane and involved in the processing of other mitochondrial proteins. The mechanism whereby paraplegin mutations cause disease is unknown.
We studied two female and two male adult patients from two Norwegian families with a combination of progressive external ophthalmoplegia and spastic paraplegia. Sequencing of SPG7 revealed a novel missense mutation, c.2102A>C, p.H 701P, which was homozygous in one family and compound heterozygous in trans with a known pathogenic mutation c.1454_1462del in the other. Muscle was examined from an additional, unrelated adult female patient with a similar phenotype caused by a homozygous c.1047insC mutation in SPG7. Immunohistochemical studies in skeletal muscle showed mosaic deficiency predominantly affecting respiratory complex I, and also complexes III and IV. Molecular studies in single, microdissected fibres showed multiple mitochondrial DNA deletions segregating at high levels (38-97%) in respiratory deficient fibres.
Our findings demonstrate for the first time that paraplegin mutations cause accumulation of mitochondrial DNA damage and multiple respiratory chain deficiencies. While paraplegin is not known to be directly associated with the mitochondrial nucleoid, it is known to process other mitochondrial proteins and it is possible therefore that paraplegin mutations lead to mitochondrial DNA deletions by impairing proteins involved in the homeostasis of the mitochondrial genome.
These studies increase our understanding of the molecular pathogenesis of SPG7 mutations and suggest that SPG7 testing should be included in the diagnostic workup of autosomal recessive, progressive external ophthalmoplegia, especially if spasticity is present.
SOURCE: PLoS One. 2014 Jan 22;9(1):e86340. doi: 10.1371/journal.pone.0086340. eCollection 2014. PMID: 24466038 [PubMed – indexed for MEDLINE] PMCID: PMC3899233
Spastic paraplegia type 7 is associated with multiple mitochondrial DNA deletions.
Wedding IM1, Koht J2, Tran GT3, Misceo D4, Selmer KK4, Holmgren A4, Frengen E4, Bindoff L3, Tallaksen CM5, Tzoulis C3.
- 1Department of Neurology, Oslo University Hospital, Ullevål, Oslo, Norway ; University of Oslo, Faculty of Medicine, Oslo, Norway.
- 2Department of Neurology, Drammen Hospital, Vestre Viken Health Trust, Norway ; University of Oslo, Faculty of Medicine, Oslo, Norway.
- 3Department of Clinical Medicine, University of Bergen, Bergen, Norway ; Department of Neurology, Haukeland University Hospital, Bergen, Norway.
- 4University of Oslo, Faculty of Medicine, Oslo, Norway ; Department of Medical Genetics, Oslo University Hospital, Ullevål, Oslo, Norway.
- 5University of Oslo, Faculty of Medicine, Oslo, Norway ; Department of Neurology, Oslo University Hospital, Ullevål, Oslo, Norway.