Whole exome sequencing reveals a homozygous C1QBP deletion as the cause of progressive external ophthalmoplegia and multiple mtDNA deletions

Whole exome sequencing (WES), analyzed with GENESIS and WeGET, revealed a homozygous deletion in the C1QBP gene in a patient with progressive external ophthalmoplegia (PEO) and multiple mtDNA deletions. The gene encodes the mitochondria-located complementary 1 Q subcomponent-binding protein, involved in mitochondrial homeostasis. Biallelic mutations in C1QBP cause mitochondrial cardiomyopathy and/or PEO with variable age of onset. Our patient showed only late-onset PEO-plus syndrome without overt cardiac involvement. Available data suggest that early-onset cardiomyopathy variants localize in important structural domains and PEO-plus variants in the coiled-coil region. Our patient demonstrates that C1QBP mutations should be considered in individuals with PEO with or without cardiomyopathy.

Progressive external ophthalmoplegia due to a recurrent de novo m.15990C>T MT-TP (mt-tRNAPro) gene variant

Progressive external ophthalmoplegia is typically associated with single or multiple mtDNA deletions but occasionally mtDNA single nucleotide variants within mitochondrial transfer RNAs (mt-tRNAs) are identified. We report a 34-year-old female sporadic patient with progressive external ophthalmoplegia accompanied by exercise intolerance but neither fixed weakness nor multisystemic involvement. Histopathologically, abundant COX-deficient fibres were present in muscle with immunofluorescence analysis confirming the loss of mitochondrial complex I and IV proteins. Molecular genetic analysis identified a rare heteroplasmic m.15990C>T mt-tRNA^Pro variant reported previously in a single patient with childhood-onset myopathy. The variant in our patient was restricted to muscle. Single muscle fibre analysis identified higher heteroplasmy load in COX-deficient fibres than COX-normal fibres, confirming segregation of high heteroplasmic load with a biochemical defect. Our case highlights the phenotypic variability typically observed with pathogenic mt-tRNA mutations, whilst the identification of a second case with the m.15990C>T mutation not only confirms pathogenicity but shows that de novo mt-tRNA point mutations can arise in multiple, unrelated patients.

Diagnosis of mitochondrial myopathies

Mitochondria are ubiquitous organelles and play crucial roles in vital functions, most importantly, the oxidative phosphorylation and energy metabolism. Therefore, mitochondrial dysfunction can affect multiple tissues, with muscle and nerve preferentially affected. Mitochondrial myopathy is a common clinical phenotype, which is characterized by early fatigue and/or fixed muscle weakness; rhabdomyolysis can seldom occur. Muscle biopsy often identifies signs of diseased mitochondria by morphological studies, while biochemical analysis may identify respiratory chain deficiencies. The clinical, morphological and biochemical data guide molecular analysis. Being the mitochondrial function under the control of both mitochondrial DNA and nuclear DNA, the search for mitochondrial DNA mutations and mitochondrial DNA quantitation, may not be sufficient for the molecular diagnosis of mitochondrial myopathies. Approximately 1500 nuclear genes can affect mitochondrial structure and function and the targeting of such genes may be necessary to reach the diagnosis. The identification of causative molecular defects in nuclear or mitochondrial genome leads to the definite diagnosis of mitochondrial myopathy.

Mitochondrial DNA abnormalities in ophthalmological disease

Mitochondrial disorders are a group of clinically heterogeneous diseases, commonly defined by lack of cellular energy due to genetic defects of oxidative phosphorylation (OXPHOS). Ocular involvement is a prominent clinical feature of mitochondrial disease. This can manifest as optic nerve dysfunction specifically involving retinal ganglion cells as typified by Leber hereditary optic neuropathy (LHON), or progressive external ophthalmoplegia (PEO) and ptosis involving the extraocular muscles which is commonly associated with either primary mitochondrial DNA (mtDNA) mutations or acquired mtDNA defects secondary to a nuclear genetic disorder of mtDNA maintenance. In this short review, we will outline the unique characteristics of mitochondrial genetic disease and its investigation with reference to the clinical features and molecular genetic abnormalities underlying mitochondrial ophthalmological disease.