Multisystem mitochondrial diseases due to mutations in mtDNA-encoded subunits of complex I.

BACKGROUND: Maternally inherited complex I deficiencies due to mutations in MT-ND genes represent a heterogeneous group of multisystem mitochondrial disorders (MD) with a unfavourable prognosis. The aim of the study was to characterize the impact of the mutations in MT-ND genes, including the novel m.13091 T > C variant, on the course of the disease, and to analyse the activities of respiratory chain complexes, the amount of protein subunits, and the mitochondrial energy-generating system (MEGS) in available muscle biopsies and cultivated fibroblasts. METHODS: The respiratory chain complex activities were measured by spectrophotometry, MEGS were analysed using radiolabelled substrates, and protein amount by SDS-PAGE or BN-PAGE in muscle or fibroblasts. RESULTS: In our cohort of 106 unrelated families carrying different mtDNA mutations, we found heteroplasmic mutations in the genes MT-ND1, MT-ND3, and MT-ND5, including the novel variant m.13091 T > C, in 13 patients with MD from 12 families. First symptoms developed between early childhood and adolescence and progressed to multisystem disease with a phenotype of Leigh or MELAS syndromes. MRI revealed bilateral symmetrical involvement of deep grey matter typical of Leigh syndrome in 6 children, cortical/white matter stroke-like lesions suggesting MELAS syndrome in 3 patients, and a combination of cortico-subcortical lesions and grey matter involvement in 4 patients. MEGS indicated mitochondrial disturbances in all available muscle samples, as well as a significantly decreased oxidation of [1-14C] pyruvate in fibroblasts. Spectrophotometric analyses revealed a low activity of complex I and/or complex I + III in all muscle samples except one, but the activities in fibroblasts were mostly normal. No correlation was found between complex I activities and mtDNA mutation load, but higher levels of heteroplasmy were generally found in more severely affected patients. CONCLUSIONS: Maternally inherited complex I deficiencies were found in 11% of families with mitochondrial diseases in our region. Six patients manifested with Leigh, three with MELAS. The remaining four patients presented with an overlap between these two syndromes. MEGS, especially the oxidation of [1-14C] pyruvate in fibroblasts might serve as a sensitive indicator of functional impairment due to MT-ND mutations. Early onset of the disease and higher level of mtDNA heteroplasmy were associated with a worse prognosis.

Thymidine kinase 2 and alanyl-tRNA synthetase 2 deficiencies cause lethal mitochondrial cardiomyopathy: case reports and review of the literature.

Cardiomyopathy is a common manifestation in neonates and infants with mitochondrial disorders. In this study, we report two cases manifesting with fatal mitochondrial hypertrophic cardiomyopathy, which include the third known patient with thymidine kinase 2 deficiency and the ninth patient with alanyl-tRNA synthetase 2 deficiency. The girl with thymidine kinase 2 deficiency had hypertrophic cardiomyopathy together with regression of gross motor development at the age of 13 months. Neurological symptoms and cardiac involvement progressed into severe myopathy, psychomotor arrest, and cardiorespiratory failure at the age of 22 months. The imaging methods and autoptic studies proved that she suffered from unique findings of leucoencephalopathy, severe, mainly cerebellar neuronal degeneration, and hepatic steatosis. The girl with alanyl-tRNA synthetase 2 deficiency presented with cardiac failure and underlying hypertrophic cardiomyopathy within 12 hours of life and subsequently died at 9 weeks of age. Muscle biopsy analyses demonstrated respiratory chain complex I and IV deficiencies, and histological evaluation revealed massive mitochondrial accumulation and cytochrome c oxidase-negative fibres in both cases. Exome sequencing in the first case revealed compound heterozygozity for one novel c.209T>C and one previously published c.416C>T mutation in the TK2 gene, whereas in the second case homozygozity for the previously described mutation c.1774C>T in the AARS2 gene was determined. The thymidine kinase 2 mutations resulted in severe mitochondrial DNA depletion (to 12% of controls) in the muscle. We present, for the first time, severe leucoencephalopathy and hepatic steatosis in a patient with thymidine kinase 2 deficiency and the finding of a ragged red fibre-like image in the muscle biopsy in a patient with alanyl-tRNA synthetase 2 deficiency.

Hereditary Multiple Exostoses: Clinical, Molecular and Radiologic Survey in 9 Families.

Hereditary multiple exostoses (HME) represents a heterogeneous group of diseases often associated with progressive skeletal deformities. Most frequently, mutations in EXT1 and EXT2 genes with autosomal dominant inheritance are responsible for HME. In our group of 9 families with HME we evaluated the clinical course of the disease and analysed molecular background using Sanger sequencing and MLPA in EXT1 and EXT2 genes. The mean age in our group of patients, when the first exostosis was recognised was 4.5 years (range 2-10 years) and the number of exostoses per one patient documented on X-ray ranged from 2 to 54. Most of the exostoses developed before the growth was completed and they were dominantly localised in the distal femurs, proximal tibia, proximal humerus and distal radius. In all patients, at least one to 8 surgeries were necessary due to complaints and local complications, but neither patient developed malignant transformation. In half of the patients, the disease resulted in short stature. DNA analyses were positive in 7 families. In five probands, different EXT1 gene mutations resulting in premature stop-codon (p.Gly124Argfs*65, p.Leu191*, p.Trp364Lysfs*11, p.Val371Glyfs*10, p.Leu490Profs*31) were found. In two probands, nonsense mutations were found in EXT2 gene (p.Val187Profs*115, p.Cys319fs*46). Five mutations have been novel and two mutations have occurred de novo in probands. Although the risk for malignant transformation is usually low, especially in patients with low number of exostoses, early diagnostics and longitudinal follow up of patients is of a big importance, because early surgery can prevent progression of secondary bone deformities.

Unique presentation of LHON/MELAS overlap syndrome caused by m.13046T>C in MTND5.

BACKGROUND: Leber hereditary optic neuropathy (LHON) and mitochondrial encephalopathy, myopathy, lactic acidosis and stroke-like episodes (MELAS) syndromes are mitochondrially inherited disorders characterized by acute visual failure and variable multiorgan system presentation, respectively. MATERIALS AND METHODS: A 12-year-old girl with otherwise unremarkable medical history presented with abrupt, painless loss of vision. Over the next few months, she developed moderate sensorineural hearing loss, vertigo, migraines, anhedonia and thyroiditis. Ocular examination confirmed bilateral optic nerve atrophy. Metabolic workup documented elevated cerebrospinal fluid lactate. Initial genetic analyses excluded the three most common LHON mutations. Subsequently, Sanger sequencing of the entire mitochondrial DNA (mtDNA) genome was performed. RESULTS: Whole mtDNA sequencing revealed a pathogenic heteroplasmic mutation m.13046T>C in MTND5 encoding the ND5 subunit of complex I. This particular variant has previously been described in a single case report of MELAS/Leigh syndrome (subacute necrotizing encephalopathy). Based on the constellation of clinical symptoms in our patient, we diagnose the condition as LHON/MELAS overlap syndrome. CONCLUSIONS: We describe a unique presentation of LHON/MELAS overlap syndrome resulting from a m.13046T>C mutation in a 12-year-old girl. In patients with sudden vision loss in which three of the most prevalent LHON mitochondrial mutations have been ruled out, molecular genetic examination should be extended to other mtDNA-encoded subunits of MTND5 complex I. Furthermore, atypical clinical presentations must be considered, even in well-described phenotypes.

Large copy number variations in combination with point mutations in the TYMP and SCO2 genes found in two patients with mitochondrial disorders.

Mitochondrial disorders are caused by defects in mitochondrial or nuclear DNA. Although the existence of large deletions in mitochondrial DNA (mtDNA) is well known, deletions affecting whole genes are not commonly described in patients with mitochondrial disorders. Based on the results of whole-genome analyses, copy number variations (CNVs) occur frequently in the human genome and may overlap with many genes associated with clinical phenotypes. We report the discovery of two large heterozygous CNVs on 22q13.33 in two patients with mitochondrial disorders. The first patient harboured a novel point mutation c.667G>A (p.D223N) in the SCO2 gene in combination with a paternally inherited 87-kb deletion. As hypertrophic cardiomyopathy (HCMP) was not documented in the patient, this observation prompted us to compare his clinical features with all 44 reported SCO2 patients in the literature. Surprisingly, the review shows that HCMP was present in only about 50% of the SCO2 patients with non-neonatal onset. In the second patient, who had mitochondrial neurogastrointestinal encephalopathy (MNGIE), a maternally inherited 175-kb deletion and the paternally inherited point mutation c.261G>T (p.E87D) in the TYMP gene were identified.