Mitochondrial encephalomyopathy due to a novel mutation in the tRNAGlu of mitochondrial DNA

Reversible infantile respiratory chain deficiency is a unique, genetically heterogenous mitochondrial disease

OBJECTIVES

Homoplasmic maternally inherited, m.14674T>C or m. 14674T>G mt-tRNA(Glu) mutations have recently been identified in reversible infantile cytochrome c oxidase deficiency (or 'benign COX deficiency'). This study sought other genetic defects that may give rise to similar presentations.

PATIENTS

Eight patients from seven families with clinicopathological features of infantile reversible cytochrome c oxidase deficiency were investigated.

METHODS

The study reviewed the diagnostic features and performed molecular genetic analyses of mitochondrial DNA and nuclear encoded candidate genes.

RESULTS

Patients presented with subacute onset of profound hypotonia, feeding difficulties and lactic acidosis within the first months of life. Although recovery was remarkable, a mild myopathy persisted into adulthood. Histopathological findings in muscle included increased lipid and/or glycogen content, ragged-red and COX negative fibres. Biochemical studies suggested more generalised abnormalities than pure COX deficiency. Clinical improvement was reflected by normalisation of lactic acidosis and histopathological abnormalities. The m.14674T>C mt-tRNA(Glu) mutation was identified in four families, but none had the m. 14674T>G mutation. Furthermore, in two families pathogenic mutations were also found in the nuclear TRMU gene which has not previously been associated with this phenotype. In one family, the genetic aetiology still remains unknown.

CONCLUSIONS

Benign COX deficiency is better described as 'reversible infantile respiratory chain deficiency'. It is genetically heterogeneous, and patients not carrying the m.14674T>C or T>G mt-tRNA(Glu) mutations may have mutations in the TRMU gene. Diagnosing this disorder at the molecular level is a significant advance for paediatric neurologists and intensive care paediatricians, enabling them to select children with an excellent prognosis for continuing respiratory support from those with severe mitochondrial presentation in infancy.

A novel mitochondrial tRNAGlu (MTTE) gene mutation causing chronic progressive external ophthalmoplegia at low levels of heteroplasmy in muscle

Mitochondrial respiratory chain defects are associated with diverse clinical phenotypes in both adults and children, and may be caused by mutations in either nuclear or mitochondrial DNA (mtDNA). We report the molecular genetic investigations of a patient with chronic progressive external ophthalmoplegia (CPEO) and myopathy where muscle biopsies taken 11 years apart revealed a progressive increase in the proportion of cytochrome c oxidase (COX)-deficient fibres. Mitochondrial genetic analysis of the early biopsy had seemingly excluded both mtDNA rearrangements and mtDNA point mutations. Sequencing mtDNA from individual COX-deficient muscle fibres in the second biopsy, however, identified an unreported m.14723T>C substitution within the mitochondrial tRNAGlu (MTTE) gene, which fulfilled all canonical criteria for pathogenicity. The m.14723T>C mutation was absent from several tissues, including muscle, from maternal relatives suggesting a de novo event, whilst quantitative analysis of the first muscle biopsy confirmed a very low level of the mutation (7% mutated mtDNA), highlighting a potential problem whereby pathogenic mtDNA mutations may remain undetected using established screening methodologies.

Mitochondrial ataxias

Mitochondrial disorders (MIDs) are an increasingly recognized condition. The second most frequently affected organ in MIDs is the central nervous system. One of the most prevalent clinical CNS manifestations of MIDs is ataxia. Ataxia may be even the dominant manifestation of a MID. This is why certain MIDs should be included in the classification of heredoataxias or at least considered as differentials of classical heredoataxias. MIDs due to mutations of the mitochondrial DNA, which develop ataxia include the MERRF, NARP, MILS, or KSS syndrome. More rarely, ataxia may be a feature of MELAS, LHON, PS, MIDD, or MSL. MIDs due to mutations of the nuclear DNA, which develop ataxia include LS, SANDO, SCAE, AHS, XSLA/A, IOSCA, MIRAS, MEMSA, or LBSL syndrome. More rarely ataxia can be found in AD-CPEO, AR-CPEO, MNGIE, DIDMOAD, CoQ-deficiency, ADOAD, DCMA, or PDC-deficiency. MIDs most frequently associated with ataxia are the non-syndromic MIDs. Syndromic and non-syndromic MIDs with ataxia should be delineated from classical heredoataxias to initiate appropriate symptomatic or supportive treatment.

The human gene map for performance and health-related fitness phenotypes: the 2006-2007 update

This update of the human gene map for physical performance and health-related fitness phenotypes covers the research advances reported in 2006 and 2007. The genes and markers with evidence of association or linkage with a performance or a fitness phenotype in sedentary or active people, in responses to acute exercise, or for training-induced adaptations are positioned on the map of all autosomes and sex chromosomes. Negative studies are reviewed, but a gene or a locus must be supported by at least one positive study before being inserted on the map. A brief discussion on the nature of the evidence and on what to look for in assessing human genetic studies of relevance to fitness and performance is offered in the introduction, followed by a review of all studies published in 2006 and 2007. The findings from these new studies are added to the appropriate tables that are designed to serve as the cumulative summary of all publications with positive genetic associations available to date for a given phenotype and study design. The fitness and performance map now includes 214 autosomal gene entries and quantitative trait loci plus seven others on the X chromosome. Moreover, there are 18 mitochondrial genes that have been shown to influence fitness and performance phenotypes. Thus,the map is growing in complexity. Although the map is exhaustive for currently published accounts of genes and exercise associations and linkages, there are undoubtedly many more gene-exercise interaction effects that have not even been considered thus far. Finally, it should be appreciated that most studies reported to date are based on small sample sizes and cannot therefore provide definitive evidence that DNA sequence variants in a given gene are reliably associated with human variation in fitness and performance traits.

A novel mutation in the mitochondrial tRNA(Pro) gene associated with late-onset ataxia, retinitis pigmentosa, deafness, leukoencephalopathy and complex I deficiency

We present a patient with ataxia, retinitis pigmentosa, dysarthria, neurosensorial deafness, nystagmus and leukoencephalopathy. A novel heteroplasmic G to A transition at nucleotide 15 975 was found, affecting the T arm of the mitochondrial (mt) tRNA(Pro) gene. A biochemical analysis of respiratory chain enzymes in muscle revealed isolated complex I deficiency. This is the fourth pathogenic tRNA(Pro) point mutation to be associated with an mt disorder. The result highlights the importance of molecular dissection of mtDNA in patients with defined mt disorder and confirms the clinical and biochemical heterogeneity associated with tRNA(Pro) mutations.