PGD and heteroplasmic mitochondrial DNA point mutations: a systematic review estimating the chance of healthy offspring

Mitochondrial disorders are often fatal multisystem disorders, partially caused by heteroplasmic mitochondrial DNA (mtDNA) point mutations. Prenatal diagnosis is generally not possible for these maternally inherited mutations because of extensive variation in mutation load among embryos and the inability to accurately predict the clinical expression. The aim of this study is to investigate if PGD could be a better alternative, by investigating the existence of a minimal mutation level below which the chance of an embryo being affected is acceptably low, irrespective of the mtDNA mutation. We performed a systematic review of muscle mutation levels, evaluating 159 different heteroplasmic mtDNA point mutations derived from 327 unrelated patients or pedigrees, and reviewed three overrepresented mtDNA mutations (m.3243AG, m.8344AG and m.8993TC/G) separately. Mutation levels were included for familial mtDNA point mutations only, covering all affected (n 195) and unaffected maternal relatives (n 19) from 137 pedigrees. Mean muscle mutation levels were comparable between probands and affected maternal relatives, and between affected individuals with tRNA- versus protein-coding mutations. Using an estimated a priori prevalence of being affected in pedigrees of 0.477, we calculated that a 95 or higher chance of being unaffected was associated with a musc Our data show, for the first time, that carriers of heteroplasmic mtDNA mutations will have a fair chance of having healthy offspring, by applying PGD. Nevertheless, our conclusions are partly based on estimations and, as indicated, do not provide absolute certainty. Carriers of mtDNA should be informed about these constraints

Large scale mtDNA sequencing reveals sequence and functional conservation as major determinants of homoplasmic mtDNA variant distribution

The mitochondrial DNA (mtDNA) is highly variable, containing large numbers of pathogenic mutations and neutral polymorphisms. The spectrum of homoplasmic mtDNA variation was characterized in 730 subjects and compared with known pathogenic sites. The frequency and distribution of variants in protein coding genes were inversely correlated with conservation at the amino acid level. Analysis of tRNA secondary structures indicated a preference of variants for the loops and some acceptor stem positions. This comprehensive overview of mtDNA variants distinguishes between regions and positions which are likely not critical, mainly conserved regions with pathogenic mutations and essential regions containing no mutations at all

Defective NDUFA9 as a novel cause of neonatally fatal complex I disease.

BACKGROUND: Mitochondrial disorders are associated with abnormalities of the oxidative phosphorylation (OXPHOS) system and cause significant morbidity and mortality in the population. The extensive clinical and genetic heterogeneity of these disorders due to a broad variety of mutations in several hundreds of candidate genes, encoded by either the mitochondrial DNA (mtDNA) or nuclear DNA (nDNA), impedes a straightforward genetic diagnosis. A new disease gene is presented here, identified in a single Kurdish patient born from consanguineous parents with neonatally fatal Leigh syndrome and complex I deficiency. METHODS AND RESULTS: Using homozygosity mapping and subsequent positional candidate gene analysis, a total region of 255.8 Mb containing 136 possible mitochondrial genes was identified. A pathogenic mutation was found in the complex I subunit encoding the NDUFA9 gene, changing a highly conserved arginine at position 321 to proline. This is the first disease-causing mutation ever reported for NDUFA9. Complex I activity was restored in fibroblasts of the patient by lentiviral transduction with wild type but not mutant NDUFA9, confirming that the mutation causes the complex I deficiency and related disease. CONCLUSIONS: The data show that homozygosity mapping and candidate gene analysis remain an efficient way to detect mutations even in small consanguineous pedigrees with OXPHOS deficiency, especially when the enzyme deficiency in fibroblasts allows appropriate candidate gene selection and functional complementation