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Polymerase gamma (POLG) is the gene most commonly involved in mitochondrial disorders with mitochondrial DNA instability and causes a wide range of diseases with recessive or dominant transmission. More than 170 mutations have been reported. Most of them are missense mutations, although nonsense mutations, splice-site mutations, small deletions and insertions have also been identified. However, to date, only one large-scale rearrangement has been described in a child with Alpers syndrome. Below, we report a large cohort of 160 patients with clinical, molecular and/or biochemical presentation suggestive of POLG deficiency. Using sequencing, we identified POLG variants in 22 patients (18 kindreds) including five novel pathogenic mutations. Two patients with novel mutations had unusual clinical presentation: the first exhibited an isolated ataxic neuropathy and the second was a child who presented with endocrine signs. We completed the sequencing step by quantitative multiplex PCR of short fluorescent fragments (QMPSF) analysis in 37 patients with either only one POLG heterozygous variant or a family history suggesting a dominant transmission. We identified a large intragenic deletion encompassing part of intron 21 and exon 22 of POLG in a child with refractory epilepsia partialis continua. In conclusion, we describe the first large French cohort of patients with POLG mutations, expanding the wide clinical and molecular spectrum observed in POLG disease. We confirm that large deletions in the POLG gene are rare events and we highlight the importance of QMPSF in patients with a single heterozygous POLG mutation, particularly in severe infantile phenotypes. [ABSTRACT FROM AUTHOR]

MFN2 and OPA1 genes encode two dynamin-like GTPase proteins involved in the fusion of the mitochondrial membrane. They have been associated with Charcot–Marie–Tooth disease type 2A and autosomal dominant optic atrophy, respectively. We report a large family with optic atrophy beginning in early childhood, associated with axonal neuropathy and mitochondrial myopathy in adult life. The clinical presentation looks like the autosomal dominant optic atrophy ‘plus’ phenotype linked to OPA1 mutations but is associated with a novel MFN2 missense mutation (c.629A>T, p.D210V). Multiple mitochondrial DNA deletions were found in skeletal muscle and this observation makes MFN2 a novel gene associated with ‘mitochondrial DNA breakage’ syndrome. Contrary to previous studies in patients with Charcot–Marie–Tooth disease type 2A, fibroblasts carrying the MFN2 mutation present with a respiratory chain deficiency, a fragmentation of the mitochondrial network and a significant reduction of MFN2 protein expression. Furthermore, we show for the first time that impaired mitochondrial fusion is responsible for a deficiency to repair stress-induced mitochondrial DNA damage. It is likely that defect in mitochondrial DNA repair is due to variability in repair protein content across the mitochondrial population and is at least partially responsible for mitochondrial DNA instability. [ABSTRACT FROM PUBLISHER]

ANT1, TWINKLE and POLG genes affect mtDNA stability and are involved in autosomal dominant PEO, while mutations in POLG are responsible for numerous clinical presentations, including autosomal recessive PEO, sensory ataxic neuropathy, dysarthria and ophthalmoparesis (SANDO), spino-cerebellar ataxia and epilepsy (SCAE) or Alpers syndrome. In this study, we report on the mutational analysis of ANT1, TWINKLE and POLG genes in 15 unrelated patients, using a dHPLC-based protocol. This series of patients illustrates the large array of clinical presentations associated with mtDNA stability defects, ranging from isolated benign PEO to fatal Alpers syndrome. A total of seven different mutations were identified in six of 15 patients (40%). Six different recessive mutations were found in POLG, one in TWINKLE while no mutation was identified in ANT1. Among the POLG mutations, three are novel and include two missense and one frameshift changes. Seventeen neutral changes and polymorphisms were also identified, including four novel neutral polymorphisms. Overall, this study illustrates the variability of phenotypes associated with mtDNA stability defects, increases the mutational spectrum of POLG variants and provides an efficient and reliable detection protocol for ANT1, TWINKLE and POLG mutational screening.European Journal of Human Genetics (2006) 14, 917–922. doi:10.1038/sj.ejhg.5201627; published online 26 April 2006 [ABSTRACT FROM AUTHOR]

OBJECTIVE--The m.3243A>G mutation in mitochondrial DNA (mtDNA) is responsible for maternally inherited diabetes and deafness (MIDD). Other mtDNA mutations are extremely rare. RESEARCH DESIGN AND METHODS--We studied a patient presenting with diabetes and deafness who does not carry the m.3243A>G mutation. RESULTS--We identified a deficiency of respiratory chain complex I in the patient's fibroblasts. mtDNA sequencing revealed a novel mutation that corresponds to an insertion of one or two cytosine residues in the coding region of the MT-ND6 gene (m.14535_14536insC or CC), leading to premature stop codons. This heteroplasmic mutation is unstable in the patient's somatic tissues. CONCLUSIONS--We describe for the first time an unstable mutation in a mitochondrial gene coding for a complex I subunit, which is responsible for the MIDD phenotype. This mutation is likely favored by the m.14530T>C polymorphism, which is homoplasmic and leads to the formation of an 8-bp polyC tract responsible for genetic instability. [ABSTRACT FROM AUTHOR]

Background: Since the advent of next generation sequencing (NGS), several studies have tried to evaluate the relevance of targeted gene panel sequencing and whole exome sequencing for molecular diagnosis of mitochondrial diseases. The comparison between these different strategies is extremely difficult. A recent study analysed a cohort of patients affected by a mitochondrial disease using a NGS approach based on a targeted gene panel including 132 genes. This strategy led to identify the causative mutations in 15.2% of cases. The number of novel genes responsible for respiratory chain deficiency increases very rapidly. Methods: In order to determine the impact of larger panels used as a first screening strategy on molecular diagnosis success, we analysed a cohort of 80 patients affected by a mitochondrial disease with a first mitochondrial DNA (mtDNA) NGS screening and secondarily a targeted mitochondrial panel of 281 nuclear genes. Results: Pathogenic mtDNA abnormalities were identified in 4.1% (1/24) of children and 25% (14/56) of adult patients. The remaining 65 patients were analysed with our targeted mitochondrial panel and this approach enabled us to achieve an identification rate of 21.7% (5/23) in children versus 7.1% (3/42) in adults. Conclusions: Our results confirm that larger gene panels do not improve diagnostic yield of mitochondrial diseases due to (i) their very high genetic heterogeneity, (ii) the ongoing discovery of novel genes and (iii) mutations in genes apparently not related to mitochondrial function that lead to secondary respiratory chain deficiency. [ABSTRACT FROM AUTHOR]

Mutations in genes coding for mitochondrial helicases such as TWINKLE and DNA2 are involved in mitochondrial myopathies with mtDNA instability in both human and mouse. We show that inactivation of Pif1 , a third member of the mitochondrial helicase family, causes a similar phenotype in mouse. pif1 − /− animals develop a mitochondrial myopathy with respiratory chain deficiency. Pif1 inactivation is responsible for a deficiency to repair oxidative stress-induced mtDNA damage in mouse embryonic fibroblasts that is improved by complementation with mitochondrial isoform mPif1 67 . These results open new perspectives for the exploration of patients with mtDNA instability disorders. [ABSTRACT FROM AUTHOR]

Abstract: Mutations within the mitochondrially encoded cytochrome b (MTCYB) gene are heteroplasmic and lead to severe exercise intolerance. We describe an unusual clinical presentation secondary to a novel homoplasmic mutation within MTCYB. The m.15635T>C transition (S297P) was carried by a newborn who presented with a polyvisceral failure. This mutation was responsible for a complex III deficiency. It was homoplasmic in all tissues tested and was undetectable in patient’s mother. Functional analyses, including studies on patient’s cybrid cell lines, demonstrate the pathogenicity of this variant. Our data show that mutations within MTCYB can be responsible for severe phenotype at birth. [Copyright &y& Elsevier]

Mutations in the CHCHD10 gene have been recently identified in a large family with a complex phenotype variably associating frontotemporal dementia (FTD) with amyotrophic lateral sclerosis (ALS), cerebellar ataxia, myopathy, and hearing impairment. CHCHD10 encodes a protein located in the mitochondrial intermembrane space and is likely involved in mitochondrial genome stability and maintenance of cristae junctions. However, the exact contribution of CHCHD10 in FTD and ALS diseases spectrum remains unknown. In this study, we evaluated the frequency of CHCHD10 mutations in 115 patients with FTD and FTD-ALS phenotypes. We identified 2 heterozygous variants in 3 unrelated probands presenting FTD and ALS, characterized by early and predominant bulbar symptoms. This study demonstrates the implication of CHCHD10 in FTD and ALS spectrum. Although the frequency of mutations is low in this series (2.6%), our work suggests that CHCHD10 mutations should be searched particularly when bulbar symptoms are present at onset. [ABSTRACT FROM AUTHOR]