Molecular and clinical genetics of mitochondrial diseases due to POLG mutations

Mutations in the POLG gene have emerged as one of the most common causes of inherited mitochondrial disease in children and adults. They are responsible for a heterogeneous group of at least 6 major phenotypes of neurodegenerative disease that include: 1) childhood Myocerebrohepatopathy Spectrum disorders (MCHS), 2) Alpers syndrome, 3) Ataxia Neuropathy Spectrum (ANS) disorders, 4) Myoclonus Epilepsy Myopathy Sensory Ataxia (MEMSA), 5) autosomal recessive Progressive External Ophthalmoplegia (arPEO), and 6) autosomal dominant Progressive External Ophthalmoplegia (adPEO). Due to the clinical heterogeneity, time-dependent evolution of symptoms, overlapping phenotypes, and inconsistencies in muscle pathology findings, definitive diagnosis relies on the molecular finding of deleterious mutations. We sequenced the exons and flanking intron region from approximately 350 patients displaying a phenotype consistent with POLG related mitochondrial disease and found informative mutations in 61 (17%). Two mutant alleles were identified in 31 unrelated index patients with autosomal recessive POLG-related disorders. Among them, 20 (67%) had Alpers syndrome, 4 (13%) had arPEO, and 3 (10%) had ANS. In addition, 30 patients carrying one altered POLG allele were found. A total of 25 novel alterations were identified, including 6 null mutations. We describe the predicted structural/functional and clinical importance of the previously unreported missense variants and discuss their likelihood of being pathogenic. In conclusion, sequence analysis allows the identification of mutations responsible for POLG-related disorders and, in most of the autosomal recessive cases where two mutant alleles are found in trans, finding deleterious mutations can provide an unequivocal diagnosis of the disease.     

Functional consequences of mitochondrial tRNATrp and tRNAArg mutations causing combined OXPHOS defects

Combined oxidative phosphorylation (OXPHOS) system deficiencies are a group of mitochondrial disorders that are associated with a range of clinical phenotypes and genetic defects. They occur in approximately 30% of all OXPHOS disorders and around 4% are combined complex I, III and IV deficiencies. In this study we present two mutations in the mitochondrial tRNA^Trp (MT-TW) and tRNA^Arg (MT-TR) genes, m.5556G>A and m.10450A>G, respectively, which were detected in two unrelated patients showing combined OXPHOS complex I, III and IV deficiencies and progressive multisystemic diseases. Both mitochondrial tRNA mutations were almost homoplasmic in fibroblasts and muscle tissue of the two patients and not present in controls. Patient fibroblasts showed a general mitochondrial translation defect. The mutations resulted in lowered steady-state levels and altered conformations of the tRNAs. Cybrid cell lines showed similar tRNA defects and impairment of OXPHOS complex assembly as patient fibroblasts. Our results show that these tRNA^Trp and tRNA^Arg mutations cause the combined OXPHOS deficiencies in the patients, adding to the still expanding group of pathogenic mitochondrial tRNA mutations.     

A study of 133 Chinese children with mitochondrial respiratory chain complex I deficiency

To investigate the clinical, enzymological and mitochondrial gene profiles of complex I deficiency in Chinese, clinical and laboratory data of the patients (79 boys, 54 girls) were retrospectively assessed. Activities of mitochondrial respiratory chain complexes in peripheral leucocytes were spectrophotometrically measured. The entire mitochondrial DNA (mtDNA) sequence was analyzed in 62 patients. Restriction fragment length polymorphism and gene sequencing analyses were performed in 15 families. Ninety‐one patients had isolated complex I deficiency; 42 had combined deficiencies of complex I and other complexes. The main clinical presentations were neuromuscular disorders (107 patients) and non‐neurological dysfunction (hepatopathy, renal damage and cardiomyopathy; 26 patients). In 32 of 62 patients who underwent mtDNA sequencing, 24 mutations were identified in 15 mitochondrial genes. The 12338T>C, 4833A>G and 14502T>C mutations were found in 12.9%, 11.3% and 4.8% patients, respectively. Seven patients had multiple mutations. Three novel mutations were identified. Chinese patients with complex I deficiency presented heterogeneous phenotypes and genotypes. Twenty‐four mutations were identified in 15 mitochondrial genes in 51.6% patients. mtDNA mutations were more common in isolated complex I deficiency than in combined complex deficiencies. The 12338T>C, 4833A>G and 14502T>C mutations were common.     

Respiratory chain complex I deficiency caused by mitochondrial DNA mutations

Defects of the mitochondrial respiratory chain are associated with a diverse spectrum of clinical phenotypes, and may be caused by mutations in either the nuclear or the mitochondrial genome (mitochondrial DNA (mtDNA)). Isolated complex I deficiency is the most common enzyme defect in mitochondrial disorders, particularly in children in whom family history is often consistent with sporadic or autosomal recessive inheritance, implicating a nuclear genetic cause. In contrast, although a number of recurrent, pathogenic mtDNA mutations have been described, historically, these have been perceived as rare causes of paediatric complex I deficiency. We reviewed the clinical and genetic findings in a large cohort of 109 paediatric patients with isolated complex I deficiency from 101 families. Pathogenic mtDNA mutations were found in 29 of 101 probands (29%), 21 in MTND subunit genes and 8 in mtDNA tRNA genes. Nuclear gene defects were inferred in 38 of 101 (38%) probands based on cell hybrid studies, mtDNA sequencing or mutation analysis (nuclear gene mutations were identified in 22 probands). Leigh or Leigh-like disease was the most common clinical presentation in both mtDNA and nuclear genetic defects. The median age at onset was higher in mtDNA patients (12 months) than in patients with a nuclear gene defect (3 months). However, considerable overlap existed, with onset varying from 0 to >60 months in both groups. Our findings confirm that pathogenic mtDNA mutations are a significant cause of complex I deficiency in children. In the absence of parental consanguinity, we recommend whole mitochondrial genome sequencing as a key approach to elucidate the underlying molecular genetic abnormality.     

The pathophysiology of mitochondrial disease as modeled in the mouse

It is now clear that mitochondrial defects are associated with a plethora of clinical phenotypes in man and mouse. This is the result of the mitochondria''s central role in energy production, reactive oxygen species (ROS) biology, and apoptosis, and because the mitochondrial genome consists of roughly 1500 genes distributed across the maternal mitochondrial DNA (mtDNA) and the Mendelian nuclear DNA (nDNA). While numerous pathogenic mutations in both mtDNA and nDNA mitochondrial genes have been identified in the past 21 years, the causal role of mitochondrial dysfunction in the common metabolic and degenerative diseases, cancer, and aging is still debated. However, the development of mice harboring mitochondrial gene mutations is permitting demonstration of the direct cause-and-effect relationship between mitochondrial dysfunction and disease. Mutations in nDNA-encoded mitochondrial genes involved in energy metabolism, antioxidant defenses, apoptosis via the mitochondrial permeability transition pore (mtPTP), mitochondrial fusion, and mtDNA biogenesis have already demonstrated the phenotypic importance of mitochondrial defects. These studies are being expanded by the recent development of procedures for introducing mtDNA mutations into the mouse. These studies are providing direct proof that mtDNA mutations are sufficient by themselves to generate major clinical phenotypes. As more different mtDNA types and mtDNA gene mutations are introduced into various mouse nDNA backgrounds, the potential functional role of mtDNA variation in permitting humans and mammals to adapt to different environments and in determining their predisposition to a wide array of diseases should be definitively demonstrated.     

Atypical case of Wolfram syndrome revealed through targeted exome sequencing in a patient with suspected mitochondrial disease

Background  

Mitochondrial diseases comprise a diverse set of clinical disorders that affect multiple organ systems with varying severity and age of onset. Due to their clinical and genetic heterogeneity, these diseases are difficult to diagnose. We have developed a targeted exome sequencing approach to improve our ability to properly diagnose mitochondrial diseases and apply it here to an individual patient. Our method targets mitochondrial DNA (mtDNA) and the exons of 1,600 nuclear genes involved in mitochondrial biology or Mendelian disorders with multi-system phenotypes, thereby allowing for simultaneous evaluation of multiple disease loci.     

Yield of mtDNA mutation analysis in 2,000 patients

The multiplex polymerase chain reaction–allele specific oligonucleotides (PCR/ASO) dot blot hybridization method was used to detect 44 mitochondrial DNA point mutations in 2,000 patients suspected as having mitochondrial DNA disorders. These point mutations are classified into four categories. Category I consists of primary disease-causing, heteroplasmic point mutations. Homoplasmic nucleotide substitutions that have been reported to be possibly disease associated are in Category II. Homoplasmic nucleotide substitutions that are thought to be benign polymorphism are included in category III. The novel nucleotide substitutions recently discovered in our laboratory by single strand conformation polymorphism analysis are in category IV. Frequencies of these 44 nucleotide substitutions in 2,000 patients and 262 control individuals were studied. The results indicated that analysis of 12 recurrent disease-causing point mutations in category I identified 5.4 % of the patients suspected as having mitochondrial DNA disorders. Since the mitochondrial disorders are a group of complex, heterogeneous, and multisystemic diseases, it is often difficult to confirm clinical diagnosis without molecular studies. Thus, the multiplex PCR/ASO method is an effective approach for initial screening of mtDNA mutations in patients suspected as having mitochondrial DNA disorders. Am. J. Med. Genet. 77:395–400, 1998. © 1998 Wiley-Liss, Inc.     

Mitochondrial diseases: an overview of genetics, pathogenesis, clinical features and an approach to diagnosis and treatment

Defects in structures or functions of mitochondria, mainly involving the oxidative phosphorylation, mitochondrial biogenesis and other metabolic pathways have been shown to be associated with a wide spectrum of clinical phenotypes. The ubiquitous nature of mitochondria and their unique genetic features contribute to the clinical, biochemical and genetic heterogenecity of mitochondrial diseases. This article focuses on the recent advances in the field of mitochondrial disorders with respect to the consequences for an advanced clinical and genetic diagnostics. In addition, an overview on recently identified genetic defects and their pathogenic molecular mechanisms are given.     

Mutations of the mitochondrial genome and its clinical expression in cardiology

One of the great challenges in molecular biology is to understand the mechanisms by which a particular genetic defect gives origin to a specific disease. Mitochondrial DNA is more susceptible than nuclear DNA to mutations. Mitochondrial mutations have been associated with a wide spectrum of disorders characterized by a complex phenotype and actually named mitochondrial cytopathies or oxidative phosphorylation diseases. The objective of this paper is to review the relevant genetic, clinical, and morphologic features of cardiac involvement in this heterogeneous but exciting group of diseases. The clinical features of cardiac involvement in mitochondrial cytopathies vary in the different subgroups of these disorders and in particular, mitochondrial mutations can causes characteristic cardiac abnormalities.     

LHON/MELAS overlap syndrome associated with a mitochondrial MTND1 gene mutation

Pathogenic point mutations in the mitochondrial MTND1 gene have previously been described in association with two distinct clinical phenotypes -- Leber hereditary optic neuropathy (LHON) and mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS). Here we report the first heteroplasmic mitochondrial DNA (mtDNA) point mutation (3376G>A) in the MTND1 gene associated with an overlap syndrome comprising the clinical features of both LHON and MELAS. Muscle histochemistry revealed subtle mitochondrial abnormalities, while biochemical analysis showed an isolated complex I deficiency. Our findings serve to highlight the growing importance of mutations in mitochondrial complex I structural genes in MELAS and its associated overlap syndromes.     

Cytochrome c oxidase deficiency

Cytochrome c oxidase (COX) is the terminal enzyme of the mitochondrial respiratory chain, catalyzing the transfer of electrons from reduced cytochrome c to molecular oxygen. It is composed of 13 structural subunits, three of which are encoded in mtDNA and form the catalytic core of the enzyme. In addition to these structural subunits, a large number of accessory factors are necessary for the assembly and maintenance of the active holoenzyme complex. Most isolated COX deficiencies are inherited as autosomal recessive disorders; mutations in the mtDNA-encoded COX subunit genes are relatively rare. These mutations are associated with a wide spectrum of clinical phenotypes ranging from isolated myopathy to multisystem disease, with onset from late childhood to adulthood. Autosomal recessive COX deficiencies generally have a very early age of onset and a fatal outcome. Several clinical presentations have been described including Leigh Syndrome, hypertrophic cardiomyopathy and myopathy, and fatal infantile lactic acidosis. Surprisingly, mutations in the nuclear-encoded structural COX subunits have not been found in association with any of these phenotypes. Mutations have, however, been identified in several COX assembly factors: SURF1 (Leigh Syndrome), SCO2 (hypertrophic cardiomyopathy), SCO1 (hepatic failure, ketoacidotic coma), and COX10 (encephalopathy, tubulopathy). As all of these assembly factors are ubiquitously expressed, the molecular basis for the different clinical presentations remains unexplained. Although the genetic defects in the majority of patients with COX deficiency are unknown, it is likely that most will be solved in the near future using functional complementation techniques.     

Quantitative multiplex PCR of short fluorescent fragments for the detection of large intragenic POLG rearrangements in a large French cohort

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.     

Organismal effects of mitochondrial dysfunction

Mitochondrial disease can lead to clinical abnormalities in any organ system. Both inherited and spontaneous disorders are known. The spontaneous forms can occur as a mitochondrial DNA (mtDNA) mutation early in embryogenesis or, later in life, as somatic mutations that accumulate with age. The inherited forms may arise from any of >100 characterized mutations in mtDNA or from >200 nuclear gene defects that affect proteins required for mitochondrial function. Most dividing cells survive and interact normally despite their mitochondrial defects. Thus post-mitotic, terminally differentiated cells are preferentially affected in mitochondrial disease. This review emphasizes cellular metabolic co-operation and the structural and biochemical diversity of mitochondria as the framework for understanding the clinical spectrum of mitochondrial disease. The principles of the mitochondrial clinical assessment scale I (MCAS-I) are presented to assist in the development of diagnostic spectra of mitochondrial disease.     

Polydactyly: how many disorders and how many genes?

Disorders that include polydactyly as a manifestation are diverse and numerous. Cataloging these disorders by phenotype and genotype demonstrates numerous overlapping phenotypes, genetic heterogeneity of phenotypes, and distinct phenotypes generated from mutations in single genes. To assess these issues, a list of disorders with polydactyly has been compiled from several sources. Among 119 disorders, 39 disorders are associated with mutations in genes, and among these, genotypic and phenotypic overlap is demonstrated. These issues highlight the need for a diagnostic system that catalogs both genotype and phenotype. Published 2002 Wiley-Liss, Inc.     

Polydactyly: How many disorders and how many genes?

Disorders that include polydactyly as a manifestation are diverse and numerous. Cataloging these disorders by phenotype and genotype demonstrates numerous overlapping phenotypes, genetic heterogeneity of phenotypes, and distinct phenotypes generated from mutations in single genes. To assess these issues, a list of disorders with polydactyly has been compiled from several sources. Among 119 disorders, 39 disorders are associated with mutations in genes, and among these, genotypic and phenotypic overlap is demonstrated. These issues highlight the need for a diagnostic system that catalogs both genotype and phenotype. Published 2002 Wiley‐Liss, Inc.     

Mutations in COX10 result in a defect in mitochondrial heme A biosynthesis and account for multiple,early-onset clinical phenotypes associated with isolated COX deficiency

Deficiencies in the activity of cytochrome c oxidase (COX) are an important cause of autosomal recessive respiratory chain disorders. Patients with isolated COX deficiency are clinically and genetically heterogeneous, and mutations in several different assembly factors have been found to cause specific clinical phenotypes. Two of the most common clinical presentations, Leigh Syndrome and hypertrophic cardiomyopathy, have so far only been associated with mutations in SURF1 or SCO2 and COX15, respectively. Here we show that expression of COX10 from a retroviral vector complements the COX deficiency in a patient with anemia and Leigh Syndrome, and in a patient with anemia, sensorineural deafness and fatal infantile hypertrophic cardiomyopathy. A partial rescue was also obtained following microcell-mediated transfer of mouse chromosomes into patient fibroblasts. COX10 functions in the first step of the mitochondrial heme A biosynthetic pathway, catalyzing the conversion of protoheme (heme B) to heme O via the farnesylation of a vinyl group at position C2. Heme A content was reduced in mitochondria from patient muscle and fibroblasts in proportion to the reduction in COX enzyme activity and the amount of fully assembled enzyme. Mutation analysis of COX10 identified four different missense alleles, predicting amino acid substitutions at evolutionarily conserved residues. A topological model places these residues in regions of the protein shown to have important catalytic functions by mutation analysis of a prokaryotic ortholog. Mutations in COX10 have previously been reported in a single family with tubulopathy and leukodystrophy. This study shows that mutations in this gene can cause nearly the full range of clinical phenotypes associated with early onset isolated COX deficiency.