A neonatal polyvisceral failure linked to a de novo homoplasmic mutation in the mitochondrially encoded cytochrome b gene

Mitochondrial Deoxyribonucleic Acid (mtDNA), Maternal Inheritance, and Their Role in the Development of Cancers: A Scoping Review

Mitochondrial DNA (mtDNA) is a small, circular, double-stranded DNA inherited from the mother during fertilization. Evolutionary evidence supported by the endosymbiotic theory identifies mitochondria as an organelle that could have descended from prokaryotes. This may be the reason for the independent function and inheritance pattern shown by mtDNA. The unstable nature of mtDNA due to the lack of protective histones, and effective repair systems make it more vulnerable to mutations. The mtDNA and its mutations could be maternally inherited thereby predisposing the offspring to various cancers like breast and ovarian cancers among others. Although mitochondria are considered heteroplasmic wherein variations among the multiple mtDNA genomes are noticed, mothers can have mitochondrial populations that are homoplasmic for a given mitochondrial mutation. Homoplasmic mitochondrial mutations may be transmitted to all maternal offspring. However, due to the complex interplay between the mitochondrial and nuclear genomes, it is often difficult to predict disease outcomes, even with homoplasmic mitochondrial populations. Heteroplasmic mtDNA mutations can be maternally inherited, but the proportion of mutated alleles differs markedly between offspring within one generation. This led to the genetic bottleneck hypothesis, explaining the rapid changes in allele frequency witnessed during the transmission of mtDNA from one generation to the next. Although a physical reduction in mtDNA has been demonstrated in several species, a comprehensive understanding of the molecular mechanisms is yet to be demonstrated. Despite initially thought to be limited to the germline, there is evidence that blockages exist in different cell types during development, perhaps explaining why different tissues in the same organism contain different levels of mutated mtDNA. In this review, we comprehensively discuss the potential mechanisms through which mtDNA undergoes mutations and the maternal mode of transmission that contributes to the development of tumors, especially breast and ovarian cancers.

Organization of the Respiratory Supercomplexes in Cells with Defective Complex III: Structural Features and Metabolic Consequences

The mitochondrial respiratory chain encompasses four oligomeric enzymatic complexes (complex I, II, III and IV) which, together with the redox carrier ubiquinone and cytochrome c, catalyze electron transport coupled to proton extrusion from the inner membrane. The protonmotive force is utilized by complex V for ATP synthesis in the process of oxidative phosphorylation. Respiratory complexes are known to coexist in the membrane as single functional entities and as supramolecular aggregates or supercomplexes (SCs). Understanding the assembly features of SCs has relevant biomedical implications because defects in a single protein can derange the overall SC organization and compromise the energetic function, causing severe mitochondrial disorders. Here we describe in detail the main types of SCs, all characterized by the presence of complex III. We show that the genetic alterations that hinder the assembly of Complex III, not just the activity, cause a rearrangement of the architecture of the SC that can help to preserve a minimal energetic function. Finally, the major metabolic disturbances associated with severe SCs perturbation due to defective complex III are discussed along with interventions that may circumvent these deficiencies.

Overview of mitochondrial germline variants and mutations in human disease: Focus on breast cancer (Review)

High lactate production in cells during growth under oxygen-rich conditions (aerobic glycolysis) is a hallmark of tumor cells, indicating the role of mitochondrial function in tumorigenesis. In fact, enhanced mitochondrial biogenesis and impaired quality control are frequently observed in cancer cells. Mitochondrial DNA (mtDNA) encodes 13 subunits of oxidative phosphorylation (OXPHOS), is present in thousands of copies per cell, and has a very high mutation rate. Mutations in mtDNA and nuclear DNA (nDNA) genes encoding proteins that are important players in mitochondrial biogenesis and function are involved in oncogenic processes. A wide range of germline mtDNA polymorphisms, as well as tumor mtDNA somatic mutations have been identified in diverse cancer types. Approximately 72% of supposed tumor-specific somatic mtDNA mutations reported, have also been found as polymorphisms in the general population. The ATPase 6 and NADH dehydrogenase subunit genes of mtDNA are the most commonly mutated genes in breast cancer (BC). Furthermore, nuclear genes playing a role in mitochondrial biogenesis and function, such as peroxisome proliferators-activated receptor gamma coactivator-1 (PGC-1), fumarate hydratase (FH) and succinate dehydrogenase (SDH) are frequently mutated in cancer. In this review, we provide an overview of the mitochondrial germline variants and mutations in cancer, with particular focus on those found in BC.

De novo mtDNA point mutations are common and have a low recurrence risk

Background

Severe, disease-causing germline mitochondrial (mt)DNA mutations are maternally inherited or arise de novo. Strategies to prevent transmission are generally available, but depend on recurrence risks, ranging from high/unpredictable for many familial mtDNA point mutations to very low for sporadic, large-scale single mtDNA deletions. Comprehensive data are lacking for de novo mtDNA point mutations, often leading to misconceptions and incorrect counselling regarding recurrence risk and reproductive options. We aim to study the relevance and recurrence risk of apparently de novo mtDNA point mutations.

Methods

Systematic study of prenatal diagnosis (PND) and recurrence of mtDNA point mutations in families with de novo cases, including new and published data. ‘De novo’ based on the absence of the mutation in multiple (postmitotic) maternal tissues is preferred, but mutations absent in maternal blood only were also included.

Results

In our series of 105 index patients (33 children and 72 adults) with (likely) pathogenic mtDNA point mutations, the de novo frequency was 24.6%, the majority being paediatric. PND was performed in subsequent pregnancies of mothers of four de novo cases. A fifth mother opted for preimplantation genetic diagnosis because of a coexisting Mendelian genetic disorder. The mtDNA mutation was absent in all four prenatal samples and all 11 oocytes/embryos tested. A literature survey revealed 137 de novo cases, but PND was only performed for 9 (including 1 unpublished) mothers. In one, recurrence occurred in two subsequent pregnancies, presumably due to germline mosaicism.

Conclusions

De novo mtDNA point mutations are a common cause of mtDNA disease. Recurrence risk is low. This is relevant for genetic counselling, particularly for reproductive options. PND can be offered for reassurance.

A novel mutation in the mitochondrial DNA cytochrome b gene (MTCYB) in a patient with Prader Willi syndrome

In recent years, it has been suggested that defects in energy metabolism may accompany Prader Willi syndrome. Mutations in the mitochondrial cytochrome b gene have been commonly associated isolated mitochondrial myopathy and exercise intolerance, rarely with multisystem disorders. The authors describe a novel mutation (mt. 15209T>C) in mitochondrial cytochrome b gene in a 2-year-old girl with Prader-Willi syndrome with a clinical history of lactic acidosis attacks, renal sodium loss, hepatopathy, progressive cerebral atrophy, and sudden death. The authors suggest that atypical clinical findings in patients with Prader-Willi syndrome should direct the physician to search for a mitochondrial disease.

A novel in-frame 18-bp microdeletion in MT-CYB causes a multisystem disorder with prominent exercise intolerance

A novel heteroplasmic mitochondrial DNA (mtDNA) microdeletion affecting the cytochrome b gene (MT-CYB) was identified in an Italian female patient with a multisystem disease characterized by sensorineural deafness, cataracts, retinal pigmentary dystrophy, dysphagia, postural and gait instability, and myopathy with prominent exercise intolerance. The deletion is 18-base pair long and encompasses nucleotide positions 15,649-15,666, causing the loss of six amino acids (Ile-Leu-Ala-Met-Ile-Pro) in the protein, but leaving the remaining of the MT-CYB sequence in frame. The defective complex III function was cotransferred with mutant mtDNA in cybrids, thus unequivocally establishing its pathogenic role. Maternal relatives failed to show detectable levels of the deletion in blood and urinary epithelium, suggesting a de novo mutational event. This is the second report of an in-frame intragenic deletion in MT-CYB, which most likely occurred in early stages of embryonic development, associated with a severe multisystem disorder with prominent exercise intolerance.

A novel mutation in the mitochondrial DNA cytochrome b gene (MTCYB) in a patient with mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes syndrome

Mutations in the mitochondrial DNA cytochrome b gene (MTCYB) have been commonly associated with isolated mitochondrial myopathy and exercise intolerance, rarely with multisystem disorders, and only once with a parkinsonism/mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes (MELAS) overlap syndrome. Here, we describe a novel mutation (m.14864 T>C) in MTCYB in a 15-year-old girl with a clinical history of migraines, epilepsy, sensorimotor neuropathy, and strokelike episodes, a clinical picture reminiscent of MELAS.  The mutation, which changes a highly conserved cysteine to arginine at amino acid position 40 of cytochrome b, was heteroplasmic in muscle, blood, fibroblasts, and urinary sediment from the patient but absent in accessible tissues from her asymptomatic mother. This case demonstrates that MTCYB must be included in the already long list of mitochondrial DNA genes that have been associated with the MELAS phenotype.

Respiratory complex III dysfunction in humans and the use of yeast as a model organism to study mitochondrial myopathy and associated diseases

The bc1 complex or complex III is a central component of the aerobic respiratory chain in prokaryotic and eukaryotic organisms. It catalyzes the oxidation of quinols and the reduction of cytochrome c, establishing a proton motive force used to synthesize adenosine triphosphate (ATP) by the F1Fo ATP synthase. In eukaryotes, the complex III is located in the inner mitochondrial membrane. The genes coding for the complex III have a dual origin. While cytochrome b is encoded by the mitochondrial genome, all the other subunits are encoded by the nuclear genome. In this review, we compile an exhaustive list of the known human mutations and associated pathologies found in the mitochondrially-encoded cytochrome b gene as well as the fewer mutations in the nuclear genes coding for the complex III structural subunits and accessory proteins such as BCS1L involved in the assembly of the complex III. Due to the inherent difficulties of studying human biopsy material associated with complex III dysfunction, we also review the work that has been conducted to study the pathologies with the easy to handle eukaryotic microorganism, the yeast Saccharomyces cerevisiae. Phenotypes, biochemical data and possible effects due to the mutations are also discussed in the context of the known three-dimensional structure of the eukaryotic complex III. This article is part of a Special Issue entitled: Respiratory complex III and related bc complexes.

Comparison of mitochondrial mutation spectra in ageing human colonic epithelium and disease: absence of evidence for purifying selection in somatic mitochondrial DNA point mutations

Human ageing has been predicted to be caused by the accumulation of molecular damage in cells and tissues. Somatic mitochondrial DNA (mtDNA) mutations have been documented in a number of ageing tissues and have been shown to be associated with cellular mitochondrial dysfunction. It is unknown whether there are selective constraints, which have been shown to occur in the germline, on the occurrence and expansion of these mtDNA mutations within individual somatic cells. Here we compared the pattern and spectrum of mutations observed in ageing human colon to those observed in the general population (germline variants) and those associated with primary mtDNA disease. The pathogenicity of the protein encoding mutations was predicted using a computational programme, MutPred, and the scores obtained for the three groups compared. We show that the mutations associated with ageing are randomly distributed throughout the genome, are more frequently non-synonymous or frameshift mutations than the general population, and are significantly more pathogenic than population variants. Mutations associated with primary mtDNA disease were significantly more pathogenic than ageing or population mutations. These data provide little evidence for any selective constraints on the occurrence and expansion of mtDNA mutations in somatic cells of the human colon during human ageing in contrast to germline mutations seen in the general population.

Mitochondrial DNA encodes essential components of the mitochondrial respiratory chain and is strictly maternally inherited, making it vulnerable to the accumulation of deleterious mutations. To avoid this, mtDNA is subjected to a bottleneck phenomenon whereby only a small number of mtDNA molecules are passed on to the oocyte precursor. These are then amplified to the required number of mtDNA molecules in the mature oocyte, meaning that any mutations may be either lost or rapidly fixed. Purifying selection is thought to be an important protective mechanism against pathogenic mtDNA mutations in the germline, as this is essential for mtDNA stability. It is unknown whether there are any such protective mechanisms in the somatic tissues. To investigate this we have compared the spectrum of mutations present in ageing human colonocytes with those population variants passed through the maternal germline and mtDNA mutations responsible for primary mtDNA disease. We show that pathogenic mtDNA mutations are present at a significantly higher frequency in the somatic cells of the human colon in contrast to variants that have passed though the germline, showing little evidence for purifying selection in the somatic tissues studied here, but strong evidence of this selective mechanism in the germline.

Neonatal onset of mitochondrial disorders in 129 patients: clinical and laboratory characteristics and a new approach to diagnosis

INTRODUCTION

Mitochondrial disorders (MD) may manifest in neonates, but early diagnosis is difficult. In this study, clinical and laboratory data were analyzed in 129 patients with neonatal onset of MD to identify any association between specific mitochondrial diseases and their symptoms with the aim of optimizing diagnosis.

MATERIALS AND METHODS

Retrospective clinical and laboratory data were evaluated in 461 patients (331 families) with confirmed MD.

RESULTS

The neonatal onset of MD was reported in 28% of the patients. Prematurity, intrauterine growth retardation and hypotonia necessitating ventilatory support were present in one-third, cardiomyopathy in 40%, neonatal seizures in 16%, Leigh syndrome in 15%, and elevated lactate level in 87%. Hyperammonemia was observed in 22 out of 52 neonates. Complex I deficiency was identified in 15, complex III in one, complex IV in 23, complex V in 31, combined deficiency of several complexes in 53, and PDH complex deficiency was identified in six patients. Molecular diagnosis was confirmed in 49 cases, including a newborn with a 9134A>G mutation in the MTATP6 gene, which has not been described previously.

CONCLUSION

The most significant finding is the high incidence of neonatal cardiomyopathy and hyperammonemia. Based on our experience, we propose a diagnostic flowchart applicable to critically ill neonates suspicious for MD. This tool will allow for the use of direct molecular genetic analyses without the need for muscle biopsies in neonates with Alpers, Barth, MILS and Pearson syndromes, SCO1, SCO2, TMEM70, ATP5E, SUCLG1 gene mutations and PDH complex deficiency.

A novel unstable mutation in mitochondrial DNA responsible for maternally inherited diabetes and deafness

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.

Mitochondrial hepatopathies in the newborn period

Mitochondrial disorders recognized in the neonatal period usually present as a metabolic crisis combined with one or several organ manifestations. Liver disorder in association with a respiratory chain deficiency may be overlooked since liver dysfunction is common in severely sick newborn infants. Lactacidosis, hypoglycemia, elevated serum transaminases and conjugated bilirubin are common signs of mitochondrial hepatopathy. Hepatosplenomegaly may occur in severe cases. A clinical picture with fetal growth restriction, postnatal lactacidosis, hypoglycemia, coagulopathy, and cholestasis, especially in combination with neurological symptoms or renal tubulopathy, should alert the neonatologist to direct investigations on mitochondrial disorder. A normal lactate level does not exclude respiratory chain defects. The most common liver manifestation caused by mutated mitochondrial DNA (deletion) is Pearson syndrome. Recently, mutations in several nuclear DNA genes have been identified that lead to mitochondrial hepatopathy, e.g. mitochondrial depletion syndrome caused by DGUOK, MPV17, SUCLG1, POLG1, or C10ORF2 mutations. A combination of lactacidosis, liver involvement, and Fanconi type renal tubulopathy is common when the complex III assembly factor BCS1L harbors mutations, the most severe disease with consistent genotype-phenotype correlation being the GRACILE syndrome. Mutations in nuclear translation factor genes (TRMU, EFG1, and EFTu) of the respiratory chain enzyme complexes have recently been identified. Diagnostic work-up of neonatal liver disorder should include assessment of function and structure of the complexes as well as mutation screening for known genes. So far, treatment is mainly symptomatic.

Mitochondrial disorders caused by mutations in respiratory chain assembly factors

Mitochondrial diseases involve the dysfunction of the oxidative phosphorylation (OXPHOS) system. This group of diseases presents with heterogeneous clinical symptoms affecting mainly organs with high energy demands. Defects in the multimeric complexes comprising the OXPHOS system have a dual genetic origin, mitochondrial or nuclear DNA. Although many nuclear DNA mutations involve genes coding for subunits of the respiratory complexes, the majority of mutations found to date affect factors that do not form part of the final complexes. These assembly factors or chaperones have multiple functions ranging from cofactor insertion to proper assembly/stability of the complexes. Although significant progress has been made in the last few years in the discovery of new assembly factors, the function of many remains elusive. Here, we describe assembly factors or chaperones that are required for respiratory chain complex assembly and their clinical relevance.