Clinical and biochemical spectrum of mitochondrial complex III deficiency caused by mutations in the BCS1L gene

Pathological variants in nuclear genes causing mitochondrial complex III deficiency: An update

Mitochondrial disorders are a group of clinically and biochemically heterogeneous genetic diseases within the group of inborn errors of metabolism. Primary mitochondrial diseases are mainly caused by defects in one or several components of the oxidative phosphorylation system (complexes I-V). Within these disorders, those associated with complex III deficiencies are the least common. However, thanks to a deeper knowledge about complex III biogenesis, improved clinical diagnosis and the implementation of next-generation sequencing techniques, the number of pathological variants identified in nuclear genes causing complex III deficiency has expanded significantly. This updated review summarizes the current knowledge concerning the genetic basis of complex III deficiency, and the main clinical features associated with these conditions.

Clinical and diagnostic characteristics of complex III mitopathy due to novel BCS1L gene mutation in a Saudi patient

Background

Of the many types of mitochondrial diseases, mutations affecting BCS1L gene are regarded as chief cause of the defective mitochondrial complex-III, affecting normal mitochondrial functioning, and leading to wide variety of phenotypes.

Case presentation

In this case report we describe a novel genotype linked to a unique phenotype in a Saudi patient born of a consanguineous marriage. Detailed genetic analysis and whole genome sequencing identified a novel homozygous missense mutation in exon 5 c.712A > G (p.Ser328Gly) of the BCS1L gene, with predicted deleterious effects on the functioning AAA⁺-ATPase domain of the protein characterized by distinct clinical presentation associated with profound multisystem involvement, conductive hearing loss, absent external auditory canal, low posterior hair line, short neck, micro and retrognathia, over riding fingers, rocker bottom foot, small phallus with bilateral absent testis (empty scrotum) and intolerable lactic acidosis.

Conclusions

A pathogenic effect of this novel BCS1L mutation was reflected in the patient with his failure to thrive and a complex clinical and metabolic phenotype.

Mitochondrial Neurodegeneration

Mitochondria are cytoplasmic organelles, which generate energy as heat and ATP, the universal energy currency of the cell. This process is carried out by coupling electron stripping through oxidation of nutrient substrates with the formation of a proton-based electrochemical gradient across the inner mitochondrial membrane. Controlled dissipation of the gradient can lead to production of heat as well as ATP, via ADP phosphorylation. This process is known as oxidative phosphorylation, and is carried out by four multiheteromeric complexes (from I to IV) of the mitochondrial respiratory chain, carrying out the electron flow whose energy is stored as a proton-based electrochemical gradient. This gradient sustains a second reaction, operated by the mitochondrial ATP synthase, or complex V, which condensates ADP and Pi into ATP. Four complexes (CI, CIII, CIV, and CV) are composed of proteins encoded by genes present in two separate compartments: the nuclear genome and a small circular DNA found in mitochondria themselves, and are termed mitochondrial DNA (mtDNA). Mutations striking either genome can lead to mitochondrial impairment, determining infantile, childhood or adult neurodegeneration. Mitochondrial disorders are complex neurological syndromes, and are often part of a multisystem disorder. In this paper, we divide the diseases into those caused by mtDNA defects and those that are due to mutations involving nuclear genes; from a clinical point of view, we discuss pediatric disorders in comparison to juvenile or adult-onset conditions. The complementary genetic contributions controlling organellar function and the complexity of the biochemical pathways present in the mitochondria justify the extreme genetic and phenotypic heterogeneity of this new area of inborn errors of metabolism known as ‘mitochondrial medicine’.

Modelling of BCS1L-related human mitochondrial disease in Drosophila melanogaster

Mutations in BCS1L are the most frequent cause of human mitochondrial disease linked to complex III deficiency. Different forms of BCS1L-related diseases and more than 20 pathogenic alleles have been reported to date. Clinical symptoms are highly heterogenous, and multisystem involvement is often present, with liver and brain being the most frequently affected organs. BCS1L encodes a mitochondrial AAA + -family member with essential roles in the latest steps in the biogenesis of mitochondrial respiratory chain complex III. Since Bcs1 has been investigated mostly in yeast and mammals, its function in invertebrates remains largely unknown. Here, we describe the phenotypical, biochemical and metabolic consequences of Bcs1 genetic manipulation in Drosophila melanogaster. Our data demonstrate the fundamental role of Bcs1 in complex III biogenesis in invertebrates and provide novel, reliable models for BCS1L-related human mitochondrial diseases. These models recapitulate several features of the human disorders, collectively pointing to a crucial role of Bcs1 and, in turn, of complex III, in development, organismal fitness and physiology of several tissues.

Defective complex III mitochondrial respiratory chain due to a novel variant in CYC1 gene masquerades acute demyelinating syndrome or Leber hereditary optic neuropathy

Complex III (CIII) is the third out of five mitochondrial respiratory chain complexes residing at the mitochondrial inner membrane. The assembly of 10 subunits encoded by nuclear DNA and one by mitochondrial DNA result in the functional CIII which transfers electrons from ubiquinol to cytochrome c. Deficiencies of CIII are among the least investigated mitochondrial disorders and thus clinical spectrum of patients with mutations in CIII is not well defined. We report on a 10-year-old girl born to consanguineous Iranian parents presenting with recurrent visual loss episodes and optic nerve contrast enhancement in brain imaging reminiscent of an acquired demyelination syndrome (i.e. optic neuritis or multiple sclerosis), who was ultimately confirmed to have a novel homozygous missense variant of unknown significance, c.949C > T; p.(Arg317Trp) in the CYC1 gene, a nuclear DNA subunit of complex III of the mitochondrial chain. Sanger sequencing confirmed the segregation of this variant with disease in the family. The effect of this variant on the protein structure was shown in-silico. Our findings, not only expand the clinical spectrum due to defects in CYC1 gene but also highlight that mitochondrial respiratory chain disorders could be considered as a potential differential diagnosis in children who present with unusual patterns of acquired demyelination syndromes (ADS). In addition, our results support the hypothesis that mitochondrial disorders might have an overlapping presentation with ADS.

Mitochondrial disorders of the OXPHOS system

Mitochondrial disorders are among the most frequent inborn errors of metabolism, their primary cause being the dysfunction of the oxidative phosphorylation system (OXPHOS). OXPHOS is composed of the electron transport chain (ETC), formed by four multimeric enzymes and two mobile electron carriers, plus an ATP synthase [also called complex V (cV)]. The ETC performs the redox reactions involved in cellular respiration while generating the proton motive force used by cV to synthesize ATP. OXPHOS biogenesis involves multiple steps, starting from the expression of genes encoded in physically separated genomes, namely the mitochondrial and nuclear DNA, to the coordinated assembly of components and cofactors building each individual complex and eventually the supercomplexes. The genetic cause underlying around half of the diagnosed mitochondrial disease cases is currently known. Many of these cases result from pathogenic variants in genes encoding structural subunits or additional factors directly involved in the assembly of the ETC complexes. Here, we review the historical and most recent findings concerning the clinical phenotypes and the molecular pathological mechanisms underlying this particular group of disorders.

Human Mitochondrial Pathologies of the Respiratory Chain and ATP Synthase: Contributions from Studies of Saccharomyces cerevisiae

The ease with which the unicellular yeast Saccharomyces cerevisiae can be manipulated genetically and biochemically has established this organism as a good model for the study of human mitochondrial diseases. The combined use of biochemical and molecular genetic tools has been instrumental in elucidating the functions of numerous yeast nuclear gene products with human homologs that affect a large number of metabolic and biological processes, including those housed in mitochondria. These include structural and catalytic subunits of enzymes and protein factors that impinge on the biogenesis of the respiratory chain. This article will review what is currently known about the genetics and clinical phenotypes of mitochondrial diseases of the respiratory chain and ATP synthase, with special emphasis on the contribution of information gained from pet mutants with mutations in nuclear genes that impair mitochondrial respiration. Our intent is to provide the yeast mitochondrial specialist with basic knowledge of human mitochondrial pathologies and the human specialist with information on how genes that directly and indirectly affect respiration were identified and characterized in yeast.

Structural snapshots of the cellular folded protein translocation machinery Bcs1

Proteins destined to various intra‐ and extra‐cellular locations must traverse membranes most frequently in an unfolded form. When the proteins being translocated need to remain in a folded state, specialized cellular transport machinery is used. One such machine is the membrane‐bound AAA protein Bcs1 (Bcs1), which assists the iron‐sulfur protein, an essential subunit of the respiratory Complex III, across the mitochondrial inner membrane. Recent structure determinations of mouse and yeast Bcs1 in three different nucleotide states reveal its homo‐heptameric association and at least two dramatically different conformations. The apo and ADP‐bound structures are similar, both containing a large substrate‐binding cavity accessible to the mitochondrial matrix space, which contracts by concerted motion of the ATPase domains upon ATP binding, suggesting that bound substrate could then be pushed across the membrane. ATP hydrolysis drives substrate release and resets Bcs1 conformation back to the apo/ADP form. These structures shed new light on the mechanism of folded protein translocation across a membrane, provide better understanding on the assembly process of the respiratory Complex III, and correlate clinical presentations of disease‐associated mutations with their locations in the 3D structure.

Structures of AAA protein translocase Bcs1 suggest translocation mechanism of a folded protein

The mitochondrial membrane-bound AAA protein Bcs1 translocate substrates across the mitochondrial inner membrane without previous unfolding. One substrate of Bcs1 is the iron-sulfur protein (ISP), a subunit of the respiratory Complex III. How Bcs1 translocates ISP across the membrane is unknown. Here we report structures of mouse Bcs1 in two different conformations, representing three nucleotide states. The apo and ADP-bound structures reveal a homo-heptamer and show a large putative substrate-binding cavity accessible to the matrix space. ATP binding drives a contraction of the cavity by concerted motion of the ATPase domains, which could push substrate across the membrane. Our findings shed light on the potential mechanism of translocating folded proteins across a membrane, offer insights into the assembly process of Complex III and allow mapping of human disease-associated mutations onto the Bcs1 structure.

Human diseases associated with defects in assembly of OXPHOS complexes

The structural biogenesis and functional proficiency of the multiheteromeric complexes forming the mitochondrial oxidative phosphorylation system (OXPHOS) require the concerted action of a number of chaperones and other assembly factors, most of which are specific for each complex. Mutations in a large number of these assembly factors are responsible for mitochondrial disorders, in most cases of infantile onset, typically characterized by biochemical defects of single specific complexes. In fact, pathogenic mutations in complex-specific assembly factors outnumber, in many cases, the repertoire of mutations found in structural subunits of specific complexes. The identification of patients with specific defects in assembly factors has provided an important contribution to the nosological characterization of mitochondrial disorders, and has also been a crucial means to identify a huge number of these proteins in humans, which play an essential role in mitochondrial bioenergetics. The wide use of next generation sequencing (NGS) has led to and will allow the identifcation of additional components of the assembly machinery of individual complexes, mutations of which are responsible for human disorders. The functional studies on patients' specimens, together with the creation and characterization of in vivo models, are fundamental to better understand the mechanisms of each of them. A new chapter in this field will be, in the near future, the discovery of mechanisms and actions underlying the formation of supercomplexes, molecular structures formed by the physical, and possibly functional, interaction of some of the individual respiratory complexes, particularly complex I (CI), III (CIII), and IV (CIV).

TTC19 Plays a Husbandry Role on UQCRFS1 Turnover in the Biogenesis of Mitochondrial Respiratory Complex III

Loss-of-function mutations in TTC19 (tetra-tricopeptide repeat domain 19) have been associated with severe neurological phenotypes and mitochondrial respiratory chain complex III deficiency. We previously demonstrated the mitochondrial localization of TTC19 and its link with complex III biogenesis. Here we provide detailed insight into the mechanistic role of TTC19, by investigating a Ttc19?/? mouse model that shows progressive neurological and metabolic decline, decreased complex III activity, and increased production of reactive oxygen species. By using both the Ttc19?/? mouse model and a range of human cell lines, we demonstrate that TTC19 binds to the fully assembled complex III dimer, i.e., after the incorporation of the iron-sulfur Rieske protein (UQCRFS1). The in situ maturation of UQCRFS1 produces N-terminal polypeptides, which remain bound to holocomplex III. We show that, in normal conditions, these UQCRFS1 fragments are rapidly removed, but when TTC19 is absent they accumulate within complex III, causing its structural and functional impairment.

Effect of High-Carbohydrate Diet on Plasma Metabolome in Mice with Mitochondrial Respiratory Chain Complex III Deficiency

Mitochondrial disorders cause energy failure and metabolic derangements. Metabolome profiling in patients and animal models may identify affected metabolic pathways and reveal new biomarkers of disease progression. Using liver metabolomics we have shown a starvation-like condition in a knock-in (Bcs1lc.232A>G) mouse model of GRACILE syndrome, a neonatal lethal respiratory chain complex III dysfunction with hepatopathy. Here, we hypothesized that a high-carbohydrate diet (HCD, 60% dextrose) will alleviate the hypoglycemia and promote survival of the sick mice. However, when fed HCD the homozygotes had shorter survival (mean ± SD, 29 ± 2.5 days, n = 21) than those on standard diet (33 ± 3.8 days, n = 30), and no improvement in hypoglycemia or liver glycogen depletion. We investigated the plasma metabolome of the HCD- and control diet-fed mice and found that several amino acids and urea cycle intermediates were increased, and arginine, carnitines, succinate, and purine catabolites decreased in the homozygotes. Despite reduced survival the increase in aromatic amino acids, an indicator of liver mitochondrial dysfunction, was normalized on HCD. Quantitative enrichment analysis revealed that glycine, serine and threonine metabolism, phenylalanine and tyrosine metabolism, and urea cycle were also partly normalized on HCD. This dietary intervention revealed an unexpected adverse effect of high-glucose diet in complex III deficiency, and suggests that plasma metabolomics is a valuable tool in evaluation of therapies in mitochondrial disorders.

Metabolic flexibility of mitochondrial respiratory chain disorders predicted by computer modelling

Mitochondrial respiratory chain dysfunction causes a variety of life-threatening diseases affecting about 1 in 4300 adults. These diseases are genetically heterogeneous, but have the same outcome; reduced activity of mitochondrial respiratory chain complexes causing decreased ATP production and potentially toxic accumulation of metabolites. Severity and tissue specificity of these effects varies between patients by unknown mechanisms and treatment options are limited. So far most research has focused on the complexes themselves, and the impact on overall cellular metabolism is largely unclear. To illustrate how computer modelling can be used to better understand the potential impact of these disorders and inspire new research directions and treatments, we simulated them using a computer model of human cardiomyocyte mitochondrial metabolism containing over 300 characterised reactions and transport steps with experimental parameters taken from the literature. Overall, simulations were consistent with patient symptoms, supporting their biological and medical significance. These simulations predicted: complex I deficiencies could be compensated using multiple pathways; complex II deficiencies had less metabolic flexibility due to impacting both the TCA cycle and the respiratory chain; and complex III and IV deficiencies caused greatest decreases in ATP production with metabolic consequences that parallel hypoxia. Our study demonstrates how results from computer models can be compared to a clinical phenotype and used as a tool for hypothesis generation for subsequent experimental testing. These simulations can enhance understanding of dysfunctional mitochondrial metabolism and suggest new avenues for research into treatment of mitochondrial disease and other areas of mitochondrial dysfunction.

Nuclear gene mutations as the cause of mitochondrial complex III deficiency

Complex III (CIII) deficiency is one of the least common oxidative phosphorylation defects associated to mitochondrial disease. CIII constitutes the center of the mitochondrial respiratory chain, as well as a crossroad for several other metabolic pathways. For more than 10 years, of all the potential candidate genes encoding structural subunits and assembly factors, only three were known to be associated to CIII defects in human pathology. Thus, leaving many of these cases unresolved. These first identified genes were MT-CYB, the only CIII subunit encoded in the mitochondrial DNA; BCS1L, encoding an assembly factor, and UQCRB, a nuclear-encoded structural subunit. Nowadays, thanks to the fast progress that has taken place in the last 3-4 years, pathological changes in seven more genes are known to be associated to these conditions. This review will focus on the strategies that have permitted the latest discovery of mutations in factors that are necessary for a correct CIII assembly and activity, in relation with their function. In addition, new data further establishing the molecular role of LYRM7/MZM1L as a chaperone involved in CIII biogenesis are provided.

Differential proteomic profiling unveils new molecular mechanisms associated with mitochondrial complex III deficiency

We have analyzed the cellular pathways and metabolic adaptations that take place in primary skin fibroblasts from patients with mutations in BCS1L, a major genetic cause of mitochondrial complex III enzyme deficiency. Mutant fibroblasts exhibited low oxygen consumption rates and intracellular ATP levels, indicating that the main altered molecular event probably is a limited respiration-coupled ATP production through the OXPHOS system. Two-dimensional DIGE and MALDI-TOF/TOF mass spectrometry analyses unambiguously identified 39 proteins whose expression was significantly altered in complex III-deficient fibroblasts. Extensive statistical and cluster analyses revealed a protein profile characteristic for the BCS1L mutant fibroblasts that included alterations in energy metabolism, cell signaling and gene expression regulation, cytoskeleton formation and maintenance, and intracellular stress responses. The physiological validation of the predicted functional adaptations of human cultured fibroblasts to complex III deficiency confirmed the up-regulation of glycolytic enzyme activities and the accumulation of branched-chain among other amino acids, suggesting the activation of anaerobic glycolysis and cellular catabolic states, in particular protein catabolism, together with autophagy as adaptive responses to mitochondrial respiratory chain dysfunction and ATP deficiency. Our data point to an overall metabolic and genetic reprogramming that could contribute to explain the clinical manifestations of complex III deficiency in patients.

BIOLOGICAL SIGNIFICANCE

Despite considerable knowledge about their genetic origins, the pathophysiological mechanisms that contribute to the clinical manifestations of mitochondrial disorders remain poorly understood. We have investigated the molecular pathways and metabolic adaptations that take place in primary skin fibroblasts from patients with mutations in the BCS1L gene, a primary cause of mitochondrial complex III enzyme deficiency. Two-dimensional DIGE together with MALDI-TOF/TOF mass spectrometry and physiological validation analyses revealed a significant metabolic and genetic reprogramming as an adaptive response to mitochondrial respiratory chain dysfunction. Our data provide information about specific protein targets that regulate the transmitochondrial functional responses to complex III deficiency, thereby opening new doors for future research.

Reprint of: Biogenesis of the cytochrome bc(1) complex and role of assembly factors

The cytochrome bc(1) complex is an essential component of the electron transport chain in most prokaryotes and in eukaryotic mitochondria. The catalytic subunits of the complex that are responsible for its redox functions are largely conserved across kingdoms. In eukarya, the bc(1) complex contains supernumerary subunits in addition to the catalytic core, and the biogenesis of the functional bc(1) complex occurs as a modular assembly pathway. Individual steps of this biogenesis have been recently investigated and are discussed in this review with an emphasis on the assembly of the bc(1) complex in the model eukaryote Saccharomyces cerevisiae. Additionally, a number of assembly factors have been recently identified. Their roles in bc(1) complex biogenesis are described, with special emphasis on the maturation and topogenesis of the yeast Rieske iron-sulfur protein and its role in completing the assembly of functional bc(1) complex. This article is part of a Special Issue entitled: Biogenesis/Assembly of Respiratory Enzyme Complexes.

Role of creatine as biomarker of mitochondrial diseases

Recent investigations have suggested creatine (Cr) as an additional biomarker of mitochondrial diseases. With the aim of corroborating previous findings, we have studied plasma Cr in a cohort of 33 patients with different mitochondrial diseases. Cr was clearly increased in 9 out of 33 patients. Therefore, positive patients represent only 28% of the total number, suggesting that Cr is not a sensitive biomarker of mitochondrial diseases although it does present an acceptable specificity (83%). High plasma Cr, together with other biomarkers, might be useful to reinforce the diagnosis of mitochondrial diseases.

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.

BCS1L gene mutation presenting with GRACILE-like syndrome and complex III deficiency

The clinical presentation of a neonate with GRACILE-like syndrome, complex III deficiency and BCS1L mutations is discussed. This case is compared and contrasted with the original Finnish reports of GRACILE syndrome and other cases with a similar phenotype. This case confirms the pathogenicity of the BCS1L gene mutation c.166C>T, and provides support for the pathogenicity of a sequence variation, c.-588T>A, previously reported.

Biogenesis of the cytochrome bc(1) complex and role of assembly factors

The cytochrome bc(1) complex is an essential component of the electron transport chain in most prokaryotes and in eukaryotic mitochondria. The catalytic subunits of the complex that are responsible for its redox functions are largely conserved across kingdoms. In eukarya, the bc(1) complex contains supernumerary subunits in addition to the catalytic core, and the biogenesis of the functional bc(1) complex occurs as a modular assembly pathway. Individual steps of this biogenesis have been recently investigated and are discussed in this review with an emphasis on the assembly of the bc(1) complex in the model eukaryote Saccharomyces cerevisiae. Additionally, a number of assembly factors have been recently identified. Their roles in bc(1) complex biogenesis are described, with special emphasis on the maturation and topogenesis of the yeast Rieske iron-sulfur protein and its role in completing the assembly of functional bc(1) complex. This article is part of a Special Issue entitled: Biogenesis/Assembly of Respiratory Enzyme Complexes.

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.

The GRACILE mutation introduced into Bcs1l causes postnatal complex III deficiency: a viable mouse model for mitochondrial hepatopathy

Mitochondrial dysfunction is an important cause for neonatal liver disease. Disruption of genes encoding oxidative phosphorylation (OXPHOS) components usually causes embryonic lethality, and thus few disease models are available. We developed a mouse model for GRACILE syndrome, a neonatal mitochondrial disease with liver and kidney involvement, caused by a homozygous BCS1L mutation (232A>G). This gene encodes a chaperone required for incorporation of Rieske iron-sulfur protein (RISP) into complex III of respiratory chain. Homozygous mutant mice after 3 weeks of age developed striking similarities to the human disease: growth failure, hepatic glycogen depletion, steatosis, fibrosis, and cirrhosis, as well as tubulopathy, complex III deficiency, lactacidosis, and short lifespan. BCS1L was decreased in whole liver cells and isolated mitochondria of mutants at all ages. RISP incorporation into complex III was diminished in symptomatic animals; however, in young animals complex III was correctly assembled. Complex III activity in liver, heart, and kidney of symptomatic mutants was decreased to 20%, 40%, and 40% of controls, respectively, as demonstrated with electron flux kinetics through complex III. In high-resolution respirometry, CIII dysfunction resulted in decreased electron transport capacity through the respiratory chain under maximum substrate input. Complex I function, suggested to be dependent on a functional complex III, was, however, unaffected.

CONCLUSION

We present the first viable model of complex III deficiency mimicking a human mitochondrial disorder. Incorporation of RISP into complex III in young homozygotes suggests another complex III assembly factor during early ontogenesis. The development of symptoms from about 3 weeks of age provides a convenient time window for studying the pathophysiology and treatment of mitochondrial hepatopathy and OXPHOS dysfunction in general.

Clinicopathologic conference: multiple fetal demises, lactic acidosis and hepatic iron accumulation

A case of a premature infant with lactic acidosis and hepatic iron accumulation, born to a mother with multiple fetal demises, is presented and discussed by both clinician and pathologist, in this traditional clinico-pathologic conference. The discussion includes the differential diagnoses of lactic acidosis and hepatic iron accumulation in infants.

Cellular pathophysiological consequences of BCS1L mutations in mitochondrial complex III enzyme deficiency

Mutations in BCS1L, an assembly factor that facilitates the insertion of the catalytic Rieske Iron-Sulfur subunit into respiratory chain complex III, result in a wide variety of clinical phenotypes that range from the relatively mild Björnstad syndrome to the severe GRACILE syndrome. To better understand the pathophysiological consequences of such mutations, we studied fibroblasts from six complex III-deficient patients harboring mutations in the BCS1L gene. Cells from patients with the most severe clinical phenotypes exhibited slow growth rates in glucose medium, variable combined enzyme deficiencies, and assembly defects of respiratory chain complexes I, III, and IV, increased H(2)O(2) levels, unbalanced expression of the cellular antioxidant defenses, and apoptotic cell death. In addition, all patients showed cytosolic accumulation of the BCS1L protein, suggestive of an impaired mitochondrial import, assembly or stability defects of the BCS1L complex, fragmentation of the mitochondrial networks, and decreased MFN2 protein levels. The observed structural alterations were independent of the respiratory chain function and ROS production. Our results provide new insights into the role of pathogenic BCS1L mutations in mitochondrial function and dynamics.

A neurological perspective on mitochondrial disease

Disruption of the most fundamental cellular energy process, the mitochondrial respiratory chain, results in a diverse and variable group of multisystem disorders known collectively as mitochondrial disease. The frequent involvement of the brain, nerves, and muscles, often in the same patient, places neurologists at the forefront of the interesting and challenging process of diagnosing and caring for these patients. Mitochondrial diseases are among the most frequently inherited neurological disorders, and can be caused by mutations in mitochondrial or nuclear DNA. Substantial progress has been made over the past decade in understanding the genetic basis of these disorders, with important implications for the general neurologist in terms of the diagnosis, investigation, and multidisciplinary management of these patients.

Long-term survival of neonatal mitochondrial complex III deficiency associated with a novel BCS1L gene mutation

Mutations of the BCS1L gene are a recognised cause of isolated respiratory chain complex III deficiency and underlie several fatal, neonatal mitochondrial diseases. Here we describe a 20-year-old Kenyan woman who initially presented as a floppy infant but whose condition progressed during childhood and adolescence with increasing muscle weakness, focal motor seizures and optic atrophy. Muscle biopsy demonstrated complex III deficiency and the pathogenicity of a novel, homozygous BCS1L mutation was confirmed by yeast complementation studies. Our data indicate that BCS1L mutations can cause a variable, neurological course which is not always fatal in childhood.

Characterization of complex III deficiency and liver dysfunction in GRACILE syndrome caused by a BCS1L mutation

A homozygous mutation in the complex III chaperone BCS1L causes GRACILE syndrome (intrauterine growth restriction, aminoaciduria, cholestasis, hepatic iron overload, lactacidosis). In control and patient fibroblasts we localized BCS1L in inner mitochondrial membranes. In patient liver, kidney, and heart BCS1L and Rieske protein levels, as well as the amount and activity of complex III, were decreased. Major histopathology was found in kidney and liver with cirrhosis and iron deposition, but of iron-related proteins only ferritin levels were high. In placenta from a GRACILE fetus, the ferrooxidases ceruloplasmin and hephaestin were upregulated suggesting association between iron overload and placental dysfunction.