A SURF1 gene mutation presenting as isolated leukodystrophy

More than Just Bread and Wine: Using Yeast to Understand Inherited Cytochrome Oxidase Deficiencies in Humans

Inherited defects in cytochrome c oxidase (COX) are associated with a substantial subset of diseases adversely affecting the structure and function of the mitochondrial respiratory chain. This multi-subunit enzyme consists of 14 subunits and numerous cofactors, and it requires the function of some 30 proteins to assemble. COX assembly was first shown to be the primary defect in the majority of COX deficiencies 36 years ago. Over the last three decades, most COX assembly genes have been identified in the yeast Saccharomyces cerevisiae, and studies in yeast have proven instrumental in testing the impact of mutations identified in patients with a specific COX deficiency. The advent of accessible genome-wide sequencing capabilities has led to more patient mutations being identified, with the subsequent identification of several new COX assembly factors. However, the lack of genotype-phenotype correlations and the large number of genes involved in generating a functional COX mean that functional studies must be undertaken to assign a genetic variant as being causal. In this review, we provide a brief overview of the use of yeast as a model system and briefly compare the COX assembly process in yeast and humans. We focus primarily on the studies in yeast that have allowed us to both identify new COX assembly factors and to demonstrate the pathogenicity of a subset of the mutations that have been identified in patients with inherited defects in COX. We conclude with an overview of the areas in which studies in yeast are likely to continue to contribute to progress in understanding disease arising from inherited COX deficiencies.

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’.

Clinical Diagnosis and Treatment of Leigh Syndrome Based on SURF1: Genotype and Phenotype

SURF1 encodes the assembly factor for maintaining the antioxidant of cytochrome c oxidase (COX) stability in the human electron respiratory chain. Mutations in SURF1 can cause Leigh syndrome (LS), a subacute neurodegenerative encephalopathy, characterized by early onset (infancy), grave prognosis, and predominant symptoms presenting in the basal ganglia, thalamus, brainstem, cerebellum, and peripheral nerves. To date, more than sixty different SURF1 mutations have been found to cause SURF1-associated LS; however, the relationship between genotype and phenotype is still unclear. Most SURF1-associated LS courses present as typical LS and cause early mortality (before the age of ten years). However, 10% of the cases present with atypical courses with milder symptoms and increased life expectancy. One reason for this inconsistency may be due to specific duplications or mutations close to the C-terminus of the SURF1 protein appearing to cause less protein decay. Furthermore, the treatment for SURF1-associated LS is unsatisfactory. A ketogenic diet is most often prescribed and has proven to be effective. Supplementing with coenzyme Q and other cofactors is also a common treatment option; however, the results are inconsistent. Importantly, anti-epileptic drugs such as valproate—which cause mitochondrial dysfunction—should be avoided in patients with SURF1-associated LS presenting with seizures.

Whole exome sequencing uncovered highly penetrant recessive mutations for a spectrum of rare genetic pediatric diseases in Bangladesh

Collectively, rare genetic diseases affect a significant number of individuals worldwide. In this study, we have conducted whole-exome sequencing (WES) and identified underlying pathogenic or likely pathogenic variants in five children with rare genetic diseases. We present evidence for disease-causing autosomal recessive variants in a range of disease-associated genes such as DHH-associated 46,XY gonadal dysgenesis (GD) or 46,XY sex reversal 7, GNPTAB-associated mucolipidosis II alpha/beta (ML II), BBS1-associated Bardet-Biedl Syndrome (BBS), SURF1-associated Leigh Syndrome (LS) and AP4B1-associated spastic paraplegia-47 (SPG47) in unrelated affected members from Bangladesh. Our analysis pipeline detected three homozygous mutations, including a novel c. 863 G > C (p.Pro288Arg) variant in DHH, and two compound heterozygous variants, including two novel variants: c.2972dupT (p.Met991Ilefs*) in GNPTAB and c.229 G > C (p.Gly77Arg) in SURF1. All mutations were validated by Sanger sequencing. Collectively, this study adds to the genetic heterogeneity of rare genetic diseases and is the first report elucidating the genetic profile of (consanguineous and nonconsanguineous) rare genetic diseases in the Bangladesh population.

Cytochrome c oxidase deficiency

Cytochrome c oxidase (COX) deficiency is characterized by a high degree of genetic and phenotypic heterogeneity, partly reflecting the extreme structural complexity, multiple post-translational modification, variable, tissue-specific composition, and the high number of and intricate connections among the assembly factors of this enzyme. In fact, decreased COX specific activity can manifest with different degrees of severity, affect the whole organism or specific tissues, and develop a wide spectrum of disease natural history, including disease onsets ranging from birth to late adulthood. More than 30 genes have been linked to COX deficiency, but the list is still incomplete and in fact constantly updated. We here discuss the current knowledge about COX in health and disease, focusing on genetic aetiology and link to clinical manifestations. In addition, information concerning either fundamental biological features of the enzymes or biochemical signatures of its defects have been provided by experimental in vivo models, including yeast, fly, mouse and fish, which expanded our knowledge on the functional features and the phenotypical consequences of different forms of COX deficiency.

SURF1 knockout cloned pigs: Early onset of a severe lethal phenotype

Leigh syndrome (LS) associated with cytochrome c oxidase (COX) deficiency is an early onset, fatal mitochondrial encephalopathy, leading to multiple neurological failure and eventually death, usually in the first decade of life. Mutations in SURF1, a nuclear gene encoding a mitochondrial protein involved in COX assembly, are among the most common causes of LS. LSSURF1 patients display severe, isolated COX deficiency in all tissues, including cultured fibroblasts and skeletal muscle. Recombinant, constitutive SURF1-/- mice show diffuse COX deficiency, but fail to recapitulate the severity of the human clinical phenotype. Pigs are an attractive alternative model for human diseases, because of their size, as well as metabolic, physiological and genetic similarity to humans. Here, we determined the complete sequence of the swine SURF1 gene, disrupted it in pig primary fibroblast cell lines using both TALENs and CRISPR/Cas9 genome editing systems, before finally generating SURF1-/- and SURF1-/+ pigs by Somatic Cell Nuclear Transfer (SCNT). SURF1-/- pigs were characterized by failure to thrive, muscle weakness and highly reduced life span with elevated perinatal mortality, compared to heterozygous SURF1-/+ and wild type littermates. Surprisingly, no obvious COX deficiency was detected in SURF1-/- tissues, although histochemical analysis revealed the presence of COX deficiency in jejunum villi and total mRNA sequencing (RNAseq) showed that several COX subunit-encoding genes were significantly down-regulated in SURF1-/- skeletal muscles. In addition, neuropathological findings, indicated a delay in central nervous system development of newborn SURF1-/- piglets. Our results suggest a broader role of sSURF1 in mitochondrial bioenergetics.

Cytochrome C oxydase deficiency: SURF1 gene investigation in patients with Leigh syndrome

Leigh syndrome (LS) is a rare progressive neurodegenerative disorder occurring in infancy. The most common clinical signs reported in LS are growth retardation, optic atrophy, ataxia, psychomotor retardation, dystonia, hypotonia, seizures and respiratory disorders. The paper reported a manifestation of 3 Tunisian patients presented with LS syndrome. The aim of this study is the MT[HYPHEN]ATP6 and SURF1 gene screening in Tunisian patients affected with classical Leigh syndrome and the computational investigation of the effect of detected mutations on its structure and functions by clinical and bioinformatics analyses. After clinical investigations, three Tunisian patients were tested for mutations in both MT-ATP6 and SURF1 genes by direct sequencing followed by in silico analyses to predict the effects of sequence variation. The result of mutational analysis revealed the absence of mitochondrial mutations in MT-ATP6 gene and the presence of a known homozygous splice site mutation c.516-517delAG in sibling patients added to the presence of a novel double het mutations in LS patient (c.752-18 A > C/c. c.751 + 16G > A). In silico analyses of theses intronic variations showed that it could alters splicing processes as well as SURF1 protein translation. Leigh syndrome (LS) is a rare progressive neurodegenerative disorder occurring in infancy. The most common clinical signs reported in LS are growth retardation, optic atrophy, ataxia, psychomotor retardation, dystonia, hypotonia, seizures and respiratory disorders. The paper reported a manifestation of 3 Tunisian patients presented with LS syndrome. The aim of this study is MT-ATP6 and SURF1 genes screening in Tunisian patients affected with classical Leigh syndrome and the computational investigation of the effect of detected mutations on its structure and functions. After clinical investigations, three Tunisian patients were tested for mutations in both MT-ATP6 and SURF1 genes by direct sequencing followed by in silico analysis to predict the effects of sequence variation. The result of mutational analysis revealed the absence of mitochondrial mutations in MT-ATP6 gene and the presence of a known homozygous splice site mutation c.516-517delAG in sibling patients added to the presence of a novel double het mutations in LS patient (c.752-18 A>C/ c.751+16G>A). In silico analysis of theses intronic vaiations showed that it could alters splicing processes as well as SURF1 protein translation.

Leigh syndrome: One disorder, more than 75 monogenic causes

Leigh syndrome is the most common pediatric presentation of mitochondrial disease. This neurodegenerative disorder is genetically heterogeneous, and to date pathogenic mutations in >75 genes have been identified, encoded by 2 genomes (mitochondrial and nuclear). More than one-third of these disease genes have been characterized in the past 5 years alone, reflecting the significant advances made in understanding its etiological basis. We review the diverse biochemical and genetic etiology of Leigh syndrome and associated clinical, neuroradiological, and metabolic features that can provide clues for diagnosis. We discuss the emergence of genotype-phenotype correlations, insights gleaned into the molecular basis of disease, and available therapeutic options.

Magnetic resonance imaging correlates of genetically characterized patients with mitochondrial disorders: A study from south India

BACKGROUND

Large studies analyzing magnetic resonance imaging correlates in different genotypes of mitochondrial disorders are far and few. This study sought to analyze the pattern of magnetic resonance imaging findings in a cohort of genetically characterized patients with mitochondrial disorders.

METHODS

The study cohort included 33 patients (age range 18 months-50 years, M:F - 0.9:1) with definite mitochondrial disorders seen over a period of 8 yrs. (2006-2013). Their MR imaging findings were analyzed retrospectively.

RESULTS

The patients were classified into three groups according to the genotype, Mitochondrial point mutations and deletions (n=21), SURF1 mutations (n=7) and POLG1 (n=5). The major findings included cerebellar atrophy (51.4%), cerebral atrophy (24.2%), signal changes in basal ganglia (45.7%), brainstem (34.2%) & white matter (18.1%) and stroke like lesions (25.7%). Spinal cord imaging showed signal changes in 4/6 patients. Analysis of the special sequences revealed, basal ganglia mineralization (7/22), lactate peak on magnetic resonance spectrometry (10/15), and diffusion restriction (6/22). Follow-up images in six patients showed that the findings are dynamic. Comparison of the magnetic resonance imaging findings in the three groups showed that cerebral atrophy and cerebellar atrophy, cortical signal changes and basal ganglia mineralization were seen mostly in patients with mitochondrial mutation. Brainstem signal changes with or without striatal lesions were characteristically noted in SURF1 group. There was no consistent imaging pattern in POLG1 group.

CONCLUSION

Magnetic resonance imaging findings in mitochondrial disorders are heterogeneous. Definite differences were noted in the frequency of anatomical involvement in the three groups. Familiarity with the imaging findings in different genotypes of mitochondrial disorders along with careful analysis of the family history, clinical presentation, biochemical findings, histochemical and structural analysis will help the physician for targeted metabolic and genetic testing.

Clinical and magnetic resonance imaging findings in patients with Leigh syndrome and SURF1 mutations

BACKGROUND

Mutation in the SURF1 is one of the most common nuclear mutations associated with Leigh syndrome and cytochrome c oxidase deficiency. This study aims to describe the phenotypic and imaging features in four patients with Leigh syndrome and novel SURF1 mutation.

METHODS

The study included four patients with Leigh syndrome and SURF1 mutations identified from a cohort of 25 children with Leigh syndrome seen over a period of six years (2006-2012). All the patients underwent a detailed neurological assessment, muscle biopsy, and sequencing of the complete mitochondrial genome and SURF1.

RESULTS

Three patients had classical presentation of Leigh syndrome. The fourth patient had a later age of onset with ataxia as the presenting manifestation and a stable course. Hypertrichosis, facial dysmorphism and hypopigmentation were the additional phenotypic features noted. On magnetic resonance imaging all patients had brainstem and cerebellar involvement and two had basal ganglia involvement in addition. The bilateral symmetrical hypertrophic olivary degeneration in these patients was striking. The SURF1 analysis identified previously unreported mutations in all the patients. On follow-up three patients expired and one had a stable course.

CONCLUSIONS

Patients with Leigh syndrome and SURF1 mutation often have skin and hair abnormalities. Bilateral symmetrical hypertrophic olivary degeneration was a consistent finding on magnetic resonance imaging in these patients.

SURF1 deficiency causes demyelinating Charcot-Marie-Tooth disease

OBJECTIVE

To investigate whether mutations in the SURF1 gene are a cause of Charcot-Marie-Tooth (CMT) disease.

METHODS

We describe 2 patients from a consanguineous family with demyelinating autosomal recessive CMT disease (CMT4) associated with the homozygous splice site mutation c.107-2A>G in the SURF1 gene, encoding an assembly factor of the mitochondrial respiratory chain complex IV. This observation led us to hypothesize that mutations in SURF1 might be an unrecognized cause of CMT4, and we investigated SURF1 in a total of 40 unrelated patients with CMT4 after exclusion of mutations in known CMT4 genes. The functional impact of c.107-2A>G on splicing, amount of SURF1 protein, and on complex IV activity and assembly was analyzed.

RESULTS

Another patient with CMT4 was found to harbor 2 additional SURF1 mutations. All 3 patients with SURF1-associated CMT4 presented with severe childhood-onset neuropathy, motor nerve conduction velocities <25 m/s, and lactic acidosis. Two patients had brain MRI abnormalities, including putaminal and periaqueductal lesions, and developed cerebellar ataxia years after polyneuropathy. The c.107-2A>G mutation produced no normally spliced transcript, leading to SURF1 absence. However, complex IV remained partially functional in muscle and fibroblasts.

CONCLUSIONS

We found SURF1 mutations in 5% of families (2/41) presenting with CMT4. SURF1 should be systematically screened in patients with childhood-onset severe demyelinating neuropathy and additional features such as lactic acidosis, brain MRI abnormalities, and cerebellar ataxia developing years after polyneuropathy.

SURF1 deficiency: a multi-centre natural history study

Background

SURF1 deficiency, a monogenic mitochondrial disorder, is the most frequent cause of cytochrome c oxidase (COX) deficient Leigh syndrome (LS). We report the first natural history study of SURF1 deficiency.

Methods

We conducted a multi-centre case notes review of 44 SURF1-deficient patients from ten different UK centres and two Australian centres. Survival data for LRPPRC-deficient LS and nuclear-encoded complex I-deficient LS patients were obtained from previous publications. The survival of SURF1-deficient patients was compared with these two groups using Kaplan-Meier survival analysis and logrank test.

Results

The majority of patients (32/44, 73%) presented in infancy (median 9.5 months). Frequent symptoms were poor weight gain (95%, median age 10 months), hypotonia (93%, median age 14 months), poor feeding/vomiting (89%, median age 10 months), developmental delay (88%, median age 14 months), developmental regression (71%, median age 19 months), movement disorder (52%, median age 24 months), oculomotor involvement (52%, median age 29 months) and central respiratory failure (78%, median age 31 months). Hypertrichosis (41%), optic atrophy (23%), encephalopathy (20%), seizures (14%) and cardiomyopathy (2%) were observed less frequently.

Lactate was elevated in CSF (mean 4.3 mmol/L) in all patients (30/30) and in blood (mean 4.4 mmol/L) in 31/38 (81%). Fibroblast COX activity was universally decreased (25/25). Normal COX histochemistry was noted in 30% of biopsies, whereas muscle COX activity was reduced in 96% (25/26). Neuroimaging demonstrated lesions characteristic of LS in 28/33 (85%) and atypical findings in 3/33 (9%). Peripheral neuropathy was present in 13/16 (81%) (demyelinating 7/16, axonal 2/16). Kaplan-Meier analysis demonstrated that SURF1-deficient patients experience longer survival (median 5.4 years, p < 0.001) compared to LRPPRC deficiency (median 1.8 years) and nuclear-encoded complex I-deficient LS (median 1.6 years). Survival >10 years was observed in 7 patients, 6 of these patients did not experience neurological regression. The most frequent mutation was c.312_320del10insAT. Five novel mutations (c.468_469delTC, c.799_800delCT, c.575G>A (p.Arg192Gln), c.751+5G>A and c.752-2A>G) were identified.

Conclusions

SURF1-deficient patients have a homogeneous clinical and biochemical phenotype. Early recognition is essential to expedite diagnosis and enable prenatal diagnosis.

Recent advances in the molecular pathogenesis of dystonia-plus syndromes and heredodegenerative dystonias

The majority of studies investigating the molecular pathogenesis and cell biology underlying dystonia have been performed in individuals with primary dystonia. This includes monogenic forms such as DYT1and DYT6 dystonia, and primary focal dystonia which is likely to be multifactorial in origin. In recent years there has been renewed interest in non-primary forms of dystonia including the dystonia-plus syndromes and heredodegenerative disorders. These are caused by a variety of genetic mutations and their study has contributed to our understanding of the neuronal dysfunction that leads to dystonia These findings have reinforced themes identified from study of primary dystonia including abnormal dopaminergic signalling, cellular trafficking and mitochondrial function. In this review we highlight recent advances in the understanding of the dystonia-plus syndromes and heredodegenerative dystonias.

Adult-onset leukodystrophies from respiratory chain disorders: do they exist?

Respiratory chain disorders (RCDs) have been included in the differential diagnosis of adult-onset leukodystrophies. Here, we first report a 32-year-old female with an atypical, adult-onset, non-syndromic RCD due to a mitochondrial DNA deletion and manifesting as complicated ataxia. A 'leukodystrophic' pattern was found on brain MRI, but it was neither isolated nor predominant because of the presence of overt basal ganglia and infratentorial lesions, which led us to the proper diagnosis. Subsequently, we evaluated our series of patients with RCDs in order to verify whether a 'leukodystrophic' pattern with little or no involvement of deep grey structures and brainstem may be found in adult-onset RCDs, as reported in children. Among 52 patients with adult-onset RCDs, no case with a 'leukodystrophic' pattern was found, apart from three cases with a classical phenotype of mitochondrial neurogastrointestinal encephalopathy. In addition, no case of RCDs was found among six cases of adult-onset leukodystrophy of unknown origin and at least one feature suggestive of mitochondrial disease. The review of the literature was in agreement with these findings. Thus, we provide evidence that, unlike in children, RCDs should not be included in the differential diagnosis of adult-onset leukodystrophies, except when there are additional MRI findings or clinical features which unequivocally point towards a mitochondrial disorder.

Infantile mitochondrial encephalopathy

Individually rare, when taken as a whole, genetic inborn errors of metabolism (IEM) account for a significant proportion of early onset encephalopathy. Prompt diagnosis is crucial to assess appropriate investigation and can sometimes warrant successful therapy. Recent improvements in technology and expansion of knowledge on the biochemical and molecular basis of these disorders allow astute child neurologists and paediatricians to improve the early diagnosis of these genetically determined defects. However, because of rarity and heterogeneity of these disorders, IEM encephalopathies are still a formidable challenge for most physicians. The most frequent cause of childhood IEM encephalopathy is mitochondrial disease, whose biochemical 'signature' is faulty energy supply due to defects of the last component of the oxidative pathways residing within mitochondria, i.e. the mitochondrial respiratory chain.

Spongy degeneration with cerebellar ataxia in Malinois puppies: a hereditary autosomal recessive disorder?

BACKGROUND

There is a high incidence of hereditary degenerative diseases of the central nervous system in purebred dogs.

HYPOTHESIS

Cerebellar ataxia in Malinois puppies, caused by degenerative changes that predominate in cerebellar nuclei and the granular cell layer, is a hereditary disorder that is distinct from cerebellar cortical abiotrophies.

ANIMALS

Thirteen Malinois puppies with cerebellar ataxia.

METHODS

Retrospective study. Records of Malinois puppies with spongy degeneration of the cerebellar nuclei were analyzed including clinical signs, histopathological changes, and pedigree data.

RESULTS

Signs of cerebellar dysfunction were observed in puppies of both sexes from 5 different litters (1995-2009) of phenotypically normal parents. Clinical signs started before the age of 2 months and resulted in euthanasia of all puppies by the age of 13 weeks. Histopathology disclosed marked bilateral spongy degeneration of the cerebellar nuclei and vacuoles in the granular cell layer and foliate white matter of the cerebellum. In some puppies, discrete vacuoles in gray and white matter were present in other parts of the brain. Furthermore, spheroids and dilated myelin sheaths were observed. Pedigree data and segregation frequency support an autosomal recessive hereditary disorder.

CONCLUSIONS AND CLINICAL IMPORTANCE

Malinois suffer from a hereditary spongiform degeneration that predominates in the cerebellum and causes an early onset of clinical signs with unfavorable prognosis. Future efforts should increase awareness among veterinarians and breeders and aim to identify underlying metabolic mechanisms and the affected genes.

Cranial magnetic resonance imaging findings in children with nonsyndromic mitochondrial diseases

Cranial magnetic resonance imaging findings suggestive of specific mitochondrial syndromes are reported. However, cranial magnetic resonance imaging features in children with nonsyndromic mitochondrial diseases are rarely described. From January 1992-September 2009, data from 33 patients with nonsyndromic mitochondrial diseases were collected. We investigated cranial magnetic resonance imaging features in children with nonsyndromic mitochondrial diseases, and identified potential diagnostic characteristics. Eleven of 33 patients (33.3%) demonstrated normal findings, and 22 (66.7%) demonstrated abnormal findings. The most common abnormal finding was cerebral atrophy, with or without other lesion sites (15/33; 45.5%). The second most common was bilateral basal ganglia involvement (6/33; 18.2%). Follow-up imaging was performed in 20 patients. Ten of these 20 (50.0%) demonstrated evolutionary changes, in which progressive global brain atrophy was evident. Three patients with normal results and one patient with cerebral atrophy on initial imaging demonstrated prominent signal changes over the basal ganglia, brainstem, gray matter, white matter, and bilateral cerebellar hemispheres on follow-up imaging. Imaging in children with nonsyndromic mitochondrial diseases may produce variable findings. Normal results and cerebral atrophy on the initial cranial magnetic resonance imaging are commonly evident in this patient group.

A de novo mutation in the adenosine triphosphatase (ATPase) 8 gene in a patient with mitochondrial disorder

Mitochondrial DNA defects were known to be associated with a wide spectrum of human diseases and patients might present a wide range of clinical features in various combinations. In the current study, we described a patient with psychomotor and neurodevelopmental delay, mild hyperintensity of posterior periventicular white matter, generalized clonic seizures, leukodystrophy, and congenital deafness. He also had tetraplegia, with central blindness and swallowing difficulty. Brain magnetic resonance imaging (MRI) showed involvement of the interpeduncular nucleus and central tegmental tract, white matter abnormalities, and cerebellar atrophy. A whole mitochondrial genome screening revealed the presence of 19 reported polymorphisms and an undescribed A to G mutation at nucleotide 8411 (p.M16V) affecting a conserved region of the mitochondrial adenosine triphosphatase (ATPase) 8 protein. This de novo mutation was detected in heteroplasmic form (97%) and was absent in 120 controls. Thus, the m.8411A>G mutation could strongly be associated with the disease in the tested patient.

SURF1 missense mutations promote a mild Leigh phenotype

SURF1 gene mutations are the most common cause of Leigh syndrome (LS), a rare progressive neurodegenerative disorder of infancy, characterized by symmetric necrotizing lesions and hypervascularity in the brainstem and basal ganglia, leading to death before the age of 4 years. Most of the reported mutations create premature termination codons, whereas missense mutations are rare. The aim of the study was to characterize the natural history of LS patients carrying at least one missense mutation in the SURF1 gene. Nineteen such patients (8 own cases and 11 reported in the literature) were compared with a reference group of 20 own c.845_846delCT homozygous patients, and with other LS(SURF-) cases described in the literature. Disease onset in the studied group was delayed. Acute failure to thrive and hyperventilation episodes were rare, respiratory failure did not appear before the age of 4 years. Dystonia, motor regression and eye movement dissociation developed slowly. The number of patients who survived 7 years of life totaled 9 out of 15 (60%) in the 'missense group' and 1 out of 26 (4%) patients with mutations leading to truncated proteins.

IN CONCLUSION

(i) The presence of a missense mutation in the SURF1 gene may correlate with a milder course and longer survival of Leigh patients, (ii) normal magnetic resonance imaging (MRI) findings, normal blood lactate value, and only mild decrease of cytochrome c oxidase (COX) activity are not sufficient reasons to forego SURF1 mutation analysis in differential diagnosis.

SURF-1 gene mutation associated with leukoencephalopathy in a 2-year-old

Mutations in the nuclear SURF-1 gene lead directly to cytochrome-c oxidase deficiency, the most common respiratory chain defect in Leigh syndrome, a neurodegenerative mitochondrial disease involving the deep gray matter and brain stem. We describe the second documented case in the literature to have a SURF-1 mutation presenting with diffuse leukodystrophy, adding to the growing number of cases of mitochondrial syndromes presenting with white matter disease. We examine magnetic resonance imaging (MRI) findings, which suggest that high-grade cytotoxic edema on diffusion-weighted imaging may be a helpful diagnostic feature in differentiating mitochondrial leukodystrophy from other, more common leukodystrophies. We show how MRI white matter findings may progress to include the brain stem, suggesting that a leukodystrophy due to respiratory chain defects can precede more classic Leigh syndrome deep gray matter radiographic findings.

Leigh's disease associated with a dorsal midbrain syndrome

Leigh's disease is a rare progressive neurodegenerative condition caused by a mitochondrial cytopathy. The authors present the case of a 9-year-old girl with dorsal midbrain syndrome causing convergence retraction nystagmus. Magnetic resonance imaging, skin biopsy, and genetic testing confirmed the cause to be Leigh's disease due to two SURF1 mutations. To the authors' knowledge, this is the first reported case of dorsal midbrain syndrome caused by a mitochondrial cytopathy.

High prevalence of SURF1 c.845_846delCT mutation in Polish Leigh patients

Leigh syndrome is a neuropathological disorder with typical morphological changes in brain, appearing regardless of diverse molecular background. One of the most common enzymatic defects in Leigh patients is cytochrome c oxidase deficiency associated with recessive mutations in the SURF1 gene. To assess the SURF1 mutation profile among Polish patients we studied 41 affected children from 34 unrelated families by PCR-SSCP and sequencing. Four novel mutations, c.39delG, c.752-1G>C, c.800_801insT, c.821A>G, and five described pathogenic changes, c.311_312insAT312_321del10, c.688C>T, c.704T>C, c.756_757delCA, c.845_846delCT, were identified in 85.3% of analysed probands. One mutation, c.845_846delCT, was identified in 77.6% of SURF1 alleles. Up to now, it has been reported only in 9% of alleles in other parts of the world. The deletion was used as LS(SURF1-) marker in population studies. Eight heterozygous carriers of the mutation were found in a cohort of 2890 samples. The estimated c.845_846delCT allele frequency is 1:357 (0.28+/-0.2%), and the lowest predicted LS(SURF1-) frequency in Poland 1:126,736.births. Relatively high frequency of LS(SURF1-) in Poland with remarkable c.845_846delCT mutation dominance allows one to start the differential diagnosis of LS in each patient of Polish (and probably Slavonic) origin from the direct search for c.845_846delCT SURF1 mutation.

Neuroimaging of mitochondrial disease

Mitochondrial disease represents a heterogeneous group of genetic disorders that require a variety of diagnostic tests for proper determination. Neuroimaging may play a significant role in diagnosis. The various modalities of nuclear magnetic resonance imaging (MRI) allow for multiple independent detection procedures that can give important anatomical and metabolic clues for diagnosis. The non-invasive nature of neuroimaging also allows for longitudinal studies. To date, no pathonmonic correlation between specific genetic defect and neuroimaging findings have been described. However, certain neuroimaging results can give important clues that a patient may have a mitochondrial disease. Conventional MRI may show deep gray structural abnormalities or stroke-like lesions that do not respect vascular territories. Chemical techniques such as proton magnetic resonance spectroscopy (MRS) may demonstrate high levels of lactate or succinate. When found, these results are suggestive of a mitochondrial disease. MRI and MRS studies may also show non-specific findings such as delayed myelination or non-specific leukodystrophy picture. However, in the context of other biochemical, structural, and clinical findings, even non-specific findings may support further diagnostic testing for potential mitochondrial disease. Once a diagnosis has been established, these non-invasive tools can also aid in following disease progression and evaluate the effects of therapeutic interventions.

Nuclear respiratory factor 1 regulates all ten nuclear-encoded subunits of cytochrome c oxidase in neurons

Cytochrome c oxidase (COX) is one of only four bigenomic proteins in mammalian cells, having ten subunits encoded in the nuclear genome and three in the mitochondrial DNA. The mechanism of its bigenomic control is not well understood. The ten nuclear subunits are on different chromosomes, and the possibility of their coordinate regulation by the same transcription factor(s) deserves serious consideration. The present study tested our hypothesis that nuclear respiratory factor 1 (NRF-1) serves such a role in subunit coordination. Following in silico analysis of murine nuclear-encoded COX subunit promoters, electrophoretic mobility shift and supershift assays indicated NRF-1 binding to all ten promoters. In vivo chromatin immunoprecipitation assays also showed NRF-1 binding to all ten promoters in murine neuroblastoma cells. Site-directed mutagenesis of putative NRF-1 binding sites confirmed the functionality of NRF-1 binding on all ten COX promoters. These sites are highly conserved among mice, rats, and humans. Silencing of NRF-1 with RNA interference reduced all ten COX subunit mRNAs and mRNAs of other genes involved in mitochondrial biogenesis. We conclude that NRF-1 plays a significant role in coordinating the transcriptional regulation of all ten nuclear-encoded COX subunits in neurons. Moreover, NRF-1 is known to activate mitochondrial transcription factors A and B, thereby indirectly regulating the expressions of the three mitochondrial-encoded COX subunits. Thus, NRF-1 and our previously described NRF-2 prove to be the two key bigenomic coordinators for transcriptional regulation of all cytochrome c oxidase subunits in neurons. Possible interactions between the NRFs will be investigated in the future.

Light and electron microscopy characteristics of the muscle of patients with SURF1 gene mutations associated with Leigh disease

AIMS

Leigh syndrome (LS) is characterised by almost identical brain changes despite considerable causal heterogeneity. SURF1 gene mutations are among the most frequent causes of LS. Although deficiency of cytochrome c oxidase (COX) is a typical feature of the muscle in SURF1-deficient LS, other abnormalities have been rarely described. The aim of the present work is to assess the skeletal muscle morphology coexisting with SURF1 mutations from our own research and in the literature.

METHODS

Muscle samples from 21 patients who fulfilled the criteria of LS and SURF1 mutations (14 homozygotes and 7 heterozygotes of c.841delCT) were examined by light and electron microscopy.

RESULTS

Diffuse decreased activity or total deficit of COX was revealed histochemically in all examined muscles. No ragged red fibres (RRFs) were seen. Lipid accumulation and fibre size variability were found in 14 and 9 specimens, respectively. Ultrastructural assessment showed several mitochondrial abnormalities, lipid deposits, myofibrillar disorganisation and other minor changes. In five cases no ultrastructural changes were found. Apart from slight correlation between lipid accumulation shown by histochemical and ultrastructural techniques, no other correlations were revealed between parameters investigated, especially between severity of morphological changes and the patient's age at the biopsy.

CONCLUSION

Histological and histochemical features of muscle of genetically homogenous SURF1-deficient LS were reproducible in detection of COX deficit. Minor muscle changes were not commonly present. Also, ultrastructural abnormalities were not a consistent feature. It should be emphasised that SURF1-deficient muscle assessed in the light and electron microscopy panel may be interpreted as normal if COX staining is not employed.

Mitochondrial Leukodystrophy: an Unusual Manifestation of Leigh's Disease. A Report of Three Cases and Review of the Literature

Leigh's disease is an inherited, progressive neurodegenerative disorder of infancy and early childhood. This metabolic disease is biochemically and genetically a heterogeneous disorder with defects involving various enzymes involved in the respiratory chain mechanism. Due to the multitude of enzyme defects known to occur in patients afflicted with Leigh's disease, this condition is known to have a variable clinical, pathological and radiological pattern of presentation. Isolated deficiency of cytochrome oxidase (COX) enzyme is one of the commonest abnormalities seen in patients afflicted by Leigh's disease. A primary white matter pattern of involvement representing a mitochondrial leukodystrophy is rare. Symmetric hyperintensities on T2WI involving the subthalamic nuclei and brainstem have been reported in patients with COX deficiency with SURF 1 mutations and are considered almost a hallmark of Leigh's disease with COX deficiency. We describe three cases of Leigh's disease with a primary white matter involvement diagnosed at our institution on the basis of clinical features, radiological appearance and laboratory findings.

Central nervous system manifestations of mitochondrial disorders

The central nervous system (CNS) is, after the peripheral nervous system, the second most frequently affected organ in mitochondrial disorders (MCDs). CNS involvement in MCDs is clinically heterogeneous, manifesting as epilepsy, stroke-like episodes, migraine, ataxia, spasticity, extrapyramidal abnormalities, bulbar dysfunction, psychiatric abnormalities, neuropsychological deficits, or hypophysial abnormalities. CNS involvement is found in syndromic and non-syndromic MCDs. Syndromic MCDs with CNS involvement include mitochondrial encephalomyopathy, lactacidosis, stroke-like episodes syndrome, myoclonic epilepsy and ragged red fibers syndrome, mitochondrial neuro-gastrointestinal encephalomyopathy syndrome, neurogenic muscle weakness, ataxia, and retinitis pigmentosa syndrome, mitochondrial depletion syndrome, Kearns-Sayre syndrome, and Leigh syndrome, Leber's hereditary optic neuropathy, Friedreich's ataxia, and multiple systemic lipomatosis. As CNS involvement is often subclinical, the CNS including the spinal cord should be investigated even in the absence of overt clinical CNS manifestations. CNS investigations comprise the history, clinical neurological examination, neuropsychological tests, electroencephalogram, cerebral computed tomography scan, and magnetic resonance imaging. A spinal tap is indicated if there is episodic or permanent impaired consciousness or in case of cognitive decline. More sophisticated methods are required if the CNS is solely affected. Treatment of CNS manifestations in MCDs is symptomatic and focused on epilepsy, headache, lactacidosis, impaired consciousness, confusion, spasticity, extrapyramidal abnormalities, or depression. Valproate, carbamazepine, corticosteroids, acetyl salicylic acid, local and volatile anesthetics should be applied with caution. Avoiding certain drugs is often more beneficial than application of established, apparently indicated drugs.

Canine spongiform leukoencephalomyelopathy is associated with a missense mutation in cytochrome b

Two families of dogs (Australian cattle dogs and Shetland sheepdogs) with an inherited "spongiform leukoencephalomyelopathy" were identified, with widespread vacuolation of white matter of the brain and spinal cord. Affected dogs of both breeds developed tremors at 2-9 weeks of age followed by progressive neurological worsening with ataxia, paresis, paralysis, spasticity, and cranial nerve dysfunction. The modes of inheritance of both families were most likely maternal. The cerebrospinal fluid (CSF) analysis showed elevated ratio of 3-OH butyrate to acetoacetic acid. Mitochondrial DNA sequencing showed a G to A transition at 14,474 nt (G14474A, GenBank accession no. NC002008 ) that results in an amino acid change of valine-98 to methionine (V98M) of mitochondrial encoded cytochrome b. Western blot analysis showed increased levels of core I and core II but decreased level of cytochrome c1 of the complex III and cytochrome c oxidase of the complex IV of the respiratory chain.

White matter involvement in mitochondrial diseases

White matter involvement is recently being realized as a common finding in mitochondrial disorders. It is considered an inherent part of the classical mitochondrial syndromes which are usually associated with alterations in the mitochondrial DNA such as: Leigh disease, Kearns-Sayre syndrome, mitochondrial encephalomyopathy lactic acidosis, and stroke like episodes, mitochondrial neuro-gastro-intestinal encephalomyopathy and Leber's hereditary optic neuropathy. White matter involvement is also described in mitochondrial disorders due to mutations in the nuclear DNA which are transmitted in an autosomal pattern. MRI findings suggestive of a mitochondrial disease are: small cyst-like lesions in abnormal white matter, involvement of both cerebral and cerebellar white matter, and a combination of a leukoencephalopathy with bilateral basal ganglia lesions. The clinical manifestations may be disproportionate to the extent of white matter involvement. Other organs may frequently be involved. The onset is often in infancy with a neurodegenerative course. The finding of a leukoencephalopathy in a patient with a complex neurologic picture and multisystem involvement should prompt a thorough mitochondrial evaluation.

Hypertrichosis in patients withSURF1 mutations

We present three patients with SURF1 mutations. In addition to Leigh syndrome all patients had hypertrichosis, a clinical sign that is not usually associated with Leigh syndrome. The hypertrichosis was not congenital and it was mainly distributed on the extremities and forehead. In addition to our three patients, we have identified five patients in the literature with hypertrichosis and Leigh syndrome due to SURF1 mutations. Since most patients had onset of hypertrichosis before the diagnosis of Leigh syndrome was made, we suggest that clinicians consider Leigh syndrome in patients with, for example, psychomotor retardation or other unspecific symptoms in combination with hypertrichosis.

Diagnostic difficulties with common SURF1 mutations in patients with cytochrome oxidase-deficient Leigh syndrome

In three unrelated patients with systemic cytochrome oxidase deficiency resulting from mutations in the SURF1 gene, the same mutation in the splice donor site of intron 3 was identified. All three patients were compound heterozygotes, two for the common insertion/deletion mutation in exon 4. In all three cases, complete definition of the causative mutations was only resolved by combined analysis of cDNA and genomic DNA. Several factors were identified that contributed to the diagnostic difficulties: preferential amplification of deleted cDNA, significant formation of heteroduplexes in cDNA PCR amplification and unequal representation of heterozygous peaks in genomic DNA sequences. These patients emphasize the need to perform mutation analysis on both cDNA and genomic DNA wherever possible.

NovelSURF1 mutation in a child with subacute encephalopathy and without the radiological features of Leigh Syndrome

Mutations in SURF1, a gene involved in cytochrome-c oxidase (COX) assembly, cause COX deficiency and Leigh Syndrome (LS). Typical presentation is in the first year of life, with failure to thrive, psychomotor regression, ataxia, signs of brainstem dysfunction, and peripheral neuropathy. Progression is rapid and patients usually die of respiratory failure before 2 years of age. LS is characterized by symmetrical bilateral lesions in the brainstem and basal ganglia, revealed premortem as signal hyperintensities in T2-weighted MRI imaging. Here, we describe a 10-year-old boy with a novel mutation in SURF1 associated with an unusually mild clinical course. At 39 months, there were no MRI lesions, and a follow-up MRI at 8 years of age showed only brainstem and cerebellar involvement without lesions in the basal ganglia or subthalamic nuclei. These data confirm that the spectrum of MRI findings in LS is variable and that SURF1 mutations should be considered in patients with encephalomyopathy and COX deficiency even when early MRI findings are negative.

Inheritance of mitochondrial disorders

Over the last decade there have been major advances in our understanding of the genetic basis of mitochondrial disease, enabling genetic counseling for patients with autosomal dominant and autosomal recessive disorders. Genetic counseling for patients with mitochondrial DNA (mtDNA) mutations is less well established. Approximately one-third of adults with a mtDNA disorder are sporadic cases, usually due to a single deletion of mtDNA. About two-thirds of adults with mtDNA disease harbor a maternally transmitted point mutation. The recurrence risks are well documented for homoplasmic mtDNA mutations causing Leber hereditary optic neuropathy, but the situation is less clear for families with heteroplasmic mtDNA disorders. Two large studies have shown that for some heteroplasmic point mutations there appears to be a relationship between the percentage level of mutant mtDNA in a mother's blood and her risk of having clinically affected offspring. The situation is less clear for other point mutations, some of which may cause sporadic disease. Recent evidence has cast light on the general principles behind the transmission of heteroplasmic mtDNA point mutations, which may be important for genetic counseling in the future.

Defects in mitochondrial respiratory complexes III and IV, and human pathologies

Here, relationships between alterations in tissue-specific content, protein structure, activity, and/or assembly of respiratory complexes III and IV induced by mutations in corresponding genes and various human pathologies are reviewed. Cytochrome bc(1) complex and cytochrome c oxidase (COX) deficiencies have been detected in a heterogeneous group of neuromuscular and non-neuromuscular diseases in childhood and adulthood, presenting a number of clinical phenotypes of variable severity. Such disorders can be caused by mutations located either in mitochondrial genes or in nuclear genes encoding structural subunits of the complexes or corresponding assembly factors/chaperones. Of the defects in mitochondrial DNA genes, mutations in cytochrome b subunit of complex III, and in structural subunits I-III of COX have been described to date. As to defects in nuclear DNA genes, mutations in genes encoding the complexes assembly factors such as the BCS1L protein for complex III; and SURF-1, SCO1, SCO2, and COX10 for complex IV have been identified so far.

Succinate in dystrophic white matter: a proton magnetic resonance spectroscopy finding characteristic for complex II deficiency

A deficiency of succinate dehydrogenase is a rare cause of mitochondrial encephalomyopathy. Three patients, 2 sisters and 1 boy from an unrelated family, presented with symptoms and magnetic resonance imaging signs of leukoencephalopathy. Localized proton magnetic resonance spectroscopy indicated a prominent singlet at 2.40ppm in cerebral and cerebellar white matter not present in gray matter or basal ganglia. The signal was also elevated in cerebrospinal fluid and could be identified as originating from the two equivalent methylene groups of succinate. Subsequently, an isolated deficiency of complex II (succinate:ubiquinone oxidoreductase) was demonstrated in 2 patients in muscle and fibroblasts. One of the sisters died at the age of 18 months. Postmortem examination showed the neuropathological characteristics of Leigh syndrome. Her younger sister, now 12 months old, is also severely affected; the boy, now 6 years old, follows a milder, fluctuating clinical course. Magnetic resonance spectroscopy provides a characteristic pattern in succinate dehydrogenase deficiency.

Infantile leukoencephalopathy owing to mitochondrial enzyme dysfunction

Mitochondrial disease is classically associated with deep gray-matter lesions. When white matter is involved, the lesions are typically subcortical and overshadowed by more significant disease in the gray matter. We report six infants in five families who developed neurodegenerative diseases characterized primarily by abnormalities in deep white-matter structures such as the periventricular region, internal capsule, and corpus callosum. Five patients had impairments of mitochondrial enzymes, including a pre-electron transport chain defect and defects in respiratory chain complexes I, III, and IV (cytochrome-c oxidase). One patient, the sibling of one of the others, was diagnosed clinically with complex III deficiency. These six patients, along with others in the literature, appear to represent a distinct syndrome of mitochondrial infantile leukoencephalopathy. Our observations suggest that infants with leukoencephalopathies, especially leukodystrophies, who do not have one of the more common causes of white-matter disease should be evaluated for mitochondrial dysfunction.

Diseases of adenosine triphosphate synthesis in children

There is an expanding understanding of primary genetic oxidative-phosphorylation disorders and the recognition of new multi-system clinical phenotypes in the energy metabolism diseases. Although initially recognized in association with mitochondrial DNA mutations, there is progress in the more laborious identification of nuclear DNA encoded genes relevant to mitochondrial structure and function. More pathogenic mitochondrial DNA and nuclear DNA mutations have been identified. Diagnosis of these disorders is often difficult and relies on a concurrence of findings, including recognition of a variety of clinical signs and symptoms, biochemical marker screening, electron transport chain enzyme measurements, and mitochondrial DNA or nuclear DNA mutation assay of genes relevant to mitochondrial structure, function or adenosine triphosphate metabolic pathways. Clinical diagnostic assessment now can be augmented by physiologic imaging techniques, including nuclear magnetic resonance spectroscopy and positron emission tomography. These capabilities should be increasingly helpful for studies of clinical progression and therapeutic intervention. Biologic studies, in families and patients, are beginning to address the factors of mitochondrial replication and segregation that underlie cellular/tissue heteroplasmy and clinical variability. Most epigenetic factors affecting organ-specific and phenotypic variability, however, remain to be elaborated.

A novel mutation in the SURF1 gene in a child with Leigh disease, peripheral neuropathy, and cytochrome-c oxidase deficiency

We report a 16-month-old boy with psychomotor regression, muscle hypotonia, peripheral neuropathy, and lactic acidosis. Brain magnetic resonance imaging showed a bilateral abnormal signal in the substantia nigra and in the subthalamic nucleus, suggestive of Leigh disease. Histochemical analysis of skeletal muscle showed decreased cytochrome-c oxidase activity. Biochemical analysis of respiratory chain enzymes in muscle homogenate and in cultured fibroblasts showed isolated cytochrome-c oxidase deficiency. Western blot analysis in fibroblasts showed the absence of Surf1 protein. Genetic analysis of the SURF1 gene revealed that the patient was compound heterozygous for a previously reported mutation at the splice-junction site of intron 3 (240 + 1G > T), and for a novel 4-bp deletion in exon 6 (531_534delAAAT). Our data further enlarge the spectrum of mutations in SURF1 gene in patients with Leigh disease and cytochrome-c oxidase deficiency, contributing to better characterization of the clinical and neuroradiologic features of this group of patients for genotype-phenotype correlations.

Mitochondrial oxidative phosphorylation system assembly in man: recent achievements

The human oxidative phosphorylation system consists of five multi-subunit complexes of which the individual subunits, with the exception of complex II, are encoded either by mitochondrial or nuclear DNA. Consequently, a deficient enzyme activity of one or more of the complexes can be caused by mitochondrial or nuclear DNA mutations. In the past 5 years numerous mutations have been found in structural nuclear oxidative phosphorylation system genes. However, in a substantial number of patients with oxidative phosphorylation system complex deficiencies, despite extensive investigations, no mutations in the mitochondrial DNA or the structural nuclear genes have been found. Genetic defects in such patients are therefore suspected at the transcriptional, translational, post-translational level or in gene products involved in the assembly of the oxidative phosphorylation system. The latter is a complicated process, as the proteins encoded by the two genomes have to be brought together in a proper stoichiometric way to form five functional complexes. In the past year substantial progress in the knowledge of the human oxidative phosphorylation assembly process has been made. Several human assembly genes have been identified, and mutations in these genes responsible for human oxidative phosphorylation system complex-related diseases have been found. In this review, we summarize our current knowledge about human oxidative phosphorylation system assembly genes in health and disease.

A mutant mitochondrial respiratory chain assembly protein causes complex III deficiency in patients with tubulopathy, encephalopathy and liver failure

Complex III (CIII; ubiquinol cytochrome c reductase of the mitochondrial respiratory chain) catalyzes electron transfer from succinate and nicotinamide adenine dinucleotide-linked dehydrogenases to cytochrome c. CIII is made up of 11 subunits, of which all but one (cytochrome b) are encoded by nuclear DNA. CIII deficiencies are rare and manifest heterogeneous clinical presentations. Although pathogenic mutations in the gene encoding mitochondrial cytochrome b have been described, mutations in the nuclear-DNA-encoded subunits have not been reported. Involvement of various genes has been indicated in assembly of yeast CIII (refs. 8-11). So far only one such gene, BCS1L, has been identified in human. BCS1L represents, therefore, an obvious candidate gene in CIII deficiency. Here, we report BCS1L mutations in six patients, from four unrelated families and presenting neonatal proximal tubulopathy, hepatic involvement and encephalopathy. Complementation study in yeast confirmed the deleterious effect of these mutations. Mutation of BCS1L would seem to be a frequent cause of CIII deficiency, as one-third of our patients have BCS1L mutations.