A novel mutation in SURF1 causes skipping of exon 8 in a patient with cytochrome c oxidase-deficient leigh syndrome and hypertrichosis

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.

Mutations in SURF1 are important genetic causes of Leigh syndrome in Slovak patients

OBJECTIVES

Leigh syndrome is a progressive early onset neurodegenerative disease typically presenting with psychomotor regression, signs of brainstem and/or basal ganglia disease, lactic acidosis, and characteristic magnetic resonance imaging findings. At molecular level, deficiency of respiratory complexes and/or pyruvate dehydrogenase complex is usually observed. Nuclear gene SURF1 encodes an assembly factor for cytochrome c-oxidase complex of the respiratory chain and autosomal recessive mutations in SURF1 are one of the most frequent causes of cytochrome c-oxidase-related Leigh syndrome cases. Here, we aimed to elucidate the genetic basis of Leigh syndrome in three Slovak families.

METHODS AND RESULTS

Three probands presenting with Leigh syndrome were selected for DNA analysis. The first proband, presenting with atypical LS onset without abnormal basal ganglia magnetic resonance imaging findings, was analyzed with whole exome sequencing. In the two remaining probands, SURF1 was screened by Sanger sequencing. Four different heterozygous mutations were identified in SURF1: c.312_321delinsAT:p.(Pro104Profs*1), c.588+1G>A, c.823_833+7del:p. (?) and c.845_846del:p.(Ser282Cysfs*9). All the mutations are predicted to have a loss-of-function effect.

CONCLUSIONS

We identified disease-causing mutations in all three probands, which points to the important role of SURF1 gene in etiology of Leigh syndrome in Slovakia. Our data showed that patients with atypical Leigh syndrome phenotype without lesions in basal ganglia may benefit from the whole exome sequencing method. In the case of probands presenting the typical phenotype, Sanger sequencing of the SURF1 gene seems to be an effective method of DNA analysis.

Mitochondrial Diseases Part I: mouse models of OXPHOS deficiencies caused by defects in respiratory complex subunits or assembly factors

Mitochondrial disorders are the most common inborn errors of metabolism affecting the oxidative phosphorylation system (OXPHOS). Because of the poor knowledge of the pathogenic mechanisms, a cure for these disorders is still unavailable and all the treatments currently in use are supportive more than curative. Therefore, in the past decade a great variety of mouse models have been developed to assess the in vivo function of several mitochondrial proteins involved in human diseases. Due to the genetic and physiological similarity to humans, mice represent reliable models to study the pathogenic mechanisms of mitochondrial disorders and are precious to test new therapeutic approaches. Here we summarize the features of several mouse models of mitochondrial diseases directly related to defects in subunits of the OXPHOS complexes or in assembly factors. We discuss how these models recapitulate many human conditions and how they have contributed to the understanding of mitochondrial function in health and disease.

Mitochondrial depletion causes neonatal-onset leigh syndrome, myopathy, and renal tubulopathy

The authors describe a newborn with postnatal myopathy who subsequently developed feeding difficulties, ophthalmoplegia, ptosis, encephalopathy, and seizures. She became ventilator dependent after sudden apnea. The myopathy was without ragged red fibers in the muscle biopsy. An electron transport chain study showed a markedly generalized low level of enzyme activity, particularly in complexes I, I + III, and IV. An initial electroencephalogram finding was normal; subsequent electroencephalograms showed suppression bursts. The mitochondrial copy number in skeletal muscle was 2% of normal.

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.

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.

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.

Tissue-specific cytochrome c oxidase assembly defects due to mutations in SCO2 and SURF1

The biogenesis of eukaryotic COX (cytochrome c oxidase) requires several accessory proteins in addition to structural subunits and prosthetic groups. We have analysed the assembly state of COX and SCO2 protein levels in various tissues of six patients with mutations in SCO2 and SURF1. SCO2 is a copper-binding protein presumably involved in formation of the Cu(A) centre of the COX2 subunit. The function of SURF1 is unknown. Immunoblot analysis of native gels demonstrated that COX holoenzyme is reduced to 10-20% in skeletal muscle and brain of SCO2 and SURF1 patients and to 10-30% in heart of SCO2 patients, whereas liver of SCO2 patients' contained normal holoenzyme levels. The steady-state levels of mutant SCO2 protein ranged from 0 to 20% in different SCO2 patient tissues. In addition, eight distinct COX subcomplexes and unassembled subunits were found, some of them identical with known assembly intermediates of the human enzyme. Heart, brain and skeletal muscle of SCO2 patients contained accumulated levels of the COX1.COX4.COX5A subcomplex, three COX1-containing subcomplexes, a COX4.COX5A subcomplex and two subcomplexes composed of only COX4 or COX5A. The accumulation of COX1.COX4.COX5A subcomplex, along with the virtual absence of free COX2, suggests that the lack of the Cu(A) centre may result in decreased stability of COX2. The appearance of COX4.COX5A subcomplex indicates that association of these nucleus-encoded subunits probably precedes their addition to COX1 during the assembly process. Finally, the consequences of SCO2 and SURF1 mutations suggest the existence of tissue-specific functional differences of these proteins that may serve different tissue-specific requirements for the regulation of COX biogenesis.

Retrospective, multicentric study of 180 children with cytochrome C oxidase deficiency

A retrospective, multicenter study of 180 children with cytochrome c oxidase (COX) deficiency analyzed the clinical features, prognosis, and molecular bases of the COX deficiency. Clinical symptoms including failure to thrive, encephalopathy, hypotony, Leigh syndrome, cardiac involvement, and hepatopathy appeared in most patients early after birth or in early childhood. Two thirds of all children died. Biochemical examination revealed an isolated COX deficiency in 101 children and COX deficiency combined with disturbances of other respiratory chain complexes in 79 children. Blood and cerebrospinal fluid lactate increased in 85% and 81% of examined cases, respectively. Pathogenic mutations in mitochondrial or nuclear DNA were established in 75 patients. Mutations in surfeit locus protein 1 gene (SURF1) were found in 47 children with Leigh syndrome; 2bp deletion 845-846delCT was found in 89% of independent alleles. Mutations in a mitochondrial copper-binding protein (SCO2) gene were found in nine children with encephalomyopathy and/or cardiomyopathy; all of them were homozygotes or heterozygotes for 1541G>A mutation. Different mitochondrial DNA (mtDNA) deletion or depletion were found in nine children, mtDNA mutation 3243A>G in six, mtDNA mutation 8363G>A in two children with Leigh syndrome and mtDNA mutations 8344A>G, and 9205-9206delTA in one child each. COX deficiency represents a heterogeneous group of diseases with unfavorable prognosis. Marked prevalence of two nuclear DNA mutations (845-846delCT in the SURF1 gene and 1541G>A in the SCO2 gene) associated with COX deficiency in a Slavonic population suggests the existence of regional differences in the genetic basis of COX deficiency.

Are mitochondria critical in the pathogenesis of Alzheimer's disease?

This review summarizes recent findings that suggest a causal connection between mitochondrial abnormalities and sporadic Alzheimer's disease (AD). Genetic causes of AD are known only for a small proportion of familial AD patients, but for a majority of sporadic AD patients, genetic causal factors are still unknown. Currently, there are no early detectable biomarkers for sporadic AD, and there is a lack of understanding of the pathophysiology of the disease. Findings from recent genetic studies of AD pathogenesis suggest that mitochondrial defects may play an important role in sporadic AD progression, and that mitochondrial abnormalities and oxidative damage may play a significant role in the progression of familial AD. Findings from biochemical studies, in vitro studies, gene expression studies, and animal model studies of AD are reviewed, and the possible contribution of mitochondrial mutations to late-onset sporadic AD is discussed.

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.

Molecular diagnostics of mitochondrial disorders

The mitochondrial respiratory chain (RC) results from the expression of both mitochondrial and nuclear genes. The number of disease-causing mutations in nuclear genes is steadily growing and mitochondrial DNA (mtDNA) deletions and mutations account for no more than 15-20% of pediatric patients. Unfortunately, the disease-causing mutations have been identified for only a small number of patients. Thus, elucidating the genetic bases of RC is both essential for genetic diagnosis of patients and for fundamental knowledge of these disorders. The molecular diagnostics of mitochondrial disorders come under both genetic diagnosis and research. Indeed, identification of a new gene in a specific patient allows to perform genetic diagnosis in other families and identification of mutations in already known disease-causing genes allows to constitute a cohort of patients for further functional studies. Thus, elucidating the genetic bases of RC deficiency is an essential task that needs the use of several appropriate strategies. Fine phenotypage of patients and candidate gene screening is the first step for the constitution of a well-characterized cohort of patients. Genetic mapping has to be used in large families. This approach is greatly enhanced in the case of consanguineous families. The consanguinity of the parents should also lead to test genetic markers surrounding the gene loci rather than to directly sequence several candidate genes. However, the main problem is encountered in the cases of sporadic cases for which no genetic approaches can be developed. In these cases, functional complementation by human chromosomes or cDNA is the only presently available strategy.

Cytochrome c Oxidase Subassemblies in Fibroblast Cultures from Patients Carrying Mutations in COX10, SCO1, or SURF1

Cytochrome c oxidase contains two redox-active copper centers (Cu(A) and Cu(B)) and two redox-active heme A moieties. Assembly of the enzyme relies on several assembly factors in addition to the constituent subunits and prosthetic groups. We studied fibroblast cultures from patients carrying mutations in the assembly factors COX10, SCO1, or SURF1. COX10 is involved in heme A biosynthesis. SCO1 is required for formation of the Cu(A) center. The function of SURF1 is unknown. Immunoblot analysis of native gels demonstrated severely decreased levels of holoenzyme in the patient cultures compared with controls. In addition, the blots revealed the presence of five subassemblies: three subassemblies involving the core subunit MTCO1 but apparently no other subunits; a subassembly containing subunits MTCO1, COX4, and COX5A; and a subassembly containing at least subunits MTCO1, MTCO2, MTCO3, COX4, and COX5A. As some of the subassemblies correspond to known assembly intermediates of human cytochrome c oxidase, we think that these subassemblies are probably assembly intermediates that accumulate in patient cells. The MTCO1.COX4.COX5A subassembly was not detected in COX10-deficient cells, which suggests that heme A incorporation into MTCO1 occurs prior to association of MTCO1 with COX4 and COX5A. SCO1-deficient cells contained accumulated levels of the MTCO1.COX4.COX5A subassembly, suggesting that MTCO2 associates with the MTCO1.COX4.COX5A subassembly after the Cu(A) center of MTCO2 is formed. Assembly in SURF1-deficient cells appears to stall at the same stage as in SCO1-deficient cells, pointing to a role for SURF1 in promoting the association of MTCO2 with the MTCO1.COX4.COX5A subassembly.

Functional alteration of cytochrome c oxidase by SURF1 mutations in Leigh syndrome

Subacute necrotising encephalomyopathy (Leigh syndrome) due to cytochrome c oxidase (COX) deficiency is often caused by mutations in the SURF1 gene, encoding the Surf1 protein essential for COX assembly. We have investigated five patients with different SURF1 mutations resulting in the absence of Surf1 protein. All of them presented with severe and generalised COX defect. Immunoelectrophoretic analysis of cultured fibroblasts revealed 85% decrease of the normal-size COX complexes and significant accumulation of incomplete COX assemblies of 90-120 kDa. Spectrophotometric assay of COX activity showed a 70-90% decrease in lauryl maltoside (LM)-solubilised fibroblasts. In contrast, oxygen consumption analysis in whole cells revealed only a 13-31% decrease of COX activity, which was completely inhibited by detergent in patient cells but not in controls. In patient fibroblasts ADP-stimulated respiration was 50% decreased and cytofluorometry showed a significant decrease of mitochondrial membrane potential DeltaPsi(m) in state 4, as well as a 2.4-fold higher sensitivity of DeltaPsi(m) to uncoupler. We conclude that the absence of the Surf1 protein leads to the formation of incomplete COX complexes, which in situ maintain rather high electron-transport activity, while their H(+)-pumping is impaired. Enzyme inactivation by the detergent in patient cells indicates instability of incomplete COX assemblies.

Down-regulation of mitochondrial cytochrome c oxidase in senescent porcine pulmonary artery endothelial cells

Cellular aging is associated with dysfunction of the mitochondrial respiration chain. Deficiency of mitochondrial cytochrome c oxidase (complex IV) plays a critical role in aging-induced mitochondrial dysfunction. We investigated whether in vitro cellular aging causes the downregulation of complex IV activity and gene expression using senescent (passage 45) and young (passage 3) pulmonary artery endothelial cells (PAEC). In senescent PAEC, the catalytic activity of complex IV decreased 84%, compared to that in young cells. Relative protein levels of complex IV subunits I and IV (complex IV S1 and S4) in senescent cells decreased 91%, compared to those in young cells. This suggests that lack of complex IV S1 and S4 in senescent cells may contribute to the deficiency of complex IV. Total steady state levels of mRNA for complex IV S1 and S4 in senescent cells were decreased to 20% and 18% of those in young cells. The relative rates of mRNA synthesis of complex IV S1 and S4 were decreased 46% and 37% in senescent cells, respectively, compared to young cells. The degradation of complex IV S1 and S4 was increased 76% and 64% in senescent cells, compared to young cells. These data indicate that mitochondrial DNA-encoded subunit I and nuclear DNA-encoded subunit IV of complex IV are downregulated through reduced synthesis and enhanced degradation of their mRNA, which may be responsible for the deficiency of complex IV in replicative senescent PAEC.

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.