Primary Coenzyme Q10 Deficiency

open4siCLINICAL CHARACTERISTICS: Primary coenzyme Q10 (CoQ10) deficiency is usually associated with multisystem involvement, including neurologic manifestations such as fatal neonatal encephalopathy with hypotonia; a late-onset slowly progressive multiple-system atrophy-like phenotype (neurodegeneration with autonomic failure and various combinations of parkinsonism and cerebellar ataxia, and pyramidal dysfunction); and dystonia, spasticity, seizures, and intellectual disability. Steroid-resistant nephrotic syndrome (SRNS), the hallmark renal manifestation, is often the initial manifestation either as isolated renal involvement that progresses to end-stage renal disease (ESRD), or associated with encephalopathy (seizures, stroke-like episodes, severe neurologic impairment) resulting in early death. Hypertrophic cardiomyopathy (HCM), retinopathy or optic atrophy, and sensorineural hearing loss can also be seen. DIAGNOSIS/TESTING: The diagnosis of primary CoQ10 deficiency in a proband is established by identification of biallelic pathogenic variants in one of the nine genes encoding proteins directly involved in the synthesis of coenzyme Q10 or by detection of reduced levels of CoQ10 (ubiquinone) in skeletal muscle or reduced activities of complex I+III and II+III of the mitochondrial respiratory chain on frozen muscle homogenates. MANAGEMENT: Treatment of manifestations: In individuals with primary CoQ10 deficiency early treatment with high-dose oral CoQ10 supplementation (ranging from 5 to 50 mg/kg/day) can limit disease progression and reverse some manifestations; however, established severe neurologic and/or renal damage cannot be reversed. ACE inhibitors may be used in combination with CoQ10 supplementation in persons with proteinuria; renal transplantation is an option for those with ESRD. Treatment of hypertrophic cardiomyopathy, retinopathy, and sensorineural hearing loss is per usual practice. Prevention of primary manifestations: Supplementation with high-dose oral CoQ10 can prevent progression of the renal disease and onset of neurologic manifestations. Surveillance: Periodic neurologic evaluation, urine analysis (for proteinuria) and renal function tests, ophthalmologic evaluation, and audiometry. Evaluation of relatives at risk: Presymptomatic diagnosis for the purpose of early treatment with CoQ10 supplementation is warranted for relatives at risk. GENETIC COUNSELING: Primary coenzyme Q10 deficiency is inherited in an autosomal recessive manner. At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Carrier testing for at-risk relatives, prenatal testing for pregnancies at increased risk, and preimplantation genetic diagnosis are possible if the pathogenic variants in a family are known.openSalviati, L; Trevisson, E; Doimo, M; Navas, PSalviati, Leonardo; Trevisson, Eva; Doimo, Mara; Navas, P

Effect of vanillic acid on COQ6 mutants identified in patients with coenzyme Q10 deficiency

Under a Creative Commons license.-- et al.Human COQ6 encodes a monooxygenase which is responsible for the C5-hydroxylation of the quinone ring of coenzyme Q (CoQ). Mutations in COQ6 cause primary CoQ deficiency, a condition responsive to oral CoQ10 supplementation. Treatment is however still problematic given the poor bioavailability of CoQ10. We employed S. cerevisiae lacking the orthologous gene to characterize the two different human COQ6 isoforms and the mutations found in patients. COQ6 isoform a can partially complement the defective yeast, while isoform b, which lacks part of the FAD-binding domain, is inactive but partially stable, and could have a regulatory/inhibitory function in CoQ10 biosynthesis. Most mutations identified in patients, including the frameshift Q461fs478X mutation, retain residual enzymatic activity, and all patients carry at least one hypomorphic allele, confirming that the complete block of CoQ biosynthesis is lethal. These mutants are also partially stable and allow the assembly of the CoQ biosynthetic complex. In fact treatment with two hydroxylated analogues of 4-hydroxybenzoic acid, namely, vanillic acid or 3-4-hydroxybenzoic acid, restored the respiratory growth of yeast δcoq6 cells expressing the mutant huCOQ6-isoa proteins. These compounds, and particularly vanillic acid, could therefore represent an interesting therapeutic option for COQ6 patients.This work has been supported by grants from Telethon Italy, Fondazione CARIPARO, and University of Padova (CPDA123573/12) (to L.S.), the Italian Ministry of Health (GR-2009-1578914) (to E.T.), Région Rhônes-Alpes CIBLE 2009 (to F.P.), Spanish FIS grant PI11-00078 (to P.N.) and Proyecto Excelencia P08-CTS-03988 (to P.N.).Open Access funded by Telethon (Italy).Peer Reviewe

Effect of vanillic acid on COQ6 mutants identified in patients with coenzyme Q10 deficiency.

International audience: Human COQ6 encodes a monooxygenase which is responsible for the C5-hydroxylation of the quinone ring of coenzyme Q (CoQ). Mutations in COQ6 cause primary CoQ deficiency, a condition responsive to oral CoQ10 supplementation. Treatment is however still problematic given the poor bioavailability of CoQ10. We employed S. cerevisiae lacking the orthologous gene to characterize the two different human COQ6 isoforms and the mutations found in patients. COQ6 isoform a can partially complement the defective yeast, while isoform b, which lacks part of the FAD-binding domain, is inactive but partially stable, and could have a regulatory/inhibitory function in CoQ10 biosynthesis. Most mutations identified in patients, including the frameshift Q461fs478X mutation, retain residual enzymatic activity, and all patients carry at least one hypomorphic allele, confirming that the complete block of CoQ biosynthesis is lethal. These mutants are also partially stable and allow the assembly of the CoQ biosynthetic complex. In fact treatment with two hydroxylated analogues of 4-hydroxybenzoic acid, namely, vanillic acid or 3-4-hydroxybenzoic acid, restored the respiratory growth of yeast Δcoq6 cells expressing the mutant huCOQ6-isoa proteins. These compounds, and particularly vanillic acid, could therefore represent an interesting therapeutic option for COQ6 patients

A two-nuclease pathway involving RNase H1 is required for primer removal at human mitochondrial OriL.

The role of Ribonuclease H1 (RNase H1) during primer removal and ligation at the mitochondrial origin of light-strand DNA synthesis (OriL) is a key, yet poorly understood, step in mitochondrial DNA maintenance. Here, we reconstitute the replication cycle of L-strand synthesis in vitro using recombinant mitochondrial proteins and model OriL substrates. The process begins with initiation of DNA replication at OriL and ends with primer removal and ligation. We find that RNase H1 partially removes the primer, leaving behind the last one to three ribonucleotides. These 5'-end ribonucleotides disturb ligation, a conclusion which is supported by analysis of RNase H1-deficient patient cells. A second nuclease is therefore required to remove the last ribonucleotides and we demonstrate that Flap endonuclease 1 (FEN1) can execute this function in vitro. Removal of RNA primers at OriL thus depends on a two-nuclease model, which in addition to RNase H1 requires FEN1 or a FEN1-like activity. These findings define the role of RNase H1 at OriL and help to explain the pathogenic consequences of disease causing mutations in RNase H1

Molecular characterization of the human COQ5 C-methyltransferase in coenzyme Q10 biosynthesis

Under a Creative Commons license.Coq5 catalyzes the only C-methylation involved in the biosynthesis of coenzyme Q (Q or ubiquinone) in humans and yeast Saccharomyces cerevisiae. As one of eleven polypeptides required for Q production in yeast, Coq5 has also been shown to assemble with the multi-subunit complex termed the CoQ-synthome. In humans, mutations in several COQ genes cause primary Q deficiency, and a decrease in Q biosynthesis is associated with mitochondrial, cardiovascular, kidney and neurodegenerative diseases. In this study, we characterize the human COQ5 polypeptide and examine its complementation of yeast coq5 point and null mutants. We show that human COQ5 RNA is expressed in all tissues and that the COQ5 polypeptide is associated with the mitochondrial inner membrane on the matrix side. Previous work in yeast has shown that point mutations within or adjacent to conserved COQ5 methyltransferase motifs result in a loss of Coq5 function but not Coq5 steady state levels. Here, we show that stabilization of the CoQ-synthome within coq5 point mutants or by over-expression of COQ8 in coq5 null mutants permits the human COQ5 homolog to partially restore coq5 mutant growth on respiratory media and Q6 content. Immunoblotting against the human COQ5 polypeptide in isolated yeast mitochondria shows that the human Coq5 polypeptide migrates in two-dimensional blue-native/SDS-PAGE at the same high molecular mass as other yeast Coq proteins. The results presented suggest that human and Escherichia coli Coq5 homologs expressed in yeast retain C-methyltransferase activity but are capable of rescuing the coq5 yeast mutants only when the CoQ-synthome is assembled.Open Access funded by Telethon (Italy).Peer Reviewe

Haploinsufficiency of COQ4 causes coenzyme Q10 deficiency

PMCID: PMC3983946.-- et al.[Background]: COQ4 encodes a protein that organises the multienzyme complex for the synthesis of coenzyme Q10 (CoQ10). A 3.9 Mb deletion of chromosome 9q34.13 was identified in a 3-year-old boy with mental retardation, encephalomyopathy and dysmorphic features. Because the deletion encompassed COQ4, the patient was screened for CoQ10 deficiency. [Methods]: A complete molecular and biochemical characterisation of the patient's fibroblasts and of a yeast model were performed. [Results]: The study found reduced COQ4 expression (48% of controls), CoQ10 content and biosynthetic rate (44% and 43% of controls), and activities of respiratory chain complex II+III. Cells displayed a growth defect that was corrected by the addition of CoQ10 to the culture medium. Knockdown of COQ4 in HeLa cells also resulted in a reduction of CoQ10. Diploid yeast haploinsufficient for COQ4 displayed similar CoQ deficiency. Haploinsufficency of other genes involved in CoQ10 biosynthesis does not cause CoQ deficiency, underscoring the critical role of COQ4. Oral CoQ10 supplementation resulted in a significant improvement of neuromuscular symptoms, which reappeared after supplementation was temporarily discontinued. [Conclusion]: Mutations of COQ4 should be searched for in patients with CoQ10 deficiency and encephalomyopathy; patients with genomic rearrangements involving COQ4 should be screened for CoQ10 deficiency, as they could benefit from supplementation.This work was supported by Telethon Italy grant no GGP09207, CARIPARO foundation, the Spanish Ministerio de Sanidad (FIS) grant no PI 08/0500, University of Padova grant no 2010-CPDA102953, Italian Ministry of Health grant no GR-2009-1578914, National Institute of Health grant nos 1R01HD057543-01 and HD 32062, and Cariplo Foundation grant no 2007.5197.Peer reviewe

Copper and bezafibrate cooperate to rescue cytochrome c oxidase deficiency in cells of patients with sco2 mutations

Background: Mutations in SCO2 cause cytochrome c oxidase deficiency (COX) and a fatal infantile cardioencephalomyopathy. SCO2 encodes a protein involved in COX copper metabolism; supplementation with copper salts rescues the defect in patients’ cells. Bezafibrate (BZF), an approved hypolipidemic agent, ameliorates the COX deficiency in mice with mutations in COX10, another COX-assembly gene. Methods: We have investigated the effect of BZF and copper in cells with SCO2 mutations using spectrophotometric methods to analyse respiratory chain activities and a luciferase assay to measure ATP production.. Results: Individual mitochondrial enzymes displayed different responses to BZF. COX activity increased by about 40% above basal levels (both in controls and patients), with SCO2 cells reaching 75-80% COX activity compared to untreated controls. The increase in COX was paralleled by an increase in ATP production. The effect was dose-dependent: it was negligible with 100 μM BZF, and peaked at 400 μM BZF. Higher BZF concentrations were associated with a relative decline of COX activity, indicating that the therapeutic range of this drug is very narrow. Combined treatment with 100 μM CuCl2 and 200 μM BZF (which are only marginally effective when administered individually) achieved complete rescue of COX activity in SCO2 cells. Conclusions: These data are crucial to design therapeutic trials for this otherwise fatal disorder. The additive effect of copper and BZF will allow to employ lower doses of each drug and to reduce their potential toxic effects. The exact mechanism of action of BZF remains to be determined

Yeast models for the study of mitochondrial genetic defects and other metabolic disorders

Yeast is a versatile tool to study the function of genes involved in mitochondrial processes and in metabolic pathways. A large number of human genes involved in such pathways have yeast orthologues; moreover, yeast is easy to manipulate and it can switch from a fermentative to a respiratory metabolism, thus permitting to study mitochondrial phenotypes. For all these reasons, we employed S. cerevisiae for the functional characterization of different genes involved in inherited metabolic and mitochondrial disorders. Gyrate atrophy of choroid and retina (GA) is an autosomal recessive disorder caused by mutations at the level of the ornithine aminotransferase (OAT), a mitochondrial matrix protein involved in the ornithine metabolism. We identified a number of missense mutations in OAT and we proved their pathogenicity in a model of S. cerevisiae deleted for CargB, the homologue of OAT. Moreover, analysis of protein stability and residual enzymatic activity permitted us to elucidate the mechanism by which the aminoacid substitution affects the protein function. However, these data did not allow to establish any genotype-phenotype correlation, suggesting that other factors than the specific OAT genotypes are responsible for the phenotypic variability present in the patients. Yeast Cox23p is a mitochondrial protein with a twin CX9C domain, involved in the COX assembly and possibly in copper homeostasis. In our lab we identified using a bioinformatics approach its human homologue, hCOX23 and characterized its function. We demonstrated using mass spectrometry that the recombinant protein binds Cu(I), providing the first direct evidence of its copper binding. Upon silencing of COX23 we could not observe a phenotype in HeLa cells, but expression of human COX23 in S. cerevisiae deleted for the corresponding gene, showed that it can vicariate for the function of yCox23, confirming its involvement in COX biogenesis. We then demonstrated that it exerts its function in the intermembrane space (IMS) and that Cmc4p, another twin CX9C protein, has overlapping functions. Since the majority of primary COX deficiencies have not a known cause, the characterization of genes involved in the COX biogenesis is of primary importance to find new possible candidates in these disorders. Coq6p is a monooxygenase involved in the synthesis of CoQ6 in yeast. Recently, point mutations in its human homologue have been associated with steroid resistant nephrotic syndrome (SRNS). We modelled the missense mutations on a yeast strain deleted for Coq6 and we demonstrated that all the human mutations reduce the ability of the human gene to rescue the phenotype of the deleted yeast. Moreover, we mutated the corresponding aminoacid on the yeast gene and we proved that all these allelic combinations retain some residual activity, supporting the notion that complete lack of CoQ biosynthesis is embryonically lethal. OPA1 is a dynamin related protein mutated in dominant optic atrophy (ADOA), the most common inherited optic neuropathy. It is involved in different processes such as mitochondrial fusion and apoptosis. In human OPA1 is present with 8 different splicing variants, each of them processed to originate a long form, attached to the inner mitochondrial membrane (IMM) and a short, soluble form, localized in the IMS. The processing of OPA1 is strictly regulated, because the ratio between the two forms is important for the different protein functions. Several proteases have been implicated in the processing. To better characterise this mechanism and to understand the role of the specific splicing isoforms, we decided to employ a model of S. cerevisiae deleted for Mgm1, the homologue of OPA1. Expression of single OPA1 splicing variants cannot rescue the phenotype of the deleted strain while a hybrid form of the protein, containing the mitochondrial targeting and the processing sequences of Mgm1, restores the growth of ΔMgm1. These data indicate that the function of the active core of OPA1 is conserved among evolution and the lack of complementation of OPA1 is probably due to a different mechanism of processing in the two organisms. The hybrid gene will represent a simple tool to study the pathogenicity of missense OPA1 mutations identified in patients with ADOA and ADOA plus. All together these data demonstrate that yeast represent a simple and effective system to characterize the function and to study the pathogenicity of a broad spectrum of conserved proteins such as the ones involved in mitochondrial respiration, mitochondrial morphology or other metabolic pathways.S. cerevisiae è un sistema molto versatile per studiare la funzione dei geni coinvolti in numerose vie mitocondriali e metaboliche. La maggior parte dei geni umani coinvolti in tali processi presentano ortologhi in lievito. Inoltre, questo organismo è facile da manipolare ed è in grado di produrre ATP sia attraverso la glicolisi che attraverso la catena respiratoria, sulla base della fonte di carbonio fornita; tale caratteristica permette lo studio fenotipi mitocondriali. Per tutte queste ragioni, abbiamo impiegato S. cerevisiae per la caratterizzazione funzionale di geni coinvolti in alcune malattie ereditarie metaboliche e mitocondriali. L’atrofia girata della retina e della coroide (GA) è una malattia autosomica recessiva causata da mutazioni a livello dell’enzima ornitina aminotransferasi (OAT), una proteina della matrice mitocondriale coinvolta nel metabolismo dell’ornitina. Abbiamo individuato una serie di mutazioni missenso nel gene OAT e ne abbiamo dimostrato la patogenicità in un modello di S. cerevisiae deleto per il gene CargB, l'omologo di OAT. Ulteriori studi sull’analisi della stabilità della proteina e la misurazione dell’attività enzimatica residua hanno permesso di chiarire il meccanismo attraverso il quale le differenti mutazioni missenso influiscono sulla funzione dell’enzima. Tuttavia questi dati non permettono di stabilire alcuna correlazione genotipo-fenotipo, suggerendo che altri fattori oltre la specifica variazione aminoacidica sono responsabili per la variabilità fenotipica osservata nei pazienti. Cox23p è una proteina di lievito localizzata nei mitocondri e coinvolta nell’ assemblaggio della COX, il complesso IV della catena respiratoria. Possiede il dominio twin CX9C, presente in altre proteine coinvolte nel trasporto del rame. Nel nostro laboratorio abbiamo identificato mediante un approccio bioinformatico il suo omologo umano, hCOX23 e ne abbiamo caratterizzato la funzione. Abbiamo dimostrato con tecniche di spettrometria di massa che la proteina ricombinante lega Cu (I), fornendo la prima prova diretta della sua abilità di legare il rame. Il silenziamento di COX23 in cellule HeLa non ha evidenziato alcun fenotipo. Al contrario, l’espressione del gene umano in un ceppo di lievito deleto per il gene corrispondente, ha dimostrato che COX23 può complementare il fenotipo, confermando il suo coinvolgimento nel processo di assemblaggio della COX. Abbiamo inoltre dimostrato che yCox23p è localizzato nello spazio intermembrana (IMS) e che Cmc4p, un'altra proteina contenente il dominio twin CX9C , ha funzioni rindondanti. Dal momento che la maggior parte dei deficit primari di COX non hanno ancora una causa nota, la caratterizzazione dei geni coinvolti nella biogenesi COX è di primaria importanza per trovare nuovi possibili candidati responsabili di queste patologia. Coq6p è una monoossigenasi coinvolta nella sintesi di CoQ6 in lievito. Recentemente mutazioni puntiformi nel suo omologo umano sono state associate con la sindrome nefrosica steroido-resistente (SRNS). Abbiamo espresso le mutazioni missenso in un ceppo di lievito deleto per il gene Coq6 e abbiamo dimostrato che tutte le mutazioni riducono o aboliscono la capacità del gene umano di complementare il fenotipo del lievito deleto. Le mutazioni umane sono state successivamente introdotte nei rispettivi residui conservati del gene di lievito. Questo ha permesso di dimostrare che tutte queste combinazioni alleliche presentano una certa attività residua. Tali dati supportano l’ipotesi che la mancanza totale di CoQ biosintesi è letale a livello embrionale. OPA1 è una proteina mitocondriale coinvolta in diversi processi cellulari tra cui la fusione mitocondriale ed l’apoptosi. Mutazioni a livello di questa proteina causano l’atrofia ottica dominante (ADOA), la più comune neuropatia ottica ereditaria. Nell’uomo il gene OPA1 è presente in 8 differenti varianti di splicing, ognuna delle quali può originare una forma lunga, attaccata alla membrana mitocondriale interna (IMM) e una forma solubile, localizzata nel IMS. Il processamento di OPA1 è strettamente regolato, in quanto il rapporto tra le due forme è importante per le funzioni della stessa. Numerose proteasi sono state indicate come coinvolte in tale processo. Per caratterizzare questo meccanismo e per comprendere il ruolo specifico di ciascuna delle isoforme di splicing, abbiamo deciso di impiegare un modello di S. cerevisiae deleto per Mgm1, l'omologo di OPA1. L’espressione delle singole varianti di splicing non è in grado di ripristinare la crescita del ceppo deleto mentre una forma ibrida della proteina, contenente la sequenza di import mitocondriale e di processamento di Mgm1, permette il recupero del fenotipo di ΔMgm1. Questi dati indicano che la funzione di OPA1 è conservata e la mancanza di complementazione di OPA1 è probabilmente dovuta ad un differente meccanismo di processamento nel lievito rispetto all’uomo. Il gene ibrido rappresenterà un semplice strumento per studiare la patogenicità di missenso OPA1 mutazioni identificate nei pazienti con ADOA e ADOA plus. Nel complesso questi dati dimostrano che il lievito rappresenta un sistema semplice ed efficace per caratterizzare la funzione e per studiare la patogenicità di un ampio spettro di proteine coinvolte nei processi di respirazione, morfologia mitocondriale e in altre vie metaboliche

Molecular Genetics of Argininosuccinic Aciduria

Argininosuccinic aciduria is an autosomal recessive disorder of the urea cycle caused by mutations in argininosuccinate lyase (ASL). Two main clinical phenotypes are reported: an acute neonatal form characterised by severe hyperammonaemia and coma, and a subacute, late-onset form which may present with relatively milder neurological symptoms. More than 120 ASL mutations have been reported so far: the majority are missense, but virtually all types of point mutations are found. Large rearrangements are rare and standard genomic deoxyribonucleic acid (DNA) analysis has a high diagnostic yield. Genotype–phenotype correlations have been difficult to establish as standard biochemical techniques are not sufficiently sensitive to measure residual activity, and other factors such as intragenic complementation, overexpression of nonfunctional ASL transcripts and environmental factors may modulate the phenotype. Clinical manifestations result from the block in the urea cycle and also from impairment of nitric oxide biosynthesis, and the therapy is aimed at restoring these two functions