Differential effect of DCA treatment on the pyruvate dehydrogenase complex in patients with severe PDHC deficiency

Phenylbutyrate and Dichloroacetate Enhance the Liquid-Stored Boar Sperm Quality via PDK1 and PDK3

Artificial insemination (AI) with liquid-stored semen is the most prevalent and efficient assisted reproduction technique in the modern pork industry. Pyruvate dehydrogenase complex component X (PDHX) was demonstrated to be associated with sperm metabolism and affected the boar sperm viability, motility, and fertility. Pyruvate Dehydrogenase Kinases (PDKs) are the key metabolic enzymes that regulate pyruvate dehydrogenase complex (PDHC) activity and also the conversion from glycolysis to oxidative phosphorylation. In the present study, two PDK inhibitors, Dichloroacetate (DCA) and Phenylbutyrate (4-PBA), were added to an extender and investigated to determine their regulatory roles in liquid-stored boar sperm at 17 °C. The results indicated that PDK1 and PDK3 were predominantly located at the head and flagella of the boar sperm. The addition of 2 mM DCA and 0.5 mM 4-PBA significantly enhanced the sperm motility, plasma membrane integrity (PMI), mitochondrial membrane potential (MMP), and ATP content. In addition, DCA and 4-PBA exerted their effects by inhibiting PDK1 and PDK3, respectively. In conclusion, DCA and 4-PBA were found to regulate the boar sperm metabolic activities via PDK1 and PDK3. These both can improve the quality parameters of liquid-stored boar sperm, which will help to improve and optimize liquid-stored boar semen after their addition in the extender.

Pearls & Oy-sters: Paroxysmal Exercise-Induced Dyskinesias Due to Pyruvate Dehydrogenase Deficiency

Paroxysmal exercise-induced movement disorders may be caused by energy metabolism disorders, such as Glut 1 deficiency, pyruvate dehydrogenase deficiency, or mitochondrial respiratory chain disorders. A 4-year-old boy with a history of febrile seizures presented with paroxysmal dystonia, triggered by exercise, or occurring at rest. Additional investigations demonstrated pallidal hyperintensities on brain MRI and low CSF glucose. Pyruvate and lactate were elevated. The clinical presentation combined with neuroimaging abnormalities and biochemical profile (the lactate/pyruvate ratio) were clues to pyruvate dehydrogenase deficiency, a treatable metabolic disorder with neurologic presentations.

Pyruvate dehydrogenase kinase regulates vascular inflammation in atherosclerosis and increases cardiovascular risk

AIMS

Recent studies have revealed a close connection between cellular metabolism and the chronic inflammatory process of atherosclerosis. While the link between systemic metabolism and atherosclerosis is well established, the implications of altered metabolism in the artery wall are less understood. Pyruvate dehydrogenase kinase (PDK)-dependent inhibition of pyruvate dehydrogenase (PDH) has been identified as a major metabolic step regulating inflammation. Whether the PDK/PDH axis plays a role in vascular inflammation and atherosclerotic cardiovascular disease remains unclear.

METHODS AND RESULTS

Gene profiling of human atherosclerotic plaques revealed a strong correlation between PDK1 and PDK4 transcript levels and the expression of pro-inflammatory and destabilizing genes. Remarkably, the PDK1 and PDK4 expression correlated with a more vulnerable plaque phenotype, and PDK1 expression was found to predict future major adverse cardiovascular events. Using the small-molecule PDK inhibitor dichloroacetate (DCA) that restores arterial PDH activity, we demonstrated that the PDK/PDH axis is a major immunometabolic pathway, regulating immune cell polarization, plaque development, and fibrous cap formation in Apoe-/- mice. Surprisingly, we discovered that DCA regulates succinate release and mitigates its GPR91-dependent signals promoting NLRP3 inflammasome activation and IL-1β secretion by macrophages in the plaque.

CONCLUSIONS

We have demonstrated for the first time that the PDK/PDH axis is associated with vascular inflammation in humans and particularly that the PDK1 isozyme is associated with more severe disease and could predict secondary cardiovascular events. Moreover, we demonstrate that targeting the PDK/PDH axis with DCA skews the immune system, inhibits vascular inflammation and atherogenesis, and promotes plaque stability features in Apoe-/- mice. These results point toward a promising treatment to combat atherosclerosis.

Comparison Between Dichloroacetate and Phenylbutyrate Treatment for Pyruvate Dehydrogenase Deficiency

Pyruvate dehydrogenase (PDH) deficiency is caused by a number of pathogenic variants and the most common are found in the PDHA1 gene. The PDHA1 gene encodes one of the subunits of the PDH enzyme found in a carbohydrate metabolism pathway involved in energy production. Pathogenic variants of PDHA1 gene usually impact the α-subunit of PDH causing energy reduction. It potentially leads to increased mortality in sufferers. Potential treatments for this disease include dichloroacetate and phenylbutyrate, previously used for other diseases such as cancer and maple syrup urine disease. However, not much is known about their efficacy in treating PDH deficiency. Effective treatment for PDH deficiency is crucial as carbohydrate is needed in a healthy diet and rice is the staple food for a large portion of the Asian population. This review analysed the efficacy of dichloroacetate and phenylbutyrate as potential treatments for PDH deficiency caused by PDHA1 pathogenic variants. Based on the findings of this review, dichloroacetate will have an effect on most PDHA1 pathogenic variant and can act as a temporary treatment to reduce the lactic acidosis, a common symptom of PDH deficiency. Phenylbutyrate can only be used on patients with certain pathogenic variants (p.P221L, p.R234G, p.G249R, p.R349C, p.R349H) on the PDH protein. It is hoped that the review would provide an insight into these treatments and improve the quality of lives for patients with PDH deficiency.

Structural and functional impact of clinically relevant E1α variants causing pyruvate dehydrogenase complex deficiency

Pyruvate dehydrogenase complex (PDC) catalyzes the oxidative decarboxylation of pyruvate to acetyl-coenzyme A, hinging glycolysis and the tricarboxylic acid cycle. PDC deficiency, an inborn error of metabolism, has a broad phenotypic spectrum. Symptoms range from fatal lactic acidosis or progressive neuromuscular impairment in the neonatal period, to chronic neurodegeneration. Most disease-causing mutations in PDC deficiency affect the PDHA1 gene, encoding the α subunit of the PDC-E1 component. Detailed biophysical analysis of pathogenic protein variants is a challenging approach to support the design of therapies based on improving and correcting protein structure and function. Herein, we report the characterization of clinically relevant PDC-E1α variants identified in Portuguese PDC deficient patients. These variants bear amino acid substitutions in different structural regions of PDC-E1α. The structural and functional analyses of recombinant heterotetrameric (αα'ββ') PDC-E1 variants, combined with molecular dynamics (MD) simulations, show a limited impact of the amino acid changes on the conformational stability, apart from the increased propensity for aggregation of the p.R253G variant as compared to wild-type PDC-E1. However, all variants presented a functional impairment in terms of lower residual PDC-E1 enzymatic activity and ≈3-100 × lower affinity for the thiamine pyrophosphate (TPP) cofactor, in comparison with wild-type PDC-E1. MD simulations neatly showed generally decreased stability (increased flexibility) of all variants with respect to the WT heterotetramer, particularly in the TPP binding region. These results are discussed in light of disease severity of the patients bearing such mutations and highlight the difficulty of developing chaperone-based therapies for PDC deficiency.

Potential role of stress-induced gluconeogenesis in disease aggravation and mortality in pyruvate dehydrogenase deficiency: A case-based hypothesis

Pyruvate dehydrogenase (PDH) deficiency is an inherited metabolic disorder caused by a defect in any subunit of the pyruvate dehydrogenase complex (PDHC), which has an essential role in glucose metabolism. The causes of disease progression in PDH deficiency are not fully understood yet. Based on repeated observations of a patient with PDH deficiency at our center, we hypothesized that stress-induced gluconeogenesis contributes to rapid exacerbation of the disease. This link has not been established previously.

Pyruvate dehydrogenase complex deficiency: updating the clinical, metabolic and mutational landscapes in a cohort of Portuguese patients

BACKGROUND

The pyruvate dehydrogenase complex (PDC) catalyzes the irreversible decarboxylation of pyruvate into acetyl-CoA. PDC deficiency can be caused by alterations in any of the genes encoding its several subunits. The resulting phenotype, though very heterogeneous, mainly affects the central nervous system. The aim of this study is to describe and discuss the clinical, biochemical and genotypic information from thirteen PDC deficient patients, thus seeking to establish possible genotype-phenotype correlations.

RESULTS

The mutational spectrum showed that seven patients carry mutations in the PDHA1 gene encoding the E1α subunit, five patients carry mutations in the PDHX gene encoding the E3 binding protein, and the remaining patient carries mutations in the DLD gene encoding the E3 subunit. These data corroborate earlier reports describing PDHA1 mutations as the predominant cause of PDC deficiency but also reveal a notable prevalence of PDHX mutations among Portuguese patients, most of them carrying what seems to be a private mutation (p.R284X). The biochemical analyses revealed high lactate and pyruvate plasma levels whereas the lactate/pyruvate ratio was below 16; enzymatic activities, when compared to control values, indicated to be independent from the genotype and ranged from 8.5% to 30%, the latter being considered a cut-off value for primary PDC deficiency. Concerning the clinical features, all patients displayed psychomotor retardation/developmental delay, the severity of which seems to correlate with the type and localization of the mutation carried by the patient. The therapeutic options essentially include the administration of a ketogenic diet and supplementation with thiamine, although arginine aspartate intake revealed to be beneficial in some patients. Moreover, in silico analysis of the missense mutations present in this PDC deficient population allowed to envisage the molecular mechanism underlying these pathogenic variants.

CONCLUSION

The identification of the disease-causing mutations, together with the functional and structural characterization of the mutant protein variants, allow to obtain an insight on the severity of the clinical phenotype and the selection of the most appropriate therapy.

TGF-β1 is a regulator of the pyruvate dehydrogenase complex in fibroblasts

TGF-β1 reprograms metabolism in renal fibroblasts, inducing a switch from oxidative phosphorylation to aerobic glycolysis. However, molecular events underpinning this are unknown. Here we identify that TGF-β1 downregulates acetyl-CoA biosynthesis via regulation of the pyruvate dehydrogenase complex (PDC). Flow cytometry showed that TGF-β1 reduced the PDC subunit PDH-E1α in fibroblasts derived from injured, but not normal kidneys. An increase in expression of PDH kinase 1 (PDK1), and reduction in the phosphatase PDP1, were commensurate with net phosphorylation and inactivation of PDC. Over-expression of mutant PDH-E1α, resistant to phosphorylation, ameliorated effects of TGF-β1, while inhibition of PDC activity with CPI-613 was sufficient to induce αSMA and pro-collagen I expression, markers of myofibroblast differentiation and fibroblast activation. The effect of TGF-β1 on PDC activity, acetyl-CoA, αSMA and pro-collagen I was also ameliorated by sodium dichloroacetate, a small molecule inhibitor of PDK. A reduction in acetyl-CoA, and therefore acetylation substrate, also resulted in a generalised loss of protein acetylation with TGF-β1. In conclusion, TGF-β1 in part regulates fibroblast activation via effects on PDC activity.

Dichloroacetate prevents TGFβ-induced epithelial-mesenchymal transition of retinal pigment epithelial cells

Proliferative retinopathies are associated with formation of fibrous epiretinal membranes. At present, there is no pharmacological intervention for the treatment of retinopathies. Cytokines such as TGFβ are elevated in the vitreous humor of the patients with proliferative vitro-retinopathy, diabetic retinopathy and age-related macular degeneration. TGFβ isoforms lead to epithelial-mesenchymal transition (EMT) or trans-differentiation of the retinal pigment epithelial (RPE) cells. PI3K/Akt and MAPK/Erk pathways play important roles in the EMT of RPE cells. Therefore, inhibition of EMT by pharmacological agents is an important therapeutic strategy in retinopathy. Dichloroacetate (DCA) is shown to prevent proliferation and EMT of cancer cell lines but its effects are not explored on the prevention of EMT of RPE cells. In the present study, we have investigated the role of DCA in preventing TGFβ2 induced EMT of RPE cell line, ARPE-19. A wound-healing assay was utilized to detect the anti-EMT effect of DCA. The expressions of EMT and cell adhesion markers were carried out by immunofluorescence, western blotting, and quantitative real-time PCR. The expression of MAPK/Erk and PI3K/Akt pathway members was carried out using western blotting. We found that TGFβ2 exposure leads to an increase in the wound healing response, expression of EMT markers (Fibronectin, Collagen I, N-cadherin, MMP9, S100A4, α-SMA, Snai1, Slug) and a decrease in the expression of cell adhesion/epithelial markers (ZO-1, Connexin 43, E-cadherin). These changes were accompanied by the activation of PI3K/Akt and MAPK/Erk pathways. Simultaneous exposure of DCA along with TGFβ2 significantly inhibited wound healing response, expression of EMT markers and cell adhesion/epithelial markers. Furthermore, DCA and TGFβ2 effectively attenuated the activation of MAPK/Erk/JNK and PI3K/Akt/GSK3β pathways. Our results demonstrate that DCA has a strong anti-EMT effect on the ARPE-19 cells and hence can be utilized as a therapeutic agent in the prevention of proliferative retinopathies.

The histidine triad nucleotide-binding protein 2 (HINT-2) positively regulates hepatocellular energy metabolism

The histidine triad nucleotide-binding protein 2 (HINT-2) is a mitochondrial adenosine phosphoramidase expressed in hepatocytes. The phenotype of Hint2 knockout ( Hint2^-/-) mice includes progressive hepatic steatosis and lysine hyperacetylation of mitochondrial proteins, which are features of respiratory chain malfunctions. We postulated that the absence of HINT-2 induces a defect in mitochondria bioenergetics. Isolated Hint2^-/- hepatocytes produced less ATP and generated a lower mitochondrial membrane potential than did Hint2^+/+ hepatocytes. In extracellular flux analyses with glucose, the basal, ATP-linked, and maximum oxygen consumption rates (OCRs) were decreased in Hint2^-/- hepatocytes and in HepG2 cells lacking HINT-2. Conversely, in HINT-2 overexpressing SNU-449 and HepG2 cells, the basal, ATP-linked, and maximum OCRs were increased. Similarly, with palmitate, basal and maximum OCRs were decreased in Hint2^-/- hepatocytes, but they were increased in HINT-2 overexpressing HepG2 cells. When assayed with radiolabeled substrate, palmitate oxidation was reduced by 25% in Hint2^-/- mitochondria. In respirometry assays, complex I- and II-driven, coupled and uncoupled respirations and complex IV KCN-sensitive respiration were reduced in Hint2^-/- mitochondria. Furthermore, HINT-2 associated with cardiolipin and glucose-regulated protein 75 kDa. Our study shows decreased electron transfer and oxidative phosphorylation capacity in the absence of HINT-2. The bioenergetics deficit accumulated over time in hepatocytes lacking HINT-2 likely leads to the secondary outcome of steatosis.-Rajasekaran, R., Felser, A., Nuoffer, J.-M., Dufour, J.-F., St-Pierre, M. V. The histidine triad nucleotide-binding protein 2 (HINT-2) positively regulates hepatocellular energy metabolism.

Haloacetic Acid Water Disinfection Byproducts Affect Pyruvate Dehydrogenase Activity and Disrupt Cellular Metabolism

The disinfection of drinking water has been a major public health achievement. However, haloacetic acids (HAAs), generated as byproducts of water disinfection, are cytotoxic, genotoxic, mutagenic, carcinogenic, and teratogenic. Previous studies of monoHAA-induced genotoxicity and cell stress demonstrated that the toxicity was due to inhibition of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), leading to disruption of cellular metabolism and energy homeostasis. DiHAAs and triHAAs are also produced during water disinfection, and whether they share mechanisms of action with monoHAAs is unknown. In this study, we evaluated the effects of mono-, di-, and tri-HAAs on cellular GAPDH enzyme kinetics, cellular ATP levels, and pyruvate dehydrogenase complex (PDC) activity. Here, treatments conducted in Chinese hamster ovary (CHO) cells revealed differences among mono-, di-, and triHAAs in their molecular targets. The monoHAAs, iodoacetic acid and bromoacetic acid, were the strongest inhibitors of GAPDH and greatly reduced cellular ATP levels. Chloroacetic acid, diHAAs, and triHAAs were weaker inhibitors of GAPDH and some increased the levels of cellular ATP. HAAs also affected PDC activity, with most HAAs activating PDC. The primary finding of this work is that mono- versus multi-HAAs address different molecular targets, and the results are generally consistent with a model in which monoHAAs activate the PDC through GAPDH inhibition-mediated disruption in cellular metabolites, including altering ATP-to-ADP and NADH-to-NAD ratios. The monoHAA-mediated reduction in cellular metabolites results in accelerated PDC activity by way of metabolite-ratio-dependent PDC regulation. DiHAAs and triHAAs are weaker inhibitors of GAPDH, but many also increase cellular ATP levels, and we suggest that they increase PDC activity by inhibiting pyruvate dehydrogenase kinase.

Histone Deacetylase Inhibitors Protect Against Pyruvate Dehydrogenase Dysfunction in Huntington's Disease

Transcriptional deregulation and changes in mitochondrial bioenergetics, including pyruvate dehydrogenase (PDH) dysfunction, have been described in Huntington's disease (HD). We showed previously that the histone deacetylase inhibitors (HDACIs) trichostatin A and sodium butyrate (SB) ameliorate mitochondrial function in cells expressing mutant huntingtin. In this work, we investigated the effect of HDACIs on the regulation of PDH activity in striatal cells derived from HD knock-in mice and YAC128 mice. Mutant cells exhibited decreased PDH activity and increased PDH E1alpha phosphorylation/inactivation, accompanied by enhanced protein levels of PDH kinases 1 and 3 (PDK1 and PDK3). Exposure to dichloroacetate, an inhibitor of PDKs, increased mitochondrial respiration and decreased production of reactive oxygen species in mutant cells, emphasizing PDH as an interesting therapeutic target in HD. Treatment with SB and sodium phenylbutyrate, another HDACI, recovered cell viability and overall mitochondrial metabolism in mutant cells. Exposure to SB also suppressed hypoxia-inducible factor-1 (HIF-1α) stabilization and decreased the transcription of the two most abundant PDK isoforms, PDK2 and PDK3, culminating in increased PDH activation in mutant cells. Concordantly, PDK3 knockdown improved mitochondrial function, emphasizing the role of PDK3 inactivation on the positive effects achieved by SB treatment. YAC128 mouse brain presented higher mRNA levels of PDK1-3 and PDH phosphorylation and decreased energy levels that were significantly ameliorated after SB treatment. Furthermore, enhanced motor learning and coordination were observed in SB-treated YAC128 mice. These results suggest that HDACIs, particularly SB, promote the activity of PDH in the HD brain, helping to counteract HD-related deficits in mitochondrial bioenergetics and motor function.SIGNIFICANCE STATEMENT The present work provides a better understanding of mitochondrial dysfunction in Huntington's disease (HD) by showing that the pyruvate dehydrogenase (PDH) complex is a promising therapeutic target. In particular, the histone deacetylase inhibitor sodium butyrate (SB) may indirectly (through reduced hypoxia-inducible factor 1 alpha stabilization) decrease the expression of the most abundant PDH kinase isoforms (e.g., PDK3), ameliorating PDH activity and mitochondrial metabolism and further affecting motor behavior in HD mice, thus constituting a promising agent for HD neuroprotective treatment.

Pyruvate dehydrogenase complex in cerebral ischemia-reperfusion injury

Pyruvate dehydrogenase (PDH) complex is a mitochondrial matrix enzyme that serves a critical role in the conversion of anaerobic to aerobic cerebral energy. The regulatory complexity of PDH, coupled with its significant influence in brain metabolism, underscores its susceptibility to, and significance in, ischemia-reperfusion injury. Here, we evaluate proposed mechanisms of PDH-mediated neurodysfunction in stroke, including oxidative stress, altered regulatory enzymatic control, and loss of PDH activity. We also describe the neuroprotective influence of antioxidants, dichloroacetate, acetyl-L-carnitine, and combined therapy with ethanol and normobaric oxygen, explained in relation to PDH modulation. Our review highlights the significance of PDH impairment in stroke injury through an understanding of the mechanisms by which it is modulated, as well as an exploration of neuroprotective strategies available to limit its impairment.

Pathogenic mechanisms underlying X-linked Charcot-Marie-Tooth neuropathy (CMTX6) in patients with a pyruvate dehydrogenase kinase 3 mutation

Charcot-Marie-Tooth disease (CMT) is the most common inherited peripheral neuropathy. An X-linked form of CMT (CMTX6) is caused by a missense mutation (R158H) in the pyruvate dehydrogenase kinase isoenzyme 3 (PDK3) gene. PDK3 is one of 4 isoenzymes that negatively regulate the activity of the pyruvate dehydrogenase complex (PDC) by reversible phosphorylation of its first catalytic component pyruvate dehydrogenase (designated as E1). Mitochondrial PDC catalyses the oxidative decarboxylation of pyruvate to acetyl CoA and links glycolysis to the energy-producing Krebs cycle. We have previously shown the R158H mutation confers PDK3 enzyme hyperactivity. In this study we demonstrate that the increased PDK3 activity in patient fibroblasts (PDK3(R158H)) leads to the attenuation of PDC through hyper-phosphorylation of E1 at selected serine residues. This hyper-phosphorylation can be reversed by treating the PDK3(R158H) fibroblasts with the PDK inhibitor dichloroacetate (DCA). In the patient cells, down-regulation of PDC leads to increased lactate, decreased ATP and alteration of the mitochondrial network. Our findings highlight the potential to develop specific drug targeting of the mutant PDK3 as a therapeutic approach to treating CMTX6.

Understanding Muscle Dysfunction in Chronic Fatigue Syndrome

Introduction. Chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME) is a debilitating disorder of unknown aetiology, characterised by severe disabling fatigue in the absence of alternative diagnosis. Historically, there has been a tendency to draw psychological explanations for the origin of fatigue; however, this model is at odds with findings that fatigue and accompanying symptoms may be explained by central and peripheral pathophysiological mechanisms, including effects of the immune, oxidative, mitochondrial, and neuronal pathways. For example, patient descriptions of their fatigue regularly cite difficulty in maintaining muscle activity due to perceived lack of energy. This narrative review examined the literature for evidence of biochemical dysfunction in CFS/ME at the skeletal muscle level. Methods. Literature was examined following searches of PUB MED, MEDLINE, and Google Scholar, using key words such as CFS/ME, immune, autoimmune, mitochondria, muscle, and acidosis. Results. Studies show evidence for skeletal muscle biochemical abnormality in CFS/ME patients, particularly in relation to bioenergetic dysfunction. Discussion. Bioenergetic muscle dysfunction is evident in CFS/ME, with a tendency towards an overutilisation of the lactate dehydrogenase pathway following low-level exercise, in addition to slowed acid clearance after exercise. Potentially, these abnormalities may lead to the perception of severe fatigue in CFS/ME.

Pyruvate dehydrogenase complex deficiency and its relationship with epilepsy frequency--An overview

The pyruvate dehydrogenase complex (PDHc) is a member of a family of multienzyme complexes that provides the link between glycolysis and the tricarboxylic acid (TCA) cycle by catalyzing the physiologically irreversible decarboxylation of various 2-oxoacid substrates to their corresponding acyl-CoA derivatives, NADH and CO2. PDHc deficiency is a metabolic disorder commonly associated with lactic acidosis, progressive neurological and neuromuscular degeneration that vary with age and gender. In this review, we aim to discuss the relationship between occurrence of epilepsy and PDHc deficiency associated with the pyruvate dehydrogenase complex (E1α subunit (PDHA1) and E1β subunit (PDHB)) and PDH phosphatase (PDP) deficiency. PDHc plays a crucial role in the aerobic carbohydrate metabolism and regulates the use of carbohydrate as the source of oxidative energy. In severe PDHc deficiency, the energy deficit impairs brain development in utero resulting in physiological and structural changes in the brain that contributes to the subsequent onset of epileptogenesis. Epileptogenesis in PDHc deficiency is linked to energy failure and abnormal neurotransmitter metabolism that progressively alters neuronal excitability. This metabolic blockage might be restricted via inclusion of ketogenic diet that is broken up by β-oxidation and directly converting it to acetyl-CoA, and thereby improving the patient's health condition. Genetic counseling is essential as PDHA1 deficiency is X-linked. The demonstration of the X-chromosome localization of PDHA1 resolved a number of questions concerning the variable phenotype displayed by patients with E1 deficiency. Most patients show a broad range of neurological abnormalities, with the severity showing some dependence on the nature of the mutation in the Elα gene, while PDHB and PDH phosphatase (PDP) deficiencies are of autosomal recessive inheritance. However, in females, the disorder is further complicated by the pattern of X-chromosome inactivation, i.e., unfavorable lyonization. Furthermore research should focus on epileptogenic animal models; this might pave a new way toward identification of the pathophysiology of this challenging disorder.

Phenylbutyrate increases pyruvate dehydrogenase complex activity in cells harboring a variety of defects

Objective

Deficiency of pyruvate dehydrogenase complex (PDHC) is the most common genetic disorder leading to lactic acidosis. PDHC deficiency is genetically heterogenous and most patients have defects in the X-linked E1-α gene but defects in the other components of the complex encoded by PDHB, PDHX, DLAT, DLD genes or in the regulatory enzyme encoded by PDP1 have also been found. Phenylbutyrate enhances PDHC enzymatic activity in vitro and in vivo by increasing the proportion of unphosphorylated enzyme through inhibition of pyruvate dehydrogenase kinases and thus, has potential for therapy of patients with PDHC deficiency. In the present study, we investigated response to phenylbutyrate of multiple cell lines harboring all known gene defects resulting in PDHC deficiency.

Methods

Fibroblasts of patients with PDHC deficiency were studied for their enzyme activity at baseline and following phenylbutyrate incubation. Drug responses were correlated with genotypes and protein levels by Western blotting.

Results

Large deletions affecting PDHA1 that result in lack of detectable protein were unresponsive to phenylbutyrate, whereas increased PDHC activity was detected in most fibroblasts harboring PDHA1 missense mutations. Mutations affecting the R349-α residue were directed to proteasome degradation and were consistently unresponsive to short-time drug incubation but longer incubation resulted in increased levels of enzyme activity and protein that may be due to an additional effect of phenylbutyrate as a molecular chaperone.

Interpretation

PDHC enzyme activity was enhanced by phenylbutyrate in cells harboring missense mutations in PDHB, PDHX, DLAT, DLD, and PDP1 genes. In the prospect of a clinical trial, the results of this study may allow prediction of in vivo response in patients with PDHC deficiency harboring a wide spectrum of molecular defects.

Disorders of mitochondrial function

PURPOSE OF REVIEW

Mitochondrial diseases are a major category of childhood illness that produce a wide variety of symptoms and multisystemic disorders. This review highlights recent clinically important developments in diagnostic evaluation and treatment of mitochondrial diseases.

RECENT FINDINGS

Major advances have been made in understanding the genetic bases of mitochondrial diseases. Molecular defects have recently been reported in mitochondrial DNA maintenance, RNA translation and protein import and in mitochondrial fusion and fission, opening new areas of cell disorder. Diagnostic testing is struggling to keep pace with these fundamental discoveries. The diagnostic approach to children suspected of mitochondrial disease is rapidly evolving but few patients have a molecular diagnosis. A better notion of the prognosis of affected children is emerging from studies of long-term outcome. Some therapeutic successes are reported, such as in coenzyme Q deficiency conditions.

SUMMARY

Mitochondrial diseases can present with signs in almost any organ. Well planned clinical evaluation is the key to successful diagnostic work-up of mitochondrial diseases. An approach is presented for further testing in specialized laboratories. Mitochondrial diseases can be caused by mutations in mitochondrial DNA or, more commonly in children, in nuclear genes. Mitochondrial DNA mutations pose special challenges for genetic counseling and prenatal diagnosis. Supportive treatment and avoidance of environmental stresses are important aspects of patient care. Specific treatment of mitochondrial diseases is in its infancy and is a major challenge for pediatric medicine.

Two silent substitutions in the PDHA1 gene cause exon 5 skipping by disruption of a putative exonic splicing enhancer

BACKGROUND

Synonymous mutations within exons may cause aberrant splicing by disrupting exonic splicing enhancer (ESE) motifs in the vicinity of non consensus splice sites. Mutational analysis of PDHA1 revealed only one silent single nucleotide substitution in exon 5 in two unrelated boys and a girl (c.483C>T and c.498C>T variants, respectively). For both patients, pyruvate dehydrogenase complex activity was low and the immunoreactive E1alpha protein was defective in cultured fibroblasts.

METHODS AND RESULTS

One of the boys was a somatic mosaic for the c.483C>T variant, as shown by the variable ratio of mutant to normal alleles in fibroblast, lymphocyte and single hair root DNA. Transcript analysis in fibroblasts from the three patients revealed the presence of both normal and truncated cDNAs, with the splicing out of exon 5 predicted to result in a frame shift and premature termination (p.Arg141AlafsX11). The treatment of fibroblasts with emetine before harvesting to prevent nonsense mRNA-mediated decay increased the amount of mutant mRNA. In silico analysis revealed that each variant disrupted a putative SRp55 binding site and that the intron 5 donor splice site (5'ss) contained a weak splicing signal. Transient transfection of COS-7 or Hela cells with hybrid minigene constructs containing wild-type or mutant PDHA1 exon 5, followed by RT-PCR demonstrated that each variant resulted in the incomplete inclusion of PDHA1 exon 5, and that this defect was corrected following the restoration of a perfect consensus sequence for the 5' splice site by site-directed mutagenesis.

CONCLUSION

These two synonymous mutations expand the spectrum of rare PDHA1 splicing mutations, all of which are located in non canonical splice sites.

Pyruvate dehydrogenase complex deficiency caused by ubiquitination and proteasome-mediated degradation of the E1 subunit

Congenital deficiencies of the human pyruvate dehydrogenase (PDH) complex are considered to be due to loss of function mutations in one of the component enzymes. Here we describe a case of PDH deficiency associated with the PDH E1beta subunit (PDHB) gene. The clinical phenotype of the patient was consistent with reported cases of PDH deficiency. Cultured skin fibroblasts demonstrated a 55% reduction in PDH activity and markedly decreased immunoreactivity for PDHB protein, compared with healthy controls. Surprisingly, nucleotide sequence analyses of cDNAs corresponding to the patient PDH E1alpha (PDHA1) and PDHB genes revealed no pathological mutations. Moreover, the relative expression level of PDHB mRNA and the rates of transcription and translation of the PDHB gene were normal. However, PDC activity could be restored in cells from this patient following treatment with MG132, a specific proteasome inhibitor, and normal levels of E1beta could be detected in MG132-treated cells. Similar results were obtained following treatment with Tyr-phostin 23 (Tyr23), a specific inhibitor of epidermal growth factor receptor-protein-tyrosine kinase (EGFR-PTK), which also restored E1beta protein levels to those in cells from healthy subjects or from patients with PDHA1 deficiency. The index patient's cells contained a high basal level of EGFR-PTK activity that correlated with the high level of ubiquitination of cellular proteins, although the total EGFR protein levels were similar to those in cells from Elalpha-deficient subjects and healthy subjects. These data indicate that PDH deficiency in our patient involves a post-translational modification in which EGFR-PTK-mediated tyrosine phosphorylation of the E1beta protein leads to enhanced ubiquitination followed by proteasome-mediated degradation. They also provide a novel mechanism accounting for congenital deficiency of the PDH complex and perhaps other inborn errors of metabolism.

Long-term outcome and clinical spectrum of 73 pediatric patients with mitochondrial diseases

OBJECTIVES

We sought to determine the clinical spectrum, survival, and long-term functional outcome of a cohort of pediatric patients with mitochondrial diseases and to identify prognostic factors.

METHODS

Medical charts were reviewed for 73 children diagnosed between 1985 and 2005. The functional status of living patients was assessed prospectively by using the standardized Functional Independence Measure scales.

RESULTS

Patients fell into 7 phenotypic categories: neonatal-onset lactic acidosis (10%), Leigh syndrome (18%), nonspecific encephalopathy (32%), mitochondrial (encephalo)myopathy (19%), intermittent neurologic (5%), visceral (11%), and Leber hereditary optic neuropathy (5%). Age at first symptoms ranged from prenatal to 16 years (median: 7 months). Neurologic symptoms were the most common (90%). Visceral involvement was observed in 29% of the patients. A biochemical or molecular diagnosis was identified for 81% of the patients as follows: deficiency of complex IV (27%), of pyruvate dehydrogenase or complex I (25% each), of multiple complexes (13%), and of pyruvate carboxylase (5%) or complexes II+III (5%). A mitochondrial DNA mutation was found in 20% of patients. At present, 46% of patients have died (median age: 13 months), 80% of whom were <3 years of age. Multivariate analysis showed that age at first symptoms was a major independent predictor of mortality: patients with first symptoms before 6 months had a highly increased risk of mortality. Cardiac or visceral involvement and neurologic crises were not independent prognostic factors. Living patients showed a wide range of independence levels that correlated positively with age at first symptoms. Among patients aged >5 years (n = 32), 62% had Functional Independence Measure quotients of >0.75.

CONCLUSIONS

Mitochondrial diseases in children span a wide range of symptoms and severities. Age at first symptoms is the strongest predictor mortality. Despite a high mortality rate in the cohort, 62% of patients aged >5 years have only mild impairment or normal functional outcome.

Therapeutic potential of dichloroacetate for pyruvate dehydrogenase complex deficiency

We reviewed the use of oral dichloroacetate (DCA) in the treatment of children with congenital lactic acidosis caused by mutations in the pyruvate dehydrogenase complex (PDC). The case histories of 46 subjects were analyzed with regard to diagnosis, clinical presentation and response to DCA. DCA decreased blood and cerebrospinal fluid lactate concentrations, and was generally well tolerated. DCA may be particularly effective in children with PDC deficiency by stimulating residual enzyme activity and, consequently, cellular energy metabolism. A controlled trial is needed to determine the definitive role of DCA in the management of this devastating disease.

Controlled clinical trial of dichloroacetate for treatment of congenital lactic acidosis in children

OBJECTIVE

Open-label studies indicate that oral dichloroacetate (DCA) may be effective in treating patients with congenital lactic acidosis. We tested this hypothesis by conducting the first double-blind, randomized, control trial of DCA in this disease.

METHODS

Forty-three patients who ranged in age from 0.9 to 19 years were enrolled. All patients had persistent or intermittent hyperlactatemia, and most had severe psychomotor delay. Eleven patients had pyruvate dehydrogenase deficiency, 25 patients had 1 or more defects in enzymes of the respiratory chain, and 7 patients had a mutation in mitochondrial DNA. Patients were preconditioned on placebo for 6 months and then were randomly assigned to receive an additional 6 months of placebo or DCA, at a dose of 12.5 mg/kg every 12 hours. The primary outcome results were (1) a Global Assessment of Treatment Efficacy, which incorporated tests of neuromuscular and behavioral function and quality of life; (2) linear growth; (3) blood lactate concentration in the fasted state and after a carbohydrate meal; (4) frequency and severity of intercurrent illnesses and hospitalizations; and (5) safety, including tests of liver and peripheral nerve function.

OUTCOME

There were no significant differences in Global Assessment of Treatment Efficacy scores, linear growth, or the frequency or severity of intercurrent illnesses. DCA significantly decreased the rise in blood lactate caused by carbohydrate feeding. Chronic DCA administration was associated with a fall in plasma clearance of the drug and with a rise in the urinary excretion of the tyrosine catabolite maleylacetone and the heme precursor delta-aminolevulinate.

CONCLUSIONS

In this highly heterogeneous population of children with congenital lactic acidosis, oral DCA for 6 months was well tolerated and blunted the postprandial increase in circulating lactate. However, it did not improve neurologic or other measures of clinical outcome.

Clinical and genetic spectrum of pyruvate dehydrogenase deficiency: Dihydrolipoamide acetyltransferase (E2) deficiency

Pyruvate dehydrogenase deficiency is a major cause of primary lactic acidosis and neurological dysfunction in infancy and early childhood. Most cases are caused by mutations in the X-linked gene for the E1alpha subunit of the complex. Mutations in DLAT, the gene encoding dihydrolipoamide acetyltransferase, the E2 core component of the complex, have not been described previously. We report two unrelated patients with pyruvate dehydrogenase deficiency caused by defects in the E2 subunit. Both patients are less severely affected than typical patients with E1alpha mutations and both have survived well into childhood. Episodic dystonia was the major neurological manifestation, with other more common features of pyruvate dehydrogenase deficiency, such as hypotonia and ataxia, being less prominent. The patients had neuroradiological evidence of discrete lesions restricted to the globus pallidus, and both are homozygous for different mutations in the DLAT gene. The clinical presentation and neuroradiological findings are not typical of pyruvate dehydrogenase deficiency and extend the clinical and mutational spectrum of this condition.

Pyruvate dehydrogenase complex: metabolic link to ischemic brain injury and target of oxidative stress

The mammalian pyruvate dehydrogenase complex (PDHC) is a mitochondrial matrix enzyme complex (greater than 7 million Daltons) that catalyzes the oxidative decarboxylation of pyruvate to form acetyl CoA, nicotinamide adenine dinucleotide (the reduced form, NADH), and CO(2). This reaction constitutes the bridge between anaerobic and aerobic cerebral energy metabolism. PDHC enzyme activity and immunoreactivity are lost in selectively vulnerable neurons after cerebral ischemia and reperfusion. Evidence from experiments carried out in vitro suggests that reperfusion-dependent loss of activity is caused by oxidative protein modifications. Impaired enzyme activity may explain the reduced cerebral glucose and oxygen consumption that occurs after cerebral ischemia. This hypothesis is supported by the hyperoxidation of mitochondrial electron transport chain components and NAD(H) that occurs during reperfusion, indicating that NADH production, rather than utilization, is rate limiting. Additional support comes from the findings that immediate postischemic administration of acetyl-L-carnitine both reduces brain lactate/pyruvate ratios and improves neurologic outcome after cardiac arrest in animals. As acetyl-L-carnitine is converted to acetyl CoA, the product of the PDHC reaction, it follows that impaired production of NADH is due to reduced activity of either PDHC or one or more steps in glycolysis. Impaired cerebral energy metabolism and PDHC activity are associated also with neurodegenerative disorders including Alzheimer's disease and Wernicke-Korsakoff syndrome, suggesting that this enzyme is an important link in the pathophysiology of both acute brain injury and chronic neurodegeneration.