Dihydrolipoyl dehydrogenase deficiency: a therapeutic trial with branched-chain amino acid restriction

Lipoamide dehydrogenase (LADH) deficiency: medical perspectives of the structural and functional characterization of LADH and its pathogenic variants

(Dihydro)lipoamide dehydrogenase (LADH) deficiency is an autosomal recessive genetic metabolic disorder. It generally presents with an onset in the neonatal age and premature death. The clinical picture usually involves metabolic decompensation and lactic acidosis that lead to neurological, cardiological, and/or hepatological outcomes. Severity of the disease is due to the fact that LADH is a common E3 subunit to the pyruvate, alpha-ketoglutarate, alpha-ketoadipate, and branched-chain alpha-keto acid dehydrogenase complexes and is also part of the glycine cleavage system; hence, a loss in LADH activity adversely affects several central metabolic pathways simultaneously. The severe clinical manifestations, however, often do not parallel the LADH activity loss, which implies the existence of auxiliary pathological pathways; stimulated reactive oxygen species (ROS) production as well as dissociation from the relevant multienzyme complexes proved to be auxiliary exacerbating pathomechanisms for selected disease-causing LADH mutations. This review provides an overview on the therapeutic challenges of inherited metabolic diseases, structural and functional characteristics of the mitochondrial alpha-keto acid dehydrogenase complexes, molecular pathogenesis and structural basis of LADH deficiency, and relevant potential future medical perspectives.

The Effects of a Ketogenic Diet on Patients with Dihydrolipoamide Dehydrogenase Deficiency

Background: Dihydrolipoamide dehydrogenase (DLD lipoamide dehydrogenase, the E3 subunit of the pyruvate dehydrogenase complex (PDHC)) is the third catalytic enzyme of the PDHC, which converts pyruvate to acetyl-CoA catalyzed with the introduction of acetyl-CoA to the tricyclic acid (TCA) cycle. In humans, PDHC plays an important role in maintaining glycose homeostasis in an aerobic, energy-generating process. Inherited DLD-E3 deficiency, caused by the pathogenic variants in DLD, leads to variable presentations and courses of illness, ranging from myopathy, recurrent episodes of liver disease and vomiting, to Leigh disease and early death. Currently, there is no consensus on treatment guidelines, although one suggested solution is a ketogenic diet (KD). Objective: To describe the use and effects of KD in patients with DLD-E3 deficiency, compared to the standard treatment. Results: Sixteen patients were included. Of these, eight were from a historical cohort, and of the other eight, four were on a partial KD. All patients were homozygous for the D479V (or D444V, which corresponds to the mutated mature protein without the mitochondrial targeting sequence) pathogenic variant in DLD. The treatment with partial KD was found to improve patient survival. However, compared to a historical cohort, the patients’ quality of life (QOL) was not significantly improved. Conclusions: The use of KD offers an advantage regarding survival; however, there is no significant improvement in QOL.

An Updated View on the Molecular Pathomechanisms of Human Dihydrolipoamide Dehydrogenase Deficiency in Light of Novel Crystallographic Evidence

Dihydrolipoamide dehydrogenase (LADH, E3) deficiency is a rare (autosomal, recessive) genetic disorder generally presenting with an onset in the neonatal age and early death; the highest carrier rate has been found among Ashkenazi Jews. Acute clinical episodes usually involve severe metabolic decompensation and lactate acidosis that result in neurological, cardiological, and/or hepatological manifestations. Clinical severity is due to the fact that LADH is a common E3 subunit to the alpha-ketoglutarate, pyruvate, alpha-ketoadipate, and branched-chain alpha-keto acid dehydrogenase complexes, and is also a constituent in the glycine cleavage system, thus a loss in LADH function adversely affects multiple key metabolic routes. However, the severe clinical pictures frequently still do not parallel the LADH activity loss, which implies the involvement of auxiliary biochemical mechanisms; enhanced reactive oxygen species generation as well as affinity loss for multienzyme complexes proved to be key auxiliary exacerbating pathomechanisms. This review provides an overview and an up-to-date molecular insight into the pathomechanisms of this disease in light of the structural conclusions drawn from the first crystal structure of a disease-causing hE3 variant determined recently in our laboratory.

Human dihydrolipoamide dehydrogenase (E3) deficiency: Novel insights into the structural basis and molecular pathomechanism

This review summarizes our present view on the molecular pathogenesis of human (h) E3-deficiency caused by a variety of genetic alterations with a special emphasis on the moonlighting biochemical phenomena related to the affected (dihydro)lipoamide dehydrogenase (LADH, E3, gene: dld), in particular the generation of reactive oxygen species (ROS). E3-deficiency is a rare autosomal recessive genetic disorder frequently presenting with a neonatal onset and premature death; the highest carrier rate of a single pathogenic dld mutation (1:94-1:110) was found among Ashkenazi Jews. Patients usually die during acute episodes that generally involve severe metabolic decompensation and lactic acidosis leading to neurological, cardiological, and/or hepatological manifestations. The disease owes its severity to the fact that LADH is the common E3 subunit of the alpha-ketoglutarate (KGDHc), pyruvate (PDHc), and branched-chain α-keto acid dehydrogenase complexes and is also part of the glycine cleavage system, hence the malfunctioning of LADH simultaneously incapacitates several central metabolic pathways. Nevertheless, the clinical pictures are usually not unequivocally portrayed through the loss of LADH activities and imply auxiliary mechanisms that exacerbate the symptoms and outcomes of this disorder. Enhanced ROS generation by disease-causing hE3 variants as well as by the E1-E2 subcomplex of the hKGDHc likely contributes to selected pathogeneses of E3-deficiency, which could be targeted by specific drugs or antioxidants; lipoic acid was demonstrated to be a potent inhibitor of ROS generation by hE3 in vitro. Flavin supplementation might prove to be beneficial for those mutations triggering FAD loss in the hE3 component. Selected pathogenic hE3 variants lose their affinity for the E2 component of the hPDHc, a mechanism which warrants scrutiny also for other E3-haboring complexes.

Conformational Change Near the Redox Center of Dihydrolipoamide Dehydrogenase Induced by NAD(+) to Regulate the Enzyme Activity

Dihydrolipoamide dehydrogenase (LipDH) transfers two electrons from dihydrolipoamide (DHL) to NAD(+) mediated by FAD. Since this reaction is the final step of a series of catalytic reaction of pyruvate dehydrogenase multi-enzyme complex (PDC), LipDH is a key enzyme to maintain the fluent metabolic flow. We reported here the conformational change near the redox center of LipDH induced by NAD(+) promoting the access of the DHL to FAD. The increase in the affinity of DHL to redox center was evidenced by the decrease in K M responding to the increase in the concentration of NAD(+) in Lineweaver-Burk plots. The fluorescence intensity of FAD transiently reduced by the addition of DHL was not recovered but rather reduced by the binding of NAD(+) with LipDH. The fluorescence decay lifetimes of FAD and Trp were prolonged in the presence of NAD(+) to show that FAD would be free from the electron transfer from the neighboring Tyrs and the resonance energy transfer efficiency between Trp and FAD lowered. These results consistently reveal that the conformation near the FAD and the surroundings would be so rearranged by NAD(+) to allow the easier access of DHL to the redox center of LipDH.

Stimulation of reactive oxygen species generation by disease-causing mutations of lipoamide dehydrogenase

We investigated pathogenic mutations relevant in dihydrolipoamide dehydrogenase (LADH; gene: Dld) deficiency, a severe human disease, to elucidate how they alter reactive oxygen species (ROS) generation and associated biophysical characteristics of LADH. Twelve known disease-causing mutants of human LADH have been expressed and purified to homogeneity from E. coli. Detailed biophysical and biochemical characterization of the mutants has been performed applying circular dichroism (CD) spectroscopy, nano-spray mass spectrometry (MS), calibrated gel filtration and flavin adenine dinucleotide-content analysis. Functional analyses revealed that four of the pathogenic mutations significantly stimulated the ROS-generating activity of LADH and also increased its sensitivity to an acidic shift in pH. LADH activity was reduced by variable extents in the mutants exhibiting excessive ROS generation. It is remarkable that in the P453L mutant, enzyme activity was nearly completely lost with a ROS-forming activity becoming dominant, whereas the G194C mutation, common among Ashkenazi Jews, resulted in no alteration in LADH activity but a gain in the ROS-generating activity. There have been neither major conformational alterations nor monomerization of the functional homodimer of LADH associated with the higher ROS-generating capacity as measured by CD spectroscopy and size-exclusion chromatography combined with nano-spray MS, respectively. The excessive ROS generation of selected LADH mutants could be an important factor in the pathology and clinical presentation of human LADH deficiency and raises the possibility of an antioxidant therapy in the treatment of this condition.

Animal models of maple syrup urine disease

Maple syrup urine disease (MSUD) is an inherited aminoacidopathy resulting from dysfunction of the branched-chain keto acid dehydrogenase (BCKDH) complex. This disease is currently treated primarily by dietary restriction of branched-chain amino acids (BCAAs). However, dietary compliance is often challenging. Conversely, liver transplantation significantly improves outcomes, but donor organs are scarce and there are high costs and potential risks associated with this invasive procedure. Therefore, improved treatment options for MSUD are needed. Development of novel treatments could be facilitated by animal models that accurately mimic the human disease. Animal models provide a useful system in which to explore disease mechanisms and new preclinical therapies. Here we review MSUD and currently available animal models and their corresponding relevance to the human disorder. Using animal models to gain a more complete understanding of the pathophysiology behind the human disease may lead to new or improved therapies to treat or potentially cure the disorder.

Relationship of causative genetic mutations in maple syrup urine disease with their clinical expression

Maple syrup urine disease [MSUD] is a rare inborn error of metabolism inherited as an autosomal recessive trait through mutations in any of three different genes that encode components of the branched chain alpha-ketoacid dehydrogenase [BCKD] complex. In this work, the genotype of affected individuals was correlated with their clinical histories. These individuals were diagnosed and followed in a single centralized clinic, and their molecular genetic characterization was done by one laboratory. Three individuals had mutant alleles in the gene for the E1alpha component, five had mutations in the gene for E1beta, and three had mutations in the gene for E2. The results emphasize the diversity of the molecular and clinical presentations for individuals with MSUD and support the complexity of diseases termed "single gene traits." Of primary importance is early identification of at risk infants through newborn screening programs to minimize many of the complications associated with this protein intolerance. Attention to abnormal neurological signs in the neonate or evidence of neurological decompensation in older infants and children by a centralized medical management team minimizes permanent brain damage and improves survival.

Deficiency of dihydrolipoamide dehydrogenase due to two mutant alleles (E340K and G101del). Analysis of a family and prenatal testing

A male child with metabolic acidosis was diagnosed as having dihydrolipoamide dehydrogenase (E3) deficiency. E3 activity of the proband's cultured fibroblasts and blood lymphocytes was 3-9% of normal, while in the parent's lymphocytes it was about 60% of normal. The proband's pyruvate dehydrogenase complex (PDC) and the alpha-ketoglutarate dehydrogenase complex activities from cultured skin fibroblasts were 12% and 6% of normal, respectively. PDC activity in the parents cultured fibroblasts was 25-31% of normal. Western and Northern blot analyses showed similar quantities of E3 protein and mRNA in cultured fibroblasts from the proband and his parents. DNA sequencing of cloned full-length E3 cDNAs, from the proband and the parents, showed two mutations on different alleles of proband were inherited from the parents. One mutation is a three nucleotide (AGG) deletion, from the mother, resulting in deletion of Gly101 in the FAD binding domain. The other mutation is a nucleotide substitution (G to A), from the father, leading to substitution of Lys for Glu340 in the central domain. The same deletion mutation was found in E3 cDNA from a chorionic villus sample and cultured fibroblasts obtained from the mother's subsequent offspring. This finding illustrates the possibility of successful prenatal diagnosis of E3 deficiency utilizing mutations characterized prior to initiation of pregnancy.

Leigh disease with deficiency of lipoamide dehydrogenase: treatment failure with dichloroacetate

A 6-month-old female infant with hypotonia and keto and lactic acidosis was diagnosed with lipoamide dehydrogenase (E3) deficiency. This enzyme is a component of the pyruvate, alpha-ketoglutarate, and branched chain alpha-ketoacid dehydrogenase complexes. At the time of diagnosis her plasma contained elevated branched chain amino acids, alanine, alloisoleucine, ketones, pyruvate, and lactate, and her urine contained elevated branched chain ketoacids and lactate. By neuroimaging she was found to have Leigh subacute necrotizing encephalomyelopathy. Modest branched-chain amino acid restriction led to the disappearance of alloisoleucine and normalization of her branched chain amino acid values, while institution of a high fat diet precipitated hypoglycemia and acidosis. A trial of lipoic acid led to a transient modest improvement in her lactic acidemia. Use of dichloroacetate to activate the pyruvate dehydrogenase complex led to a significant decline in lactate levels, but this was also transient. The patient had significant growth failure despite a high carbohydrate, high calorie diet, yet remained clinically well until 28 months of age when she developed acute acidosis and brainstem dysfunction and died.

Congenital lacticacidemia caused by lipoamide dehydrogenase deficiency with favorable outcome

A 5-year-old boy had recurrent vomiting and lethargy with lacticacidemia and ketoacidemia since birth. Lipoamide dehydrogenase deficiency was found in muscle and fibroblasts. Therapy with sodium dichloroacetate, thiamine, and carnitine was associated with reduction of the severity and frequency of the decompensation episodes and near normal plasma lactate levels. At 5 years of age, the patient has normal cognitive function and moderate motor impairment.

Identification of two missense mutations in a dihydrolipoamide dehydrogenase-deficient patient

The molecular basis of dihydrolipoamide dehydrogenase (E3; dihydrolipoamide:NAD+ oxidoreductase, EC 1.8.1.4) deficiency in an E3-deficient patient was studied. Fibroblasts cultured from the patient contained only approximately 6% of the E3 activity of cells from a normal subject. Western and Northern blot analyses indicated that, compared to control cells, the patient's cells had a reduced amount of protein but normal amounts of E3 mRNA. Direct sequencing of E3 cDNA derived from the patient's RNA as well as each of the subclones of the cDNA revealed that the patient had two substitution mutations in the E3 coding region. One mutation changed a single nucleotide from A to G, resulting in substitution of Glu (GAA) for Lys-37 (AAA). The other point mutation was a nucleotide change from C to T, resulting in the substitution of Leu (CTG) for Pro-453 (CCG). These mutations appear to be significant in that they alter the active site and possibly the binding of FAD.

Investigation of enzyme defects in children with lactic acidosis

Screening for enzyme deficiencies was carried out in cultured skin fibroblasts and leukocytes of 19 patients with lactic acidosis and neurological problems. Pyruvate carboxylase deficiency was demonstrated in three cases. Reduced pyruvate oxidation was found in seven cultures; six showed no significant stimulation of the oxidation rate by methylene blue and in three a decreased pyruvate dehydrogenase complex activity was confirmed. Methylene blue restored a near normal oxidation rate in the seventh culture which had decreased cytochrome c oxidase activity.

Recurrent, familial Reye-like syndrome with a new complex amino and organic aciduria

Five of 13 siblings from a Jewish-Ashkenazi family suffered from recurrent Reye-like episodes. During attacks, these patients excreted alpha-keto-adipic, alpha-hydroxy-adipic, and alpha-aminoadipic acids, branched-chain keto acids and saccharopine in addition to lactic, pyruvic, and dicarboxylic acids characteristic of Reye syndrome. The serum concentrations of citrulline and alpha-amino-adipic acid were elevated and carnitine was at the upper limit of the normal range. Serum acetoacetate level was 4-5 times the beta-hydroxybutyrate level, but the pyruvate/lactate ratio was normal. Notably, plasma ketone bodies were lower than expected from the degree of catabolism. When the patients were symptom-free, no abnormal amino or organic acids in serum or urine were detected. These findings might be interpreted as a functional impairment at three different biochemical sites: fatty acid beta-oxidation, dehydrogenase complexes of the pyruvic, alpha-ketoglutaric, alpha-ketoadipic, and branched-chain keto acids, and pyruvate carboxylase. We suggest that in this hereditary disorder a toxic substance, exogenously or endogenously derived, interfered at multiple sites in different metabolic pathways.