Maple syrup urine disease in treated patients: biochemical and oxidative stress profiles

Treatment of maple syrup urine disease: Benefits, risks, and challenges of liver transplantation

Maple syrup urine disease (MSUD) is caused by a deficiency in the activity of the branched-chain α-ketoacid dehydrogenase (BCKD) complex, promoting the accumulation of the branched-chain amino acids (BCAA) leucine, isoleucine, and valine, as well as their respective α-keto acids. MSUD is an autosomal recessive hereditary metabolic disorder characterized by ketoacidosis, ataxia, coma, and mental and psychomotor retardation. The mechanisms involved in the brain damage caused by MSUD are not fully understood. Early diagnosis and treatment, as well as proper control of metabolic decompensation crises, are crucial for patients' survival and for a better prognosis. The recommended treatment consists of a high-calorie diet with restricted protein intake and specific formulas containing essential amino acids, except those accumulated in MSUD. This treatment will be maintained throughout life, being adjusted according to the patients' nutritional needs and BCAA concentration. Because dietary treatment may not be sufficient to prevent neurological damage in MSUD patients, other therapeutic strategies have been studied, including liver transplantation. With transplantation, it is possible to obtain an increase of about 10% of the normal BCKD in the body, an amount sufficient to maintain amino acid homeostasis and reduce metabolic decompensation crises. However, the experience related to this practice is very limited when considering the shortage of liver for transplantation and the risks related to the surgical procedure and immunosuppression. Thus, the purpose of this review is to survey the benefits, risks, and challenges of liver transplantation in the treatment of MSUD.

Effective immobilization of lactate dehydrogenase onto mesoporous silica

This study presents that covalent immobilization technique has been utilized for the immobilization of l-lactate dehydrogenase (l-LDH) from porcine on mesoporous silica. To develop mesoporous silica as support material for use in l-LDH immobilization, the particle surfaces were functionalized with 3-aminopropyltrimethoxysilane and further conjugated with glutaraldehyde. The effect of some parameters such as glutaraldehyde concentration, immobilization pH, initial enzyme concentration, and immobilization time was investigated and the optimum conditions for these parameters were determined as 1% (w/v), pH 8.0, 1 mg/ml, and 120 min, respectively. The maximum working pH and temperature for the oxidation of lactate to pyruvate reaction were determined as 10.0 and 35°C for free and 9.0 and 40°C for immobilized l-LDH, respectively. The kinetic parameters (K_m and V_max ) of l-LDH for the oxidation of lactate to pyruvate reaction were examined as 1.02 mM and 7.58 U/mg protein for free and 0.635 mM and 1.7 U/mg protein for immobilized l-LDH, respectively. Moreover, the immobilized l-LDH was 1.3-fold more stable than free l-LDH at 25°C according to calculated t_1/2 values. The immobilized l-LDH retained 80% of its initial activity in a batch reactor after 14 reuses.

Maple syrup urine disease in Brazilian patients: variants and clinical phenotype heterogeneity

Background

Maple syrup urine disease (MSUD) is an autosomal recessive inherited metabolic disease caused by deficient activity of the branched-chain α-keto acid dehydrogenase (BCKD) enzymatic complex. BCKD is a mitochondrial complex encoded by BCKDHA, BCKDHB, DBT, and DLD genes. MSUD is predominantly caused by Variants in BCKDHA, BCKDHB, and DBT genes encoding the E1α, E1β, and E2 subunits of BCKD complex, respectively. The aim of this study was to characterize the genetic basis of MSUD by identifying the point variants in BCKDHA, BCKDHB, and DBT genes in a cohort of Brazilian MSUD patients and to describe their phenotypic heterogeneity. It is a descriptive cross-sectional study with 21 MSUD patients involving molecular genotyping by Sanger sequencing.

Results

Eight new variants predicted as pathogenic were found between 30 variants (damaging and non-damaging) identified in the 21 patients analyzed: one in the BCKDHA gene (p.Tyr120Ter); five in the BCKDHB gene (p.Gly131Val, p.Glu146Glnfs * 13, p.Phe149Cysfs * 9, p.Cys207Phe, and p.Lys211Asn); and two in the DBT gene (p.Glu148Ter and p.Glu417Val). Seventeen pathogenic variants were previously described and five variants showed no pathogenicity according to in silico analysis.

Conclusion

Given that most of the patients received late diagnoses, the study results do not allow us to state that the molecular features of MSUD variant phenotypes are predictive of clinical severity.

Brain Branched-Chain Amino Acids in Maple Syrup Urine Disease: Implications for Neurological Disorders

Maple syrup urine disease (MSUD) is an autosomal recessive disorder caused by decreased activity of the branched-chain α-ketoacid dehydrogenase complex (BCKDC), which catalyzes the irreversible catabolism of branched-chain amino acids (BCAAs). Current management of this BCAA dyshomeostasis consists of dietary restriction of BCAAs and liver transplantation, which aims to partially restore functional BCKDC activity in the periphery. These treatments improve the circulating levels of BCAAs and significantly increase survival rates in MSUD patients. However, significant cognitive and psychiatric morbidities remain. Specifically, patients are at a higher lifetime risk for cognitive impairments, mood and anxiety disorders (depression, anxiety, and panic disorder), and attention deficit disorder. Recent literature suggests that the neurological sequelae may be due to the brain-specific roles of BCAAs. This review will focus on the derangements of BCAAs observed in the brain of MSUD patients and will explore the potential mechanisms driving neurologic dysfunction. Finally, we will discuss recent evidence that implicates the relevance of BCAA metabolism in other neurological disorders. An understanding of the role of BCAAs in the central nervous system may facilitate future identification of novel therapeutic approaches in MSUD and a broad range of neurological disorders.

Melatonin ameliorates oxidative stress and DNA damage of rats subjected to a chemically induced chronic model of Maple Syrup Urine Disease

Maple Syrup Urine Disease (MSUD) is an inborn error of metabolism caused by a deficiency of branched α-ketoacid dehydrogenase complex (BCKDC) activity. Branched-chain amino acids (BCAA) accumulation is, at least in part, responsible for neurological disturbances characteristic of this metabolic disorder. Experimental studies demonstrated that high levels of BCAA induce brain oxidative stress. Considering that many antioxidants are obtained from the diet, the dietary restriction in MSUD patients probably produce deficiency of vitamins and micronutrients involved in antioxidant defenses. Supplementation with synthetic melatonin has been used to prevention and treatment of pathological conditions, including brain diseases. In this study, we aimed at investigating the potential neuroprotective effect of melatonin treatment in a MSUD experimental model. Infant rats (7 day old) received twice daily subcutaneous injections of a BCAA pool (0.21472 g/kg, 190 mmol/L leucine, 59 mmol/L isoleucine and 69 mmol/L valine in saline solution (15.8 µL/g per weight/injection) or saline alone, and supplemented with melatonin (10 mg/kg, intraperitoneal) for 21 days. Oxidative stress parameters, i.e. antioxidant enzyme activity, reactive species production and damage to lipids and proteins, were assessed in the cerebral cortex, hippocampus and striatum at twenty-eight days of age. In addition, the damage to blood cell DNA was evaluated. The chronic administration of BCAA pool in infant rats induced significant oxidative stress (p < 0.05) - such as oxidation of lipids and proteins, imbalance in antioxidant enzymes activities - damages in DNA (p < 0.05) and in brain structures (cerebral cortex, hippocampus and striatum). Notably, melatonin supplementation was able to ameliorate the oxidative (p < 0.05) and antioxidant (p < 0.05) parameters in the brain and blood of the rat model of MSUD. Our results show that melatonin could be a promising therapeutic agent for MSUD.

Investigation of L - Carnitine Concentrations in Treated Patients with Maple Syrup Urine Disease

Maple syrup urine disease (MSUD), also known as branched-chain α ketoaciduria, is a metabolic disorder caused by an inborn deficiency in the activity of the branched-chain α-ketoacid dehydrogenase complex. Severe neurological damage occurs in most patients with MSUD although the exact mechanism of neurotoxicity still remains unknown. Studies have suggested that neuropathology in patients with MSUD may be related to oxidative stress. L - carnitine mediates the transport of fatty acids into the mitochondria that are required for β-oxidation and ATP production. Along with the important roles it plays in lipid metabolism, L-carnitine also protects tissues from oxidative damage through its antioxidant properties. The study included a total of 15 patients with MSUD who attended regular follow-up visits, and 15 age-matched healthy control subjects, and aimed to investigate L - carnitine levels in treated patients with MSUD and healthy control subjects. L - carnitine levels were found to be significantly lower in the patient group than in the healthy controls. No significant correlation was identified between the plasma branched-chain amino acids leucine, isoleucine, valine, and L - carnitine levels. Patients with MSUD can be treated with adjuvant therapy with L - carnitine supplementation.

DNA damage induced by alloisoleucine and other metabolites in maple syrup urine disease and protective effect of l-carnitine

Maple syrup urine disease (MSUD) is an inherited deficiency of the branched-chain α-keto dehydrogenase complex, characterized by accumulation of the branched-chain amino acids (BCAAs) and their respective branched chain α-keto-acids (BCKAs), as well as by the presence of alloisoleucine (Allo). Studies have shown that oxidative stress is involved in the pathophysiology of MSUD. In this work, we investigated using the comet assay whether Allo, BCAAs and BCKAs could induce in vitro DNA damage, as well as the influence of l-Carnitine (L-Car) upon DNA damage. We also evaluated urinary 8-hydroxydeoguanosine (8-OHdG) levels, an oxidative DNA damage biomarker, in MSUD patients submitted to a restricted diet supplemented or not with L-Car. All tested concentrations of metabolites (separated or incubated together) induced in vitro DNA damage, and the co-treatment with L-Car reduced these effects. We found that Allo induced the higher DNA damage class and verified a potentiation of DNA damage induced by synergistic action between metabolites. In vivo, it was observed a significant increase in 8-OHdG levels, which was reversed by L-Car. We demonstrated for the first time that oxidative DNA damage is induced not only by BCAAs and BCKAs but also by Allo and we reinforce the protective effect of L-Car.

Acute and long-term effects of intracerebroventricular administration of α-ketoisocaproic acid on oxidative stress parameters and cognitive and noncognitive behaviors

Maple Syrup Urine Disease (MSUD) is biochemically characterized by elevated levels of leucine, isoleucine and valine, as well as their corresponding transaminated branched-chain α-keto acids in tissue and biological fluids. Neurological symptoms and cerebral abnormalities, whose mechanisms are still unknown, are typical of this metabolic disorder. In the present study, we evaluated the early effects (1 h after injection) and long-term effects (15 days after injection) of a single intracerebroventricular administration of α-ketoisocaproic acid (KIC) on oxidative stress parameters and cognitive and noncognitive behaviors. Our results showed that KIC induced early and long-term effects; we found an increase in TBARS levels, protein carbonyl content and DNA damage in the hippocampus, striatum and cerebral cortex both one hour and 15 days after KIC administration. Moreover, SOD activity increased in the hippocampus and striatum one hour after injection, whereas after 15 days, SOD activity decreased only in the striatum. On the other hand, KIC significantly decreased CAT activity in the striatum one hour after injection, but 15 days after KIC administration, we found a decrease in CAT activity in the hippocampus and striatum. Finally, we showed that long-term cognitive deficits follow the oxidative damage; KIC induced impaired habituation memory and long-term memory impairment. From the biochemical and behavioral findings, it we presume that KIC provokes oxidative damage, and the persistence of brain oxidative stress is associated with long-term memory impairment and prepulse inhibition.

Apoptotic signaling pathways induced by acute administration of branched-chain amino acids in an animal model of maple syrup urine disease

Maple Syrup Urine Disease (MSUD) is an inborn error of metabolism caused by a deficiency of the branched-chain α-keto acid dehydrogenase complex activity. This blockage leads to accumulation of the branched-chain amino acids leucine, isoleucine and valine, as well as their corresponding α-keto acids and α-hydroxy acids. The affected patients present severe neurological symptoms, such as coma and seizures, as well as edema and cerebral atrophy. Considering that the mechanisms of the neurological symptoms presented by MSUD patients are still poorly understood, in this study, protein levels of apoptotic factors are measured, such as Bcl-2, Bcl-xL, Bax, caspase-3 and -8 in hippocampus and cerebral cortex of rats submitted to acute administration of branched-chain amino acids during their development. The results in this study demonstrated that BCAA acute exposure during the early postnatal period did not significantly change Bcl-2, Bcl-xL, Bax and caspase-8 protein levels. However, the Bax/Bcl-2 ratio and procaspase-3 protein levels were decreased in hippocampus. On the other hand, acute administration of BCAA in 30-day-old rats increase in Bax/Bcl-2 ratio followed by an increased caspase-3 activity in cerebral cortex, whereas BCAA induces apoptosis in hippocampus through activation and cleavage of caspase-3 and -8 without changing the Bax/Bcl-2 ratio. In conclusion, the results suggest that apoptosis could be of pivotal importance in the developmental neurotoxic effects of BCAA. In addition, the current studies also suggest that multiple mechanisms may be involved in BCAA-induced apoptosis in the cerebral cortex and hippocampus.

A 3-methylcrotonyl-CoA carboxylase deficient human skin fibroblast transcriptome reveals underlying mitochondrial dysfunction and oxidative stress

Isolated 3-methylcrotonyl-CoA carboxylase (MCC) deficiency is an autosomal recessive inherited metabolic disease of leucine catabolism with a highly variable phenotype. Apart from extensive mutation analyses of the MCCC1 and MCCC2 genes encoding 3-methylcrotonyl-CoA carboxylase (EC 6.4.1.4), molecular data on MCC deficiency gene expression studies in human tissues is lacking. For IEMs, unbiased '-omics' approaches are starting to reveal the secondary cellular responses to defects in biochemical pathways. Here we present the first whole genome expression profile of immortalized cultured skin fibroblast cells of two clinically affected MCC deficient patients and two healthy individuals generated using Affymetrix(®)HuExST1.0 arrays. There were 16191 significantly differentially expressed transcript IDs of which 3591 were well annotated and present in the predefined knowledge database of Ingenuity Pathway Analysis software used for downstream functional analyses. The most noticeable feature of this MCCA deficient skin fibroblast transcriptome was the typical genetic hallmark of mitochondrial dysfunction, decreased antioxidant response and disruption of energy homeostasis, which was confirmed by mitochondrial functional analyses. The MCC deficient transcriptome seems to predict oxidative stress that could alter the complex secondary cellular response that involve genes of the glycolysis, the TCA cycle, OXPHOS, gluconeogenesis, β-oxidation and the branched-chain fatty acid metabolism. An important emerging insight from this human MCCA transcriptome in combination with previous reports is that chronic exposure to the primary and secondary metabolites of MCC deficiency and the resulting oxidative stress might impact adversely on the quality of life and energy levels, irrespective of whether MCC deficient individuals are clinically affected or asymptomatic.

Catabolic Defect of Branched-Chain Amino Acids Promotes Heart Failure

BACKGROUND

Although metabolic reprogramming is critical in the pathogenesis of heart failure, studies to date have focused principally on fatty acid and glucose metabolism. Contribution of amino acid metabolic regulation in the disease remains understudied.

METHODS AND RESULTS

Transcriptomic and metabolomic analyses were performed in mouse failing heart induced by pressure overload. Suppression of branched-chain amino acid (BCAA) catabolic gene expression along with concomitant tissue accumulation of branched-chain α-keto acids was identified as a significant signature of metabolic reprogramming in mouse failing hearts and validated to be shared in human cardiomyopathy hearts. Molecular and genetic evidence identified the transcription factor Krüppel-like factor 15 as a key upstream regulator of the BCAA catabolic regulation in the heart. Studies using a genetic mouse model revealed that BCAA catabolic defect promoted heart failure associated with induced oxidative stress and metabolic disturbance in response to mechanical overload. Mechanistically, elevated branched-chain α-keto acids directly suppressed respiration and induced superoxide production in isolated mitochondria. Finally, pharmacological enhancement of branched-chain α-keto acid dehydrogenase activity significantly blunted cardiac dysfunction after pressure overload.

CONCLUSIONS

BCAA catabolic defect is a metabolic hallmark of failing heart resulting from Krüppel-like factor 15-mediated transcriptional reprogramming. BCAA catabolic defect imposes a previously unappreciated significant contribution to heart failure.

Brain-blood amino acid correlates following protein restriction in murine maple syrup urine disease

Background

Conventional therapy for patients with maple syrup urine disease (MSUD) entails restriction of protein intake to maintain acceptable levels of the branched chain amino acid, leucine (LEU), monitored in blood. However, no data exists on the correlation between brain and blood LEU with protein restriction, and whether correction in blood is reflected in brain.

Methods

To address this question, we fed intermediate MSUD mice diets of 19% (standard) and 6% protein, with collection of sera (SE), striata (STR), cerebellum (CE) and cortex (CTX) for quantitative amino acid analyses.

Results

LEU and valine (VAL) levels in all brain regions improved on average 28% when shifting from 19% to 6% protein, whereas the same improvements in SE were on average 60%. Isoleucine (ILE) in brain regions did not improve, while the SE level improved 24% with low-protein consumption. Blood-branched chain amino acids (LEU, ILE, and VAL in sera (SE)) were 362-434 μM, consistent with human values considered within control. Nonetheless, numerous amino acids in brain regions remained abnormal despite protein restriction, including glutamine (GLN), aspartate (ASP), glutamate (GLU), gamma-aminobutyric acid (GABA), asparagine (ASN), citrulline (CIT) and serine (SER). To assess the specificity of these anomalies, we piloted preliminary studies in hyperphenylalaninemic mice, modeling another large neutral aminoacidopathy. Employing an identical dietary regimen, we found remarkably consistent abnormalities in GLN, ASP, and GLU.

Conclusions

Our results suggest that blood amino acid analysis may be a poor surrogate for assessing the outcomes of protein restriction in the large neutral amino acidopathies, and further indicate that chronic neurotransmitter disruptions (GLU, GABA, ASP) may contribute to long-term neurocognitive dysfunction in these disorders.

Neurological damage in MSUD: the role of oxidative stress

Maple syrup urine disease (MSUD) is a metabolic disease caused by a deficiency in the branched-chain α-keto acid dehydrogenase complex, leading to the accumulation of branched-chain keto acids and their corresponding branched-chain amino acids (BCAA) in patients. Treatment involves protein-restricted diet and the supplementation with a specific formula containing essential amino acids (except BCAA) and micronutrients, in order to avoid the appearance of neurological symptoms. Although the accumulation of toxic metabolites is associated to appearance of symptoms, the mechanisms underlying the brain damage in MSUD remain unclear, and new evidence has emerged indicating that oxidative stress contributes to this damage. In this context, this review addresses some of the recent findings obtained from cells lines, animal studies, and from patients indicating that oxidative stress is an important determinant of the pathophysiology of MSUD. Recent works have shown that the metabolites accumulated in the disease induce morphological alterations in C6 glioma cells through nitrogen reactive species generation. In addition, several works demonstrated that the levels of important antioxidants decrease in animal models and also in MSUD patients (what have been attributed to protein-restricted diets). Also, markers of lipid, protein, and DNA oxidative damage have been reported in MSUD, probably secondary to the high production of free radicals. Considering these findings, it is well-established that oxidative stress contributes to brain damage in MSUD, and this review offers new perspectives for the prevention of the neurological damage in MSUD, which may include the use of appropriate antioxidants as a novel adjuvant therapy for patients.

Hepatocyte transplantation (HTx) corrects selected neurometabolic abnormalities in murine intermediate maple syrup urine disease (iMSUD)

Skvorak et al. [1] demonstrated the therapeutic efficacy of HTx in a murine model of iMSUD, confirming significant metabolic improvement and survival. To determine the effect of HTx on extrahepatic organs, we examined the metabolic effects of HTx in brain from iMSUD animals. Amino acid analysis revealed that HTx corrected increased ornithine, partially corrected depleted glutamine, and revealed a trend toward alloisoleucine correction. For amino acid and monoamine neurotransmitters, decreased GABA was partially corrected with HTx, while the l-histidine dipeptide of GABA, homocarnosine, was decreased in iMSUD mice and hypercorrected following HTx. Elevated branched-chain amino acids (BCAA; leucine, isoleucine, and valine) in MSUD can deplete brain tyrosine and tryptophan (the precursors of monoamine neurotransmitters, dopamine (DA) and serotonin (5-hydroxytryptamine; 5-HT)) through competition via the large neutral amino acid transporter. HTx corrected decreased DA levels and the DA metabolite, 3-methoxytyramine, and partially corrected the DA intermediate 3,4-dihydroxyphenylacetate (DOPAC) and 5-HT levels, despite normal tyrosine and tryptophan levels in iMSUD mouse brain. We further observed enhanced intracellular turnover of both DA and 5-HT in iMSUD mouse brain, both of which partially corrected with HTx. Our results suggest new pathomechanisms of neurotransmitter metabolism in this disorder and support the therapeutic relevance of HTx in iMSUD mice, while providing proof-of-principle that HTx has corrective potential in extrahepatic organs.

Protein phosphatase 2Cm is a critical regulator of branched-chain amino acid catabolism in mice and cultured cells

The branched-chain amino acids (BCAA) are essential amino acids required for protein homeostasis, energy balance, and nutrient signaling. In individuals with deficiencies in BCAA, these amino acids can be preserved through inhibition of the branched-chain-alpha-ketoacid dehydrogenase (BCKD) complex, the rate-limiting step in their metabolism. BCKD is inhibited by phosphorylation of its E1alpha subunit at Ser293, which is catalyzed by BCKD kinase. During BCAA excess, phosphorylated Ser293 (pSer293) becomes dephosphorylated through the concerted inhibition of BCKD kinase and the activity of an unknown intramitochondrial phosphatase. Using unbiased, proteomic approaches, we have found that a mitochondrial-targeted phosphatase, PP2Cm, specifically binds the BCKD complex and induces dephosphorylation of Ser293 in the presence of BCKD substrates. Loss of PP2Cm completely abolished substrate-induced E1alpha dephosphorylation both in vitro and in vivo. PP2Cm-deficient mice exhibited BCAA catabolic defects and a metabolic phenotype similar to the intermittent or intermediate types of human maple syrup urine disease (MSUD), a hereditary disorder caused by defects in BCKD activity. These results indicate that PP2Cm is the endogenous BCKD phosphatase required for nutrient-mediated regulation of BCKD activity and suggest that defects in PP2Cm may be responsible for a subset of human MSUD.