Management of maple syrup urine disease in Canada. Committee for improvement of Hereditary Disease Management.
Nine patients with classic maple syrup urine disease (MSUD) and four with variant forms are under care at five treatment centres in the network affiliated with the National Food Distribution Centre for the Treatment of Hereditary Metabolic Diseases (the "Food Bank"). Diagnosis was made by clinicians and not from mass screening programs. MSUD requires complex emergency treatment to prevent severe neurologic damage, but effective management is compatible with normal growth and development. Long-term treatment requires continuous monitoring of the response to diets restricted in branched-chain amino acids; semisynthetic diet products free of branched-chain amino acids, provided by the Food Bank, are essential. Centralized treatment programs reduce the cost of treatment and maximize the potential benefits. The leucine requirement for adequate somatic growth during infancy in MSUD was found to be 200 to 600 mg/d; this range is lower than that estimated for infants with an intact leucine catabolic outflow pathway. The requirements for isoleucine and valine in infancy were also found to be lower than published values for normal infants. (+info)
Significance of L-alloisoleucine in plasma for diagnosis of maple syrup urine disease.
BACKGROUND: The significance of plasma L-alloisoleucine, which is derived from L-isoleucine in vivo, for diagnosis of maple syrup urine disease (MSUD) was examined. METHODS: Branched-chain L-amino acids were measured by automatic amino acid analysis. RESULTS: Alloisoleucine reference values in plasma were established in healthy adults [1.9 +/- 0.6 micromol/L (mean +/- SD); n = 35], children 3-11 years (1.6 +/- 0.4 micromol/L; n = 17), and infants <3 years (1.3 +/- 0.5 micromol/L; n = 37). The effect of dietary isoleucine was assessed in oral loading tests. In controls receiving 38 micromol (n = 6; low dose) and 1527 micromol (n = 3; high dose) of L-isoleucine per kilogram of body weight, peak increases of plasma isoleucine were 78 +/- 24 and 1763 +/- 133 micromol/L, respectively; the peak increase of alloisoleucine, however, was negligible for low-dose (<0.3 micromol/L) and minor for high-dose (5. 5 +/- 2.1 micromol/L) load. In patients with diabetes mellitus, ketotic hypoglycemia, phenylketonuria, and obligate heterozygous parents of MSUD patients, alloisoleucine was not significantly different from healthy subjects. Therefore, a plasma concentration of 5 micromol/L was used as a cutoff value. In patients with classical MSUD (n = 7), alloisoleucine was beyond the cutoff value in 2451 of 2453 unselected samples. In patients with variant MSUD (n = 9), alloisoleucine was >5 micromol/L in all samples taken for establishment of diagnosis and in 94% of the samples taken for treatment control (n = 624). With the other branched-chain amino acids, the frequency of diagnostically significant increases was <45%. CONCLUSIONS: The present findings indicate that plasma L-alloisoleucine above the cutoff value of 5 micromol/L is the most specific and most sensitive diagnostic marker for all forms of MSUD. (+info)
Crystal structure of human branched-chain alpha-ketoacid dehydrogenase and the molecular basis of multienzyme complex deficiency in maple syrup urine disease.
BACKGROUND: Mutations in components of the extraordinarily large alpha-ketoacid dehydrogenase multienzyme complexes can lead to serious and often fatal disorders in humans, including maple syrup urine disease (MSUD). In order to obtain insight into the effect of mutations observed in MSUD patients, we determined the crystal structure of branched-chain alpha-ketoacid dehydrogenase (E1), the 170 kDa alpha(2)beta(2) heterotetrameric E1b component of the branched-chain alpha-ketoacid dehydrogenase multienzyme complex. RESULTS: The 2.7 A resolution crystal structure of human E1b revealed essentially the full alpha and beta polypeptide chains of the tightly packed heterotetramer. The position of two important potassium (K(+)) ions was determined. One of these ions assists a loop that is close to the cofactor to adopt the proper conformation. The second is located in the beta subunit near the interface with the small C-terminal domain of the alpha subunit. The known MSUD mutations affect the functioning of E1b by interfering with the cofactor and K(+) sites, the packing of hydrophobic cores, and the precise arrangement of residues at or near several subunit interfaces. The Tyr-->Asn mutation at position 393-alpha occurs very frequently in the US population of Mennonites and is located in a unique extension of the human E1b alpha subunit, contacting the beta' subunit. CONCLUSIONS: Essentially all MSUD mutations in human E1b can be explained on the basis of the structure, with the severity of the mutations for the stability and function of the protein correlating well with the severity of the disease for the patients. The suggestion is made that small molecules with high affinity for human E1b might alleviate effects of some of the milder forms of MSUD. (+info)
Branched chain amino acids induce apoptosis in neural cells without mitochondrial membrane depolarization or cytochrome c release: implications for neurological impairment associated with maple syrup urine disease.
Maple syrup urine disease (MSUD) is an inborn error of metabolism caused by a deficiency in branched chain alpha-keto acid dehydrogenase that can result in neurodegenerative sequelae in human infants. In the present study, increased concentrations of MSUD metabolites, in particular alpha-keto isocaproic acid, specifically induced apoptosis in glial and neuronal cells in culture. Apoptosis was associated with a reduction in cell respiration but without impairment of respiratory chain function, without early changes in mitochondrial membrane potential and without cytochrome c release into the cytosol. Significantly, alpha-keto isocaproic acid also triggered neuronal apoptosis in vivo after intracerebral injection into the developing rat brain. These findings suggest that MSUD neurodegeneration may result, at least in part, from an accumulation of branched chain amino acids and their alpha-keto acid derivatives that trigger apoptosis through a cytochrome c-independent pathway. (+info)
Chloride-dependent inhibition of vesicular glutamate uptake by alpha-keto acids accumulated in maple syrup urine disease.
Maple syrup urine disease is a metabolic disorder caused by mutations of the branched chain keto acid dehydrogenase complex, leading to accumulation of alpha-keto acids and their amino acid precursors in the brain. We now report that alpha-ketoisovaleric, alpha-keto-beta-methyl-n-valeric and alpha-ketoisocaproic acids accumulated in the disease inhibit glutamate uptake into rat brain synaptic vesicles. The alpha-keto acids did not affect the electrochemical proton gradient across the membrane, suggesting that they interact directly with the vesicular glutamate carrier. Chloride anions have a biphasic effect on glutamate uptake. Low concentrations activate the uptake (0.2 to 8 mM), while higher concentrations are inhibitory. The alpha-keto acids inhibited glutamate uptake by a new mechanism, involving a change in the chloride dependence for the activation of glutamate uptake. The activation of glutamate uptake by low chloride concentrations was shifted toward higher concentrations of the anion in the presence of alpha-keto acids. Inhibition by alpha-keto acids was abolished at high chloride concentrations (20 to 80 mM), indicating that alpha-keto acids specifically change the stimulatory effect of low chloride concentrations. High glutamate concentrations also reduced the inhibition by alpha-keto acids, an effect observed both in the absence and in the presence of low chloride concentrations. The results suggest that in addition to their possible pathophysiological role in maple syrup urine disease, alpha-keto acids are valuable tools to study the mechanism of vesicular transport of glutamate. (+info)
Rare etiology of autosomal recessive disease in a child with noncarrier parents.
A child with maple syrup urine disease type 2 (MSUD2) was found to be homozygous for a 10-bp MSUD2-gene deletion on chromosome 1. Both purported parents were tested, and neither carries the gene deletion. Polymorphic simple-sequence repeat analyses at 15 loci on chromosome 1 and at 16 loci on other chromosomes confirmed parentage and revealed that a de novo mutation prior to maternal meiosis I, followed by nondisjunction in maternal meiosis II, resulted in an oocyte with two copies of the de novo mutant allele. Fertilization by a sperm that did not carry a paternal chromosome 1 or subsequent mitotic loss of the paternal chromosome 1 resulted in the propositus inheriting two mutant MSUD2 alleles on two maternal number 1 chromosomes. (+info)
Gene preference in maple syrup urine disease.
Untreated maple syrup urine disease (MSUD) results in mental and physical disabilities and often leads to neonatal death. Newborn-screening programs, coupled with the use of protein-modified diets, have minimized the severity of this phenotype and allowed affected individuals to develop into productive adults. Although inheritance of MSUD adheres to rules for single-gene traits, mutations in the genes for E1alpha, E1beta, or E2 of the mitochondrial branched-chain alpha-ketoacid dehydrogenase complex can cause the disease. Randomly selected cell lines from 63 individuals with clinically diagnosed MSUD were tested by retroviral complementation of branched-chain alpha-ketoacid dehydrogenase activity to identify the gene locus for mutant alleles. The frequencies of the mutations were 33% for the E1alpha gene, 38% for the E1beta gene, and 19% for the E2 gene. Ten percent of the tested cell lines gave ambiguous results by showing no complementation or restoration of activity with two gene products. These results provide a means to establish a genotype/phenotype relationship in MSUD, with the ultimate goal of unraveling the complexity of this single-gene trait. This represents the largest study to date providing information on the genotype for MSUD. (+info)
Biochemical basis of type IB (E1beta ) mutations in maple syrup urine disease. A prevalent allele in patients from the Druze kindred in Israel.
Maple syrup urine disease (MSUD) is a metabolic disorder associated with often-fatal ketoacidosis, neurological derangement, and mental retardation. In this study, we identify and characterize two novel type IB MSUD mutations in Israeli patients, which affect the E1beta subunit in the decarboxylase (E1) component of the branched-chain alpha-ketoacid dehydrogenase complex. The recombinant mutant E1 carrying the prevalent S289L-beta (TCG --> TTG) mutation in the Druze kindred exists as a stable inactive alphabeta heterodimer. Based on the human E1 structure, the S289L-beta mutation disrupts the interactions between Ser-289-beta and Glu-290-beta', and between Arg-309-beta and Glu-290-beta', which are essential for native alpha(2)beta(2) heterotetrameric assembly. The R133P-beta (CGG --> CCG) mutation, on the other hand, is inefficiently expressed in Escherichia coli as heterotetramers in a temperature-dependent manner. The R133P-beta mutant E1 exhibits significant residual activity but is markedly less stable than the wild-type, as measured by thermal inactivation and free energy change of denaturation. The R133P-beta substitution abrogates the coordination of Arg-133-beta to Ala-95-beta, Glu-96-beta, and Ile-97-beta, which is important for strand-strand interactions and K(+) ion binding in the beta subunit. These findings provide new insights into folding and assembly of human E1 and will facilitate DNA-based diagnosis for MSUD in the Israeli population. (+info)