Intermediate Maple Syrup Urine Disease Research Articles

Initial Studies to Cure Maple Syrup Urine Disease (MSUD) in a Mouse Model

MSUD is a genetic disorder caused by deficiency of branched-chain α-keto acid dehydrogenase (BCKDH). Current unsatisfactory treatments require improved therapies to combat this disease. To develop novel therapies, we created murine models of MSUD (BMC Med. Gen. 2006, 7:33). In the current study, further characterization of the intermediate MSUD model revealed brain glutamate and glutamine levels reduced by 50% and GABA reduced by 20%. These findings are associated with neuropathology suggesting cerebral edema and are consistent with human MSUD. We then attempted to correct the disease phenotype using recombinant adeno-associated virus (rAAV) gene therapy. The rAAV vector was engineered to contain human BCKDH E2 subunit cDNA, a mouse albumin promoter to direct expression of E2 protein in the liver, and packaged into AAV2 and AAV8 serotypes. The viral vector (1011 v.g./kg) was administered by IP injection at 3 weeks of age. Some animals were also given a second injection at 7 weeks of age. Preliminary studies suggested increased BCKDH enzyme activity in AAV8 treated animals. Both AAV2 and AAV8 increased survival compared to saline treated mice but failed to normalize blood branched-chain amino acid levels. Based on these data we conclude that MSUD is a promising candidate for gene therapy. This work was supported by NIH grant DK057386, the Laverty Foundation, and the MSUD Family Support Group.

Production and characterization of murine models of classic and intermediate maple syrup urine disease

BackgroundMaple Syrup Urine Disease (MSUD) is an inborn error of metabolism caused by a deficiency of branched-chain keto acid dehydrogenase. MSUD has several clinical phenotypes depending on the degree of enzyme deficiency. Current treatments are not satisfactory and require new approaches to combat this disease. A major hurdle in developing new treatments has been the lack of a suitable animal model.MethodsTo create a murine model of classic MSUD, we used gene targeting and embryonic stem cell technologies to create a mouse line that lacked a functional E2 subunit gene of branched-chain keto acid dehydrogenase. To create a murine model of intermediate MSUD, we used transgenic technology to express a human E2 cDNA on the knockout background. Mice of both models were characterized at the molecular, biochemical, and whole animal levels.ResultsBy disrupting the E2 subunit gene of branched-chain keto acid dehydrogenase, we created a gene knockout mouse model of classic MSUD. The homozygous knockout mice lacked branched-chain keto acid dehydrogenase activity, E2 immunoreactivity, and had a 3-fold increase in circulating branched-chain amino acids. These metabolic derangements resulted in neonatal lethality. Transgenic expression of a human E2 cDNA in the liver of the E2 knockout animals produced a model of intermediate MSUD. Branched-chain keto acid dehydrogenase activity was 5–6% of normal and was sufficient to allow survival, but was insufficient to normalize circulating branched-chain amino acids levels, which were intermediate between wildtype and the classic MSUD mouse model.ConclusionThese mice represent important animal models that closely approximate the phenotype of humans with the classic and intermediate forms of MSUD. These animals provide useful models to further characterize the pathogenesis of MSUD, as well as models to test novel therapeutic strategies, such as gene and cellular therapies, to treat this devastating metabolic disease.

Molecular and biochemical basis of intermediate maple syrup urine disease. Occurrence of homozygous G245R and F364C mutations at the E1 alpha locus of Hispanic-Mexican patients.

Maple syrup urine disease (MSUD) is caused by a deficiency of the mitochondrial branched-chain alpha-keta acid dehydrogenase (BCKAD) complex. The multienzyme complex comprises five enzyme components, including the E1 decarboxylase with a heterotetrameric (alpha 2 beta 2) structure. Four unrelated Hispanic-Mexican MSUD patients with the intermediate clinical phenotype were diagnosed 7 to 22 mo after birth during evaluation for developmental delay. Three of the four patients were found homozygous for G to A transition at base 895 (exon 7) of the E1 alpha locus, which changes Gly-245 to Arg (G245R) in that subunit. The remaining patient was homozygous for T to G transversion at base 1,253 in the E1 alpha gene, which converts Phe-364 to Cys (F364C) in the gene product. Transfection studies in E1 alpha-deficient lymphoblasts indicate that both G245R and F364C mutant E1 alpha subunits were unable to significantly reconstitute BCKAD activity. Western blotting showed that both mutant E1 alpha subunits in transfected cells failed to efficiently rescue the normal E1 beta through assembly. The putative assembly defect was confirmed by pulse-chase labeling of E1 subunits in a chaperone-augmented bacterial overexpression system. The kinetics of initial assembly of the G245R E1 alpha subunit with the normal E1 beta was shown to be slower than the normal E1 alpha. No detectable assembly of the F364C E1 alpha with normal E1 beta was observed during the 2 h chase. Small amounts of recombinant mutant E1 proteins were produced after 15 h induction with isopropyl thiogalactoside and exhibited very low or no E1 activity. Our study establishes that G245R and F364C mutations in the E1 alpha subunit disrupt both the E1 heterotetrameric assembly and function of the BCKAD complex. Moreover, the results suggest that the G245R mutant E1 alpha allele may be important in the Hispanic-Mexican population.

A distinct variant of intermediate maple syrup urine disease

Branched chain alpha-ketoacid dehydrogenase (BCKAD) deficiency, or maple syrup urine disease (MSUD), can be categorized as classical, intermediate, intermittent or thiamine responsive, based on generally concordant in vitro BCKAD activity and severity of phenotype. We present clinical and enzymatic data on a boy with intermediate maple syrup urine disease, and suggest that he represents a novel category of mutation. He presented at age 10 months in ketoacidotic coma, with a history of irritability, poor feeding and growth and developmental delay. Branched chain amino acid restriction effected normal growth and developmental parameters by age 42 months. In contrast to previous patients with intermediate MSUD, his fibroblasts and fibroblast extracts failed to decarboxylate [1-14C]-alpha-ketoisovalerate (KIV). The defect is not in mitochondrial transport of substrate, but rather in the catalytic activity of the E1 component of the BCKAD. Disrupted cells of the proband exhibited negligible BCKAD activity over a wide range of keto acid substrate concentrations, irrespective of the presence of added thiamine pyrophosphate (TPP). These results differ from the sigmoidal kinetics observed using classical MSUD extracts, and the hyperbolic kinetics with control preparations under the same assay conditions. We propose that the structurally altered enzyme possesses reduced but not negligible activity in vivo, and exists as an unstable complex in vitro under assay conditions used, even in the presence of added TPP.