ENU mutagenesis identifies mice with mitochondrial branched-chain aminotransferase deficiency resembling human maple syrup urine disease
Jer-Yuarn Wu, Hsiao-Jung Kao, Sing-Chung Li, Robert Stevens, Steven Hillman, David Millington, Yuan-Tsong Chen
Jer-Yuarn Wu, Hsiao-Jung Kao, Sing-Chung Li, Robert Stevens, Steven Hillman, David Millington, Yuan-Tsong Chen
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Article Metabolism

ENU mutagenesis identifies mice with mitochondrial branched-chain aminotransferase deficiency resembling human maple syrup urine disease

Abstract

Tandem mass spectrometry was applied to detect derangements in the pathways of amino acid and fatty acid metabolism in N-ethyl-N-nitrosourea–treated (ENU-treated) mice. We identified mice with marked elevation of blood branched-chain amino acids (BCAAs), ketoaciduria, and clinical features resembling human maple syrup urine disease (MSUD), a severe genetic metabolic disorder caused by the deficiency of branched-chain α-keto acid dehydrogenase (BCKD) complex. However, the BCKD genes and enzyme activity were normal. Sequencing of branched-chain aminotransferase genes (Bcat) showed no mutation in the cytoplasmic isoform (Bcat-1) but revealed a homozygous splice site mutation in the mitochondrial isoform (Bcat-2). The mutation caused a deletion of exon 2, a marked decrease in Bcat-2 mRNA, and a deficiency in both BCAT-2 protein and its enzyme activity. Affected mice responded to a BCAA-restricted diet with amelioration of the clinical symptoms and normalization of the amino acid pattern. We conclude that BCAT-2 deficiency in the mouse can cause a disease that mimics human MSUD. These mice provide an important animal model for study of BCAA metabolism and its toxicity. Metabolomics-guided screening, coupled with ENU mutagenesis, is a powerful approach in uncovering novel enzyme deficiencies and recognizing important pathways of genetic metabolic disorders.

Authors

Jer-Yuarn Wu, Hsiao-Jung Kao, Sing-Chung Li, Robert Stevens, Steven Hillman, David Millington, Yuan-Tsong Chen

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Body weight and amino acid changes with diet restricted in BCAAs. (a) Bo...
Body weight and amino acid changes with diet restricted in BCAAs. (a) Body weight changes of five mice in two litters: affected males (filled squares and filled triangles); affected female (filled circles); unaffected siblings (male, open triangles; female, open circles). Black arrows indicate when the restricted diet (MSUD diet/regular diet, 2:1) was initiated. Open arrows denote that diet was further changed to MSUD diet/regular diet, 3:1. One mouse (filled triangle) died accidentally at day 89. (b) Changes in amino acid concentrations. Light gray bars, affected mice with regular diet; dark gray bars, MSUD diet/regular diet, 2:1; black bars, MSUD diet/regular diet, 3:1; white bars, control ENU mice with regular diet.