Mice homozygous for the Dbt (E2) targeted mutation and carrying both the LAP-tTA and TRE-E2 transgenes are viable and fertile. The E2-targeted mutation leads to absence of branched-chain keto acid dehydrogenase (BCKDH) activity and E2 protein in liver tissue, but this absence is rescued by the two transgenes: liver-directed expression of the modified human BCKDH E2 subunit from the complimentary "Tet-off" transgenes abrogates the severity of Maple Syrup Urine Disease (MSUD) phenotype observed in E2-deficient single mutant mice. Triple mutant mice are a model for intermediate MSUD (iMSUD); BCKDH activity is only 5-6% of that found in wildtype mice. This low level of BCKDH activity is sufficient to allow survival, but insufficient to normalize circulating branched chain amino acids levels. Because these mice have near normal amounts of E2 protein, but only 5-6% of normal BCKDH enzyme activity, it is probable that the c-myc tag at the carboxy-terminus of the human E2 transgene interferes with enzymatic activity. The donating investigator reports that addition of the tetracycline analog doxycycline does not affect MSUD phenotype rescue. These mutant mice may be useful in studying metabolic disease, biochemistry, and both the complete/classic and intermediate forms of Maple Syrup Urine Disease.

In these triple mutant mice the absence of keto acid dehydrogenase (BCKDH) activity and E2 protein in liver tissue due to the Dbttm1Geh knock-out mutation is rescued by the two transgenes, rendering this strain as a useful model for Maple Syrup Urine Disease (MSUD).