Contribution of Brain Intrinsic Branched-Chain Amino Acid Metabolism in a Novel Mouse Model of Maple Syrup Urine Disease

Maple syrup urine disease (MSUD) results from loss of branched-chain ketoacid dehydrogenase (BCKDH) activity, the committed, rate-limiting step of branched-chain amino acid (BCAA) oxidation. Current treatments, including a low protein diet and liver transplantation, improve peripheral biochemistry and limit episodes of metabolic decompensation but do not fully prevent chronic neuropsychiatric symptoms. The mechanisms underlying chronic neurologic phenotypes remain poorly understood. Currently available MSUD mouse models do not survive long enough to evaluate chronic central nervous system (CNS) pathology. To investigate if loss of brain-intrinsic BCAA metabolism contributes to chronic neurologic disease, we developed a new brain-specific knockout mouse model of MSUD. First, we generated a mouse harboring a floxed Dbt allele (Dbt^flox/flox). Then we crossed this line with Cre recombinase driver lines to induce loss of Dbt expression in (1) all developing CNS cell populations (2) neurons alone or (3) astrocytes alone. We found that brain-specific KO mice have elevations in BCAA levels in cortex that are exacerbated by a high protein diet. They also have secondary changes in amino acids in brain that are important for neuronal function, including glutamine and glycine. These metabolic differences result in subtle functional deficits as measured by electroencephalogram and behavioral testing. Astrocyte and neuron-specific KO mice each also demonstrate mild biochemical features of MSUD in the cortex, suggesting that both cell populations may contribute to disease pathology. Collectively, these data suggest that therapies targeting the CNS directly, in addition to the periphery, may improve outcomes in MSUD.