A Knock-out Mouse Model for Methylmalonic Aciduria Resulting in Neonatal Lethality

Methylmalonic aciduria is a human autosomal recessive disorder of organic acid metabolism resulting from a functional defect in the activity of the enzyme methylmalonyl-CoA mutase. Based upon the homology of the human mutase locus with the mouse locus, we have chosen to disrupt the mouse mutase locus within the critical CoA binding domain using gene-targeting techniques to create a mouse model of methylmalonic aciduria. The phenotype of homozygous knock-out mice (mut-/-) is one of early neonatal lethality. Mice appear phenotypically normal at birth and are indistinguishable from littermates. By 15 h of age, they develop reduced movement and suckle less. This is followed by the development of abnormal breathing, and all of the mice with a null phenotype die by 24 h of age. Urinary levels of methylmalonic and methylcitric acids are grossly increased. Measurement of acylcarnitines in blood shows elevation of propionylcarnitine with no change in the levels of acetylcarnitine and free carnitine. Incorporation of [14C]propionate in primary fibroblast cultures from mut-/- mice is reduced to approximately 6% of normal level, whereas there is no detectable synthesis of mut mRNA in the liver. This is the first mouse model that recapitulates the key phenotypic features of mut0 methylmalonic aciduria.