Limited accumulation of damaged proteins in L-isoaspartyl (D-aspartyl) O-methyltransferase-deficient mice

L-Isoaspartyl (D-aspartyl) O-methyltransferase (PCMT1) can initiate the conversion of damaged aspartyl and asparaginyl residues to normal L-aspartyl residues. Mice lacking this enzyme (Pcmt1-/- mice) have elevated levels of damaged residues and die at a mean age of 42 days from massive tonic-clonic seizures. To extend the lives of the knockout mice so that the long term effects of damaged residue accumulation could be investigated, we produced transgenic mice with a mouse Pcmt1 cDNA under the control of a neuron specific promoter. Pcmt1 transgenic mice that were homozygous for the endogenous Pcmt1 knockout mutation (transgenic Pcmt1-/- mice) had brain PCMT1 activity levels that were 6.5-13% those of mild-type mice but had little or no activity in other tissues. The transgenic Pcmt1-/- mice lived, on average, B-fold longer than nontransgenic Pcmt1-/- mice and accumulated only half as many damaged aspartyl residues in their brain proteins. The concentration of damaged residues in heart, testis, and brain proteins in transgenic Pcmt1-/- mice initially increased with age but unexpectedly reached a plateau by 100 days of age. Urine from Pcmt1-/- mice contained increased amounts of peptides with damaged aspartyl residues, apparently enough to account for proteins that were not repaired intracellularly. In the absence of PCMT1, proteolysis may limit the intracellular accumulation of damaged proteins but less efficiently than in wild-type mice having PCMT1-mediated repair.