357. A Mouse Model of Cobalamin A (cblA) Class Isolated Methymalonic Academy (MMA) Provides Unique Platform for Testing Gene and Cell Therapies

Abstract

Isolated Methylmalonic Acidemia (MMA) comprises a relatively common and heterogeneous group of inborn errors of metabolism. Most affected individuals display severe multisystemic disease with episodes of metabolic instability, chronic renal disease, and neurological complications. The most severe clinical presentations are typically associated with complete loss of methylmalonyl-CoA mutase (MUT) enzymatic activity. MUT converts methylmalonyl-CoA to succinyl-CoA within mitochondria but defects in the transport or synthesis of its cobalamin co-factor also variably impair MUT activity. Previous mouse models of Mut MMA generated by our group have been critical to developing gene therapy for MMA but have either displayed neonatal lethality or required concomitant transgenesis for viability. Thus, the need for ameliorated models to further investigate disease-associated pathophysiology and facilitate gene therapy studies exists. We have therefore constructed a mouse model of an attenuated form of isolated MMA, the cblA subtype, by using homologous recombination to create a deletion allele of Mmaa, the enzyme that performs the gated transfer of adenosylcobalamin to Mut and protects Mut from oxidative inactivation. Patients with the cblA subtype of MMA can have variable presentations, spanning the full spectrum of MMA associated symptoms and pathology, yet always harbor an element of clinical and biochemical responsiveness to injectable B12.Mmaa-/- mice were born in mendelian proportions, exhibited decreased survival after weaning (P<0.0001), weighed 50% less than littermates at 6 months (P<0.0001), lacked immunoreactive Mmaa and Mmaa mRNA in multiple tissues, including the liver, and manifested severe metabolic perturbations. Plasma methylmalonic acid concentrations ranged between 63.88-1641.91 µM, representing a 62-fold or more elevation over age matched littermates. Mmaa-/- mice also display diminished 1-C-13 propionate oxidative capacity (P<0.0001) and hepatic complex IV activity (P=0.002). Electron microscopy confirmed the presence of hepatic megamitochondria and abnormal proximal renal tubular mitochondria. The glomerular filtration rate, measured using FITC-sinistrin decay, showed that Mmaa-/- mice have diminished kidney function compared to heterozygous littermates. Following hydroxycobalamin administration, Mmaa-/- mice had improved survival and increased weight (P=0.0003), which was associated with improved 1-13C-propionate oxidative capacity.This new mouse model fully recapitulates the clinical and biochemical features seen in cblA patients, and provides a robust platform for testing gene and cell therapies for MMA because affected mice are relatively severe but survive until weaning. These animals also offer the opportunity to longitudinally examine disease biomarkers and more easily assay vectors across a wide age range.