A mouse model of cblA class isolated methylmalonic acidemia (MMA) displays reduced survival, growth failure, renal disease and secondary mitochondrial dysfunction. M. W. Epping1, C. X. Wang1, P. M. Zerfas2, G. Elliot1, L. Li3, I. Manoli1, C. P. Venditti1 1) Genetics and Molecular Biology Branch, NHGRI, NIH, Bethesda, MD; 2) Office of Research Services, Division of Veterinary Resources, NIH, Bethesda, MD; 3) Division of Nephrology and Hypertension, Georgetown University Medical Center, Washington, DC.
The Methylmalonic Aciduria cbl¬A Type (MMAA) gene product is responsible for the gated transfer of adenosylcobalamin to methylmalonyl-CoA mutase (MUT) and the protection of MUT from oxidative inactivation. Mutations in the MMAA gene cause the cblA complementation class of isolated MMA that typically has a milder clinical course than MUT deficiency, although is still associated with chronic renal failure, basal ganglia stroke, and optic nerve atrophy (ONA). Knock out murine models of Mut display immediate neonatal lethality and, while valuable for the testing of gene therapy, a need to model an ameliorated form of MMA to discern disease pathophysiology exists. Mmaa-/- mice were generated on a C57B6/Sv129 background and harbor a reporter-tagged deletion in Mmaa that removes exons 3-5 and replaces them with a LacZ-IRES-neo cassette. Mmaa-/- mice were born in Mendelian proportions. The homozygous mutants exhibited decreased survival, with 10% reaching two months of age, and were significantly growth retarded, even when maintained on high fat and carbohydrate diet (P<0.05). Immunoreactive MMAA was not detected in liver extracts from Mmaa-/- mice indicating the knock out allele is a null. The concentrations of plasma methylmalonic acid were significantly higher in Mmaa-/- mice, ranging between 63.88-1641.91 M, compared to the heterozygous range of 5.36-13.56 M (P < 0.0001). Stable isotope studies indicated that the mutant mice exhibit a diminished capacity to oxidize 1-C-13 propionate (P < 0.0001). Additionally, the glomerular filtration rate, measured with FITC-sinistrin, showed that Mmaa-/- mice have kidney function diminished to 45% of their heterozygous littermates. Consistent with the organ-specific mitochondrial dysfunction of MMA, electron microscopy confirmed the presence of megamitochondria, with aberrant cristae, and abnormal structure in the hepatocytes, proximal renal tubular epithelial cells, and brown fat. Regular subcutaneous injection of vitamin B12 (OHcbl) improved survival, weight gain, and propionate oxidation in Mmaa-/- males, which maintain shorter survival periods than females. Mmaa-/- mice represent a promising model for examining the pathophysiology and disease manifestations of vitamin-B12-responsive MMA, including hepatorenal disease, metabolic stroke, and ONA, and will allow the development and testing of gene and small molecule therapies for the disease.
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