46. The First Viable Mouse Model of cblC Type Combined Methylmalonic Acidemia and Homocysteinemia: AAV Gene Therapy Rescues Neonatal Lethality and Provides Insight into Disease-Associated Retinal Degeneration

Combined methylmalonic acidemia and homocysteinemia, cblC type (cblC), is the most common inborn error of cobalamin metabolism and is caused by mutations in the MMACHC gene. MMACHC transports and processes intracellular cobalamin (vitamin B12) into its two active cofactors, 5’-adenosylcobalamin and methylcobalamin, necessary for the enzymatic reactions of methylmalonyl-CoA mutase and methionine synthase, respectively. Mutations in MMACHC result in methylmalonic acidemia, hyperhomocysteinemia and hypomethioninemia. Disease manifestations include growth failure, anemia, heart defects, developmental delay and a progressive maculopathy and pigmentary retinopathy that causes blindness, usually by the end of the first decade. Despite the use of conventional therapies including cobalamin injections and other cofactors, the manifestions of cblC, in particular the eye disease, remain unresolved with treatment. In order to explore disease pathophysiology and develop AAV gene therapy for cblC deficiency, we first created a viable mouse model using TALENs to edit the murine Mmachc gene, near the location of the most common mutation in humans, c. 271dupA. Two alleles were investigated: c. 165_166delAC p. P56CfsX4 (Δ2) and c. 162_164delCAC p. S54_T55delinsR (Δ3). MmachcΔ2/Δ3 and MmachcΔ3/Δ3 homozygous mutant mice displayed reduced survival, severe growth retardation, and massive metabolic perturbations. The median survival was less than 7 days with 90% of the mutant mice perishing before 3 weeks (Δ2 n=13; Δ3 n=42). The weights of MmachcΔ3/Δ3 mice were reduced relative to heterozygote and wild type littermates (n=15, p<0.0001). MmachcΔ2/Δ2 and MmachcΔ3/Δ3 mice (n=4, n=6) recapitulated the biochemical features of cblC, with significantly elevated plasma methylmalonic acid, homocysteine, cystathionine and decreased methionine compared to littermate controls (n=7) (p<0.05 for all metabolites). To assess the potential for gene therapy as a treatment for cblC, we generated two AAV vectors: rAAVrh10-CBA-mMmachc and rAAV9-CBA-hMMACHC and compared AAV with biweekly injections of OH-cobalamin, the standard therapy. MmachcΔ3/Δ3 mice were then treated with a single vector dose (1 × 1011 GC) delivered via intrahepatic injection in the neonatal period. MmachcΔ3/Δ3 mice treated with AAV vectors (AAVrh10 n=11, AAV9 n=5) displayed dramatically improved clinical appearance with improved growth (p= 0.0568), and increased survival (p<0.0001 for both vectors), with the oldest treated mutants currently living beyond 9 months. Successful gene therapy in the MmachcΔ3/Δ3 mice also enabled us to model, for the first time, cblC associated ocular pathology: surviving MmachcΔ3/Δ3 mice displayed thinning of the outer nuclear layer and shortening of photoreceptor outer segments, consistent with the pathology described in patients. Our results demonstrate that AAV gene delivery of MMACHC represents a new therapy for cblC which can treat the systemic, and possibly ocular, manifestations of this relatively common and devastating inborn error of metabolism.