353. 1-13C-Propionate Oxidation as a Measure of Methylmalonyl-CoA Mutase (MUT) Activity in Methylmalonic Acidemia

Isolated methylmalonic acidemia (MMA) is a grave autosomal recessive inborn error of metabolism, caused by a defect in methylmalonyl-CoA mutase (MUT). In some severe patients, liver (LT), kidney (KT), or combined liver and kidney (LKT) transplantation has been used as a surgical treatment for MMA. Previous studies have shown that 1-13C-propionate oxidation can be used to measure whole body MUT activity in gene therapy treated and transgenic MMA mice and have encouraged translation to patients, with the aim of using this approach as an outcome measure in future gene therapy clinical trials.We therefore investigated 1-13C-propionate oxidation in 29 patients with isolated MMA (mut0 N=17, mut− N=6, cblA N=5, cblB N=1, age range: 4-41 years), including 3 KT, 1 LT, and 4 LKT recipients, as well as one patient who received a heterotopic liver allograft post KT (hLKT). In addition, 7 healthy volunteers (3 males, 4 females, age range: 21-44 years) and 8 heterozygote controls (age range:35-62 years) were tested to establish test reproducibility. Baseline CO2 production of the participants was measured using an indirect calorimetry cart before sodium 1-13C-propionate was administered as an oral or G-tube bolus, and breath samples were collected serially over 2 hours via disposable breath collection kits to measure 13CO2 enrichment and subsequently propionate oxidation.Propionate oxidation was decreased in all non-transplanted MMA patients compared with controls (p<0.0001). Both LT and LKT recipients demonstrated complete restoration of oxidation rates to control levels (p=NS), while only minimal oxidation was observed in the single hLKT recipient. KT recipients responsive to vitamin B-12 supplementation (N=2) with cblA MMA showed oxidation rates similar to controls. However, the remaining KT recipient, a severely affected mut0 patient carrying two stop mutations, showed almost no metabolism of label.Repeat testing conducted both on healthy volunteers in triplicate and 7 MMA patients (6 mut, 1 cblA) in duplicate supported excellent reproducibility. Notably, similar oxidation rates were observed in 2 patients despite markedly different plasma methylmalonic acid concentrations between one assessment and the next (1741 vs. 2246 and 2260 vs. 719 µmol/L). This suggests that measured 1-13C-propionate oxidation may be a more consistent and reproducible clinical biomarker than plasma metabolites that depend, in part, on protein intake and kidney function.Our results demonstrate that 1-13C-propionate oxidation, measured using breath testing, has great potential as an outcome measure for therapeutic interventions aimed at increasing hepatic MUT activity, such as gene or cell therapy. It is safe, non-radioactive, minimally invasive, well tolerated by very young patients, and can be frequently repeated over the course of treatment to evaluate the magnitude and sustainability of whole body in vivo MUT activity, with every patient's baseline values serving as an individual pre-treatment control. Furthermore, the method is readily applicable to the study of other inborn errors of metabolism affecting propionate oxidation, such as propionic acidemia.