A Prospective Study of Pediatric Participants up to 16 Years of Age With Methylmalonic Acidemia (MMA) Due to Mutations in the MMUT Gene

Inclusion Criteria: 1. Aged ≤16 years at screening visit 2. With or without previous liver (or combined liver/kidney) transplantation at time of screening (note: number of transplanted participants capped at n=15) 3. Confirmed laboratory diagnosis of Isolated MMA caused by mutations in the MMUT gene (NOTE: if a historical genetic mutational analysis report was available at Screening visit but was not from a CLIA/ISO15189 approved laboratory, a confirmatory sample will be taken during the study. However the original lab report will be adequate for study eligibility consideration).. 4\. Severe MMA phenotype. * For untransplanted participants, all of the following criteria (a-c) must be met to qualify as "severe" MMA phenotype: 1. Serum methylmalonic acid (sMMA) level of \>100 µmol/L at the Screening visit 2. An unscheduled ER visit, hospitalization or requirement for use of the sick day diet regimen in the 12 months prior to the screening visit 3. Considered to potentially require future liver transplantation to improve metabolic stability in accordance with MMA transplantation guidelines (Baumgartner 2014, Forny 2021, Sen 2023) * For participants with previous liver or combined liver and kidney transplantation, the phenotype of the participant will be judged by the Investigator to be severe if both of the following were applicable prior to transplantation: 1. Pre-transplant serum methylmalonic acid (sMMA) level of \>100 µmol/L AND 2. Transplantation was conducted to improve metabolic stability in accordance with MMA transplantation guidelines (Baumgartner 2014, Forny 2021, Sen 2023). NOTE: sMMA pre-transplant measure must have been obtained within 6 months prior to transplant. An alternative blood MMA concentration may be used (e.g. plasma MMA, or dry blood spot), if sMMA is unavailable. Details of the assay used must be provided and additional samples will be collected during the study to allow comparison to sMMA results. Exclusion Criteria: 1. Participant/parent/legal guardian/caregiver not willing to consent to participate 2. Current participation in another interventional or therapeutic study 3. Prior participation in a gene therapy clinical trial including mRNA therapy 4. Participants who, in the opinion of the Site Investigator, would be unable or unsuitable to participate in the demands of the study, for example but not limited to participants unable to travel to protocol study visits or participants under palliative care. 5. For participants who are post-liver transplant only, participant will be excluded if no prior pre-liver transplant measurements of blood MMA are available.