This 3-part, Phase 1/2 study is designed to characterize the safety, tolerability, and pharmacological activity (as assessed by biomarker measurements) and to determine the selected dose of mRNA-3927 in participants with genetically confirmed propionic acidemia (PA). After establishing a dose with an acceptable safety and pharmacodynamic (PD) response for participants ≥1 year of age in Part 1, participants will be enrolled in Part 2 (which will serve as the pivotal study) to allow for determination of the efficacy, safety, and PD of mRNA-3927. Part 3 will evaluate the safety, efficacy and PD response of mRNA-3927 in infants (\<1 year of age).