Open-Label Study of mRNA-3927 in Participants With Propionic Acidemia

Inclusion Criteria:

Participants ≥1 year of age are eligible to be included in the study only if all of the following criteria apply:

• ≥ 8 years of age at the time of consent/assent if enrolled as 1 of the first 2participants in Part 1.

• ≥1 year of age at the time of consent/assent if enrolled after the first 2participants in Part 1.

• Confirmed diagnosis of PA based on diagnosis by molecular genetic testing viacentral laboratory (PCCA and/or PCCB mutations).

• Part 2 only: At least one documented MDE in the 12-month period before consent.

Participants <1 Year of Age :

• Identification by newborn screening shortly after birth or having suspected PA bypresenting with a spectrum of metabolic symptoms, and having a sibling diagnosedwith PA. Participant may enter the Screening Period while awaiting genetic testingresults, provided that all other eligibility criteria are met but would not beenrolled until diagnosis of PA is confirmed.

• For infants in the neonatal intensive care unit (NICU) only: ≥37 weeks gestationalage at the time of birth without other conditions/comorbidities that in the opinionof the Investigator may interfere with the interpretation of study results.

• Body weight ≥3 kilograms (kg) at Screening.

• At least 1 documented PA-related event prior to Screening defined as the followingcriteria:

• Clinical signs of metabolic deterioration consistent with PA (for example,vomiting, not feeding well/poor suck, heavy breathing, lethargy, absence ofproper perfusion, abnormal movements including bicycling, abnormal tone, lowbody temperature, seizure[s]), OR

• Meeting the criteria of MDE definition, OR

• Evidence of laboratory abnormalities as evidenced by at least one of thefollowing:

• Metabolic acidosis with elevated anion gap.

• Acute hyperammonemia.

• Neutropenia or thrombocytopenia.

Exclusion Criteria:

Participants of all ages are excluded from the study if during Screening any of the following criteria apply:

• Any individual with laboratory abnormalities considered to be clinically significant (for example, markedly out of range, associated with clinical symptoms) in theInvestigator or Sponsor's opinion that could interfere with or limit theparticipation in the study.

• Estimated glomerular filtration rate (eGFR) <30 milliliters (mL)/minute/1.73 squaremeter (m^2) for participants of all ages receiving chronic dialysis.

• History of organ transplantation or planned organ transplantation during the periodof study participation.

• Corrected QT interval (QTc) >480 milliseconds (ms) using Bazett's correction.

• Grade 3 or 4 heart failure according to the Modified Ross Heart FailureClassification for Children or the New York Heart Association Classification.

• Pregnant or breastfeeding.

• Other clinically significant conditions that in the Investigator's opinion couldinterfere with the safety of the participant, the interpretation of study results,or limit the participation in the study.