Moderate-to-severely impaired cardiac function with LVEF \< 45% by ECHO.

Clinically significant arrhythmia by Holter monitor.

QTcF \> 450 msec

Moderate to severe chronic kidney disease with estimated glomerular filtration rate (eGFR) \< 60 mL/min/1.73m2.

Exposure to any investigational therapy, apart for a COVID-19 vaccine, within the past 6 months prior to study entry.

Exposure to gene therapy for PA or MMA at any time prior to study entry.

History of organ transplantation (Part A and B only)

History of severe allergic or anaphylactic reactions to any of the components of HST5040.

Study of HST5040 in Subjects With Propionic or Methylmalonic Acidemia