Abstract
Carbamoyl phosphate synthetase 1 (CPS1) deficiency (OMIM#237300) is a rare inherited disorder due to complete or partial lack of the CPS1 enzyme. Polymyositis is a relatively rare systemic inflammatory autoimmune disease. Here, we report a 59-year-old Japanese woman diagnosed with late-onset CPS1 deficiency during polymyositis treatment. The polymyositis appeared two years before the diagnosis of CPS1 deficiency. Prednisolone (PSL) at 35 mg/day initial dosage, promptly alleviated the symptoms. However, the patient, without apparent cause, suddenly developed confusion progressing to unconsciousness and coma. Upon admission, the patient's plasma ammonia levels were 458 μg/dL (269 μM). Plasma amino acid analysis revealed decreased citrulline levels and elevated glutamine levels. Genetic analysis of CPS1 (OMIM *608307) showed homozygosity for the likely pathogenic variant c.2397G > A (p.Met799Ile), leading to the diagnosis of CPS1 deficiency. The patient responded to pharmacotherapy and continuous hemodialysis. However, the patient experienced hyperammonemia decompensation events while on pharmacotherapy at home, which were successfully managed with emergency treatment and/or hemodialysis. Subsequently, after liver transplantation, the patient's plasma ammonia levels consistently remained at normal. This case illustrates late-onset CPS1 deficiency manifested in an adult treated with PSL for polymyositis, and the cure of its enzyme deficiency by live-donor liver transplantation.