A Four-Year Prospective Pilot Study of Newborn Screening for Late-Onset Proximal Urea-Cycle Disorders in Hyogo Prefecture in Japan.

Proximal urea-cycle disorders (PUCDs), including N-acetylglutamate synthase deficiency (NAGSD), ornithine transcarbamylase deficiency (OTCD), and carbamoyl phosphate synthase 1 deficiency (CPS1D), cause hyperammonemia and impair neurological outcomes. Early detection of late-onset forms allows presymptomatic intervention to prevent hyperammonemia; however, reliable newborn screening (NBS) markers are lacking. This prospective pilot study in Hyogo Prefecture, Japan, evaluated hypocitrullinemia as a screening marker for late-onset PUCDs. Newborns with citrulline levels below the 0.05th percentile on NBS between June 2020 and May 2024 were enrolled in the study. Confirmatory diagnosis of PUCDs was performed using plasma amino acids, urinary organic acids, and genetic testing. During the first period (101,172 newborns), 11 newborns exhibited hypocitrullinemia; 10 underwent further evaluation. One newborn was diagnosed with CPS1D (compound heterozygous CPS1 variants); another was later diagnosed with Leigh syndrome. The remaining eight cases were false positives, often associated with prematurity, poor feeding, or gastrointestinal disorders. A second dried blood spot (DBS) card protocol was introduced in the second period (34,694 newborns), reducing false positives. One neonatal-onset OTCD case was detected, and citrulline levels were normalized in six of the seven other cases. In summary, hypocitrullinemia can identify presymptomatic PUCDs, and requesting a second DBS card reduces false positives, supporting its feasibility for incorporation into NBS programs.