Diagnosis of citrullinemia type 1 carriage after identification of mild citrullinemia on neonatal screening

Asymptomatic ASS1 carriers with high blood citrulline levels

INTRODUCTION

Citrullinemia Type 1 (CTLN1) is an autosomal recessive disorder caused by variants in the ASS1 gene. This study intends to clarify the etiology of false positives in newborn screening for citrullinemia.

METHOD

Newborns who had elevated dried-blood spot citrulline levels were enrolled, and medical records were reviewed retrospectively. Common ASS1 variants were screened using high-resolution melting analysis.

RESULT

Between 2011 and 2021, 130 newborns received confirmatory testing for citrullinemia, 4 were found to be patients for CTLN1; 11 were patients with citrin deficiency; and 49 newborns were confirmed to be carrying one pathogenic ASS1 variant. The incidence of CTLN1 was 1 in 188,380 (95% confidence interval: 1 in 73,258 to 1 in 484,416). All ASS1 variants studied in this cohort were located in exons 11 to 15, which encode the tetrameric interface regions of the ASS1 protein. Among 10 ASS1 carriers with elevated citrulline levels and complete sequence data, four (40%) revealed additional non-benign ASS1 variants; in contrast, only 2 of the 26 controls (7.7%), with normal citrulline levels, had additional ASS1 variants.

CONCLUSION

Heterozygote ASS1 variants may lead to a mild elevation of blood citrulline levels: about 2-6 times the population mean. Molecular testing and family studies remain critical for precise diagnosis, genetic counseling, and management.

The diagnostic challenge of mild citrulline elevation at newborn screening

Citrulline is a target analyte measured at expanded newborn screening (NBS) and its elevation represents a biomarker for distal urea cycle disorders and citrin deficiency. Altered ratios of citrulline with other urea cycle-related amino acids are helpful for the differential diagnosis. However, the use of cut-off values in screening programmes has raised the issue about the interpretation of mild elevation of citrulline levels detected at NBS, below the usual range observed in the "classical/severe" forms of distal urea cycle disorders and in citrin deficiency. Herein, we report ten subjects with positive NBS for a mild elevation of citrulline (<100 μmol/L), in whom molecular investigations revealed carriers status for argininosuccinate synthase deficiency, a milder form of argininosuccinate lyase deficiency and two other diseases, lysinuric protein intolerance and dihydrolipoamide dehydrogenase deficiency, not primarily affecting the urea cycle. To guide the diagnostic process, we have designed an algorithm for mild citrulline elevation (<100 μmol/L) at NBS, which expands the list of disorders to be included in the differential diagnosis.