Mild/Variant cases of argininosuccinic acid lyase deficiency by newborn screening using argininosuccinic acid as a primary marker

Background: Many newborn screening (NBS) programs include argininosuccinic acid lyase deficiency (ASLD), a treatable urea cycle disorder, which can present as severe neonatal and late-onset forms. Citrulline (CIT) may not detect all cases of ASLD if used as primary NBS marker. Since 2004, Tennessee (USA) NBS has used argininosuccinic acid (ASA) as primary analyte and CIT as secondary for ASLD. We herein report 3 mild/variant cases with positive NBS for ASA elevation with later confirmed diagnosis.
Case study and results: 1) NBS on 9-day-old girl reveals ASA 5.13 (cutoff <0.33 µmol/L), ASA/arginine (ARG) 0.14 (cutoff<0.13), CIT 111.33 (cutoff<62 µmol/L). Confirmatory plasma amino acids (PAA) revealed ASA 37 (normal (nl) <2 µmol/L), CIT 79 (nl 10-34 µmol/L). Heterozygous variants, c.35G>A (p. Arg12 Gln), pathogenic, c.655G>C (p. Glu219Gln), likely pathogenic in ASL gene confirmed ASLD. At 4-years-old she is growing and developing well. 2) NBS on 2-day-old girl revealed ASA 0.98 (cut-off <0.29) with nl CIT. Confirmatory PAA showed ASA 60 (nl <2 µmol/L), CIT 161 (nl 10-34 µmol/L). ASL gene testing revealed one pathogenic missense variant c. 857A>G (p. Gln288Arg). She is now 5-years-old and healthy. 3) NBS on 2-day-old boy reveals ASA 0.98 (cutoff<0.81 µmol/L), ASA/Arg 0.14 (cutoff<0.21) with nl CIT. Confirmatory PAA showed ASA 12 (nl <2 µmol/L), CIT 67 nl (10-34 µmol/L). Urinary ASA 1053 (nl <77 µmol/L). Molecular testing on ASL gene reveals one pathogenic missense variant c. 35G>A (p. Arg12 Gln).
Conclusion and discussion All 3 cases are biochemically diagnosed and treated including diet restriction due to consistent ASA and CIT elevations. These cases illustrate ASA should be used as primary NBS marker over CIT for ASLD. If CIT was used as the only biomarker for ASLD in NBS, the two variant cases might have been called falsely negative. Our observation also reiterates that urine ASA is more sensitive than plasma as confirmatory testing for ASLD.