Analysis of Enzyme Activity and Cellular Function for the N80S and S480F Asparagine Synthetase Variants Expressed in a Child with Asparagine Synthetase Deficiency

Int J Mol Sci. 2022 Dec 29;24(1):559. doi: 10.3390/ijms24010559.


Asparagine Synthetase Deficiency (ASNSD) is a disease caused by mutations in asparagine synthetase (ASNS). Newborns exhibit microcephaly, intractable epileptic-like seizures, progressive brain atrophy, and axial hypotonia. ASNSD results in global developmental delays and premature death. The present report describes a 9-year-old child who is a compound heterozygote with ASNS mutations c.1439C > T and c.239A > G leading to variants p.S480F and p.N80S, respectively. When grown in a complete culture medium, primary fibroblasts from the child contained ASNS mRNA and protein levels similar to an unrelated wild-type fibroblast cell line. When the child’s fibroblasts were cultured for up to 72 h in a medium lacking asparagine, proliferation was reduced by about 50%. Purification of ASNS proteins harboring either the S480F or the N80S substitution had reduced enzymatic activity by 80% and 50%, respectively. Ectopic expression of either variant in ASNS-null Jensen rat sarcoma (JRS) cells did not support proliferation in the absence of medium-supplied asparagine, whereas expression of wild-type enzyme completely restored growth. These studies add to the list of pathogenic ASNS variants and use enzyme activity and protein expression in ASNS-null cells to expand our knowledge of the biological impact of mutations in the ASNS gene.

Keywords: amino acid; brain metabolism; cell proliferation; epilepsy; inborn error of metabolism; metabolism.

Publication types

  • Case Reports

MeSH terms

  • Asparagine / genetics
  • Aspartate-Ammonia Ligase* / genetics
  • Atrophy
  • Child
  • Humans
  • Intellectual Disability* / genetics
  • Microcephaly* / genetics
  • Nervous System Malformations*
  • Seizures / genetics


  • Asparagine
  • Aspartate-Ammonia Ligase
  • ASNS protein, human