4.5 Article

Kinetic mutations in argininosuccinate synthetase deficiency: characterisation and in vitro correction by substrate supplementation

Journal

JOURNAL OF MEDICAL GENETICS
Volume 53, Issue 10, Pages 710-719

Publisher

BMJ PUBLISHING GROUP
DOI: 10.1136/jmedgenet-2016-103937

Keywords

citrullinemia type 1; argininosuccinate synthetase 1; substrate supplementation; oxaloacetate; hyperammonemia

Funding

  1. Swiss National Science Foundation [310030_153196]
  2. Swiss National Science Foundation (SNF) [310030_153196] Funding Source: Swiss National Science Foundation (SNF)

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Background Citrullinemia type 1 is an autosomal-recessive urea cycle disorder caused by mutations in the ASS1 gene and characterised by increased plasma citrulline concentrations. Of the approximate to 90 argininosuccinate synthetase (ASS) missense mutations reported, 21 map near the substrate (aspartate or citrulline) binding site, and thus are potential kinetic mutations whose decreased activities could be amenable to substrate supplementation. This article aims at characterising these 21 ASS mutations to prove their disease-causing role and to test substrate supplementation as a novel therapeutic approach. Methods We used an Escherichia coli expression system to study all potentially kinetic ASS mutations. All mutant enzymes were nickel-affinity purified, their activity and kinetic parameters were measured using tandem mass spectrometry and their thermal stability using differential scanning fluorimetry. Structural rationalisation of the effects of these mutations was performed. Results Of the characterised mutants, 13 were totally inactive while 8 exhibited decreased affinity for aspartate and citrulline. The activity of these eight kinetic mutations could be rescued to approximate to 10-99% of the wild-type using high l-aspartate concentrations. Conclusions Substrate supplementation raised in vitro the activity of eight citrullinemia type 1 mutations with reduced affinity for aspartate. As a direct translation of these results to the clinics, we propose to further evaluate the use of oxaloacetate, a nitrogen-free aspartate precursor and already available medical food (anti-ageing and brain stimulating, not considered as a drug by the US Food and Drug Administration), in patients with citrullinemia type 1 with decreased aspartate affinity. Although only patients with kinetic mutations would benefit, oxaloacetate could offer a safe novel treatment.

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