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

Abstract

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.

Early prediction of phenotypic severity in Citrullinemia Type 1

“…Moreover, the observation that residual enzymatic ASS1 activity does not allow to predict the exact NH 4 + max at initial decompensation as well as the exact number of HAEs within the group of patients below the threshold of 8% indicates that – besides residual enzymatic activity – further precipitating factors (e.g., severity of catabolic state, delay between first symptoms and treatment initiation, compliance to long‐term treatment) might determine the severity of metabolic decompensations, as reflected by the scattered data in this group. Another possible explanation might be, that a subset of patients have “leaky mutations” associated with altered kinetic properties (Km mutations) due to decreased substrate binding of the ASS1 enzyme, which needs to be addressed in future research.…”

Mutations in the Human Argininosuccinate Synthetase (ASS1) Gene, Impact on Patients, Common Changes, and Structural Considerations

“…Several patients remained asymptomatic until a catabolic situation (during and after surgery, post‐partum period or fever) [Häberle et al., , ; Enns et al., ; Berning et al., ], which sometimes lead to a fatal hyperammonemia. Most of the mutations found in these patients were proven to have altered kinetic parameters, and under shortage of aspartate, they exhibited life‐threatening decreased activities (Table , [Diez‐Fernandez et al., ]). In these cases, only complete characterization of the recombinant mutant enzyme forms [Berning et al., ; Diez‐Fernandez et al., ] allows to establish any solid correlation.…”

“…However, two important facts must be kept in mind when comparing these results. First, that the determination of enzyme activity was carried out by different methods, either using direct enzyme analysis in liver tissue or an indirect enzyme assay based on the incorporation of 14 C-citrulline [Brown and Cohen, 1959;Saheki et al, 1981;Kleijer et al, 1984;Kobayashi et al, 1994] or using purified recombinant enzyme forms and measuring the direct formation of argininosuccinic acid by tandem mass spectrometry [Berning et al, 2008;Diez-Fernandez et al, 2016]. Second, the time of presentation of the disease does not correlate with the severity of the crisis.…”

“…Most of the mutations found in these patients were proven to have altered kinetic parameters, and under shortage of aspartate, they exhibited life-threatening decreased activities (Table 1, [Diez-Fernandez et al, 2016]). In these cases, only complete characterization of the recombinant mutant enzyme forms [Berning et al, 2008;Diez-Fernandez et al, 2016] allows to establish any solid correlation.…”

Translational Metabolism: A multidisciplinary approach towards precision diagnosis of inborn errors of metabolism in the omics era

“…Effect of different missense mutations in argininosuccinate synthetase (ASS) on its kinetic parameters. Data taken from Diez‐Fernandez et al …”