Precision medicine in rare disease: Mechanisms of disparate effects of N -carbamyl- l -glutamate on mutant CPS1 enzymes

Abstract

This study documents the disparate therapeutic effect of N-carbamyl-L-glutamate (NCG) in the activation of two different disease-causing mutants of carbamyl phosphate synthetase 1 (CPS1). We investigated the effects of NCG on purified recombinant wild-type (WT) mouse CPS1 and its human corresponding E1034G (increased ureagenesis on NCG) and M792I (decreased ureagenesis on NCG) mutants. NCG activates WT CPS1 sub-optimally compared to NAG. Similar to NAG, NCG, in combination with MgATP, stabilizes the enzyme, but competes with NAG binding to the enzyme. NCG supplementation activates available E1034G mutant CPS1 molecules not bound to NAG enhancing ureagenesis. Conversely, NCG competes with NAG binding to the scarce M792I mutant enzyme further decreasing residual ureagenesis. These results correlate with the respective patient’s response to NCG. Particular caution should be taken in the administration of NCG to patients with hyperammonemia before their molecular bases of their urea cycle disorders is known.

N‐carbamoylglutamate‐responsive carbamoyl phosphate synthetase 1 (CPS1) deficiency: A patient with a novel CPS1 mutation and an experimental study on the mutation's effects

“…Indeed, it has been used in single cases of ornithine transcarbamylase deficiency and citrullinemia type 1 . Observations in CPS1D indicated that the deficiency must be partial to be NCG‐sensitive, and suggested that the specific mutations found in each patient can determine the degree of response to NCG . The homozygous p.(Pro1211Arg) novel mutation found in our patient is shown here to cause partial deficiency and was associated with partial susceptibility to NCG.…”

“…Thus, a corollary of all the above is that potential chaperoning effects of NCG should be kept in mind because they may be crucial in determining NCG effectiveness in CPS1D. As mutation effects on CPS1 stability may not be easily predictable, and given the poor general prognosis of CPS1D, it appears reasonable to give patients a clinical trial of NCG to assess the clinical responsiveness to this orphan drug, taking due caution with monitoring for potential negative effects of NCG as observed in a single CPS1D patient …”

“…Although advocated for many years, there are few reports on the use of NCG in CPS1D . Results in five late‐onset CPS1D patients were encouraging, although a paradoxical negative biochemical effect in one patient led to propose individual CPS1 mutations‐based counselling . We report here a patient with genetically characterized partial but severe CPS1D due to homozygosity for the novel c.3632C>G, p.(Pro1211Arg) mutation that was controlled for a long period with merely oral NCG, arginine, and moderate protein restriction.…”

“…11,12 Results in five lateonset CPS1D patients were encouraging, 12 although a paradoxical negative biochemical effect in one patient led to propose individual CPS1 mutations-based counselling. 13 We report here a patient with genetically characterized partial but severe CPS1D due to homozygosity for the novel c.3632C>G, p.(Pro1211Arg) mutation that was controlled for a long period with merely oral NCG, arginine, and moderate protein restriction. Our case, with normal neurodevelopmental outcome, illustrates how a CPS1 mutation having no apparent direct effects on NAG binding to the enzyme could be sensitive to NCG administration, supporting the convenience of clinically testing the efficacy of NCG in every patient with partial CPS1D.…”

Cell and Gene Therapy for Carbamoyl Phosphate Synthetase 1 Deficiency

“…Recently, basic studies have shown that N-carbamyl-L-glutamate (NCG), the chemical analog of CPS1's essential activator, can improve and protect the function of mutant CPS1. Clinical studies have also shown that NCG had beneficial effects on CPS1-deficient patients with some special mutations, such as the p.Glu1034Gly mutation [3]. The treatment outcomes depends on disease severity.…”