Clinical and structural insights into potential dominant negative triggers of proximal urea cycle disorders.

Despite biochemical and genetic testing being the golden standards for identification of proximal urea cycle disorders (UCDs), genotype-phenotype correlations are often unclear. Co-occurring partial defects affecting more than one gene have not been demonstrated so far in proximal UCDs. Here, we analyzed the mutational spectrum of 557 suspected proximal UCD individuals. We probed oligomerizing forms of NAGS, CPS1 and OTC, and evaluated the surface exposure of residues mutated in heterozygously affected individuals. BN-PAGE and gel-filtration chromatography were employed to discover protein-protein interactions within recombinant enzymes. From a total of 281 confirmed patients, only 15 were identified as "heterozygous-only" candidates (i.e. single defective allele). Within these cases, the only missense variants to potentially qualify as dominant negative triggers were CPS1 p.Gly401Arg and NAGS p.Thr181Ala and p.Tyr512Cys, as assessed by residue oligomerization capacity and surface exposure. However, all three candidates seem to participate in critical intramolecular functions, thus, unlikely to facilitate protein-protein interactions. This interpretation is further supported by BN-PAGE and gel-filtration analyses revealing no multiprotein proximal urea cycle complex formation. Collectively, genetic analysis, structural considerations and in vitro experiments point against a prominent role of dominant negative effects in human proximal UCDs.

The Effect of S-Adenosylmethionine on Self-Mutilation in a Patient with Lesch-Nyhan Disease.

BACKGROUND: Lesch-Nyhan disease (LND) is an X-chromosomal disorder of purine metabolism characterized by hyperuricemia, dystonia, and self-mutilation, leading to an extremely high burden of disease in affected patients and families. Although allopurinol therapy can control hyperuricemia, it has no effect on self-mutilation and neurological symptoms. Single reports describe a beneficial effect of S-adenosylmethionine (SAM) on the neurological symptoms, which motivated us to evaluate this alternative treatment. METHODS: We performed a double-blind placebo-controlled trial to analyze the effects of SAM on self-mutilation attempts in a male patient affected by LND. The trial lasted for 282 days and comprised three alternating verum and placebo periods of 50 days each. The mother of the patient recorded attempts of self-mutilation during the entire trial. RESULTS: While verum and placebo were both well tolerated, a total of 1,762 events of self-mutilation were recorded, of which 1,281 events were in the placebo period and 481 in the verum period. The daily mean of events was 8.6 with placebo and 4.5 with SAM corresponding to a 50 % decrease in self-mutilation events under SAM treatment (p < 0.05). CONCLUSION: The results of this double-blind placebo-controlled single-case trial suggest that SAM can have a beneficial effect on self-mutilation in patients with LND, possibly by replenishing the purine pool in affected brain cells.