Exp Clin Endocrinol Diabetes 2014; 122 - P038
DOI: 10.1055/s-0034-1372055

CaSR mutants disrupting the disulfide bond between C582 and C568 in the cysteine-rich domain of the CaSR do not act as dominant negative mutants

S Letz 1, U Elbelt 2, CJ Strasburger 2, C Haag 3, E Schulze 3, K Frank-Raue 3, F Raue 3, B Mayr 1, C Schöfl 1
  • 1Universitätsklinikum Erlangen, Division of Endocrinology and Diabetes, Department of Medicine I, Erlangen, Germany
  • 2Charité, Department of Endocrinology and Metabolic Diseases, Berlin, Germany
  • 3Endocrine Practice, Heidelberg, Germany

Context: Heterozygous inactivating mutations of the CaSR result in familial hypocalciuric hypercalcemia (FHH), whereas homozygous inactivating mutations lead to neonatal severe hyperparathyroidism (NSHPT). In one NSHPT patient, however, only a heterozygous mutation of cysteine 582, which forms a disulfide bond with cysteine 568 was reported. A dominant negative effect of the mutant on the wildtype receptor can be suspected, possibly through aberrant behavior of the free cysteine 568. We recently found a heterozygous CaSR mutation at C582 in an adult patient with FHH.

Subjects and Results: A 44 year old patient presented with persistent hypercalcemia of 2.8 mM after revision parathyroidectomy. Laboratory studies revealed a calcium/creatinine clearance ratio of 0.008 and a heterozygous C582R mutation in the CaSR gene. The patient was briefly treated with cinacalcet, which increased urinary calcium excretion from 2.3 mmol/d to 4.2 mmol/d, but did not significantly change serum calcium levels.

Functional in vitro analyses of the C582R mutant revealed a complete loss of function. Coexpression with wildtype CaSR, however, restored signaling to a large extent (EC50 C582R/wt 3.3 mM; EC50 wt 3.0 mM). In addition the allosteric calcimimetic NPS-R568 was able to augment signaling function in the C582R mutant both alone and coexpressed with wildtype CaSR. Furthermore a heterozygous mutation of C568, the disulfide bonding partner of C582 also leads to FHH and very similar functional consequences in vitro.

Conclusion: Mutations that disrupt the disulfide bond between C582 and C568 in the cysteine-rich domain of the CaSR do not act as dominant negative mutants. The loss of function is probably caused by the loss of stability in the internal architecture of the cysteine-rich domain. External stabilization through dimerization with wildtype receptor or binding of an allosteric agonist can partially restore signaling function.