The calcitonin-CGRP gene in the infantile hypercalcaemia/Williams-Beuren syndrome

Calcium metabolism in Williams-Beuren syndrome

Increased 1,25-dihydroxyvitamin D levels and decreased basal and calcium-stimulated calcitonin serum levels have been found in children with Williams-Beuren syndrome (WBS). To determine whether isolated or combined disturbances of secretion or action of the calcium-regulating hormones may cause the tendency to hypercalcemia in WBS, we investigated several aspects of calcium metabolism in 27 normocalcemic children and adults, aged 2 to 47 years, with WBS. With the exception of slightly decreased 25-hydroxyvitamin D and slightly increased calcitonin in serum, all measured basal indexes of calcium and bone metabolism, including the serum levels of intact parathyroid hormone and 1,25-dihydroxyvitamin D, were comparable to control values. Total and extractable calcitonin, the latter representing the monomeric and biologically important form of the hormone, showed the same relative increase after a low-dose calcium infusion in patients and control subjects, indicating a normal capacity of the calcitonin-producing C cells of the thyroid gland in WBS. Furthermore, exogenous parathyroid hormone induced a normal response of 1,25-dihydroxyvitamin D, cyclic adenosine monophosphate, and phosphate excretion, indicating a normal response of the renal 25-hydroxyvitamin D-1 alpha-hydroxylase and the renal receptor-adenylate cyclase system to parathyroid hormone. These findings suggest that neither deficient calcitonin secretion nor increased renal sensitivity to parathyroid hormone is a feature of WBS in normocalcemic patients.

Myopathy in Williams-Beuren syndrome

Williams-Beuren syndrome (WBS) is a disorder of unknown aetiology. The classical features of the syndrome include a typical ('elfin') facies, mental retardation and heart defects. Myopathy has not so far been part of the spectrum of WBS. We studied six patients with WBS aged 3-25 years, five of whom showed clinical and morphological evidence of myopathy. The clinical manifestations of myopathy included hypotonia in infancy, walking delay, joint contractures, scoliosis, and increased exhaustion on exertion. These symptoms were present in variable expression but part of a typical postural pattern. Examination of muscle biopsies showed lipid storage in four patients and increased variability of fibre size in three. In one patient a muscle biopsy gave normal results. Biochemical investigation in four patients with morphological evidence of lipid storage in muscle revealed muscle carnitine deficiency in three. In addition, enzyme activities of fatty acid beta-oxidation were low in one of two specimens tested. It is concluded that a clinically relevant myopathy is part of the multi-system manifestation of WBS and a clinical trial of carnitine supplementation is justified.

Characterization of the calcitonin/CGRP gene in Williams syndrome

We have investigated the possibility of mutations in the calcitonin/calcitonin gene related peptide (CGRP) gene in children with Williams syndrome. Involvement of the calcitonin/CGRP gene in Williams syndrome is postulated on the basis that Williams syndrome children often have infantile hypercalcemia and deficient expression of calcitonin, a hormone that lowers serum calcium levels. To test the hypothesis that mutations in the calcitonin/CGRP gene might be responsible for the reduced calcitonin levels, we examined the calcitonin/CGRP gene structure in Williams syndrome children. Analysis of white blood cell DNA by Southern blot hybridizations in 5 individuals did not show any detectable large deletions or rearrangements in the calcitonin/CGRP gene locus. The possibility of small deletions or point mutations within the exon encoding the mature calcitonin hormone is unlikely based on ribonuclease protection assays with patient DNA amplified by the polymerase chain reaction (PCR) technique. These findings suggest that the calcitonin deficiency might be due either to mutations elsewhere in the gene or to defects in the cellular machinery needed for calcitonin synthesis and/or secretion.