Cinacalcet Treatment in an Adolescent With Concurrent 22q11.2 Deletion Syndrome and Familial Hypocalciuric Hypercalcemia Type 3 Caused by AP2S1 Mutation

CONTEXT

The 22q11.2 deletion syndrome (DS) is a common multiple anomaly syndrome in which typical features include congenital heart defects, facial dysmorphism, and palatal anomalies. Hypocalcemia due to hypoparathyroidism is a common endocrine manifestation resulting from variable parathyroid hypoplasia, but hypercalcemia has not previously been reported in 22q11.2 DS.

CASE DESCRIPTION

Our patient is a 16-year-old adolescent male with dysmorphic facial features and delayed motor and speech development. At 2 years of age, 22q11.2 DS was confirmed by fluorescence in situ hybridization. In contrast to hypoparathyroidism that is usually seen in 22q11.2 DS, this patient had early childhood-onset hypercalcemia with inappropriately high PTH levels and hypocalciuria. Genomic DNA was obtained from the proband and screened for calcium-sensing receptor (CASR) mutations with negative results. No parathyroid tissue could be localized by imaging or surgical exploration. As a result of symptomatic hypercalcemia, the patient was treated with a calcimimetic (cinacalcet). During the treatment, plasma calcium normalized with mild symptoms of hypocalcemia. After discontinuation of cinacalcet, calcium returned to high pretreatment levels. Further DNA analysis of adaptor protein-2 σ subunit (AP2S1) showed a heterozygous missense mutation c.44 G>T, resulting in a p.R15L substitution; the mutation was absent in the healthy parents and two siblings.

CONCLUSIONS

Hypercalcemia in our patient with 22q11.2 DS could be explained by the de novo mutation in AP2S1. Identification of a genetic cause for hypercalcemia is helpful in guiding management and avoiding unnecessary treatment.

Codon Arg15 mutations of the AP2S1 gene: common occurrence in familial hypocalciuric hypercalcemia cases negative for calcium-sensing receptor (CASR) mutations

CONTEXT

Familial hypocalciuric hypercalcemia (FHH) is an autosomal dominant disorder with three known subtypes: FHH1, FHH2, and FHH3. About 65% of FHH cases are FHH1, caused by inactivating mutations of the calcium-sensing receptor (CASR) gene. FHH3 was recently found to be caused by codon Arg15 (p.R15) mutations in the adaptor-related protein complex 2, σ-2 subunit that interacts with the CaSR and is encoded by the AP2S1 gene.

OBJECTIVE

The objective of the study was to assess the prevalence of AP2S1 mutations, and describe the phenotype of FHH3, in an independent cohort of FHH subjects lacking CASR mutations.

PATIENTS AND METHODS

Thirty-nine patients presenting with some combination of hypercalcemia, hypermagnesemia, nonsuppressed serum PTH levels, and reduced urinary calcium excretion were studied. Exon 2 of the AP2S1 gene was PCR amplified from patient genomic DNA and Sanger sequenced. The presence of p.R15 mutations was confirmed by restriction enzyme analysis.

RESULTS

Five of the 39 subjects had AP2S1 p.R15 mutations, a frequency of 13%. The three recurrent mutations reported previously were all found in our cohort (p.R15C in two, p.R15L in two, and p.R15H in one subject). The FHH3 phenotype did not differ materially from that of FHH1 due to CASR mutations.

CONCLUSIONS

The results affirm that a significant number of patients suspected of having FHH but proven negative for CASR mutation have AP2S1 p.R15 mutations. Screening for AP2S1 p.R15 mutations in such cases should be considered, given the clinical benefits (avoiding unnecessary parathyroidectomy) that have already been demonstrated for CASR screening in FHH1.