Identification of a novel calcium-sensing receptor gene mutation causing familial hypocalciuric hypercalcemia by single-strand conformation polymorphism analysis.

Calcium-sensing receptor gene (CASR) mutations that alter the function of the G protein coupled Ca (2+)-sensing receptor are reported in patients with familial hypocalciuric hypercalcemia (FHH), autosomal dominant hypocalcemia (ADH), and neonatal severe hyperparathyroidism (NSHPT). In search for novel disease causing mutations in the CASR gene, we screened exons 2 - 7 of the CASR gene of a family with FHH using single-strand conformation polymorphism analysis. We identified a novel CASR mutation (c.518 T > C; L173 P) in exon 4 encoding for the extracellular domain of the Ca (2+)-sensing receptor. This region seems to represent a hot spot within the CASR gene with at least 13 reported disease causing mutations thus far.     

Severe hypercalcemia in a 9-year-old Brazilian girl due to a novel inactivating mutation of the calcium-sensing receptor

Familial hypocalciuric hypercalcemia (FHH) and neonatal severe hyperparathyroidism (NSHPT) are consequent to inactivating mutations of the calcium-sensing receptor (CaR) gene. FHH is usually associated with heterozygous inactivating mutations of the CaR gene, whereas NSHPT is usually due to homozygous inactivation of the CaR gene. FHH is generally asymptomatic and is characterized by mild to moderate lifelong hypercalcemia, relative hypocalciuria, and normal intact PTH, whereas individuals with NSHPT frequently show life-threatening hypercalcemia. In this study, we report a novel inactivating mutation of the CaR gene, identified in a 9-yr-old Brazilian girl who was found to be severely hypercalcemic during investigation of a 6-month history of headaches and vomits. Direct sequencing of the CaR gene from this patient showed a novel homozygous mutation (L13P) in exon 2. Functional characterization by intracellular calcium measurement by fluorometry showed that the mutant receptor had a dose-response curve shifted to the right relative to that of wild type. The proband's consanguineous parents, who had mild asymptomatic hypercalcemia, showed the same mutation in the heterozygous form. The mutation described in this study is the inactivating missense mutation present at the most N-terminal end among the known CaR missense mutations. This study reinforces the fact that patients with homozygous inactivation of the CaR gene may present with severe hypercalcemia in different phases of life.     

Identification and functional characterization of novel calcium-sensing receptor mutations in familial hypocalciuric hypercalcemia and autosomal dominant hypocalcemia

Familial hypocalciuric hypercalcemia (FHH), neonatal severe hyperparathyroidism (NSHPT), and autosomal dominant hypocalcemia (ADH), in which calcium homeostasis is disordered, are associated with mutations in the calcium-sensing receptor (CASR). Six unrelated kindreds with FHH and/or NSHPT and two unrelated kindreds with ADH were studied. Direct sequence analysis of the exons of the CASR gene identified heterozygous mutations in six of the kindreds with FHH and in one of those with ADH. We performed functional analyses on the novel missense and insertion/frameshift mutants by transiently transfecting wild-type and mutant CASRs tagged with a c-Myc epitope in human embryonic kidney (HEK293) cells. All mutant receptors were expressed at a similar level to that of the wild type; however, whereas mutants R220W and A835T (the ADH mutant) were fully glycosylated and were visualized on the cell surface, glycosylation of mutants G549R and C850-851 ins/fs was impaired, resulting in reduced cell surface staining. In fura-2-loaded HEK293 cells expressing the wild-type or mutant receptors, the inactivating R220W mutant produced a significant shift to the right relative to the wild-type CASR in the cytosolic calcium response to increasing extracellular calcium concentrations and the G549R and C850-851 ins/fs mutants were without detectable activity. The activating A835T mutation resulted in a shift to the left in the cytosolic calcium response to extracellular calcium concentrations relative to the wild type. Our studies have identified novel CASR mutations that alter the function of the CASR in several different ways.     

Identification and functional characterization of a novel mutation in the calcium-sensing receptor gene in familial hypocalciuric hypercalcemia: modulation of clinical severity by vitamin D status

CONTEXT: Familial hypocalciuric hypercalcemia (FHH) is a benign condition associated with heterogeneous inactivating mutations in the calcium-sensing receptor (CASR) gene. OBJECTIVE: The objective of the study was to identify and characterize a CASR mutation in a moderately hypercalcemic, hyperparathyroid individual and his family and assess the influence of vitamin D status on the clinical expression of the defect. SUBJECTS: We studied a kindred with FHH, in which the proband (a 34-yr-old male) was initially diagnosed with primary hyperparathyroidism due to frankly elevated serum PTH levels. METHODS: CASR gene mutation analysis was performed on genomic DNA of the proband and family members. The mutant CASR was functionally characterized by transient transfection studies in kidney cells in vitro. RESULTS: A novel heterozygous mutation (F180C, TTC>TGC) in exon 4 of the CASR gene was identified. Although the mutant receptor was expressed normally at the cell surface, it was unresponsive with respect to intracellular signaling (MAPK activation) to increases in extracellular calcium concentrations. The baby daughter of the proband presented with neonatal hyperparathyroidism with markedly elevated PTH. Vitamin D supplementation of both the proband and the baby resulted in reduction of serum PTH levels to the normal range. The serum calcium level remained at a constant and moderately elevated level. CONCLUSION: The identification of a novel CASR gene mutation established the basis of the hypercalcemia in the kindred. Concomitant vitamin D deficiency modulates the severity of the presentation of FHH.     

Molecular genetic analysis of the calcium sensing receptor gene in patients clinically suspected to have familial hypocalciuric hypercalcemia: phenotypic variation and mutation spectrum in a Danish population

CONTEXT: The autosomal dominantly inherited condition familial hypocalciuric hypercalcemia (FHH) is characterized by elevated plasma calcium levels, relative or absolute hypocalciuria, and normal to moderately elevated plasma PTH. The condition is difficult to distinguish clinically from primary hyperparathyroidism and is caused by inactivating mutations in the calcium sensing receptor (CASR) gene. OBJECTIVE: We sought to define the mutation spectrum of the CASR gene in a Danish FHH population and to establish genotype-phenotype relationships regarding the different mutations. DESIGN AND PARTICIPANTS: A total of 213 subjects clinically suspected to have FHH, and 121 subjects enrolled as part of a family-screening program were studied. Genotype-phenotype relationships were established in 66 mutation-positive index patients and family members. MAIN OUTCOME MEASURES: We determined CASR gene mutations, and correlating levels of plasma calcium (albumin corrected), ionized calcium (pH 7.4), and PTH were measured. RESULTS: We identified 22 different mutations in 39 FHH families. We evaluated data on circulating calcium and PTH for 11 different mutations, representing a spectrum of clinical phenotypes, ranging from calcium concentrations moderately above the upper reference limit, to calcium levels more than 20% above the upper reference limit. Furthermore, the mean plasma PTH concentration was within the normal range in eight of 11 studied mutations, but mild to moderately elevated in families with the mutations p.C582Y, p.C582F, and p.G553R. CONCLUSIONS: The present data add 19 novel mutations to the catalog of inactivating CASR mutations and illustrate a variety of biochemical phenotypes in patients with the molecular genetic diagnosis FHH.     

Familial hypocalciuric hypercalcemia and other disorders with resistance to extracellular calcium.

Cloning of the CaR has increased understanding of the normal control of mineral ion homeostasis and has clarified the pathophysiology of PTH-dependent hypercalcemia. Cloning of the CaR has enabled identification of FHH and NSHPT as inherited conditions with generalized resistance to Ca2+o, which is caused in many cases by inactivating mutations in the CaR gene. In most kindreds with FHH, there is resetting of Ca2+o to a mildly elevated level that does not require an increase in the circulating level of PTH above the normal range to maintain it. FHH is not accompanied by the usual symptoms, signs, and complications of hypercalcemia. The kidney participates in the genesis of the hypercalcemia in FHH by avidly reabsorbing Ca2+; consequently, there is no increased risk of forming urinary calculi in most cases. Generally, there is no compelling rationale for attempting to lower the level of Ca2+o in these patients to a nominal normal level. In contrast, in primary hyperparathyroidism, the Ca2+o resistance is limited to the pathologic parathyroid glands, and the rest of the body suffers the consequences of high circulating levels of calcium, PTH, or both. In this condition, removal of the offending parathyroid glands is often the treatment of choice. Parathyroidectomy may also be appropriate in disorders with generalized resistance to Ca2+o owing to inactivating CaR mutations in the following special circumstances: in selected families with FHH in which there is unusually severe hypercalcemia, frankly elevated PTH levels, or atypical features such as hypercalciuria; in cases of NSHPT with severe hypercalcemia and hyperparathyroidism; and in the occasional mild case of homozygous FHH owing to CaR mutations that confer mild-to-moderate resistance to Ca2+o that escapes clinical detection in the neonatal period. As discussed elsewhere in this issue, selective calcimimetic CaR activators are being tested in clinical trials, which potentiate the activation of the CaR by Ca2+o, thereby resetting the elevated set point for Ca2+o-regulated PTH release in primary and secondary hyperparathyroidism toward normal. It is hoped that these agents may become an effective medical therapy for the acquired Ca2+o resistance in primary and secondary hyperparathyroidism and perhaps for that present in the unusual cases of FHH and NSHPT, resetting the "calciostat" downward and thereby reducing Ca2+o and PTH toward normal.     

钙敏感受体与甲状旁腺疾病

钙敏感受体（CASR）为机体维持钙稳态、保持正常生理功能的关键环节。CASR的活化可抑制甲状旁腺激素分泌,减少。肾小管钙的重吸收,促进甲状腺C细胞降钙素分泌。CASR失活性突变可引起家族性良性低尿钙性高钙血症（FBHH）及新生儿严重甲状旁腺功能亢进症（NSHFF）,激活性突变引起常染色体显性遗传性低钙血症伴高尿钙（ADHH）。获得性甲状旁腺功能亢进时病变甲状旁腺中CASR表达减少,CASR基因多态性可能与甲状旁腺功能亢进症的临床表现严重程度相关。CASR也是自身免疫性甲状旁腺功能减退症的关键自身抗原。     

A novel mutation in the calcium-sensing receptor gene in a Chinese subject with persistent hypercalcemia and hypocalciuria

Familial hypocalciuric hypercalcemia (FHH) is an autosomal dominant disorder characterized by high penetrance of relatively benign, lifelong persistent hypercalcemia and hypocalciuria. By contrast, neonatal severe hyperparathyroidism represents a life-threatening form of hypercalcemia that can cause the early newborn mortality if immediate intervention is not undertaken. Both disorders are due to inactivation mutation of the human calcium-sensing receptor (CaSR) gene on chromosome 3q21-24. Up to now, more than 30 mutations in the CaSR gene associated with FHH have been described. In this study, we analyzed one 79-yr-old male with hypocalciuric hypercalcemia without siblings or children to compare with an additional group of 50 normal Chinese subjects in Taiwan. DNA sequence analysis of the CaSR gene was performed. The result showed that the proband had a heterozygous nonsense mutation in exon 7 of the CaSR gene at codon 648 (CGA-->TGA/Arg-->Ter). This mutation, located in the COOH-terminal of the first intracellular loop of the CaSR, predicts a markedly truncated protein. We have identified a novel R648X mutation in the CaSR gene in one patient with FHH in Taiwan     

A novel mutation in Ca2+-sensing receptor gene in familial hypocalciuric hypercalcemia

Missense mutations in the calcium-sensing receptor (CaSR) gene have previously been identified in patients with familial hypocalciuric hypercalcemia (FHH) and neonatal severe hyperparathyroidism. We identified a newborn with hypercalcemia in our hospital by mass screening. The family members were studied, and we found a novel CaSR missense mutation with polymerase chain reaction single-strand conformational polymorphism analysis. The mother, grandmother, and aunt of the baby all had FHH. A heterozygous missense mutation in exon 6 that substitutes a glutamic acid for the glycine at codon 557 (Gly557 Glu), which corresponds to the extracellular domain of CaSR, was identified and shown to cosegregate with the disease. Identification of the mutation responsible for the FHH phenotype in this family may facilitate rapid testing of individuals at risk for FHH.     

Mutations in the calcium-sensing receptor: a new genetic risk factor for chronic pancreatitis?

OBJECTIVE: In 2003 we identified a family with familial hypocalciuric hypercalcemia (FHH) (heterozygous CASR gene mutation L173P) and a mutation in the pancreatic secretory trypsin inhibitor gene (SPINK1) (N34S). While family members with an isolated calcium-sensing receptor gene (CASR) mutation remained healthy, a combination of the CASR and SPINK1 gene mutation caused chronic pancreatitis (CP). We thus speculate that the combination of two genetic defects affecting calcium homeostasis and pancreatic enzyme activation might represent a novel approach in chronic inherited pancreatic disease. We therefore sought to explore whether CASR gene mutations were prevalent in a cohort of patients with CP and confirmed SPINK1 mutations. MATERIAL AND METHODS: A cohort of 19 families (n=170) with a history of idiopathic CP (ICP) was screened for mutations within the CASR gene; 104 members of that cohort had a mutation (N34S) within the SPINK1 gene and 66 of those were suffering from CP. The entire CASR gene was screened for single strand conformation polymorphism under varying polyacrylamide gel conditions and subjected to direct dideoxy nucleotide sequencing of amplified cDNA. RESULTS: Single-strand conformation polymorphisms were observed in 59 samples, clustering of exons 3, 4 and 7. DNA sequence analysis revealed a yet unreported missense mutation in exon 7 (R896H) and two conservative mutations in exon 4 (F391F) and exon 7 (E790E). Furthermore, an intronic polymorphism in nucleotide position 493-19 G>A was detected in 19 out of 170 members of that cohort. CONCLUSIONS: We identified three novel calcium-sensing receptor gene mutations (1 missense mutation, 2 silent mutations and 1 intronic polymorphism) in a cohort of 19 families with ICP. In particular, the kindred with the R896H mutation presenting with a similar pedigree to the family described above may indicate a role for CASR gene mutations in SPINK1-related CP. Again, only the patient with the combination of both CASR and N34S SPINK1 gene mutation developed pancreatitis, whereas in the healthy parents and children only an isolated CASR or N34S SPINK1 gene mutation could be detected. We suggest that the CASR gene is a novel yet undetected co-factor in a multifactorial genetic setting of SPINK1-related pancreatitis that alters the susceptibility for pancreatitis in these patients.     

An adult patient with severe hypercalcaemia and hypocalciuria due to a novel homozygous inactivating mutation of calcium-sensing receptor

Inactivating mutations in the calcium-sensing receptor (CaSR) cause familial hypocalciuric hypercalcaemia (FHH) and neonatal severe hyperparathyroidism (NSHPT). Earlier investigations showed patients with FHH are heterozygous, and NSHPT are homozygous for inactivating mutations. However, one adult patient with severe hypercalcaemia and hypocalciuria has been reported to have a homozygous inactivating mutation in CaSR (Pro39Ala). This suggested that mutant CaSR in this patient had some residual activity and hypercalcaemia was not so severe as to be fatal. However, the function of this mutant CaSR was not evaluated. In the present study, we describe a novel homozygous mutation in an adult patient with severe hypercalcaemia and hypocalciuria, and evaluate the function of the mutant CaSRs. The DNA sequence of CaSR gene was determined by direct sequencing of the polymerase chain reaction product. The function of mutant CaSR was analysed by creating mutant cDNAs by in vitro mutagenesis, transfection of mutant cDNAs into HEK293 cells and measuring intracellular ionized Ca in response to changes in extracellular Ca. A 26-year-old Japanese woman showed marked hypercalcaemia with an elevated parathyroid hormone (PTH) level. Her consanguineous parents had asymptomatic hypercalcaemia with relative hypocalciuria. The proband had a homozygous mutation at codon 27 of CaSR gene (CAA-->CGA, Gln27Arg). Her parents were heterozygous for this mutation. EC50 for Ca of this mutant CaSR (GIn27Arg) was 4.9 mM. EC50 of another mutant CaSR (Pro39Ala) whose homozygous mutation was discovered in an adult patient was 4.4 mM. These EC50s were significantly higher than that of wild-type CaSR (3.7} 0.1 mM), but were the lowest among the reported EC50s for inactivating mutations of CaSR. These results indicate that serum Ca and PTH levels are determined by residual function of mutant CaSR in patients with homozygous mutation in CaSR, and that patients having homozygous mutant CaSRs with mild dysfunction do not suffer from fatal hypercalcaemia in infancy and can survive into adulthood.     

Calcium metabolism and endocrine functions in a family with familial hypocalciuric hypercalcemia.

OBJECTIVE: We report two Hungarian patients with familial hypocalciuric hypercalcemia (FHH) caused by a mutation of the calcium-sensing receptor (CaSR) at codon 55. The proband and her father were heterozygous for this mutation. DESIGN: We performed detailed clinical and laboratory assessments of this family to characterize the effects of CaSR mutation on several endocrine organs expressing CaSR. RESULTS: Interestingly, we could not detect any failure in the function of any tissues we examined, except in serum calcium levels. CONCLUSIONS: To our knowledge, this has been the first report from Eastern and Central Europe showing P55 L mutation of the CaSR, as well as the first publication discussing the effect of this mutation on several endocrine systems containing CASR.     

Vitamin D metabolism in familial benign hypercalcemia (hypocalciuric hypercalcemia) differs from that in primary hyperparathyroidism

We compared serum concentrations of immunoreactive PTH and plasma levels of vitamin D metabolites in 11 patients with adenomatous primary hyperparathyroidism and 32 individuals with the syndrome of familial benign hypercalcemia or familial hypocalciuric hypercalcemia (FHH). Serum immunoreactive PTH was elevated in the hyperparathyroid group but indistinguishable from control in FHH, despite comparable degrees of hypercalcemia. Plasma 25-hydroxyvitamin D concentrations were normal in both groups, but plasma 1,25-dihydroxyvitamin D levels in FHH were significantly lower than control (P less than 0.0025) or hyperparathyroid (P less than 0.01) values. FHH is pathogenetically distinct from primary hyperparathyroidism and should not be thought of simply as a variant of that condition.     

Neonatal severe hyperparathyroidism associated with a novel de novo heterozygous R551K inactivating mutation and a heterozygous A986S polymorphism of the calcium-sensing receptor gene

INTRODUCTION: Neonatal severe hyperparathyroidism (NSHPT) is induced by inactivating mutations of human calcium-sensing receptor (CaSR). Only three heterozygous de novo inactivating mutations of CaSR causing NSHPT have been described. We report the case of a now 11-year-old boy with NSHPT and we characterize a novel inactivating mutation along with the results of some functional analyses. PATIENT AND METHODS: As a neonate the patient presented the clinical syndrome of NSHPT. At 6 years of age persisting hypercalcaemia without clinical symptoms was documented, and the patient remained completely symptom free without parathyroid surgery until his present age of 11 years. The entire coding region of the CaSR gene of the patient and his family members was sequenced. Functional investigation was performed in HEK-293 cells, transiently transfected with wild type and mutant CaSR plasmid constructs. RESULTS: Sequence analysis revealed a novel de novo heterozygous mutation at codon 551 (AGG-->AAG), predicting a change of arginine to lysine (R551K) and a known heterozygous polymorphism (A986S) on the same allele, which was inherited from the father. We demonstrated that the novel R551K mutation significantly reduced the calcium sensitivity of CaSR (EC50: from 3.38 +/- 0.62-6.10 +/- 0.83 mmol/l), which was not alleviated by the simultaneous presence of A986S polymorphism. CONCLUSIONS: We present the fourth NSHPT case induced by a novel de novo heterozygous inactivating mutation (R551K) of the CaSR gene. The disease gradually reverted to a symptomless, benign condition resembling familial hypocalciuric hypercalcaemia without any surgical intervention.     

Familial hypercalcemia and hypercalciuria caused by a novel mutation in the cytoplasmic tail of the calcium receptor

Familial hyperparathyroidism (HPT), characterized by hypercalcemia and hypercalciuria, and familial benign hypocalciuric hypercalcemia (FHH) are the most common causes of hereditary hypercalcemia. The calcium-sensing receptor (CaR) regulates PTH secretion and renal calcium excretion. Heterozygous inactivating mutations of the gene cause FHH, whereas CaR gene mutations have not been demonstrated in HPT. In a kindred with 20 affected individuals, the hypercalcemic disorder segregated with inappropriately higher serum PTH and magnesium levels and urinary calcium levels than in unaffected members. Subtotal parathyroidectomy revealed parathyroid gland hyperplasia/adenoma and corrected the biochemical signs of the disorder in seven of nine individuals. Linkage analysis mapped the condition to markers flanking the CaR gene on chromosome 3q. Sequence analysis revealed a mutation changing phenylalanine to leucine at codon 881 of the CaR gene, representing the first identified point mutation located within the cytoplasmic tail of the CaR. A construct of the mutant receptor (F881L) was expressed in human embryonic kidney cells (HEK 293), and demonstrated a right-shifted dose-response relationship between the extracellular and intracellular calcium concentrations. The hypercalcemic disorder of the present family is caused by an inactivating point mutation in the cytoplasmic tail of the CaR and displays clinical characteristics atypical of FHH and primary HPT.     

The effect of calcium-sensing receptor gene polymorphisms on serum calcium levels: a familial hypocalciuric hypercalcemia family without mutation in the calcium-sensing receptor gene

Familial hypocalciuric hypercalcemia (FHH) is a benign syndrome with elevated levels of serum calcium, relative hypocalciuria, and non-suppressed serum parathyroid hormone (PTH) levels. FHH usually occurs by a heterozygous mutation of the calcium sensing receptor (Casr), but some FHH patients show no mutations of the Casr. We encountered a unique FHH family in which the proband and her mother had many calcium deposits on their skin. The proband was medicated with Levothyroxine for hypothyroidism due to an iodine transport defect (ITD). We searched for mutation of the Casr, but found none. The only change distinguishing the proband and her mother from her father was at codon 990, reported to be a polymorphic site. We investigated the frequency of polymorphism at codon 990, but a significant relationship between the three genotypes and the serum calcium concentration was excluded. At the other polymorphic sites at codon 536, 926, 986, and 1011, polymorphisms were rare in Japanese, and we could not confirm a significant relationship. In conclusion, mutation in the Casr gene alone does not explain all cases of FHH. The other mechanisms should be identified.     

Familial benign hypocalciuric hypercalcemia and neonatal hyperparathyroidism

Conclusion In summary, both FBHH and NSHPT represent the phenotypic expression of inactivating mutations of the CaSR in most cases, FBHH usually presenting with mild asymptomatic lifelong hypercalcemia and NSHPT with generally more severe, potentially lethal hypercalcemia, which, in cases of homozygous or compound heterozygous CaSR mutations, carries a high morbidity. Additional molecular studies should elucidate the basis for variations in the phenotypic and biochemical abnormalities observed with both FBHH and NSHPT and identify the disease genes on chromosome 19 that can also cause FBHH in rare cases.     

Familial hypocalciuric hypercalcemia. Report of a new family

We report a new family with familial hypocalciuric hypercalcemia (FHH) composed by 55 living members. Of 38 studied, 10 have been found to be affected by FHH. Differences between FHH and primary hyperparathyroidism are emphasized; lack of clinical features, relative hypocalciuria for the concomitant hypercalcemia and low phosphate excretion index. The PTH and urinary cAMP are normal. It is noteworthy that the disease is benign. None of our patients have undergone surgery, and all of them are asymptomatic.     

Novel mutations in the calcium-sensing receptor gene associated with biochemical and functional differences in familial hypocalciuric hypercalcaemia

OBJECTIVE: Heterozygous inactivating mutations of the calcium-sensing receptor (CaR) gene cause familial hypocalciuric hypercalcaemia (FHH), a generally benign disorder characterized by mild to moderate PTH-dependent hypercalcaemia. We aimed to identify the causative CaR mutations in three families with FHH and examine the correlation between type of mutation and biochemical and/or functional phenotypes. PATIENTS, DESIGN AND MEASUREMENTS: The CaR gene from FHH family members was assessed for mutations by direct DNA sequencing and mutations were confirmed by restriction enzyme analysis. Functional studies on two missense mutations were conducted by introducing them by site-directed mutagenesis into the CaR cloned into a mammalian expression vector, and assessing calcium responsiveness using an inositol phosphate (IP) assay in HEK293 cells. Biochemical data from patients heterozygous for each type of mutant were correlated with functionality. RESULTS: Two novel nonsense mutations (R25stop and K323stop) and one novel missense mutation (G778D) were identified. The G778D mutant receptor and another mutation identified in an earlier study (L174R) demonstrated a complete lack of Ca2+ responsiveness using the IP assay. When cotransfected with wild-type receptor, the mutant receptors demonstrated a dominant-negative effect on wild-type receptor response, with L174R having a more pronounced effect than G778D. Significantly more severe hypercalcaemia and a trend towards higher PTH levels were observed in patients heterozygous for CaR mutants with a stronger dominant-negative effect. CONCLUSIONS: Naturally occurring CaR mutations with differences in dominant-negative effect on wild-type receptor demonstrate differences in biochemical severity in FHH.     

A novel mutation of the calcium sensing receptor gene is associated with chronic pancreatitis in a family with heterozygous SPINK1 mutations

Background

The role of mutations in the serine protease inhibitor Kazal type 1 (SPINK1) gene in chronic pancreatitis is still a matter of debate. Active SPINK1 is thought to antagonize activated trypsin. Cases of SPINK1 mutations, especially N34S, have been reported in a subset of patients with idiopathic chronic pancreatitis. However, the inheritance pattern is still unknown. Some cases with N34S heterozygosity have been reported with and without evidence for CP indicating neither an autosomal recessive nor dominant trait. Therefore SPINK1 mutations have been postulated to act as a disease modifier requiring additional mutations in a more complex genetic model. Familial hypocalciuric hypercalcemia (FHH) caused by heterozygous inactivating mutations in the calcium sensing receptor (CASR) gene is considered a benign disorder with elevated plasma calcium levels. Although hypercalcemia represents a risk factor for pancreatitis, increased rates of pancreatitis in patients with FHH have not been reported thus far.

Methods

We studied a family with a FHH-related hypercalcemia and chronic pancreatitis. DNA samples were analysed for mutations within the cationic trypsinogen (N29I, R122H) and SPINK1 (N34S) gene using melting curve analysis. Mutations within CASR gene were identified by DNA sequencing.

Results

A N34S SPINK1 mutation was found in all screened family members. However, only two family members developed chronic pancreatitis. These patients also had FHH caused by a novel, sporadic mutation in the CASR gene (518T>C) leading to an amino acid exchange (leucine->proline) in the extracellular domain of the CASR protein.

Conclusion

Mutations in the calcium sensing receptor gene might represent a novel as yet unidentified predisposing factor which may lead to an increased susceptibility for chronic pancreatitis. Moreover, this family analysis supports the hypothesis that SPINK1 mutations act as disease modifier and suggests an even more complex genetic model in SPINK1 related chronic pancreatitis.