Sequencing the CaSR locus in Pakistani stone formers reveals a novel loss-of-function variant atypically associated with nephrolithiasis

Background Nephrolithiasis (NL) affects 1 in 11 individuals worldwide and causes significant morbidity and cost. Common variants in the calcium sensing receptor gene (CaSR) have been associated with NL. Rare inactivating CaSR variants classically cause hyperparathyroidism, hypercalcemia and hypocalciuria. However, NL and familial hypercalciuria have been paradoxically associated with select inactivating CaSR variants in three kindreds from Europe and Australia. Methods To discover novel NL-associated CaSR variants from a geographically distinct cohort, 57 Pakistani families presenting with pediatric onset NL were recruited. The CaSR locus was analyzed by directed or exome sequencing. Results We detected a heterozygous and likely pathogenic splice variant (GRCh37 Chr3:122000958A>G; GRCh38 Chr3:12228211A>G; NM_000388:c.1609-2A>G) in CaSR in one family with recurrent calcium oxalate stones. This variant would be predicted to cause exon skipping and premature termination (p.Val537Metfs*49). Moreover, a splice variant of unknown significance in an alternative CaSR transcript (GRCh37 Chr3:122000929G>C; GRCh38 Chr3:122282082G >C NM_000388:c.1609-31G >C NM_001178065:c.1609-1G >C) was identified in two additional families. Conclusions Sequencing of the CaSR locus in Pakistani stone formers reveals a novel loss-of-function variant, expanding the connection between the CaSR locus and nephrolithiasis.

A Case Report of Calcium-Sensing Receptor Gene Variant and Primary Hyperparathyroidism

Introduction: Primary hyperparathyroidism (PHPT) is an endocrinopathy that results from excessive production of parathyroid hormone from one or more overactive parathyroid gland(s). An estimated 90% of PHPT cases are sporadic, and 10% are inherited, comprising familial hyperparathyroidism (FHP). In FHP syndromes, variable clinical features are partly attributed to genetic heterogeneity. While there is not a consensus whether Familial hypocalciuric hypercalcemia (FHH) is a distinct entity or a form of FHP, accurate diagnosis is critical in guiding proper decision-making. The utility of genetic testing is multi-fold: help inform most likely diagnosis, guide prognosis and management, and inform familial recurrence risk. We report a patient with early-onset hypercalcemia, post subtotal parathyroidectomy, notable family history, with a likely pathogenic variant in the calcium-sensing receptor gene, CASR. Case Presentation: A 42 yo, female with history of nephrolithiasis and hypercalcemia diagnosed in her 20’s with PHPT was referred to our endocrinology clinic. Preoperative work up showed calcium of 11.1mg/dL, albumin 4.2g/dL, PTH of 177pg/mL, creatinine 0.92mg/dL. Sestamibi showed persistent activity in the mid to inferior aspect of right thyroid lobe suspicious for parathyroid adenoma. She underwent 3 gland parathyroidectomy and pathology showed mildly hypercellular parathyroid in left superior & right superior gland, normocellular left inferior gland. PTH levels normalized post-surgery. PTH levels normalized post surgery. Genetic evaluation was done, given her early-onset hypercalcemia, multi-gland involvement, and notable family history (maternal grandfather with parathyroidectomy for hypercalcemia and mother reportedly undergoing a hyperparathyroidism workup). Invitae hyperparathyroidism genetic panel revealed a likely pathogenic variant in CASR (c.659G>A; p.R220Q). Discussion: To date, our case with PHPT is the second report of the likely pathogenic variant CASR (c.659G>A; p.R220Q), which affects a conserved extracellular domain residue, thought to be important in sensing extracellular calcium. This variant has been previously reported in another family with FHH with 5 affected relatives. The reported 29-year old proband also had recurrent pancreatitis, which resolved with subtotal parathyroidectomy, but remained hypercalcemic. Despite the marked phenotypic heterogeneity in our patient and the case with FHH in terms of clinical presentation and response to treatment with surgery; both shared a history of hyperparathyroidism and a family history of hypercalcemia, suggesting contributions from the same CASR gene variant. With limited existing data about the variable expressivity of variants in genes implicated in the pathogenesis of both FHH and FHP; our case adds value to the existing evidence that supports its possible genetic contribution to PHPT

Double jeopardy: a patient’s tale of two concurrent hypercalcaemic syndromes

Primary hyperparathyroidism (PHPT) is the most common cause of parathyroid hormone (PTH) dependent hypercalcaemia, however there are few reported cases of its co-occurrence in patients with familial hypocalciuric hypercalcaemia (FHH). This case highlights the challenges in managing a rare case of dual pathology. A 49-year-old Caucasian woman with symptoms of hypercalcaemia presented with an adjusted serum calcium of 2.77 mmol/L and PTH of 11.5 pmol/L. Neck ultrasound and sestamibi scan were concordant with a left lower parathyroid adenoma, and a preoperative dual-energy X-ray absorptiometry scan confirmed osteopenia. Parathyroidectomy resulted in a PTH reduction from 11.5 pmol/L to 2.7 pmol/L. Interestingly, her lowest pre-operative adjusted serum calcium of 2.67 mmol/L remained unchanged 14 months post-parathyroidectomy. Twenty-four hours urine calcium:creatinine clearance ratio performed postoperatively was low and sequencing analysis of the calcium-sensing receptor gene confirmed the coexistence of FHH. Although surgery is not indicated in FHH, parathyroidectomy may help reduce hypercalcaemia and its associated complications if there is coexistent PHPT.

Heterozygous Mutation (Q459R) in the Calcium-Sensing Receptor Gene Causes Familial Hypocalciuric Hypercalcemia 1 (FHH1)

Context Several heterozygous loss-of-function mutations in the calcium-sensing receptor gene (CASR) leading to elevated ionized serum calcium and familial hypocalciuric hypercalcemia 1 (FHH1) have been characterized. Few mutations are not pathogenic, and previous studies suggested that the Q459R mutation does not result in an FHH1 phenotype. Objective We identified a family with a heterozygous CASR Q459R mutation and characterized their calcium homeostasis and the pathophysiological mechanisms of a homozygous and heterozygous Q459R mutation in vitro. Design The index patient and her family had clinical, biochemical, and genetic analyses performed. In vitro functional characterization of homozygous and heterozygous (Q459R) mutations was conducted by determining CaSR cell-surface expression and inositol monophosphate (IP1) signaling in transiently transfected human embryonic kidney 293A (HEK293A) cells. Results All 3 heterozygous carriers had mild asymptomatic hypercalcemia, hypocalciuria, and 2 had elevated serum parathyroid hormone (PTH). In vitro characterization in HEK293A cells revealed that CASR Q459R is a loss-of-function mutation with no impact on cell-surface expression. Cells with the homozygous Q459R genotype had significantly reduced calcium potency of IP1 signaling compared to wild type, whereas the heterozygous Q459R also had lower calcium potency albeit not significantly different from wild type. Conclusion A loss-of-function Q459R mutation in CASR in a family caused FHH1 characterized by elevated ionized calcium and PTH and low calcium excretion. The marked presence of CaSR at the membrane and inhibition of IP1 signaling in vitro suggest that calcimimetics may be functional in patients with this mutation, which seems to be a mild loss-of-function mutation associated with autosomal dominant transmission of FHH1.