Familial Hypocalciuric Hypercalcemia(FHH)

CASR DisordersGita Majdi, M.D, MRCP (UK), FRCPC,ABIM

PGY5 EndocrinologyFeb 2015

Objectives:

• Introduction to FHH and CaSR disorders

• Presentaion

• Genetics

• Diagnosis

Calcium Metabolism

Introduction

Familial hypocalciuric hypercalcemia(FHH):• Autosomal dominant disorder• Mild hypercalcemia • low urinary calcium excretion (mean urinary calcium:creatinine clearance

ratio, <0.01) • Normal circulating parathyroid hormone concentrations in 80% of patients• High-normal to frankly elevated serum magnesium levels .( Because the

calcium-sensing receptor is a cation receptor, urinary magnesium excretion parallels calcium excretion and is therefore low in FHH in contrast to primary hyperparathyroidism).

•

Presentations:

• Generally asymptomatic• Although pancreatitis or chondrocalcinosis may develop in some

affected adults. • Familial hypocalciuric hypercalcemia is genetically heterogeneous,

with three reported variants. • The usual lack of high serum PTH concentrations may contribute to

the benign clinical course of FHH, as compared with primary hyperparathyroidism

Clinical Findings:

• These features are present from infancy, and most cases are associated with inactivating mutations of the gene for the calcium-sensing receptor, which is present in the parathyroid glands and in the thick ascending limb of the loop of Henle.

• Homozygous mutations of this receptor give rise to neonatal severe hyperparathyroidism, which is an extreme form of hyperparathyroidism and hypercalcemia that can be fatal in infancy.

• Since the inappropriately normal or the frankly elevated parathyroid hormone levels in these conditions result from a defect in calcium sensing that affects all parathyroid tissue, subtotal parathyroidectomy does not usually lead to long-term normocalcemia.

Pathophysiology

• Familial hypocalciuric hypercalcemia is genetically heterogeneous, with three reported variants.

• Types 1, 2, and 3.

• However, some individuals with loss-of-function CaSR mutations remain normocalcaemic.

• In addition, there is genetic heterogeneity amongst the forms of FHH.

Genetics:

• Type 1 is due to loss-of-function mutations of the calcium-sensing receptor (encoded by CASR)

• Type 2 is due to GNA11 Mutation • Type 3 is associated with adaptor-related protein complex 2, sigma 1

subunit (AP2S1) mutations, which alter calcium-sensing receptor endocytosis.

CaSR :

• Serum calcium regulates the secretion of parathyroid hormone by the parathyroid glands — a fact that implies the presence of a calcium sensor on the surface of the parathyroid cell.

FHH: CASR disorder

• Many cases of familial hypocalciuric hypercalcemia are caused by heterozygous germline inactivating mutations in the gene encoding the calcium-sensing receptor (CASR).

CaSR: History

• Most researchers in this field assumed that the calcium sensor would turn out to be some kind of calcium-regulated channel.

CaSR:

• Calcium-sensing receptor was first identified in 1993.

CASR History:

• This large class of receptors is exemplified by the thyrotropin receptor.

• The identification of the calcium-sensing receptor as a G protein–coupled receptor was surprising because at the time, no small cation such as calcium had been shown to be capable of acting as a ligand for such a receptor.

GPCR

• G proteins (guanine nucleotide-binding proteins) are heterotrimers composed of guanosine triphosphate–binding alpha subunits and tightly linked beta and gamma subunits. They couple a vast array of receptors (G-protein–coupled receptors).

GPCR

GPCR

15 human alpha-subunit genes.Some, such as GNAS( guanine nucleotide binding protein, alpha stimulating) are expressed ubiquitously.

Others are expressed only in specialized cells. Gs (the G protein encoded by GNAS) couples many hormone and neurotransmitter receptors to cyclic AMP stimulation, and it was the first G protein to be associated with human disease

GPCR:

• Germline mutations that inactivate Gαs (the G-protein subunit αs) were shown to cause the prototypical hormone-resistance disorder, pseudohypoparathyroidism.

• Somatic activating mutations cause sporadic endocrine tumors and the McCune–Albright syndrome.

CaSR:

• The calcium-sensing receptor is a G-protein–coupled receptor that is highly expressed in parathyroid and renal tubular cells, but it is also detectable in many other tissues.

CaSR

• A decade has passed, and all now accept the fact that serum calcium binds to the extracellular portion of the calcium-sensing receptor and relays intracellular signals (such as intracellular calcium, protein kinase C, and phosphoinositides) that, in turn, suppress the secretion of parathyroid hormone, with a resultant decrease in the serum calcium concentration.

• Conversely, when the serum calcium concentration falls, parathyroid hormone secretion is stimulated, resulting in an increase in serum calcium.

CaRS

The calcium-sensing receptor is a G-protein–coupled receptor that is highly expressed in parathyroid and renal tubular cells, but it is also detectable in many other tissues

CaSR

• The human CaSR gene is located on chromosome 3q21.1 and loss-of-function CaSR mutations have been reported in the hypercalcaemic disorders of familial benign (hypocalciuric) hypercalcaemia (FHH, FBH or FBHH) and neonatal severe primary hyperparathyroidism (NSHPT).

• However, some individuals with loss-of-function CaSR mutations remain normocalcaemic.

CASR mutations:

• About 200 different mutations of the CaSR have been identified .

• A summary of CaSR mutations is maintained at the calcium-sensing receptor database [CASRdb] at McGill University.

FHH1:Inactivating Mutation

• In most cases, FHH results from inactivating mutations in the CaSR, whose gene resides on the long arm of chromosome 3 (3q21.1)

• This form of FHH is now called FHH1.• Loss-of-function mutations of the calcium-sensing receptor, a guanine

nucleotide–binding protein (G-protein)–coupled receptor that signals through the G-protein subunit α11 (Gα11).

FHH2

• The form of FHH (FHH2) arising from the short arm of chromosome 19 results from inactivating mutations in G alpha 11, one of the guanine nucleotide binding (G) proteins linking the CaSR to activation of phospholipase C, which contributes to inhibition of PTH release at elevated extracellular calcium concentrations .

GNA11 Mutation in a Patient with Familial Hypocalciuric Hypercalcemia Type 2.

FHH3

• The form of FHH (FHH3) that is linked to the long arm of chromosome 19 results from missense mutations of adaptor-related protein complex 2, sigma 1 subunit (AP2S1) , which participates in clathrin-mediated endocytosis of G protein-coupled receptors.

• Mutations in AP2S1 decrease the sensitivity of the CaSR-expressing cells to extracellular calcium and modify the receptor’s endocytosis.

CASR Disorders

• Calcium directly activates the calcium-sensing receptor, inhibiting parathyroid hormone secretion in the parathyroids and decreasing renal calcium reabsorption.

• Loss of one functional CASR allele in familial hypocalciuric hypercalcemia reduces the sensitivity of the parathyroid and renal cells to calcium; this leads to hypercalcemia and to hypocalciuria relative to the filtered load of calcium.

• Homozygous loss-of-function mutations in CASR cause severe, potentially lethal neonatal hyperparathyroidism that may necessitate total parathyroidectomy.

CARS Disorders:

• Conversely, heterozygous germline activating mutations in CASR cause a form of autosomal dominant hypocalcemia and hypoparathyroidism.

• Such mutations cause increased parathyroid and renal sensitivity to calcium, so that parathyroid hormone is suppressed at low serum levels of calcium, often with hypercalciuria relative to the filtered load of calcium.

CARS: Gain of Function Mutation

• Gain-of-function mutations of the gene for the calcium-sensing receptor cause hypocalcemia with normal serum parathyroid hormone concentrations, a combination that suggests an abnormality in the set point of the calcium-sensing receptor.

• Expression studies confirmed that these mutations cause the receptor to respond to lower-than-normal concentrations of calcium, corresponding to a leftward shift in the dose–response curve for calcium-dependent responses.

• Patients with these mutations also have hypercalciuria, and some have nephrocalcinosis and renal impairment, raising the possibility that abnormalities of the calcium-sensing receptor may contribute to idiopathic hypercalciuria.

CaSR: Summary

• Thus, the majority of FHH patients have loss-of-function CaSR mutations, and this is referred to as FHH type 1.

• Gain-of-function CaSR mutations have been shown to result in autosomal dominant hypocalcaemia with hypercalciuria (ADHH) and Bartter's syndrome type V.

• CaSR auto-antibodies have been found in FHH patients who did not have loss-of-function CaSR mutations, and in patients with an acquired form (i.e. autoimmune) of hypoparathyroidism.

• Thus, abnormalities of the CaSR are associated with three hypercalcaemic and three hypocalcaemic disorders

CaRS: Loss of Function Mutation,FHH• This mild and typically asymptomatic disorder is converted into a

severe, life-threatening disorder when two inactivated calcium-sensing–receptor alleles are present.

• Newborns with this condition, called neonatal severe hyperparathyroidism, have severe hypercalcemia (serum calcium concentration,3.7 to 5.0 mmol per liter), as well as marked increases in the parathyroid hormone concentration and parathyroid mass, and require urgent parathyroidectomy.

Acquired Hypocalciuric Hypercalcemia• Since G protein–coupled receptors reside on the cell surface, they

may be the targets of autoantibodies and cellular immune attack.• A good example is the thyrotropin receptor, the target of

hyperthyroidism-inducing autoantibodies (thyroid-stimulating immunoglobulins) in Graves' disease and of autoimmune, cell-mediated thyroid destruction and hypothyroidism in Hashimoto's thyroiditis.

• Could this kind of scenario occur in parathyroid disease?

An acquired hypocalciuric hypercalcemia autoantibody induces allosteric transition among active human Ca-sensing receptor conformationsNoriko Makita, Junichiro Sato, Katsunori Manaka, Yuki Shoji, Atsuro Oishi, Makiko Hashimoto, Toshiro Fujita, and Taroh Iiri *Author Affiliations

Communicated by Henry R. Bourne, University of California, San Francisco, CA, February 11, 2007 (received for review October 10, 2006)

Case

A 74-year-old man, who had been receiving treatment for hypertension, presented with:• hypercalcemia (Ca 12.1 mg/dl, normal 8.4–9.7),• Mild hypophosphatemia (P 2.8 mg/dl)• elevated PTH levels (intact PTH 120 pg/ml) without any clinical symptoms.• Normal level of Vit D,and PTHrP levels of <1.0 pmol/liter suggesting primary

hyperparathyroidism.• However, his urinary calcium excretion was remarkably and neither

ultrasound examination of the neck nor parathyroid scan showed any evidence of parathyroid adenoma. Furthermore, a bone mineral was normal.

Cse:

• The clinical data obtained for this patient were therefore deemed to be compatible with a diagnosis of familial hypocalciuric hypercalcemia, which shows an autosomal dominant pattern of inheritance and is characterized by mild to moderate hypercalcemia in a relatively hypocalciuric setting, and also by normal or only slightly elevated circulating levels of PTH, although still inappropriate.

• Previous laboratory data for this individual, however, ruled out this genetic disease.

• Until the age of 68, his serum calcium and phosphate had been within normal ranges (. Although at the age of 69 he was found to have hypercalcemia (11.1 mg/dl) with slightly decreased IP (2.9 mg/dl), he was not further examined until the present investigation.

AHH:

• Patient’s symptoms regressed following glucocorticoid. • Medical history of patient provided evidence of autoimmune

dysregulation including psoriasis, adult-onset asthma, Cooms'-positive hemagglutination, rheumatoid arthritis, and autoimmune hypophysitis.

• These findings suggested that the blocking autoantibody against CaSR was in fact responsible for the AHH in this patient.

AHH

• Blocking autoantibody against CaSR has been isolated and reported to cause acquired hypocalciuric hypercalcemia (AHH).

• The autoantibody itself (IgG4) inhibits CaSR signaling pathways, inositol phosphate (IP) accumulation and ERK phosphorylation, by binding to specific CaSR sites.

Diagnosis:

Combination of• clinical suspicion• biochemical testing• genetic analysis

Diagnosis:

• It is important to distinguish asymptomatic primary hyperparathyroidism (PHPT) from FHH because FHH is a benign inherited condition that typically does not require parathyroidectomy, nor will it be routinely cured by it.

• Hypercalcemia with "normal" serum PTH concentrations occurs in approximately 10 percent of patients with primary hyperparathyroidism, which is a much more common cause of hypercalcemia than FHH, and 15 to 20 percent of patients with FHH may have a mildly elevated PTH concentration.

Diagnosis: Ca/Cr

• This ratio is calculated from the results of a 24-hour urine collection and simultaneously measured total serum calcium and creatinine concentrations, using the following formula:

• Ca/Cr clearance ratio = [24-hour Urine Ca x serum Cr] ÷ [Serum Ca x 24-hour Urine Cr]

Diagnosis:

• Urinary calcium excretion is low in patients with FHH.• The 24-hour urinary calcium excretion is typically below 200 mg/day (5

mmol/day). • 40 percent of patients with primary hyperparathyroidism have

hypercalciuria (24-hour calcium excretion above 250 mg [6.2 mmol] in women and 300 mg [7.5 mmol] in men).

• When evaluating patients suspected of having FHH, it is important to exclude other factors causing hypocalciuria.

• These include vitamin D deficiency and/or very low calcium intake, mild renal insufficiency, and treatment with thiazides or lithium (both of which are hypocalciuric).

Diagnosis:

• The Ca/Cr clearance ratio is less than 0.01 in approximately 80 percent of patients with FHH, indicating that more than 99 percent of the filtered calcium has been reabsorbed despite the presence of hypercalcemia.

• In patients with primary hyperparathyroidism, however, the Ca/Cr clearance ratio is most often >0.02.

• In a study that reevaluated the discriminative power of the Ca/Cr clearance ratio, a value of 0.0115, similar to that proposed earlier provided optimal discrimination between FHH and primary hyperparathyroidism.

Genetic Testing

• As many as one-third of families with FHH linked to chromosome 3(FHH 1) do not have a detectable mutation within the coding region of the gene or its intron-exon boundaries.

• It is thought that these cases harbor mutations in regulatory regions of the gene affecting its level of expression.

• Genetic testing by mutational analysis of G alpha 11 and AP2S1 is currently available only on a research basis for the diagnosis of FHH2 and FHH3, respectively, but may eventually be useful in some cases to establish a definitive diagnosis of FHH in a patient without an identifiable mutation in the CaSR gene.

Management:

• In most cases of FHH, parathyroid surgery is neither curative nor appropriate, although in the uncommon cases with atypical features, such as pancreatitis, hypercalciuria, and/or overt hyperparathyroidism, subtotal parathyroidectomy has in some cases corrected these clinical and biochemical abnormalities.

• In NSHPT due to homozygous inactivating CaSR mutations, total parathyroidectomy is usually the therapy of choice. Treatment of these patients prior to surgery with a bisphosphonate or cinacalcet may be helpful in stabilizing them medically.

Treatment of CaSR disorders

• CaSR modulators that function as agonists (calcimimetics) or antagonists (calcilytics) are in development for the treatment of select disorders of calcium and parathyroid hormone regulation.

• One calcimimetic agent (cinacalcet) is approved in the United States for the treatment of secondary hyperparathyroidism in patients with chronic kidney disease receiving dialysis, for the treatment of hypercalcemia in patients with parathyroid carcinoma, and for the treatment of severe hypercalcemia in patients with primary hyperparathyroidism unable to undergo parathyroidectomy.

• It has also been approved for use in other forms of primary hyperparathyroidism in Europe.

Cinacalcet (Sensipar) is indicated

for the treatment of secondary

hyperparathyroidism in patients

with chronic kidney disease who

are on dialysis.

Clinical trial evidence supports

that cinacalcet (Sensipar) improves

biochemical markers of

hyperparathyroidism (i.e., blood

levels of the parathyroid hormone,

calcium and phosphorus).

The cost of cinacalcet (Sensipar)

ranges from $4,000 to $23,500 per year,

depending on the dosage.

Treatment of CaSR

• Oral CaSR antagonists (calcilytics) are in development for the treatment of osteoporosis . Administration leads to a transient rise in endogenous parathyroid hormone, similar to intermittently administered exogenous parathyroid hormone .

Summary

• Familial hypocalciuric hypercalcemia (FHH) is due to an inactivating mutation in the calcium-sensing receptor in the parathyroid glands and the kidneys .

• A family history of hypercalcemia, especially in young children, and the absence of symptoms and signs of hypercalcemia (such as anorexia, neuromuscular symptoms, and polyuria) are characteristic of this disorder.

• Fifteen to 20 percent of patients with FHH may have a mildly elevated PTH concentration . In these individuals, it may be difficult to distinguish asymptomatic primary hyperparathyroidism from FHH.

• It is important to make this distinction, however, because FHH is a benign inherited condition that typically does not require parathyroidectomy, nor will it be cured by it.

Sources:

• Translational Implications of the Parathyroid Calcium ReceptorAndrew F. Stewart, M.D.,N Engl J Med 2004; 351:324-326July 22, 2004Mutations Affecting G-Protein Subunit α11 in Hypercalcemia and HypocalcemiaM. Andrew Nesbit, Ph.D., Fadil M. Hannan, D.Phil., F.R.C.Path., Sarah A. Howles, B.M., B.Ch., Valerie N. Babinsky, M.Sc., Rosie A. Head, M.A., Treena Cranston, B.Sc., Dip.R.C.Path., Nigel Rust, M.Phil., Maurine R. Hobbs, Ph.D., Hunter Heath, III, M.D., and Rajesh V. Thakker, M.D.N Engl J Med 2013; 368:2476-2486June 27, 2013

Sources:

• Differentiating familial hypocalciuric hypercalcemia from primary hyperparathyroidism,Endocr Pract. 2013 Jul-Aug;19(4):697-702.

• Cell Calcium. 2004 Mar;35(3):275-82