Calcium homeostasis
The present discussion is about the recent articles published on NEJM, concerning Hypoparathyroidism and Hypercalciuria. They represent on line material of my studies on these topics, as I’ve done a summary on my recent letter to the Editor on Brief Report on NHERF1 gene mutations and parathyroid hormone by Prié Dominique working in Paris, France at Hopithal Necker – Enfants Malades.
Calcium crystallization process
Excessively high concentration of calcium ions in the urine is one identifiable and correctable factor in stone formation. Calcium stone formation is a process of mineral crystallization in body tissue or fluid.
Inorganic crystal are shaped to become an integral part of organic tissue to provide strength and hardness. Thsese inorganic substances are capable of reversible interactions with biomolecules so that the crystalline structures can be remodeled for physiological needs.
Calcium salts have an highly adaptable coordination geometry, that greatly facilitates the protein binding, in its solid state or solution, adapting theirself to irregular geometry of proteins.
The physical properties of bone and teeth result from the activities of proteins that functions as the organic-inorganic interface.
Proteins share specific domains that specifically are able to interact with calcium crystals. The sequence below seems to be specific in these extracellular matrix proteins:
Aspartate-phosphoserine-phosphoserine-glutamate-glutamate
(DpSpSEE)
The motif described in the saliva protein “statherin” is also found in other calcium crystal-interacting proteins, such as osteopontin.
Interestingly unlike EF domain hand that binds ionic calcium, this structure specifically binds to solid phase calcium phosphate crystals and it is conserved in all phylogenetically evolved forms of life in the heart from invertebrates, such as crustaceans, to higher vertebrates, such as humans.
Physiologic crystallization includes formation of exoskeleton, pearl, endoskeleton, and dentition, whereas pathophysiologic crystallization includes pyroposphate arhtropathy, pigmented gallstones, vascular calcifications, and urolithiasis.
Serum calcium concentration is tightly regulated in humans, so that also small decrement in its concentration can led to clinical manifestations of “tetania”, as we have in hypoparathyroidism where Trousseau and Kwostek signs are present as clinical maifestations of altered muscle cells contraction regulations.
On other side increments of calcium levels can lead to increased urinary calcium secretions with kidney function alterations, hypergastrinemia with gastric ulcer formation, hypertension, bone reabsorption with osteoporomalacia and bone fractures, parodontopathies with alterations in theet adherence to bone stucture in particular of mandible (inferior dental arc), as mention the more important clinical manifestations.
The calcium homeostasis is under control of four main organs, bowel and intestinal system, parathyroid glands, bone tissues, and kidneys. Interestingly small changes in calcium levels can be evident such as alterations in acid-base equilibrium (i.e blood PH), due to important action exerted on these physicochemical equilibrium by renal cells activity of secretion and reabsorption. So that small changes in blood PH have to be resetted by intervention of kidney system ( metabolic acidosis or alkalosis) and only after by lung gas exchanges ( respiratory acidosis or alkalosis).
The secretion and reabsorption of calcium ions by kidney, in that view, is essential to human life. When these equilibrium is altered the first alteration we see is an excessive excretion or loss of calcium ions in urine, so that we call hypercalciuria.
Isolated hypercalciuria per se is not detrimental, but clinician interest in hypercalciuria concerns the complications that include mainly nephrolithiasis and nephrocalcinosis.
Stones formation
It has been suggested that the importance of hypercalciuria versus hyperoxaluria in calcium oxalate stones formers is equal; so that both urinary concentration of calcium and oxalate are important contributing factors in formation of kidney stones.
Kidney stones have a lifetime incidence of up to 13% in USA; in at least 70% of cases the stones are formed by calcium oxalate crystals, often with calcium phosphate or sodium urate.
For a stone to form there must be “supersaturation” a chemical condition dependent from PH, Ionic strength and Ionic concentration; in the presence of a “nidus” the nucleation process occurs, where the “nidus” is formed by extracellular matrix components or cellular debris. The subsequent step is the formation of of a true stone by crystal growth and aggregation.
The molecular mechanisms underlying the stone formation are described as:
. heterogeneous nucleation: in which the initial ion complex is attached to a foreign surface
. homogeneous nucleation: in which stones are formed independently from a nucleating surface
The heterogeneous nucleation occurs more often, requiring less energy and so at low level of supersaturation.
Some “factors” control the nucleation and crystal growth processes such as lowering supersaturation energy required and the presence of chemical inhibitors of crystal growth such as:
. pyrophosphate inorganic
. citrate
. glycoproteins
The majority of stone formers are defined such as affected by ”idiopathic hypercalciuria” . Any analysis of hypercalciuria should take into account a Pak pioneering work of 1975 introducing a tripartite classification of hypercalciuria:
- Absorptive hypercalciuria
- Reabsorptive hypercalciuria
- Renal hypercalciuria
From pathophysiological point of view and also by genetic view, this classification may seem to be very important and today useful.
Accordingly the extracellular fluid compartment can be regulated by the exchanges with three systems:
- Intestinal system
- Bone system
- Kidney system
Where the action of main hormones secreted and regulated by parathyroid glands is exerted : parathormone, 1,25 dihydroxy vitamin D3. Interestingly the physiological action exerted by the third hormone “calcitonin” in humans is not relevant, whereas in fish living in water mabient rich in calcium salts, this hormene is very important.
A possible classification of clinical parameters avaible if we consider renal hypercalciuria is the present:
- Parathyroid hormone and 1,25 vitmian D3 are higher than expected
- Hypercalciuria is inappropriate for the slightly elevated parathyroid homone, normal serum calcium, and normal filtered calcium.
- Persistent hypercalciuria is present even during fasting
- Increased bone resorption markers and/or reduced bone mineral density are present.
Interestingly half of patients labeled as havng idipatic hypercalciuria shown a family history of kidney stones. However the genetic rules observed by hypercalciuric patients don’t follow the Mendelian pattern of inheritance, but it seems likely a variable under the effect of
- Polygenic influence
- Polymorphism if a single gene locus (heterogeneity)
- Secondary and compensatory influences by three systems before described
- Under influece of external non genetic factors in particular dietary and lifestyle factors
So that also calciuria can be considered a continous variable with a polygenic determination and those phennotypic expression is modulated by non genetic environmental factors such as blodd pressure and body mass.
Parathyroid hormone hyposecretion or hypoactivity
Post-surgical
Radiation induced
Metastatic infiltration
Autoimmune (isolated or combined with polyglandular endocrine defects)
Autoimmune Polyglandular Syndrome type 1
It is linked to chromosome 21q22.3 coding for AIRE gene, inherited as autosomal recessive moitety. Loss-of-function mutation in AIRE, a zinc finger transcription factor present in thymus and lymph nodes, it is critical in mediating central tolerance by the thymus. NALP-5 is an intracellular signalling molecule strongly expressed in the parathyroid, and it can be target of specific parathyroid autoantigens in patients affected by APS-1. Autoantibodies to NALP5 were found in 49% of patients with APS-1 and hypoparathyroidism.
Clinical picture is variably present in people concentrated in Finnish, Iranian Jewis, and Sardinian populations, presenting more than 58 mutations. Classic triad is represented by:
- Mucocutaneous candidiasis
- Adrenal insufficiency
- Hypoparathyroidism
(any of these two conditions are suffcient to formulate the diagnosis of APS-1). Other different features include hypogonadism, type 1 diabetes, hypothyroidism, vitiligo, alopecia, keratoconjuntivitis, hepatitis, pernicious anemia, and malabsorption. More than 80% of patients with APS-1 have hypoparathyroidism, as sole endocrinopathy. Typically the disease is presented in childhood or adolescence, but patients with only one disease manifestation is folowed long-term for the appearance of other signs of disease.
Deposition of heavy metals
Thalassemia for iron excess
Hemochromathosis
Wilson’s disease
Severe magnesium depletion
alchoolism, malnutrition, malabsorption, diabetes, metabolic acidosis, renal disorders leading to magnesium wasting (pyelonephritis, postostructive nephropathy, renal tubular acidosis, acute tubular necrosis, drugs toxicity ( diuretics, cisplatinum, aminoglycoside antibiotics, amphotericin B, cyclosporin)
Primary renal magnesium wasting or familial hypomagnesiemia with hypercalciuria and nephrocalcinosis (OMIM 248250) due to mutations in genes coding for parcellin-1 and claudin 16
Hypermagnesiemia
On patients receiving tocolytic therapy or in patients with chronic kidney disease receiving magnesium supplements, antiacids or laxatives
Genetic disorders of PTH biosynthesis and parathyroid gland development
PTH gene mutations
Familial Isolated hypoparathyroidism
It is linked to chromosomal alteration of gene coding for pre-pro-PTH located on chromosome 11p15, and it is inherited in an autosomal recessive fashion. Mutations in signal peptides, disrupting PTH secretion, or in a donor splice site of the PTH gene, leading to skipping of PTH exon-2, which contains the initiation codon and signal peptide, are the molecular gene derangements accounting for the clinical picture. Very low or undetectable levels of of PTH and symptomatic hypocalcemia are main features of this syndromic complex.
Instead of mutations in signal peptides, we can have on the same chromosomal locus point mutations in the signal sequence of the pre-pro PTH that prevents processing and translocation of PTH across endoplasmic reticulum and memebrane exocytosis. Mutant PTH is believed to be trapped into endoplasmic reticulum inside cells; resulting stress in endoplasmic reticulum is thought to predispose cells to undergo to apoptosis.
Large deletions in transcription factor for gene coding for PTH called Glial cell Missing B or 2 transcription factor coded on chromosome 6p23-p24 (GCMB or GCM2) are autosomal recessive transmitted gene mutations responsible for forms of familial hypoparathyroidism due to large deletions of these transcription factors with subsequent loss-of-function mutation or point-mutations in the DNA-binding domain of these transcription factors. Leading to loss of transactivating capacity. Interestingly the two transcription factors are highly expressed in parathyroid cells and they controls the embryologic development of parathyroid glands
X-linked Hypoparathyroidism
The X linked recessive mutations affecting the chromosome Xq26-27 involve deletions or insertions of genetic material from chromosome 2p25.3 to chromosome Xq27.1, causing a position effect on regulatory elements controlling SOX3 transcription factor. SOX3 transcription factoris believed to be expressed during developement of parathyroid glands and its mutations cause parahtyroid agenesis of these glands.
Hypoparathyroidism may is a part of complex genetic syndromes:
Familial hypocalcemia with hypercalciuria:
The gene locus responsible is located on chromosome 3q13 and it is coding for calcium sensing receptor, the mutation is transmitted as autosomal dominant form and the phenotypic appearance of affected patients is cause by a gain-of-function mutation in calcium sensing receptor leading to milf hypocaòcemia and hypomagnesemia with hypocalciuria. Mutant receptors caused a left shifted set point for PTH secretion , definied as extracellualr calcium level necessary for half maximal suppression of PTH secretion. Most than 40 mutations have been identified at present, some of them responsible for the Batter’s syndrome type 5 (OMIM 601199 ).
Constitutive active Calcium Sensing Receptors
Most commonly coused by mutations and rarely caused by acquired antibodies that stimulates the calcium sensing receptor; appears to be among the most common causes of hypoparathyroidism.
Syndrome of hypoparathyroidism, deafness, and renal anomalies.
This syndromic complex is linked to mutations on chromosome 10p14-10pter, coding for the transcription factor GATA3. This mutation is inherited through an autosomal dominant form and interfere with the ability of GATA3 to bind to DNA or other transcriptional complexes. GATA3 is a transcription factor known to be highly expressed in in parathyroid glands, kidney and otic-vescicles during organ development precesses. So that clinical alterations characterizing this syndromic complex are hypoparathyroidism, bilateral sensorineural deafness, and renal anomalies or disfunction.
Syndrome of hypoparathyroidism with growth retardation, mental diseases and dysmorphism.
- Kenny-Caffey syndrome
- Sanjad-Sakati syndrome
The syndromic complex is due to mutations affecting chromosome 1q42-q43 coding for transcription factor TBCE and transmitted as autosomal recessive trait. TBCE mutations cause loss of function and alter the assembly of microtubules in affected tissues. Kenny-Caffey syndrome is presented such as hypoparathyroidism probably due to agenesia of the glands, shorth stature, osteosclerosis, cortical bone thickening, calcifications of basal ganglia, ocular abnormalities; whereas Sanjad-Sakati syndrome is characterized by parathyroid aplasia, growth failure, ocular amalformations, microencephaly, and mental retardation.
DiGeorge Syndrome or VeloCardioFacial Syndrome
An heterozygous deletion of chromosome 22q11.2 coding for the transcription factor TBX1 is the known cause of this syndrome. Loss of function mutation of TBX1 is responsible for loss of adjuvating action by TBX1 on other transcription factors known to be involved into the development of thymus and parathyroid glands. Embriological alterations are demonstrated to occurs in these patients in the formation and development of thrid and fourth branchial pouches. Wide spectrum of phenotypc expression, may include conotruncal cardiac defects, parathyroid and thymic hypoplasia, neurocognitive problems, and palatal, renal, ocular, and skeletal abnormalities. Hypocalccemia (in 50% of patients) can be transient or permanent and can develop in adulthood. A screening test si available with confirmed dletion by FISH technique.
Mithocondrial disorders with hypoparathyroidism
- Kearns-Sayre syndrome
- MELAS syndrome
- MTPDS syndrome
Are known syndromic complexes due to deletions, mutations, rearrangments and duplications in the mitochondrial genome. These diseases are inherited uniquely by maternal cells (as all mithocondrial structures) and hypopararhytoidism can be present with various syndromic complexes:
In Kearns-Sayre syndrome with progressive external ophtalmoplegy, pigmentary retinopathy, hearth block or cardiomegaly, diabetes. In MELAS syndrome with diabetes only In MTPDS with fatty acids oxidation alterations, peripheral europathy, retinopathy, acute fatty liver in pregnancy.
Resistance to PTH action
Pseudo-Hypoparathyroisim Type 1a
The disease is due to inactivating mutation in the gene coding for the subunit alfa of G protein coupled with PTH Receptor (GNAS gene on chromosome 20q13.3).
GNAS gene is able to code for the a-subunit of the stimulatory G protein (GSa) and it is located on Chromosome 20q11, where 13 exons are present with differnt promoter regions. It is well demonstrated that this protein is linked to many transmembrane receptors such as Parathyroid hormone receptor, TSH Receptor, FSH and LH Receptors, GH Receptor.
During the past few years it became apparent that GNAS gene enchodes not only for for GSa but also for several splice variants:
1. XLas (paternal allele)
2. NESP55 neurosecretory protein (maternal allele)
3. A/B (1A) (paternal allele)
4. Antisense transcript
Later it was demosntrated that the alternative exons and their promoter regions are “methylated” on one parental allele, giving rise only to “non-methylated” allele transcription.
Moreover, in most tissues the transcripts encoding GSa are derived from both alleles; whereas in a few tissues such as
. proximal renal tubular cells
. adipocytes
. pituitary cells
GSa appears to be expressed only from maternal allele.
In the type 1a the mutation is an heterozygous inactivating mutation transmitted with autosomal dominant pattern with maternal transmission of the biochemical phenotype. Clinical features include those described first as Albright’s Hereditary Osteodystrophy such as round facies, mental retardation, frontal bossing, shorth stature, obesity, brachydactyly, ectopic ossification, hypocalcemia, hyperphopshatemia, evelated PTH levels, hypothyroidism, hypogonadism.
Pseudohypoparathyroidism Type 1b
The disease is due to a partenally imprinted defect in G protein due to methylation defect in exon A and exon B, this alterations lead to a selective resistance only to parathyroid hormone and not to other G coupled receptors linking other hormones. So the features are not those present in classic Albright Hereditary Osteodystrophy but hypoparathyroidism with elevalted PTH values, hypocacemia, hyperphosphatemia and elevated levels of urinary cAMP after administration of PTH.
Pseudohypoparathyroidism Type 2 or Pseudo-pseudo-hypoparathyroidism
It is due to GNAS inactivating mutation paternally inherited; however a resistance to PTH is present so that patients secrete normal urinary cAMP levels but not phosphaturic responses to PTH. It can have inheried or sporadic occurrence.
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Calcium homeostasis
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