Genetic Disorders of Calcium and Phosphorus Metabolism Related with Parathyroid Glands

Abstract

Calcium (Ca), phosphorus (phosphate, HPO4), and magnesium (Mg) are essential nutrients that are critical for the structural integrity and functions of the body. Therefore, disorders of calcium and phosphorus metabolism lead to serious and even life-threatening consequences such as skeletal and cardiovascular morbidity. Parathyroid hormone (PTH), calcitonin, and the active form of vitamin D (calcitriol, 1,25-dihydroxyvitamin D3) hormones are the main hormones that are responsible for regulating the calcium and phosphorus level in the blood. Hypoparathyroidism is due to insufficient circulating parathyroid hormone levels characterized by hypocalcemia and hyperphosphatemia. Besides being an isolated condition or a component of a complex syndrome, the causes of hypoparathyroidism are rarely genetic. Primary hyperparathyroidism is a disorder that results in excessive, uncontrolled production of parathyroid hormone. Rarely, primary hyperparathyroidism caused by genetic disorders is associated with an inherited familial germline mutation syndrome such as familial isolated hyperparathyroidism and multiple endocrine neoplasia type 1 and type 2A. Although genetic disorders are not the most common cause of hyper/hypoparathyroidism, molecular analyses have identified an increasing number of genes that cause loss or gain of function of genes related to calcium and phosphorus metabolism.

1. Introduction

Around 99% of total body calcium is present in the bones with the remaining 1% being found in the extracellular fluid and cellular organelles. Approximately 50% of total serum calcium is bound to plasma proteins, mostly albumin and globulin, and 5% is complex with citrate, lactate, and bicarbonate. The other 45% of serum calcium is an ionized and biologically active form of calcium. Serum total and ionized calcium concentrations are associated with levels of albumin, creatinine, parathyroid hormone (PTH), phosphate, and serum pH. Ca²⁺ serum concentration is regulated by a combined system including the extracellular calcium-sensing receptor (CaSR); PTH and its receptor (PTH/PTH-related protein PTHrP-1R); calcitonin and its receptor; and vitamin D hormone system that acts on the intestinal tract, bone, and kidney. The absorption of calcium is enhanced in the intestinal tract, renal tubule, and bones in response to calcitriol (1,25OH2D3) and/or PTH. PTH and calcitriol mobilize calcium from the hydroxyapatite apatite crystal, and calcitonin secreted by the parafollicular C cells of the thyroid gland suppresses PTH secretion. PTH increases concentrations of plasma Ca²⁺, lowers serum phosphate values, accelerates the synthesis of calcitriol, and stimulates both anabolic and catabolic effects on bone [1, 2].

The parathyroid glands develop together with the thymus from the third and fourth pharyngeal pouch. Hypoparathyroidism, also called PTH deficiency, causes hypocalcemia, hyperphosphatemia, and increased neuromuscular irritability [3]. It is usually caused by the postoperative complications of head and neck surgery in an individual. The other most common cause of hypoparathyroidism is autoimmune destruction of parathyroid tissue associated with autoimmune disorders that cause destruction or damage to the glands. In infancy, hypoparathyroidism due to delayed developmental maturation of the parathyroid is usually transient and can be resolved within the first few weeks [4, 5]. When hypoparathyroidism is persistent, it may result; from an error in embryogenesis or destruction of the parathyroid glands, disorders resulting in impaired PTH synthesis or secretion, or peripheral unresponsiveness to PTH.

2. Disorders of parathyroid gland formation

2.1 22q11 deletion syndrome

22q11.2 deletion syndrome, also known as Di George syndrome, is the most common human contiguous gene deletion caused by a microdeletion of chromosome 22 with a wide phenotypic spectrum that is highly variable even within the family. It is characterized by particularly conotruncal cardiac defects, immune deficiency, palatal anomalies, craniofacial dysmorphism, and hypoparathyroidism. Additionally, patients with 22q 11.2 deletion may have feeding difficulties, poor growth, intellectual disability, and psychiatric problems [6]. Most patients with 22q11 deletion have heterozygous 2.5–3 Mb deletions including TBX1 gene, which encodes the T-box transcription factor gene 1. Patients with point mutations in TBX1 manifest many of the clinical findings of cases with 22q11 deletions [7]. This microdeletion syndrome usually occurs de novo but can also be inherited from a parent (5–10%). The estimate that DiGeorge syndrome affects one in every 2000–4000 live births may be an underestimate as it is based on major birth defects. Because some people with deletions will have few symptoms, these cases may go undiagnosed. Hypoparathyroidism in patients with 22q11.2 deletion results from aplasia or hypoplasia of parathyroid glands. It has been reported that 30–60% of patients with hypoparathyroidism 22q11.2 causing hypocalcemia are present especially in the neonatal period [8]. Many forms of hypoparathyroidism including late-onset, transient, latent, and recurrent types have been reported in patients with 22q11 deletion syndrome [9]. Although calcium homeostasis generally improves with age, hypocalcemia tends to occur in adolescence, pregnancy, and later in life can occur during times of stress (for example, surgery or serious illness [10, 11]). It has been reported that 80% of adults with 22q11.2 deletion patients may have hypocalcemia, hypoparathyroidism, hypothyroidism, and hypomagnesemia [12]. Typical features of 22q11 deletion include thymic aplasia with impaired T-cell mediated immunity, conotruncal cardiac defects, cleft palate, and dysmorphic facies with mid-face hypoplasia and tubular nose [13].

2.2 CHARGE syndrome – CHD7, SEMA3E

CHARGE syndrome is an acronym for Coloboma, Heart disease, Atresia of the choanae, Retarded growth and mental development, Genital anomalies, Ear malformations, and hearing loss with an incidence of 1 in 8,500–10,000 [14]. Hypoparathyroidism due to parathyroid hypoplasia may accompany as a component of CHARGE syndrome. Most patients have a heterozygous loss-of-function mutation within the coding region of the Chromodomain helicase DNA binding-7 (CHD7) gene on chromosome 8q1 [15]. CHD7 is a member of the Chromodomain helicase DNA-binding protein (CHD) family of ATP-dependent chromatin remodelers, which catalyze nucleosome movement on DNA. CHD7 functions as a positive regulator of ribosomal RNA (rRNA) biogenesis in the nucleolus and uses the energy of ATP to remodel nucleosomes. CHARGE is an autosomal dominant disease often caused by a de novo pathogenic variant. Rarely, the patients with CHARGE Syndrome inherit the mutation from an affected parent. A less common cause is due to abnormalities involving semaphorin 3E (SEMA3E) that are involved in embryonic development for neural guidance [16].

2.3 Barakat syndrome (Hypoparathyroidism, Deafness and Renal dysplasia syndrome HDRS)

Hypoparathyroidism, sensorineural deafness, and renal dysplasia syndrome (HDRS), also known as Barakat syndrome, is an autosomal dominant disorder caused by a monoallelic mutation in the GATA3 gene on chromosome 10p14 [17, 18]. The GATA3 protein is one of the transcription factors required for the development of the pharyngeal sacs and the differentiation and organization of the parathyroid glands. It is expressed in parathyroid glands as well as in the thymus, kidney, inner ear, and central nervous system. GATA3 interacts with two known transcriptional regulators of parathyroid development, GCM2 (isolated parathyroid aplasia) and MAFB, and synergistically stimulates the promoter of the PTH gene, thereby activating PTH gene transcription and regulating PTH gene expression. Barakat syndrome is characterized by the classic triad of hypoparathyroidism, sensorineural deafness, and/or renal disease. Renal manifestations may include renal dysplasia, progressive renal failure, proteinuria, glomerulonephritis, and renal agenesis. Renal abnormalities are observed in 60% of patients and are highly variable, with only a small proportion (9%) of cases progressing to end-stage renal disease [19]. Additional clinical features include congenital heart disease, hypogonadotropic hypogonadism, polycystic ovaries, retinitis pigmentosa, and intellectual disability. There is wide phenotypic variability with hypoparathyroidism ranging from asymptomatic or transient neonatal hypocalcemia to severe symptomatic or persistent hypocalcemia. Sensorineural hearing loss is typically discovered during infancy or childhood is usually bilateral and present in more than 95% of cases [20].

2.4 Sanjad-Sakati Syndrome (Hypoparathyroidism-Retardation-Dysmorphism syndrome HRDS)

The Sanjad-Sakati syndrome of congenital hypoparathyroidism, mental retardation, and dysmorphism (HRD, OMIM 241410) is an autosomal recessive disorder caused by biallelic loss-of-function mutations in TBCE (tubulin-specific chaperone E) gene on chromosome 1q42 [21]. Facial dysmorphic features include a prominent forehead, deep-set eyes, microphthalmia, depressed nasal bridge, beak nose, long philtrum, thin upper lip, micrognathia, bifid uvula, and ear abnormalities. Most patients have intrauterine and postnatal growth retardation, developmental delay, hypocalcemic seizures [22]. TBCE gene encodes a chaperone protein required for formation, folding, and stability of alpha-tubulin subunits and the formation of alpha-beta-tubulin heterodimers. Kenny-Caffey syndrome (OMIM 244460), allelic to Sanjad-Sakati syndrome, manifests cortical thickening and medullary stenosis of the long bones, osteosclerosis of the skull, and recurrent bacterial infections [23].

2.5 Mitochondrial disorders associated with hypoparathyroidism

Hypoparathyroidism may accompany mitochondrial disorders whose typical clinical features are lactic acidosis, cataracts, sensorineural deafness, and myopathy/ophthalmoplegia. The mitochondrial disorders associated with hypoparathyroidism include Kearns-Sayre syndrome, mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS), long-chain 3-hydroxyacyl-CoA dehydrogenase (LCAHD), medium-chain acyl-CoA dehydrogenase deficiency (MCADD), and combined mitochondrial trifunctional protein (MTP) [24].

3. Disorders of parathyroid gland destruction

Autoimmune polyendocrine syndrome 1 (APS1) is an autosomal recessive disorder characterized by the classic triad of autoimmune polyendocrinopathy, mucocutaneous candidiasis, and ectodermal dystrophy (APECED). There are many additional autoimmune endocrinopathies including hypoparathyroidism, hypothyroidism, insulin-dependent diabetes mellitus, primary adrenal insufficiency, and ovarian/testicular failure. Less often non-endocrine signs are pernicious anemia, hepatitis, keratoconjunctivitis, vitiligo, alopecia, malabsorption, and metaphyseal dysplasia. APS1 is caused by homozygous or compound heterozygous loss-of-function variants of the autoimmune regulator gene (AIRE, OMIM 607358) [25]. Almost all women with APECED and 80% of affected men develop hypoparathyroidism. Despite wide variability in clinical expression, there is no significant association between genotype and phenotype. The diagnosis of APECED usually requires the presence of two of the three main findings (mucocutaneous candidiasis, hypoparathyroidism, and hypoadrenocorticism), but sometimes just one of the three findings alone may be the sole manifestation of an inactivating mutation in AIRE. Later manifestations of APECED involve esophageal and oral squamous cell carcinoma, asplenia, and interstitial nephritis [26].

4. Disorders resulting in impaired PTH synthesis or secretion

The parathyroid glands develop together with the thymus from the third and fourth pharyngeal sacs under the impact of multiple transcription factors including T-box 1 (encoded by TBX1), GATA-binding factor/protein 3 (encoded by GATA3), glial cell missing, drosophila, homolog 2 (encoded by GCM2), and V-MAF musculoaponeurotic fibrosarcoma oncogene family, protein B (encoded by MAFB), an essential factor that supports in the separation of parathyroid glands from thymus and migration to the thyroid. Variants of GATA3 are associated with the syndrome of hypoparathyroidism, sensorineural deafness, and renal disease, whereas mutations in GCM2 result in both familial hypo- and hyperparathyroidism [27, 28]. Familial isolated congenital hypoparathyroidism can be transmitted as an autosomal dominant, autosomal recessive, or X-linked recessive trait resulting from loss-of-function mutations in genes necessary for differentiation of the parathyroid glands causing congenital aplasia or hypoplasia of these structures. Familial isolated hypoparathyroidism is caused by heterozygous, homozygous, or compound heterozygous mutation in the parathyroid hormone gene PTH (Familial isolated congenital hypoparathyroidism 1) gene, GCM2 gene (Familial isolated congenital hypoparathyroidism 2) [29, 30].

4.1 PTH gene

Familial isolated hypoparathyroidism-1 (FIH1) is caused by heterozygous, homozygous, or compound heterozygous mutation in the parathyroid hormone gene (PTH) on chromosome 11p15 [31]. PTH gene is composed of three exons, where its second and third exons of PTH encode the prepro-PTH sequence of 115 amino acids that is processed to intact 84 amino acids PTH that is released from the parathyroid glands in response to decreasing serum concentrations of Ca²⁺, detected by the CaSR expressed on the plasma membrane of the parathyroid gland chief cell. Familial isolated hypoparathyroidism 1 is caused by monoallelic/biallelic mutations in the PTH gene located on chromosome 11p15.3 that impair synthesis of PTH. Since the PTH gene was discovered, only eight pathogenic variants in PTH gene have been identified. A few patients with FIH1 have been associated with heterozygous Cys18Arg mutation in PTH gene that mutation induces endoplasmic reticulum stress and subsequent apoptosis in parathyroid cells. A donor splice site mutation at the nucleotide 1 of intron 2 of PTH leads to exon 2 skipping, with loss of the initiator codon (ATG), the resultant mutant allele cannot initiate the translation of PTH mRNA into the pre-pro-PTH protein, and the translocation of the PTH peptide through the ER prior to secretion [32].

4.2 GCM2 gene

Familial isolated hypoparathyroidism-2 (FIH2) is caused by homozygous mutation in the glial cells missing transcription factor-2 gene (GCM2) on chromosome 6p24. Patients with familial isolated hypoparathyroidism-2 (FIH2) usually present with seizures, caused by hypocalcemia, in early life. Serum parathyroid hormone (PTH; 168450) levels are low to undetectable. GCM2 is a transcription factor whose expression is restricted to the parathyroid glands [30]. Biallelic loss-of-function mutations in exons 2, 3, and 5 of GCM2 result in hypoparathyroidism. Mutations in GCM2 exons 2 and 3 (encoding DNA binding and transactivation domain 1) lead to impaired protein synthesis and stability and autosomal recessive transmission of congenital hypoparathyroidism, whereas those in exon 5 (encoding transactivation domain 2) lead to mutations with a dominant negative effect and autosomal dominant transmission of this disorder. Expression of GCM2 occurs immediately after specification of parathyroid cells and is dependent on the normal transcriptional function of the mutated gene, GATA3, in patients with Barakat syndrome. Gain-of-function mutations of GCM2 are associated with hyperparathyroidism [33].

5. Disorders of peripheral unresponsiveness to PTH

5.1 Pseudohypoparathyroidism Type 1a (PHP1a) and Albright Hereditary Osteodystrophy (AHO)

Pseudohypoparathyroidism type 1a (PHP1a), Albright hereditary osteodystrophy (AHO), and Pseudopseudohypoparathyroidism (PPHP) are caused by heterozygous inactivating mutations of the GNAS gene, which encodes the Gsα subunit necessary for peptide hormone signal transduction of GCPRs. Patients with PHP1a have end-organ resistance to PTH with hypocalcemia, hyperphosphatemia, and elevated PTH levels [34]. In addition, patients present with short stature, craniofacial anomalies, shortened fingers, and short fourth and fifth metacarpals characterized by heterotopic ossifications called Albright hereditary osteodystrophy. Patients show resistance to hormones whose receptors bind to Gs such as GHRH, TSH, gonadotropin, calcitonin, and hypothalamic neurotransmitters. PHP1a patients also have neurocognitive deficiency and obesity reflecting the effect of Gαs in the brain. The reason for these two different manifestations of the same gene defect is due to a complex genomic imprinting mechanism that controls the transcription of the GNAS gene [35, 36]. Patients with a GNAS mutation on a maternal allele will develop a more severe form of Gsα deficiency with hormone resistance (PHP1a), whereas patients with identical mutations on the paternal GNAS allele will have a milder condition with normal hormone response (PPHP) [37].

5.2 Parathyroid-hormone-related peptide receptors PTHrP

The type 1 PTH receptor (PTHR1) is activated by PTH and parathyroid-hormone-related peptide (PTHrP) and mediates PTH effects in bone and kidney. Mutations in PTHR1 have been reported in two types of skeletal dysplasia with different clinical manifestations, Metaphyseal chondrodysplasia, Murk Jansen type, a dominant disorder resulting from a gain-of-function mutations and accelerated chondrocyte differentiation, and short-limbed dwarfism in Blomstrand chondrodysplasia, a recessive lethal disorder resulting from loss-of-function mutations [38].

5.3 Calcium sensing receptor (CASR)-related disease

The synthesis and secretion of PTH by parathyroid gland chief cells are regulated by Ca²⁺ concentrations acting through the CaSR that either enhance (when Ca²⁺ concentrations are low) or repress (when they are high) transcription of PTH and secretion of PTH; Ca²⁺ values also modulate the rate of chief cell proliferation, a response also mediated by the CaSR. The extracellular calcium-sensing receptor (CaSR) is a G-protein-coupled receptor (GPCR) that is mostly expressed in the parathyroid and kidneys. CaSR allows regulation of parathyroid hormone (PTH) secretion and renal tubular calcium re-absorption in response to changes in extracellular calcium concentrations [39]. There are calcium-sensing receptor (CASR)-related diseases associated with hypercalcemic and hypocalcemic disorders. While inactivating CaSR mutations are associated with hypercalcemic disorders of familial benign hypercalcemia (FBH), neonatal severe primary hyperparathyroidism (NSHPT), adult primary hyperparathyroidism, and autoimmune hypocalciuric hypercalcemia (AHH); activating CaSR mutations result in autosomal-dominant hypocalcemia with hypercalciuria (ADHH) and Bartter-like syndrome. Furthermore, CaSR auto-antibodies have been found in patients with hypercalcemia or hypocalcemia.

CaSR abnormalities are associated with four hypercalcemic disorders, which are familial benign hypercalcemia (FBH), neonatal severe primary hyperparathyroidism (NSHPT), adult primary hyperparathyroidism, and autoimmune hypocalciuric hypercalcemia (AHH). CaSR abnormalities are associated with three hypocalcemic disorders, which are autosomal-dominant hypocalcemic hypercalciuria (ADHH, Bartter syndrome type V (i.e., ADHH with a Bartter-like syndrome)), and a form of autoimmune hypoparathyroidism (AH) due to CaSR autoantibodies [40].

5.4 Hyperparathyroidism

Pituitary adenomas occur in isolation or may be part of a genetic syndrome, such as multiple endocrine neoplasia type 1 (MEN1) or McCune-Albright syndrome. MEN1 is an inherited tumor syndrome characterized by glandular hyperplasia and benign or malignant neoplasms occurring in two or more endocrine glands, classically the parathyroids, pituitary, and neuroendocrine pancreas [41]. Patients are also at risk for developing adrenocortical tumors, pheochromocytoma (PHEO), extra-abdominal neuroendocrine tumors, benign tumors of the skin (angiofibromas, lipomas, collagenomas, lipomas), and central nervous system tumors (meningiomas and ependymomas). The MEN1 gene is a tumor suppressor gene that encodes a nuclear protein, which plays a role in transcriptional regulation, genome stability, cell division, and proliferation. Most patients with MEN1 inherit the mutation from an affected parent, but about 10% of individuals with MEN1 represent de novo. An inherited heterozygous germline MEN1 mutation is insufficient to induce tumor formation. Therefore, a somatic mutation (the second hit) in the wild-type MEN1 allele is required to cause disease. Primary Hyperparathyroidism (PHPT) is the most common and initial endocrine manifestation of MEN1. MEN1 patients have multigland parathyroid hyperplasia rather than single gland adenomas. Most children with MEN1-related PHPT are asymptomatic, and nephrolithiasis is the major hallmark of symptomatic disease [42].

MEN2A is characterized by medullary thyroid carcinoma (MTC), PHEO, and primary hyperparathyroidism. MEN2A is caused by heterozygous gain-of-function mutations in the RET proto-oncogene. More than 90% of RET mutation carriers have a high penetrance for developing thyroid cancer. MEN 2B is characterized by clinically aggressive MTC, pheochromocytoma, a Marfanoid body habitus, mucosal neuromas, and intestinal tumors.

Pathogenic variants involving cysteine codons 609, 618, and 620 in exon 10 of RET are associated with MEN2A, while the RET germline pathogenic variant p.Met918Thr is associated with MEN2B only. More than 90% of RET mutation carriers have a high penetrance for developing thyroid cancer. Most MEN2B cases arise as a result of a de novo mutation, with the child having unaffected parents [43].

6. Conclusion

Although genetic disorders are not the most common cause of hyper/hypoparathyroidism, molecular analyses help us understand mutations in genes involved in calcium and phosphorus metabolism. Furthermore, clarifying the genetic etiopathogenesis of hyper/hypoparathyroidism may contribute to management, prevention of comorbidities, and genetic counseling.