Genes implicated in syndromic parathyroid neoplasia and related hypercalcemic states.
Regulation of serum calcium in vertebrates is maintained by the actions of the parathyroid glands working in concert with vitamin D and critical target tissues that include the renal tubules, the small intestine, and bone cells. The parathyroid glands release parathyroid hormone (PTH) into the systemic circulation as is required in order to maintain the serum calcium concentration within a narrow physiologic range. Excessive secretion of PTH from one or more abnormal parathyroid glands however results in primary hyperparathyroidism (HPT), a metabolic disease typically associated with abnormally elevated serum calcium. Although HPT is typically a sporadic disease, it can represent a manifestation of an inherited syndrome. Many sporadic parathyroid tumors result from inactivating mutations in tumor suppressor genes that were first discovered by the analysis of genomic DNA from patients with HPT as part of an inherited syndrome. Somatic and inherited alterations in DNA encoding proto-oncogenes can also cause parathyroid neoplasia. Two promising future approaches for the discovery of novel genes pertinent to parathyroid tumor development are the analysis of acquired genetic alterations in DNA isolated from parathyroid tumors and the investigation of familial HPT in kindreds lacking germline mutation in the known genes predisposing to HPT.
- multiple endocrine neoplasia
- jaw tumor syndrome
- tumor suppressor
The inappropriate or excessive secretion of parathyroid hormone (PTH) from one or multiple abnormal parathyroid glands typically results in hypercalcemia and the disorder of mineral metabolism called primary hyperparathyroidism (HPT) . Most cases of HPT are sporadic (~95%). Among the small remaining fraction of patients with an inherited basis for HPT, most harbor germline mutation of a known parathyroid tumor susceptibility gene (listed in Table 1). In spite of their infrequency, study of the genetics of these uncommon inherited syndromes has yielded substantial insight into the etiology of both sporadic and familial parathyroid tumor development. Since the release of PTH from parathyroid cells involves close regulation by the calcium-sensing receptor (CASR), a cell surface transmembrane receptor of the G protein-coupled receptor family C , the germline mutation of the CASR and other genes mediating its signaling can also result in inherited syndromes characterized by hypercalcemia and circulating levels of PTH that are elevated or inappropriately normal. This chapter will summarize current knowledge of the clinical genetics and molecular pathophysiology of HPT that results from both benign and malignant parathyroid gland neoplasia.
|Gene||Corresponding protein||Chromosomal location||Associated hyperparathyroid syndrome: main syndromic manifestations||Features of syndromic parathyroid tumors|
|Menin||11q13.1||Multiple endocrine neoplasia type 1 (MEN1): anterior pituitary, parathyroid, enteropancreatic, foregut carcinoid tumors||Multiple, asymmetric tumors typical (>99% benign)|
|Parafibromin||1q31.2||Hyperparathyroidism-jaw tumor syndrome: fibro-osseous jaw, parathyroid, uterine tumors; renal cysts||Single tumor common (~20% malignant)|
|P27(Kip1)||12p13.1||Multiple endocrine neoplasia type 4 (MEN4): anterior pituitary, other involvement varies||Single to multiple glands (benign in reports to date); can be recurrent|
|Glial cells missing transcription factor 2||6p24.2||Familial isolated primary hyperparathyroidism||Single to multiple glands|
|Calcium-sensing receptor||3q13.33-q21.1||Familial hypocalciuric hypercalcemia type 1 (FHH1) with heterozygous inactivation; neonatal severe hyperparathyroidism (NSHPT) with homozygous inactivation||FHH1: near-normal size and surgical pathology; altered serum calcium set-point for PTH release|
NSHPT: marked enlargement of multiple glands by polyclonal (non-neoplastic) mechanism
|G protein α11 subunit||19p13.3||Familial hypocalciuric hypercalcemia type 2 (FHH2)||ND|
|Adaptor protein-2 sigma subunit||19q13.32||Familial hypocalciuric hypercalcemia type 3 (FHH3): hypercalcemia more severe than in FHH1||ND|
|c-Ret||10q11.21||Multiple endocrine neoplasia type 2A: medullary thyroid cancer, pheochromocytoma, parathyroid tumors||Single tumor common (>99% benign)|
|Cyclin D1||11q13.3||NA (to date, only implicated in sporadic parathyroid tumors)||NA (to date, only implicated in sporadic parathyroid tumors)|
2. The evolution of calcium regulation in vertebrates
In sea water the concentration of elemental calcium is approximately 10 mM. As a result, early eukaryotes living in a marine environment had easy access to calcium. Given this abundant supply of extracellular calcium, numerous intracellular processes evolved in simple eukaryotes that depended on this divalent cation. Such calcium-dependent processes were preserved in metazoans. Thus marine chordates and early vertebrate fish depended on calcium for cellular processes such as membrane permeability, neurotransmitter release, intracellular second messenger signaling, muscular contraction, neuromuscular excitability, and the actions of multiple calcium-dependent enzymes. Calcium’s particular coordination chemistry facilitated many proteins’ ability to reversibly bind divalent calcium ions, thus enabling signaling through such binding .
Calcium is much scarcer on land compared to the marine environment. As lobe-finned fish, marine vertebrates believed to be the ancestors of the early amphibians, began to explore the periphery of the terrestrial environment, evolutionary pressure to develop a system of internal calcium balance mounted. A system of internal calcium homeostasis at the organismal level would ensure the continued preservation and function of numerous cellular and tissue operations that vitally depended on calcium.
Metabolically-active trabecular or cancellous bone in lobe-finned fish and associated hematopoietic bone marrow likely co-evolved . These developments probably both lightened overall skeletal mass and provided a reliable internal source of calcium as a basis for calcium homeostasis. The lightening of skeletal mass was critical since lobe-finned fish and early amphibians had to come to terms with full gravitational force in their terrestrial movements, no longer buoyed by surrounding seawater in accordance with Archimedes’ principle . The potential significance of the close physical apposition of hematopoietic bone marrow to spongiform bone, inferred from X-ray synchrotron microtomography of fossilized lobe-finned fish humerus , is suggested by the realization that osteoclasts, cells uniquely specialized to mobilize ionized calcium via resorption of bone, develop from hematopoietic stem cell precursors . In contrast, osteoblasts, which lay down osteoid and mineralize bone, derive from mesenchymal stem cells which are abundant in non-hematopoietic bone marrow.
Although analogs of Gcm2, Gata3, CaSR, PTH, and other genes associated with the development and function of human parathyroid glands are expressed in the fish gills, actual parathyroid glands are first seen in amphibians [7, 8, 9]. Complete surgical excision of parathyroid gland tissue in amphibians, reptiles, birds, and mammals results in tetany and death.
3. The pathophysiology of primary hyperparathyroidism
PTH secretion from cells of the parathyroid glands is finely regulated in response to changes in the ambient ionized calcium level in order to maintain the circulating calcium concentration within a defined physiologic range. The G protein-coupled CASR is a critical regulator of PTH secretion and is located on the plasma membrane of chief cells in the parathyroid glands [10, 11]. In a classic endocrine negative feedback loop, the active form of cholecalciferol, 1,25-dihydroxyvitamin D, whose synthesis is stimulated by PTH acting on proximal renal tubular cells, inhibits PTH biosynthesis and release from parathyroid cells [12, 13, 14, 15]. The simultaneous demonstration of elevated serum calcium with an inappropriately normal or elevated PTH is a typical clinical definition of HPT . The vast majority of parathyroid tumors are adenomas (i.e. benign tumors), with parathyroid cancer accounting for less than 1% of HPT in most series.
Most cases of HPT are sporadic with inherited forms of HPT representing only 2–5% of cases. As illustrated in Table 1, research into the molecular pathophysiology of this small subcategory of cases has notwithstanding yielded important understanding with respect to the genes and pathways that promote parathyroid tumorigenesis. Multiple endocrine neoplasia type 1 (MEN1), multiple endocrine neoplasia type 2A (MEN2A), the hyperparathyroidism-jaw tumor syndrome (HPT-JT), and familial isolated hyperparathyroidism (FIHP) are the most common inherited disorders associated with HPT [17, 18, 19, 20, 21]. Familial hypocalciuric hypercalcemia (FHH) is a related and largely benign autosomal dominant condition characterized by lifelong asymptomatic hypercalcemia. Often mis-diagnosed as HPT, in FHH the PTH-dependent hypercalcemia does not correct with partial or even subtotal parathyroidectomy . The relevance of these inherited disorders to the underlying molecular pathogenetic alterations in parathyroid tumorigenesis will be discussed in more detail below.
4. Oncogenes and proto-oncogenes
Mutant genes that drive cell growth are called oncogenes and represent one potential molecular mechanism for tumor development. Oncogenes are mutationally activated versions of naturally occurring genes, called proto-oncogenes, which under normal conditions positively regulate cell division and/or cell growth . Oncogenes represent gain-of-function mutants or overexpressed forms of proto-oncogenes that can induce cell growth and cell division, often in a tissue-specific fashion, resulting in tumor formation. Proto-oncogenes often encode proteins that are involved in mitogenic signal transduction. In the context of currently recognized familial cancer syndromes, germline mutational activation of proto-oncogenes is rare as an etiology compared to the inactivation of tumor suppressor genes (see below). Constitutive proliferative signaling resulting from the germline activation of most proto-oncogenes would presumably be deleterious to embryonic and fetal development.
5. The role of tumor suppressor genes in tumor development
Alfred Knudson proposed another model for tumor development based on the study of retinoblastoma disease patterns nearly 50 years ago . Sporadic retinoblastoma is usually monocular. Familial retinoblastoma, though rare compared to the sporadic form, is more frequently binocular and has a much earlier age of onset. The “two-hit” hypothesis of tumor development, as proposed by Knudson, hypothesizes that two events (or “hits”) in a parental cell confer a selective growth advantage and result in that cell’s clonal expansion .
Newer clinical and molecular genetic insight that has emerged since his original proposal allow us to update Knudson’s concept. In many hereditary tumor syndromes, an inherited germline DNA mutation that affects one copy of a tumor suppressor gene represents the first “hit” or event and is present throughout all cells of the affected offspring. The greater likelihood of any particular cell acquiring a “second hit”, i.e. a somatic mutation in the second allele of the same tumor suppressor gene that was heretofore unaffected, accounts for the earlier age of onset and predisposition for bilateral and multifocal disease in hereditary tumor syndromes. This “second hit” in somatic DNA, that disables the remaining wild-type allele, typically results from a deletion that involves a portion or the entirety of a chromosome. In the familial tumor syndromes MEN1 and HPT-JT, inactivating mutation that involves both alleles of the
6. Multiple endocrine neoplasia type 1 (MEN1)
MEN1 is the most common hereditary cause of primary hyperparathyroidism . The syndrome of MEN1 is characterized by the predisposition to develop tumors derived from cells in the anterior pituitary, parathyroid glands, and endocrine cells present in the gut and pancreatic islets (such as gastrinomas, and pancreatic neuroendocrine tumors such as insulinomas) . Tumors in several other endocrine organs and non-endocrine tumors such as lipomas, angiofibromas, and leiomyomas affecting the esophagus, uterus, and/or ureters for example, can also be associated with the syndrome . HPT is the most penetrant hormonal feature of MEN1.
Familial MEN1 is characterized by autosomal dominant transmission. The predisposition to tumor development in one of the tissues characteristically involved in the MEN1 syndrome is caused by germline inactivating mutation in one copy of the
Conventional DNA sequencing of tumor DNA has identified somatic
Mutation of the
7. The hyperparathyroidism-jaw tumor syndrome (HPT-JT)
HPT-JT is a familial syndrome with variable and incomplete penetrance transmitted in an autosomal dominant fashion. The key clinical features of HPT-JT include HPT, jaw tumors (fibro-osseous tumors involving the maxilla and/or mandible, formally classified as cemento-ossifying fibromas , and distinct from so called “brown” tumors sometimes associated with HPT), renal cysts or tumors and uterine tumors in women [43, 44, 45]. HPT is the most penetrant feature of HPT-JT and is usually the presenting manifestation. In contrast to MEN1, parathyroid cancer is frequent in HPT-JT, affecting some 20% or more of those with HPT [43, 44, 45, 46].
In the majority of HPT-JT kindreds, a germline loss-of-function mutation of the
8. Multiple endocrine neoplasia type 4 (MEN4)
MEN4 is a syndrome originally described by Pellegata and coworkers in a multi-generational family with features resembling MEN1, including a proband with a growth hormone-secreting pituitary adenoma and HPT, but lacking germline
Following the original report by Pellegata et al. , several groups have investigated a possible role for
Recent evidence supports the characterization of
9. Familial isolated hyperparathyroidism (FIHP)
By definition, FIHP is a non-syndromic category of familial HPT describing families that contain two or more members with HPT but which lack the specific features of MEN1, MEN2A, HPT-JT or FHH (Figure 1) . FIHP is genetically heterogeneous and is a diagnosis of exclusion. While at the time of initial ascertainment germline mutation of
Missense variants in GCM2, a transcription factor homologous to the Drosophila “glial cells missing” (gcm) gene and required for parathyroid gland development, were recently described in the germline DNA of eight unrelated families with FIHP . Previous studies showed that germline dominant-negative and loss-of-function mutations in GCM2 were associated with autosomal dominant and autosomal recessive familial isolated hypoparathyroidism, respectively [79, 80]. The two rare germline
10. Familial hypocalciuric hypercalcemia (FHH)
FHH is a condition of PTH-dependent hypercalcemia, often resembling HPT, that is clinically benign and genetically heterogeneous (Table 1) . Following partial or subtotal parathyroidectomy, affected patients from FHH kindreds almost always remain hypercalcemic. FHH is transmitted in an autosomal dominant fashion and usually causes mild hypercalcemia with relative hypocalciuria. The hypercalcemia seen in FHH is highly penetrant across all ages, including in infants [22, 83]. The majority of cases of FHH result from heterozygous germline inactivating mutation of the
Loss of surface expression of the CASR protein has been documented in parathyroid adenomas and may contribute to the altered calcium set point and impaired calcium-mediated negative feedback on the release of PTH typical of such adenomas. Decreased
Type 2 FHH (FHH2) resulting from germline loss-of-function mutation of
11. Multiple endocrine neoplasia type 2A (MEN2A)
MEN2A is a familial cancer syndrome characterized by a predisposition to the development of medullary thyroid cancer (MTC), pheochromocytoma (typically benign and often bilateral), and primary HPT. In the context of MEN2A, HPT is usually mild and resembles sporadic HPT. HPT in MEN2A is almost always results from benign parathyroid disease. MEN2A is an autosomal dominant disorder that results from germline gain-of-function mutation in the
Germline oncogenic mutations of
12. Parathyroid tumorigenesis involving the CCND1 oncogene
The discovery of the
While activating CCND1 missense mutations have not been observed in sporadic parathyroid tumors , overexpression of
13. Other genes involved in parathyroid tumorigenesis
Recurrent mutations in a subset of genes likely relevant to parathyroid tumorigenesis have been identified by WES analysis of DNA derived from sporadic parathyroid tumors. Eight out of 193 sporadic parathyroid tumors analyzed by WES demonstrated the Y641N missense mutation in the
Soong and Arnold used WES analysis of DNA extracted from 19 parathyroid adenomas and matching germline DNA to identify somatic mutations in
WES analysis of 22 blood-sporadic parathyroid adenoma tumor pairs from a Chinese patient cohort identified recurrent mutations of
While inherited forms of HPT represent only a small fraction of cases (<5%), study of the molecular pathophysiology of these uncommon familial syndromes has yielded substantial insight into the genetic etiology of both sporadic and familial parathyroid disease and resulted in the identification of genes such as
The existence of currently unidentified parathyroid tumor suppressors and oncogenes is also suggested by analysis of parathyroid tumors using techniques such as comparative genomic hybridization (CGH) to identify specific chromosomal regions harboring loss or gain of DNA. Several investigators have documented recurrent loss of DNA at the 1p, 6q, 9p, and 13q chromosomal loci in parathyroid tumors, indicating the potential presence there of novel parathyroid tumor suppressor genes [114, 115, 116, 117]. The potential presence of novel oncogenes at chromosomal loci 9q, 16p, 19p, and Xq is suggested by results demonstrating specific chromosomal gain at these loci in benign or malignant parathyroid tumors [114, 116, 117, 118].
Next-generation sequencing analysis including WES of parathyroid neoplasms is an auspicious approach for the identification of novel acquired and germline gene variations that predispose to the development of HPT and parathyroid neoplasia. The apparent validation of this line of investigation by the identification of
The author wishes to thank the members of the Metabolic Diseases Branch, NIDDK for many helpful discussions and suggestions. The Intramural Research Program of the National Institute of Diabetes and Digestive and Kidney Diseases (ZIA DK043012-18) supported this research. The author declares no competing financial interests.
Conflict of interest
The author declares no conflict of interest.