Familial Hyperparathyroidism.

HPT-JT was first recognized as a form of familial HPT that was genetically distinct from MEN1 and MEN2A in 1990 by Gene Jackson and co-workers ( 90 ). Central to this recognition was re-evaluation of a family, first described three decades earlier ( 91 ), in which occurrence of multiple ossifying fibromas of the maxilla and mandible in two affected members of the third generation was seen to be similar to the jaw tumors of four of five affected members of the first generation. As noted originally by Jackson and co-workers ( 90 ), the maxillary and mandibular tumors seen in HPT-JT are distinct from the “brown tumors” of HPT (often seen in the context of osteitis fibrosa cystica and metabolically severe HPT), because the former can appear and/or enlarge in affected members in the absence, or following surgical correction of, HPT. Furthermore, whereas the “brown tumors” of HPT can occur anywhere in the axial or appendicular skeleton, the jaw tumors found in HPT-JT kindreds are restricted to the maxilla and mandible. The jaw tumors in HPT-JT are histologically distinct fibro-osseous lesions without the abundant multinucleated giant cells seen in “brown tumors,” and have been formally classified as cemento-ossifying fibromas ( 92 ). The exact cell of origin from which cemento-ossifying fibromas derive in HPT-JT is not precisely known, but they are generally believed to be mesodermal odontogenic tumors derived from the mesenchymal blast cells of the periodontal ligament, with the potential to form fibrous tissue, cementum, and bone or a combination thereof ( 93 , 94 ). HPT-JT is an autosomal dominant inherited syndrome with variable penetrance and expressivity. HPT is the most penetrant feature of HPT-JT and is the manifestation that most often brings carriers to medical attention. Besides HPT, the key clinical features of HPT-JT include cemento-ossifying fibromas restricted to the maxilla and mandible (as described above), renal lesions, and uterine tumors in women ( 90 , 95 – 97 ). In contrast to sporadic HPT and MEN1, parathyroid cancer is relatively frequent in the context of HPT-JT, and affects ~20% of those with HPT ( 90 , 95 , 96 , 98 , 99 ). In the preponderance of HPT-JT kindreds, a germline inactivating mutation of the CDC73 gene (formerly called HRPT2 ) on the long arm of chromosome 1 can be identified ( 19 , 100 ) The CDC73 gene encodes parafibromin, a protein of 531 residues, that is considered to be a tumor suppressor protein because germline mutation predicted to cause loss-of-function predisposes to the neoplastic expressions of HPT-JT ( 100 ). The majority of germline CDC73 mutations in kindreds with HPT-JT are predicted to inactivate gene function via frameshift or nonsense mutation, with only a minority of the variants encoding missense mutations ( 99 , 101 ). Partial or complete deletion of the CDC73 gene has also been described in patients and kindreds with HPT-JT ( 102 – 105 ). The high frequency of parathyroid cancer is a hallmark of HPT-JT. A genotype-phenotype correlation has been observed among CDC73 -mutation carriers such that those with frameshift, nonsense or deletion mutations are nearly 7-fold more likely to develop parathyroid cancer than patients harboring missense CDC73 mutations ( 99 ). In apparently sporadic cases of parathyroid cancer, mutations of CDC73 are frequently identified ( 106 – 110 ). Interestingly, germline loss-of-function mutation in CDC73 may be found in some 25% of patients with seemingly sporadic parathyroid carcinoma, suggesting that such patients may have either de novo germline mutation in CDC73 or else a forme fruste of HPT-JT ( 19 , 107 , 108 ). A potential genotype-phenotype correlation has also been observed with respect to certain renal manifestations of HPT-JT. Mixed epithelial and stromal tumor of the kidney (MEST), a rare type of renal tumor, has been associated with HPT-JT and/or CDC73 germline mutation in at least two kindreds ( 111 , 112 ). MEST is a renal neoplasm, characterized by cystic structures lined by epithelium and admixed with ovarian-type stroma, that must be diagnosed differentially from cystic hamartoma of the renal pelvis, adult type mesoblastic nephroma, and leiomyomatous renal hamartoma ( 113 , 114 ). MEST in the context of HPT-JT and/or CDC73 germline mutation appears to correlate with a specific CDC73 missense mutation in which the initiator methionine of parafibromin is replaced with isoleucine, i.e. the Met1Ile genotype ( 100 , 111 , 112 ). Wilms tumor has been identified in members of several families with HPT-JT ( 115 , 116 ). Renal cysts are also reported to be part of the clinical spectrum of HPT-JT and CDC73 germline mutation ( 19 ). No genotype-phenotype correlation has been reported for Wilms tumor and renal cysts in the context of CDC73 germline mutation. The presence of uterine manifestations of HPT-JT was first elucidated by Bradley et al. who recognized a high frequency of menorrhagia often leading to early hysterectomy among affected women and adult female carriers ( 95 ). There was a range of uterine pathology among women with HPT-JT who underwent hysterectomy that included adenosarcomas, adenofibromas, leiomyomas, adenomyosis, and endometrial hyperplasia ( 95 ). The uterine manifestations of HPT-JT significantly reduced the reproductive fitness of affected women. Lifelong monitoring for uterine tumors with routine gynecologic care and pelvic ultrasound examination as clinically indicated has been recommended for women with HPT-JT, starting at reproductive age ( 19 ). A subset of kindreds classified as FIHP have been shown to harbor germline CDC73 mutation, suggesting that incompletely penetrant HPT-JT can phenocopy FIHP (see below and Figure 2 ). Approximately 20% of obligate or genetically confirmed CDC73 mutation-positive subjects lack any clinical manifestations of HPT-JT at the time of kindred ascertainment ( 99 ), in line with the variable penetrance and expressivity of CDC73 mutation. Lifelong surveillance of initially asymptomatic CDC73 mutation carriers is recommended since the penetrance of the manifestations of HPT-JT increases with age ( 117 ). The relationship among familial forms of hyperparathyroidism that may present as familial isolated hyperparathyroidism (FIHP) as a Venn diagram. The large dashed circle represents the set of patients that can present with a provisional diagnosis of FIHP at the time of initial ascertainment. This includes patients with FIHP who have been evaluated for, but lack findings diagnostic of, MEN1, FHH, and HPT-JT (nonsyndromic FIHP; in a solid circle). Approximately 18% of nonsyndromic FIHP kindreds harbor germline gain-of-function mutations in GCM2 (inner dotted circle) (see text), whereas the remainder have currently unknown genetic etiologies. Subsets of patients with incomplete expression of MEN1, FHH and HPT-JT (the total set of patients in each syndrome represented by a solid circle) can also present with the FIHP phenotype (and thus overlap with the large dashed circle). The distinction between the nonsyndromic FIHP category and the syndromic categories arbitrarily depends on the depth and rigor of evaluation and the sensitivity of diagnostic tests used to detect the syndrome, testing that can include germline gene mutational analysis. MEN2A is a familial form of hyperparathyroidism that seldom if ever presents as FIHP, with patients usually coming to medical attention for signs and symptoms of medullary thyroid cancer and/or pheochromocytoma. Within each circle representing a defined syndrome are included the genetic locus (or loci in the case of FHH; see text) of the syndromic trait and the associated gene product. The causative gene for HPT-JT that encodes parafibromin is CDC73 , formerly called HRPT2 . The relationship among the patient sets illustrated as circles in this diagram is intended to be qualitative and neither the area of each circle nor the area of overlap between circles has any quantitative significance.

Typically evidenced by elevated serum calcium, primary hyperparathyroidism (HPT) is a disorder of mineral metabolism caused by the inappropriate or excessive secretion of parathyroid hormone (PTH) from one or several abnormal parathyroid glands ( 1 ). The majority of cases of HPT are sporadic and arise or occur randomly with no apparent predisposition (~95%). With respect to the remaining < 5% of patients with a familial predisposition to develop HPT, most carry germline mutation of a gene known to confer susceptibility to parathyroid tumor development (listed in Table 1 ). Even though these disorders are rare, study of the molecular genetics of these uncommon familial HPT syndromes has provided considerable insight into the molecular pathophysiology of both sporadic and familial parathyroid neoplasia. Since the release of parathyroid hormone (PTH) from parathyroid chief cells is tightly regulated by the calcium-sensing receptor (CASR), a cell surface-expressed G protein-coupled receptor (GPCR) belonging to GPCR family C ( 2 ), mutation in the germline of CASR or other genes transducing and propagating the CASR signal can also result in heritable syndromes associated with hypercalcemia and PTH levels that are high or inappropriately normal. This chapter will review and summarize current knowledge of the molecular pathophysiology and clinical genetics of familial syndromes that predispose to parathyroid gland neoplasia and HPT. Hereditary Hyperparathyroidism. HPT, primary hyperparathyroidism; TS, tumor supressor gene, LoF, loss of function; NET, neuroendocrine tumor; PO, proto-oncogene; GoF, gain of function; MEST, mixed epithelial and stromal tumor [of the kidney]; ICSST, impaired calcium sensing or signal transduction.

Cellular activation of tumorigenesis requires transformation of normal cell into a neoplastic derivative. This process is typically regulated by genes that encode proteins that help control cell growth and proliferation. Tumorigenesis initiates when the balance between cell growth and inhibition is lost, and genes that positively regulate growth (proto-oncogenes) or inhibit growth (tumor suppressor genes) are either constitutively activated or inactivated, respectively. The most common mechanism for tumor growth in hereditary tumor syndromes, which account for ~5–10% of all cancer, is inactivation of tumor suppressor genes. Knudson’s “two‐hit hypothesis” has been fundamental for understanding tumor suppressor genes and familial tumor-predisposing syndromes. Before the advancement of molecular genetics, Knudson used mathematical modelling to compare the clinical presentation of sporadic and inherited cases of retinoblastoma (RB) in children to better understand the latency of the disease ( 23 ). By evaluating the time at which both eyes would be affected by retinoblastoma if one or two hits to DNA were required, Knudson accurately predicted the relative chance of this occurring. The conclusion that retinoblastoma required two-hits, one from the germline (inherited) and a subsequent second hit to inactivate the gene (termed “loss of heterozygosity” or LOH), was confirmed with the discovery of the RB gene in 1986 and identification of LOH in the tumor tissue ( 24 ). The most common tumor suppressor genes such as p53, RB , and PTEN are frequently identified in various tumor syndromes ( 25 ). A similar observation of the two-hit hypothesis holds true for inherited causes of hyperparathyroidism due to tumor suppressor genes such as MEN1 and CDKN1B ( Table 1 ).

JB and WS conceived, planned, and executed the writing of this chapter. All authors contributed to the article and approved the submitted version.

FHH describes a condition of PTH-dependent hypercalcemia, resembling and in the differential diagnosis of HPT, that is typically benign ( Table 1 ) ( 22 , 129 ). The condition, also known as “familial benign hypercalcemia”, is genetically heterogeneous and results from mutations that cause parathyroid gland insensitivity to extracellular calcium with a resulting rightward shift of the set point for suppression by calcium of PTH secretion. As a result of this intrinsic insensitivity of the parathyroid cells to suppression by circulating calcium, affected patients from FHH kindreds almost always remain hypercalcemic following partial or subtotal parathyroidectomy. FHH is inherited in an autosomal dominant manner and usually results in mild hypercalcemia, non-suppressed or mildly elevated PTH, and relative hypocalciuria. The hypercalcemia observed in FHH is highly penetrant across the age spectrum, including in neonates and infants ( 22 , 130 ). The majority of cases of FHH results from heterozygous germline loss-of-function mutation of the calcium-sensing receptor, encoded by the CASR gene on the long arm of chromosome 3 ( 4 , 131 ). FHH due to inactivating mutation in the CASR gene is classified as type 1 FHH (FHH1) and is the most common type ( 132 ). Neonatal severe primary hyperparathyroidism (NSHPT) that presents as severe hypercalcemia, typically occurring in the first 6 months of life, is a rare autosomal recessive disorder that most often is a consequence of the compound heterozygous or homozygous inheritance of two mutationally-inactivated CASR alleles ( 133 ). While true parathyroid tumors demonstrate cellular monoclonality, the hyperfunctioning parathyroid tissue removed from a patient with NSHPT demonstrated generalized polyclonal hyperplasia by molecular genetic analysis, illustrating the non-neoplastic quality of the abnormal parathyroids that result from CASR loss-of-function mutation ( 134 ). Reduced cell surface expression of the CASR protein has been demonstrated in parathyroid adenomas and may explain the rightward shift of the calcium set point and the impaired calcium-mediated suppression of parathyroid hormone release typical of such adenomas. Decreased CASR transcript expression, but not loss-of-heterozygosity at the CASR chromosomal locus, has been demonstrated in benign parathyroid tumors ( 135 ). In molecular genetic studies of sporadic parathyroid tumors reported to date, somatic inactivation of the CASR gene has not been described ( 136 , 137 ). A minority of families with FHH result from germline mutations in genes other than CASR on chromosome 3q. Type 2 FHH (FHH2) results from inactivating mutation in the germline of the G protein α11 subunit encoded by GNA11 ( 138 , 139 ). GNA11 encodes the α-subunit of the signal-transducing G protein downstream of the CASR ( 140 ). The CASR allosteric activator cinacalcet has been reported to correct the hypercalcemia of patients with FHH2 due to GNA11 mutation ( 141 ). Type 3 FHH (FHH3) results from germline loss-of-function mutation in adaptor related protein complex 2 subunit sigma 1 (AP2S1), an adaptor protein involved in clathrin-mediated endocytosis and encoded by AP2S1 on chromosome 19q ( 142 – 145 ). In FHH3 the vast majority of inactivating mutations in AP2S1 involve missense substitutions of codon Arg-15, but analysis of large scale exome datasets suggests that other residues may be more rarely involved ( 146 ). Mutant forms of AP2S1 involving codon Arg-15 that result in FHH3 may exert dominant-negative effects on CASR signaling ( 144 ). In molecular genetic analyses of sporadic parathyroid tumors, somatic loss-of-function mutation in the genes encoding GNA11 and AP2S1 have thus far not been described.

Pellegata and coworkers first described MEN4 in a multi-generational kindred with manifestations overlapping those of MEN1 but whose affected members lacked germline MEN1 mutation ( 74 , 75 ). A germline heterozygous stop-gain mutation in the cyclin dependent-kinase inhibitor p27(Kip1), encoded by CDKN1B , was identified in the proband with acromegaly and HPT and in several other members of this kindred ( 74 ). Genetic analysis of rats with a multi-tumor syndrome phenotype called multiple endocrine neoplasia X (MENX) had previously drawn attention to the Cdkn1b locus ( 74 , 76 ). The rat MENX phenotype was discovered accidently when some rats in a Sprague–Dawley colony were observed to spontaneously develop multiple endocrine tumors ( 77 ). The MENX phenotype in rats was manifested by parathyroid hyperplasia, multifocal anterior pituitary adenoma, adrenal and extra-adrenal pheochromocytoma, thyroid C-cell hyperplasia, and pancreatic islet cells hyperplasia. MENX in rats was recessively inherited and caused by a frameshift mutation in Cdkn1b ( 74 , 76 ). In the study by Pellegata et al., only the proband among members of the MEN4/MENX kindred described was reported to have HPT ( 74 ). Prior to 2019, reports of kindreds demonstrating segregation of a mutation in  CDKN1B with an MEN4 phenotype across multiple generations were quite limited. Several groups investigated for a possible role for CDKN1B mutation in parathyroid neoplasia following the original report by Pellegata et al. ( 74 ). Several studies examined MEN1 mutation-negative patients and kindreds expressing MEN1-like tumors and harboring germline mutation in CDKN1B and which thus could be considered as MEN4 ( 74 , 78 – 85 ). Apart from the verification of HPT in association with CDKN1B mutation in a pair of monozygotic twins ( 79 ), none of these reports prior to 2019 had described kindreds with more than one individual with HPT proven to segregate with the CDKN1B mutation. In 2019 however, a report by Frederiksen et al. described a large multi-generational Danish MEN4 family in which HPT occurred in 13 members and segregated with a germline frameshift CDKN1B mutation ( 86 ). Molecular genetic analysis of sporadic parathyroid adenomas demonstrating that CDKN1B mutation can be both somatic and clonal further supports the characterization of CDKN1B as a gene predisposing to the development of primary parathyroid tumors ( 87 , 88 ). A recent study suggests that germline CDKN1B mutation can also present as apparently sporadic, isolated pediatric Cushing’s disease ( 89 ).

Even though inherited forms of HPT represent only a small proportion of total cases (< 5%), investigation into the molecular basis of these rare familial syndromes has resulted in considerable insight into the genetics and pathophysiology of both sporadic and familial HPT and spotlighted the importance of genes such as MEN1, CDC73, CASR, GNA11, AP2S1, CDKN1B , and GCM2 . It seems very likely that gain- or loss-of-function mutation of other genes, currently unrecognized, can also promote parathyroid tumor formation. As an illustration of this, the risk predisposing to the development of parathyroid tumors in most FIHP kindreds appears to result from the germline mutation of genes currently not recognized to play a role in parathyroid neoplasia. This inference follows from the fact that nearly 70% of families initially characterized with an FIHP phenotype in several clinical studies that screened for germline MEN1 , CASR , and CDC73/HRPT2 gene mutation, were found to have no currently recognized syndromic or genetic basis ( Figure 2 ) ( 20 , 119 – 121 ). From the FIHP families which are MEN1 , CASR , and CDC73/HRPT2 mutation-negative, estimates are that approximately 20% carry germline gain-of-function mutations in the GCM2 proto-oncogene ( 21 ). The clear implication is that some 80% of FIHP families have no currently defined genetic etiology for their predisposition to HPT. The analysis of DNA extracted from parathyroid tumors using methods such as comparative genomic hybridization (CGH) to highlight particular chromosomal regions that have lost or gained segments of DNA also implies the presence of currently unidentified parathyroid tumor suppressors and oncogenes. Several research groups have demonstrated recurrent DNA loss at chromosomal locations 1p, 6q, 9p, and 13q in parathyroid tumors, indicating the possible presence there of currently unrecognized parathyroid tumor suppressor genes ( 147 – 150 ). On the other hand, the demonstration of specific chromosomal gain at chromosomal loci 9q, 16p, 19p, and Xq in benign or malignant parathyroid tumors suggests the possible presence at these loci of novel parathyroid oncogenes ( 147 , 149 – 151 ). What clinical, demographic, or historical considerations should persuade the practitioner to screen individual patients for a genetic basis for their HPT, even if they lack obvious features of a syndrome? As was discussed above in the case of tumor suppressor genes, germline mutation predisposes to earlier onset of disease so very often it is the younger cohort of seemingly sporadic patients with HPT who are considered for such genetic screening. Skandarajah and co-workers performed a retrospective genetic analysis of 21 patients who had undergone surgery for HPT before the age of 40. Although none had suspicious personal or family histories suggestive of MEN1, one patient, who had a double parathyroid adenoma at surgery, was found to have a germline MEN1 frameshift mutation ( 152 ). In another study from Carling and colleagues, 86 patients ≤ 45 years of age with seemingly sporadic HPT underwent genetic analysis for genes predisposing to HPT. Eight of the 86 patients (~ 9%) were found to harbor germline loss-of-function mutations in known HPT susceptibility genes: four MEN1 , three CASR , and one CDC73 ( 153 ). An expert panel of physicians, surgeons, and geneticists assembled to provide clinical practice guidelines for MEN1 suggested that gene mutational testing be performed on patients presenting with seemingly sporadic HPT younger than 40 years of age due to multi-gland parathyroid disease ( 47 ). A recent retrospective study of 121 patients screened for familial forms of HPT found that with respect to sole risk factors, a positive family history, but not age at diagnosis or the finding of multiglandular parathyroid disease, was strongly predictive of a pathogenic germline variant in a HPT susceptibility gene ( 132 ).

When a familial form HPT is suspected based on patient or family medical history, and/or one or more suspicious non-parathyroid features, certain key clinical or laboratory findings can help in the differential diagnosis. A history of pituitary adenomas, pNETs, bronchial carcinoid, or duodenal endocrine tumors in the patient or first-degree relatives favor MEN1. Suspicion of MEN2A is heightened if a history of MTC or pheochromocytoma is documented in the patient or a family member. History of parathyroid cancer, maxillary or mandibular tumors, Wilms tumor, or uterine abnormalities requiring early hysterectomy in the patient or another member of the kindred, should elevate the possibility of HPT-JT. Persistent hypercalcemia following parathyroidectomy, hypocalciuria, hypermagnesemia, and hypercalcemia in family members younger than 10 years of age could be clues to the diagnosis of FHH. Genetic testing, conveniently performed with a gene panel that includes MEN1, CDC73, CASR, GNA11, AP2S1, CDKN1B , and GCM2 , can help clarify the diagnosis and establish the genetic etiology. Recommendations for the testing of younger patients with seemingly sporadic HPT are described below.

JB is employed by the company AstraZeneca. The remaining author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The handling editor declared a past co-authorship with the authors JB and WS.

Another potential molecular mechanism for tumor development involves mutant genes called oncogenes that drive cell growth. Oncogenes derive from naturally occurring genes, called proto-oncogenes. Typically proto-oncogenes are genes which positively regulate cell division and/or cell growth under normal conditions ( 26 ). Examples of proto-oncogenes include SRC , HRAS , KRAS , NRAS , WNT1 , and MYC . Oncogenes result from the mutational activation (e.g. via the acquisition of mutations that result in constitutive signaling activity) or overexpression of proto-oncogenes that can induce cell division and cell growth, causing tumor formation often in a tissue-specific fashion. The gene products encoded by proto-oncogenes often belong to mitogenic signaling pathways. With respect to the etiologies of currently understood inherited cancer syndromes, germline mutational gain-of-function of proto-oncogenes is rare compared to loss-of-function mutations in tumor suppressor genes (see below). Constitutive mitogenic signaling resulting from the germline gain-of-function of most proto-oncogenes is likely to be incompatible with normal embryonic and fetal development which requires extremely precise regulation of intercellular communication.

The Intramural Research Program of the National Institute of Diabetes and Digestive and Kidney Diseases (ZIA DK043012-18) supported this research.