**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 [12–15]. The simultaneous demonstration of elevated serum calcium with an inappropriately normal or elevated PTH is a typical clinical definition of HPT [16]. 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 [17–21]. 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 [22]. 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 [23]. Oncogenes represent gain-of-function mutants or overexpressed forms of protooncogenes 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 protooncogenes 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 [24]. 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 [25].

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

**117**

*Familial Syndromes of Primary Hyperparathyroidism DOI: http://dx.doi.org/10.5772/intechopen.93036*

"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 *MEN1* and the *CDC73/HRPT2* tumor suppressor genes, respectively, can often be found in parathyroid tumor-derived DNA. In such patients, the first "hit", namely a loss-of-function mutation of the relevant tumor

MEN1 is the most common hereditary cause of primary hyperparathyroidism [26]. 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) [27]. 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 [27]. HPT is the most penetrant hormonal

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 *MEN1* gene on chromosome 11q13 [28]. As of 2015, 576 unique germline mutations in *MEN1* were reported from patients and families with MEN1 [29]. The study of DNA derived from pituitary, parathyroid, and entero-pancreatic tumors from MEN1 patients has shown that most syndromic tumors possess an acquired deletion or other inactivating mutation of the second, wild-type *MEN1* allele [18, 30]. Approximately 10% of patients with MEN1 on a clinical basis are germline *MEN1*

Conventional DNA sequencing of tumor DNA has identified somatic *MEN1* mutation in up to 35% of sporadic parathyroid adenomas [31–35]. In studies testing for loss-of-heterozygosity (LOH) in sporadic parathyroid adenomas, the frequency of LOH at the *MEN1* locus on chromosome 11q13 ranged from 26 to 37%. Using whole exome sequencing (WES) methodology, somatic *MEN1* mutation was found in some 35% of parathyroid benign tumors, comparable to results using conventional Sanger DNA sequencing [36, 37]. As mentioned above, HPT is the most penetrant feature of MEN1 and is usually the initial manifestation. As a result, true MEN1 families may sometimes be initially mis-assigned a clinical diagnosis of familial isolated hyperparathyroidism (FIHP) if only younger affected members are

Mutation of the *MEN1* gene is only rarely associated with parathyroid carcinoma. The occurrence of parathyroid carcinoma in the context of familial MEN1 is extremely uncommon. Fewer than 20 patients with HPT due to parathyroid cancer in the context of the MEN1 syndrome have been reported [38]. LOH analysis of parathyroid tumor-extracted DNA has shown that DNA loss at the location of the *MEN1* gene on chromosome 11q, though frequently seen in benign parathyroid tumors, is quite uncommon in parathyroid carcinomas [39]. Recent studies that use next-generation WES of tumor-derived DNA to profile parathyroid cancers did not

considered at the time that the family is ascertained (see **Figure 1**).

report any somatic mutations in *MEN1* [40, 41].

suppressor gene, can frequently be demonstrated in the germline DNA.

**6. Multiple endocrine neoplasia type 1 (MEN1)**

feature of MEN1.

mutation-negative.

*Familial Syndromes of Primary Hyperparathyroidism DOI: http://dx.doi.org/10.5772/intechopen.93036*

*Mineral Deficiencies - Electrolyte Disturbances, Genes, Diet and Disease Interface*

parathyroid cancer accounting for less than 1% of HPT in most series.

discussed in more detail below.

and fetal development.

**4. Oncogenes and proto-oncogenes**

1,25-dihydroxyvitamin D, whose synthesis is stimulated by PTH acting on proximal renal tubular cells, inhibits PTH biosynthesis and release from parathyroid cells [12–15]. The simultaneous demonstration of elevated serum calcium with an inappropriately normal or elevated PTH is a typical clinical definition of HPT [16]. The vast majority of parathyroid tumors are adenomas (i.e. benign tumors), with

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 [17–21]. 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 [22]. The relevance of these inherited disorders to the underlying molecular pathogenetic alterations in parathyroid tumorigenesis will be

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 [23]. Oncogenes represent gain-of-function mutants or overexpressed forms of protooncogenes 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 protooncogenes 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

**5. The role of tumor suppressor genes in tumor development**

advantage and result in that cell's clonal expansion [25].

Alfred Knudson proposed another model for tumor development based on the study of retinoblastoma disease patterns nearly 50 years ago [24]. 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

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

**116**

"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 *MEN1* and the *CDC73/HRPT2* tumor suppressor genes, respectively, can often be found in parathyroid tumor-derived DNA. In such patients, the first "hit", namely a loss-of-function mutation of the relevant tumor suppressor gene, can frequently be demonstrated in the germline DNA.
