Primary hyperparathyroidism (PHPT) is a common endocrine disease characterized by parathyroid hormone excess and hypercalcemia and caused by hypersecreting parathyroid glands. Familial PHPT occurs in an isolated nonsyndromal form, termed familial isolated hyperparathyroidism (FIHP), or as part of a syndrome, such as multiple endocrine neoplasia type 1 or hyperparathyroidism-jaw tumor syndrome. The specific genetic or other cause(s) of FIHP are unknown. We performed exome sequencing on germline DNA of eight index-case individuals from eight unrelated kindreds with FIHP. Selected rare variants were assessed for co-segregation in affected family members and screened for in an additional 32 kindreds with FIHP. In eight kindreds with FIHP, we identified three rare missense variants in GCM2, a gene encoding a transcription factor required for parathyroid development. Functional characterization of the GCM2 variants and deletion analyses revealed a small C-terminal conserved inhibitory domain (CCID) in GCM2. Two of the three rare variants were recurrent, located in the GCM2 CCID, and found in seven of the 40 (18%) kindreds with FIHP. These two rare variants acted as gain-of-function mutations that increased the transcriptional activity of GCM2, suggesting that GCM2 is a parathyroid proto-oncogene. Our results demonstrate that germline-activating mutations affecting the CCID of GCM2 can cause FIHP.

Context:Germline gain-of-function variants in the transcription factor GCM2 were found in 18% of kindreds with familial isolated hyperparathyroidism (FIHP). These variants [c.1136T>A (p.Leu379Gln) and c.1181A>C (p.Tyr394Ser)] were located in a 17-amino acid transcriptional inhibitory domain named C-terminal conserved inhibitory domain (CCID).Objective:We investigated the ethnicity of individuals with germline variants in the GCM2 CCID in our primary hyperparathyroidism (PHPT) patient samples and in the Genome Aggregation Database.Design:Ethnicity information was obtained from an in-house clinical database and genetic counseling. Sanger sequencing of blood DNA was used to determine the genotype of the GCM2 CCID region. Luciferase reporter assays were performed to determine the functional impact of GCM2 variants.Setting and Patients:National Institute of Diabetes and Digestive and Kidney Diseases endocrine clinic is a service that accepts PHPT referral patients.Results:The GCM2 p.Tyr394Ser variant was found in 41% [95% confidence interval (CI), 22% to 64%] of Ashkenazi Jewish (AJ) kindreds with FIHP and in 27% (95% CI, 17% to 40%) of AJ patients with sporadic PHPT. The p.Tyr394Ser variant was also found in sporadic PHPT patients of European ancestry, but at a lower prevalence. The p.Leu379Gln variant was found in 8% (95% CI, 1% to 26%) of European kindreds with FIHP and 0.5% (95% CI, 0% to 3.0%) of sporadic PHPT cases of European ancestry. The sporadic PHPT patients with GCM2-activating variants often had multigland involvement or postoperative recurrent or persistent disease.Conclusions:Specific GCM2-activating variants enriched among various ethnic backgrounds could contribute to a large number of cases with FIHP or sporadic PHPT.

GCM2-associated primary hyperparathyroidism patients have greater preoperative parathyroid hormone levels, a greater rate of multigland disease, a lesser rate of biochemical cure, and a substantial risk of parathyroid carcinoma. Knowledge of these clinical characteristics could optimize the surgical management of GCM2-associated familial isolated hyperparathyroidism.

In addition to a family history of PHPT, male sex, age 45 years and younger, and presence of multigland disease, should prompt physicians to offer the opportunity for genetic counseling and testing, as it could influence the management of patients with PHPT.

Mutation of the CDC73 gene, which encodes parafibromin, has been linked with parathyroid cancer. However, no correlation between genotypes of germline CDC73 mutations and the risk of parathyroid cancer has been known. In this study, subjects with germline CDC73 mutations were identified from the participants of two clinical protocols at National Institutes of Health (Discovery Cohort) and from the literature (Validation Cohort). The relative risk of developing parathyroid cancer was analyzed as a function of CDC73 genotype, and the impact of representative mutations on structure of parafibromin was compared between genotype groups. A total of 419 subjects, 68 in Discovery Cohort and 351 in Validation Cohort, were included. In both cohorts, percentages of CDC73 germline mutations that predicted significant conformational disruption or loss of expression of parafibromin (referred as ‘high-impact mutations’) were significantly higher among the subjects with parathyroid cancers compared to all other subjects. The Kaplan-Meier analysis showed that high-impact mutations were associated with a 6.6-fold higher risk of parathyroid carcinoma compared to low-impact mutations, despite a similar risk of developing primary hyperparathyroidism between two groups. Disruption of the C-terminal domain (CTD) of parafibromin is directly involved in predisposition to parathyroid carcinoma, since only the mutations impacting this domain were associated with an increased risk of parathyroid carcinoma. Structural analysis revealed that a conserved surface structure in the CTD is universally disrupted by the mutations affecting this domain. In conclusion, high-impact germline CDC73 mutations were found to increase risk of parathyroid carcinoma by disrupting the CTD of parafibromin.