Novel Glial Cells Missing-2 (GCM2) variants in parathyroid disorders

Germline mutations of GCM2 cause a novel variant of hereditary primary hyperparathyroidism

Primary hyperparathyroidism (pHPT) occurs as hereditary disease in approximately 10% of cases. GCM2 germline mutations have been recently described as responsible for the development of a novel variant of hereditary pHPT. This study aimed to determine the features of GCM2-related pHPT. Demographics, laboratory, and surgical data were assessed in a series of 17 index cases carrying GCM2 mutations undergoing surgery for pHPT. The GCM2 germline pathogenic variant c.1181 A>C p.(Tyr394Ser) was detected in 59% of cases. GCM2-related pHPT was diagnosed at a median age of 57 years (range 32-82) with a Female/Male ratio 1.8. Preoperative median calcemia was 2.89 mmol/L (range 2.69-3.8). Family history of pHPT was absent in 65% of cases. Complete clinical, surgical and follow-up data were available for 13 patients. At initial surgery, bilateral neck exploration with subtotal parathyroidectomy was performed in 46% of patients; achieving cure in all cases at a median follow-up of 51 months (range 7-60). In the remaining cases undergoing selective parathyroidectomy, a persistent pHPT occurred in 3 cases; recurrent pHPT in 1 patient (after a disease-free interval of 4 years) while 3 are disease free at a mean follow-up of 21 months. Thus, at an overall prolonged follow-up (median 48 months, range 7-216), multiglandular involvement occurred in 77% of cases. GCM2 germline mutations may cause hereditary pHPT, even if it may mimic sporadic variant due to the absence of familial history and late onset. The main feature is multiglandular involvement, needing bilateral neck exploration and subtotal parathyroidectomy to achieve long-term cure.

Heritable hyperparathyroidism: Genetic insights and clinical implications

Familial or heritable hyperparathyroidism (FHPT) is seen in approximately 10-15 % of patients with primary hyperparathyroidism (PHPT). Once the diagnosis of PHPT is established, consideration of heritable forms should be made in patients with positive family history, young onset, multi-glandular disease, and recurrent or persistent disease. FHPT encompasses both syndromic and non-syndromic forms. Syndromic forms include multiple endocrine neoplasia (MEN) types 1, 2, 3 and 4, hyperparathyroidism-jaw tumor syndrome, hereditary pheochromocytoma and paraganglioma, and the more recently reported, Birt-Hogg-Dubé (BHD) syndrome, and X-linked intellectual disability syndrome. Non-syndromic forms include familial hypocalciuric hypercalcemia (FHH)- types 1,2, and 3, neonatal severe hyperparathyroidism, GCM2-mediated hyperparathyroidism, transient neonatal hyperparathyroidism, and familial isolated hyperparathyroidism. In this review we aim to review the heritable forms of PHPT and highlight the important genetics insights and clinical implications.

Progress report on multiple endocrine neoplasia type 1

Multiple endocrine neoplasia type 1 (MEN1) syndrome is an autosomal dominant disorder caused by a germline pathogenic variant in the MEN1 tumor suppressor gene. Patients with MEN1 have a high risk for primary hyperparathyroidism (PHPT) with a penetrance of nearly 100%, pituitary adenomas (PitAd) in 40% of patients, and neuroendocrine neoplasms (NEN) of the pancreas (40% of patients), duodenum, lung, and thymus. Increased MEN1-related mortality is mainly related to duodenal-pancreatic and thymic NEN. Management of PHPT differs from that of patients with sporadic disease, as the surgical approach in MEN1-related PHPT includes near-total or total parathyroidectomy because of multigland hyperplasia in most patients and the consequent high risk of recurrence. NEN management also differs from patients with sporadic disease due to multiple synchronous and metasynchronous neoplasms. In addition, the lifelong risk of developing NEN requires special considerations to avoid excessive surgeries and to minimize damage to the patient's function and well-being. This progress report will outline current insights into surveillance and management of the major clinical manifestation of MEN1 syndrome in children and adults with MEN1 diagnosis. In addition, we will discuss MEN1-like clinical presentation with negative MEN1-genetic workup and future clinical and research directions.

Heterogeneous Transcriptional Landscapes in Human Sporadic Parathyroid Gland Tumors

The expression of several key molecules is altered in parathyroid tumors due to gene mutations, the loss of heterozygosity, and aberrant gene promoter methylation. A set of genes involved in parathyroid tumorigenesis has been investigated in sporadic parathyroid adenomas (PAds). Thirty-two fresh PAd tissue samples surgically removed from patients with primary hyperparathyroidism (PHPT) were collected and profiled for gene, microRNA, and lncRNA expression (n = 27). Based on a gene set including MEN1, CDC73, GCM2, CASR, VDR, CCND1, and CDKN1B, the transcriptomic profiles were analyzed using a cluster analysis. The expression levels of CDC73 and CDKN1B were the main drivers for clusterization. The samples were separated into two main clusters, C1 and C2, with the latter including two subgroups of five PAds (C2A) and nineteen PAds (C2B), both differing from C1 in terms of their lower expression of CDC73 and CDKN1B. The C2A PAd profile was also associated with the loss of TP73, an increased expression of HAR1B, HOXA-AS2, and HOXA-AS3 lncRNAs, and a trend towards more severe PHPT compared to C1 and C2B PAds. C2B PAds were characterized by a general downregulated gene expression. Moreover, CCND1 levels were also reduced as well as the expression of the lncRNAs NEAT1 and VLDLR-AS1. Of note, the deregulated lncRNAs are predicted to interact with the histones H3K4 and H3K27. Patients harboring C2B PAds had lower ionized and total serum calcium levels, lower PTH levels, and smaller tumor sizes than patients harboring C2A PAds. In conclusion, PAds display heterogeneous transcriptomic profiles which may contribute to the modulation of clinical and biochemical features. The general downregulated gene expression, characterizing a subgroup of PAds, suggests the tumor cells behave as quiescent resting cells, while the severity of PHPT may be associated with the loss of p73 and the lncRNA-mediated deregulation of histones.

Familial states of primary hyperparathyroidism: an update

BACKGROUND

Familial primary hyperparathyroidism (PHPT) includes syndromic and non-syndromic disorders. The former are characterized by the occurrence of PHPT in association with extra-parathyroid manifestations and includes multiple endocrine neoplasia (MEN) types 1, 2, and 4 syndromes, and hyperparathyroidism-jaw tumor (HPT-JT). The latter consists of familial hypocalciuric hypercalcemia (FHH) types 1, 2 and 3, neonatal severe primary hyperparathyroidism (NSHPT), and familial isolated primary hyperparathyroidism (FIHP). The familial forms of PHPT show different levels of PHPT penetrance, developing earlier and with multiglandular involvement compared to sporadic counterpart. All these diseases exhibit Mendelian inheritance patterns, and for most of them, the genes responsible have been identified. DNA testing for predisposing mutations is helpful in index cases or in individuals with a high suspicion of the disease. Early recognition of hereditary disorders of PHPT is of great importance for the best clinical and surgical approach. Genetic testing is useful in routine clinical practice because it will also involve appropriate screening for extra-parathyroidal manifestations related to the syndrome as well as the identification of asymptomatic carriers of the mutation.

PURPOSE

The aim of the review is to discuss the current knowledge on the clinical and genetic profile of these disorders along with the importance of genetic testing in clinical practice.

Dicer-Mediated mTORC1 Signaling and Parathyroid Gland Integrity and Function

Key Points

Background

Secondary hyperparathyroidism of CKD contributes significantly to patient morbidity and mortality. The underlining mechanisms of CKD-induced secondary hyperparathyroidism remain elusive. We previously demonstrated that PT-Dicer −/− mice, with parathyroid-specific deletion of the microRNA (miRNA)-processing enzyme Dicer and consequently miRNA, maintain normal basal serum parathyroid hormone (PTH) levels but do not develop secondary hyperparathyroidism induced by CKD. In addition, we showed that the parathyroid mechanistic target of rapamycin complex 1 (mTORC1) pathway is activated in CKD. We now explored the roles of Dicer/miRNA and mTORC1 in parathyroid development and function.

Methods

We generated mice with parathyroid-specific Dicer (PT-Dicer −/− ), mechanistic target of rapamycin (PT-mTOR −/− ), or tuberous sclerosis complex 1 (PT-Tsc1 −/− ) deficiency combined with yellow fluorescent protein (YFP) or tdTomato expression to identify the parathyroids by fluorescence microscopy. CKD was induced by an adenine-rich high-phosphate diet.

Results

Despite normal basal serum PTH levels, PT-Dicer −/− mice displayed apoptotic loss of intact parathyroid glands postnatally and reduced mechanistic target of rapamycin activity. PT-mTOR −/− mice lacked intact parathyroid glands yet maintained normal serum PTH levels, mirroring the phenotype of PT-Dicer −/− mice. Conversely, PT-Tsc1 −/− mice with hyperactivated mTORC1 exhibited enlarged glands along with elevated basal serum PTH and calcium levels. Significantly, PT-Dicer −/− ;Tsc1 −/− double knockout mice preserved intact parathyroid glands and reinstated CKD-induced secondary hyperparathyroidism.

Conclusions

mTORC1 operates downstream of Dicer and miRNA in the parathyroid and is essential for maintaining postnatal parathyroid gland integrity throughout life and for the pathogenesis of CKD-induced secondary hyperparathyroidism.

Identification of conserved skeletal enhancers associated with craniosynostosis risk genes

Craniosynostosis, defined by premature fusion of one or multiple cranial sutures, is a common congenital defect affecting more than 1/2000 infants and results in restricted brain expansion. Single gene mutations account for 15%-20% of cases, largely as part of a syndrome, but the majority are nonsyndromic with complex underlying genetics. We hypothesized that the two noncoding genomic regions identified by a GWAS for craniosynostosis contain distal regulatory elements for the risk genes BMPER and BMP2. To identify such regulatory elements, we surveyed conserved noncoding sequences from both risk loci for enhancer activity in transgenic Danio rerio. We identified enhancers from both regions that direct expression to skeletal tissues, consistent with the endogenous expression of bmper and bmp2. For each locus, we also found a skeletal enhancer that also contains a sequence variant associated with craniosynostosis risk. We examined the activity of each enhancer during craniofacial development and found that the BMPER-associated enhancer is active in the restricted region of cartilage closely associated with frontal bone initiation. The same enhancer is active in mouse skeletal tissues, demonstrating evolutionarily conserved activity. Using enhanced yeast one-hybrid assays, we identified transcription factors that bind each enhancer and observed differential binding between alleles, implicating multiple signaling pathways. Our findings help unveil the genetic mechanism of the two craniosynostosis risk loci. More broadly, our combined in vivo approach is applicable to many complex genetic diseases to build a link between association studies and specific genetic mechanisms.

Molecular Genetic Aspects of Sporadic Multiglandular Primary Hyperparathyroidism

Multiglandular primary hyperparathyroidism (MGD) represents a rare form of primary hyperparathyroidism (PHPT). MGD is associated with hereditary PHPT, but the sporadic MGD is more common and affects a similar patient profile as single gland parathyroid disease (SGD). The distinction between SGD and MGD is of great clinical importance, especially for the strategy of parathyroidectomy. Based on the limited knowledge available, MGD is likely to be a genetically heterogeneous disease resulting from the interaction of germline and somatic DNA mutations together with epigenetic alterations. Furthermore, these events may combine and occur independently in parathyroid tumors within the same individual with MGD. Gene expression profiling has shown that SGD and MGD may represent distinct entities in parathyroid tumorigenesis. We are waiting for studies to analyze exactly which genes are different in SGD and MGD in order to identify potential biomarkers that can distinguish between the two forms of the disease.

GCM2 p.Tyr394Ser variant in Ashkenazi Israeli patients with suspected familial isolated hyperparathyroidism

Context

A germline mutation can be identified in up to 10% of patients with primary hyperparathyroidism (PHPT). In 2017, a high frequency of the GCM2 [(NM_ 004752.4) c.1181A> C; p.Tyr394Ser; rs142287570] variant was reported in PHPT Ashkenazi Jews (AJ).

Objective

To evaluate the presence of the GCM2 p.Tyr394Ser variant in Israeli patients addressed for genetic evaluation to characterize their phenotype and clinical management.

Method

Patients with PHPT who underwent addressed for genetic screening for suspected familial hypocalciuric hypercalcemia (FHH), a family history of isolated hyperparathyroidism (FIHP), or failed parathyroidectomy with persistent PHPT were recruited. Those with normal initial selected gene sequencing or hyperparathyroid genetic panel completed the GCM2 p.Tyr394Ser variant sequencing. The prevalence of this variant was evaluated using our local genomic database.

Results

A total of 42 single individuals from unrelated kindreds were evaluated. A disease-causing mutation was found in 11 (26.1%) patients: 10 were diagnosed with FHH (eight CASR and two AP2S1 mutations), and one patient had a CKN2B mutation. In 28 of the remaining patients, the GCM2 p.Tyr394Ser variant was positive in three (10.7%), and all were AJ. Within AJ (15/28, 53.5%), the rate of the p.Tyr394Ser variant was 3/15 (20%), and of those, two had a history of familial isolated hyperparathyroidism. Multi-glandular parathyroid adenoma/hyperplasia was also observed in two of these patients. No clinical or laboratory findings could discriminate patients with the GCM2 p.Tyr394Ser variant from those with FHH. Cinacalcet normalized the calcium levels in one patient. The prevalence of the GCM2 p.Tyr394Ser variant in 15,407 tests in our local genomic database was 0.98%.

Conclusion

In contrast to previous observations, the GCM2 p.Tyr394Ser variant-associated phenotype may be mild in AJ with FIHP, sometimes mimicking FHH. Because surgery may be curative, surgeons should be aware of the possibility of multiple gland diseases in these patients. The clinical spectrum and clinical utility of screening for this variant warrant further investigation.

A Knock-In Mouse Model of the Gcm2 Variant p.Y392S Develops Normal Parathyroid Glands

Context

The glial cells missing 2 (GCM2) gene functions as a transcription factor that is essential for parathyroid gland development, and variants in this gene have been associated with 2 parathyroid diseases: isolated hypoparathyroidism in patients with homozygous germline inactivating variants and primary hyperparathyroidism in patients with heterozygous germline activating variants. A recurrent germline activating missense variant of GCM2, p.Y394S, has been reported in patients with familial primary hyperparathyroidism.

Objective

To determine whether the GCM2 p.Y394S missense variant causes overactive and enlarged parathyroid glands in a mouse model.

Methods

CRISPR/Cas9 gene editing technology was used to generate a mouse model with the germline heterozygous Gcm2 variant p.Y392S that corresponds to the human GCM2 p.Y394S variant. Wild-type (Gcm2+/+) and germline heterozygous (Gcm2+/Y392S) mice were evaluated for serum biochemistry and parathyroid gland morphology.

Results

Gcm2 +/Y392S mice did not show any change compared to Gcm2+/+ mice in serum calcium and parathyroid hormone levels, parathyroid gland histology, cell proliferation, or parathyroid gland size.

Conclusion

The mouse model of the p.Y392S variant of Gcm2 shows that this variant is tolerated in mice, as it does not increase parathyroid gland cell proliferation and circulating calcium or PTH levels. Further investigation of Gcm2+/Y392S mice to study the effect of this variant of Gcm2 on early events in parathyroid gland development will be of interest.

Molecular and Clinical Spectrum of Primary Hyperparathyroidism

Recent data suggest an increase in the overall incidence of parathyroid disorders, with primary hyperparathyroidism (PHPT) being the most prevalent parathyroid disorder. PHPT is associated with morbidities (fractures, kidney stones, chronic kidney disease) and increased risk of death. The symptoms of PHPT can be nonspecific, potentially delaying the diagnosis. Approximately 15% of patients with PHPT have an underlying heritable form of PHPT that may be associated with extraparathyroidal manifestations, requiring active surveillance for these manifestations as seen in multiple endocrine neoplasia type 1 and 2A. Genetic testing for heritable forms should be offered to patients with multiglandular disease, recurrent PHPT, young onset PHPT (age ≤40 years), and those with a family history of parathyroid tumors. However, the underlying genetic cause for the majority of patients with heritable forms of PHPT remains unknown. Distinction between sporadic and heritable forms of PHPT is useful in surgical planning for parathyroidectomy and has implications for the family. The genes currently known to be associated with heritable forms of PHPT account for approximately half of sporadic parathyroid tumors. But the genetic cause in approximately half of the sporadic parathyroid tumors remains unknown. Furthermore, there is no systemic therapy for parathyroid carcinoma, a rare but potentially fatal cause of PHPT. Improved understanding of the molecular characteristics of parathyroid tumors will allow us to identify biomarkers for diagnosis and novel targets for therapy.

One half-century of advances in the evaluation and management of disorders of bone and mineral metabolism in children and adolescents

The past 50 years of research in pediatric bone and mineral metabolism have led to remarkable progress in the identification and characterization of disorders that affect the developing skeleton. Progress has been facilitated through advances in both technology and biology and this paper provides a brief description of some but not all of the key findings, including identification of the calcium sensing receptor and the polypeptides parathyroid hormone and parathyroid hormone-related protein as well as their shared receptor and signal generating pathways; the elucidation of vitamin D metabolism and actions; discovery of fibroblast growth factor 23 (FGF23), the sodium-phosphate co-transporters and the other components that regulate phosphate metabolism. Moreover, the past half-century of research has led to the delineation of the molecular bases for genetic forms of hypoparathyroidism, pseudohypoparathyroidism, and primary hyperparathyroidism as well as the determination of the genetic causes of osteogenesis imperfecta, osteopetrosis, hypophosphatasia, and other disorders of mineral/bone homeostasis. During the next decade we expect that many of these fundamental discoveries will lead to the development of innovative treatments that will improve the lives of children with these disorders.

Clinical and genetic analysis of atypical parathyroid adenoma compared with parathyroid carcinoma and benign lesions in a Chinese cohort

CONTEXT

The malignant potential and molecular signature of atypical parathyroid adenoma (APA) remain elusive. Data from Asia are still lacking.

DESIGN AND SETTING

This was a retrospective study on a large APA cohort in a single center from mainland China.

METHODS

A total of 320 patients with primary hyperparathyroidism (PHPT), containing 79 APA, 79 Parathyroid cancer (PC) and 162 benign lesions cases, were enrolled after surgery for collection of clinical data and genetic analysis.

RESULTS

APA patients showed earlier mean onset age than benign group (46.9 ± 17.1 vs. 52.0 ± 14.3 yrs). Less bone involvement and gastrointestinal symptoms were presented in APA compared to PC (35.4% vs. 62.0%, and 17.7% vs. 41.8%), while more urolithiasis was seen in APA than in benign lesions (57.0% vs. 29.6%). The APA group had moderate hypercalcemia (mean 3.02 ± 0.44mmol/L) with elevated serum PTH (median 593.0pg/ml) and proportion of hypercalcemic crisis as 22.8%, all higher than those of benign lesions but lower than those of PC group. The recurrence/no remission rate of the APA group was significantly lower than that of the PC and similar to the benign group (5.1% vs. 31.6% vs. 3.1%). Germline CDC73 mutation was the most common molecular abnormality in both PC and APA subjects. APA patients with nonsynonymous germline variants showed earlier onset age (28.5 ± 16.9 vs. 48.1 ± 17.7 yrs) and more cases developing no remission/recurrence (25.0% vs. 0.0%).

CONCLUSIONS

Patients with APA presented clinical and biochemical characteristics much less severe than PC and resembling the benign neoplasms, with a relatively good prognosis. Germline gene variations were associated with earlier onset and probably more recurrence of PHPT in APA.