Parathyroid Adenoma Cell Models for Research

Parathyroid adenoma is the most common cause of primary hyperparathyroidism (PHPT), accounting for approximately 85-90% of cases. The global prevalence of PHPT varies, with estimates ranging from 0.1% to 0.5% in the general population, and higher rates (up to 2-3%) in postmenopausal women. According to the World Health Organization (WHO) classification of tumors of endocrine organs (4th edition, 2017), parathyroid adenoma is a benign neoplasm, but it can cause significant morbidity due to hypercalcemia, including nephrolithiasis, osteoporosis, and neurocognitive symptoms. The 5-year survival for benign parathyroid adenoma is essentially 100% after surgical resection, but the disease burden is substantial due to its high prevalence and associated complications. The National Cancer Institute (NCI) Surveillance, Epidemiology, and End Results (SEER) database does not track benign tumors, but the incidence of PHPT has been reported to be increasing, partly due to routine calcium screening. Research on parathyroid adenoma is crucial to understand the molecular drivers of tumorigenesis, improve diagnostic markers, and develop targeted therapies for atypical or malignant cases.

Parathyroid adenoma is an ideal model for studying endocrine tumorigenesis due to its well-defined clinical presentation, relatively simple genetic landscape compared to malignant tumors, and the availability of normal parathyroid tissue as a comparator. Key research questions include: What are the initiating events in parathyroid adenoma formation? How do mutations in genes like MEN1, CCND1, and CDC73 contribute to clonal expansion? What are the mechanisms of calcium-sensing receptor (CASR) dysregulation? The disease is also valuable for studying the effects of gene dosage and haploinsufficiency, as many driver mutations are loss-of-function in tumor suppressor genes. Public datasets, such as those from The Cancer Genome Atlas (TCGA) and COSMIC, provide genomic and transcriptomic data, although parathyroid adenoma is not as extensively covered as other cancers. Nonetheless, targeted sequencing studies have identified recurrent alterations, making it a tractable system for functional validation using gene-edited cell models.

Parathyroid adenoma arises from dysregulation of cell proliferation and calcium homeostasis. The major pathways include:

• • MEN1-encoded menin pathway: Menin is a tumor suppressor that regulates gene transcription, histone modification, and genome stability. Loss of menin leads to unchecked cell proliferation.
• • Cyclin D1/CCND1 pathway: Overexpression of cyclin D1 promotes G1/S transition, driving cell cycle progression. This can occur via chromosomal rearrangement or amplification.
• • Wnt/β-catenin signaling: Aberrant activation of β-catenin has been observed in some adenomas, leading to increased transcription of proliferative genes.
• • Calcium-sensing receptor (CASR) signaling: Reduced expression or function of CASR impairs the ability of parathyroid cells to sense extracellular calcium, leading to inappropriate PTH secretion and hypercalcemia.

These pathways are not mutually exclusive and may cooperate in tumorigenesis.

Based on data from COSMIC and targeted sequencing studies (e.g., from NCBI Gene and ClinVar), the following genetic alterations are frequently observed in parathyroid adenomas:

Note: Frequencies vary by study population and detection method. Data are derived from COSMIC and NCBI Gene.

The deregulated signaling networks in parathyroid adenoma include:

• • Wnt/β-catenin pathway: Key nodes include β-catenin (CTNNB1), APC, and GSK3β. Mutations or altered expression can lead to nuclear accumulation of β-catenin and activation of TCF/LEF transcription factors.
• • PI3K/AKT/mTOR pathway: Activation of this pathway promotes cell survival and proliferation. PTEN loss or PIK3CA mutations may contribute.
• • MAPK/ERK pathway: Growth factor signaling via RAS/RAF/MEK/ERK can be upregulated, though direct mutations are less common.
• • Cell cycle regulators: Besides CCND1, alterations in CDKN1B (p27) and CDKN2C (p18) have been reported, leading to loss of cell cycle inhibition.

These networks interact, and their dysregulation contributes to the clonal expansion of parathyroid cells.

Unlike common cancers, there are few well-characterized parathyroid adenoma cell lines. The most widely used is the bovine parathyroid cell line, but human lines are limited. Organoid models derived from human parathyroid adenomas are emerging as valuable tools. The following table summarizes available cell lines and organoid models:

Organoids offer advantages such as 3D architecture, long-term culture, and the ability to model tumor heterogeneity. However, their use is still in early stages.

Animal models for parathyroid adenoma are limited but include:

• • Genetically engineered mouse models (GEMMs): Conditional knockout of Men1 in parathyroid cells (e.g., using PTH-Cre) leads to parathyroid hyperplasia and adenoma formation.
• • Xenograft models: Subcutaneous or renal capsule implantation of human parathyroid adenoma cells or organoids into immunodeficient mice (e.g., NSG mice) to study tumor growth and response to therapy.
• • Induced models: Chronic calcium or vitamin D deficiency can induce secondary hyperparathyroidism, but not true adenoma.

These models are useful for studying tumorigenesis and testing therapeutic interventions, but they have limitations in recapitulating the human disease.

Gene-edited cell models, particularly CRISPR-based isogenic lines, are powerful tools for studying parathyroid adenoma. These models allow precise introduction or correction of mutations in a controlled genetic background, enabling functional studies. Examples include:

• • MEN1 knockout cell lines: Generated by CRISPR-Cas9-mediated disruption of MEN1 in a suitable cell line (e.g., a parathyroid-derived or HEK293T background) to study loss of menin function.
• • CCND1 overexpression lines: Knock-in of a constitutively active CCND1 allele or overexpression construct to mimic the rearrangement seen in adenomas.
• • CDC73 knockout lines: To study the role of parafibromin loss.
• • CASR mutant lines: Introduction of inactivating mutations to study calcium sensing.

These gene-edited models are commercially available from various sources, and sequence-verified clones accelerate research by providing reproducible, isogenic controls. They are essential for validating driver mutations, understanding signaling pathways, and screening therapeutic compounds.

Gene-edited cells enable functional validation of candidate driver genes identified from genomic studies. For example:

• • MEN1 knockout: Demonstrates that loss of menin leads to increased proliferation and altered gene expression, confirming its tumor suppressor role.
• • CDC73 knockout: Shows how loss of parafibromin affects chromatin remodeling and cell growth.
• • CCND1 overexpression: Validates the oncogenic role of cyclin D1 in driving cell cycle progression.

These models allow researchers to study the downstream effects of specific mutations in a controlled system, complementing data from patient samples.

Isogenic cell line pairs (e.g., wild-type vs. MEN1 knockout) are ideal for drug screening. They can be used to:

• • Identify compounds that selectively inhibit mutant cells, providing a therapeutic window.
• • Study mechanisms of resistance to drugs targeting calcium-sensing or PTH secretion.
• • Test combination therapies that exploit synthetic lethal interactions.

For example, a MEN1 knockout line could be used to screen for compounds that induce apoptosis specifically in menin-deficient cells, potentially leading to targeted therapies for parathyroid adenomas.

CRISPR-based synthetic lethality screens can identify genes that are essential only in the context of specific mutations. For parathyroid adenoma, such screens could:

• • Identify vulnerabilities in MEN1-deficient cells, such as dependencies on specific DNA repair pathways.
• • Discover biomarkers that predict response to therapy.
• • Uncover novel therapeutic targets for atypical or malignant parathyroid tumors.

Gene-edited models provide the necessary isogenic background to perform these screens with high specificity.

The following public databases provide valuable data for parathyroid adenoma research: