Although the molecular pathogenesis of parathyroid neoplasia in PHPT has been linked to abnormal expression of genes involved in cell growth (e.g., cyclin D1, retinoblastoma, and β-catenin), the molecular basis of abnormal calcium sensing by calcium-sensing receptor (CaSR) and PTH hypersecretion in PHPT are incompletely understood.
Indeed, EZH2 contributed to the total level of aberrantly accumulated transcriptionally active (nonphosphoylated) β-catenin in the parathyroid tumor cells.
Aberrant accumulation of beta-catenin is very common in parathyroid tumors, and is caused by stabilizing homozygous mutation in 7.3% of Swedish pHPT patients.
Expression of the truncated LRP5 receptor was required to maintain the nonphosphorylated active beta-catenin level, transcription activity of beta-catenin, MYC expression, parathyroid cell growth in vitro, and parathyroid tumor growth in a xenograft severe combined immunodeficiency (SCID) mouse model.
Dysfunction of the Wnt signaling pathway is an established pathogenetic contributor to human tumorigenesis and, recently, the role of stabilizing mutations in beta-catenin, a cause of abnormal Wnt signaling, has been examined in parathyroid tumors with conflicting results.
The presence of normal allelic deletions of the APC gene in bilateral parathyroid tumors and nuclear staining of beta-catenin in the pancreatic tumor in addition to the germline mutations suggests that functional loss of the APC gene played an important role not only in familial adenomatous polyposis but also in the MEN1-related tumors in this case.