Para This, Fibromin That: The Role of CDC73 in Parathyroid Tumors and Familial Tumor Syndromes

CDC73 alterations are associated with three main parathyroid lesions according to the World Health Organization (WHO) classification of tumors of the endocrine system. These include hyperparathyroidism-jaw tumor (HPT-JT) syndrome-associated adenomas, atypical parathyroid tumors (APTs), and parathyroid carcinomas (PCs). The loss of nuclear parafibromin expression, which serves as a surrogate marker for the underlying CDC73 alteration, encompasses these tumors under the term parafibromin-deficient parathyroid tumors. They have distinct morphologic features of more abundant eosinophilic cytoplasm with perinuclear clearing surrounding a large nucleus as well as prominent dilated branching "hemangiopericytoma-like" vasculature and a thick capsule as well as variably sized cystic spaces. These tumors include cases that show unequivocal histologic features fulfilling the criteria for PCs with growing data indicating a higher rate of recurrence or metastasis compared with parafibromin intact PCs. More importantly, the loss of parafibromin expression can be used in clinical practice to recognize APTs that fall short of a conclusive diagnosis of PCs, but clinically behave akin to them. Moreover, recognizing these tumors can lead to an underlying germline mutation and a diagnosis of HPT-JT, which impacts long-term treatment and surveillance for patients and close family.