Parafibromin-deficient (HPT-JT Type, CDC73 Mutated) Parathyroid Tumors Demonstrate Distinctive Morphologic Features

Abstract

The gene CDC73 (previously known as HRPT2) encodes the protein parafibromin. Biallelic mutation of CDC73 is strongly associated with malignancy in parathyroid tumors. Heterozygous germline mutations cause hyperparathyroidism jaw tumor syndrome,which is associated with a high life-time risk of parathyroid carcinoma. Therefore loss of parafibromin expression by immunohistochemistry may triage genetic testing for hyperparathyroidism jaw tumor syndrome and be associated with malignant behavior in atypical parathyroid tumors. We share our experience that parafibromin-negative parathyroid tumors show distinctive morphology. We searched our institutional database for parathyroid tumors demonstrating complete loss of nuclear expression of parafibromin with internal positive controls. Forty-three parafibromin-negative tumors from 40 (5.1%) of 789 patients undergoing immunohistochemistry were identified. Thirty-three (77%) were external consultation cases; the estimated incidence in unselected tumors was 0.19%. Sixteen (37.2%) fulfilled World Health Organization 2017 criteria for parathyroid carcinoma and 63% had serum calcium greater than 3mmol/L. One of 27 (3.7%) noninvasive but parafibromin-negative tumors subsequently metastasized. Parafibromin-negative patients were younger (mean, 36 vs. 63 y; P<0.001) and had larger tumors (mean, 3.04 vs. 0.62 g; P<0.001). Not all patients had full testing, but 26 patients had pathogenic CDC73 mutation/deletions confirmed in tumor (n=23) and/or germline (n=16). Parafibromin-negative tumors demonstrated distinctive morphology including extensive sheet-like rather than acinar growth, eosinophilic cytoplasm, nuclear enlargement with distinctive coarse chromatin, perinuclear cytoplasmic clearing, a prominent arborizing vasculature, and, frequently, a thick capsule. Microcystic change was found in 21 (48.8%). In conclusion, there are previously unrecognized morphologic clues to parafibromin loss/CDC73 mutation in parathyroid tumors which, given the association with malignancy and syndromic disease, are important to recognize.

FIGURE 1

We emphasize that parafibromin can be a difficult stain to perform and interpret and often different conditions are required to achieve a workable result. This figure shows parafibromin IHC from the same tumor performed on the same block initially at primary diagnosis (A, B) and repeated 8 years later for this study (C, D). When first performed, all non-neoplastic cells are completely negative with crisp nuclear staining in internal positive controls (A, B). C, When repeated on archived material, a greater concentration of primary antibody was required to achieve expression in internal positive controls resulting in nonspecific cytoplasmic staining but still completely absent nuclear staining in neoplastic cells. D, The internal controls are weaker in some areas on repeat staining but still positive. Parafibromin IHC, original magnifications A) 200x, B) and C) 400x, D) 600x.

FIGURE 2

In accordance with the WHO 2017 classification the diagnosis of parathyroid carcinoma was restricted to cases that demonstrated unequivocal invasive growth. Hematoxylin and eosin, whole mount.

FIGURE 3

Parafibromin-deficient (HPT-JT type, CDC73 mutated) parathyroid tumors are characterized by cells with eosinophilic cytoplasm demonstrating a sheet-like growth pattern. At this magnification both the prominent arborizing vasculature and the distinctive perinuclear cytoplasmic clearing are also evident. Hematoxylin and eosin, original magnification 100x.

FIGURE 4

Parafibromin-deficient (HPT-JT type, CDC73 mutated) parathyroid tumors demonstrate eosinophilic cytoplasm, but these tumor cells usually lack the distinct cytoplasmic granularity of usual oxyphil cells and are notable for their relative nuclear enlargement and perinculear cytoplasmic clearing. Hematoxylin and eosin, original magnification 200x.

FIGURE 5

In some cases, both multinucleation and nuclear atypia were present. The nuclear atypia was sometimes associated with smudged chromatin and relatively preserved N/C ratios imparting an ancient quality. An atypical mitotic figure is noted in the upper right quadrant. Hematoxylin and eosin, original magnification 400x.

FIGURE 6

Patient 5’s case was initially classified as adenoma at first excision (A–C) but subsequently demonstrated unequivocal metastasis warranting classification as carcinoma (D–F). At initial resection, the tumor came out easily but was fragmented (A) making assessment of the interface with non-neoplastic tissue difficult. B, However, no unequivocal invasive growth was evident. C, At high power the primary tumor demonstrated cytologic features of parafibromin deficiency, including nuclear enlargement with relatively preserved N/C ratios and eosinophilic but not oxyphilic cytoplasm. D, At recurrence, the tumor was resected from the level 3/4 lymph nodes, effectively excluding seeding of benign disease from the previous operation. The recurrence demonstrated an unequivocal invasive growth pattern into soft tissue (E), but it still showed similar cytologic features to that seen in the original tumor (F). Hematoxylin and eosins, original magnifications. B) 100x, C) 400x, E) 100x , F) 200x.