CDC73 c.1155-3A>G is a pathogenic variant that causes aberrant splicing, disrupted parafibromin expression, and hyperparathyroidism-jaw tumor syndrome

Abstract

Germline and somatic pathogenic variants in the CDC73 gene, encoding the nuclear protein parafibromin, increase the risk for parathyroid carcinoma and cause hereditary primary hyperparathyroidism (PHPT) syndromes known as familial isolated hyperparathyroidism (FIHP) and hyperparathyroidism-jaw tumor syndrome (HPT-JT). The identification of pathogenic germline variants in PHPT-susceptibility genes can influence surgical planning for parathyroidectomy, guide screening for potential syndromic manifestations, and identify/exonerate at-risk family members. Numerous types of pathogenic germline variants have been described for CDC73-related conditions, including deletion, truncating, missense, and splice site mutations. Here, we report identification of a non-coding germline CDC73 variant (CDC73 c.1155-3A > G), previously categorized as a variant of uncertain significance (VUS), in a family with HPT-JT. This variant, found in two family members with PHPT, altered CDC73 splicing in peripheral blood cells and disrupted parafibromin immunostaining in associated parathyroid adenomas, strongly evidencing its pathogenicity. Sestamibi scintigraphy yielded nondiagnostic localization results for both patients' parathyroid adenomas, consistent with prior studies suggesting lower sensitivity for small or cystic lesions. Our findings demonstrate key aspects of CDC73-related disorders, highlight the diagnostic value of RNA testing, and exemplify the importance of obtaining a thorough, three-generational family history.

Keywords: Cdc73, aberrant splicing; HPT-JT; RNA sequencing; primary hyperparathyroidism.

Graphical Abstract

Figure 1

Case I parathyroid localization studies. (A) Thyroid US showed a heterogenous hypoechoic lesion posterior to the left thyroid lobe measuring 0.9 cm deep × 0.9 cm wide × 1.5 cm long (arrow, sagittal view). (B) Internal vascularity within the lesion was evident using doppler imaging (arrow, sagittal view). (C) On technetium-99m sestamibi scintigraphy physiologic uptake or the radiotracer was seen in the thyroid gland on the immediate images (arrow). (D) One hour delayed sestamibi images showed poor washout of radiotracer (arrow) but no areas of focal radiotracer retention. (E, F) No focal areas of uptake were observed on SPECT/CT images in the region correlating with the lesion identified on US (arrows).

Figure 2

Sashimi plots drawn from RNA-sequencing data. (A) A sample of the proband’s peripheral blood demonstrated the native splice site (arrow, ~80% use) and introduction of a cryptic splice site (~20% use). Read densities at splice junction sites are shown; junction reads are plotted as arcs. (B) A zoomed-in image of the splice junction site reveals the insertion of two intronic nucleotides (dashed box) in the affected family members but not in an unaffected control patient (proband, (top); sister, (middle); control, (bottom)).

Figure 3

Case 1 left upper parathyroideçtomy pathology. (A) Gross pathology following surgical parathyroid exploration in which the left upper parathyroid gland (arrow) and a portion of the thymus were removed. The excised parathyroid gland measured 1.5 × 1.2 × 0.7 cm and weighed 0.95 g. (B) H&E-stained sections revealed hypercellular parathyroid parenchyma consistent with a parathyroid adenoma. (C) Immunohistochemical staining for parafibromin revealed focally weak to negative nuclear expression in lesional cells (two-toned arrows) with retained staining in surrounding cells (internal positive control, solid arrows).

Figure 4

Case 2 parathyroid localization studies. (A) Thyroid US revealed a left inferior thyroid lobe lesion measuring 1.5 cm deep × 1.6 cm wide × 2.0 cm long (arrow, sagittal view). The lesion had a complex cystic and solid appearance with layering internal debris. (B) The solid component of the lesion demonstrated vascularity (arrow, sagittal view with Doppler). (C) Sestamibi scintigraphy showed physiologic radiotracer uptake of the thyroid gland on the immediate images (arrow). (D) One hour delayed sestamibi images showed radiotracer washout without areas of focal sestamibi retention (arrow). (E) The lesion observed on thyroid US correlated with a hypoattenuating lesion posterior to the left thyroid lobe on the CT scan that was done for (arrow). (F) The hypoattenuating lesion did not have increased radiotracer uptake on the fused SPECT/CT image (arrow).

Figure 5

Case 2 left lower parathyroidectomy pathology. (A) The specimen measured 2.5 × 1.5 × 1.2 cm, weighed 2.8 g, and contained a cystic structure consisting of dark brown fluid. (B) H&E-stained sections displayed hypercellular parathyroid parathyroid with associated areas of fibrosis and cyst formation. No significant cytologic atypia, mitotic activity, or angioinvasion was identified. (C) Immunohistochemical staining for parafibromin revealed negative nuclear expression in the lesional cells (two-toned arrows) and retained staining in surrounding cells (internal positive control, solid arrows).