OR23-08 Immunohistochemical and Single-cell Transcriptomic Analysis Reveals Molecular and Cellular Alterations in Normal, Suppressed, and Adenomatous Parathyroid Glands

Abstract Disclosure: T. Glinin: None. E. Khanafshar: None. A. Shubina: None. J.A. Sosa: Novo Nordisk, AstraZeneca, Eli Lilly & Company, Exelixis, Inc.. J. Koh: None. Primary hyperparathyroidism (PHPT) is caused by excessive parathyroid hormone (PTH) secretion, predominantly due to single-gland adenomas that arise in one of the four parathyroid glands (PTGs). In the presence of a dominant hypersecretory adenoma, the remaining normal PTGs become functionally suppressed, with attenuated PTH output and reduced gland size. Despite its potential significance, the mechanisms of physiological suppression remain poorly understood. We report the first characterization of suppressed PTGs using single-nuclei RNA sequencing (snRNAseq) and immunofluorescence (IF) analysis. During procurement, we identified a cadaveric donor with previously undiagnosed single gland PHPT (PTH = 140 pg/ml, Ca++ = 11.7 mg/dL, affected gland mass = 638 mg). The contralateral normal PTGs from this donor exhibited significantly reduced PTH secretion (99 vs 272 pg/ml/15 min in the tumor), consistent with physiological suppression. Comparison to normal PTGs from non-PHPT individuals by snRNAseq revealed a distinct transcriptomic signature of suppression in all five of the core parathyroid cell populations identified in our previous studies. To assess whether these markers were altered at the protein level in pathological PTGs, we analyzed 7 normal glands, 5 PHPT adenomas, 5 secondary HPT (SHPT) glands, and one suppressed gland by IF, quantitating signal using background-corrected mean fluorescence intensity (MI-N) normalized to nuclear count across >1000 cells per sample. Consistent with the snRNAseq data, protein levels of the LDL receptor-related protein 2 (LRP2), the most highly upregulated gene in suppressed PTGs (3.7-fold increase, p<2e-290), manifested low baseline expression in normal PTGs, while suppressed glands showed significantly increased levels and SHPT glands exhibited near-complete loss (p=0.009). Similarly, the sorbin and SH3 domain containing 1 (SORBS1) transcript increased 2.6-fold in suppressed glands relative to normal tissue (p<2e-290); SORBS1-positive cells comprised 48% of normal PTGs but declined significantly in PHPT (9%; p=0.01) and SHPT (0.4%; p=0.0035). Conversely, suppression downregulated pappalysin 1 (PAPPA) and RNA exonuclease 5 (REXO5) gene expression (p=3.4e-190 and p=1.5e-240, respectively). PAPPA-positive cells were reduced in PHPT (16% vs 40% in normal tissue; p=0.026) and nearly absent in SHPT (0.4%; p=0.003). REXO5-positive cells were rare in all conditions but decreased further in SHPT (0.1% vs. 5% in normal tissue; p=0.002). These findings suggest that suppression engages a distinct gene expression program that is disrupted in hypersecretory adenomas and SHPT glands. Gaining a better understanding of the pathways natively employed in the parathyroid to suppress PTH secretion could reveal potential mechanisms that can be activated as an alternative, non-surgical approach for mitigating PTH hypersecretion. Presentation: Sunday, July 13, 2025