Identifying TMEM127-deficient pheochromocytomas/paragangliomas via RET overexpression by immunohistochemistry

CONTEXT: Pheochromocytomas and paragangliomas (PPGLs) are rare, genetically diverse tumors originating from the adrenal medulla or extra-adrenal paraganglia, respectively. Knowledge of the pathogenic status of genetic variants, especially in the 35-40% of patients with germline changes, impacts management and family surveillance. However, interpreting variants of uncertain significance (VUS) remains challenging, particularly for poorly characterized genes such as TMEM127, and often requires additional testing. We recently reported that TMEM127 loss promotes RET accumulation by reducing its degradation. OBJECTIVE: We evaluated RET expression by immunohistochemistry (IHC) as a potential aid to highlight TMEM127 dysfunction in PPGLs carrying TMEM127 germline variants. METHODS: We performed RET IHC in 104 formalin-fixed and paraffin-embedded (FFPE) sections of clinically and genetically diverse PPGLs and generated histochemical scores (H-S) manually for membrane (MH-S), cytoplasm (CH-S) and their sum (total TH-S), and digitally for CH-S and TH-S. RESULTS: Tumors driven by TMEM127 variants carried the highest RET expression scores (151.8 ± 62), predominantly MH-S, when compared with other PPGL genotypes, including RET pathogenic disruptions (69.9 ± 96.8, adjusted p = 0.03) or tumors of undefined genotype (40.8 ± 69, adjusted p = 0.0001). Digital scoring showed high correlation with manual H-S (0.74 and 0.80 for C-HS and TH-S, respectively). RET membrane immunoreactivity also distinguished PPGLs carrying likely damaging from non-disrupting TMEM127 variants. CONCLUSIONS: These findings point to increased RET membrane expression as a promising biomarker for loss-of-function TMEM127 variants in PPGLs. If validated in independent cohorts, RET IHC could be an effective tool for assessing the functional implications of PPGLs from patients carrying TMEM127 VUS.