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. It is critically important to establish the pathogenic status of genetic variants, especially for the 35-40% patients carrying a germline change, as it impacts patient management and family surveillance. However, determining the clinical impact of variants of uncertain significance (VUS) remains challenging and requires additional testing. This is especially relevant for genes for which limited functional information is available, such as the TMEM127 gene. We recently reported that loss of TMEM127 promotes RET accumulation by reducing its degradation. Objective : Here, 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 applied histochemical scoring (HS) for membrane (MH-S) and cytoplasm (CH-S) staining. Results : Tumors driven by TMEM127 variants carried the highest RET expression scores (151.8±62), predominantly MH-S, when compared with all other PPGL genotypes, including those with RET pathogenic disruptions (69.9±96.8, adjusted p=0.03) or tumors of undefined genotype (40.8±69, adjusted p=0.0001). RET membrane immunoreactivity also distinguished PPGLs carrying TMEM127 germline variants with likely damaging effects from non-disrupting 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.