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

## TL;DR

This study shows that high RET protein levels in tumor cells can help identify pheochromocytomas/paragangliomas caused by TMEM127 gene defects.

## Contribution

RET immunohistochemistry is proposed as a novel diagnostic tool to detect TMEM127 dysfunction in PPGLs.

## Key findings

- TMEM127-deficient tumors showed significantly higher RET expression compared to other PPGL genotypes.
- RET membrane staining had 100% specificity and 91% sensitivity for TMEM127 pathogenic variants.
- RET IHC can help distinguish non-disrupting TMEM127 variants from variants of uncertain significance.

## Abstract

Pheochromocytomas and paragangliomas (PPGLs) are rare, genetically diverse tumors originating from the adrenal medulla or extra-adrenal paraganglia, respectively. Defining a pathogenic variant is critical for patient management and family surveillance, particularly for the 35–40% of patients carrying a germline variant, including those in the TMEM127 gene. However, determining the functional impact of some variants remains challenging and requires additional testing. We recently reported that loss of TMEM127 promotes RET accumulation by reducing its degradation. Here, we evaluated RET expression by immunohistochemistry (IHC) as a potential aid to highlight TMEM127 dysfunction in PPGLs. 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. Tumors driven by TMEM127 variants carried the highest RET expression scores (151.8 ± 62), predominantly MH-S, when compared with other PPGL genotypes, including those with RET pathogenic disruptions (69.9 ± 96.8, adjusted p = 0.04) or tumors of undefined genotype (40.8 ± 69, adjusted p = 0.0003), reflecting high specificity (100%) and sensitivity (91%). RET membrane immunoreactivity also distinguished PPGLs carrying non-disrupting TMEM127 variants from variants of uncertain significance (VUS) with likely damaging effects. These results point to high RET membrane expression as a biomarker for pathogenic TMEM127 and suggest that RET IHC may also assist in interpreting the functional impact of PPGLs carrying TMEM127 VUS.