MDH2 and hereditary pheochromocytoma-paraganglioma: Cluster 1 PPGLs include pheochromocytomas and abdominal paragangliomas carrying the highest risk of metastatic spread, especially the TCA cycle aberrant tumors carrying genetic variants in enzymes regulating the TCA cycle, including SDHA, SDHB, SDHAF2, FH, MDH2, ISH1 and SLC25A11 [73].