Molecular epidemiology of IDH2 hotspot mutations in cancer and immunohistochemical detection of R172K, R172G, and R172M variants.

IDH1/2 hotspot mutations occur in glioma, cholangiocarcinoma, chondrosarcoma, sinonasal carcinoma, and T-cell lymphoma and have diagnostic, prognostic, and/or therapeutic value. Availability of immunohistochemistry (IHC) protocols for specific IDH2 mutation detection is limited. A targeted exome sequencing assay MSK-IMPACT cohort comprising >38,000 cancer cases was explored for the presence of IDH1/2 mutations in solid malignancies and select T-cell lymphomas. Seventy-four formalin-fixed paraffin-embedded IDH1/2-mutated (n = 62) and wild-type (n = 12) samples were used for testing and optimization of anti-IDH2 monoclonal antibodies (mAbs) 14H7, 3C11, and MMab1 targeting R172K, R172G, and R172M mutant proteins, respectively. IDH1/2 mutations were common in glioma (26.8% and 1.6%), intrahepatic cholangiocarcinoma (23.1% and 5.7%), chondrosarcoma (19.4% and 10.7%), sinonasal undifferentiated/large-cell neuroendocrine carcinoma (0% and 84.2%), angioimmunoblastic T-cell lymphoma (0% and 22%), and peripheral T-cell lymphoma (0 and 5.1%). In other cancers, IDH2 mutations were rare. IDH2 R172 variants included R172K (39%), R172S (29%), R172W (12%), R172G (10%), R172M (5%), and R172T (4%). 14H7, 3C11, and MMab1 detected all IDH2 R172K, R172G, and R172M, respectively, and produced a crisp, granular cytoplasmic staining pattern. 3C11 was also positive in 5 of 6 IDH1 R132G mutants showing a homogeneous, smooth cytoplasmic staining. All 3 mAbs were negative in other IDH1/2 mutant or wild-type cases. IHC using mAbs 14H7, 3C11, and MMab1 can facilitate molecular diagnosis as a reliable, fast, and inexpensive alternative for specific IDH2 variant detection. Given the distinct distribution of IDH2 R172 mutations in cancers, these mAbs could also serve as useful pathologic diagnostic markers.