Correlation between RICTOR overexpression and amplification in advanced solid tumors

Rapamycin-insensitive companion of mTOR (RICTOR) is a key component of mammalian target of rapamycin (mTOR) complex 2 (mTORC2), and promotes cellular proliferation and survival through the activation of downstream AGC kinase family members. The amplification of RICTOR has been proposed as a therapeutically relevant genomic alteration. However, other than next-generation sequencing, precise diagnostic methods to detect RICTOR amplification in advanced solid cancers have not been fully explored. We performed immunohistochemistry (IHC) analysis on solid tumor tissues from 435 cancer patients. Overexpression of RICTOR was found in 213 cases (49.0 %: 1+, 29.4 %; 2+, 15.2 %; 3+, 4.4 %) consisting of 111 colorectal cancers, 42 gastric cancers, 16 renal cell carcinomas, 8 soft tissue sarcomas, 6 hepatocellular carcinomas, 6 cholangiocarcinomas, 4 lung cancers, and 37 other tumors. RICTOR overexpression was heterogeneous (stained < 50 % of the tumor volume) in 32.4 % (12/37) of IHC-positive cases. We performed fluorescence in situ hybridization (FISH) in 37 RICTOR-overexpressed IHC-positive cases (1+, 12; 2+, 11; 3+, 14) and 13 IHC-negative solid tumors. FISH enabled us to detect RICTOR amplification in 7/12 (58.3 %) IHC 1+, 10/11 (90.9 %) IHC 2+, and 11/14 (78.6 %) IHC 3+ cases. In total, there was amplification in 75.7 % (n = 28) of the RICTOR-overexpressed cases, according to FISH. There was RICTOR amplification in only 7.7 % of the RICTOR IHC-negative cases. RICTOR amplification was significantly more common in IHC-positive cases than in IHC-negative cases (p < 0.0001). The IHC results correlated well with those of FISH (r = 0.60). RICTOR overexpression is more common in solid tumors than previously reported in cases detected by next-generation sequencing. This discrepancy may be caused by intratumoral heterogeneity. In conclusion, heterogeneous RICTOR overexpression is common in solid tumors and RICTOR IHC can be used as a screening tool to detect RICTOR amplification.