NRASQ61R mutation-specific immunohistochemistry is highly specific for either NRASQ61R or KRASQ61R mutation in colorectal carcinoma

Anti–epidermal growth factor receptor–targeted therapy is only indicated in RAS wild-type colorectal carcinomas (CRCs). It is recommended that both NRAS and KRAS mutation testing to be performed before a CRC is considered RAS wild-type. Given that mutation-specific immunohistochemistry (IHC) has been shown to be sensitive and specific for the detection of NRASQ61R mutations in melanoma, we assessed the specificity of NRASQ61R mutation-specific IHC in CRC. IHC was performed on tissue microarrays containing 2823 consecutive CRC undergoing surgery with curative intent using a novel mutation-specific antibody to the protein produced by the NRASQ61R mutation (clone SP174). Tissue microarrays were assessed by 2 observers and all IHC-positive or equivocal cases were repeated on whole sections to confirm the result. Positive cases then underwent molecular testing by matrix-assisted laser desorption/ionization-time of flight polymerase chain reaction. In total, 22 of 2823 (0.8%) CRCs demonstrated confirmed positive staining with complete interobserver concordance. RAS mutations were confirmed in all IHC-positive CRCs. In total, 11 cases harbored the NRASQ61R mutation. Surprisingly, 11 cases demonstrated the KRASQ61R mutation. We conclude that mutation-specific IHC with this currently available NRASQ61R antibody is highly specific for the presence of either NRASQ61R or KRASQ61R mutations in CRC. We caution that we did not assess the sensitivity of IHC and that this antibody does not detect other RAS mutations. Therefore, negative staining does not exclude a clinically significant RAS mutation. However, positive staining confirms the presence of an NRASQ61R or KRASQ61R mutation without the need for further molecular testing.