Discordance in RAS mutations between primary colon tumor and metastases: a real event or a matter of methodology?

Background: Analysis of K- and N-RAS mutations is mandatory before planning treatment of metastatic colorectal cancer, because only RAS wild-type (WT) patients can benefit from treatment with anti-EGFR monoclonal antibodies (cetuximab and panitumumab).
Case Report: Here we report the case of a 69-year-old male patient affected by metastatic sigmoid cancer. He underwent left hemicolectomy, and histology diagnosed a well-differentiated, pT4, node-positive adenocarcinoma; KRAS analysis performed with direct sequencing identified a mutation in exon 2 of the KRAS gene (GGT->GTT). After first-line chemotherapy with FOLFOX6 plus bevacizumab, the patient underwent surgical resection of residual liver metastases. Histology showed metastatic deposits from colic adenocarcinoma with extensive coagulative necrosis. Mutational analysis of the KRAS gene was also performed on liver metastases by pyrosequencing assay, and no mutation was identified. Due to the discordant results (GGT->GTT exon 2 KRAS mutation in the primary tumor, and KRAS-WT in the liver metastases), mutational analysis on liver metastasis was repeated using next-generation sequencing and enriching the sample in tumor cells by manual microdissection; the same type of mutation of the primary tumor (GGT->GTT exon 2 KRAS gene) was confirmed.
Conclusions: Accurate tissue sampling and adequately sensitive assays are essential to correctly identify colorectal cancer patients who can be treated with an anti-EGFR monoclonal antibody.