PO-505 Prognostic impact of KRAS splicing in microsatellite stable colorectal cancer

IntroductionMutations in the KRAS oncogene represent one of the most common genetic alterations in colorectal cancer (CRC). KRAS is expressed as two transcript variants caused by alternative splicing, KRAS-4A and KRAS-4B, which both give rise to oncogenic proteins when KRAS is mutated. While KRAS mutations are negative predictors of response to anti-EGFR therapy in metastatic disease, and shown to be associated with poor prognosis in the microsatellite stable (MSS) subtype, little is known about the clinical relevance of KRAS splice variants in CRC. Here, we evaluated the prognostic impact of KRAS splicing in relation to KRAS mutation status in primary MSS CRC.Material and methodsA total of 258 primary MSS CRCs and 41 normal mucosa samples from a population-representative series of patients treated at the Oslo University Hospital, were subjected to exon-resolution and splicing-sensitive expression analysis using microarrays (Affymetrix Human Transcriptome Arrays 2.0) and/or RNA sequencing (Illumina HiSeq 2500). The study was approved by the Medical and Health Research Ethics, South Eastern Norway, and written informed consent was obtained from all patients.Results and discussionsAnalysis of the relative expression level of KRAS-4A and KRAS-4B revealed that KRAS splicing was altered in MSS CRC compared to normal colonic mucosa. There was no association between KRAS splicing and KRAS mutation status. However, gene set enrichment analysis of a KRAS activity signature revealed a mutation-dependent impact of KRAS splicing on downstream signalling, specific to the KRAS wild-type subgroup. In concordance, survival analysis of patients with stage I-III tumours revealed KRAS splicing to be associated with overall survival in KRAS wild-type (HR: 2.36, 95% CI: 1.07–5.18, p=0.033), and not in KRAS mutant cases (HR: 0.69, 95% CI: 0.32–1.48, p=0.337, P interaction test = 0.026). The prognostic association was retained in multivariable analysis including age, stage, gender and location (HR: 2.68, 95% CI: 1.18–6.09, p=0.018), indicating KRAS splicing to be an independent prognostic marker in KRAS wild-type MSS CRC.ConclusionOur results indicate that KRAS has prognostic value beyond mutation status in MSS CRC, and that the prognostic impact of KRAS splicing is specific to the clinically relevant KRAS wild-type subgroup.