**4. The consensus molecular subtypes of colorectal cancer**

More recently, in order to resolve inconsistencies in subclassification systems and to aid clinical translation, the CRC research community formed an international consortium dedicated to large-scale data sharing and analytics [22]. After analysing the independent transcriptomic-based classification systems (which comprised 18 CRC datasets and 4151 patients in total) and using unsupervised clustering techniques, four robust consensus molecular subtypes (CMSs) with distinguishing features were proposed. Tumours with mixed features (approximately 13%) were thought to represent a transition phenotype or intratumoural heterogeneity. **Table 3** summarises the main biological, molecular, clinical and prognostic associations of the four consensus subtypes.

In regard to genomic aberrations, CMS1 samples were hypermutated and encompassed the majority of MSI-H tumours. This group also displayed widespread hypermethylation and low prevalence of SCNAs. CMS2 and CMS4 subgroups displayed higher CIN via high SCNA counts. CMS3 samples consisted of fewer SCNAs than other CIN tumours, a significant proportion (30%) of hypermutated tumours


#### **Table 3.**

*The four consensus molecular subtypes of CRC.*

and intermediate levels of gene hypermethylation [22]. Despite clear enrichment of certain gene mutations within CMS groups, such as high rates of BRAF mutation in CMS1 and KRAS mutations in CMS3, no single genetic aberration was found to be limited to one subtype, and no subtype was defined by a single molecular event. Further integrative genomic analysis did not draw any clear associations either, highlighting the poor genotype–phenotype correlation in this cancer.

Further exploration of gene expression data revealed insight into the underlying biology of the subtypes: CMS1 samples showed strong immune activation and infiltration with CD4+ T helper cells, CD8+ cytotoxic T cells and natural killer (NK) cells along with strong activation of immune evasion pathways. CMS2 showed marked upregulation of Wnt and MYC downstream targets and higher expression of oncogenes EGFR, ERBB2, insulin-like growth factor 2 (IGF-2), insulin receptor substrate 2 (IRS-2) and transcription factor hepatocyte nuclear factor 4α (HNF4α). CMS3 samples showed enrichment for multiple metabolism signatures which are keeping with the described notion that activating KRAS mutations induce prominent metabolic adaptation [23, 24]. CMS4 tumours showed upregulation of genes associated with epithelial-mesenchymal transition (EMT), such as transforming growth factor β (TGF-β) and integrins, as well as stromal invasion.
