Classification of colorectal cancer based on correlation of clinical, morphological and molecular features

Over the last 20 years it has become clear that colorectal cancer (CRC) evolves through multiple pathways. These pathways may be defined on the basis of two molecular features: (i) DNA microsatellite instability (MSI) status stratified as MSI‐high (MSI‐H), MSI‐low (MSI‐L) and MS stable (MSS), and (ii) CpG island methylator phenotype (CIMP) stratified as CIMP‐high, CIMP‐low and CIMP‐negative (CIMP‐neg). In this review the morphological correlates of five molecular subtypes are outlined: Type 1 (CIMP‐high/MSI‐H/BRAF mutation), Type 2 (CIMP‐high/MSI‐L or MSS/BRAF mutation), Type 3 (CIMP‐low/MSS or MSI‐L/KRAS mutation), Type 4 (CIMP‐neg/MSS) and Type 5 or Lynch syndrome (CIMP‐neg/MSI‐H). The molecular pathways are determined at an early evolutionary stage and are fully established within precancerous lesions. Serrated polyps are the precursors of Types 1 and 2 CRC, whereas Types 4 and 5 evolve through the adenoma–carcinoma sequence. Type 3 CRC may arise within either type of polyp. Types 1 and 4 are conceived as having few, if any, molecular overlaps with each other, whereas Types 2, 3 and 5 combine the molecular features of Types 1 and 4 in different ways. This approach to the classification of CRC should accelerate understanding of causation and will impact on clinical management in the areas of both prevention and treatment.