Molecular pathology of colorectal carcinoma. A systematic review centred on the new role of the pathologist.

Colorectal carcinoma (CRC) is the second most frequent malignant disease in developed countries. Many aetiological factors have been reported in CRC development including genetic or non-genetic (environmental) elements. Independently of these, three groups of alterations have been implicated: 1) chromosomal instability (CIN); 2) microsatellite instability (MSI); 3) CpG island methylator phenotype (CIMP). A different multistep association between these alterations contributes to determine three distinct developmental pathways: traditional, alternative and serrated. Each genotypic CRC assessment is associated with specific morphologic or clinical features. Pathologists have to consider the morphologic and clinical features of each CRC when study tumours with molecular tests. Chromatin remodelling is extremely dynamic and depends on several DNA-based processes, such as transcription, DNA repair and replication. The recent results with whole genome sequencing in a vast array of cancers have provided a catalogue of genetic lesions in chromatin modifiers that were previously unappreciated. It has revealed surprising facts about mutations in several SWI/ SNF complex members in many malignancies including CRC. The loss of INI1 expression is detected at a low rate in CRC and may be associated with differentiation grade and survival. Accumulating evidence suggests a critical role of the epithelial mesenchymal transition (EMT) in cancer progression. Some results support the existence of crosstalk between EMT and epigenetic modifications in the MSI-CRC group. We have summarized the role of genetic/epigenetic changes in the origin of the multiple CRC pathway, taking into account current knowledge of pathogenesis and feasibility of designing novel therapeutic approaches.