The chromosomal instability pathway in colon cancer

Abstract

The acquisition of genomic instability is a crucial feature in tumor development and there are at least 3 distinct pathways in colorectal cancer pathogenesis: the chromosomal instability (CIN), microsatellite instability, and CpG island methylator phenotype pathways. Most cases of colorectal cancer arise through the CIN pathway, which is characterized by widespread imbalances in chromosome number (aneuploidy) and loss of heterozygosity. It can result from defects in chromosomal segregation, telomere stability, and the DNA damage response, although the full complement of genes underlying CIN remains incompletely described. Coupled with the karyotypic abnormalities observed in CIN tumors are the accumulation of a characteristic set of mutations in specific tumor suppressor genes and oncogenes that activate pathways critical for colorectal cancer initiation and progression. Whether CIN creates the appropriate milieu for the accumulation of these mutations or vice versa remains a provocative and unanswered question. The goal of this review is to provide an updated perspective on the mechanisms that lead to CIN and the key mutations that are acquired in this pathway.

Figure 1. Multistep genetic model of colorectal carcinogenesis

The initial step in colorectal tumorigenesis is the formation of aberrant crypt foci (ACF). Activation of the Wnt signaling pathway can occur at this stage as a result of mutations in the Adenomatous polyposis coli (APC) gene. Progression to larger adenomas and early carcinomas requires activating mutations of the proto-oncogene KRAS, mutations in TP53, and loss of heterozygosity at chromosome 18q. Mutational activation of PIK3CA occurs late in the adenoma–carcinoma sequence in a small proportion of colorectal cancers. Chromosomal instability is observed in benign adenomas and increases in tandem with tumor progression.

Figure 2. Regulation of sister chromatid separation at the metaphase-anaphase transition

In prometaphase, highly condensed chromosomes establish bipolar attachments to the mitotic spindle. Unattached or malorientated chromosomes generate a signal to delay the onset of anaphase until all pairs of sister chromatids are properly aligned on the metaphase plate. This signal is transduced by a relay of spindle-checkpoint proteins, including MAD1, MAD2, BUB1, BUBR1, BUB3 and centrosome protein E (CENP-E), which inhibits cell division cycle 20 (CDC20)-mediated activation of an E3 ubiquitin ligase, the anaphase promoting complex/cyclosome (APC/C). Following attachment and alignment of all the chromosomes at metaphase, the checkpoint signal is silenced and APC/C initiates the ubiquitin-dependent degradation of securin and activation of separase. Separase in turn cleaves a multiprotein complex termed cohesin, which creates physical links between sister chromatids to initiate anaphase.

Figure 3. The Wnt signaling pathway in the “OFF” and “ON” states

In the absence of a Wnt signal, the destruction complex containing adenomatous polyposis coli (APC), glycogen synthase kinase 3β (GSK-3β) and casein kinase 1α/ε (CK1α/ε) on an axin-conductin scaffold targets the degradation of cytoplasmic β-catenin in a proteasome-dependent manner. In the nucleus, Wnt target genes are also kept silent by the repressor Groucho interacting with DNA-bound T cell factor (TCF). In the presence of a Wnt ligand, occupancy of the receptors Frizzled (Frz) and coreceptor low-density lipoprotein receptor-related protein (LRP) triggers the phosphorylation of the cytoplasmic tail of LRP by CK1 and GSK-3β as well as the disheveled (Dsh)-dependent recruitment of axin on phosphorylated LRP. Phosphorylation of β-catenin no longer occurs, and the increased cytoplasmic levels of β-catenin translocate to the nucleus, where the transcription of multiple genes is initiated through displacement of Groucho and the interaction of β-catenin with the T-cell factor (TCF)/lymphoid enhancer factor (LEF) family of transcription factors.

Figure 4. The RAS signaling pathway

Growth factors binding to their cell surface receptors activate guanine exchange factors (GEF) such as SOS (son of sevenless) that are attached by the adaptor protein GRB2 (growth-factor-receptor bound protein 2). SOS stimulates the release of bound GDP from RAS, and it is exchanged for GTP, leading to the active RAS-GTP conformation. The GTPase-activating proteins (GAP) can bind to RAS-GTP and accelerate the conversion of RAS-GTP to RAS-GDP (guanosine diphosphate), which terminates signaling. Mutated RAS is constitutively active in the RAS-GTP conformation. Activated RAS regulates multiple cellular functions through effectors including the Raf-MEK-ERK pathway, PI3K, RALGDS, RALGDS-like gene (RLG) and RGL2.