Clinical relevance of microsatellite instability in colorectal cancer

Abstract

Microsatellite instability (MSI) is a clonal change in the number of repeated DNA nucleotide units in microsatellites. It arises in tumors with deficient mismatch repair due to the inactivation of one of the four mismatch repair genes: MSH2, MLH1, MSH6, and PMS2. In order to determine the MSI status of a tumor, microdissection and polymerase chain reaction-based detection strategies are required. For practical purposes, MSI is equivalent to the loss of staining by immunohistochemistry (IHC) of one of the mismatch repair genes since both signify an abnormality in mismatch repair. Of all colorectal cancers (CRCs), 15% to 20% display MSI or abnormal IHC (often referred to as microsatellite instability [MIN] pathway). The remaining 80% to 85% of CRCs are microsatellite stable but most are characterized by chromosomal instability (CIN pathway). Almost all Lynch syndrome tumors have MSI or abnormal IHC and they account for up to one third of all MIN CRCs (3% to 5% of all CRCs). The remaining MIN tumors are sporadic as a result of somatic inactivation of the MLH1 gene caused by methylation of its promoter. Thus, the presence of a MSI/IHC abnormality prompts further investigations to diagnose Lynch syndrome, whereas its absence excludes Lynch syndrome. We recommend screening all CRC tumors for IHC or MSI. MIN tumors have a more favorable outcome than CIN tumors, and fluorouracil-based adjuvant chemotherapy does not improve the outcome of stage II or stage III MIN tumors. More data are needed to determine how best to treat patients with stage II and stage III MIN CRCs.

Fig 1.

Tracings of polymerase chain reaction (PCR) amplicons containing microsatellite D2S123, a highly polymorphic (CA)n dinucleotide. One PCR primer was end labeled with fluorescent dye, the amplicon was run on a sequencer, and it was analyzed by the Genotyper software. Lengths of the amplicons containing the microsatellite and the strength of the signal are indicated. The upper tracing is from blood DNA displaying heterozygosity, with one allele measuring 164 bp and the other allele measuring 184 bp in length (arrows). The lower tracing is from the same patient's colorectal tumor showing two new alleles, which measure 162 bp and 178 bp in length (arrows). This means that both of the germline alleles were mutated in the tumor. The persistence of the germline alleles most likely emanates from noncancerous tissue present in the tumor specimen (eg, tumor-infiltrating lymphocytes, stroma, blood vessels).

Fig 2.

Microsatellite instability (MSI) determination of two microsatellites in the same, multiplexed run. The upper tracing is from blood DNA showing (A)n mononucleotide marker BAT26 at 120 bp and (CA)n dinucleotide marker D2S123 at 170 bp. The patient is homozygous for both markers. The lower tracing is from the same patient's tumor DNA. Arrows point to the new alleles in BAT26 at 114 bp and in D2S123 at 172 and 160 bp. The fact that D2S123 displays two novel alleles suggests that the tumor consists of at least two different clonal expansions. Note the “stutter” bands that occur both in the dinucleotide marker and, more strongly, in the mononucleotide marker. These are believed to be produced during the polymerase chain reaction and do not occur in vivo.

Fig 3.

Schematic classification of colorectal cancers (CRCs). MSI, microsatellite instability.