Clinical Implications

Microsatellite instability (MSI) results from the systematic accumulation of deletions/insertions in short repetitive DNA sequences in tumor cells with a deficient mismatch repair (MMR) system. MSI occurs in approximately 15% of all colorectal cancers and is clinically useful to identify patients with hereditary nonpolyposis colorectal cancer (HNPCC, Lynch Syndrome) caused by germline mutations of MMR genes. MSI status may also predict cancer response/resistance to certain chemotherapies.

To standardize MSI analysis, a 1997 National Cancer Institute (NCI) workshop recommended a reference panel of five microsatellite markers for the detection of MSI and established MSI classification guidelines based on the results. The reference panel, referred to as the Bethesda panel, consists of two mononucleotide loci (Big Adenine Tract or BAT-25 and BAT-26) and three dinucleotide loci (D2S123, D5S346 and D17S250). Using the Bethesda panel, cancers with instability at 2 or more of these loci were interpreted as MSI-high (MSI-H), and cancers with no instability at any of the five loci were considered Microsatellite Stable (MSS). At a further follow-up NCI workshop, it was recognized that the sensitivity and specificity of the original Bethesda panel might be limited due to the inclusion of dinucleotide repeats, which are less sensitive and specific than mononucleotide repeats for the identification of cancers with MMR deficiencies. It has been suggested that a panel of quasimonomorphic mononucleotide repeats, may be sensitive and specific enough for the detection of MSI-high cancers and may obviate the need for normal matching DNA for the tumors being tested.

Specimen Requirements

Acceptable specimens for the assay are formalin-fixed, paraffin-embedded colorectal carcinoma tissue specimens with a fixation time of 6-48 hours.

Volume

One representative paraffin block is preferred. Alternatively, for resection specimens a minimum of 2 unstained tissue sections (5 µm thick; 50-600mm²) is required.

Storage and Shipment Instructions

Maintain and ship specimens at ambient temperature.

Limitations

Insufficient tumor content may not allow the detection of MSI instability: 20% of tumor cells is required. Tumor content is evaluated prior to analysis and macrodissection is performed. Fixatives other than formalin or prolonged fixation time may give rise to inadequate results.

Turn-Around Time

Three to five business days for respectively slides and paraffin blocks.