Microsatellite Instability

MSI testing and/or immunocytochemistry for mismatch repair proteins MLH1, MSH2, MSH6, and PMS2 should normally be ordered first as part of initial testing (screening) for possible defects in the DNA mismatch repair system. Testing for mutations in the mismatch repair genes is available as a separate test. MSI immunohistochemistry can be concurrently ordered on the same sample.

Defective mismatch repair results in inaccurate copying of repetitive DNA sequences during cell division, causing alterations in the length of short tandem repeat regions (microsatellites) in tumor DNA. This is detected by analyzing the length of a panel of microsatellite sequences using Illumina sequencing (Salipante 2014).

Indications for MSI Testing

• Guide treatment with checkpoint blockade immunotherapy
• Workup of possible Lynch Syndrome (HNPCC)
• Workup of possible Muir-Torre Syndrome in patients with sebaceous neoplasms
• Prognosis in sporadic colorectal cancer

Follow up testing for MSI positive tumors when Lynch Syndrome is suspected

BRAF Mutations [BRAF] V600E mutations are present in about 50% of sporadic MSI colorectal cancer and are absent in Lynch Syndrome.

Lynch Syndrome Workup

Defects in DNA mismatch repair resulting in MSI are present in hereditary non-polyposis colorectal cancer (HNPCC, Lynch syndrome), as well as variants such as Muir-Torre syndrome, and in about 10-15% of sporadic colorectal cancers as well as numerous additional cancer types.

Muir-Torre Syndrome (MTS) is a rare variant of Lynch syndrome (HNPCC) characterized by the additional presence of sebaceous neoplasms. Given that 30-60% of patients presenting with sebaceous neoplasms have Muir-Torre Syndrome (Abbas et al. 2009) and that cutaneous manifestations are the first neoplasm diagnosed in 20% of patients with said syndrome (Ponti et al. 2005), it is suggested that microsatellite instability testing be performed in all patients with sebaceous neoplasms, regardless of family history.

Cancer Treatment with Immunotherapy

MSI is also used to guide treatment with checkpoint blockade immunotherapy in patients with advianced cancer. The U.S. FDA has approved the use of pembrolizumab for adult and pediatric solid tumors with MSI in patients with unresectable or metastatic cancers that have progressed following prior treatments and who have no satisfactory alternative treatment options OR in colorectal cancer with MSI that has progressed following treatment with a oxaliplatin, fluoropyrimidine, and irinotecan.

Microsatellite stable.

No instability detected.

Requirements for Specimen Selection

• To ensure clinically relevant results, the most recent and/or metastatic sample is preferred to older specimens, provided sufficient tumor is present (see point 2).
• To ensure detection of all types of mutations there should be at least 10% tumor cells in the tissue area processed for DNA for mutation detection and 20% tumor cells for microsatellite instability evaluation. If there is more than one tissue block, please provide the block that has the greatest percentage of neoplastic nuclei.
• Tissue samples and pathology reports will be reviewed by directors upon receipt for acceptability prior to testing. Director consultation for tissue selection is available if needed (contact Genetics lab).

Specimen Types

Tissue samples

Send one of the following:

1. Slides: 1 slide at 4-micron thickness stained with hematoxylin-and-eosin (H&E) AND 10 unstained, non-baked slides at 10-micron thickness (a minimum of 5 unstained slides is acceptable). Unstained slides can be on charged or uncharged slides.
2. Tissue Blocks: Provide complete formalin-fixed tissue block containing tumor tissue. Tissue block will be returned at completion of testing.
3. Fresh/frozen tissue: 5 microgram tissue in cell culture medium or frozen tissue stored at -20C. Tumor percentage will not be determined prior to sequencing studies.

NOTE: In order to ensure that enough DNA is obtained, the minimum acceptable tissue area is 10 square millimeters when ten 10-micron slides are supplied (1 cubic millimeter of tissue).

Purified DNA

5 micrograms ANDa reference hematoxylin-and-eosin (H&E) stained slide and pathology report required.

Bone Marrow

1 to 2 mL Bone Marrow in LAVENDER TOP (EDTA) tube

Blood

6 mL blood in LAVENDER TOP (EDTA) tube.

Alternative specimens may be acceptable with approval (contact: 206-598-1149).

For ADD-ON after prior testing, contact Genetics lab.

Unacceptable samples

We cannot accept decalcified samples or tissue samples treated with fixatives other than formalin.

Quantity:

Requested:

• Tissue: 10 unstained slides (10-micron thickness) plus one H&E-stained slide.
• Extracted DNA: 5 microgram Bone Marrow: 2 mL
• Blood: 6 mL

Minimum:

• Tissue: 5 unstained slides (10-micron thickness) plus one H&E-stained slide.
• Extracted DNA: 100-250 nanograms Bone Marrow: 1 mL
• Blood: 3 mL

Genetics Requisition

Providers with access to the UW implementation of Epic (i.e., FHCC, HMC, SCCA, UWMC, UWNW) may order this test using the order "UW Genetics and Solid Tumor Test Request." See tip sheet for more information (internal link).

Attach a copy of the pathology report for the tumor sample being submitted.

Outside Laboratories: Ship at room temperature. Stability: unstained slides or tissue blocks stable at room temperature for at least 2 years.