Test Name: PMS-2
Microsatellite instability (MSI) is characterized by genome-wide alterations in short, repetitive DNA sequences. It is caused by defects in the nucleotide mismatch repair (MMR) system1. Biologically, defective MMR results in a general increase in the mutation rate and the development of a “mutator phenotype”1,2,3,4,5. In colorectal cancer (CRC), high-level MSI was first described in tumors from patients with hereditary non-polyposis colorectal cancer (HNPCC). In about 70% of cases, the HNPCC syndrome develops as a result of an inherited germline mutation of one allele, followed by a somatic mutation of the other allele in one of several mismatch repair genes: hMSH2, hMLH1, hPMS1, hPMS2, hMSH6, and hMLH32,6. 95% percent of the mutations occur in hMSH2 or hMLH12,6,7. Most colorectal carcinomas are thought to be of the chromosomal instable (CIN) or microsatellite stable (MSS) genotype. However, approximately 15% are thought to be of the MSI genotype, of which the HNPCC cases represent less than one-third2,7. The MSS and MSI tumors also seem to differ in clinicopathologic features. The MSI tumors are more often located in the proximal colon and may be synchronous. On histologic examination, they are more often mucinous or poorly differentiated. The patients with MSI-type colorectal carcinomas are generally thought to have a better prognosis than patients with MSS-type colorectal carcinomas. On the other hand, MSS tumors are more often located in the distal colon and represent typical adenocarcinomas6. Although the results published so far have been conflicting, some studies suggest that patients with MSS-type colorectal carcinomas seem to have a greater benefit from adjuvant chemotherapy (with 5-FU) than patients with MSI-type colorectal carcinomas. In contrast, there is some evidence that MSI-CRCs respond better to CPT-11 (irrinotecan) than MSS-CRCs
A formalin-fixed, paraffin-embedded (FFPE) tissue block is preferred specimen type or one (1) unbaked, unstained slide cut at 4-5 microns for H&E staining (required) and two to three (2-3) positively charged unstained slides cut at 3-4 microns for each test/antibody ordered.
Storage and Transport
Use cold pack during transport of paraffin blocks and unstained slides. Cold pack should not be placed in direct contact with the specimen during shipping
Specimen Rejection Criteria
Turn Around Time (TAT): 2-3 Days
CPT Codes*: 88341, 88342, 88360
*The CPT codes provided with our Test Descriptions are based on AMA guidelines and are for informational purposes only. Correct CPT coding is the sole responsibility of the billing party. Please direct any questions regarding coding to the payor being billed.