Quality Control in Microsatellite Instability Testing.
Angela N Bartley, Rajyalakshmi Luthra, Devki Saraiya, Russell Broaddus. The University of Texas MD Anderson Cancer Center, Houston
Background: Microsatellite instability (MSI) analysis and immunohistochemistry (IHC) for DNA mismatch repair (MMR) gene products are well-accepted methods in the evaluation of cancers for Lynch Syndrome (LS). Additionally, MSI in sporadic colon cancer has important prognostic and therapeutic implications. The concordance between PCR-based MSI analysis and IHC is typically high. However, whether in a large molecular diagnostics lab or a small community practice, problem cases will arise and are not well-described in the literature. We summarize our findings to document the frequency and outcomes of such cases.
Design: The records of 736 patients who underwent MSI analysis and/or IHC from 2002 to 2010 were reviewed. Patients with both MSI and IHC analysis (n=628) were subsequently studied in detail. Discordance was defined as a discrepancy between the result of MSI and IHC. Problem was defined as a case with indeterminate or questionable IHC for one or more of the MMR genes. All discordant/problematic results were re-reviewed by two pathologists.
Results: Discordances (13) and problems (10) were identified in 23/628 (3.7%) of the cases. Most (12/13) discordances were detected in MSI-high cancers with positive IHC for the four MMR proteins, MLH1, MLH2, MSH6 and PMS2. Following genetic counseling, 67% of those who underwent germline MMR testing were identified to have MMR mutations. The ten problematic cases were grouped as selection error (2), pathologist error in IHC interpretation (6), or unusual pattern of IHC expression (2). Selection error cases involved patients with multiple synchronous MSI-high and MS-stable colon cancers. Pathologist error cases involved difficulties with MSH6 IHC interpretation and overlooking a lack of positive internal control staining. One unusual staining pattern identified included a case with heterogeneous MLH1 and PMS2 staining, later explained by methylation of MLH1. The second unusual pattern case showed expression of MLH1 and MSH2 with complete loss of MSH6 and PMS2 in a colorectal tumor that was MSI-high.
Conclusions: Through this quality control review, several recurring sources of problems in laboratory testing for DNA MMR genes were identified including selection and interpretation error and unusual IHC patterns. Many of these problems are correctable via pathologist education. Recognition of discordant cases and referral for germline mutation analysis is important, as a relatively high percentage of these patients will have germline MMR mutations.
Category: Quality Assurance
Monday, February 28, 2011 1:00 PM
Poster Session II # 227, Monday Afternoon