[627] Concordance between Microsatellite Instability (MSI) Testing and Mismatch Repair Protein Immunohistochemistry (MMR IHC) and Analysis of Discordant Cases.

James P Grenert, Peggy Conrad, Jonathan P Terdiman, Yunn-Yi Chen. UCSF, San Francisco, CA

Background: MSI and MMR IHC testing are both widely used to screen patients for Lynch syndrome. Few studies have directly compared the results of MSI and IHC performed with current methodologies (mononucleotide repeat markers, four-protein IHC). Using a series of tumors from patients at elevated risk for Lynch syndrome, we looked at the concordance rate of these tests and analyzed discordant cases to look for trends and to determine if performing both assays provides any benefit over using a single assay.
Design: Cancers were selected on the basis of either histologic or clinical findings suggesting Lynch syndrome. 286 tumors (152 GI, 128 GYN, 2 breast, 1 ea brain, skin, kidney, unknown) were tested with both MSI and IHC. MSI testing used five mononucleotide markers (Promega MSI Analysis System). MMR IHC was performed with stains for MLH1, MSH2, MSH6, PMS2. Agreement between tests is defined as MSS/MSI-L and all proteins present; or MSI-H and any protein absent. For cases with discordance, available clinical history and MMR gene methylation/sequencing results were obtained. Additionally, some cases with absent MSH6 were re-stained with an alternate MSH6 antibody, and neoadjuvantly-treated rectal tumors were re-tested on an untreated sample when available.
Results: Overall, 68 tumors (23.8%) were abnormal by at least one test. This includes 49 tumors (17.1%) that tested abnormal by both methods, and 19 tumors (6.6%) abnormal by only one test. MSI and IHC gave the same result in 266 cases (93%), 14 cases (5.0%) had discordant results, and 6 cases (2.1%) had equivocal IHC results. Of discordant cases, 10 could be resolved with additional testing or history and showed that MSI was correct in 8 of these, while IHC was correct in 2 (TABLE). Of equivocal IHC cases, 2 were shown to have a correct MSI result.

Discordant Cases

	n	MSI correct	IHC correct	Unknown
MSI-H, IHC normal	5	3	0	2
MSS, IHC abnormal	9	5	2	2
Total	14	8	2	4

Conclusions: MSI and MMR IHC have a high rate of concordance when a mononucleotide repeat panel and four-protein IHC are used. However, some patients with defective MMR, including those with Lynch syndrome, are identifiable with only one of these methods. For patients with an elevated suspicion of Lynch syndrome, testing with both MSI and IHC improved sensitivity and specificity over a single test. Additionally, MSI can clarify ambiguous IHC results, such as may be seen post-treatment or with weak MSH6 staining.
Category: Gastrointestinal

Wednesday, March 2, 2011 9:30 AM

Poster Session V # 38, Wednesday Morning