Can Microsatellite Status of Colorectal Cancer be Reliably Assessed after Neoadjuvant Therapy?

Purpose: Determination of microsatellite instability (MSI) by PCR is the gold standard; however, immunohistochemistry (IHC) of mismatch repair (MMR) proteins is frequently performed instead. The reliability of these methods on post-neoadjuvant-therapy specimens is unknown.  We examined the effect of neoadjuvant therapy on MSI results by PCR and IHC. Experimental design: A total of 239 colorectal cancers resected after neoadjuvant therapy were assessed for MSI with PCR and IHC. PCR and IHC results for matched paired pre- and post-treatment specimens were compared. In parallel, two isogenic cell lines conditioned for MMR functioning and two different patient-derived xenografts (PDX) were exposed to chemotherapy, radiation or both. We also examined whether establishment of PDXs induced MSI changes in five tumors. IHC and MSI were tested after treatment to assess for changes. Results: We identified paired pre- and post-treatment specimens for 37 patients: 2 with PCR only, 34 with IHC only, and 1 with both. All three patients with PCR had microsatellite stable pre- and post-treatment specimens. Of the 35 patients with IHC, 30 had intact MMR proteins in pre- and post-treatment specimens, 1 had equivocal MLH1 staining in the pre-treatment and loss in the post-treatment specimen, and 4 had intact pre-treatment MSH6 but variable post-treatment staining. In the experimental setting, no changes in MSI status were detected after treatment or tumor implantation in animals. Conclusions: Our findings show that expression of MMR proteins, commonly MSH6, can change after neoadjuvant therapy and confirm PCR as the gold-standard test for MSI after neoadjuvant therapy.

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