Evaluating Mismatch Repair Deficiency in Colorectal Cancer Biopsy Specimens

Mismatch repair (MMR) testing on all new cases of colorectal cancer (CRC), has for now been preferably performed on surgical specimens as more tissue is available, however, new clinical trials for the use of immune check-point inhibitors in the neoadjuvant setting require MMR testing on biopsy samples. This study aims at identifying advantages, disadvantages and any potential pitfalls in MMR evaluation on biopsy tissue and how to cope with them. The study is prospective-retrospective, recruiting 141 biopsies (86 proficient (p)MMR and 55 deficient (d)MMR) and 97 paired surgical specimens (48 pMMR and 49 dMMR). In biopsy specimens, a high number of indeterminate stains was observed, in particular for MLH1 (31 cases – 56.4%). The main reasons were a punctate nuclear expression of MLH1, enhanced MLH1 nuclear expression compared to internal controls or both (making MLH1 loss difficult to interpret), which can be solved by reducing primary incubation times for MLH1. Cases with adequate immunostains have a mean number of > 5 biopsies compared to ≤ 3 biopsies in inadequate cases. Conversely, surgical specimens rarely suffer from indeterminate reactions while weaker staining intensity (p < 0.007) for MLH1 and PMS2, and increased grade of patchiness (p < 0.0001) was seen. Central artefacts were almost exclusive to surgical specimens. MMR status classification was possible in 92/97 matched biopsy/resection specimen cases, and all of these were concordant (47 pMMR and 45 dMMR). Evaluation of MMR status on CRC biopsy samples is feasible and correlation between biopsy and surgical samples is excellent, if pitfalls in interpretation are known.