High concordance rate of capillary electrophoresis workflow for microsatellite instability analysis and mismatch repair (MMR) immunostaining in colorectal carcinoma

Microsatellite instability (MSI) is the primary predictive biomarker for therapeutic efficacies of cancer immunotherapies. Establishment of the MSI detection methods with high sensitivity and accessibility is important. Because MSI is mainly caused by defects in DNA mismatch repair (MMR), immunohistochemical (IHC) staining for the MMR proteins has been widely employed to predict the responses to immunotherapies. Thus, due to the high sensitivity of PCR, the MSI-PCR analysis has also been recommended as the primary approach as MMR IHC. This study aimed to develop a sensitive and convenient platform for daily MSI-PCR services. The routine workflow used a non-labeling QIAxcel capillary electrophoresis system which did not need the fluorescence labeling of the DNA products or usage of a multi-color fluorescence reader. Furthermore, the 15 and 1000 bp size alignment markers were used to precisely detect the size of the DNA product. A cohort of 336 CRC cases was examined by MSI-PCR on the five mononucleotide MSI markers recommended by ESMO. The PCR products were analyzed in the screening gels, followed by high-resolution gel electrophoresis for confirmation if needed. In the MSI-PCR tests, 90.1% (303/336) cases showed clear major shift patterns in the screening gels, and only 33 cases had to be re-examined using the highresolution gels. The cohort was also analyzed by MMR IHC is, which revealed 98.5% (331/ 336) concordance with MSI-PCR. In the five discordant cases, 4 (3 MSI-L and 1 MSS) showed MSH6 loss. Besides, one case exhibited MSI-H but no loss in the MMR IHC. Further NGS analysis, in this case, found that missense and frameshift mutations in the PMS2 and MSH6 genes occurred, respectively. In conclusion, the non-labeling MSI-PCR capillary electrophoresis revealed high con cordance with the MMR IHC analysis and is cost- and timeeffective. Therefore, it shall be highly applicable in clinical laboratories.