For metastatic colorectal cancer, is it possible for the tumor to be mismatch repair proficient at primary site but mismatch repair deficient at metastatic site?

This is a very interesting question, and most of the time we think the MSI/MMR status between the primary site and the metastatic lesion is consistent. However, some studies suggest there could be some inconsistent results in colorectal cancer patient.

In a small study with 40 metastatic colorectal cancer patients, 30 cases were found to have both pMMR in the primary and the metastatic sites. Four cases were both dMMR in primary and metastatic tumors. There were six cases found to have pMMR in primary tumor while dMMR in metastatic tumor (Wang et al., Biosci Rep 2019).

Another study compared the MMR status between primary and matched metastatic site in colorectal cancer patients. This study included 369 metastatic colorectal cancer patients;...