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MMR results are inconsistent with MSI status, what are the reasons?
MMR results are inconsistent with MSI status, what are the reasons?
I. What is MMR?
Various errors occur in the replication of DNA due to external or internal factors, such as single/double-strand breaks, base mismatches, cross-linking damage, etc. Fortunately, the human body has a self-repair and regulatory system for DNA damage, which allows the normal functioning of the organism to be maintained. The mismatch repair system (MMR) can repair base mismatch errors that occur during DNA replication, thereby maintaining genomic stability.
Members of the mismatch repair system family include proteins such as MLH1, MSH2, MSH6, and PMS2. The mismatch repair protein family consists of two complexes with mismatch repair functions, with MLH1 and MLH2 being the main functional proteins within these complexes.
Mismatch repair system deficiency (dMMR, deficient mismatch repair) is due to abnormalities in the function of these proteins. The expression of the above four MMR proteins can usually be detected by immunohistochemistry (IHC). The absence of any one MMR protein indicates dMMR, while normal expression of all four proteins indicates pMMR.
II. What is MSI?
Microsatellites (MS) are short and repetitive DNA sequences, commonly composed of double-base CA/GA/GT or single-base A/T tandem repeats, usually with 1-6 base pairs as a repeating unit.
Earlier, we also mentioned that DNA can undergo some errors during the replication process. Considering that microsatellites, these tandem repeat sequences, make DNA more prone to "fatigue" during self-replication, the likelihood of errors occurring is even greater.
If, at this point, the mismatch repair system malfunctions as well, the replication errors introduced by microsatellites cannot be promptly corrected and continue to accumulate. This leads to changes in the length or base composition of microsatellite sequences. This phenomenon is referred to as microsatellite instability (MSI).
▲Microsatellite stability and microsatellite instability
III. How to Detect and Evaluate MSI (Microsatellite Instability) in MSI Status Assessment?
01. Multiplex Fluorescent PCR Capillary Electrophoresis Method
This method is currently recognized as the "gold standard" for MSI detection. It typically involves testing MSI sites in both normal and tumor tissues, and then determining the MSI status of the tumor sample by comparing the number of microsatellite instability points. Domestic expert consensus recommends the use of a combination of 5 to 7 microsatellite sites (BAT-25, BAT-26, NR-21, NR-22, NR-24, NR-27, MONO-27) for MSI detection. The interpretation of MSI results generally follows: 2 or more markers showing instability indicate MSI-H; 1 marker showing instability indicates MSI-L; and all sites stable indicate MSS.
02. High-Throughput Sequencing (NGS) Method
With the breakthrough development of high-throughput sequencing technology and the increasing richness of clinical applications, NGS has its unique advantages in detecting MSI status. It can simultaneously assess a larger number of microsatellite sequences and provide additional valuable information from other tests.
IV. Comparative Analysis of Various Methodologies for MSI Status Evaluation
▲Chinese Expert Consensus on Microsatellite Instability Detection in Colorectal Cancer and Other Related Solid Tumors
V. dMMR does not equal MSI-HThe relationship between MSI and MMR is close; to some extent, dMMR ≈ MSI-H, and pMMR ≈ MSS + MSI-L. However, due to differences in the detection subjects and detection technology platforms for MMR and MSI, there is an issue of incomplete conformity.
VI. Inconsistencies between MMR Protein Detection and MSI Status and Corresponding Reasons
01
dMMR, but MSS
MMR protein loss (MSH6 protein), but its function is compensated by (MSH3 protein), and it can still repair mismatched genes. However, since MSH3 protein is not included in IHC detection, the result remains MSS.
02
pMMR, but MSI-H
MMR gene undergoes missense mutation, and the MMR protein function may be abnormal, but its antigen structure may not be affected. In this case, IHC detects that MMR protein is expressed normally, i.e., pMMR, but its mismatch repair function is abnormal, leading to MSI-H detection results.
MSI-H caused by mutations in genes other than MMR genes (such as POLE/D).
VII. How to Handle Inconsistencies between MMR Protein Detection and MSI Status?
The "Chinese Expert Consensus on Microsatellite Instability Detection in Colorectal Cancer and Other Related Solid Tumors" suggests that if a patient undergoes both IHC-MMR protein expression and DNA analysis (PCR or NGS) for MSI status testing, and the results are inconsistent, the third method (NGS or PCR) can be considered for verification (recommended by some experts).
SpaceGen's MSI product can simultaneously test 34 MSI loci with a high fault-tolerant rate, providing strong support for the evaluation of immunotherapy efficacy in common solid tumors.
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