Microsatellites are short, repetitive segments of DNA found throughout an organism’s genome. They consist of 1 to 6 base pairs repeated multiple times. The term “instability” refers to changes in the length of these repetitive DNA sequences, indicating errors in the DNA copying process.
The Science of Microsatellite Instability
Microsatellites are particularly susceptible to errors during DNA replication. During this copying, DNA polymerase, the enzyme responsible for building new DNA strands, can sometimes slip. This slippage can lead to the accidental insertion or deletion of repeat units within the microsatellite sequences.
While these errors occur frequently, the body possesses a corrective mechanism known as the DNA mismatch repair (MMR) system. The MMR system acts like a cellular proofreader, identifying and correcting mistakes that arise during DNA replication by recognizing mispaired bases and small insertions or deletions, excising the incorrect section, and inserting the correct sequence.
When the MMR system functions properly, it maintains the stability of the genome, including the microsatellite regions. Microsatellite instability (MSI) occurs when the MMR system is impaired, meaning that replication errors, especially those in microsatellites, are not corrected and accumulate. Several genes are integral to the MMR system, including MLH1, MSH2, MSH6, and PMS2. When these genes are mutated or silenced, the MMR system fails, leading to the characteristic length changes observed in microsatellites.
MSI and Cancer Development
Microsatellite instability, resulting from a failed DNA mismatch repair system, directly connects to certain cancers. When the MMR system fails, mutations accumulate not only in microsatellites but also in other genes. These additional mutations can affect genes that control cell growth, division, and death, thereby contributing to the uncontrolled cell proliferation characteristic of cancer.
High microsatellite instability (MSI-H) is a characteristic of some tumors. MSI-H tumors often arise from a deficient MMR system, due to inherited genetic mutations (e.g., Lynch Syndrome). Lynch Syndrome, also known as hereditary non-polyposis colorectal cancer (HNPCC), is a common inherited cancer predisposition. MSI-H is also observed in a significant portion of sporadic colorectal cancers, typically due to the silencing of the MLH1 gene.
Beyond colorectal cancer, MSI-H is found in various other tumor types, including endometrial, gastric, and ovarian cancers. MSI-H is associated with tumor characteristics like poor differentiation and increased tumor-infiltrating lymphocytes. This profile indicates a unique cancer development pathway compared to tumors without MSI.
Testing for Microsatellite Instability
Detecting microsatellite instability in tumor samples helps understand a cancer’s characteristics. Two laboratory methods are commonly used for this purpose: Polymerase Chain Reaction (PCR)-based testing and Immunohistochemistry (IHC). These methods provide different but complementary information about the tumor’s MMR status.
PCR-based testing directly analyzes the DNA to identify changes in microsatellite length. DNA is extracted from tumor and normal tissue for comparison. Fluorescently labeled primers are used to amplify specific microsatellite regions, and changes in the size of these amplified fragments indicate instability. Results are categorized as MSI-High (MSI-H) if two or more markers show instability, MSI-Low (MSI-L) if one marker is unstable, or Microsatellite Stable (MSS) if no markers show instability.
Immunohistochemistry (IHC) assesses the presence or absence of the MMR proteins (MLH1, MSH2, MSH6, PMS2) within tumor cells. Antibodies stain for these proteins; loss of expression suggests a deficient MMR system. While IHC is often used as an initial screening method, PCR-based testing functionally measures the DNA changes caused by MMR deficiency. Combining both methods can enhance the accuracy of characterization.
MSI and Cancer Treatment Decisions
Identifying microsatellite instability in a tumor guides cancer treatment. Tumors with MSI-High status often possess a higher number of genetic mutations compared to microsatellite stable tumors. This elevated mutational burden can lead to the production of abnormal proteins, known as neoantigens, which the immune system may recognize as foreign.
Because of these neoantigens, MSI-H tumors are more likely to be responsive to a type of treatment called immunotherapy, specifically immune checkpoint inhibitors. These medications work by unleashing the body’s own immune system to better identify and attack cancer cells. The U.S. Food and Drug Administration has approved certain immune checkpoint inhibitors, such as pembrolizumab, for the treatment of MSI-H or mismatch repair deficient solid tumors across various cancer types.
Conversely, MSI-H tumors have been observed to respond poorly to conventional chemotherapy regimens, particularly those based on 5-fluorouracil. Therefore, MSI testing plays a role in personalized cancer medicine, helping oncologists make informed decisions about therapeutic strategies for individual patients. This allows for a tailored approach, potentially leading to better outcomes for patients with MSI-H cancers.