Microsatellite instability cancer – Diagnostics – Overview of Information and Clinical Research

Microsatellite instability cancer diagnostics involves specialized testing to detect changes in the DNA of tumor cells. These tests help doctors understand the genetic features of a cancer, guide treatment decisions, and identify patients who may benefit from certain therapies. Understanding when and how this testing is performed can empower patients to participate more fully in their healthcare journey.

Introduction: Who Should Undergo Diagnostics

Not everyone with cancer needs to be tested for microsatellite instability, but specific groups of patients are more likely to benefit from this diagnostic evaluation. Testing is particularly important for people diagnosed with colorectal cancer, as approximately 15 to 20 percent of these tumors show high microsatellite instability.^[3] This percentage is significant enough that many medical guidelines now recommend testing for all colorectal cancer patients, regardless of their age or family history.

Beyond colorectal cancer, microsatellite instability testing is also advisable for patients with endometrial cancer, gastric cancer, and certain other gastrointestinal cancers. Around one in three endometrial cancers and one in seven colorectal, stomach, and ovarian cancers show high microsatellite instability.^[14] The presence of this genetic feature can significantly influence which treatments work best, making early identification crucial for optimal care.

People who have a family history of cancer, especially if multiple relatives have been diagnosed with colorectal or endometrial cancer at young ages, should discuss microsatellite instability testing with their doctor. This pattern may suggest Lynch syndrome, an inherited condition that increases cancer risk and is strongly associated with microsatellite instability.^[8] In such cases, testing serves a dual purpose: it helps guide immediate treatment decisions and can reveal whether other family members might benefit from genetic counseling.

Patients whose cancer has spread or metastasized may also be candidates for microsatellite instability testing, particularly if standard treatments have not been effective. The reason is that tumors with high microsatellite instability often respond well to a type of treatment called immunotherapy, which harnesses the body’s immune system to fight cancer.^[9] Knowing the microsatellite status before starting treatment allows doctors to select therapies with the highest chance of success.

⚠️ Important

Microsatellite instability testing should ideally be performed on tumor tissue obtained during biopsy or surgery, not on normal tissue. The test looks specifically for changes that occur in cancer cells, so having adequate tumor material is essential for accurate results. If you’re planning surgery or a biopsy, ask your doctor whether the tissue sample will be tested for microsatellite instability.

Classic Diagnostic Methods

There are two main approaches to testing for microsatellite instability, each with its own strengths and applications. The first method looks directly at the DNA repair machinery, while the second examines the consequences when that machinery fails. Understanding both methods helps explain why doctors might choose one over the other in different situations.

Immunohistochemistry, often abbreviated as IHC, is one of the most commonly used techniques for detecting problems with DNA repair. This test examines tumor tissue under a microscope after treating it with special dyes that attach to specific proteins involved in DNA repair. The human body normally uses a system called mismatch repair, or MMR, to fix errors that occur when cells copy their DNA.^[3] The mismatch repair system includes several key proteins with technical names like MLH1, MSH2, MSH6, and PMS2. When immunohistochemistry testing is performed, pathologists look to see whether these proteins are present in the tumor cells.

If one or more of these mismatch repair proteins are missing from the tumor tissue, the test result is described as deficient mismatch repair, or dMMR. This finding strongly suggests that the tumor has microsatellite instability. The beauty of immunohistochemistry is that it can be performed relatively quickly on tissue that has already been collected during routine biopsy or surgery. Most pathology laboratories are equipped to run these tests, making them widely accessible.^[7]

The second major testing approach is called polymerase chain reaction, or PCR-based microsatellite instability testing. Instead of looking at repair proteins, this method directly examines the microsatellites themselves. Microsatellites are short, repetitive sequences of DNA scattered throughout the genome, typically consisting of one to six DNA building blocks repeated many times.^[7] These areas are particularly prone to errors when the mismatch repair system isn’t working properly. Think of microsatellites like a stuttering pattern in the genetic code—when the repair system fails, these stutters can lengthen or shorten in tumor cells compared to normal cells from the same person.

PCR-based testing compares the length of specific microsatellite sequences between tumor tissue and normal tissue from the same patient. Laboratories typically examine a standard panel of five microsatellite markers. Based on how many of these markers show changes, tumors are classified into categories. If two or more of the five markers show instability, the tumor is designated as microsatellite instability-high, or MSI-H. If only one marker shows changes, it’s called microsatellite instability-low, or MSI-L. If none show changes, the tumor is microsatellite stable, or MSS.^[3] For treatment purposes, MSI-L and MSS tumors are generally grouped together because they behave similarly.

More recently, next-generation sequencing, or NGS, has emerged as a third option for detecting microsatellite instability. This advanced technology can read millions of DNA sequences simultaneously, providing a comprehensive view of genetic changes in a tumor. Next-generation sequencing can detect microsatellite instability while also identifying other genetic alterations that might be relevant for treatment decisions.^[7] Because it provides so much information in a single test, NGS is becoming increasingly popular, especially in larger cancer centers. However, it typically requires more sophisticated equipment and expertise than the other methods.

Each testing method has advantages and limitations. Immunohistochemistry is relatively inexpensive and quick, making it a good first-line test. It can also provide clues about which specific repair gene might be affected, which can be helpful if Lynch syndrome is suspected. However, in rare cases, mismatch repair proteins might be present but not functioning properly, leading to false results. PCR-based testing directly measures the genetic instability, providing strong evidence of mismatch repair problems. It works well even with small tissue samples. Next-generation sequencing offers the most comprehensive genetic information but is more expensive and time-consuming.^[7]

In practice, many laboratories use immunohistochemistry as the initial screening test because of its convenience and cost-effectiveness. If the results are unclear or if additional genetic information is needed, PCR-based testing or next-generation sequencing might follow. Some guidelines recommend using both immunohistochemistry and PCR-based testing together to maximize accuracy.^[20] The choice of method often depends on the laboratory’s capabilities, the amount of tissue available, and what other genetic information might be needed for treatment planning.

Diagnostics for Clinical Trial Qualification

When patients with microsatellite instability-high cancers are being considered for clinical trials, the diagnostic requirements become more specific and standardized. Clinical trials test new treatments or new combinations of existing treatments, and they require consistent, reliable methods of identifying which patients are eligible to participate. The way microsatellite instability is tested for trial enrollment has evolved as our understanding of the condition has grown.

Most clinical trials that focus on immunotherapy treatments for cancers with mismatch repair problems require confirmation of microsatellite instability status using validated testing methods. The most widely accepted approaches are immunohistochemistry for mismatch repair proteins and PCR-based microsatellite instability testing using standardized marker panels.^[9] Trial protocols typically specify which method must be used and where the testing must be performed, as consistency across all trial participants is essential for interpreting results.

For immunohistochemistry-based enrollment, trials usually require testing for all four major mismatch repair proteins: MLH1, MSH2, MSH6, and PMS2. A tumor is considered deficient in mismatch repair if any one of these proteins is absent from the tumor cell nuclei while remaining present in surrounding normal tissue, which serves as an internal control.^[8] The test must be performed by a laboratory that is certified to meet specific quality standards, ensuring that results are reliable and reproducible.

When PCR-based testing is used for trial qualification, the analysis must include examination of at least five standard microsatellite markers. These markers have been carefully selected because they are particularly sensitive to mismatch repair deficiency. The test compares DNA extracted from tumor tissue with DNA from the patient’s normal tissue, typically obtained from blood or non-cancerous tissue from the same biopsy specimen.^[3] A tumor showing instability at two or more of the five markers meets the criteria for microsatellite instability-high status and trial eligibility.

Some clinical trials now accept next-generation sequencing results as proof of microsatellite instability status. In these cases, the sequencing platform must have been validated specifically for detecting microsatellite instability, and the analysis must examine a sufficient number of microsatellite loci across the genome. Different sequencing platforms use different computational methods to assess microsatellite instability, so trial protocols often specify which platforms or algorithms are acceptable.^[7] This level of detail ensures that all patients enrolled in the trial truly have the molecular features the treatment is designed to target.

The tissue sample used for testing must meet certain quality standards for trial enrollment. The tumor content—the percentage of cancer cells in the sample—typically needs to be high enough to ensure accurate results, often at least 20 percent tumor cells. The tissue must also be properly preserved, usually as formalin-fixed, paraffin-embedded blocks, which is the standard method for storing biopsy and surgical specimens.^[7] Fresh tissue is sometimes preferred for certain advanced testing methods, but this is less common because it requires special handling and immediate processing.

⚠️ Important

If you’re interested in participating in a clinical trial, don’t assume that microsatellite instability testing you’ve already had will automatically qualify you. Different trials may have different testing requirements. Your doctor will need to verify that your previous test meets the specific trial’s criteria, or you may need to undergo additional testing at a designated laboratory. Keep all your pathology reports and testing documentation, as these will be essential for the enrollment process.

Results from nodal tissue—cancer that has spread to lymph nodes—or metastatic tissue can be used for microsatellite instability testing in clinical trials, not just the primary tumor. This flexibility is important because microsatellite instability status is generally consistent throughout a patient’s disease, whether testing the original tumor or areas where cancer has spread.^[8] This means that even if the primary tumor is no longer available for testing, samples from other cancer sites can provide the needed information for trial qualification.

The timeline for obtaining test results is another practical consideration for clinical trial enrollment. Immunohistochemistry results typically become available within a few days to a week after the tissue sample reaches the laboratory. PCR-based testing may take slightly longer, often one to two weeks. Next-generation sequencing generally requires two to three weeks or sometimes longer, depending on the complexity of the analysis and the laboratory’s workflow.^[7] Patients and doctors need to factor in these timeframes when considering trial participation, especially if treatment needs to begin quickly.

Some clinical trials are investigating whether testing additional markers, such as tumor mutational burden (TMB), can help predict which patients with microsatellite instability will respond best to treatment. Tumor mutational burden refers to the total number of mutations present in tumor DNA. Tumors with high microsatellite instability tend to have high tumor mutational burden because the faulty DNA repair system allows many errors to accumulate.^[9] When both features are present, the cancer may be particularly vulnerable to immunotherapy. However, tumor mutational burden testing is not yet a standard requirement for most microsatellite instability-focused trials.

Prognosis and Survival Rate

Prognosis

The outlook for patients with microsatellite instability-high cancer varies depending on the type of cancer and the stage at which it is detected. Interestingly, the prognosis often differs from cancers without this feature, and in many cases, it can be more favorable. For colorectal cancer specifically, patients with high microsatellite instability tend to have better outcomes after surgery compared to those with microsatellite stable tumors. This is partly because microsatellite instability-high tumors are often less likely to spread to distant organs.^[8]

Several factors influence the prognosis for patients with microsatellite instability cancer. The stage at diagnosis remains critically important—earlier-stage cancers generally have better outcomes regardless of microsatellite status. The specific location of the tumor within the colon or rectum can also matter, as can the patient’s overall health and age. Additionally, whether the microsatellite instability is related to Lynch syndrome or occurs sporadically can affect long-term outcomes, as Lynch syndrome patients face ongoing risk of developing new cancers in other organs.

The response to treatment is a major factor in determining prognosis. Microsatellite instability-high tumors have shown remarkable responses to immunotherapy drugs, particularly checkpoint inhibitors that help the immune system recognize and attack cancer cells. Patients whose tumors respond well to immunotherapy often experience prolonged disease control.^[9] However, the response to traditional chemotherapy is more complex, with some studies suggesting that certain chemotherapy regimens may be less effective in microsatellite instability-high colorectal cancers, particularly when used without additional agents.

Survival Rate

Specific survival statistics for microsatellite instability-high cancers depend heavily on the cancer type and stage. In colorectal cancer, approximately 15 to 20 percent of tumors show high microsatellite instability.^[3] When detected at early stages and treated appropriately, these tumors often have favorable survival rates. Patients with stage II microsatellite instability-high colorectal cancer have been reported to have particularly good outcomes with surgery alone, sometimes with five-year survival rates exceeding those of microsatellite stable tumors at the same stage.

For advanced or metastatic disease, the introduction of immunotherapy has significantly changed survival prospects for patients with microsatellite instability-high tumors. Clinical trials have demonstrated that substantial percentages of patients with advanced microsatellite instability-high cancers experience disease control lasting for extended periods when treated with checkpoint inhibitor immunotherapy.^[9] The exact survival figures continue to evolve as more patients receive these newer treatments and longer follow-up data becomes available.

It’s important to understand that survival statistics represent averages from large groups of patients and may not accurately predict any individual’s outcome. Many factors beyond microsatellite instability status influence survival, including the specific characteristics of each person’s cancer, their overall health, how well they tolerate treatment, and the expertise of their medical team. Advances in treatment continue to improve outcomes, meaning that statistics based on patients treated several years ago may not reflect the current situation for newly diagnosed patients.

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