Microsatellite instability-high (MSI-H)

What it is

Microsatellite instability-high (MSI-H) is a tumour biomarker showing defective DNA mismatch repair; about 15% of colorectal cancers are MSI-H. Microsatellites are short, repeated DNA sequences scattered through the genome. When the mismatch repair (MMR) system is not working properly, errors build up in these repeated regions, and the tumour is classified as MSI-H if instability is found at a high level.

MSI-H is not a disease by itself. It is a laboratory finding in a cancer sample that can help with diagnosis, prognosis, inherited cancer risk assessment, and treatment planning. MSI-H is closely related to deficient mismatch repair, often written as dMMR. In practice, many clinicians use MSI-H and dMMR together because they usually point to the same biology, although they are measured by different methods.

MSI-H is especially important in colorectal and endometrial cancers, but it can also be found in stomach, small bowel, pancreatic, prostate, and other solid tumours. In some patients, MSI-H raises concern for Lynch syndrome, an inherited condition caused by germline mutations in MMR genes such as MLH1, MSH2, MSH6, and PMS2.

A simple way to think about it is:

Term	What it means
MSI-H	High level of instability in microsatellite DNA regions
dMMR	Mismatch repair proteins are absent or not functioning properly
MSS	Microsatellite stable, meaning no significant instability detected
MSI-L	Low microsatellite instability; less clinically useful than MSI-H

How it works

The normal MMR system acts as a proofreading mechanism during DNA replication. It corrects small insertion and deletion errors, especially in repetitive DNA sequences like microsatellites. If this repair system fails, the tumour accumulates many mutations.

This can happen for two main reasons:

1. Inherited cause: a person is born with a harmful mutation in an MMR gene, as in Lynch syndrome.
2. Sporadic cause: the tumour acquires changes that switch off an MMR gene, most commonly MLH1 promoter hypermethylation in colorectal cancer.

Because MSI-H tumours often carry many mutations, they may produce more abnormal proteins, called neoantigens, that the immune system can recognise. This is one reason MSI-H status has become important in oncology: some MSI-H or dMMR cancers are more likely to respond to immune checkpoint inhibitors than tumours that are microsatellite stable.

MSI-H can also influence how a cancer behaves. In colorectal cancer, MSI-H tumours often have distinct pathological features and may have a different prognosis from microsatellite-stable tumours, especially in early-stage disease.

Evidence and uses

MSI-H is best established in colorectal cancer, where it has diagnostic, prognostic, and predictive value. Reviews of the evidence consistently report that roughly 15% of colorectal cancers are MSI-H. A smaller proportion of metastatic colorectal cancers are MSI-H compared with early-stage disease.

Common clinical uses include:

1. Screening for Lynch syndrome: Universal tumour testing for MMR deficiency or MSI is recommended in many colorectal and endometrial cancers because it can identify patients who need genetic counseling and germline testing.
2. Helping classify the tumour: MSI-H supports the diagnosis of a mismatch repair-deficient cancer subtype.
3. Prognostic information: In stage II colorectal cancer, MSI-H is often associated with a better overall prognosis than microsatellite-stable disease.
4. Treatment selection: MSI-H or dMMR status can help identify patients who may benefit from immunotherapy in certain settings.

For colorectal cancer, MSI-H status has long been linked to distinct outcomes and treatment patterns. Some studies suggest stage II MSI-H colon cancers may derive less benefit from fluorouracil-only adjuvant chemotherapy than microsatellite-stable tumours, though treatment decisions depend on the full clinical picture and current oncology guidelines.

MSI-H is now also relevant across tumour types. Tissue-agnostic immunotherapy approvals have made MSI-H a biomarker that matters beyond the organ where the cancer started. That said, response rates still vary by cancer type, disease burden, prior treatment, and other tumour features.

In India, MSI/MMR testing is increasingly used at tertiary cancer centres, particularly for colorectal and endometrial cancers and when immunotherapy is being considered. Access can vary by city, hospital, and whether testing is done by immunohistochemistry, PCR, or next-generation sequencing.

Diagnosis / how it's measured

MSI-H is measured on tumour tissue, not by symptoms alone. The main testing methods are:

Test method	What it checks	Typical result
Immunohistochemistry (IHC)	Presence or absence of MMR proteins MLH1, MSH2, MSH6, PMS2	Loss of one or more proteins suggests dMMR
PCR-based MSI testing	Instability in selected microsatellite markers	Classified as MSI-H, MSI-L, or MSS
Next-generation sequencing (NGS)	Broader genomic analysis including MSI status	May report MSI-H and other actionable findings

IHC and PCR are complementary. IHC shows which repair proteins are missing, which can guide germline testing. PCR directly measures instability in microsatellite regions. NGS can provide MSI status along with other biomarkers, but availability and cost are more variable.

If MLH1/PMS2 loss is seen on IHC in colorectal or endometrial cancer, additional testing such as MLH1 promoter methylation or BRAF mutation analysis may help distinguish sporadic tumours from Lynch syndrome-related cancers. A positive MSI-H result does not automatically mean a person has Lynch syndrome, but it should prompt consideration of genetic counseling when appropriate.

When to see a clinician

Patients should discuss MSI/MMR testing with their oncologist if they have colorectal, endometrial, gastric, or another solid tumour where the result could affect treatment or inherited risk assessment. It is also worth asking about MSI/MMR status if a pathology report mentions loss of MLH1, MSH2, MSH6, or PMS2, or if there is a strong family history of colorectal, endometrial, ovarian, stomach, or related cancers.

See a genetics specialist or genetic counselor if tumour testing suggests possible Lynch syndrome, especially when cancer occurs at a younger age or in multiple relatives. Family members may also need risk assessment if an inherited MMR gene mutation is confirmed.

Limitations and open questions

MSI-H is a useful biomarker, but it is not perfect. Different test methods can occasionally give discordant results, and pre-analytic issues such as poor tissue quality can affect accuracy. MSI-H and dMMR overlap strongly, but they are not absolutely identical categories.

MSI-H also does not guarantee response to immunotherapy. Some MSI-H tumours do not respond, and some responses are not durable. Researchers are still studying why resistance happens and which additional biomarkers, such as tumour mutational burden or immune microenvironment features, might improve prediction.

Most of the strongest evidence comes from colorectal and endometrial cancer. For rarer tumour types, evidence is still developing. Access, cost, and laboratory standardisation remain practical issues in many settings, including parts of India.

Finally, MSI-H is a tumour characteristic, not a complete treatment plan. It should be interpreted together with the cancer type, stage, pathology, imaging, performance status, and family history.