MSI(Panel) - CB-Gene

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MSI(Panel) Testing Services

Definition

Microsatellite instability (MSI) refers to a phenomenon in which microsatellite lengths change during cell division due to defects in the DNA mismatch repair (MMR) system. Microsatellites are short tandem repeats (typically 1-6 base pairs) that are ubiquitous throughout the genome. Normally, errors during DNA replication are promptly corrected by the MMR system. However, when MMR genes (such as MLH1, MSH2, MSH6, and PMS2) are mutated or silenced, errors accumulate, leading to variations in microsatellite lengths and the MSI phenotype.

MSI was first discovered in Lynch syndrome (hereditary nonpolyposis colorectal cancer) and is now known to occur in a variety of tumor types, including endometrial, gastric, and pancreatic cancers. Based on the number of unstable sites, MSI can be categorized as highly unstable (MSI-H), poorly unstable (MSI-L), and stable (MSS). The MSI-H phenotype is often associated with a better response to immunotherapy.

Principles of NGS-Based MSI Detection

Traditional methods for MSI detection include immunohistochemistry (IHC) detection of MMR protein expression and PCR amplification fragment length analysis. However, next-generation sequencing (NGS)-based methods have emerged as an important alternative in recent years.
The core principle is to simultaneously assess a large number of microsatellite loci through high-throughput sequencing and compare the differences in allele repeat unit numbers between tumor and normal tissues through bioinformatics analysis. NGS panels typically include dozens or even hundreds of microsatellite loci. By calculating the proportion of unstable loci, a sample is classified as MSI-H, MSI-L, or MSS.

Compared to traditional PCR methods that only detect a few landmark loci, NGS covers a wider range of loci, significantly improving detection sensitivity and specificity. Furthermore, NGS can simultaneously detect the mutation status and methylation levels of MMR genes, providing a more comprehensive molecular understanding of the mechanisms of MSI.

NGS-MSI Detection Workflow

Sample Collection and DNA Extraction

DNA is extracted from paraffin-embedded (FFPE) sections of the patient's tumor tissue or fresh tissue, and a matched normal tissue (or blood sample) is collected as a control.

Library Construction and Sequencing

A gene panel containing microsatellite loci is used to capture target regions, construct sequencing libraries, and sequence them on a high-throughput sequencing platform.

Bioinformatics Analysis

* Sequence Alignment: Sequencing reads are aligned to a reference genome.

* Microsatellite Analysis: Identify target microsatellite loci and count their repeat units.

* Instability Site Counting: Comparing tumor and normal samples to determine the number of sites with length variations.

Result Interpretation and Reporting: A clinical test report is issued based on pre-defined thresholds (e.g., ≥30% instability is considered MSI-H), and may incorporate information such as MMR gene variants and tumor mutational burden (TMB).

Advantages

High Sensitivity and Specificity

Simultaneous detection of multiple loci significantly reduces false-negative and false-positive rates.

Multi-Gene Parallel Analysis

A single test can provide information on MSI status, MMR gene mutations, TMB, and other features, improving detection efficiency.

Compatibility with Various Sample Types

Compatible with FFPE samples, facilitating retrospective studies and routine clinical applications.

Automation and Standardization

Bioinformatics pipelines enable automated analysis, reducing human error and facilitating standardized results and cross-platform comparisons.

Cost-Effectiveness

As the cost of NGS decreases, its overall cost-effectiveness has gradually surpassed that of traditional testing methods.

Application Scenarios

NGS-MSI testing has a wide range of applications in clinical practice and research:

Lynch syndrome screening: MSI-H is a key hallmark of Lynch syndrome, and NGS testing can be used to assess genetic risk for patients and their families.

Immunotherapy Guidance: MSI-H tumors have a high mutational burden and numerous neoantigens, and are therefore well-responsive to PD-1/PD-L1 inhibitors. The FDA has approved pembrolizumab for the treatment of all MSI-H solid tumors.

Prognosis: In colorectal cancer, patients with MSI-H generally have a better prognosis, but may have lower sensitivity to certain chemotherapy drugs (such as 5-FU).

Scientific Research: NGS-MSI facilitates in-depth exploration of the role of MSI in tumorigenesis and progression, as well as its interaction with the immune microenvironment.

NGS technology provides a more comprehensive, accurate, and efficient solution for MSI testing, driving the development of tumor molecular profiling and the application of personalized treatment. With the further optimization of sequencing technology and bioinformatics tools, NGS-MSI testing is expected to become a routine component of clinical tumor diagnosis, bringing the benefits of precision treatment to more patients.

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