Genome-wide methylation profiling is transforming the way clinicians approach brain tumor diagnosis. By analyzing epigenetic patterns across the genome, this advanced testing method provides a new level of precision, helping resolve ambiguous cases, reduce diagnostic variability, and support more confident clinical decision-making in complex CNS tumors.
Accurately diagnosing brain tumors has long been one of the most complex challenges in pathology. With more than 80 recognized central nervous system CNS tumor types, many of which share overlapping features, traditional diagnostic approaches can leave uncertainty, particularly in rare or ambiguous cases.
Genome-wide methylation profiling represents a significant step forward. By analyzing epigenetic patterns across the tumor genome, this approach provides a new layer of insight that can help refine tumor classification, reduce diagnostic variability, and support more confident clinical decision-making.
A persistent diagnostic challenge
Brain tumors encompass a wide spectrum of diseases, and even experienced pathologists may encounter only a limited number of rare tumor types throughout their careers. As a result, distinguishing between tumor subtypes, or even reaching a definitive diagnosis, can be difficult using conventional methods alone.
“These tumors can be challenging,” explains Kenneth Aldape, M.D., neuropathologist at Mayo Clinic Laboratories. “There are more than 80 types of CNS tumors, and many are uncommon. An individual pathologist might see only a few examples over an entire career, so it can be difficult to keep them all straight.”
This complexity can have meaningful implications for patient care. When diagnoses are uncertain or inconsistent, treatment decisions may be affected.
“When diagnostic errors are made, there can be clinical consequences,” Dr. Aldape continues. “Patients may be overtreated or undertreated depending on the diagnosis.”
Adding a new layer of molecular insight
Genome-wide methylation profiling offers a different way to approach this problem. Rather than focusing on individual mutations or chromosomal changes, the test evaluates patterns of DNA methylation across hundreds of thousands of sites in the genome. These patterns function as a kind of molecular “fingerprint” that reflects both the tumor’s origin and its biological behavior.
“DNA methylation is an epigenetic modification that acts on top of the genetic code,” explains Cris Ida, M.D., molecular pathologist and neuropathologist at Mayo Clinic Laboratories. “These patterns are stable and generate a unique signature for each tumor, which we can use as a molecular diagnostic biomarker.”
This approach has become increasingly important in recent years. Within the World Health Organization (WHO) classification of CNS tumors, methylation profiling is now recognized as a key molecular biomarker and, in some cases, essential for defining tumor entities. Ongoing guidance from expert groups, including cIMPACT-NOW featuring Dr. Aldape, has further reinforced its role in improving diagnostic accuracy, particularly in challenging cases.
How the test works
The process begins with DNA extracted from tumor tissue. After preparation, the sample undergoes genome-wide methylation analysis, generating a comprehensive profile of methylation patterns.
That profile is then evaluated using a machine learning-based classifier trained on thousands of reference tumor samples.
“The classifier compares the sample to a large dataset of known tumor types and determines which one it most closely matches,” says Dr. Aldape. “It also provides a confidence score that tells you how strong that match is.”
The result is a predicted tumor classification grounded in large-scale molecular data, helping to reduce subjectivity and variability.
To ensure clinical reliability, Mayo Clinic Laboratories uses a conservative reporting approach called Nearest Neighbor Analysis. This is a Mayo-developed validation method that complements the NIH classifier by comparing each tumor to its closest molecular matches, supporting greater confidence in the final diagnosis.
“Methylation profiling has identified new tumor subgroups, refined existing classifications, and even uncovered entirely new tumor types.”
Cris Ida, M.D.
Strengthening diagnostic accuracy
One of the most significant advantages of genome-wide methylation profiling is its ability to clarify difficult or ambiguous cases.
Traditional diagnostic methods, including histology, immunohistochemistry, and targeted molecular testing, can sometimes produce inconclusive or conflicting results. Methylation profiling adds a complementary layer of information, helping to resolve those uncertainties.
“In some cases, you can perform multiple molecular tests and still not arrive at a definitive diagnosis,” says Dr. Ida. “Methylation profiling can provide that final piece of information, allowing for a more complete and accurate integrated diagnosis.”
It can also reveal distinctions that are not visible through conventional approaches.
“Methylation profiling has identified new tumor subgroups, refined existing classifications, and even uncovered entirely new tumor types,” Dr. Ida adds.
For clinicians and patients, that level of precision is critical.
“Everything starts with the diagnosis,” says Dr. Ida. “When we are more precise, we better understand the disease process and can support more informed treatment decisions.”
“The number one challenge in molecular diagnostics is time, when you have all options available at once, you can shorten the time to diagnosis, which is especially important for aggressive tumors.”
Kenneth Aldape, M.D.
Part of a comprehensive testing strategy
Importantly, genome-wide methylation profiling is not intended to replace existing diagnostic tools. Instead, it complements them, working alongside techniques such as DNA & RNA next-generation sequencing and chromosomal microarray analysis.
Each method provides different types of information, and in many cases, they are most effective when used together.
“There are certain tumors where you don’t know ahead of time which platform will provide the answer,” says Dr. Aldape. “It may come from sequencing, it may come from chromosomal analysis, or it may come from methylation.”
To address this uncertainty, and reduce delays in diagnosis, clinicians may choose to order multiple tests concurrently.
“The number one challenge in molecular diagnostics is time,” Dr. Aldape explains. “When you have all options available at once, you can shorten the time to diagnosis, which is especially important for aggressive tumors.”
Complete biomarker CNS tumor testing
The DNA methylation profile has been recognized as one of four essential biomarkers for integrated histomolecular diagnosis of CNS tumors by the World Health Organization, and methylation profiling has been recommended as an ancillary tool to assist in diagnosis for tumor types that can be substratified by methylation, and for diagnostically difficult cases. Mayo Clinic Laboratories is currently the only large commercial laboratory in the U.S. that offers all four types of molecular biomarker tumor testing for CNS tumors, which, in addition to DNA methylation profile (Mayo ID: MTNON), includes mutations, DNA and fusions, and copy number variants.
Bringing research into clinical practice
The development of genome-wide methylation testing reflects years of research and collaboration, including foundational work at the National Cancer Institute (NCI).
“We began by studying the methylation signatures of different tumor types and found that these patterns correlated strongly with specific diagnoses,” says Dr. Aldape, who helped develop the underlying classifier. “From there, we were able to build a system that could classify new cases based on those signatures.”
Bringing this approach into the clinical laboratory required extensive validation and careful integration into existing workflows, ensuring that the test meets the standards required for patient care.
At Mayo Clinic Laboratories, this work also fills an important gap.
“Methylation profiling completes the set of molecular biomarkers included in the current WHO classification,” Dr. Ida notes. “It allows us to offer a more comprehensive approach to CNS tumor testing.”
Looking ahead
While the current focus is on CNS tumors, the underlying approach has broader potential.
Researchers are already exploring methylation-based classification in other tumor types, with the goal of extending this method across oncology.
“We’re hoping to expand this approach to other cancers, such as sarcomas and kidney tumors,” says Dr. Aldape. “Ultimately, you could imagine having classifiers for many different organ systems.”
For now, genome-wide methylation profiling represents a meaningful advancement in neuro-oncology, one that brings greater clarity to some of the most complex diagnostic challenges in medicine.
“At the end of the day, what clinicians want is an answer,” Dr. Aldape says. “This test helps provide that answer with greater confidence, while also generating data that can advance our understanding of these diseases in the future.”
To learn more, listen to the Diagnostics in Practice episode featuring Mayo Clinic experts discussing this test.
