Diffuse Midline Glioma, H3 K27-Altered: Understanding Your Pathology Report

by Jason Wasserman MD PhD FRCPC
April 24, 2026

Diffuse midline glioma, H3 K27-altered, is a type of brain tumor that develops from glial cells, the supporting cells of the central nervous system. It belongs to a larger group of tumors called diffuse gliomas. Diffuse gliomas are infiltrative, which means the tumor cells spread into the normal brain tissue around them and cannot be fully separated from it — unlike another group of gliomas called circumscribed gliomas (such as pilocytic astrocytoma), which have a clear border and can often be completely removed by surgery. For diffuse midline glioma, the infiltrative growth pattern has a major consequence: complete surgical removal is not possible, and treatment relies on a combination of radiation therapy, clinical trial participation, and, for a specific subgroup, targeted drug therapy.

As its name suggests, diffuse midline glioma arises in the midline structures of the central nervous system — most often the brainstem and pons (the bridge-like structure that connects the brainstem to the cerebellum), the thalamus (a deep central part of the brain that relays signals between different regions), or the spinal cord. When the tumor arises in the pons, it is often called a diffuse intrinsic pontine glioma (DIPG), and families may hear this older term used interchangeably with diffuse midline glioma.

Diffuse midline glioma is defined not only by where it grows but by a specific genetic change affecting a protein called histone H3. This change disrupts how genes are turned on and off in the tumor cells and is what gives the disease its name. Diffuse midline glioma is classified as World Health Organization (WHO) grade 4 — the highest grade used for central nervous system tumors — and is one of the most serious brain tumors diagnosed in children and young adults.

This article will help you understand the findings in the pathology report — what each term means and why it matters for the care of the patient, most often a child.

What are the symptoms of diffuse midline glioma?

The symptoms of diffuse midline glioma depend on where the tumor is growing. Because these tumors affect deep, central parts of the nervous system that control many essential functions, symptoms often develop quickly — usually over weeks to a few months.

Tumors in the pons or brainstem classically cause three types of problems, which often appear together:

• Cranial nerve problems — double vision, crossed eyes, drooping of one side of the face, difficulty swallowing, or slurred speech. These symptoms result from pressure on the nerves that leave the brainstem and control movements of the eyes, face, and mouth.
• Long tract signs — weakness or stiffness in the arms or legs, reduced coordination, and abnormal reflexes. These symptoms reflect pressure on the nerve pathways that carry signals between the brain and the body.
• Ataxia — problems with balance, coordination, and walking steadily. Affected children may begin to walk unsteadily, fall more often, or seem clumsy.

Tumors in the thalamus can cause headache, vomiting, and drowsiness (from a build-up of fluid and pressure inside the skull called hydrocephalus), as well as weakness or numbness on one side of the body. Tumors that involve both sides of the thalamus (bithalamic tumors) may also cause changes in alertness, memory, or behavior.

Tumors in the spinal cord can cause back or neck pain, weakness in the arms or legs, numbness, and problems with bladder or bowel control.

What causes diffuse midline glioma?

Diffuse midline glioma is caused by genetic changes in glial cells in the midline of the developing nervous system. The most important change is a mutation in one of the genes that makes histone H3, a protein that helps package DNA inside the cell nucleus. Histones work like spools around which DNA is wound, and small chemical tags attached to histones help control which genes are turned on or off in each cell.

In most diffuse midline gliomas, the histone H3 mutation changes a single building block of the histone protein at position 27 — a lysine is replaced by a methionine. This change is called the H3 K27M mutation. It removes one of the most important chemical tags that normally keeps certain genes switched off, leading to widespread changes in gene expression. In a smaller number of tumors, the H3 gene itself is normal, but a protein called EZHIP is overproduced and has a similar effect. A separate, rarer subgroup is driven by changes in a gene called EGFR.

For most patients, these genetic changes occur by chance during development and are not passed from parent to child. There is no known environmental cause, and nothing the parents did or did not do caused the tumor to form.

A very small number of diffuse midline gliomas develop in the setting of an inherited condition. Inherited conditions are caused by a genetic change that is present in every cell of the body from birth and can be passed from parent to child. The inherited conditions rarely linked to diffuse midline glioma include:

• Li-Fraumeni syndrome — caused by a change in the TP53 gene. This syndrome increases the risk of many cancers, including brain tumors, sarcomas, breast cancer, and leukemia.
• Constitutional mismatch repair deficiency (CMMRD) — caused by inherited changes in both copies of one of the DNA repair genes MLH1, MSH2, MSH6, or PMS2. CMMRD can cause brain tumors, blood cancers, and colon cancer in childhood, and may produce skin findings similar to neurofibromatosis type 1.

Because most diffuse midline gliomas are not associated with an inherited condition, genetic testing of the blood is not routine but may be offered when a family history of cancer is present or when clinical features suggest a cancer predisposition syndrome.

How common is diffuse midline glioma?

Diffuse midline glioma is rare overall but accounts for a substantial share of brain tumors in children. Pontine diffuse midline glioma (diffuse intrinsic pontine glioma, or DIPG) alone accounts for about 10–15% of all childhood brain tumors and about 75% of all brainstem tumors in children. Thalamic diffuse midline gliomas are less common, accounting for about 1–5% of childhood brain tumors and about a quarter of all thalamic tumors. Spinal diffuse midline gliomas make up about 40% of spinal cord gliomas in children and adults.

Diffuse midline glioma occurs most often between ages 5 and 10, with slightly different age patterns across molecular subgroups. The tumor can also occur in adolescents and adults, especially in the thalamus or spinal cord. Boys and girls are affected equally.

How is the diagnosis made?

The diagnosis of diffuse midline glioma usually begins when magnetic resonance imaging (MRI) of the brain, performed because of symptoms, reveals an infiltrative mass in the pons, thalamus, or another midline location. On MRI, these tumors often have a characteristic appearance — for example, a pontine diffuse midline glioma typically enlarges and distorts the pons and may surround the basilar artery, the large blood vessel that runs in front of it. However, imaging alone cannot establish the specific tumor type or identify the molecular changes needed to guide treatment.

For decades, pontine tumors in particular were often diagnosed on imaging alone because of concerns about the safety of obtaining tissue from the brainstem. This has changed. Modern biopsy techniques have made it possible to safely sample tumors in most midline locations, and a biopsy is now recommended in most cases. The reason is straightforward: the treatment landscape has changed, and knowing the exact molecular subgroup of the tumor is now essential to guide treatment and to determine eligibility for targeted therapy and clinical trials. In most cases, tissue is obtained through a stereotactic biopsy — a minimally invasive procedure in which a thin needle is guided into the tumor using imaging to sample a small amount of tissue. Full surgical removal is not attempted because of the diffuse, infiltrative nature of the tumor and the risk of injury to vital surrounding structures.

Under the microscope, a pathologist sees tumor cells infiltrating the normal brain tissue. The cells vary in appearance — some tumors have small, uniform cells, while others have larger or more varied cells with astrocyte-like, oligodendrocyte-like, or giant-cell features. Mitotic figures (cells caught in the act of dividing) are common. Features that indicate aggressive behavior in other gliomas, such as necrosis (areas of dead tumor) and microvascular proliferation (abnormal new blood vessel growth), may or may not be present. Unlike most other high-grade gliomas, these features are not required for diagnosis — the diagnosis of diffuse midline glioma depends on the tumor’s location and its molecular findings, not on how aggressive the cells look.

To confirm the diagnosis, the pathologist uses a combination of immunohistochemistry (a laboratory test that uses antibodies to detect specific proteins in the tumor cells) and molecular testing. The two most important immunohistochemistry tests are an antibody that recognizes the abnormal H3 K27M protein and an antibody that detects the loss of a chemical tag called H3 K27 trimethylation. Together, these two stains allow the pathologist to identify the tumor even when only a tiny amount of tissue is available. Additional testing, described in the biomarker section below, identifies the specific molecular subgroup.

After the initial diagnosis, imaging of the spine is usually performed to check for tumor spread through the cerebrospinal fluid (the clear fluid that surrounds the brain and spinal cord). A lumbar puncture to examine the cerebrospinal fluid for tumor cells may also be performed. Diffuse midline gliomas can spread along the surface of the brain and spinal cord, particularly in more advanced stages.

Molecular subgroups of diffuse midline glioma

Diffuse midline glioma, H3 K27-altered, is now divided into four molecular subgroups. These subgroups behave slightly differently, tend to occur at different ages, and may be eligible for different treatments or clinical trials. All four are considered WHO grade 4 and share the same overall treatment approach, but the molecular subgroup is an important part of the pathology report.

H3.3 K27M-mutant

H3.3 K27M-mutant diffuse midline glioma is the most common subgroup. It is driven by a mutation in the H3F3A gene, which makes a form of histone H3 called H3.3. This subgroup typically occurs in children aged 7–8 years and can arise in the pons, thalamus, or spinal cord. It is often accompanied by mutations in other genes, such as TP53, which can contribute to resistance to radiation therapy. This subgroup has the shortest overall survival among the four subgroups.

H3.1 or H3.2 K27M-mutant

H3.1 or H3.2 K27M-mutant diffuse midline glioma is driven by a mutation in one of the HIST1H3B, HIST1H3C, or HIST2H3C genes, which make different forms of histone H3. This subgroup typically occurs in younger children (around 5 years of age) and most commonly arises in the pons. It is often accompanied by mutations in genes of the PI3K or MAPK cell signaling pathways, and in a gene called ACVR1. Patients in this subgroup tend to have slightly longer survival than those with H3.3 K27M-mutant tumors.

H3-wildtype with EZHIP overexpression

In this subgroup, the histone H3 gene itself is not mutated. Instead, a protein called EZHIP is produced at abnormally high levels and causes a similar effect on gene regulation as the H3 K27M mutation. This is the rarest of the four subgroups. It most often occurs in young children and typically arises in the pons. Survival in this subgroup is similar to that of the H3.1 or H3.2 K27M-mutant subgroup.

EGFR-mutant

EGFR-mutant diffuse midline glioma is driven by changes in the EGFR gene, which normally helps cells grow and divide. This subgroup most often arises in the thalamus, often involving both sides (a bithalamic tumor), and typically occurs in children aged 7–8 years. Although the underlying genetic change is different, these tumors still show loss of the H3 K27 trimethylation tag and are grouped with the other three subgroups as diffuse midline glioma, H3 K27-altered. Research is underway to determine whether EGFR-targeted drugs may be useful in this subgroup.

WHO grade

The World Health Organization (WHO) assigns tumors of the central nervous system a grade from 1 to 4 that reflects how the tumor is expected to behave. All diffuse midline gliomas, H3 K27-altered, are assigned WHO grade 4 — the highest grade used for central nervous system tumors. This grade reflects the aggressive, infiltrative nature of the disease and is assigned regardless of how the tumor cells look under the microscope. Unlike most other high-grade gliomas, features such as necrosis and abnormal blood vessel growth are not required to assign this grade; the diagnosis and grade are determined by the molecular findings together with the tumor’s midline location.

Biomarker and molecular testing

Molecular testing is an essential part of the workup for diffuse midline glioma. The results confirm the diagnosis, identify the specific molecular subgroup, determine eligibility for targeted therapy, and guide enrolment in clinical trials.

H3 K27M immunohistochemistry

Immunohistochemistry with an antibody that recognizes the abnormal H3 K27M protein is the most common first-line test. A positive result — strong staining of tumor cell nuclei — strongly supports the diagnosis. Because the test is sensitive and can detect even a small number of tumor cells mixed in with normal brain tissue, it works well even on small biopsy samples. A negative result does not rule out the diagnosis, because tumors driven by EZHIP overexpression or EGFR mutation do not have the H3 K27M protein.

H3 K27 trimethylation (H3 K27me3) immunohistochemistry

H3 K27 trimethylation is a chemical tag that is normally present on histone H3. In diffuse midline glioma, H3 K27-altered, this tag is lost, whether the cause is an H3 K27M mutation, EZHIP overexpression, or an EGFR mutation. Immunohistochemistry for H3 K27 trimethylation shows loss of staining in tumor cell nuclei, with preserved staining in surrounding normal cells that serve as an internal control. This test is especially useful when H3 K27M immunohistochemistry is negative, as it can still identify the tumor as H3 K27-altered.

Sequencing of histone H3 genes

When the immunohistochemical pattern is consistent with a diffuse midline glioma, DNA sequencing is often performed to identify the specific histone H3 gene mutation. This distinguishes the H3.3 K27M-mutant subgroup from the H3.1 or H3.2 K27M-mutant subgroup and may influence eligibility for certain clinical trials.

EGFR testing

Tumors that show loss of H3 K27 trimethylation but no H3 K27M mutation should be tested for changes in the EGFR gene. Most EGFR-mutant tumors have small insertions or duplications in the part of the gene that encodes the tyrosine kinase region, while others have specific point mutations in the part that encodes the outer (extracellular) part of the protein. This testing is important because the EGFR-mutant subgroup is recognized as a distinct tumor subgroup and may be eligible for EGFR-targeted clinical trials.

Additional molecular testing

Testing for mutations in TP53, ATRX, PPM1D, ACVR1, PIK3CA, PIK3R1, PTEN, and other genes may be performed as part of a comprehensive next-generation sequencing panel. These results do not change the diagnosis but may provide prognostic information (for example, TP53 mutations are associated with resistance to radiation therapy) or identify targets for clinical trials. MGMT promoter methylation, which is important for determining response to temozolomide in other high-grade gliomas, is rarely present in diffuse midline glioma.

DNA methylation profiling

DNA methylation refers to small chemical tags attached to DNA that help control which genes are turned on or off. Different tumor types have distinct methylation patterns, almost like a fingerprint. DNA methylation profiling compares a tumor’s pattern to a large reference database and can confirm the diagnosis of diffuse midline glioma and identify its molecular subgroup. This test is increasingly used in specialized centers for difficult cases or when tissue is limited.

Germline genetic testing

Germline genetic testing — a genetic test performed on blood or saliva to look for changes present throughout the body — is not routinely performed in most patients with diffuse midline glioma. However, it may be recommended when there is a personal or family history of other cancers, when features suggest Li-Fraumeni syndrome or constitutional mismatch repair deficiency, or when a tumor testing result points to a possible inherited change. Genetic counseling is an important part of this process.

For more information about biomarkers and molecular testing across all cancer types, visit the Biomarkers and Genetic Testing section.

What is the prognosis?

The prognosis for diffuse midline glioma, H3 K27-altered, is serious. Most patients live less than two years after diagnosis, and overall survival depends on the molecular subgroup and other factors. Reported median survival figures include:

• H3.3 K27M-mutant — approximately 11 months.
• H3.1 or H3.2 K27M-mutant — approximately 15–16 months.
• H3-wildtype with EZHIP overexpression — similar to H3.1 or H3.2 K27M-mutant, approximately 15–16 months.
• EGFR-mutant (bithalamic) — approximately 10–14 months.

Additional features that may influence outcome include:

• Age — children younger than 3 or older than 10 years of age tend to have slightly longer survival than children in the typical age range for this tumor.
• Symptom duration — patients whose symptoms developed slowly over more than 24 weeks before diagnosis tend to have longer survival than those with a rapid onset.
• TP53 mutation — tumors with a TP53 mutation tend to respond less well to radiation therapy.
• Response to radiation — patients whose tumor shrinks or stabilizes after radiation therapy generally have a longer interval before the tumor grows again.

It is important to emphasize that these numbers describe averages across groups of patients. Individual outcomes can vary substantially, and treatment is advancing. Participation in a clinical trial — including the recent introduction of targeted therapy for H3 K27M-mutant tumors — has meaningfully expanded treatment options for the first time in decades.

What happens after the diagnosis?

Diffuse midline glioma is managed by a team of specialists that includes a neurosurgeon, a pediatric neuro-oncologist (or adult neuro-oncologist for adolescent and adult patients), a radiation oncologist, a neuropathologist, and a neuroradiologist. Palliative care, psychosocial support, and rehabilitation specialists are also part of the team from the beginning and help the patient and family with symptoms, quality of life, and planning. A geneticist or genetic counselor is involved when an inherited condition is suspected.

Because complete surgical removal is not possible, treatment relies on a combination of radiation, targeted therapy (for eligible patients), and clinical trials:

• Radiation therapy — the mainstay of first-line treatment. A typical course delivers focused radiation to the tumor over several weeks. Radiation can shrink the tumor, relieve symptoms, and extend survival, but it is not curative. Most patients experience at least some symptomatic improvement after radiation.
• Dordaviprone (Modeyso, formerly called ONC201) — an oral targeted therapy that received accelerated approval from the U.S. Food and Drug Administration in August 2025 for patients aged 1 year and older with H3 K27M-mutant diffuse midline glioma whose tumor has progressed after prior therapy (usually radiation). This was the first systemic drug ever approved for diffuse midline glioma. In clinical trials, about 22% of patients with H3 K27M-mutant tumors had their tumor shrink in response to dordaviprone, and responses lasted a median of about 10 months. Dordaviprone is not a cure, but it represents a meaningful new option for many patients. Approval in Canada and other countries is evolving; ask your treatment team about current access.
• Clinical trials — participation in a clinical trial is strongly encouraged for patients with diffuse midline glioma. Many trials are investigating new drugs, combinations, and approaches, including immunotherapies, vaccines, convection-enhanced drug delivery, and additional targeted therapies for specific molecular subgroups.
• Chemotherapy — standard chemotherapy drugs such as temozolomide have not been shown to improve survival in diffuse midline glioma and are generally not part of standard first-line care outside of clinical trials.
• Re-irradiation — for patients whose tumor grows after initial radiation, a second course of radiation can sometimes be given and may provide additional symptom relief.

Follow-up includes regular MRI scans to monitor the tumor’s response to treatment and to detect any growth. Long-term effects of the tumor and its treatment — on hormones, vision, hearing, cognitive function, balance, and mobility — are managed by the multidisciplinary team. Neuropsychology, physical therapy, occupational therapy, speech therapy, and educational support are important parts of survivorship care.

Because diffuse midline glioma often progresses despite treatment, many families also engage with palliative care early in the course of illness. Palliative care focuses on comfort, symptom management, and emotional and spiritual support, and is compatible with ongoing cancer-directed treatment. Early palliative care involvement has been shown to improve quality of life for both patients and families.

Questions to ask your doctor

• Where exactly is the tumor, and how does the location affect what treatments are possible?
• Has a biopsy been done or is one planned, and what molecular testing will be performed?
• Which molecular subgroup does the tumor belong to — H3.3 K27M-mutant, H3.1 or H3.2 K27M-mutant, H3-wildtype with EZHIP overexpression, or EGFR-mutant?
• Were any additional mutations identified, such as a TP53 mutation, that affect prognosis or treatment?
• What does radiation therapy involve, and when will it start?
• Is dordaviprone (Modeyso) an option for us, and if so, when would we start it?
• What clinical trials are available, and should we consider one now or after radiation?
• Where will the tumor be treated, and would a referral to a specialized pediatric neuro-oncology center be helpful?
• Should we pursue genetic testing for an inherited condition?
• What symptoms should we watch for, and what should we do if they appear?
• How often will follow-up MRI scans be done, and how will we know if the tumor is responding to treatment?
• What supportive care services are available — physical therapy, occupational therapy, psychology, education, and palliative care?
• How can we best support the patient’s quality of life during treatment?
• What should we tell siblings, classmates, and other family members?

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