Histone H3 K27M and Diffuse Midline Glioma
Diffuse midline glioma, H3 K27-altered, is an aggressive tumour of the brainstem, thalamus and spinal cord that occurs mostly in children. In the majority of cases it is caused by a single amino acid substitution, lysine to methionine at position 27, in a histone H3 gene. That one change rewires gene expression across the genome.
Quick Answer
Diffuse midline glioma, H3 K27-altered, is an aggressive tumour of the brainstem, thalamus and spinal cord that occurs mostly in children. In the majority of cases it is caused by a single amino acid substitution, lysine to methionine at position 27, in a histone H3 gene. That one change rewires gene expression across the genome.
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Cancer Epigenetics and Chromatin Regulators
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The K27M mutation usually affects just one of the many histone H3 genes, most often H3-3A encoding H3.3, so the altered protein is a small fraction of total histone H3. Despite this, it acts dominantly: the methionine residue sequesters and inhibits the EZH2 enzyme of PRC2 across the nucleus.
The consequence is a global loss of the repressive H3K27me3 mark, with focal retention at some sites. Genes that should be silenced during normal brain development are inappropriately available for transcription.
Why It Defines a Tumour Type
The 2021 World Health Organization classification of central nervous system tumours makes H3 K27 alteration a defining feature of diffuse midline glioma, grade 4, regardless of histological appearance. The diagnosis can be supported by immunohistochemistry for loss of H3K27me3 and for the mutant protein.
Prognosis remains poor, with median survival generally under a year, and radiotherapy is the main treatment that reliably provides temporary benefit.
Therapeutic Approaches Under Study
Because the lesion is epigenetic, strategies have focused on restoring the balance of histone marks, for example with inhibitors of histone deacetylases or of the H3K27 demethylase, and on targeting downstream dependencies. Imipridone compounds and several immunotherapy approaches are also in trials.
None is an established standard of care, and the blood-brain barrier and tumour location make drug delivery a persistent obstacle.
How the Diagnosis Is Made and Monitored
Diagnosis combines imaging of a midline location with tissue confirmation: sequencing for the H3 K27M mutation plus immunohistochemistry showing loss of H3K27me3 and, where available, staining for the mutant protein. A small number of H3-wild-type tumours reach the same diagnostic category through EZHIP overexpression or specific EGFR alterations.
Circulating tumour DNA in cerebrospinal fluid, and sometimes plasma, can detect the H3 K27M mutation and is being studied for monitoring when repeat biopsy is not feasible. Response assessment otherwise relies on serial MRI, which is complicated by treatment-related change after radiotherapy.
Key Takeaways
- ·A single H3 K27M substitution inhibits PRC2 and globally reduces the repressive H3K27me3 mark.
- ·H3 K27 alteration defines diffuse midline glioma, grade 4, in the current WHO classification.
- ·Treatment is largely radiotherapy; epigenetic and immunotherapy strategies remain investigational.
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Frequently asked questions
How can one mutation in one histone gene affect the whole genome?
The K27M protein acts dominantly: its methionine residue sequesters and inhibits the EZH2 enzyme across the nucleus, causing a global loss of the repressive H3K27me3 mark even though the mutant histone is a small fraction of the total.
Does H3 K27M change the diagnosis?
Yes. In the current WHO classification, H3 K27 alteration defines diffuse midline glioma, grade 4, regardless of how the tumour looks under the microscope.
Is there an effective drug for it?
Not yet. Radiotherapy provides temporary benefit; epigenetic drugs such as HDAC inhibitors and several immunotherapy approaches are in trials, hindered partly by drug delivery to the brainstem.
References
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