IDH1 and IDH2 Mutations: A Metabolic Change That Rewires the Epigenome

A Gain-of-Function Mutation

Normally IDH1 (in the cytoplasm) and IDH2 (in mitochondria) convert isocitrate to alpha-ketoglutarate. Cancer-associated mutations at a single arginine residue, IDH1 R132 or IDH2 R140 and R172, change the active site.

The mutant enzyme instead reduces alpha-ketoglutarate to D-2-hydroxyglutarate, an oncometabolite that accumulates to very high levels.

How 2-Hydroxyglutarate Drives Cancer

2-hydroxyglutarate structurally resembles alpha-ketoglutarate and competitively inhibits a family of enzymes that depend on it, including the TET DNA demethylases and the Jumonji histone demethylases.

The result is widespread DNA and histone hypermethylation, which locks in an immature gene-expression state and blocks differentiation. In blood and glial precursors this expands a stem or progenitor pool that can progress to cancer.

Where the Mutations Occur

IDH1 R132 defines most lower-grade gliomas and secondary glioblastomas and is a favourable prognostic marker within glioma. IDH1 and IDH2 mutations occur in around 10 to 20 percent of acute myeloid leukaemia, in cholangiocarcinoma, chondrosarcoma and angioimmunoblastic T-cell lymphoma.

They are usually early, truncal events and are generally mutually exclusive with each other.

IDH Inhibitors

Small molecules that selectively inhibit the mutant enzyme (ivosidenib for mutant IDH1, enasidenib for mutant IDH2, and vorasidenib, a brain-penetrant dual inhibitor) lower 2-hydroxyglutarate and can restore differentiation.

Ivosidenib and enasidenib are approved in IDH-mutant acute myeloid leukaemia, ivosidenib also in IDH1-mutant cholangiocarcinoma, and vorasidenib in IDH-mutant grade 2 glioma after showing delayed progression. In leukaemia, a differentiation syndrome can occur as blasts mature.

Interpretation Notes

An IDH result should specify the gene and codon, since inhibitor selectivity is mutation-specific and IDH1 R132H has a validated antibody while other substitutions need sequencing.

In glioma, IDH status is part of the formal diagnostic classification, not just a treatment biomarker.

Key Takeaways

• ·Mutant IDH1/IDH2 gains the ability to make the oncometabolite 2-hydroxyglutarate.
• ·2-hydroxyglutarate inhibits demethylases, causing hypermethylation and a differentiation block.
• ·The mutations define most lower-grade gliomas and occur in AML, cholangiocarcinoma and others.
• ·Mutation-selective IDH inhibitors are approved in AML, cholangiocarcinoma and grade 2 glioma.