Question 2: Causal Relationships and Cellular Metabolism Task: Explain the causal chain starting from a gain-of-function mutation in IDH1 or IDH2 to the resulting "differentiation block" observed in leukemic cells. In your answer, specifically address the role of the oncometabolite 2-hydroxyglutarate (2-HG), its effect on the TET family erasers, and how this metabolic shift ultimately prevents stem cells from committing to a specific lineage

What initiates the causal chain leading to differentiation block in IDH-mutant leukemia?

A gain-of-function (neomorphic) mutation in IDH1 (R132) or IDH2 (R172) that alters the enzyme's catalytic preference.

What does the mutated IDH1/IDH2 enzyme produce?

The oncometabolite 2-hydroxyglutarate (2-HG) instead of converting isocitrate to α-ketoglutarate (α-KG).

What levels does 2-HG reach in mutant cells?

Millimolar (mM) levels.

How does 2-HG affect α-KG-dependent enzymes?

It is structurally similar to α-KG and acts as a competitive inhibitor.

What TET family enzymes does 2-HG inhibit?

TET1, TET2, and TET3. They are epigenetic "erasers" required for active DNA demethylation.

What step of demethylation does 2-HG block at TET enzymes?

The conversion of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC).

What global epigenetic effect does TET inhibition by 2-HG cause?

Global DNA hypermethylation.

What histone demethylases does 2-HG also inhibit?

α-KG-dependent lysine demethylases (KDMs) such as KDM6A/UTX.

What happens to repressive histone marks when KDMs are inhibited?

They accumulate because they cannot be removed. Examples include H3K9me3 and H3K27me3.

What is the combined epigenetic effect of 2-HG?

Both DNA hypermethylation and accumulation of repressive histone marks at gene promoters.

What types of genes become silenced by this epigenetic reprogramming?

Genes involved in regulating normal development and lineage commitment.

How does this epigenetic reprogramming cause a differentiation block?

Stem and progenitor cells are trapped in a state of abnormal self-renewal because genes needed to respond to differentiation cues are epigenetically locked, so cells cannot commit to a specific linea