Question 5: Contrast and Deduction of "The Tight Lock" Task: Contrast the physiological "reversibility" of gene silencing mediated by Polycomb Group (PcG) complexes with silencing mediated by DNA Methyltransferases (DNMTs). Deduction: If a patient is treated with a PcG inhibitor after a gene has already transitioned to a hypermethylated state (the "tight lock"), explain why this therapy would likely fail to reactivate the gene. What would be the logically necessary "next step" in a combination therapy to successfully restore gene expression?.

What is the physiological reversibility of PcG-mediated silencing?

It is a "poised" state that is physiologically reversible, allowing developmental genes to be rapidly reactivated or permanently silenced upon differentiation cues.

What complex writes the H3K27me3 mark?

PRC2 (Polycomb Repressive Complex 2).

How is DNMT-mediated silencing different from PcG-mediated silencing?

DNMT silencing via DNA hypermethylation is a far more stable and heritable form of repression, described as a "tight lock."

What is "epigenetic switching"?

The process where relatively reversible PcG-mediated repression is replaced by the more stable "tight lock" of DNA hypermethylation.

Why would a PcG inhibitor (like EZH2 inhibitor) fail to reactivate a gene that is already hypermethylated?

Because the primary barrier is no longer PcG complexes but DNA methylation itself. Methylated DNA is resistant to PcG re-recruitment, and inhibiting a writer that is no longer the dominant repressive force will not remove the DNA methylation lock.

What happens to PcG complexes and H3K27me3 marks once DNA methylation occurs at a promoter?

They may be completely or quantitatively lost.

[;; What is the logically necessary first step in combination therapy to restore expression of a hypermethylated gene?

Why are HDAC inhibitors often needed after DNMT inhibitors?

Because DNA methylation and histone deacetylation (mediated by HDAC1/2) collaborate to maintain stable silencing. Blocking HDAC activity after demethylation leads to additive or synergistic gene re-expression.

What do DNMT inhibitors plus HDAC inhibitors achieve together?

They re-establish an "open" chromatin configuration by demethylating the DNA and relaxing chromatin through hyperacetylation, making the promoter accessible to transcriptional machinery.

Give examples of DNMT inhibitors.

Azacitidine or Decitabine.

Give examples of HDAC inhibitors.

Entinostat or Vorinos