Questions: DNA Methylation and Epigenetic Gene Silencing

Question 1

A tumor suppressor gene is silenced in cancer cells, but DNA sequencing confirms the gene's coding sequence is completely intact. Which mechanism most likely explains the silencing?

• A. Deletion of DNMT1, which prevents maintenance methylation and thus activates silencing
• B. Hypermethylation of the CpG island in the gene's promoter region, recruiting chromatin-condensing complexes
• C. Hypomethylation of the gene body, which reduces transcription elongation efficiency
• D. Loss of histone acetylation marks globally across all chromosomes

Answer: (B)

Aberrant hypermethylation of CpG islands at tumor suppressor promoters is one of the most common epigenetic events in cancer. In normal cells, CpG islands near gene promoters are typically unmethylated, keeping the gene accessible. When they become methylated, methyl-binding proteins (MeCP2, MBDs) recruit HDACs and histone methyltransferases, condensing chromatin and silencing the gene — functionally equivalent to deleting it, but without altering the DNA sequence. This is why the sequence is intact but the gene is off.

Question 2

Which best describes how DNA methylation silences a gene — the primary mechanism of action?

• A. The methyl group directly modifies the mRNA transcript, producing a truncated nonfunctional protein
• B. Methylated cytosines spontaneously mutate to thymine over time, eventually destroying the gene permanently
• C. The methyl group is large enough to sterically block RNA polymerase from binding the promoter directly
• D. Methyl-binding proteins recognize methylated CpGs and recruit histone deacetylases and chromatin-remodeling complexes, compacting the chromatin into a transcriptionally silent state

Answer: (D)

The primary silencing cascade operates through methyl-binding proteins, not direct steric blockade. When CpGs are methylated, MeCP2 and MBD proteins bind the methyl groups and recruit HDACs (which strip activating acetyl groups from histones) and histone methyltransferases (which add repressive methyl marks to histones). The resulting compact, heterochromatic structure buries the promoter and prevents transcription initiation. Direct steric blockade by the methyl group itself is a secondary, less important effect.

Question 3

DNA methylation patterns are heritable through cell division because DNMT1 recognizes hemimethylated DNA (one strand methylated, one not) after replication and methylates the new strand to restore the original pattern.

• T. True
• F. False

Answer: True

This is the maintenance methylation mechanism — the key to epigenetic inheritance. When DNA replicates, the new strand is initially unmethylated, producing hemimethylated duplexes. DNMT1 has a strong preference for hemimethylated over unmethylated DNA and is recruited to replication forks, where it faithfully copies the parental methylation pattern onto the new strand. This is why a liver cell divides to produce liver cells: the methylation patterns silencing neuron-specific genes are propagated to every daughter cell.

Question 4

DNA methylation is a permanent, irreversible epigenetic modification because it involves a covalent chemical change to the DNA molecule.

• T. True
• F. False

Answer: False

Methylation is covalent but reversible. Active demethylation is carried out by TET enzymes (which oxidize 5-methylcytosine through intermediates that are removed by base excision repair), and passive demethylation occurs when DNMT1 is absent or inhibited during replication (new strands remain unmethylated and the pattern dilutes). Critically, methylation does NOT alter the DNA sequence — only a cytosine base modification is added or removed, leaving the sequence intact. This reversibility distinguishes epigenetic silencing from mutation and underlies the therapeutic strategy of DNMT inhibitors like azacitidine.

Question 5

Explain how DNA methylation differs from a genetic mutation in terms of effect on DNA sequence, heritability, and reversibility — and why these differences matter for cancer therapy.

Model answer: A genetic mutation changes the DNA sequence itself — a permanent alteration to the bases that is inherited by all descendant cells and cannot be corrected without genome editing. DNA methylation adds a methyl group to cytosine without changing the underlying sequence; the information content of the DNA is unchanged. Like mutations, methylation patterns are heritable through cell division (via DNMT1 maintenance). Unlike mutations, they are reversible: DNMT inhibitors (azacitidine, decitabine) block DNMT1, causing passive loss of methylation during replication, which can reactivate silenced tumor suppressor genes. This is a clinically validated cancer therapy — targeting epigenetic silencing rather than the DNA sequence itself.

The reversibility of methylation makes it an attractive therapeutic target that mutation repair is not. It also underlies the distinction between epigenetics (heritable regulatory states not encoded in sequence) and genetics (heritable sequence information). Both are involved in cancer, but only epigenetic silencing can be reversed pharmacologically without genome editing.