RNA Interference and Epigenetics in Disease Revision Notes | A Level AQA Biology

1. Why Gene Silencing Matters

Your genome contains around 20,000 protein-coding genes, yet a liver cell and a neuron contain identical DNA. The difference lies not in the sequence but in which genes are switched on or off — and two of the most powerful silencing mechanisms are RNA interference and epigenetic modification.

When these silencing systems malfunction, the consequences can be severe. Tumour-suppressor genes that should be active get permanently switched off; oncogenes that should be silent become overexpressed. Understanding these mechanisms is therefore central to understanding how cancer develops — and how it might be treated.

2. RNA Interference — Silencing After Transcription

Picture a cell that has just transcribed a gene into mRNA. The mRNA is heading towards the ribosome, ready to be translated into protein. RNA interference (RNAi) intercepts that journey. It is a post-transcriptional mechanism — the DNA has already been read, but the protein never gets made.

The key players are small RNA molecules, typically around 21–23 nucleotides long, known as small interfering RNA (siRNA) or microRNA (miRNA). These molecules are double-stranded when first produced. An enzyme complex called DICER cleaves longer double-stranded RNA precursors into these short fragments.

One strand of the siRNA/miRNA is then loaded into a protein complex called the RNA-induced silencing complex (RISC). The guide strand within RISC is complementary to the target mRNA sequence. When RISC encounters the matching mRNA, it binds to it — and the mRNA is either cleaved and degraded, or its translation is blocked. Either way, no protein is produced from that transcript.

RNAi occurs in eukaryotes — including plants, animals, and fungi — and has also been identified in some prokaryotes, though the mechanism in prokaryotes differs and is less well characterised. For AQA purposes, the key point is that translation of mRNA produced from target genes is inhibited by RNAi in eukaryotes and some prokaryotes.

There is an important distinction worth noting here. RNAi does not alter the DNA sequence — the gene is still present and can still be transcribed. The silencing happens downstream, at the level of the mRNA. This makes RNAi a form of post-transcriptional regulation, not a genetic mutation.

3. Epigenetics — Silencing at the Level of Chromatin

Now that you have seen how RNAi silences genes after transcription, consider a different question: what if a gene could be prevented from being transcribed in the first place? That is the territory of epigenetics — changes to gene expression that do not involve changing the DNA sequence, but instead alter how accessible the DNA is to transcription machinery.

DNA in eukaryotic cells is wound around proteins called histones, forming a structure called chromatin. When chromatin is tightly condensed (heterochromatin), RNA polymerase and transcription factors cannot access the promoter region, so transcription cannot occur. When chromatin is relaxed (euchromatin), genes are accessible and can be expressed. Epigenetic modifications shift chromatin between these two states.

Two parallel inhibitory routes are central to the AQA specification: DNA methylation and histone deacetylation. Both result in chromatin condensation and reduced transcription, but they act through different molecular mechanisms.

3a. DNA Methylation

DNA methylation involves the addition of a methyl group ($-CH_3$) to a cytosine base in the DNA strand, typically where cytosine is followed by guanine (a CpG site). This is carried out by enzymes called DNA methyltransferases. The methyl groups do not change the base sequence — adenine still pairs with thymine, guanine still pairs with cytosine — but they change how the DNA interacts with proteins.

Heavily methylated regions of DNA attract proteins that compact the chromatin further. Transcription factors are physically blocked from binding to the promoter. The result is that the gene is silenced — it cannot be transcribed, so no mRNA is produced and no protein is made. Methylation patterns can be passed on when cells divide, which is why epigenetic silencing can be heritable at the cellular level.

3b. Histone Deacetylation

Histones carry positively charged amino acid residues (particularly lysine) on their tails. When acetyl groups are added to these residues (acetylation), the positive charge is neutralised. This reduces the attraction between the histone and the negatively charged DNA backbone, causing chromatin to relax — and genes in that region become accessible for transcription.

Deacetylation is the reverse: enzymes called histone deacetylases (HDACs) remove the acetyl groups. The positive charge on the histone tail is restored, the histone binds more tightly to the DNA, chromatin condenses, and transcription is repressed. Deacetylation therefore silences genes by making the DNA physically inaccessible.

4. Epigenetics in Disease — The Cancer Connection

With the mechanisms of methylation and deacetylation established, you can now see exactly why they are so relevant to cancer. Cancer is fundamentally a disease of uncontrolled cell division, and that loss of control often traces back to two categories of genes: tumour-suppressor genes (which normally restrain cell division) and proto-oncogenes (which normally promote it in a controlled way).

In many cancers, tumour-suppressor genes are found to be hypermethylated — their promoter regions carry an abnormally high density of methyl groups. This silences the tumour-suppressor gene, removing the brake on cell division. The cell divides uncontrollably, and a tumour can develop. Crucially, this silencing has occurred without any mutation to the DNA sequence itself.

The reverse pattern is also seen: proto-oncogenes can become hypomethylated (less methylated than normal), which relaxes chromatin at their promoter and increases their expression. Overactive proto-oncogenes drive excessive cell proliferation. Both hypermethylation of tumour suppressors and hypomethylation of oncogenes are hallmarks of many cancer types, including colorectal, breast, and lung cancers.

Histone deacetylation contributes similarly. Aberrant HDAC activity can silence tumour-suppressor genes by condensing chromatin at their loci. Because these are epigenetic changes rather than mutations, they are — in principle — reversible. This reversibility is what makes epigenetic mechanisms such attractive therapeutic targets.

5. Therapeutic Applications — Treating Disease Through Epigenetics and RNAi

Because epigenetic changes are reversible, drugs that target the enzymes responsible for methylation and deacetylation have genuine therapeutic potential. Two classes of drugs are particularly relevant: DNA methyltransferase inhibitors (which reduce methylation and can reactivate silenced tumour-suppressor genes) and HDAC inhibitors (which prevent deacetylation, keeping chromatin relaxed and restoring expression of silenced genes).

HDAC inhibitors such as vorinostat have been approved for certain cancers, including cutaneous T-cell lymphoma. By blocking HDAC enzymes, these drugs maintain histone acetylation, keep chromatin open at tumour-suppressor loci, and restore the cell's own braking mechanisms on division. This represents a fundamentally different approach from conventional chemotherapy, which typically targets DNA replication directly.

RNAi also has therapeutic potential. Synthetic siRNA molecules can be designed to be complementary to the mRNA of a disease-causing gene — for example, an overexpressed oncogene. When delivered into cells, the siRNA is loaded into RISC and directs degradation of the target mRNA, reducing production of the harmful protein. The challenge lies in delivery: getting siRNA into the correct cells in the body without triggering immune responses remains an active area of research.

It is worth pausing to compare these two therapeutic strategies. Epigenetic drugs act broadly across the genome — they alter the methylation or acetylation state at many loci simultaneously, which can have off-target effects. RNAi-based therapies are highly sequence-specific: a given siRNA will only silence the gene whose mRNA it is complementary to. This specificity is a significant advantage, though delivery remains a practical obstacle.

6. Exam Technique — Scoring Full Marks on These Topics

Questions on RNAi and epigenetics frequently appear as extended-response items worth 4–6 marks, often using the command words 'explain', 'evaluate', or 'compare'. The marking schemes reward precise mechanistic language, not vague generalisations. The difference between a 3-mark and a 6-mark answer is almost always the level of mechanistic detail.

When asked to 'explain' how methylation silences a gene, you need a chain of causation: methylation added to cytosine at CpG sites → chromatin condenses → transcription factors cannot bind promoter → no transcription → no mRNA → no protein. Each arrow in that chain is a potential mark. Stopping at 'methylation prevents transcription' will not score full marks.

For 'evaluate' questions on epigenetic treatments, structure your answer as: mechanism of the drug → how this addresses the cancer biology → limitation or counter-consideration → conclusion. Examiners reward balanced judgements, not one-sided descriptions.

Worked example: Constructing a 6-Mark Evaluate Answer: Epigenetic Therapy for Cancer

Evaluate the use of HDAC inhibitors as a treatment for cancer. [6 marks]

1. HDAC inhibitors block histone deacetylase enzymes, preventing the removal of acetyl groups from histone tails. — This earns the first mechanism mark. Be specific — name the enzyme class being inhibited.
2. Acetyl groups remain on the histones, reducing the positive charge on histone tails. The attraction between histones and the negatively charged DNA backbone decreases, so chromatin remains in a relaxed (euchromatin) state. — This earns the chromatin-structure mark. The examiner wants the electrostatic reasoning, not just 'chromatin opens'.
3. Transcription factors and RNA polymerase can now access the promoter regions of tumour-suppressor genes that were previously silenced by deacetylation. These genes are reactivated, producing tumour-suppressor proteins that inhibit uncontrolled cell division. — This earns the therapeutic-effect mark. Use the word 'reactivate' to signal understanding that the gene was silenced, not deleted.
4. HDAC inhibitors such as vorinostat have been approved for cutaneous T-cell lymphoma, demonstrating clinical efficacy. — A named example strengthens an evaluate answer and signals application-level knowledge.
5. However, HDAC inhibitors act across the genome rather than at a single gene locus. This lack of specificity means that genes other than tumour suppressors may also be reactivated, potentially including genes with harmful effects. Side effects are therefore a significant concern. — Evaluation requires a counter-point. This is where many students lose marks — they describe the mechanism but never weigh up the evidence.
6. Overall, HDAC inhibitors represent a promising approach because they exploit the reversibility of epigenetic silencing to restore normal cell-cycle control. Their clinical use is currently limited to specific cancer types where the benefit-to-risk ratio is favourable, and further research is needed to improve target specificity. — A conclusion that weighs the evidence and reaches a judgement is required for full marks on an evaluate question. Avoid sitting on the fence — commit to a reasoned position.

Answer: A full 6-mark answer covers: HDAC inhibitor mechanism (blocks deacetylation) → chromatin remains relaxed → tumour-suppressor genes reactivated → cell division controlled → limitation (genome-wide effects, off-target risks) → balanced conclusion.

Taken together, RNAi and epigenetic regulation represent two distinct but complementary layers of gene expression control. RNAi operates post-transcriptionally, targeting mRNA for degradation or translational repression. Epigenetic mechanisms operate at the transcriptional level, controlling whether a gene can be transcribed at all. Both are disrupted in disease, and both are being exploited as therapeutic targets — making them not only central to the AQA specification but to the future of precision medicine.