A 28-year-old man from Bangalore is diagnosed with β-thalassemia major after presenting with severe hemolytic anemia, splenomegaly, and transfusion-dependent anemia since childhood. Molecular analysis reveals a mutation in the polyadenylation signal (AATAAA → AATGAA) in the 3′ untranslated region of the β-globin gene. Which of the following best explains why this mutation causes β-thalassemia despite the coding sequence being intact?

Explanation

## Polyadenylation Signal Mutations and mRNA Stability ### Normal Polyadenylation Process **Key Point:** The polyadenylation signal (AATAAA or variants) is located 10–30 nucleotides upstream of the cleavage site in the 3′ UTR. It is recognized by cleavage and polyadenylation specificity factor (CPSF), which recruits endonucleases to cleave the transcript and poly(A) polymerase to add ~200 adenine residues. ### Consequence of AATAAA → AATGAA Mutation 1. **Loss of canonical signal recognition:** The mutated sequence (AATGAA) is not recognized by CPSF 2. **Transcriptional read-through:** RNA polymerase II continues transcribing past the normal termination point 3. **Unstable transcript:** The mRNA lacks the poly(A) tail, which normally: - Protects the 3′ end from exonucleases - Enhances translation efficiency - Increases mRNA half-life 4. **Rapid degradation:** The unpolyadenylated transcript is rapidly degraded by 3′→5′ exonucleases 5. **Result:** Severe reduction in β-globin mRNA and protein levels → β-thalassemia major **High-Yield:** Polyadenylation signal mutations account for ~5–10% of β-thalassemia cases. They are a classic example of how mutations outside the coding sequence can cause severe disease. ### Molecular Mechanism Diagram ```mermaid flowchart TD A["Normal β-globin gene<br/>with AATAAA signal"]:::outcome --> B["CPSF recognizes<br/>polyadenylation signal"]:::action B --> C["3' cleavage and<br/>poly-A addition"]:::action C --> D["Stable mRNA<br/>with poly-A tail"]:::outcome D --> E["Efficient translation<br/>Normal β-globin levels"]:::outcome F["Mutant: AATGAA signal"]:::urgent --> G["CPSF does NOT<br/>recognize signal"]:::action G --> H["Transcriptional<br/>read-through"]:::action H --> I["No cleavage,<br/>no polyadenylation"]:::action I --> J["Unstable mRNA<br/>3' exonuclease degradation"]:::urgent J --> K["Severe ↓ β-globin mRNA<br/>and protein"]:::urgent K --> L["β-Thalassemia major"]:::outcome ``` ### Why Coding Sequence Remains Intact The mutation is in the **3′ UTR**, not in the exons encoding the β-globin protein. The coding sequence is transcribed normally, but the transcript is unstable and degraded before it can be translated efficiently. **Clinical Pearl:** β-Thalassemia mutations can be classified by mechanism: - **Transcriptional:** Promoter mutations, TATA box mutations - **RNA processing:** Splice site mutations, polyadenylation signal mutations - **Translational:** Start codon mutations, nonsense mutations - **Post-translational:** Mutations affecting protein stability Polyadenylation mutations are RNA processing defects that reduce mRNA stability rather than affecting the protein itself. ### Comparison with Other 3′ UTR Mutations | Mutation Type | Location | Effect | Result | | --- | --- | --- | --- | | **Polyadenylation signal** | 3′ UTR (10–30 nt before cleavage site) | Loss of CPSF recognition, read-through, no poly-A | Unstable mRNA, severe ↓ protein | | **Cryptic polyadenylation** | 3′ UTR (upstream) | Premature cleavage | Truncated mRNA, loss of stability elements | | **Iron-responsive element (IRE)** | 5′ UTR | Altered iron regulation | Variable translation based on iron status | **Mnemonic:** **PASS** — Polyadenylation signal, Absent → Stability lost, Severe degradation.