Which of the following is the most common type of RNA splicing defect associated with β-thalassemia mutations in Indian populations?

Explanation

## RNA Splicing Defects in β-Thalassemia: Indian Epidemiology ### Prevalence of Splice Site Mutations in Indian β-Thalassemia **Key Point:** Mutations affecting the 5' splice site (donor site) consensus sequence of intron 1 or intron 2 are the most common molecular defects causing β-thalassemia in Indian populations, accounting for approximately 30–40% of all β-thalassemia mutations. **High-Yield:** The canonical 5' splice site consensus is **GU** at the beginning of the intron. Mutations at the first (G) or second (U) nucleotide of the intron, or at the −1 position of the exon, severely impair or abolish splicing, leading to exon skipping and absent or truncated β-globin mRNA. ### Mechanism of 5' Splice Site Mutations 1. **Disruption of snRNP Recognition:** The U1 snRNP (small nuclear ribonucleoprotein) recognizes the 5' splice site via base-pairing. Mutations in the GU dinucleotide or flanking positions prevent U1 binding. 2. **Exon Skipping:** The spliceosome cannot assemble properly, leading to skipping of the downstream exon and frameshift or nonsense mutations in the mature mRNA. 3. **Reduced β-Globin Synthesis:** Truncated or absent β-globin protein results in ineffective erythropoiesis and hemolytic anemia. ### Comparative Table: β-Thalassemia Molecular Defects | Defect Type | Mechanism | Frequency in Indian Populations | Example | | --- | --- | --- | --- | | **5' splice site mutations (intron 1/2)** | Disrupts GU consensus; exon skipping | 30–40% | IVS1-1 (G→A), IVS2-1 (G→A) | | Cryptic splice site activation | Creates alternative splice site in exon | 10–15% | IVS1-130 (G→C) activates cryptic 3' site | | 3' splice site mutations | Disrupts AG consensus; exon skipping | 5–10% | Rare in Indian populations | | Exon deletions | Complete loss of exonic sequence | <5% | Large deletions affecting exons 2–3 | **Clinical Pearl:** The IVS1-1 (G→A) mutation at the 5' splice site of intron 1 is the **single most common β-thalassemia mutation in Indian populations**, present in 15–20% of all β-thalassemia alleles. This mutation completely abolishes normal splicing, resulting in β°-thalassemia (no β-globin production). **Mnemonic:** **5' Site Splice = Severe Splicing Shutdown** — mutations at the 5' splice site consensus (GU) are the most frequent and most severe splicing defects in Indian β-thalassemia. ### Why 5' Splice Site Mutations Are Most Common - **Structural Vulnerability:** The 5' splice site GU dinucleotide is highly conserved and critical for U1 snRNP binding; even single-nucleotide changes are catastrophic. - **High Mutation Rate:** Introns 1 and 2 of the β-globin gene are hotspots for point mutations in Indian populations due to founder effects and genetic drift. - **Complete Loss of Function:** These mutations typically result in β°-thalassemia (complete absence of β-globin), whereas some other mutations (e.g., cryptic splice sites) may allow partial splicing and produce β+-thalassemia. [cite:Weatherall Disorders of the Globin Genes Ch 8; Huisman The Hemoglobinopathies and Thalassemias 2e]