Which is the most common type of RNA processing defect causing β-thalassemia major in the Indian population?

Explanation

## Most Common RNA Processing Defect in β-Thalassemia ### Epidemiology in Indian Population **Key Point:** Mutations affecting the 5' splice site of the β-globin pre-mRNA account for approximately 30–40% of β-thalassemia cases in India, making it the single most common molecular defect in this population. ### Common β-Thalassemia Mutations in India | Mutation Type | Frequency in India | Mechanism | Clinical Phenotype | |---|---|---|---| | **5' splice site mutations** | **30–40%** | Exon skipping, intron retention | β-thalassemia major/intermedia | | TATA box mutations | 15–20% | Reduced transcription initiation | β-thalassemia intermedia | | 3' UTR mutations | 10–15% | mRNA instability, reduced translation | β-thalassemia intermedia | | Polyadenylation defects | 5–10% | Impaired mRNA processing, degradation | β-thalassemia major | | Gene deletions | 5–10% | Complete loss of β-globin | β-thalassemia major | ### Specific 5' Splice Site Mutations in β-Thalassemia **High-Yield:** The most frequent 5' splice site mutation in Indian β-thalassemia is **IVS1-5 (G→A)**, located at position +5 of intron 1. This single nucleotide substitution prevents U1 snRNP recognition, causing exon 1 skipping and production of a non-functional β-globin transcript. ### Pathophysiology of Splicing Defects 1. **Normal splicing:** U1 snRNP recognizes GU at intron start → spliceosome assembly → exon ligation 2. **With IVS1-5 G→A mutation:** Altered consensus sequence → U1 snRNP fails to bind → exon 1 skipped 3. **Result:** Truncated β-globin protein (if any translation occurs) → hemolysis, anemia 4. **Severity:** Depends on whether cryptic splice sites activate (producing some normal transcript) **Clinical Pearl:** Patients with 5' splice site mutations typically present with β-thalassemia major (severe phenotype) because the mutation completely abolishes normal splicing, leaving no functional β-globin mRNA. **Mnemonic:** **IVS-5-G→A** — **I**ntron **V**ariant **S**ite at position **5**, **G** to **A** substitution is the **most common Indian mutation**. ### Why 5' Splice Site Mutations Predominate - Highest mutation rate among all splicing signals (as discussed in Question 1) - Single nucleotide changes have maximal impact on U1 snRNP binding - No redundancy or compensatory mechanisms - Geographic clustering: IVS1-5 G→A is endemic in Mediterranean and Indian populations