A 32-year-old woman from Mumbai presents to the genetics clinic with a family history of progressive muscle weakness. Her father and paternal uncle both developed symptoms in their 40s and are now wheelchair-bound. Genetic testing reveals a mutation in the DMD gene (dystrophin gene) affecting the splice donor site of exon 45. The mutation does not completely prevent splicing but causes aberrant splicing with inclusion of a cryptic exon. Which of the following best explains the molecular consequence of this mutation and the resulting clinical phenotype?

Explanation

## Molecular Basis of the Mutation **Key Point:** A mutation at the splice donor site (GT sequence at the beginning of an intron) prevents recognition by the spliceosome, forcing the machinery to use a cryptic (downstream) splice site. ### Consequence of Cryptic Splice Site Activation When the normal splice donor site is mutated: 1. The spliceosome cannot recognize the canonical GT dinucleotide 2. It scans downstream and identifies a cryptic splice site within exon 45 or the following intron 3. This causes **out-of-frame splicing** — the exon junction occurs at an abnormal position 4. The reading frame is disrupted, introducing a **frameshift mutation** 5. This leads to premature termination codons (PTCs) and truncated, non-functional dystrophin **High-Yield:** Splice site mutations (especially at ±1 or ±2 positions from exon–intron boundaries) are among the most deleterious mutations because they disrupt the normal reading frame and almost always result in severe loss-of-function phenotypes. ### Clinical Correlation | Feature | Duchenne (DMD) | Becker (BMD) | | --- | --- | --- | | **Dystrophin** | Absent or <3% | 3–20% or truncated but partially functional | | **Reading frame** | Out-of-frame | In-frame | | **Onset** | <5 years | 5–25 years | | **Severity** | Severe, wheelchair by 12 years | Mild to moderate, variable progression | **Clinical Pearl:** The "reading frame rule" predicts phenotype severity: out-of-frame deletions/mutations → DMD (severe); in-frame → BMD (mild). A cryptic splice site causing frameshift produces a severe DMD phenotype, consistent with the family history of early-onset, progressive weakness. ### Why This Is Not Exon Skipping Exon skipping (option A) would occur if the entire exon 45 were excluded from the transcript. This would be in-frame (if exon 45 is a multiple of 3 nucleotides) and produce a milder BMD phenotype. However, the mutation described is at the splice donor site, not a complete exon deletion — the cryptic site causes **aberrant splicing within or near the exon**, not its exclusion. **Mnemonic:** **CFDS** — Cryptic Frameshift Donor Site → Severe phenotype.