A 28-year-old Indian male presents with progressive hearing loss and vision problems since age 20. Audiometry shows bilateral sensorineural hearing loss, and fundoscopy reveals retinitis pigmentosa. Genetic testing identifies a mutation in the PRPH2 gene (peripherin-2), which encodes a component of photoreceptor outer segment discs. RNA sequencing of his fibroblasts shows aberrant splicing of PRPH2 pre-mRNA, with skipping of exon 3. Which of the following best explains how this splicing defect contributes to the disease phenotype?

Explanation

## Splicing Defects and Nonsense-Mediated Decay: PRPH2 Mutations ### Background: Exon Skipping and Its Consequences **Key Point:** When an exon is skipped during pre-mRNA splicing, the most common outcome depends on whether the skipped exon's nucleotide length is a multiple of 3. If it is NOT a multiple of 3, the reading frame is disrupted, introducing a **premature termination codon (PTC)**. Even if the exon IS a multiple of 3 (in-frame), the resulting protein may still be non-functional — but the dominant molecular mechanism for disease in most splicing mutations is **NMD**, not in-frame domain deletion. ### Why Option C is Correct Exon 3 skipping in PRPH2 pre-mRNA introduces a **premature termination codon (PTC)** due to a frameshift caused by the altered reading frame at the new exon-exon junction. This PTC is recognized by the **nonsense-mediated decay (NMD)** surveillance pathway: 1. During the pioneer round of translation, ribosomes encounter the PTC upstream of an exon-junction complex (EJC) 2. UPF1, UPF2, and UPF3 proteins are recruited, triggering mRNA degradation 3. The aberrant PRPH2 mRNA is rapidly degraded before functional protein can accumulate 4. Loss of functional peripherin-2 → failure of photoreceptor outer segment disc assembly → photoreceptor degeneration → retinitis pigmentosa ### NMD Pathway Summary | Step | Component | Role | |---|---|---| | PTC recognition | Ribosome + UPF1 | Stalls at premature stop codon | | EJC interaction | UPF2/UPF3 | Signals downstream EJC presence | | mRNA degradation | Decapping enzymes, exonucleases | Rapid mRNA turnover | | Protein loss | — | No functional peripherin-2 produced | **High-Yield:** NMD is the **most common and well-established** consequence of exon skipping mutations that introduce PTCs. This is a **loss-of-function** mechanism via mRNA surveillance, not structural domain deletion. ### Why Other Options Are Incorrect - **Option A:** While U1 snRNP recruitment failure at the 5' splice site is a valid mechanism for splice site mutations, the question states the result is exon 3 *skipping* — the question asks what the skipping *contributes* to disease, not the upstream cause. - **Option B:** An in-frame deletion removing a TM domain is theoretically possible if exon 3 is exactly divisible by 3, but this specific mechanism for PRPH2 exon 3 is not established in standard biochemistry textbooks (Harrison's, Robbins), and NMD is the more generalizable and exam-relevant mechanism for exon skipping. - **Option D:** Read-through into the 3' UTR producing a toxic gain-of-function domain is not a recognized consequence of exon skipping; this is not a standard molecular mechanism. ### Clinical Pearl Retinitis pigmentosa (RP) caused by PRPH2 mutations presents with: - Progressive night blindness (rods affected first) - Peripheral visual field constriction - Bone-spicule pigmentation on fundoscopy - Sensorineural hearing loss in syndromic forms (as in this vignette) **Mnemonic:** **Exon Skip → Frame Shift → PTC → NMD → No Protein → No Function** — The NMD pathway is the cell's quality control mechanism that degrades mRNAs harboring PTCs, resulting in loss-of-function disease. [cite: Strachan & Read, Human Molecular Genetics; Harrison's Principles of Internal Medicine; Robbins & Cotran Pathologic Basis of Disease]