A four-year-old child is diagnosed with Duchenne muscular dystrophy, an X-linked recessive disorder. Genetic anylysis shows that the patient&;s gene for the muscle protein dystrophin contains a mutation in its promoter region. What would be the most likely effect of this mutation?
**Core Concept**
Duchenne muscular dystrophy (DMD) is an X-linked recessive disorder caused by mutations in the dystrophin gene, leading to the absence or severe deficiency of the dystrophin protein. The dystrophin gene contains 79 exons, and mutations in this gene can occur in various regions, including the promoter region. The promoter region is essential for the initiation of gene transcription, as it provides the binding site for RNA polymerase and other transcription factors.
**Why the Correct Answer is Right**
A mutation in the promoter region of the dystrophin gene would disrupt the binding of RNA polymerase and other transcription factors, making it difficult for the gene to initiate transcription. As a result, the transcription of the dystrophin gene would be reduced or completely halted. This would lead to a deficiency in dystrophin protein production, which is characteristic of DMD. The initiation of transcription is a critical step in gene expression, and mutations in the promoter region can have severe consequences on gene expression.
**Why Each Wrong Option is Incorrect**
* **Option A:** Tailing of dystrophin mRNA will be defective - The mutation in the promoter region would not directly affect the tailing of dystrophin mRNA, which is a post-transcriptional modification that occurs after transcription.
* **Option B:** Capping of dystropin mRNA will be defective - Similarly, the mutation in the promoter region would not directly affect the capping of dystrophin mRNA, which is another post-transcriptional modification.
* **Option C:** Termination of dystrophin transcription will be deficient - While mutations in the dystrophin gene can affect transcription termination, a mutation in the promoter region would more likely affect the initiation of transcription.
**Clinical Pearl / High-Yield Fact**
The promoter region is a critical regulatory element in gene expression, and mutations in this region can have severe consequences on gene expression, leading to various genetic disorders. In the case of DMD, mutations in the promoter region can lead to a deficiency in dystrophin protein production, resulting in muscle weakness and degeneration.
**Correct Answer:**
✓ Correct Answer: D. Initiation of dystrophin transcription will be deficient.