## Investigation of Choice: DNA Sequencing of Regulatory Regions ### Why This Is Correct **Key Point:** HPFH is caused by mutations in *cis*-acting transcriptional regulatory elements (promoter, silencer regions) of the γ-globin gene cluster, NOT coding sequence mutations. These regulatory mutations prevent the normal silencing of fetal hemoglobin in adulthood. **High-Yield:** The most specific investigation to identify transcription regulation defects is **direct DNA sequencing** of: - γ-globin promoter (−200 to −30 bp region) - Upstream regulatory elements (−1000 to −500 bp) - Silencer sequences in the 3′ UTR and intergenic regions This reveals point mutations (e.g., −175 T→C, −198 C→T) that disrupt transcription factor binding sites and prevent normal developmental silencing of fetal globin. ### Diagnostic Algorithm ```mermaid flowchart TD A[Elevated HbF on electrophoresis]:::outcome --> B{Suspected transcription defect?}:::decision B -->|Yes| C[DNA sequence regulatory regions]:::action C --> D[Identify mutations in promoter/silencers]:::outcome B -->|No| E[Sequence coding region for deletions]:::action E --> F[Identify structural variants]:::outcome ``` ### Why Sequencing Targets Regulatory DNA | Feature | Location | Mutation Type | Detection Method | |---------|----------|---------------|------------------| | Promoter mutations | −200 to −30 bp | Point mutations | DNA sequencing | | Silencer mutations | 3′ UTR, intergenic | Point mutations | DNA sequencing | | Deletions | Large regions | Structural variants | Southern blot OR NGS | | Coding mutations | Exons | Missense/nonsense | DNA sequencing | **Clinical Pearl:** HPFH patients have NO hemolytic anemia or clinical symptoms because fetal hemoglobin production is benign and compensatory. The diagnosis is purely molecular/genetic. ### Citation Context [cite:Molecular Biology of the Gene, Watson et al. Ch 17 — Eukaryotic Transcriptional Regulation]
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