A 28-year-old man with suspected hereditary hemochromatosis (HFE gene mutation) has been referred for genetic confirmation. Initial PCR-based screening for the two common HFE mutations (C282Y and H63D) was negative. Which investigation is most appropriate to detect rare or novel HFE mutations?

Explanation

## Investigation of Choice for Detecting Rare HFE Mutations ### Clinical Scenario Common HFE mutations (C282Y, H63D) are negative, but clinical suspicion remains high. The patient likely carries a rare or novel mutation not detected by targeted PCR screening. ### Why Direct DNA Sequencing is Optimal **Key Point:** Direct DNA sequencing (Sanger or NGS) is the gold standard for identifying rare, novel, or atypical variants when targeted mutation screening is negative but clinical suspicion persists. **High-Yield:** When standard PCR panels fail to identify mutations in a clinically suspected genetic disorder, sequencing the entire coding sequence and regulatory regions of the gene is the next logical step—it is unbiased and detects all sequence variants. ### Comparison of Techniques for Rare Mutation Detection | Technique | Detects | Best Use | Limitation in This Case | |-----------|---------|----------|-------------------------| | **DNA Sequencing** | All point mutations, indels, splice variants | Rare/novel mutations; comprehensive | Gold standard—no limitation | | RFLP | Only mutations creating/abolishing restriction sites | Known mutations with known sites | Misses most rare variants; requires prior knowledge | | Southern Blot | Large deletions, rearrangements, copy number changes | Structural variants | Insensitive for point mutations; labour-intensive | | qRT-PCR | mRNA expression level | Measuring transcript abundance | Detects quantity, not sequence; cannot identify mutations | ### Mechanism of Sequencing for Rare Mutations 1. **PCR amplification** of all HFE exons (or whole gene) 2. **Sanger sequencing** reads the DNA sequence base-by-base 3. **Sequence alignment** to reference HFE sequence identifies all variants 4. **Variant classification** determines pathogenicity (benign vs. disease-causing) 5. **Functional studies** may confirm pathogenicity of novel mutations **Clinical Pearl:** Hereditary hemochromatosis is genetically heterogeneous—approximately 5–10% of clinically affected patients carry mutations in non-HFE genes (HJV, HAMP, TfR2, FPN1) or rare HFE variants. Sequencing is essential when common mutations are excluded. **Mnemonic: "SEQUENCE for RARE" — When Screening Fails** - **S**equencing = unbiased, detects all variants - **E**xons amplified by PCR - **Q**uery the entire gene, not just known sites - **U**nknown variants require comprehensive approach - **E**xclusion of common mutations → think rare/novel - **N**ext step after negative targeted screening - **C**omprehensive coverage of coding and regulatory regions - **E**fficient single assay for variant identification