A 28-year-old man from Bangalore presents to the hematology clinic with a family history of sickle cell disease. His mother is a known carrier, and his father is unaffected. The patient is asymptomatic but wants to understand his genetic status. Molecular testing is ordered to detect the point mutation (A→T transversion at codon 6 of β-globin) that causes sickle cell disease. The laboratory initially performs PCR to amplify the β-globin gene region, then performs a second technique to identify the specific nucleotide change. Which of the following is the most appropriate second technique for identifying this point mutation after PCR amplification?

Explanation

## Detecting the Sickle Cell Mutation: RFLP Analysis ### Clinical Context The sickle cell mutation (GAG → GTG at codon 6 of β-globin) creates or destroys a restriction enzyme recognition site. After PCR amplification of the β-globin region, the next step is to identify which allele(s) the patient carries. RFLP is a classical, rapid, and cost-effective method for this. ### Why RFLP is the Answer **Key Point:** The sickle cell mutation (A→T at the second nucleotide of codon 6) destroys the recognition site for the restriction enzyme **MstII** (or similar enzymes like DdeI). The normal allele is cut; the sickle allele is not. **High-Yield:** RFLP exploits the fact that point mutations can create or destroy restriction enzyme recognition sites, producing different fragment patterns on gel electrophoresis. ### Mechanism of RFLP for Sickle Cell Detection 1. **Normal β-globin (GAG)**: MstII recognizes and cuts the normal sequence → produces smaller fragments 2. **Sickle β-globin (GTG)**: MstII site is destroyed → PCR product remains uncut (larger fragment) 3. **Heterozygote (carrier)**: Both cut and uncut fragments appear on gel **Clinical Pearl:** A simple agarose gel showing fragment patterns immediately tells you: - **Two bands** (cut + uncut) = heterozygous carrier - **Only uncut band** = homozygous sickle cell disease - **Only cut bands** = homozygous normal ### Why Other Techniques Are Suboptimal for This Scenario | Technique | Why Not Ideal for Sickle Cell Screening | |-----------|------------------------------------------| | **Direct DNA sequencing** | Accurate but expensive, slow, and overkill for a known mutation; not cost-effective for population screening | | **Northern blot** | Detects mRNA, not DNA; requires fresh tissue; cannot distinguish genotypes reliably; time-consuming | | **Heteroduplex analysis + DGGE** | More complex; requires specialized equipment; slower than RFLP; better for unknown mutations | **Mnemonic:** **RFLP = Restriction Fragment Length Polymorphism** — uses restriction enzymes to cut DNA at specific sites; mutations that destroy the site produce longer fragments. ### Workflow: PCR → RFLP → Interpretation ```mermaid flowchart TD A[Patient blood sample]:::outcome --> B[PCR amplification of β-globin region]:::action B --> C[PCR product ~1.3 kb]:::outcome C --> D[Digest with MstII restriction enzyme]:::action D --> E[Agarose gel electrophoresis]:::action E --> F{Fragment pattern?}:::decision F -->|Two bands: 1.3 kb + 0.9 kb| G[Heterozygous carrier]:::outcome F -->|Only 1.3 kb uncut| H[Homozygous sickle cell disease]:::outcome F -->|Only 0.9 kb cut| I[Homozygous normal]:::outcome G --> J[Genetic counseling: 50% risk to offspring if partner is normal]:::action H --> K[Clinical monitoring: risk of vaso-occlusive crises]:::action I --> L[No further action needed]:::action ``` **Tip:** RFLP was the gold standard for sickle cell screening before DNA sequencing became affordable. It remains widely used in resource-limited settings because it requires only PCR, restriction enzymes, and agarose gel — no expensive sequencing equipment.