A 34-year-old Indian woman with a history of recurrent infections presents with fatigue and jaundice. Laboratory findings show: Hb 9.8 g/dL, MCV 78 fL, reticulocyte count 6%, and peripheral smear reveals polychromasia with occasional spherocytes. Flow cytometry-based RBC osmotic fragility test is positive. Her 8-year-old son has similar clinical features. Genetic testing reveals a mutation in the G6PD gene (hemizygous in the son, heterozygous in the mother). Which of the following best explains why this patient's haemolytic episodes are typically milder and less frequent than those of her son?

Explanation

## X-Linked Inheritance and G6PD Expression in Females ### Understanding X-Inactivation (Lyonization) **Key Point:** G6PD deficiency is an X-linked recessive disorder. Heterozygous females undergo random X-inactivation, resulting in two populations of RBCs: some expressing normal G6PD (from the active normal X chromosome) and some expressing deficient G6PD (from the active mutant X chromosome). **High-Yield:** This mosaicism protects heterozygous females from severe, recurrent haemolytic episodes because approximately 50% of their RBCs retain normal G6PD activity and can generate adequate NADPH to resist oxidative stress. ### Comparison: Heterozygous Female vs. Hemizygous Male | Feature | Heterozygous Female | Hemizygous Male | |---------|-------------------|------------------| | **G6PD-expressing RBCs** | ~50% (from active normal X) | 100% (single X chromosome) | | **G6PD-deficient RBCs** | ~50% (from active mutant X) | 100% (single X chromosome) | | **Average G6PD activity** | ~50% of normal | <10% of normal (depending on mutation) | | **Haemolytic episodes** | Milder, less frequent, triggered only by severe oxidative stress | Severe, recurrent, triggered by mild oxidative stress | | **Baseline haemolysis** | Usually absent or minimal | May be present even at baseline | | **Response to fava beans** | Mild to moderate haemolysis | Acute, life-threatening haemolysis | **Clinical Pearl:** The severity of G6PD deficiency in heterozygous females depends on the **skew of X-inactivation**. If inactivation is heavily skewed toward the normal X chromosome, the female may be clinically asymptomatic. If skewed toward the mutant X, she may have symptoms approaching those of hemizygous males. ### Molecular Basis of X-Inactivation ```mermaid flowchart TD A[Heterozygous Female<br/>X^N X^m]:::outcome --> B[Early Development<br/>Random X-inactivation]:::action B --> C[Cell Population 1<br/>X^N active, X^m inactivated]:::outcome B --> D[Cell Population 2<br/>X^m active, X^N inactivated]:::outcome C --> E[Normal G6PD expression<br/>NADPH production intact]:::action D --> F[Deficient G6PD expression<br/>NADPH production impaired]:::action E --> G[RBCs resistant to oxidative stress]:::outcome F --> H[RBCs susceptible to oxidative stress]:::outcome G --> I[Overall RBC population<br/>~50% protected]:::action H --> I I --> J[Milder haemolytic episodes<br/>than hemizygous males]:::outcome ``` **Mnemonic:** **LION** = **L**yonization (X-inactivation) **I**n **O**ne **N**ormal allele (heterozygous females). This protects them from severe haemolysis. ### Why This Patient's Course is Milder 1. **Mosaic RBC population:** Approximately 50% of her RBCs express normal G6PD and can generate sufficient NADPH to maintain GSH pools and defend against oxidative stress. 2. **Functional redundancy:** Even mild oxidative stress will preferentially affect the G6PD-deficient RBCs; the normal RBCs remain protected. 3. **Higher threshold for haemolysis:** A more severe oxidative insult is required to trigger haemolysis in heterozygous females compared to hemizygous males. 4. **Son's severe phenotype:** Her son, being hemizygous, has 100% of his RBCs expressing the mutant G6PD allele, making him vulnerable to haemolysis from even mild oxidative triggers. **Warning:** Do not confuse X-inactivation with gene dosage compensation. X-inactivation is the mechanism by which females achieve approximate dosage compensation with males for X-linked genes, but it results in cellular mosaicism, not uniform reduction of gene expression.