A 34-year-old Indian woman with a history of recurrent infections and delayed wound healing presents with oral ulcers and bleeding gums. Investigations reveal low white blood cell count (2.8 × 10⁹/L) and low platelet count (85 × 10⁹/L). A bone marrow biopsy shows maturation arrest in myeloid and megakaryocytic lineages. Genetic testing confirms a mutation in the NADPH oxidase complex. Which metabolic pathway is primarily impaired in this patient, and what is the consequence?

Explanation

## Chronic Granulomatous Disease (CGD) and the Pentose Phosphate Pathway ### Clinical Presentation and Diagnosis This patient presents with **Chronic Granulomatous Disease (CGD)**, a primary immunodeficiency characterized by: - Recurrent infections (catalase-positive organisms: *Staphylococcus aureus*, *Burkholderia cepacia*, *Serratia*, *Nocardia*, *Aspergillus*) - Delayed wound healing - Oral ulcers and gingivitis - Cytopenias (due to bone marrow involvement) - Defective NADPH oxidase complex **Key Point:** CGD is caused by mutations in genes encoding NADPH oxidase subunits (most commonly *CYBB* on the X chromosome, or *CYBA*, *NCF1*, *NCF2*, *NCF4* on autosomes). ### Role of the Pentose Phosphate Pathway in Phagocytes The PPP is the **primary source of NADPH** in phagocytes. NADPH is absolutely essential for: 1. **NADPH oxidase (NOX2) function** — Catalyzes the respiratory burst: $$2O_2 + NADPH \xrightarrow{NADPH\,oxidase} 2O_2^{\bullet-} + NADP^+ + H^+$$ 2. **Superoxide generation** — O₂⁻ is converted to other ROS (H₂O₂, HOCl) by myeloperoxidase and other enzymes 3. **Microbial killing** — ROS directly damage bacterial cell walls, DNA, and proteins ### Biochemical Mechanism of Defective Microbial Killing ```mermaid flowchart TD A[Glucose-6-phosphate]:::outcome --> B[Pentose Phosphate Pathway]:::action B --> C[NADPH generation]:::outcome C --> D[NADPH oxidase complex activated]:::action D --> E[Superoxide anion O₂⁻ produced]:::outcome E --> F[Conversion to H₂O₂ and HOCl]:::action F --> G[Microbial killing]:::outcome H[NADPH oxidase mutation]:::urgent --> I[No superoxide generation]:::urgent I --> J[Loss of respiratory burst]:::urgent J --> K[Impaired microbial killing]:::urgent K --> L[Recurrent infections]:::urgent ``` ### Diagnostic Confirmation | Test | Finding in CGD | |------|----------------| | **Nitroblue tetrazolium (NBT) test** | No colour change (cannot generate O₂⁻) | | **Dihydrorhodamine (DHR) flow cytometry** | No fluorescence (no ROS production) | | **Phagocytic function** | Normal migration and chemotaxis; defective killing | | **Respiratory burst** | Absent | **High-Yield:** CGD patients have **normal phagocytosis** (they can engulf bacteria) but **defective intracellular killing** because they cannot generate the ROS needed to destroy microbes. **Clinical Pearl:** Patients with CGD benefit from prophylactic antibiotics (trimethoprim-sulfamethoxazole) and interferon-gamma therapy, which enhances NADPH oxidase expression in residual functional cells. **Mnemonic:** **PPP → NADPH → NOX2 → ROS → Killing** — Pentose Phosphate Pathway provides NADPH for NADPH Oxidase to generate Reactive Oxygen Species for microbial killing.