A 2-year-old girl from Tamil Nadu presents with failure to thrive, recurrent respiratory infections, and progressive loss of developmental milestones. On examination, she has marked hepatosplenomegaly and a vertical supranuclear gaze palsy. Bone marrow aspiration shows foamy macrophages. Filipin staining of cultured fibroblasts shows abnormal cholesterol accumulation. Which enzyme deficiency is responsible for this disorder?

Explanation

## Clinical Diagnosis: Niemann-Pick Disease Type C **Key Point:** Niemann-Pick disease type C (NP-C) is characterized by a defect in intracellular cholesterol trafficking, NOT a lysosomal hydrolase deficiency. This is a critical distinction in the lysosomal storage disorder spectrum. ### Pathophysiology of NP-C **High-Yield:** NP-C results from mutations in NPC1 or NPC2 genes, which encode proteins responsible for cholesterol egress from lysosomes. Without functional transport, cholesterol accumulates intracellularly, particularly in lysosomes and late endosomes. ### Diagnostic Features: NP-C vs. Other Niemann-Pick Types | Feature | NP-C (Cholesterol Transport) | NP-A (Acid Sphingomyelinase) | NP-B (Acid Sphingomyelinase) | |---------|------------------------------|------------------------------|------------------------------| | **Enzyme deficiency** | NPC1/NPC2 protein (transporter) | Acid sphingomyelinase | Acid sphingomyelinase (partial) | | **Substrate** | Cholesterol (+ sphingomyelin) | Sphingomyelin | Sphingomyelin | | **Vertical supranuclear gaze palsy** | YES (hallmark) | No | No | | **Hepatosplenomegaly** | Marked | Marked | Marked | | **Neurological onset** | Variable (1–6 years typical) | Early (3–6 months) | Later (infancy–childhood) | | **Cherry-red spot** | Rare | Yes (type A) | No | | **Filipin staining** | Abnormal cholesterol accumulation | Normal pattern | Normal pattern | | **Foamy macrophages** | Yes | Yes | Yes | **Clinical Pearl:** The **vertical supranuclear gaze palsy** (inability to look upward voluntarily, but preserved reflex upward gaze) is a neurological hallmark of NP-C and helps distinguish it from types A and B. ### Why Filipin Staining is Diagnostic Filippin is a fluorescent dye that binds free cholesterol. In NP-C fibroblasts, filipin staining reveals: - Abnormal accumulation of cholesterol in lysosomes and late endosomes - Perinuclear and cytoplasmic fluorescence pattern - This pattern is **pathognomonic** for NP-C and is the gold-standard diagnostic test **Mnemonic: NPC Hallmarks** — **N**eurological (vertical gaze palsy), **P**rogressive (hepatosplenomegaly), **C**holesterol (accumulation on filipin staining) ### Clinical Presentation Timeline ```mermaid flowchart TD A[Niemann-Pick Disease Type C]:::outcome --> B[Defective cholesterol transport<br/>NPC1/NPC2 mutation]:::outcome B --> C[Cholesterol accumulation<br/>in lysosomes & late endosomes]:::outcome C --> D{Age of onset?}:::decision D -->|Early infantile<br/>< 1 year| E[Severe hepatosplenomegaly<br/>Rapid neurodegeneration]:::urgent D -->|Late infantile<br/>1-6 years| F[Progressive gaze palsy<br/>Ataxia, cognitive decline]:::action D -->|Juvenile/Adult<br/>> 6 years| G[Slow neurological decline<br/>Psychiatric symptoms]:::action E --> H[Vertical supranuclear gaze palsy<br/>Foamy macrophages<br/>Filipin+ fibroblasts]:::outcome F --> H G --> H ``` **High-Yield:** NP-C is the most common lysosomal storage disorder presenting with vertical supranuclear gaze palsy. This single finding should trigger immediate filipin staining and NPC1/NPC2 genetic testing. [cite:Robbins 10e Ch 5]