A 28-year-old man from Delhi presents with recurrent infections, hepatosplenomegaly, and anaemia. Bone marrow examination shows abnormal lipid-laden macrophages (Gaucher cells). Gaucher disease (β-glucosidase deficiency) is suspected. Genetic testing reveals a compound heterozygous genotype with one known pathogenic allele and one novel variant of uncertain significance (VUS) in the GBA gene. The laboratory has already performed PCR and direct sequencing. What is the most appropriate next step to determine the pathogenicity of the novel VUS?

Explanation

## Interpreting Variants of Uncertain Significance (VUS) in Monogenic Disease ### Clinical Context Gaucher disease is an autosomal recessive lysosomal storage disorder caused by loss-of-function mutations in the GBA gene (encoding β-glucosidase). The patient has a compound heterozygous genotype: one known pathogenic allele and one novel VUS. Simply sequencing the variant is not enough—its functional consequence must be determined to confirm disease causation. ### Why Functional Assays + In Silico Prediction is the Best Next Step **Key Point:** A VUS cannot be classified as pathogenic based on sequencing alone. The ACMG/AMP guidelines (2015) require integration of: 1. **Functional evidence** — does the variant impair enzyme activity or protein stability? 2. **In silico predictions** — computational tools (SIFT, PolyPhen-2, CADD) assess conservation and structural impact 3. **Population frequency** — is the variant absent in healthy controls? 4. **Segregation data** — does it co-segregate with disease in the family? **High-Yield:** For lysosomal storage disorders like Gaucher disease, functional assays are particularly valuable because: - β-glucosidase activity can be directly measured in patient fibroblasts or lymphoblasts - Protein expression and stability can be assessed by Western blot - Substrate accumulation can be quantified - These data directly link the VUS to the biochemical phenotype ### Comparison of Approaches | Approach | Determines Pathogenicity | Mechanism | Timeframe | Cost | Utility | |----------|--------------------------|-----------|-----------|------|----------| | **Functional assays + in silico** | ✓ Yes, gold standard | Direct enzyme/protein analysis | 2–4 weeks | Moderate–High | **Best for VUS classification** | | Southern blot | ✗ No | Detects large deletions only | 5–7 days | Moderate | Rearrangements, not point mutations | | Allele-specific PCR + segregation | ~ Partial | Segregation pattern only | 1–2 weeks | Low | Supports pathogenicity if segregates; not sufficient alone | | Repeat sequencing | ✗ No | Confirms sequence, not function | 1–2 weeks | Low | Rules out technical error; does not assess pathogenicity | **Clinical Pearl:** In Gaucher disease, a VUS that reduces β-glucosidase activity to <30% of normal is considered pathogenic. Functional data are often more informative than segregation analysis, especially in small families or de novo variants. ### Workflow for VUS Classification ```mermaid flowchart TD A[Novel VUS identified by sequencing]:::outcome --> B[Check population databases]:::action B --> C{Rare in healthy controls?}:::decision C -->|No| D[Likely benign polymorphism]:::outcome C -->|Yes| E[Perform functional assays]:::action E --> F[Measure β-glucosidase activity]:::action F --> G{Activity < 30% of normal?}:::decision G -->|Yes| H[Perform in silico prediction]:::action G -->|No| I[Likely benign]:::outcome H --> J{Predicted deleterious?}:::decision J -->|Yes| K[Classify as pathogenic]:::outcome J -->|No| L[Classify as VUS]:::outcome E --> M[Assess protein expression/stability]:::action M --> N{Protein misfolded or unstable?}:::decision N -->|Yes| H N -->|No| I ``` **Mnemonic:** **FAP** — **F**unctional assays + **A**ssessment of **P**rotein (and in silico prediction) for VUS classification. [cite:Robbins 10e Ch 5; Harrison 21e Ch 355]