A 28-year-old woman from Kerala presents with a 6-month history of progressive joint pain, particularly in the knees and ankles, along with recurrent epistaxis and easy bruising. On examination, she has hyperextensible skin that returns slowly to normal after pinching, and her sclerae appear bluish. Genetic testing reveals a mutation in the COL1A1 gene. Which of the following biochemical defects best explains her clinical presentation?

Explanation

## Clinical Diagnosis: Ehlers-Danlos Syndrome Type I (Classical EDS) ### Key Features in This Case - **Hyperextensible skin** with slow recoil (hallmark of EDS) - **Blue sclerae** (thin sclera allows visualization of underlying choroid) - **Joint hypermobility** with pain - **Easy bruising and bleeding** (fragile blood vessels) - **COL1A1 mutation** (encodes α1 chain of type I collagen) ### Biochemical Basis of the Defect **Key Point:** Mutations in COL1A1 or COL1A2 genes result in structurally abnormal type I collagen molecules. These mutations cause: 1. Substitution of glycine residues (which normally occupy every 3rd position in the Gly-X-Y repeat) 2. Disruption of the tight triple helix packing 3. Unstable collagen molecules that cannot form proper cross-links 4. Reduced tensile strength of connective tissues ### Why This Is Different from Other Collagen Defects | Condition | Genetic Defect | Biochemical Problem | Clinical Feature | |-----------|---|---|---| | **EDS Type I (Classical)** | COL1A1/COL1A2 mutation | Abnormal collagen structure | Hyperextensible skin, blue sclerae, joint hypermobility | | **Scurvy** | Vitamin C deficiency | Defective hydroxylation of Pro/Lys | Bleeding gums, poor wound healing | | **Lysyl Oxidase Deficiency** | LOX gene mutation | Defective cross-linking | Vascular rupture, skeletal deformities | | **Osteogenesis Imperfecta** | COL1A1/COL1A2 mutation | Abnormal collagen (quantitative) | Bone fragility, blue sclerae | **High-Yield:** The **Gly-X-Y tripeptide repeat** is the hallmark of collagen structure. Glycine (smallest amino acid) must occupy every 3rd position to fit in the interior of the triple helix. Any substitution disrupts the helix. ### Clinical Pearl Blue sclerae appear in both EDS Type I and Osteogenesis Imperfecta because both involve type I collagen defects, but the mechanisms differ: - **EDS:** Structurally abnormal collagen → thin, fragile sclera - **OI:** Quantitatively reduced collagen → thin sclera with visible choroid **Mnemonic:** **EDS-VASCULAR** = Ehlers-Danlos Syndrome features: - **E**xtensible skin - **D**islocations (joint) - **S**clerae (blue) - **V**ascular fragility - **A**bnormal collagen structure - **S**low skin recoil - **C**utaneous manifestations - **U**nstable triple helix - **L**ack of proper cross-linking - **A**rthralgias - **R**ecurrent bleeding [cite:Robbins 10e Ch 7]