A 35-year-old Indian female presents with progressive joint pain, skin hyperelasticity, and easy bruising. She has a history of recurrent joint dislocations since childhood. Skin biopsy shows abnormal collagen fibrils with irregular diameter and spacing. Genetic testing reveals a mutation in the COL5A1 gene encoding type V collagen. Which structural level of collagen is most directly affected by this mutation?

Explanation

## Clinical Presentation: Ehlers–Danlos Syndrome Type II (Classical EDS) The vignette describes **Ehlers–Danlos Syndrome (EDS) Type II (classical)**, caused by mutations in genes encoding type V collagen (COL5A1 or COL5A2). Type V collagen is a quantitatively minor but structurally critical collagen that regulates fibril diameter and organization. ### Collagen Structure Hierarchy ```mermaid flowchart TD A["Primary Structure<br/>(Amino acid sequence)<br/>Gly-X-Y repeats"]:::action --> B["Secondary Structure<br/>(Polyproline II helix)<br/>Individual α-chain"]:::action B --> C["Tertiary Structure<br/>(Triple helix)<br/>3 α-chains intertwine"]:::action C --> D["Quaternary Structure<br/>(Fibril assembly)<br/>Multiple molecules cross-link"]:::action E["COL5A1 Mutation"]:::urgent --> A A -->|"Disrupts Gly-X-Y<br/>pattern"| F["Abnormal chain folding<br/>& stability"]:::outcome F --> G["Defective triple helix<br/>& fibril organization"]:::outcome ``` ### Why Primary Structure? **Key Point:** COL5A1 mutations disrupt the **primary structure** (amino acid sequence) of the type V collagen α1-chain. Collagen has a unique primary structure requirement: the **Gly-X-Y tripeptide repeat**, where every third residue must be glycine (the smallest amino acid) to fit in the interior of the triple helix. Mutations in COL5A1 that: - Substitute glycine residues → destabilize triple helix formation - Alter X or Y positions (proline/hydroxyproline) → impair cross-linking and stability - Introduce premature stop codons → truncated, non-functional chains ### Downstream Consequences | Structural Level | Consequence | |---|---| | **Primary** | Abnormal amino acid sequence (Gly-X-Y disruption) | | **Secondary** | Impaired polyproline II helix formation in affected chains | | **Tertiary** | Unstable or malformed triple helix | | **Quaternary** | Irregular fibril diameter, poor cross-linking, weak fibrils | ### Clinical Manifestations **High-Yield:** Type V collagen regulates fibril diameter in type I collagen fibrils. Its deficiency → abnormally large, disorganized fibrils → tissue fragility: - **Skin hyperelasticity** (stretchy, velvety skin) - **Easy bruising & poor wound healing** (fragile fibrils) - **Joint hypermobility & dislocations** (defective ligament collagen) - **Abnormal collagen fibril morphology on biopsy** (irregular diameter) **Clinical Pearl:** EDS Type II is distinguished from Type III (vascular EDS, COL3A1 mutations) by the absence of vascular rupture risk and the presence of skin hyperelasticity and joint hypermobility.