In osteogenesis imperfecta, mutations in genes encoding type I collagen result in which primary structural defect?

Explanation

## Osteogenesis Imperfecta: Molecular Pathology **Key Point:** Osteogenesis imperfecta (OI) is caused by mutations in COL1A1 or COL1A2 genes encoding the α1 and α2 chains of type I collagen. The most common defect is **glycine substitution** in the Gly-X-Y tripeptide repeat, disrupting triple helix stability. **High-Yield:** The Gly-X-Y repeat is the fundamental structural motif of collagen: - Every third residue MUST be glycine (smallest amino acid, fits in the interior of the triple helix) - Substitution of glycine with a larger residue (e.g., alanine, serine) causes steric clash - Results in local unwinding, kinks, and destabilization of the entire triple helix - Even a single glycine substitution can have dominant-negative effects (mutant chains poison wild-type chains) **Mnemonic:** **GXY Rule** = Glycine-X-Y tripeptide repeat is mandatory; Glycine substitution = structural collapse **Clinical Pearl:** OI severity correlates with the position and nature of the glycine substitution: - Substitutions near the N-terminus or C-terminus → milder phenotype - Substitutions in the central 1α1 domain → severe phenotype (OI type II, lethal in utero) - Glycine → cysteine substitutions → very severe (disulfide bonding disrupts helix) ## Differential Diagnosis of Collagen Defects | Defect | Enzyme/Gene | Mechanism | Disease | |--------|-----------|-----------|----------| | Glycine substitution | COL1A1/COL1A2 | Triple helix destabilization | Osteogenesis imperfecta | | Proline/lysine hydroxylation | Prolyl/lysyl hydroxylase (vitamin C) | Reduced cross-linking, helix instability | Scurvy | | Lysyl oxidase deficiency | Lysyl oxidase (copper) | Impaired cross-linking | Menkes disease, copper deficiency | | Procollagen peptidase deficiency | ADAMTS2 | Impaired N-propeptide removal | Ehlers-Danlos syndrome type VII | **Why Glycine Substitution is the Answer:** OI mutations predominantly affect the structural integrity of the collagen triple helix by replacing glycine (the only amino acid small enough to fit in the helix interior) with larger residues. This is the defining molecular pathology of OI.