A 35-year-old man with a known diagnosis of osteogenesis imperfecta (OI) presents with recurrent bone fractures and blue sclerae. His father had similar symptoms and died of a basilar skull fracture following minor head trauma. To confirm the diagnosis and determine the severity of the genetic defect, which investigation is most appropriate?

Explanation

## Investigation of Choice for Osteogenesis Imperfecta **Key Point:** Genetic testing for COL1A1 and COL1A2 mutations is the definitive confirmatory test for osteogenesis imperfecta (OI) and allows classification into severity subtypes (Types I–IV). ### Pathophysiology of OI Osteogenesis imperfecta is caused by mutations in genes encoding type I collagen (the primary structural collagen in bone, skin, and sclera): - **COL1A1** (chromosome 17q21.33) — ~90% of OI cases - **COL1A2** (chromosome 7q21.3) — ~10% of OI cases Mutations lead to: 1. Reduced quantity of type I collagen (quantitative defects) 2. Abnormal structure/cross-linking of collagen (qualitative defects) 3. Defective mineralization and fragile bone matrix ### OI Classification & Genetic Correlation | Type | Severity | Inheritance | Genetic Feature | Clinical Features | |---|---|---|---|---| | **I** | Mild | AD | Quantitative (null allele) | Blue sclerae, normal stature, mild fractures | | **II** | Lethal | AD (de novo) | Qualitative (glycine substitution) | Perinatal lethal, severe deformities | | **III** | Severe | AD (often de novo) | Qualitative | Severe deformities, short stature, normal sclerae | | **IV** | Moderate | AD | Mixed | White sclerae, moderate deformities | **High-Yield:** Glycine substitutions in the collagen triple helix (especially in the Y position of Gly-X-Y repeats) cause severe qualitative defects and are associated with lethal or severe OI. Null alleles (nonsense mutations, frameshifts) cause milder quantitative defects. ### Why Genetic Testing? 1. **Definitive diagnosis** — confirms OI vs. other causes of fragile bones (hypophosphatasia, rickets) 2. **Severity stratification** — guides prognosis and management intensity 3. **Genetic counselling** — determines recurrence risk (95% of Type I is autosomal dominant; ~10% are de novo mutations) 4. **Family screening** — identifies asymptomatic carriers or mildly affected relatives 5. **Therapeutic planning** — informs use of bisphosphonates, growth hormone, or future gene therapy **Clinical Pearl:** Blue sclerae in OI result from thinness of the sclera, allowing visualization of the underlying choroid vasculature. This finding is most prominent in Type I OI and less common in Types III and IV. **Warning:** Serum alkaline phosphatase and calcium are typically normal in OI (unlike rickets or metabolic bone disease). Urinary hydroxyproline is elevated but non-specific and does not identify the genetic defect.