Osteogenesis imperfecta type 1: An inherited connective tissue disorder featuring bone fragility and blue sclerae (blue whites of the eyes). This is the classic form of "brittle bone disease."
Osteogenesis imperfecta type 1 is an autosomal dominant trait. (One copy of the mutant gene is enough to cause the disease in males and females in successive generations.)
The disease is characterized by easy fracturing of bones, growth deficiency, abnormal teeth (the look as if they have been sandblasted), thin skin, blue sclerae and overly extensible joints. Only 10% of patients have their first fractures noted at birth, about 25% in the first year, about 50% in the preschool years, and the balance in the early school years. The chance of fractures decreases after adolescence. Common problems also include the development of bowing of the legs, curvature of the spine (scoliosis and kyphosis), umbilical and inguinal hernias and mild mitral valve prolapse (flopping down of the leaflets of the mitral heart valve). Hearing impairment begins in the third decade due to otosclerosis (a disorder of the bones of the middle ear).
This disease (also called osteogenesis imperfecta tarda or Lobstein disease) is usually compatible with life and with ambulation. The condition results from mutations that impair the production of type I collagen, a critical component of connective tissue. Mutations responsible for osteogenesis imperfecta type 1 have been identified in both the COL1A1 and COL1A2 genes. (COL1A1 is the gene for type 1 alpha-1 chain of collagen (COL1A1) and is located in chromosome17q21.31-q22.05. COL1A2 is the gene for type 2 alpha-chain of collagen (COL1A2) which is in chromosome 7q22.1).