Osteogenisis Imperfecta

Osteogenisis Imperfecta

DEFINITION

Osteogenisis imperfecta (OI) is "a rare genetic disorder of collagen synthesis associated with broad spectrum of musculoskeletal problems, most notably bowing and fractures of the extremities, muscle weakness, ligamentous laxity, and spinal deformities."

(Binder, 386). Other collagen-containing extraskeletal tissues, such as the sclerae, the teeth, and the heart valves are also affected to a variable degree. OI has a "common feature of bony fragility associated with defective formation of collagen by osteoblasts and fibroblasts." (Smith, 1983, 13) This disease, involving defective development of the connective tissues, is usually the result of the autosomal dominant gene, but can also be the result of the autosomal recessive gene. Spontaneous mutations are common and the clinical presentation of the disease remains to be quite broad. (Binder, 386)

OI is most commonly referred to as "brittle bones", but other names include: fragilitas ossium, hypolasia of the mesenchyme, and osteopsathyrosis. Osteogenisis imperfecta is still not completely understood, and while there have been advances in diagnosing the disease, treatment is still limited.

CAUSES

Osteogenisis imperfecta is the result of mutations in the genes for type I collagen.

In the mild dominantly inherited form of OI (type I), " a non-functional allele for the alpha 1 (I) chain halves collagen synthesis," (Smith, 1995, 169) and is largely responsible for the inheritance. Single base mutations in the codon for glycine causes lethal (type II) OI by wrecking the formation of the collagen triple helix. Types III and IV are the "less dram- atic outcomes of similar glycine mutations in either the alpha 1 (I) or the alpha 2(I) chains.(Smith, 1995, 169)

The clinical signs can be caused from defective osteoblastic activity and defective mesenchymal collagen (embryonic connective tissue) and its derivatives, such as sclera, bones, and ligaments. The reticulum fails to differentiate into mature collagen or the collagen develops abnormally. This causes immature and coarse bone formation and thinning. (Loeb, 755)

SIGNS AND SYMPTOMS

The signs and symptoms of OI vary greatly depending on the type. The most commonly used classification is the Sillence (type I to IV):

Type I is the mildest form of OI and is inherited as an autosomal dominant tra...

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