What causes osteogenesis imperfecta (OI)?

Updated: Feb 24, 2020
  • Author: Manoj Ramachandran, MBBS, MRCS, FRCS; Chief Editor: Harris Gellman, MD  more...
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Answer

Type 1 collagen is a triple helix formed by two copies of the alpha1 chain and one copy of the alpha2 chain. The COL1A gene on chromosome 17 encodes the pro-alpha1 chain, and the COL2A gene on chromosome 2 encodes the pro-alpha2 chain. [13]

The gene sequence coding for the triple-helix domain has a repeating motif of (Gly-X-Y)(n), where X is commonly hydroxyproline and Y is commonly hydroxylysine. Glycine, being the smallest of all amino acids, fits into the core of the superhelix when the chains wind around each other; therefore, glycine plays an important role in the superhelix formation.

In 85-90% of cases, the gene mutation occurs in the region where the exon and intron splice sites are sequenced. All current mutations for type 1 collagen and their associated phenotypes can be found in the Human Type 1 Collagen Mutation Database.

In OI due to quantitative defects of type 1 collagen, mutations are usually found on the COL1A gene. The mutations result in the production of a premature stop codon or a microsense frame shift, which leads to mutant messenger RNA (mRNA) in the nucleus. However, the cytoplasm contains normal alpha1 mRNA; therefore, reduced amounts of structurally normal collagen are produced.

In OI due to qualitative defects of type 1 collagen, autosomal dominant mutations are found on either the COL1A or the COL1B gene. The mutations result in the production of a mixture of normal and mutant collagen chains. Substitution of a larger amino acid (eg, cysteine or alanine) for glycine results in abnormal helix formation, but these chains can combine with normal chains to produce type 1 collagen. The type 1 collagen thus formed is functionally impaired because of the mutant chain; this is the so-called dominant negative mechanism.

Wallace et al, in a report on three patients who had type I OI and primary open angle glaucoma (POAG), identified two novel mutations in COL1A1 in these individuals. [14] They suggested that some mutations in COL1A1 may be causative for OI and POAG. Alternatively, susceptibility genes may interact with mutations in COL1A1 to cause POAG.


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