Mutations in the genes coding for the proα1 and proα2 chains of type I procollagen have been found in many patients with osteogenesis imperfecta (OI), a heritable disorder of connective tissue. The severity of the disease varies between families and even among members of the same family. This phenotypic variability covers a spectrum extending from very mild forms that cannot be easily detected to perinatally lethal forms. One explanation for this phenotypic variability is the nature of the mutation in the type I procollagen genes. Another explanation is mosaicism. Here we report on 2 families with propositi who have OI, whereas their mothers had a milder form of the disease. In one family, the molecular defect was previously shown to be a substitution of α1904 by cysteine [Constantinou et al., 1990]. The biochemical phenotype was characterized by significant post-translational over-modification of the mutated type 1 collagen molecules which also had a 3-4°C decrease in their thermal unfolding. Also, secretion of the procollagen into the culture media was delayed. In the second family, the proposita's muscle fibroblasts synthesized and secreted type I procollagen molecules that were highly over-modified along the entire length of their triple-helical domain. Cells from the mother also synthesized normal and over-modified protein, although the amount of over-modified protein was less than that synthesized by her daughter's cells. The exact molecular defect has not yet been defined. The simplest explanation for the phenotypic variability seen within these 2 families is that the mothers of the propositi are mosaic for their respective mutations so that not all of their somatic cells produce abnormal type I procollagen. Hence, one additional explanation for the phenotypic variability in OI is the abundance of the mutated allele or the degree of mosaicism in the patient's population of cells so that the first affected individual in a family may be mildly affected by a mutation which in a heterozygous form is potentially lethal in his or her offspring.
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