Osteogenesis imperfecta type IV. Biochemical confirmation of genetic linkage to the pro alpha 2(I) gene of type I collagen.

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Osteogenesis imperfecta (01) is a heterogeneous disorder of type I collagen of which OI type I, an autosomal dominant condition, is the mildest and most common form. Affected individuals have blue sclerae, normal stature, bone fragility without significant deformity and osteopenia. Fibroblasts from most affected individuals produce about half the expected amount of structurally normal type I collagen as a result of decreased synthesis of one of its constituent chains, proal(I), but the nature of the mutations which result in 01 type I are unknown. We describe a three generation family with OI type I in which all affected members have one normal COLlA1 allele and another from which the intragenic Eco RI restriction site near the 3' end of the gene is missing. Amplification by polymerase chain reaction and sequence determination of the normal allele and of the mutant allele in the domain that normally contains the Eco RI site demonstrated a 5-bp deletion frop the mutant allele. The deletion changes the translational readingframe beginning at the Eco RI site and predicts the synthesis of a proal(I) chain that extends 84 amino acids beyond the normal termination. Although the mutant proal(I) chain is synthesized in an in vitro translation system, we are unable to detect its presence in intact cells, suggesting that it is unstable and rapidly destroyed in one of the cell's degradative pathways. Our analysis of individuals with 01 type I from 20 families indicates that this is a unique mutation and suggests that the phenotype can result from multiple mechanisms that decrease the synthesis of normal type I procollagen molecules, including those that alter protein stability. (J. Clin. Invest. 1990. 85:282-290.) common form, 01 type I, is characterized by normal or near normal stature, bone fragility without significant deformity, osteopenia, blue sclerae, and inheritance in an autosomal dominant fashion (1). Adult onset hearing loss and/or dentinogenesis imperfecta are seen in some kindreds. Family studies have demonstrated linkage of the 01 type I phenotype to restriction fragment length polymorphisms in COLlA1, the gene encoding the proal(I) chain of type I collagen, in the majority of cases, and to sites in COLlA2, the gene encoding the proa2(I) chain of type I collagen, in the minority (2-4).Little is known about the nature ofthe mutations that result in 01 type I, except that dermal fibroblasts from affected individuals produce and efficiently secrete about half the expected amount of structurally normal type I collagen (4, 5). The abnormality in type I collagen production is reflected in altered ratios of type I to type III collagen in skin (4), and represents the most characteristic biochemical correlate ofthe phenotype. Barsh et al. (6) demonstrated that the disturbance in type I collagen production is due to decreased synthesis of proal(I) chains in the patients they studied. Dermal fibroblasts from these individuals synthesized proa 1(I) and proa2(I) chains in a 1:1 ratio instead of the expected 2...

Section: Resultsmentioning

confidence: 83%

mentioning

confidence: 99%

Frameshift mutation near the 3' end of the COL1A1 gene of type I collagen predicts an elongated Pro alpha 1(I) chain and results in osteogenesis imperfecta type I.

Willing

Cohn²,

Byers³

1990

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“…A 4-bp deletion near the 3' end of the coding region of the proa2(I) chain that altered the last 12 residues of the chain and prevented its assembly into type I procollagen molecules produced an OI type III phenotype in the homozygous state but had little phenotypic effect in the heterozygotes (13,30). Deletion of single exons encoding domains near the amino-terminal end of the triple helix of the proa2(I) chain (11) or near the center of the triple helix (31) have produced mild-to-moderate phenotypes, as has a substitution of arginine for glycine near the carboxyl-terminal end of the triple-helical domain of proa2(I) (32). This list, however, emphasizes the difficulty in providing a cohesive understanding of the genotype-phenotype relationship in the mild-tomoderate 01 phenotypes.…”

Section: Discussionmentioning

confidence: 99%

Osteogenesis imperfecta. The position of substitution for glycine by cysteine in the triple helical domain of the pro alpha 1(I) chains of type I collagen determines the clinical phenotype.

Starman¹,

Eyre²,

Charbonneau³

et al. 1989

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“…Eight probands with EDS or 01 had mutations that caused in-phase deletions of amino acid sequences in either the proal(I) or proa2(I) chain of type I procollagen (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18). One proband with moderately severe OI had a homozygous mutation that deleted 4 bp of coding sequence and thereby altered the sequence of the last 33 amino acids in the C-propeptide of the proa2(I) chain (19-21).…”

Section: Introductionmentioning

confidence: 99%

A lethal variant of osteogenesis imperfecta has a single base mutation that substitutes cysteine for glycine 904 of the alpha 1(I) chain of type I procollagen. The asymptomatic mother has an unidentified mutation producing an overmodified and unstable type I procollagen.

Constantinou

Nielsen²,

Prockop³

1989

A fraction of the proal(I) and proa2(I) chains in type I procollagen synthesized by the fibroblasts from a proband with a lethal variant of osteogenesis imperfecta were overmodified by posttranslational reactions. After digestion with pepsin, some of the al(I) chains were recovered as disulfide-linked dimers. Mapping of cyanogen bromide peptides indicated that the disulfide link was contained in al-CB6, the cyanogen bromide fragment containing amino acid residues 823-1014 of the al(I) chain. Nucleotide sequencing of cDNA clones demonstrated a substitution of T for G that converted glycine 904 of the al(I) chain to cysteine.A large fraction of the type I procollagen synthesized by the proband's fibroblasts had a thermostability that was 340C lower than the normal type I procollagen as assayed by brief proteinase digestion. In addition, the type I procollagen synthesized by the prob.nd's fibroblasts was secreted with an abnormal kinetic pattern in that there was a lag period of about 30 min in pulse-chase experiments. The mutation of glycine to cysteine was not found in type I procollagen synthesized by fibroblasts from the proband's parents. Therefore, the mutation was a sporadic one. However, the mother's fibroblasts synthesized a type I procollagen in which part of the proa chains were overmodified and had a lower thermostability. Therefore, the proband may have inherited a mutated allele for type I procollagen from her mother that contributed to the lethal phenotype. The mother was asymptomatic. She was somewhat short and had slightly blue sclerae but no definitive signs of a connective tissue abnormality. The observations on the mother indicated, therefore, that a mutation that causes synthesis of a type I procollagen with a lowered thermal stability does not necessarily produce a heritable disorder of connective tissue.