Segregation analysis of dominant osteogenesis imperfecta in Italy

Extracellular matrix deposition in cultured dermal fibroblasts from four probands affected by osteogenesis imperfecta

Type I procollagen biosynthesis and matrix deposition were studied in cultured fibroblasts of four probands affected by Osteogenesis Imperfecta and in whom the mutations have been characterized. The mutations along the triple helix altered all biochemical parameters considered, i.e. thermal stability, kinetics of procollagen secretion and rate of maturation from procollagen to collagen. The biochemical findings were peculiar for each case considered, but there was no correlation between biochemical parameters and clinical phenotype. In all our probands, regardless of the clinical severity, mutant chains appeared in the insoluble matrix formed by fibroblasts cultured in the presence of dextran sulfate. The densitometric scanning revealed a relative increased amount of fibronectin, suggesting that the matrix contained a lower quantity of type I collagen. Furthermore, the amount of mutant chains found in the insoluble fraction was clearly less than expected, considering that 75% of new synthesized trimers are abnormal. Therefore, in the presence of a mutation, the protein available for extracellular matrix formation is reduced and the mutant trimers incorporated in the matrix probably interfere with normal fibril performance. The abnormal fibril morphology has a dramatic effect in bone, interfering presumably with a correct mineral deposition and interactions with non/collagenous bone proteins.

Haplotype analysis of collagen type I genes in the general population and in osteogenesis imperfecta families

The allele frequencies of 2 new polymorphic markers of collagen type I proalpha 1 (COL1A1) and proalpha 2 (COL1A2) genes were determined in a random sample of chromosomes by polymerase chain reaction. The minor allele frequencies were 0.27 for COL1A1/+88Mn1I, and 0.39 for COL1A2/1446 PvuII RFLPs, respectively. These 2 polymorphisms increased the combined (PIC) values we previously determined in the Italian population with Southern blotting procedures, from 0.71 at the COL1A1 locus to 0.81, and from 0.73 at the COL1A2 locus to 0.88, respectively. With a combination of these markers, we have carried out the segregation analysis of 4 new families in which osteogenesis imperfecta (OI) segregated as a dominant trait. The disease segregated with COL1A1 in 2 OI type I families, and with COL1A2 in one OI type IV family. In one OI type I family the concordant locus was uncertain. This analysis was extended to the 7 dominant OI families we previously reported: in 3 out of 11 pedigrees either locus still could not be excluded, indicating the need for more genetic markers. COL1A1 and COL1A2 haplotype frequencies were compared in normal and OI chromosomes: no preferential association of the disease with a given haplotype was detected. The correlation between affected locus and clinical aspects is discussed.

Linkage analysis in dominantly inherited osteogenesis imperfecta

The only serious attempts at linkage in osteogenesis imperfecta (OI) have shown that the disease is linked to type 1 collagen genes in all families studied in which it segregrates as a clear mendelian dominant trait. For prenatal diagnosis the probability that a new family is linked can be taken as greater than 0.95 and this figure is augmented as more meioses are studied. Some phenotype correlations, notably between the OI type IV phenotype and linkage to COL1A2 and between presenile hearing loss in OI type I and linkage to COL1A1, can be used to improve risk estimates substantially in families where there are no segregation data to distinguish whether COL1A1 or COL1A2 is the mutant locus.