Ruth F. Pereira

Transgenic mice expressing a partially deleted gene for type I procollagen (COL1A1). A breeding line with a phenotype of spontaneous fractures and decreased bone collagen and mineral.

A line of transgenic mice was prepared that expressed moderate levels of an internally deleted human gene for the pro alpha 1(I) chain of type I procollagen. The gene construct was modeled after a sporadic in-frame deletion of the human gene that produced a lethal variant of osteogenesis imperfecta by causing biosynthesis of shortened pro alpha 1(I) chains. 89 transgenic mice from the line were examined. About 6% had a lethal phenotype with extensive fractures at birth, and 33% had fractures but were viable. The remaining 61% of the transgenic mice had no apparent fractures as assessed by x ray examination on the day of birth. Brother-sister matings produced eight litters in which approximately 40% of the mice had the lethal phenotype, an observation indicating that expression of the exogenous gene was more lethal in putative homozygous mice from the line. Examination of femurs from the transgenic mice indicated that the bones were significantly shorter in length and had a decrease in wet weight, mineral content, and collagen content. However, there was no statistically significant change in the mineral to collagen ratio. Biomechanical measurements on femurs from the mice at 6 wk indicated a decrease in force and energy to failure. There was also a decrease in strain to failure and an increase in Youngs modulus of elasticity, observations indicating increased brittleness of bone matrix. The results suggested that the transgenic mice may be an appropriate model for testing potential therapies for osteogenesis imperfecta. They may also be a useful model for studying osteoporosis.

An ultrastructural, microanalytical, and spectroscopic study of bone from a transgenic mouse with a COL1.A1 pro-alpha-1 mutation

A line of transgenic mice have been investigated that expressed moderate levels of an internally deleted human gene for the pro alpha (I) chain of type I procollagen. These mice expressed the gene at approximately 50% that of the endogenous gene. The gene construct was modeled after a sporadic in-frame deletion of the human gene that produced a lethal variant of osteogenesis imperfecta by causing biosynthesis of shortened pro alpha (I) chains. Periera et al. (1993) reported extensive fracturing in these mice with femurs that were shorter in length and bone that had decreased ash weight, mineral, and collagen content. These workers demonstrated an increased brittleness in bone using biomechanical measurements. The functional consequences of these mutant genes were examined in both transgenic and in normal littermate mice to determine if a valid model at the ultrastructural and analytical level had been produced for OI. X-ray microanalysis of bone mineral demonstrated a significantly lower calcium-to-phosphorus (Ca/P) molar ratio in transgenic mouse bone than in normal littermates; this was a feature of human OI bone. Fourier transform infrared spectroscopy confirmed that the mineral present was apatitic in nature despite the lower Ca/P molar ratio. Alizarin red skeletal staining showed the presence of multiple fracture calluses on the ribs and on the long bones of some of the transgenic mice, this was not seen on normal littermates. No light microscopic differences were observed between normal and transgenic mice; however, many ultrastructural correlates with human OI were observed in the transmission electron microscope. Anomalous fibrils associated with type I collagen, and an amorphous calcified material was observed lining the cartilage, extending beyond the lamina limitans in young transgenic mice.

Phenotypic variability and incomplete penetrance of spontaneous fractures in an inbred strain of transgenic mice expressing a mutated collagen gene (COL1A1).

Phenotype variability and incomplete penetrance are frequently observed in human monogenic diseases such as osteogenesis imperfecta. Here an inbred strain of transgenic mice expressing an internally deleted gene for the pro alpha 1(I) chain of type I procollagen (COL1A1) was bred to wild type mice of the same strain so that the inheritance of a fracture phenotype could be examined in a homogeneous genetic background. To minimize the effects of environmental factors, the phenotype was evaluated in embryos that were removed from impregnated females 1 d before term. Examination of stained skeletons from 51 transgenic embryos from 11 separate litters demonstrated that approximately 22% had a severe phenotype with extensive fractures of both long bones and ribs, approximately 51% had a mild phenotype with fractures of ribs only, and approximately 27% had no fractures. The ratio of steady-state levels of the mRNA from the transgene to the level of mRNA from the endogenous gene was the same in all transgenic embryos. The results demonstrated that the phenotypic variability and incomplete penetrance were not explained by variations in genetic background or levels in gene expression. Instead, they suggested that phenotypic variation is an inherent feature of expression of a mutated collagen gene.