Conservation of the sizes of 53 introns and over 100 intronic sequences for the binding of common transcription factors in the human and mouse genes for type II procollagen (COL2A1)

Over 11,000 bp of previously undefined sequences of the human COL2A1 gene were defined. The results made it possible to compare the intron structures of a highly complex gene from man and mouse. Surprisingly, the sizes of the 53 introns of the two genes were highly conserved with a mean difference of 13%. After alignment of the sequences, 69% of the intron sequences were identical. The introns contained consensus sequences for the binding of over 100 different transcription factors that were conserved in the introns of the two genes. The first intron of the gene contained 80 conserved consensus sequences and the remaining 52 introns of the gene contained 106 conserved sequences for the binding of transcription factors. The 5'-end of intron 2 in both genes had a potential for forming a stem loop in RNA transcripts.

Expression of type VI, IX and XI collagen genes and alternative splicing of type II collagen transcripts in fracture callus tissue in mice

The levels of six mRNAs coding for constituent alpha-chains of three minor collagens of cartilage were analyzed in an experimental fracture model in normal and transgenic Del1 mice harboring a deletion mutation of exon 7 in the type II collagen gene. Reduced and retarded chondrogenesis in Del1 mice was evident in callus samples as reduced mRNA levels for the cartilage specific type IX and XI collagens at days 7 and 9 of fracture healing. Analysis of the calluses for alternative splicing of pro alpha 1(II) collagen mRNA also suggested retarded chondrogenesis in Del1 calluses. Another developmentally regulated step in limb development, a switch between alternative promoters of the alpha 1(IX) collagen gene, was also seen during fracture healing but was less obvious in Del1 calluses. Finally, the current data suggest that the abnormality in bone remodelling in Del1 mice involves activation of the genes coding for alpha 1(XI) and alpha 2(VI) collagens.

Transgenic mice with deficiencies in cartilage collagens: possible models for gene therapy

We address three issues that are important when considering somatic gene therapy approaches to osteoarthritis (OA) and related syndromes. First, only those diseases for which a precise molecular etiology has been established should be contemplated for somatic gene therapy. Second, DNA sequences should be identified that restrict expression of correcting genes to chondrocytes; we discuss the use of transgenic mice to identify such sequences. Third, we emphasize the usefulness of establishing animal models that mimic human OA syndromes by genetic manipulations. These transgenic models should be essential for testing gene therapy approaches in vivo.

Characterization of primary cultures of chondrocytes from type II collagen/beta-galactosidase transgenic mice

Studies on the function of extracellular matrix components of cartilages and on chondrocyte-specific regulatory mechanisms will benefit from approaches in which transgenic mice and cell cultures will complement each other. We therefore established and extensively characterized primary cultures of mouse chondrocytes isolated from rib growth plates of newborn mice harboring a transgene in which type II collagen gene regulatory sequences were driving expression of an E. coli beta-galactosidase reporter gene. Primary chondrocytes expressed a fully differentiated phenotype in monolayer culture, producing mRNAs for the collagen types II, IX and X, and for the transgene. Transgenic cells also synthesized high levels of E. coli beta-galactosidase, easily quantifiable and also detectable in individual cells by X-gal staining. When chondrocytes were isolated from transgenic mice in which beta-galactosidase was fused to the product of the neomycin resistance gene, they displayed resistance to G418. After one to two weeks in culture, chondrocytes progressively lost expression of the transgenes, in parallel with that of cartilage-specific genes, and started expressing high levels of type I collagen RNA. The use of transgenic chondrocytes allowed us to easily score phenotypic changes by assaying beta-galactosidase activity and neomycin resistance. Cultures of mouse chondrocytes, such as those reported here, should also help characterize biochemically the phenotypes of other transgenic mice in studies of genetic diseases of cartilages and of mechanisms involved in chondrogenesis.

Retarded chondrogenesis in transgenic mice with a type II collagen defect results in fracture healing abnormalities

We have examined the biological and biomechanical consequences of defective type II collagen production for fracture repair employing a genetically engineered mouse line Del1 which was generated by microinjection of a 39-kb mouse pro alpha 1(II) collagen gene construct containing a deletion of exon 7 and intron 7 (Metsäranta et al. [1992] J. Cell Biol. 118:203-212). Standardized tibial fractures were produced in transgenic Del1 mice and their nontransgenic littermates were used as controls. The fracture callus tissues were analyzed at days 7, 9, 14, 28, and 42 using radiography, histomorphometry, biomechanical testing, and Northern analysis of mRNAs for several tissue-specific matrix components. Deficient production of cartilage in Del1 mice resulted in reduced radiographic callus size, smaller cross-sectional area, and impaired biomechanical properties when compared with fractures of nontransgenic control mice. The differences were most evident in 14-day fracture calluses. Consequently mRNAs for cartilage-specific type IX and X collagens and aggrecan were also reduced in Del1 calluses. Levels of type II collagen mRNAs were unaffected since the mutated transgene produced additional type II collagen mRNA molecules. Further abnormalities in the fracture repair process of Del1 mice were observed in callus remodeling. In the control animals a typical feature of external callus remodeling was reduction of callus size during endochondral ossification between days 14 and 28. Such reduction was not observed in the transgenic mice. Histological examination of fracture calluses suggested also a reduction in trabecular surface area, which was found to be even more pronounced in metaphyseal bone of Del1 mice. Despite these differences the biomechanical properties of the calluses in the two groups became similar by day 28 of fracture healing. The results thus suggest that reduced chondrogenesis due to the presence of mutated transgenes in Del1 mice not only causes a temporary impairment in biomechanical properties of healing fractures but also affects later stages of callus remodeling.

Type II collagen mutations in rare and common cartilage diseases

Cartilage diseases include a wide variety of clinical phenotypes from common osteoarthrosis to several different types of chondrodysplasias, i.e. 'disorders of cartilage', of which more than 100 different have been described. Patients frequently suffer from various symptoms affecting their joints and/or the growth of their long bones. The amount of hyaline cartilage at articular surfaces is often diminished and structurally abnormal. The surface of the cartilage may have an irregular appearance with defects extending into the subchondral bone. The major constituents of this hyaline cartilage are collagens and proteoglycans, the most abundant protein being type II collagen. It is a homotrimer of three identical alpha-chains, which are encoded by a single gene on human chromosome 12. The gene for type II collagen therefore became a likely candidate for some forms of chondrodysplasias and cartilage degeneration. Recently, both linkages and exclusions between this gene and various cartilage diseases have been reported and a growing number of mutations within the gene have also been identified.

The exon structure of the mouse alpha 2(IX) collagen gene shows unexpected divergence from the chick gene

One cosmid and two overlapping phage clones covering the entire mouse alpha 2(IX) collagen gene including 12 kilobase pairs (kb) of 5'- and 8 kb of 3'-flanking sequences were isolated from two genomic libraries. The overall gene structure was determined by restriction mapping and nucleotide sequencing. The gene spans 16 kb from the start of transcription to the polyadenylation site and contains 32 exons. It codes for a mRNA of 3 kb that translates into a polypeptide of 688 amino acids. The intron-exon junctions and mRNA structure were confirmed by amplification of cDNA made for mouse cartilage RNA. The coding sequence of the mouse alpha 2(IX) collagen gene shows marked similarities to those for other type IX collagen chains. Although the overall exon-intron organization of the mouse gene is very similar to the chick alpha 2(IX) gene, some unexpected differences were observed at the splice junctions. Split codons characteristic for the central triple helical domain of the chick were not found in the mouse gene that thus exhibited a long stretch of exons with sizes that are multiples of 9 base pairs in this domain. The promoter of the mouse alpha 2(IX) collagen gene contains some G + C-rich elements including three Sp1 consensus recognition sites and a far upstream CCAAT box but no TATAA box. Both primer extension and RNase protection assays revealed several transcription start sites within 418 base pairs of the promoter. The present study reports the first complete nucleotide sequence of any type IX collagen gene and forms the basis for comparative structural studies on this collagen type and for experiments involving transgenic mice.

Abnormal craniofacial morphology and cartilage structure in transgenic mice harboring a Gly --> Cys mutation in the cartilage-specific type II collagen gene

Gross morphology and histology of the craniofacial complex was studied in the offspring of two transgenic founder mice, Gly85-1 and Gly85-3, carrying several copies of a mouse type II collagen transgene that causes a single amino acid substitution (Gly-->Cys) at position 85 of the triple helix. The newborn transgenic mice had a short snout and mandible, a protruding tongue, a cleft of the secondary palate with the tongue situated between the shelves, and a doming cranial vault. Radiological examination revealed that the cranial base of the transgenic mice was shorter and its posterior part downward bent; in addition both the palate and the cribriform plate were less extended in relation to the cranial base than in the controls. Histologically the midline cartilaginous structures were composed of densely packed enlarged chondrocytes in a reduced extracellular matrix containing abnormally thick collagen fibrils. With the exception of the zone of hypertrophic chondrocytes the matrix also showed a loss of glycosaminoglycans. The cellular architecture of the basicranial synchondroses was disorganized, and the nasocerebrally oriented collagen fibrils formed unevenly distributed aggregates. The craniofacial morphology described here for the Gly85 mice shares features typical for other mouse mutations causing short limbed dwarfism. The observations indicate that defective cartilage production causes disproportionate craniofacial growth. Transgenic mice with specific mutations in cartilage-specific genes should therefore be useful for elucidating the complex mechanisms involved in determining the craniofacial growth.

Assembly of cartilage collagen fibrils is disrupted by overexpression of normal type II collagen in transgenic mice

Cartilage collagen fibrils, which are characterized by their thin, uniform diameters, are formed of a multicomponent system of three collagen types (II, IX, and XI) and interacting proteoglycans. We have used a genetic approach to test whether the proper assembly of this multiprotein structure was altered by overexpression of one of its normal components. Here we show that in transgenic mice in which the normal mouse alpha 1(II) collagen is overexpressed, thick abnormal collagen fibrils are generated. Mice that showed the highest expression of the transgene also displayed a larger proportion of abnormal fibrils and died at birth. We propose that an imbalance among the constituents of the cartilage collagen fibrils disrupts the mechanism that controls their assembly. The results show the applicability of the transgenic mice system to studies of complex multicomponent protein assemblies in intact animals.

The mouse collagen X gene: complete nucleotide sequence, exon structure and expression pattern

Overlapping genomic clones covering the 7.2 kb mouse alpha 1(X) collagen gene, 0.86 kb of promoter and 1.25 kb of 3'-flanking sequences were isolated from two genomic libraries and characterized by nucleotide sequencing. Typical features of the gene include a unique three-exon structure, similar to that in the chick gene, with the entire triple-helical domain of 463 amino acids coded by a single large exon. The highest degree of amino acid and nucleotide sequence conservation was seen in the coding region for the collagenous and C-terminal non-collagenous domains between the mouse and known chick, bovine and human collagen type X sequences. More divergence between the sequences occurred in the N-terminal non-collagenous domain. Similarity between the mammalian collagen X sequences extended into the 3'-untranslated sequence, particularly near the polyadenylation site. The promoter of the mouse collagen X gene was found to contain two TATAA boxes 159 bp apart; primer extension analyses of the transcription start site revealed that both were functional. The promoter has an unusual structure with a very low G + C content of 28% between positions -220 and -1 of the upstream transcription start site. Northern and in situ hybridization analyses confirmed that the expression of the alpha 1(X) collagen gene is restricted to hypertrophic chondrocytes in tissues undergoing endochondral calcification. The detailed sequence information of the gene is useful for studies on the promoter activity of the gene and for generation of transgenic mice.

Chondrodysplasia in transgenic mice harboring a 15-amino acid deletion in the triple helical domain of pro alpha 1(II) collagen chain

We have generated transgenic mice by microinjection of a 39-kb mouse pro alpha 1(II) collagen gene construct containing a deletion of exon 7 and intron 7. This mutation was expected to disturb the assembly and processing of the homotrimeric type II collagen molecule in cartilage. Expression of transgene mRNA at levels equivalent or higher than the endogenous mRNA in the offspring of two founder animals resulted in a severe chondrodysplastic phenotype with short limbs, hypoplastic thorax, abnormal craniofacial development, and other skeletal deformities. The affected pups died at birth due to respiratory distress. Light microscopy of epiphyseal growth plates of transgenic pups demonstrated a marked reduction in cartilaginous extracellular matrix and disruption of the normal organization of the growth plate. The zone of proliferating chondrocytes was greatly reduced whereas the zone of hypertrophic chondrocytes was markedly increased extending deep into the diaphysis suggestive of a defect in endochondral ossification. Electron microscopic examination revealed chondrocytes with extended RER, a very severe reduction in the amount of cartilage collagen fibrils, and abnormalities in their structure. We postulate that the deletion in the alpha 1(II) collagen acts as a dominant negative mutation disrupting the assembly and secretion of type II collagen molecules. The consequences of the mutation include interference with normal endochondral ossification. These mice constitute a valuable model to study the mechanisms underlying human chondrodysplasias and normal bone formation.

Structural analysis of the regulatory elements of the type-II procollagen gene. Conservation of promoter and first intron sequences between human and mouse

Transcription of the type-II procollagen gene (COL2A1) is very specifically restricted to a limited number of tissues, particularly cartilages. In order to identify transcription-control motifs we have sequenced the promoter region and the first intron of the human and mouse COL2A1 genes. With the assumption that these motifs should be well conserved during evolution, we have searched for potential elements important for the tissue-specific transcription of the COL2A1 gene by aligning the two sequences with each other and with the available rat type-II procollagen sequence for the promoter. With this approach we could identify specific evolutionarily well-conserved motifs in the promoter area. On the other hand, several suggested regulatory elements in the promoter region did not show evolutionary conservation. In the middle of the first intron we found a cluster of well-conserved transcription-control elements and we conclude that these conserved motifs most probably possess a significant function in the control of the tissue-specific transcription of the COL2A1 gene. We also describe locations of additional, highly conserved nucleotide stretches, which are good candidate regions in the search for binding sites of yet-uncharacterized cartilage-specific transcription regulators of the COL2A1 gene.

Transgenic mice as models for heritable diseases

The development of methods for introduction of foreign DNA stably into genome of experimental animals has opened new possibilities to study the effects of mutations in complex gene-protein systems at the level of the entire organism. Information from such experiments is directly applicable to understanding of pathogenetic mechanisms in hereditary diseases and to designing of new therapeutic approaches. Techniques are currently available for studying both dominant mutations, introduced usually by microinjection, and recessive mutations introduced by homologous recombination employing the pluripotent embryonic stem cells. It remains to be emphasized, however, that the information which can be obtained with these techniques is optimal when sufficient background information is available on the protein system in question. The purpose of this review is to describe how the transgenic mouse methodology has increased our understanding of molecular basis of diseases both at the level of protein function and gene regulation.

Specific hybridization probes for mouse alpha 2(IX) and alpha 1(X) collagen mRNAs

We have used polymerase chain reaction (PCR) technology and available cross-species sequence information to construct cDNA probes for mouse alpha 2(IX) and alpha 1(X) collagen transcripts. Sequencing confirmed the identification of the clones. Northern analysis proved sufficient divergence of the cloned sequences from other collagen transcripts: specific detection of the mouse 2.9 kb alpha 2(IX) and 3.3 kb alpha 1(X) collagen mRNAs was seen under normal hybridization and washing conditions.

Mouse type II collagen gene. Complete nucleotide sequence, exon structure, and alternative splicing

Several overlapping clones covering the entire mouse type II collagen gene including 10 kilobases (kb) of 5'- and 15 kb of 3'-flanking sequences were isolated from a cosmid library. The overall gene structure was determined by restriction mapping and sequencing. The gene spans 28.9 kb from the start of transcription to the polyadenylation site and contains 54 exons. It codes for a major mRNA species of 4910 bases which translates into a polypeptide of 1419 amino acids. A less abundant RNA species of 5110 bases contains additional sequences corresponding to an alternatively spliced exon 2. Except for the amino-terminal propeptide (N-propeptide) domain the exon-intron organization of the mouse pro alpha 1(II) collagen gene is remarkably similar to genes for other fibrillar collagen types. The overall identity of the coding sequences of the mouse and human type II collagen genes is 89% at the nucleotide level, but only 37 amino acid changes occur within the mature alpha 1(II) collagen chains between mouse and man. Intron sizes are also conserved between the mouse and human genes but not with the chick alpha 1(II) gene. The promoter of the mouse type II collagen gene is similar to those of the rat and human genes containing a TATA box and several G + C-rich elements but no CCAAT box. The 3'-untranslated sequence contains two regions of high homology between chick, mouse, bovine, and human genes preceding the major polyadenylation site. Additional size variation in the mRNA arises from the use of a minor polyadenylation signal. Information on conserved noncoding sequences will help in studies on the regulation of the pro alpha 1(II) collagen gene. Detailed knowledge of the gene is also necessary for site-directed mutagenesis and work with transgenic mice.

Specific hybridization probes for mouse type I, II, III and IX collagen mRNAs

We have constructed DNA probes for the specific detection of mouse pro alpha 1(I), pro alpha 1(II), pro alpha 1(III) and alpha 1(IX) collagen transcripts. To avoid cross-hybridization the probes for fibrillar collagens cover mainly sequences in the 3' untranslated region of the gene. Sequencing and Northern analysis confirmed that the clones share minimal sequence similarity and detect only the specific mRNAs under normal hybridization and washing conditions. The clone for mouse alpha 1(IX) collagen covers coding sequences but is sufficiently divergent from other collagen transcripts to allow specific detection of the corresponding mRNA.

Localization of osteonectin expression in human fetal skeletal tissues by in situ hybridization

The expression of osteonectin gene was studied in developing human fetuses by Northern analysis and in situ hybridization. The highest levels of osteonectin mRNA were detected in RNA extracted from calvarial bones, growth plates, and skin. Low mRNA levels were present in several parenchymal tissues. In situ hybridization of developing long bones revealed three cell types with high osteonectin mRNA levels: osteoblasts, cells of the periosteum, and hypertrophic chondrocytes. Weaker signals were detected in osteocytes, fibroblasts of tendons, ligaments and skin, and in cells of the epidermis. Apart from the hypertrophic chondrocytes, only low osteonectin mRNA levels were seen in cartilage. The localization of osteonectin mRNA in fetal growth plates is consistent with the hypothesis that the protein plays a role in the mineralization of bone and cartilage matrices.