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)

Up-regulation of type II collagen gene by 17β-estradiol in articular chondrocytes involves Sp1/3, Sox-9, and estrogen receptor α

The existence of a link between estrogen deprivation and osteoarthritis (OA) in postmenopausal women suggests that 17β-estradiol (17β-E2) may be a modulator of cartilage homeostasis. Here, we demonstrate that 17β-E2 stimulates, via its receptor human estrogen receptor α 66 (hERα66), type II collagen expression in differentiated and dedifferentiated (reflecting the OA phenotype) articular chondrocytes. Transactivation of type II collagen gene (COL2A1) by ligand-independent transactivation domain (AF-1) of hERα66 was mediated by "GC" binding sites of the -266/-63-bp promoter, through physical interactions between ERα, Sp1/Sp3, Sox9, and p300, as demonstrated in chromatin immunoprecipitation (ChIP) and Re-Chromatin Immuno-Precipitation (Re-ChIP) assays in primary and dedifferentiated cells. 17β-E2 and hERα66 increased the DNA-binding activities of Sp1/Sp3 and Sox-9 to both COL2A1 promoter and enhancer regions. Besides, Sp1, Sp3, and Sox-9 small interfering RNAs (siRNAs) prevented hERα66-induced transactivation of COL2A1, suggesting that these factors and their respective cis-regions are required for hERα66-mediated COL2A1 up-regulation. Our results highlight the genomic pathway by which 17β-E2 and hERα66 modulate Sp1/Sp3 heteromer binding activity and simultaneously participate in the recruitment of the essential factors Sox-9 and p300 involved respectively in the chondrocyte-differentiated status and COL2A1 transcriptional activation. These novel findings could therefore be attractive for tissue engineering of cartilage in OA, by the fact that 17β-E2 could promote chondrocyte redifferentiation.

KEY MESSAGES

17β-E2 up-regulates type II collagen gene expression in articular chondrocytes. An ERα66/Sp1/Sp3/Sox-9/p300 protein complex mediates this stimulatory effect. This heteromeric complex interacts and binds to Col2a1 promoter and enhancer in vivo. Our findings highlight a new regulatory mechanism for 17β-E2 action in chondrocytes. 17β-E2 might be an attractive candidate for cartilage engineering applications.

Alternative splicing of type II procollagen: IIB or not IIB?

Over two decades ago, two isoforms of the type II procollagen gene (COL2A1) were discovered. These isoforms, named IIA and IIB, are generated in a developmentally-regulated manner by alternative splicing of exon 2. Chondroprogenitor cells synthesize predominantly IIA isoforms (containing exon 2) while differentiated chondrocytes produce mainly IIB transcripts (devoid of exon 2). Importantly, this IIA-to-IIB alternative splicing switch occurs only during chondrogenesis. More recently, two other isoforms have been reported (IIC and IID) that also involve splicing of exon 2; these findings highlight the complexities involving regulation of COL2A1 expression. The biological significance of why different isoforms of COL2A1 exist within the context of skeletal development and maintenance is still not completely understood. This review will provide current knowledge on COL2A1 isoform expression during chondrocyte differentiation and what is known about some of the mechanisms that control exon 2 alternative splicing. Utilization of mouse models to address the biological significance of Col2a1 alternative splicing in vivo will also be discussed. From the knowledge acquired to date, some new questions and concepts are now being proposed on the importance of Col2a1 alternative splicing in regulating extracellular matrix assembly and how this may subsequently affect cartilage and endochondral bone quality and function.

Expression of two novel alternatively spliced COL2A1 isoforms during chondrocyte differentiation

Alternative splicing of the type II procollagen gene (COL2A1) is developmentally regulated during chondrogenesis. Type IIA procollagen (+ exon 2) is synthesized by chondroprogenitor cells while type IIB procollagen (- exon 2) is synthesized by differentiated chondrocytes. Here, we report expression of two additional alternatively spliced COL2A1 isoforms during chondrocyte differentiation of bone marrow-derived mesenchymal stem cells (MSCs). One isoform, named IIC, contains only the first 34 nucleotides of exon 2 by the use of an alternative 5' splice site, resulting in a premature termination codon and possible nonsense-mediated decay of IIC mRNA. Low levels of the IIC isoform were detected by RT-PCR and Southern analysis of COL2A1 cDNA amplified from differentiating rabbit and human MSCs. A second novel transcript, named IID, arises by the use of another 5' alternative splice site in intron 2. The IID isoform contains exon 2 plus 3 nucleotides, resulting in the insertion of an additional amino acid. The IID isoform was co-expressed with the IIA isoform during chondrogenesis, and was approximately one-third as abundant. Deletion of the IIC alternative 5' splice site from a COL2A1 mini-gene construct resulted in a significant increase in the IIA:IIB ratio. A mutant mini-gene that inhibited production of the IID isoform, however, had differential effects on the production of the IIA and IIB isoforms: this was apparently related to the differentiation status of the cell type used. These data suggest that COL2A1 mRNA abundance and other aspects of chondrocyte differentiation may be regulated by the use of these previously undetermined alternative splice sites.

Nuclear protein TIA-1 regulates COL2A1 alternative splicing and interacts with precursor mRNA and genomic DNA

The RNA-binding protein TIA-1 (T-cell-restricted intracellular antigen-1) functions in regulating post-transcriptional mechanisms, including precursor mRNA (pre-mRNA) alternative splicing and mRNA translation. Utilizing a mini-gene consisting of part of the type II procollagen gene (COL2A1), we show that TIA-1 interacts with a conserved AU-rich cis element in COL2A1 intron 2 and modulates alternative splicing of exon 2. This unique, highly conserved cis element containing stem-loop secondary structure was previously identified in our laboratory as an essential motif that controls the developmentally regulated exon 2 splicing switch during chondrogenesis (McAlinden, A., Havlioglu, N., Liang, L., Davies, S. R., and Sandell, L. J. (2005) J. Biol. Chem. 280, 32700-32711). In vivo binding of endogenous TIA-1 to the AU-rich cis element in COL2A1 pre-mRNA was confirmed by the ribonucleoprotein immunoprecipitation assay. Importantly, we also show that TIA-1 interacts with the equivalent DNA sequence with a preference for single-stranded rather than double-stranded DNA. Chromatin immunoprecipitation assays (including an additional RNase step) confirmed this interaction in vivo. Competition assays showed that TIA-1 apparently binds with higher affinity to DNA than to RNA. Finally, we show that this strong DNA-TIA-1 interaction can be disrupted by an RNA polymerase during active transcription. This suggests a potentially novel, dual role for TIA-1 in shuttling between DNA and RNA ligands to co-regulate COL2A1 expression at the level of transcription and pre-mRNA alternative splicing.

Alternative splicing of type II procollagen exon 2 is regulated by the combination of a weak 5' splice site and an adjacent intronic stem-loop cis element

Alternative splicing of the type II procollagen gene (COL2A1) is developmentally regulated during chondrogenesis. Chondroprogenitor cells produce the type IIA procollagen isoform by splicing (including) exon 2 during pre-mRNA processing, whereas differentiated chondrocytes synthesize the type IIB procollagen isoform by exon 2 skipping (exclusion). Using a COL2A1 mini-gene and chondrocytes at various stages of differentiation, we identified a non-classical consensus splicing sequence in intron 2 adjacent to the 5' splice site, which is essential in regulating exon 2 splicing. RNA mapping confirmed this region contains secondary structure in the form of a stem-loop. Mutational analysis identified three cis elements within the conserved double-stranded stem region that are functional only in the context of the natural weak 5' splice site of exon 2; they are 1) a uridine-rich enhancer element in all cell types tested except differentiated chondrocytes; 2) an adenine-rich silencer element, and 3) an enhancer cis element functional in the context of secondary structure. This is the first report identifying key cis elements in the COL2A1 gene that modulate the cell type-specific alternative splicing switch of exon 2 during cartilage development.

The role of sequence variations within the genes encoding collagen II, IX and XI in non-syndromic, early-onset osteoarthritis

OBJECTIVE

We sought to determine whether sequence variations in cartilage collagen genes are associated with primary, early-onset osteoarthritis (OA).

METHODS

The cartilage collagen genes, COL2A1, COL9A1, COL9A2, COL9A3, COL11A1 and COL11A2, were screened for sequence variations in 72 Finnish probands and one US family with primary early-onset hip and/or knee OA. In addition, allelic association studies were performed using six to 12 common polymorphisms from each gene by genotyping 72 OA patients and 103 controls.

RESULTS

Altogether 239 sequence variations were found, of which 16 were not present in the controls. Seven of the unique variations, four in COL11A1, two in COL11A2 and one in COL2A1, were studied further, because they resulted in the substitution of conserved amino acids or were predicted to affect mRNA splicing. Co-segregation of a sequence variation and the phenotype was found in all four families available for study. Association analysis failed to identify any common predisposing alleles.

CONCLUSIONS

Early-onset OA demonstrates locus and allelic heterogeneity since the identified variations were in three different collagen genes and each of the six probands had a different mutation. It is also possible that some OA cases represent the mild end of the chondrodysplasia phenotypic spectrum. The major susceptibility alleles in this form of OA, however, remain to be identified.

The homeobox transcription factor Barx2 regulates chondrogenesis during limb development

Among the many factors involved in regulation of chondrogenesis, bone morphogenetic proteins (BMPs) and members of the Sox and homeobox transcription factor families have been shown to have crucial roles. Of these regulators, the homeobox transcription factors that function during chondrogenesis have been the least well defined. We show here that the homeobox transcription factor Barx2 is expressed in primary mesenchymal condensations, digital rays, developing joints and articular cartilage of the developing limb, suggesting that it plays a role in chondrogenesis. Using retroviruses and antisense oligonucleotides to manipulate Barx2 expression in limb bud micromass cultures, we determined that Barx2 is necessary for mesenchymal aggregation and chondrogenic differentiation. In accordance with these findings, Barx2 regulates the expression of several genes encoding cell-adhesion molecules and extracellular matrix proteins, including NCAM and collagen II (Col2a1) in the limb bud. Barx2 bound to elements within the cartilage-specific Col2a1 enhancer, and this binding was reduced by addition of Barx2 or Sox9 antibodies, or by mutation of a HMG box adjacent to the Barx2-binding element, suggesting cooperation between Barx2 and Sox proteins. Moreover, both Barx2 and Sox9 occupy Col2a1 enhancer during chondrogenesis in vivo. We also found that two members of the BMP family that are crucial for chondrogenesis, GDF5 and BMP4, regulate the pattern of Barx2 expression in developing limbs. Based on these data, we suggest that Barx2 acts downstream of BMP signaling and in concert with Sox proteins to regulate chondrogenesis.

A stop codon mutation in COL11A2 induces exon skipping and leads to non-ocular Stickler syndrome

Mutations in COL11A2 cause a spectrum of phenotypes affecting chondrogenic tissues. We analyzed this gene by conformation sensitive gel electrophoresis (CSGE) and sequencing in a family with non-ocular Stickler syndrome, and found a heterozygous C --> T mutation in exon 57 + 13 in affected members, resulting in Arg893Stop codon. Since heterozygous nonsense mutations in COL11A2 do not usually lead to any obvious phenotype, all exons and exon boundaries of COL11A2 in the sample of the propositus were sequenced. Because no disease-associated alterations were found, we performed RT-PCR analysis on the RNA. Analysis showed skipping of exon 57 in one allele, resulting in an inframe deletion of 54 bp or 18 amino acids, which would explain the phenotype observed in the family. Thus, the exon skipping resulted from a nonsense-associated altered splicing (NAS). This article contains supplementary material, which may be viewed at the American Journal of Medical Genetics website at http://www.interscience.wiley.com/jpages/0148-7299/suppmat/index.html.

Sequence variations in the collagen IX and XI genes are associated with degenerative lumbar spinal stenosis

BACKGROUND

Degenerative lumbar spinal stenosis (LSS) is usually caused by disc herniation or degeneration. Several genetic factors have been implicated in disc disease. Tryptophan alleles in COL9A2 and COL9A3 have been shown to be associated with lumbar disc disease in the Finnish population, and polymorphisms in the vitamin D receptor gene (VDR) (FokI and TaqI), the matrix metalloproteinase-3 gene (MMP-3) and an aggrecan gene (AGC1) VNTR have been reported to be associated with disc degeneration. In addition, an IVS6-4 a>t polymorphism in COL11A2 has been found in connection with stenosis caused by ossification of the posterior longitudinal ligament in the Japanese population.

OBJECTIVE

To study the role of genetic factors in LSS.

METHODS

29 Finnish probands were analysed for mutations in the genes coding for intervertebral disc matrix proteins, COL1A1, COL1A2, COL2A1, COL9A1, COL9A2, COL9A3, COL11A1, COL11A2, and AGC1. VDR and MMP-3 polymorphisms were also analysed. Sequence variations were tested in 56 Finnish controls.

RESULTS

Several disease associated alleles were identified. A splice site mutation in COL9A2 leading to a premature translation termination codon and the generation of a truncated protein was identified in one proband, another had the Trp2 allele, and four others the Trp3 allele. The frequency of the COL11A2 IVS6(-4) t allele was 93.1% in the probands and 72.3% in controls (p = 0.0016). The differences in genotype frequencies for this site were less significant (p = 0.0043).

CONCLUSIONS

Genetic factors have an important role in the pathogenesis of LSS.

Regulation of protein diversity by alternative pre-mRNA splicing with specific focus on chondrogenesis

Analysis of the human genome has dramatically demonstrated that the majority of protein diversity is generated by alternative splicing of pre-mRNA. This powerful and versatile mechanism controls the synthesis of functionally different protein isoforms that may be required during specific stages of development from a single gene. Consequently, ubiquitous and/or tissue-specific RNA splicing factors that regulate this splicing mechanism provide the basis for defining phenotypic characteristics of cells during differentiation. In this review, we will introduce the basic mechanisms of pre-mRNA alternative splicing, describe how this process is regulated by specific RNA splicing factors, and relate this to various systems of cell differentiation. Chondrogenesis, a well-defined differentiation pathway necessary for skeletogenesis, will be discussed in detail, with focus on some of the alternatively-spliced proteins known to be expressed during cartilage development. We propose a heuristic view that, ultimately, it is the regulation of these RNA splicing factors that determines the differentiation status of a cell. Studying regulation at the level of pre-mRNA alternative splicing will provide invaluable insights into how many developmental mechanisms are controlled, thus enabling us to manipulate a system to select for a specific differentiation pathway.

A missense mutation in the mouse Col2a1 gene causes spondyloepiphyseal dysplasia congenita, hearing loss, and retinoschisis

A missense mutation in the mouse Col2a1 gene has been discovered, resulting in a mouse phenotype with similarities to human spondyloepiphyseal dysplasia (SED) congenita. In addition, SED patients have been identified with a similar molecular mutation in human COL2A1. This mouse model offers a useful tool for molecular and biological studies of bone development and pathology.

INTRODUCTION

A new mouse autosomal recessive mutation has been discovered and named spondyloepiphyseal dysplasia congenita (gene symbol sedc).

MATERIALS AND METHODS

Homozygous sedc mice can be identified at birth by their small size and shortened trunk. Adults have shortened noses, dysplastic vertebrae, femora, and tibias, plus retinoschisis and hearing loss. The mutation was mapped to Chr15, and Col2a1 was identified as a candidate gene.

RESULTS

Sequence analyses revealed that the affected gene is Col2a1, which has a missense mutation at exon 48 causing an amino acid change of arginine to cysteine at position 1417. Two human patients with spondyloepiphyseal dysplasia (SED) congenita have been reported with the same amino acid substitution at position 789 in the human COL2A1 gene.

CONCLUSIONS

Thus, sedc/sedc mice provide a valuable model of human SED congenita with molecular and phenotypic homology. Further biochemical analyses, molecular modeling, and cell culture studies using sedc/sedc mice could provide insight into mechanisms of skeletal development dependent on Col2a1 and its role in fibril formation and cartilage template organization.

Egr-1 mediates transcriptional repression of COL2A1 promoter activity by interleukin-1beta

Following induction and activation of the early growth response (Egr)-1 transcription factor in human chondrocytes, interleukin-1beta (IL-1beta) suppresses the expression of the type II collagen gene (COL2A1), associated with induction of Egr-1 binding activity in nuclear extracts. The COL2A1 proximal promoter contains overlapping binding sites for Egr-1 and Sp1 family members at -119/-112 bp and -81/-74 bp. Mutations that block binding of Sp1 and Sp3 to either site markedly reduce constitutive expression of the core promoter. IL-1beta-induced Egr-1 binds strongly to the -119/-112 bp site, and mutations that block Egr-1 binding prevent inhibition by IL-1beta. Cotransfection with pCMV-Egr1 potentiates the inhibition of COL2A1 promoter activity by IL-1beta, whereas overexpression of dominant-negative Egr-1 mutant, Wilm's tumor-1 (WT1)/Egr1, Sp1, or Sp3 reverses the inhibition by IL-1beta. Cotransfection of pGL2-COL2/Gal4, in which we substituted the critical residue for Egr-1 binding with a Gal4 binding domain and a pCMV-Gal4-Egr1 chimera permits an inhibitory response to IL-1beta that is reversed by overexpression of Gal4-CBP. Our results indicate that IL-1beta-induced activation of Egr-1 binding is required for inhibition of COL2A1 proximal promoter activity and suggest that Egr-1 acts as a repressor of a constitutively expressed collagen gene by preventing interactions between Sp1 and the general transcriptional machinery.

Collagen XI sequence variations in nonsyndromic cleft palate, Robin sequence and micrognathia

Cleft palate is a common birth defect, but its etiopathogenesis is mostly unknown. Several studies have shown that cleft palate has a strong genetic component. Robin sequence consists of three of the following four findings: micrognathia, glossoptosis, obstructive apnea, and cleft palate. While cleft palate is mainly nonsyndromic, about 80 percent of Robin sequence cases are associated with syndromes. Mutations in genes coding for cartilage collagens II and XI, COL2A1, COL11A1 and COL11A2, have been shown to cause chondrodysplasias that are commonly associated with Robin sequence, micrognathia or cleft palate. We therefore analyzed a cohort of 24 patients with nonsyndromic Robin sequence, 17 with nonsyndromic cleft palate and 21 with nonsyndromic micrognathia for mutations in COL11A2. A total of 23 Robin sequence patients were also analyzed for mutations in COL2A1 and COL11A1. We detected two disease-associated mutations in patients with Robin sequence, an Arg to stop codon mutation in COL11A2 and a splicing mutation in COL11A1. Two putatively disease-associated sequence variations were found in COL11A1 in Robin sequence patients, one in COL11A2 in a patient with micrognathia and one in COL2A1 in two patients with Robin sequence. The results showed that sequence variations in these genes can play a role in the etiology of Robin sequence, cleft palate and micrognathia but are not common causes of these phenotypes.

Genomic organization of the human COL3A1 and COL5A2 genes: COL5A2 has evolved differently than the other minor fibrillar collagen genes

We report here on the complete structure of the human COL3A1 and COL5A2 genes. Collagens III and V, together with collagens I, II and XI make up the group of fibrillar collagens, all of which share a similar structure and function; however, despite the similar size of the major triple-helical domain, the number of exons coding for the domain differs between the genes for the major fibrillar collagens characterized so far (I, II, and III) and the minor ones (V and XI). The main triple-helical domain being encoded by 49-50 exons, including the junction exons, in the COL5A1, COL11A1 and COL11A2 genes, but by 43-44 exons in the genes for the major fibrillar collagens. Characterization of the genomic structure of the COL3A1 gene confirmed its association with the major fibrillar collagen genes, but surprisingly, the genomic organization of the COL5A2 gene was found to be similar to that of the COL3A1 gene. We also confirmed that the two genes are located in tail-to-tail orientation with an intergenic distance of approximately 22 kb. Phylogenetic analysis suggested that they have evolved from a common ancestor gene. Analysis of the genomic sequences identified a novel single nucleotide polymorphism and a novel dinucleotide repeat. These polymorphisms should be useful for linkage analysis of the Ehlers-Danlos syndrome and related disorders.

Col2-GFP reporter marks chondrocyte lineage and chondrogenesis during mouse skeletal development

Mice were generated in which a Col2-GFP transgene serves as a reporter for the chondrocyte lineage and for chondrogenesis in live embryos and newborn pups. Cells actively engaged in chondrogenesis were identified by confocal optical sectioning within their native environments in embryos and in thick tissue slices. Chondrocytes exhibiting GFP fluorescence were purified from rib cages by high-speed cell sorting of crude cell suspensions. Intensity of fluorescence correlated with biosynthesis of procollagen II in these cells. The use of these mice and their cells provides a novel approach for studying chondrocyte differentiation and chondrogenesis during skeletal development.

Autosomal recessive disorder otospondylomegaepiphyseal dysplasia is associated with loss-of-function mutations in the COL11A2 gene

Otospondylomegaepiphyseal dysplasia (OSMED) is an autosomal recessive skeletal dysplasia accompanied by severe hearing loss. The phenotype overlaps that of the autosomal dominant disorders-Stickler and Marshall syndromes-but can be distinguished by disproportionately short limbs, severe hearing loss, and lack of ocular involvement. In one family with OSMED, a homozygous Gly-->Arg substitution has been described in COL11A2, which codes for the alpha2 chain of type XI collagen. We report seven further families with OSMED. All affected individuals had a remarkably similar phenotype: profound sensorineural hearing loss, skeletal dysplasia with limb shortening and large epiphyses, cleft palate, an extremely flat face, hypoplasia of the mandible, a short nose with anteverted nares, and a flat nasal bridge. We screened affected individuals for mutations in COL11A2 and found different mutations in each family. Individuals from four families, including three with consanguineous parents, were homozygous for mutations. Individuals from three other families, in whom parents were nonconsanguineous, were compound heterozygous. Of the 10 identified mutations, 9 are predicted to cause premature termination of translation, and 1 is predicted to cause an in-frame deletion. We conclude that the OSMED phenotype is highly homogenous and results from homozygosity or compound heterozygosity for COL11A2 mutations, most of which are predicted to cause complete absence of alpha2(XI) chains.

Splicing mutations of 54-bp exons in the COL11A1 gene cause Marshall syndrome, but other mutations cause overlapping Marshall/Stickler phenotypes

Stickler and Marshall syndromes are dominantly inherited chondrodysplasias characterized by midfacial hypoplasia, high myopia, and sensorineural-hearing deficit. Since the characteristics of these syndromes overlap, it has been argued whether they are distinct entities or different manifestations of a single syndrome. Several mutations causing Stickler syndrome have been found in the COL2A1 gene, and one mutation causing Stickler syndrome and one causing Marshall syndrome have been detected in the COL11A1 gene. We characterize here the genomic structure of the COL11A1 gene. Screening of patients with Stickler, Stickler-like, or Marshall syndrome pointed to 23 novel mutations. Genotypic-phenotypic comparison revealed an association between the Marshall syndrome phenotype and splicing mutations of 54-bp exons in the C-terminal region of the COL11A1 gene. Null-allele mutations in the COL2A1 gene led to a typical phenotype of Stickler syndrome. Some patients, however, presented with phenotypes of both Marshall and Stickler syndromes.

Complete sequence of the 23-kilobase human COL9A3 gene. Detection of Gly-X-Y triplet deletions that represent neutral variants

We report the complete sequence of the human COL9A3 gene that encodes the alpha3 chain of heterotrimeric type IX collagen, a member of the fibril-associated collagens with interrupted triple helices family of collagenous proteins. Nucleotide sequencing defined over 23,000 base pairs (bp) of the gene and about 3000 bp of the 5'-flanking sequences. The gene contains 32 exons. The domain and exon organization of the gene is almost identical to a related gene, the human COL9A2 gene. However, exon 2 of the COL9A3 gene codes for one -Gly-X-Y- triplet less than exon 2 of the COL9A2 gene. The difference is compensated by an insertion of 9 bp coding for an additional triplet in exon 4 of the COL9A3 gene. As a result, the number of -Gly-X-Y- repeats in the third collagenous domain remains the same in both genes and ensures the formation of an in-register triple helix. In the course of screening this gene for mutations, heterozygosity for separate 9-bp deletions within the COL1 domain were identified in two kindreds. In both instances, the deletions did not co-segregate with any disease phenotype, suggesting that they were neutral variants. In contrast, similar deletions in triple helical domain of type I collagen are lethal. To study whether alpha3(IX) chains with the deletion will participate in the formation of correctly folded heterotrimeric type IX collagen, we expressed mutant alpha3 chains together with normal alpha1 and alpha2 chains in insect cells. We show here that despite the deletion, mutant alpha3 chains were secreted as heterotrimeric, triple helical molecules consisting of three alpha chains in a 1:1:1 ratio. The results suggest that the next noncollagenous domain (NC2) is capable of correcting the alignment of the alpha chains, and this ensures the formation of an in-register triple helix.

Restoration of normal bone development by human homologue of collagen type II (COL2A1) gene in Col2a1 null mice

Development of the vertebrate skeleton is a highly complex process in which collagen type II plays a vital role in the formation of long bones via endochondral ossification. Collagen type II, which is encoded by a single COL2A1/ Col2a1 gene, is the most abundant structural protein in the cartilage matrix, where it undergoes complex interactions with several other proteins. The sequence of mature collagen type II chains, each with about 1,100 amino acids, is conserved between different mammalian species. There are 37 amino acid positions that are different between mouse and human collagen type II. Previously, we have demonstrated that transgenic mice, in which Col2a1 gene is knocked out, exhibit a lethal phenotype due to the absence of endochondral bone formation. To investigate whether the biological role of collagen type II is conserved between the species, human COL2A1 gene was expressed in Col2a1 null mice by crossing with transgenic mice in which human COL2A1 gene was integrated. The collagen type II from human gene rescued the lethal phenotype in null mice, indicating that the biological function of collagen type II is conserved between human and mouse. The animals exhibited normal endochondral bone formation and a normal growth plate in tibio-tarsal joint. Chondrocytes isolated from the cartilage of these mice secreted human protein, suggesting that the animals incorporated heterologous protein to form cartilage which is essentially "humanized." The animals reached puberty and produced normal progeny. A completely normal phenotype in newborns indicates that human COL2A1 gene is expressed properly both temporally and spatially. These animals may be useful to generate models to study the effect of COL2A1 mutations on skeletal development in humans by introducing mutated gene constructs either into embryos or by crossing with transgenic animals with COL2A1 mutations.

Analysis of the COL1A1 and COL1A2 genes by PCR amplification and scanning by conformation-sensitive gel electrophoresis identifies only COL1A1 mutations in 15 patients with osteogenesis imperfecta type I: identification of common sequences of null-allele mutations

Although >90% of patients with osteogenesis imperfecta (OI) have been estimated to have mutations in the COL1A1 and COL1A2 genes for type I procollagen, mutations have been difficult to detect in all patients with the mildest forms of the disease (i.e., type I). In this study, we first searched for mutations in type I procollagen by analyses of protein and mRNA in fibroblasts from 10 patients with mild OI; no evidence of a mutation was found in 2 of the patients by the protein analyses, and no evidence of a mutation was found in 5 of the patients by the RNA analyses. We then searched for mutations in the original 10 patients and in 5 additional patients with mild OI, by analysis of genomic DNA. To assay the genomic DNA, we established a consensus sequence for the first 12 kb of the COL1A1 gene and for 30 kb of new sequences of the 38-kb COL1A2 gene. The sequences were then used to develop primers for PCR for the 103 exons and exon boundaries of the two genes. The PCR products were first scanned for heteroduplexes by conformation-sensitive gel electrophoresis, and then products containing heteroduplexes were sequenced. The results detected disease-causing mutations in 13 of the 15 patients and detected two additional probable disease-causing mutations in the remaining 2 patients. Analysis of the data developed in this study and elsewhere revealed common sequences for mutations causing null alleles.

Structural organization of the human and mouse laminin beta2 chain genes, and alternative splicing at the 5' end of the human transcript

We have determined the structural organization of the human and mouse genes that encode the laminin beta2 chain (s-laminin), an essential component of the basement membranes of the neuromuscular synapse and the kidney glomerulus. The human and mouse genes have a nearly identical exon-intron organization and are the smallest laminin chain genes characterized to date, due to the unusually small size of their introns. The laminin beta2 chain genes of both species consist of 33 exons that span </=12 kilobase pairs of genomic DNA. The exon-intron pattern of the laminin beta2 chain gene is also highly similar to that of the human genes encoding the homologous laminin beta1 and beta3 chains. The putative promoter regions of the human and mouse laminin beta2 chain genes have features characteristic of the promoters of genes that have a limited tissue expression. Considerable conservation of the intron sequences of the mouse and human genes was observed. The first intron of the human gene, located 1 base pair upstream of the translation start codon, contains a non-consensus 5' splice site. This intron was shown to be inefficiently spliced in humans, suggesting that post-transcriptional mechanisms may be involved in the regulation of laminin beta2 chain gene expression.

Developmental expression of a type II collagen/beta-galactosidase fusion gene in transgenic mice

The correct temporal and spatial expression of the type II collagen gene is believed to be important for normal development and growth of the skeleton and the eye, i.e., tissues where the protein product is predominantly found. To study transcriptional activation of type II collagen gene in skeletal and nonskeletal tissues we produced transgenic mice carrying murine proalpha1(II) collagen/beta-galactosidase fusion gene constructs. The expression of the fusion gene was found to depend on the presence of intron 1 deleted failed to reveal any beta-galactosidase activity confirming the important role of regulatory sequences within intron 1 of the gene. High-level expression of the functional construct was clearly confined to cartilaginous tissues but transient low-level expression was also observed in extraskeletal locations, such as the developing brain and the notochord. The results demonstrate that the regulatory elements in the proalpha1(II) collagen/beta-galactosidase fusion gene construct confer both temporal and spatial specificity indistinguishable from that of the endogenous proalpha1(II) collagen gene as determined by the presence of the corresponding mRNA by in situ hybridization. Furthermore the beta-galactosidase activity correlated well with the progression of chondrogenesis as seen by staining of whole mouse embryos with Alizarin red S and Alcian blue in the hybrid mouse strain used for microinjections. The transgenic mouse line produced should prove useful for studies on various aspects of chondrogenesis. Furthermore, the data shows that the regulatory elements present in the construct are sufficient for targetting the expression of other genes in cartilage.