Isolation and characterization of genomic DNA coding for alpha 2 type I collagen

The chick alpha2(I) collagen gene contains two functional promoters, and its expression in chondrocytes is regulated at both transcriptional and post-transcriptional levels

Embryonic chick cartilages contain transcripts derived from the alpha2(I) collagen gene, although type I collagen is not normally found in these tissues; most of these RNAs are alternative transcripts initiating within intron 2. Use of the internal start site results in replacement of exons 1 and 2 with a previously undescribed exon and a change in the translational reading frame; thus, the alternative transcript cannot encode alpha2(I) collagen. We have demonstrated that production of the alternative transcript is due to activation of an internal promoter in chondrocytes and have identified a 179-base pair domain that is required for its activity. Furthermore, we have shown that the alternative transcript resulting from activation of the internal promoter turns over relatively rapidly; thus, the steady-state level of this transcript is less than predicted based on the transcription rate. The upstream promoter is only partially repressed in chondrocytes, suggesting that the lack of authentic alpha2(I) collagen mRNA may also be due in part to decreased mRNA stability. Thus, repression of alpha2(I) collagen synthesis in cartilage involves both transcriptional and post-transcriptional mechanisms. In contrast, repression of alpha1(I) collagen synthesis appears to be mediated primarily at the level of transcription.

Changes in ribosomal RNA, poly(A)+ RNA, and collagen alpha 2(I) mRNA synthesis and processing during hypertonic treatment of cultured chick embryo cells

The effect of hypertonic conditions on RNA synthesis in cultured chick embryo cells was examined. The appearance of newly synthesized 28 S, 18 S, and 4 S and 5 S RNA into the cytoplasm was found to be decreased by hypertonic conditions. The appearance of newly synthesized poly(A)+ RNA into the cytoplasm was also found to be depressed. To examine the behavior of a specific mRNA, nuclear and cytoplasmic levels of procollagen alpha 2(I) mRNA were measured during high salt treatment. While nuclear levels of this mRNA were found to increase, those of the cytoplasm fell markedly. S1 nuclease digestion studies of an intron flanked by two exons revealed that the pro alpha 2(I) collagen nuclear RNA that accumulated under hypertonic conditions was spliced. The nuclear accumulation of mRNA appears therefore to be due to a hypertonic block of nuclear-cytoplasmic transport, and not to an inhibition of RNA splicing.

Tissue specificity of type I collagen gene expression is determined at both transcriptional and post-transcriptional levels

We analyzed the control of type I collagen synthesis in four kinds of differentiated cells from chicken embryos which synthesize very different amounts of the protein. Tendon, skin, and smooth muscle cells were found to have identical amounts of type I collagen RNAs; however, the RNAs had inherently different translatabilities, which were observed both in vivo and in vitro. Chondrocytes also had substantial amounts of type I collagen RNAs, even though they directed no detectable synthesis of the protein either in vivo or in vitro. Type I collagen RNAs in chondrocytes display altered electrophoretic mobilities, suggesting that in these cells the reduction in translational efficiency may be mediated in part by changes in the RNA structure. These data indicate that control of type I collagen gene expression is a complex process which is exerted at both transcriptional and post-transcriptional levels.

Tissue specificity of type I collagen gene expression is determined at both transcriptional and post-transcriptional levels

We analyzed the control of type I collagen synthesis in four kinds of differentiated cells from chicken embryos which synthesize very different amounts of the protein. Tendon, skin, and smooth muscle cells were found to have identical amounts of type I collagen RNAs; however, the RNAs had inherently different translatabilities, which were observed both in vivo and in vitro. Chondrocytes also had substantial amounts of type I collagen RNAs, even though they directed no detectable synthesis of the protein either in vivo or in vitro. Type I collagen RNAs in chondrocytes display altered electrophoretic mobilities, suggesting that in these cells the reduction in translational efficiency may be mediated in part by changes in the RNA structure. These data indicate that control of type I collagen gene expression is a complex process which is exerted at both transcriptional and post-transcriptional levels.

DNase I sensitivity of the alpha 2(I) collagen gene: correlation with its expression but not with its methylation pattern

The chromatin structure of the chick alpha 2(I) collagen gene was probed with DNase I. Because our previous work strongly suggested that the 5' end of this gene is not methylated whereas the rest of the gene is methylated whether or not the gene is expressed, we compared the relative DNase I sensitivity of the methylated and unmethylated segments. Both regions demonstrate similar relative DNase I sensitivities within a given tissue. In chromatin of chick embryo fibroblasts, we find a DNase I hypersensitive site which maps between 100 and 300 bp preceding the start of transcription. This site is not found in brain chromatin but is present in chick embryo fibroblasts transformed by Rous Sarcoma virus although the rate of transcription of the alpha 2(I) collagen gene is greatly reduced in these cells. Hence, the mechanism responsible for the large decrease in alpha 2(I) collagen gene expression in RSV transformed cells is different from the mechanism that is responsible for the presence of a DNase I hypersensitive site in the promoter. Furthermore, changes in the DNase I sensitivity of the chromatin of the alpha 2(I) collagen promoter occur without changes in the methylation pattern of the gene.

Restriction fragment length polymorphism associated with the pro alpha 2(I) gene of human type I procollagen. Application to a family with an autosomal dominant form of osteogenesis imperfecta

One cloned complementary DNA and one genomic subclone were used to detect restriction fragment length polymorphism associated with the pro alpha 2(I) gene for human type I procollagen. The restriction fragments obtained from examination of 30-122 chromosomes confirmed previous indications that the pro alpha 2(I) gene is found in a single copy in the human haploid genome. One highly polymorphic site was detected with EcoRI in the 5'-half of the gene. The restriction site polymorphism at the site had an allelic frequency of 0.38, and it generated two fragments of 10.5 and 3.5 kilobase in homozygous individuals. The restriction fragment length polymorphism generated at the EcoRI site was used to study affected and non-affected individuals in four generations of a family with an autosomal dominant form of osteogenesis imperfecta. The data demonstrated a linkage of the phenotype to a pro alpha 2(I) allele with a lod score of 2.41 at a recombination fraction (theta) of 0. The data therefore provided presumptive evidence that osteogenesis imperfecta in this family is caused by a mutation in the pro alpha 2(I) gene or some contiguous region of the genome. The relatively high frequency of polymorphism at the EcoRI site makes it useful for studying a broad range of genetic disorders in which mutations in type I procollagen are suspected. In addition, the polymorphic site should provide useful markers for linkage studies with other loci located on human chromosome 7.

Abnormal alpha 2-chain in type I collagen from a patient with a form of osteogenesis imperfecta

Dermal fibroblasts in culture from a woman with a mild to moderate form of osteogenesis imperfecta synthesize two species of the pro alpha 2-chain of type I procollagen. One chain is normal. The abnormal chain has a slightly faster mobility than normal during electrophoresis in sodium dodecyl sulfate polyacrylamide gels. Analysis of cyanogen bromide peptides of the pro alpha-chain, the alpha-chain, and of the mammalian collagenase cleavage products of the pro alpha- and alpha-chains indicates that the abnormality is confined to the alpha 2(I)CB4 fragment and is consistent with loss of a short triple-helical segment. Type I collagen production was decreased, perhaps because the molecules that contained the abnormal chain were unstable, with a resultant alteration in the ratio of type III to type I collagen secreted into culture medium. Collagen fibrils in bone and skin had a normal periodicity but their diameters were 50% of control; the bone matrix was undermineralized. The structural abnormality in the alpha 2(I)-chain in this patient may affect molecular stability, intermolecular interactions, and collagen-mineral relationships that act to decrease the collagen content of tissues and affect the mineralization of bone.

Human type I procollagen genes are located on different chromosomes

A recombinant plasmid containing sequences complementary to human pro-alpha l(I) collagen mRNA was used for the chromosomal assignment of the pro-alpha l(I) collagen gene. Restriction endonuclease analysis of DNA from mouse-human and Chinese hamster-human somatic cell hybrids revealed cosegregation with human chromosome 17. Hybrids containing derivative chromosomes with a t(2;17)(q14;q21) translocation showed cosegregation of the pro-alpha l(I) gene with the segment 17q21 leads to qter. In situ hybridization on human metaphasic chromosomes confirmed this conclusion.

The collagen alpha-2 chain gene

A number of DNA sequences specific for collagen messenger RNAs and genes have been isolated, cloned in bacterial plasmids or bacteriophages, and studied in detail. Such sequences have been used to study regulatory mechanisms underlying the production of type I collagen in fibroblasts in culture, fibroblasts after viral transformation, and in tissues and organs during embryonic and fetal development. It is clear that a variety of mechanisms, transcriptional, translational and post-translational, are used by cells to regulate collagen production. The study of isolated collagen gene fragments coding for the alpha 2 collagen chain in sheep and chick have shown that many genes are very large, and are interrupted by as many as 50 intervening sequences. Additionally, the structure of the genes in the regions coding for the helical regions of the protein provides evidence that collagen genes may have arisen from the reduplication of a DNA segment containing a primordial collagen gene sequence. The availability of specific cloned collagen gene sequences will allow the precise chromosomal location of the collagen genes as well as the number and the linkage relationships between these genes. In addition, genetic disorders of connective tissue where alterations in collagen structure are implicated will now be amenable to analysis at the DNA level.

Unusual methylation pattern of the alpha 2 (l) collagen gene

We studied the methylation pattern of the alpha 2 (type 1) collagen gene in DNA from five cell types with varying rates of type I collagen synthesis: chick embryo fibroblast (CEF), CEF transformed by Rous sarcoma virus, erythrocyte, brain and sperm. The methylation-sensitive restriction enzymes, Msp I, Hpa II, Ava I and Sma I were used to detect methylation in three regions of the alpha 2 (type I) collagen gene: a 5.7 kb region, which includes the start site of transcription and the first two exons of the collagen gene; a 5.2 kb region containing exons in the middle of the gene; and a 3.5 kb region containing exons in the 3' portion of the gene. The DNA around the start site of transcription is not methylated whether or not the cells synthesize collagen. In contrast, the DNA from the central and 3' region of the gene is methylated to about the same extent whether or not the cells make collagen. Our data indicate that a gene that is methylated can be actively transcribed and that the level of expression of the alpha 2 (type I) collagen gene seems to be independent of methylation.

Significant progress in dermatologic research since 1977

Some active fields of experiment dermatology have been selected to demonstrate the interaction between basic research and clinical dermatology. The identification of the Langerhans cell, the typing of mononuclear cells, and the identification of T cell growth factors have significant implication in contact dermatitis, lymphomas, etc. The group of papovaviruses is better defined in relationship to the type of disease that they produce and its oncogenic potential. Various types of vasculitis are better understood, thanks to research in humoral immunity and complement activation. Melanogenesis and its control by peptides is progressing. New specific proteins have been identified in the connective tissues, and their role has been clarified. Identification of specific proteins of keratinocytes and study of differentiation of these cells have provided useful information and some skin disorders. The control of epidermal cell proliferation and differentiation, through membrane receptors, growth factors, and intracellular enzymes, is progressively giving clues to the understanding of genetic disorders, cancers, the effect of retinoids and phototherapy.

Marfan syndrome: abnormal alpha 2 chain in type I collagen

Cells in culture from a woman with a variety of the Marfan syndrome produce two species of the alpha 2 chains of type I collagen. One alpha 2 chain appears normal; the abnormal chain has a higher apparent molecular weight than normal and migrates more slowly during electrophoresis in sodium dodecyl sulfate/polyacrylamide gels. A similar change in electrophoretic behavior is seen in the prepro alpha 2 chain and the pN alpha 2 chain (which contains the amino-terminal extension). Asymmetric cleavage of the pepsin-treated procollagens with a fibroblast collagenase locates the abnormal segment amino terminal to the cleavage site, and analysis of cyanogen bromide peptides of collagenase cleavage peptides and of whole collagens indicates that the abnormal segment is in either the alpha 2CB3 peptide or the short segment of alpha 2CB5 amino terminal to the collagenase site of the altered alpha 2 chain. The higher apparent molecular weight is consistent with the insertion of a small peptide fragment of approximately 20 amino acids. This alteration in chain size has marked effects on crosslinking because collagen from the patient's skin was 5-10 times more extractable in nondenaturing solvents than that from control skins. Although the abnormal chain was not found in several other individuals with the Marfan syndrome, these findings suggest that the phenotype may be the expression of a variety of primary structure alterations in the chains of type I collagen that interfere with normal crosslink formation.

Structure of the promoter for chicken alpha 2 type I collagen gene

The chicken alpha 2 type I collagen gene is 38 kilobases long and its coding information is subdivided into more than 50 exons. In the current study, we used primer extension and S1 nuclease mapping to determine the sequence of the 5' end of alpha 2 collagen mRNA and to locate the start site for transcription of the alpha 2 collagen gene. The DNA sequence around the start site for transcription shows a typical Goldberg-Hogness sequence, 5' T-A-T-A-A-A-T 3', between -33 and -26 and a 5' G-C-C-C-A-T-T 3' sequence ("CAT" box) between -84 and -78. Three AUGs are found in the initial portion of the mRNA, the first from +54 to +56, the second from +117 to +119, and the third from +134 to +136. The first two AUGs are followed by short coding sequences that could specify a hexapeptide a tetrapeptide, respectively. Only the third AUG is followed by an open reading frame coding for a sequence that presents considerable homology with the previously determined amino acid sequence of prepro alpha 1 collagen. In the promoter region sequence there are several extensive dyads of symmetry. Three of these inverted repeats which precede the start site for transcription overlap each other and may have a role in the developmental regulation of this gene.

Decrease in the levels of nuclear RNA precursors for alpha 2 collagen in Rous sarcoma virus transformed fibroblasts

We have examined the levels of type I alpha 2 collagen RNA precursors, containing both intron and exon sequences in nuclear RNA preparations of chick embryo fibroblasts (CEF) and of CEF transformed by the Schmidt-Rupin strain of Rous sarcoma virus (RSV). We have used two different fragments of chick alpha 2 collagen genomic DNA as hybridization probes in S1 mapping experiments. Each of these DNA probes contains an entire intron. Our results indicate that the levels of the primary transcript of alpha 2 collagen RNA are much lower in RSV-CEF than in CEF. They suggest, but do not prove that the effect of the transforming protein p60src on the synthesis of alpha 2 collagen is mediated by a transcriptional control mechanism.

Fine structural analysis of the chicken pro alpha 2 collagen gene

Forty-two kilobase pairs of cloned chicken DNA containing 80% of the pro alpha 2 (type I) collagen gene and 8 kilobase pairs of 3' flanking sequences have been isolated. Detailed analysis of these clones indicates that this collagen gene spans approximately 40 kilobase pairs of DNA and contains on the order of 50 introns. The fine structure of 40% of the pro alpha 2 gene, including its 3' end, was determined by Southern blot restriction endonuclease mapping using a 2.6-kilobase pair procollagen cDNA clone pCg45, as a probe, and by DNA sequence determination of more than 2 kilobase pairs of this part of the genome. Exons in the triple-helical coding region are all multiples of the 9 base pairs coding for the Gly-X-Y triplet and vary in size from 45 to 108 base paris. The sequences of all six exons in a 3.8-kilobase pair EcoRI fragment were determined. One of these, a 249-base pair exon, joins the collagen domains; it codes for the last 15 amino acids of the triple-helical coding region, the telopeptide, and the first 53 amino acids of the carboxy-terminal propeptide.

Identification of a Balb/c mouse pro alpha 1(I) procollagen gene: evidence for insertions or deletions in gene coding sequences

We report the first isolation and identification of a mouse genomic fragment encoding amino acid sequences for the pro alpha 1(I) chain of type I procollagen. The DNA sequence of eight coding sequences is presented; five of these are 54 bp and three are 108 bp in length. Together these specify 198 amino acids which are 94% homologous to the corresponding bovine pro alpha 1(I) chain protein sequences. Each of the eight coding sequences is flanked by appropriate splice-junction sequences that exhibit considerable sequence complementarity to the rat small nuclear U1a RNA. In the 198 codons examined in this mouse genomic clone, the preferred codons for glycine and alanine are GGU (46/67) and GCU (23/30), respectively. This is in contrast to the codon usage reported for the chicken pro alpha 1(I) cDNA clone (Fuller and Boedtker, 1981). The examined coding sequences exhibit considerable nucleotide homology in both end-to-end and in staggered alignments. Based on an analysis of this homology data, a model is presented for the generation of 108-bp coding sequences from 54-bp units by two successive homologous recombinational events within coding sequences. Alternatively, the 108-bp units may have arisen by precise deletions of an intervening sequence between 54-bp coding sequences. Evidence supporting this is provided by a comparison of pro alpha 1(I) and pro alpha 2(I) genes. In the mouse pro alpha 1(I) gene amino acids 856-891 are encoded in a 108-bp unit; the chicken pro alpha 2(I) gene these residues are encoded in two 54-bp coding sequences. In addition, the coding sequences for nearly 50% of the alpha domain are condensed in the pro alpha 1(I) gene into a region approximately one half the size occupied by the comparable sequences in the pro alpha 2(I) gene.

The collagen gene: evidence for its evolutinary assembly by amplification of a DNA segment containing an exon of 54 bp

We have determined the size and sequence of 8 exons in the gene that specifies chick type 1 alpha 2 collagen. These 8 exons represent three different segments of the gene, but all encode information for the helical portion of the protein. Seven exons have a length of 54 bp, and the 8th has a size of 99 bp. The sequences within these exons vary except for th glycine codons, which occur every third triplet. Each exon begins with a glycine codon and ends with a triplet that precedes a glycine codon. The size and the sequences of the introns do not show any homology except at their ends. Of 7 introns examined the first six bases at the 5' end of 5 of these are identical. The sequences at the 3' end of the introns also show homologies. Our results imply that the ancestral gene for collagen arose by multiple duplications of a single genetic unit containing a 54 bp condig segment. The sequences within these exons drifted by successive point mutations and in some cases by additions or deletions of 9 bp or multiples of 9 bp.

Isolation and characterization of overlapping genomic clones covering the chicken alpha 2 (type I) collagen gene

A series of overlapping recombinant clones, which cover the alpha 2 (type I) collagen gene, have been isolated by stepwise screening of two libraries of chicken genomic DNA fragments. The first genomic clone was isolated by using a cloned cDNA containing alpha 2 collagen DNA sequences as hybridization probe. The other clones were obtained by a sequence of screenings using defined fragments of the successive genomic clones as hybridization probes. Several types of experiments indicated that the DNA of these clones are truly overlapping and span 55 kilobase pairs of contiguous DNA sequences in the chicken genome. Sequence analysis of small DNA segments of some of these clones confirm that they contain coding sequences which specify alpha 2 collagen. Electron microscopic analysis of hybrids between type I alpha 2 collagen mRNA and the overlapping genomic clones indicates that the chicken alpha 2 collagen gene has a length of at least 37 kilobases, about 7.4 times longer than the corresponding translatable cytoplasmic mRNA. The coding information for alpha 2 collagen is distributed in more than 50 coding sequences which are interrupted by intervening sequences of various sizes. The structure of the gene implies that the conversion of precursor RNA to mature mRNA for alpha 2 collagen includes at least 50 splicing events.