Microfibrils 10 -12 nm in diameter are found in elastic and non-elastic tissues with fibrillin as a major component. Little is known about the supramolecular structure of these microfibrils and the protein interactions it is based on. To identify protein binding ligands of fibrillin-1, we tested binding of recombinant fibrillin-1 peptides to different extracellular matrix proteins in solid phase assays. Among the proteins tested, only fibulin-2 showed significant binding to rF11, the N-terminal half of fibrillin-1, in a calcium-dependent manner. Surface plasmon resonance demonstrated high affinity binding with a K d ‫؍‬ 56 nM. With overlapping recombinant fibrillin-1 peptides, the binding site for fibulin-2 was narrowed down to the N terminus of fibrillin-1 (amino acid positions 45-450). Immunofluorescence in tissues demonstrated colocalization of fibrillin and fibulin-2 in skin, perichondrium, elastic intima of blood vessels, and kidney glomerulus. Fibulin-2 was not present in ocular ciliary zonules, tendon, and the connective tissue around kidney tubules and lung alveoli, which all contain fibrillin. Immunogold labeling of fibulin-2 on microfibrils in skin was found preferentially at the interface between microfibrils and the amorphous elastin core, suggesting that in vivo the interaction between fibrillin-1 and fibulin-2 is regulated by cellular expression and deposition as well as by protein-protein interactions.

Ullrich syndrome is a recessive congenital muscular dystrophy affecting connective tissue and muscle. The molecular basis is unknown. Reverse transcription-PCR amplification performed on RNA extracted from fibroblasts or muscle of three Ullrich patients followed by heteroduplex analysis displayed heteroduplexes in one of the three genes coding for collagen type VI (COL6). In patient A, we detected a homozygous insertion of a C leading to a premature termination codon in the triple-helical domain of COL6A2 mRNA. Both healthy consanguineous parents were carriers. In patient B, we found a deletion of 28 nucleotides because of an A --> G substitution at nucleotide -2 of intron 17 causing the activation of a cryptic acceptor site inside exon 18. The second mutation was an exon skipping because of a G --> A substitution at nucleotide -1 of intron 23. Both mutations are present in an affected brother. The first mutation is also present in the healthy mother, whereas the second mutation is carried by their healthy father. In patient C, we found only one mutation so far-the same deletion of 28 nucleotides found in patient B. In this case, it was a de novo mutation, as it is absent in her parents. mRNA and protein analysis of patient B showed very low amounts of COL6A2 mRNA and of COL6. A near total absence of COL6 was demonstrated by immunofluorescence in fibroblasts and muscle. Our results demonstrate that Ullrich syndrome is caused by recessive mutations leading to a severe reduction of COL6.

Mutations in the extracellular matrix molecule collagen VI underlie the congenital muscular dystrophy types Ullrich and Bethlem. Establishing the origin of collagen VI in muscle is important for understanding the pathophysiology of these diseases and for developing future treatment approaches involving cell-specific delivery. Because the cells that produce collagen VI cannot be identified by histologic analysis, we examined the production of collagen VI in pure cultures of primary myogenic cells and muscle interstitial fibroblasts from limb muscle of neonatal mice. Immunofluorescence staining and Western blot analysis revealed secretion and matrix deposition of collagen VI by interstitial fibroblasts but not by myogenic cells in vitro. Using Northern blot and real-time reverse-transcriptase-polymerase chain reaction analysis for the collagen VI genes col6a1, col6a2, col6a3, transcript levels for the 3 mRNAs were high in interstitial fibroblasts, whereas in primary myogenic cells, they were indistinguishable from background. Furthermore, retention of mutant collagen VI in muscle from 3 patients with collagen VI mutation was identified in interstitial fibroblastic cells but not in their myofibers. These results suggest that interstitial fibroblasts but not myogenic cells contribute significantly to the deposition of collagen VI in the extracellular matrix in skeletal muscle and imply major roles of this cell type and the extracellular matrix in the pathogenesis of these diseases.

Nucleotide sequences were determined for two cloned cDNAs encoding for over three-fourths of the pro alpha 1 (I) chain of type I procollagen from man. Comparison with previously published data on amino acid sequences of the alpha 1 (I) chain of type I collagen made it possible to examine mutations in the transcribed products of the gene which have occurred during the evolution of man, calf, rat, mouse, and chick. Comparison of the nucleotide sequences with the corresponding sequences of cDNAs from chick [Fuller, F., & Boedtker, H. (1981) Biochemistry 20, 996] and with cDNAs for the pro alpha 2(I) chain from man [Bernard, M.P., Myers, J. C., Chu, M.-L., Ramirez, F., Eikenberry, E. F., & Prockop, D. J. (1983) Biochemistry 22, 1139] demonstrated that selective pressure during evolution for 250 million or more years acted more strongly on the structure of the pro alpha 1 (I) chain than on the pro alpha 2(I) chain. To improve the reliability of the comparison, the nucleotide sequences were examined with a modification of previous procedures for evaluating mutations in replacement sites and silent sites. The corrected divergence for replacement sites between the alpha 1 (I) chains was 6 +/- 0.8% whereas it was 15 +/- 1.9% for the alpha 2(I) chains. The C-propeptide domain of the pro alpha 1 (I) chain was also highly conserved with a corrected divergence at replacement sites of 5 +/- 0.9%, a value that was not distinguishable from the value previously found for the C-propeptide of the pro alpha 2(I) chain. Therefore, a large part of the structure of both C-propeptides appears to be under selective pressure. Inspection of changes in the C-propeptide of the pro alpha 1 (I) chain suggested that there was a highly conserved region around the carbohydrate attachment site similar to the highly conserved region of 37 amino acids previously found in the C-propeptide of the pro alpha 2(I) chain. Two statistical tests, however, were unable to confirm nonrandom distribution of changes in the C-propeptide of the pro alpha 1(I) chain. The same tests established the presence of a nonrandom distribution in nucleotide changes of the C-propeptide of the pro alpha 2(I) chain. The 3'-noncoding region of the cDNA for pro alpha 1(I) of human type I procollagen showed no homology with the same region in the chick.(ABSTRACT TRUNCATED AT 400 WORDS)

Mutations involving elastic tissue proteins result in a broad spectrum of phenotypes affecting skin, skeleton, ocular and vascular structures, including tortuous blood vessels and cutis laxa. Here we report on a female newborn with apparently long fingers, aortic aneurysm, tortuous pulmonary arteries and mild generalized lax skin. She died at 27 days of age due to severe respiratory distress and inoperable systemic vascular abnormalities. Skin biopsy showed marked paucity and fragmentation of elastic fibers and autopsy revealed occlusion of the pulmonary artery. DNA analysis identified compound heterozygous mutations ((c.835C > T (p.R279C)/c.1070_1073dupCCGC) in fibulin-4, a recently recognized elastic fiber associated protein. Analyses of dermal fibroblasts from the patient indicated that fibulin-4 mRNAs with the 4-bp duplication transcribed from one allele are probably subject to nonsense-mediated decay, whereas synthesis and secretion of the missense R279C fibulin-4 protein from the other allele is severely impaired. Immunostaining demonstrated a total absence of fibulin-4 fibers in the extracellular matrix deposited by the patient's fibroblasts. Our studies provide evidence that deficiency in fibulin-4 leads to a perinatal lethal condition associated with elastic tissue abnormalities.

Nucleotide sequences were determined for cloned cDNAs encoding for more than half of the pro alpha 2 chain of type I procollagen from man. Comparisons with previously published data on homologous cDNAs from chick embryos made it possible to examine evolution of the gene in two species which have diverged for 250-300 million years. The amino acid sequence of the alpha-chain domain supported previous indications that there is a strong selective pressure to maintain glycine as every third amino acid and to maintain a prescribed distribution of charged amino acids. However, there is little apparent selective pressure on other amino acids. The amino acid sequence of the C-propeptide domain showed less divergence than the alpha-chain domain. The 5' end or N terminus of the human C-propeptide, however, contained an insert of 12 bases coding for 4 amino acids not found in the chick C-propeptide. About 100 amino acid residues from the N terminus, two residues found in the chick sequence were missing from the human. In the second half of the C-propeptide, there was complete conservation of a 37 amino acid sequence and conservation of 50 out of 51 amino acids in the same region, an observation which suggested that the region serves some special purpose such as directing the association of one pro alpha 2(I) C-propeptide with two pro alpha 1(I) C-propeptides so as to produce the heteropolymeric structure of type I procollagen. In addition, comparison of human and chick DNAs for pro alpha 2(I) revealed three different classes of conservation of nucleotide sequence which have no apparent effect on the structure of the protein: a preference for U on the third base position of codons for glycine, proline, and alanine; a high degree of nucleotide conservation in the 51 amino acid highly conserved region of the C-propeptide; a high degree of nucleotide conservation in the 3'-noncoding region. These three classes of nucleotide conservation may reflect unusual features of collagen genes, such as their high GC content or their highly repetitive coding sequences.

The Bethlem myopathy is a rare autosomal dominant proximal myopathy characterized by early childhood onset and joint contractures. Evidence for linkage and genetic heterogeneity has been established, with the majority of families linked to 21q22.3 and one large family linked to 2q37, implicating the three type VI collagen subunit genes, COL6A1 (chromosome 21), COL6A2 (chromosome 21) and COL6A3 (chromosome 2) as candidate genes. Mutations of the invariant glycine residues in the triple-helical domain-coding region of COL6A1 and COL6A2 have been reported previously in the chromosome 21-linked families. We report here the identification of a G-->A mutation in the N-terminal globular domain-coding region of COL6A3 in a large American pedigree (19 affected, 12 unaffected), leading to the substitution of glycine by glutamic acid in the N2 motif, which is homologous to the type A domains of the von Willebrand factor. This mutation segregated to all affected family members, to no unaffected family members, and was not identified in 338 unrelated Caucasian control chromosomes. Thus mutations in either the triple-helical domain or the globular domain of type VI collagen appear to cause Bethlem myopathy.

No abstract