PERSPECTIVE SERIES Matricellular proteins E. Helene Sage, Series Editor The Ehlers-Danlos syndrome: on beyond collagens Jau-Ren Mao and James Bristow Department of Pediatrics and Cardiovascular Research Institute, University of California-San Francisco, San Francisco, California, USA Address correspondence to: James Bristow, University of California-San Francisco Laurel Heights Campus, 3333 California Street, Box 1245, Rm 150K, San Francisco, California 94118, USA. Phone: (415) 476-0911; Fax: (415) 476-3586; E-mail: [email protected]. The Ehlers-Danlos syndrome (EDS) is a clinically and remodeling. These processes are of critical importance genetically heterogeneous connective tissue disorder during development, wound healing, and aging. affecting as many as 1 in 5,000 individuals (1). EDS is characterized in its most common form by hyperexten- Collagen metabolism and molecular mechanisms sibility of the skin, hypermobility of joints often result- of EDS ing in dislocations, and tissue fragility exemplified by The concept that EDS is a disorder of fibrillar colla- easy bruising, atrophic scars following superficial injury, gen metabolism is well supported by identification and premature rupture of membranes during pregnan- of specific defects in the collagen biosynthetic path- cy. The recognition of frequent ultrastructural abnor- way that produce clinically distinct forms of EDS. malities of collagen fibrils in EDS patients led to the These are briefly reviewed below and in Figure 1. For concept that EDS is a disorder of fibrillar collagen more complete reviews of collagen biochemistry and metabolism (2). Following the identification of specific clinical aspects of EDS, the reader is directed to mutations in the genes encoding collagen types I, III, recent reviews (1, 2). and V, as well as several collagen processing enzymes, There are three fundamental mechanisms of disease the EDS classification scheme was collapsed into six dis- known to produce EDS: deficiency of collagen-pro- tinct clinical syndromes (3), emphasizing the molecular cessing enzymes, dominant-negative effects of mutant basis of each form (Table 1). collagen α-chains, and haploinsufficiency. The two Heterogeneity between the several clinical syndromes known examples of deficient enzyme activity leading to both complicates the diagnosis of EDS and makes EDS are lysyl-hydroxylase deficiency and procollagen accurate diagnosis imperative. Ultimately, one would peptidase deficiency. In the first case, the inability to like to be able to establish a molecular diagnosis for hydroxylate lysine residues precludes normal inter- each EDS patient. This is a laudable goal because it may molecular cross-linking of collagen trimers, and in the allow improved genetic counseling through correlation second instance, absence of procollagen peptidase pre- of mutant genotypes with specific outcomes or com- vents normal proteolytic cleavage of the NH2-terminus plications. However, as outlined below, the molecular of procollagen chains. In both circumstances the mor- defects described to date are not sufficient to explain phology and strength of the collagen fibril is compro- disease in many EDS patients, including those with the mised (Figure 2b), explaining the severe and early clin- most common classical and hypermobility types. As a ical findings. Because half-normal enzyme activity is result, the search for EDS genes recently has expanded sufficient for normal collagen processing, both of these beyond the collagens and collagen-modifying genes. An conditions are recessive. understanding of the complete complement of genes Dominant-negative mutations in collagen I, III, and V and proteins involved in EDS and the precise mecha- cause several different forms of EDS (Figure 1). The nisms by which they cause disease may teach us much most clinically significant of these are COL3A1 muta- about normal collagenous matrix deposition and tions causing the potentially lethal vascular form of Table 1 The Villefranche classification of EDS EDS type Clinical findings Inheritance Gene defects Classical (I/II) Skin and joint hypermobility, atrophic scars, easy bruising Autosomal dominant COL5A1, COL5A2 Hypermobility (III) Joint hypermobility, pain, dislocations Autosomal dominant Unknown Vascular (IV) Thin skin, arterial or uterine rupture, bruising, small joint hyperextensibility Autosomal dominant COL3A1 Kyphoscoliosis (VI) Hypotonia, joint laxity, congenital scoliosis, ocular fragility Autosomal recessive Lysyl-hydroxylase Arthrochalasia (VIIa,b) Severe joint hypermobility, skin mild, scoliosis, bruising Autosomal dominant COL1A1, COL1A2 Dermatosparaxsis(VIIc) Severe skin fragility, cutis laxa, bruising Autosomal recessive Procollagen N-peptidase The Journal of Clinical Investigation | May 2001 | Volume 107 | Number 9 1063 Matricellular proteins PERSPECTIVE SERIES E. Helene Sage, Series Editor ease is still unknown. Because collagen III is an impor- tant component of the arterial and bowel walls, these tissues are also frequently affected. Exon-skipping and missense mutations in the COL5A1 and COL5A2 genes have also been reported to cause classical EDS (5, 6), presumably through domi- nant-negative effects. However, two recent reports demonstrated that COL5A1 haploinsufficiency appears to be more common (7, 8). These studies showed that one-third of a subset of classical EDS patients that could be evaluated (because they are heterozygous for polymorphisms of COL5A1) did not express the mutant allele. Mutant COL5A1 transcripts are pre- sumably degraded through non-sense–mediated RNA decay. Truncating mutations were identified subse- quently in most haploinsufficient patients. The dra- Figure 1 matic clinical effect of COL5A1 haploinsufficiency is The biosynthetic pathway for the fibrillar collagens expressed in skin, surprising, given that collagen V is a quite minor con- identifying steps that are affected in different forms of EDS. (I) Collagen stituent of collagen in skin, tendon, and ligament, but gene transcription is highly regulated, but haploinsufficiency for COL5A1 the clinical phenotype clearly supports an important is uncompensated and leads to a reduction in COL5A1 mRNA and α1(V) function for collagen V in the maintenance of biome- procollagen chains. This accounts for 30–50% of classical EDS cases. (II) chanical integrity of collagenous matrix. Many proline and lysine residues in the translated procollagen chains are One clearly established function of collagen V is lim- hydroxylated by lysyl- and proline hydroxylases. Hydroxylation is essen- itation of fibril diameter. When mixtures of denatured tial for subsequent crosslinking and lysyl-hydroxylase deficiency causes the kyphoscoliosis form of EDS. (III) Procollagen α-chains are assembled collagen I and collagen V are renatured in vitro, fibrils into trimers within the rough endoplasmic reticulum (RER). Mutations of smaller size are produced with increasing collagen V in COL3A1 that interrupt the triple helical structure prevent normal pro- content (9). Indeed, the ultrastructural hallmark of cessing and secretion of collagen III, causing the vascular form of EDS. classical EDS is a 25% increase in fibril diameter and (IV) In the ECM, the NH2- and COOH-terminal propeptides are cleaved rare composite fibrils, often called “collagen cauliflow- by specific peptidases. Dominant mutations in COL1A1 and COL1A2 can ers” (Figure 2c). While the effect of collagen V on fibril prevent cleavage and cause arthrochalasia, while recessive loss of the diameter is intrinsic to its structure and does not N-procollagen peptidase cause dermatosparaxis. (V) Collagen molecules require enzymatic activity or the presence of noncolla- self-assemble into heterotypic fibrils. Dominant-negative mutations in gen proteins, the mechanism by which collagen V reg- COL5A1 and COL5A2 alter fibril assembly and cause some cases of clas- ulates fibril size is uncertain. The α1(V) chain has a sical EDS. (VI) Collagen fibrils are deposited in tissue-specific arrange- ments in close association with many fibril-associated proteins and pro- larger than usual NH2-terminal nonhelical domain, teoglycans. Because new fibrils are laid down in close association with and pepsin cleavage of this domain eliminates the the fibroblast cell membrane, interactions between the fibril and the cell effect of collagen V on fibril diameter (10). Further, are important and may involve direct interaction with collagens and/or immunohistochemical studies demonstrate that the matricellular proteins, including tenascin-X (TNX). triple helical region of collagen V is buried within the fibril, and the amino terminus of α1(V) protrudes from the fibril surface. Because the amino terminus of α1(V) EDS. The reported COL3A1 mutations are either exon- contains numerous sulfated tyrosine residues, this skipping mutations or missense mutations that disrupt domain carries a significant negative charge. It is pos- the collagen triple helix (4). Because collagen III consists tulated that this domain’s bulk or negative charge den- of homotrimers and the normal and mutant pro α1(III) sity limits growth of the nascent fibril once a critical chains appear to be produced in equal abundance, near- number of collagen V trimers are incorporated. ly 90% of all α1(III) trimers will contain one or more It is not clear how the relatively minor abnormalities in mutant α-chains. Trimers containing mutant chains are fibril structure, often affecting less than 5% of fibrils, not secreted and are either