Structure of the human aggrecan gene: exon-intron organization and association with the protein domains

A rare ACAN non-canonical splicing-site intron variant results in familial short stature

OBJECTIVE

ACAN gene variants, prevalent monogenic defects linked to short stature, are characterized by impaired cartilage generation in growth plates. We aimed to unravel the genetic basis of short stature in a specific pedigree by investigating the role of a novel non-canonical splicing-site variant, c.630-13G > A, within the ACAN gene.

METHOD

Sanger sequencing was used for pedigree verification, and the effects of this variant on mRNA splicing were analyzed through minigene assay.

RESULTS

The study revealed that this variant led to the creation of a previously unreported splice site in the fourth intron, resulting in the incorporation of an 11 bp sequence from the intron into the final transcript. This alteration led to a frameshift and formation of a premature termination codon, impacting the structure of the aggrecan protein.

CONCLUSIONS

We document the pathogenicity of an ACAN non-canonical splicing-site variant, emphasizing the significance of considering intronic variants during genetic testing.

Genotype and phenotype in patients with ACAN gene variants: Three cases and literature review

OBJECTIVE

To characterize the phenotype spectrum, diagnosis, and response to growth-promoting therapy in patients with ACAN variants causing familial short stature.

METHODS

Three families with ACAN variants causing short stature were reported. Similar cases in the literature were summarized, and the genotype and phenotype were analyzed.

RESULTS

Three novel heterozygous variants, c.757+1G>A, (splicing), c.6229delG, p.(Asp2078Tfs*1), and c.6679C>T, p.(Gln2227*) in the ACAN gene were identified. A total of 314 individuals with heterozygous variants from 105 families and 8 individuals with homozygous variants from 4 families were confirmed to have ACAN variants from literature and our 3 cases. Including our 3 cases, the variants reported comprised 33 frameshift, 39 missense, 23 nonsense, 5 splicing, 4 deletion, and 1 translocation variants. Variation points are scattered throughout the gene, while exons 12, 15, and 10 were most common (25/105, 11/105, and 10/105, respectively). Some identical variants existing in different families could be hot variants, c.532A>T, p.(Asn178Tyr), c.1411C>T, p.(Gln471*), c.1608C>A, p.(Tyr536*), c.2026+1G>A, (splicing), and c.7276G>T, p.(Glu2426*). Short stature, early-onset osteoarthritis, brachydactyly, midfacial hypoplasia, and early growth cessation were the common phenotypic features. The 48 children who received rhGH (and GnRHa) treatment had a significant height improvement compared with before (-2.18 ± 1.06 SD vs. -2.69 ± 0.95 SD, p < 0.001). The heights of children who received rhGH (and GnRHa) treatment were significantly improved compared with those of untreated adults (-2.20 ± 1.10 SD vs. -3.24 ± 1.14 SD, p < 0.001).

CONCLUSION

Our study achieves a new understanding of the phenotypic spectrum, diagnosis, and management of individuals with ACAN variants. No clear genotype-phenotype relationship of patients with ACAN variants was found. Gene sequencing is necessary to diagnose ACAN variants that cause short stature. In general, appropriate rhGH and/or GnRHa therapy can improve the adult height of affected pediatric patients caused by ACAN variants.

Short stature associated with a novel mutation in the aggrecan gene: A case report and literature review

BACKGROUND

Mutations in the aggrecan (ACAN) gene are identified in patients with: spondyloepiphyseal dysplasia, Kimberley type; short stature with advanced bone age (BA); in the presence or absence of heterozygous ACAN mutation-induced early-onset osteoarthritis and/or osteochondritis dissecans; and spondyloepimetaphyseal dysplasia, ACAN type. Heterozygous mutations contribute to spondyloepiphyseal dysplasia, Kimberley type (MIM#608361), which is a milder skeletal dysplasia. In contrast, homozygous mutations cause a critical skeletal dysplasia, which is called spondyloepimetaphyseal dysplasia, ACAN type (MIM#612813). Lately, investigations on exome and genome sequencing have shown that ACAN mutations can also lead to idiopathic short stature with or without an advanced BA, in the presence or absence of early-onset osteoarthritis and/or osteochondritis dissecans (MIM#165800). We herein reported a heterozygous defect of ACAN in a family with autosomal dominant short stature, BA acceleration, and premature growth cessation.

CASE SUMMARY

A 2-year-old male patient visited us due to growth retardation. The patient presented symmetrical short stature (height 79 cm, < -2 SD) without facial features and other congenital abnormalities. Whole-exome sequencing revealed a heterozygous pathogenic variant c. 871C>T (p. Gln291*) of ACAN, which was not yet reported in cases of short stature. This mutation was also detected in his father and paternal grandmother. According to the Human Gene Mutation Database, 67 ACAN mutations are registered. Most of these mutations are genetically inheritable, and very few children with short stature are associated with ACAN mutations. To date, heterozygous ACAN mutations have been reported in approximately 40 families worldwide, including a few individuals with a decelerated BA.

CONCLUSION

Heterozygous c. 871C>T (p. Gln291*) variation of the ACAN gene was the disease-causing variant in this family. Collectively, our newly discovered mutation expanded the spectrum of ACAN gene mutations.

Description of the molecular and phenotypic spectrum in Chinese patients with aggrecan deficiency: Novel ACAN heterozygous variants in eight Chinese children and a review of the literature

OBJECTIVE

This study analyzed eight Chinese short stature children with aggrecan deficiency, and aimed to investigate potential genotype-phenotype correlations, differences in clinical characteristics between the Chinese and the Western populations, and effectiveness of recombinant human growth hormone therapy in patients with ACAN variants through a review of the literature.

METHODS

Pediatric short stature patients with ACAN heterozygous variants were identified using whole-exome sequencing. Subsequently, a literature review was carried out to summarize the clinical features, genetic findings, and efficacy of growth-promoting therapy in patients with ACAN variants.

RESULTS

We identified seven novel ACAN mutations and one recurrent variant. Patients in our center manifested with short stature (average height SDS: -3.30 ± 0.85) with slight dysmorphic characteristics. The prevalence of dysmorphic features in the Chinese populations is significantly lower than that in the Western populations. Meanwhile, only 24.24% of aggrecan-deficient Chinese children showed significantly advanced bone age (BA). Promising therapeutic benefits were seen in the patients who received growth-promoting treatment, with an increase in growth velocity from 4.52 ± 1.00 cm/year to 8.03 ± 1.16 cm/year.

CONCLUSION

This study further expanded the variation spectrum of the ACAN gene and demonstrated that Chinese children with short stature who carried ACAN heterozygous variants exhibited early growth cessation, which may remain unnoticed by clinicians as most of these children had very mild dysmorphic characteristics and showed BA that was consistent with the chronological age. Genetic testing may help in the diagnosis.

Identification of a heterozygous ACAN mutation in a 15-year-old boy with short stature who presented with advanced bone age: a case report and review of the literature

Longitudinal bone growth is primarily mediated by the growth plate, which is a specialized cartilaginous structure. Aggrecan, encoded by ACAN, is a primary proteoglycan component of the extracellular matrix in both the growth plate and articular cartilage. Aggrecanopathies have emerged as a phenotype of genetic skeletal disease in humans. A heterozygous ACAN mutation causes short stature, premature growth cessation, and accelerated bone age maturation. We report the case of a 15-year-old boy with familial short stature, with height of 149 cm (Korean standard deviation score [SDS] of -3.6) and weight of 50.5 kg (-1.48 SDS). He presented with mild midfacial hypoplasia, frontal bossing, a broad chest, and a short neck. The father's and mother's heights were 150 cm (-4.8 SDS) and 153 cm (-1.69 SDS), respectively. The patient's bone age was 2-3 years more advanced than his chronological age, and no endocrine abnormalities were detected. Wholeexome sequencing followed by Sanger sequencing revealed a heterozygous ACAN mutation, c.512C>T (p.Ala171Val), in both the proband and his father. Short stature is generally associated with a delayed bone age, and this case suggests that ACAN mutations may be the most likely etiology among patients with short stature and an advanced bone age and should warrant early treatment.

Growth-Promoting Therapies May Be Useful In Short Stature Patients With Nonspecific Skeletal Abnormalities Caused By Acan Heterozygous Mutations: Six Chinese Cases And Literature Review

OBJECTIVE

There are numerous reasons for short stature, including mutations in osteochondral development genes. ACAN, one such osteochondral development gene in which heterozygous mutations can cause short stature, has attracted attention from researchers in recent years. Therefore, we analyzed six cases of short stature with heterozygous ACAN mutations and performed a literature review.

METHODS

Clinical information and blood samples from 6 probands and their family members were collected after consent forms were signed. Gene mutations in the probands were detected by whole-exome sequencing. Then, we searched the literature, performed statistical analyses, and summarized the characteristics of all reported cases.

RESULTS

We identified six novel mutations in ACAN: c.1411C>T, c.1817C>A, c.1762C>T, c.2266G>C, c.7469G>A, and c.1733-1G>A. In the literature, more than 200 affected individuals have been diagnosed genetically with a similar condition (height standard deviation score [SDS] -3.14 ± 1.15). Among affected individuals receiving growth-promoting treatment, their height before and after treatment was SDS -2.92±1.07 versus SDS -2.14±1.23 (P<.001). As of July 1, 2019, a total of 57 heterozygous ACAN mutations causing nonsyndromic short stature had been reported, including the six novel mutations found in our study. Approximately half of these mutations can lead to protein truncation.

CONCLUSIONS

This study used clinical and genetic means to examine the relationship between the ACAN gene and short stature. To some extent, clear diagnosis is difficult, since most of these affected individuals' characteristics are not prominent. Growth-promoting therapies may be beneficial for increasing the height of affected patients.

ABBREVIATIONS

AI = aromatase inhibitor; ECM = extracellular matrix; GnRHa = gonadotropin-releasing hormone analogue; IQR = interquartile range; MIM = Mendelian Inheritance in Man; PGHD = partial growth hormone deficiency; rhGH = recombinant human growth hormone; SDS = standard deviation score; SGA = small for gestational age; SGHD = severe growth hormone deficiency.

The matrix proteins aggrecan and fibulin-1 play a key role in determining aortic stiffness

Stiffening of the aorta is an important independent risk factor for myocardial infarction and stroke. Yet its genetics is complex and little is known about its molecular drivers. We have identified for the first time, tagSNPs in the genes for extracellular matrix proteins, aggrecan and fibulin-1, that modulate stiffness in young healthy adults. We confirmed SNP associations with ex vivo stiffness measurements and expression studies in human donor aortic tissues. Both aggrecan and fibulin-1 were found in the aortic wall, but with marked differences in the distribution and glycosylation of aggrecan reflecting loss of chondroitin-sulphate binding domains. These differences were age-dependent but the striking finding was the acceleration of this process in stiff versus elastic young aortas. These findings suggest that aggrecan and fibulin-1 have critical roles in determining the biomechanics of the aorta and their modification with age could underpin age-related aortic stiffening.

Clinical Characterization of Patients With Autosomal Dominant Short Stature due to Aggrecan Mutations

CONTEXT

Heterozygous mutations in the aggrecan gene (ACAN) cause autosomal dominant short stature with accelerated skeletal maturation.

OBJECTIVE

We sought to characterize the phenotypic spectrum and response to growth-promoting therapies.

PATIENTS AND METHODS

One hundred three individuals (57 females, 46 males) from 20 families with autosomal dominant short stature and heterozygous ACAN mutations were identified and confirmed using whole-exome sequencing, targeted next-generation sequencing, and/or Sanger sequencing. Clinical information was collected from the medical records.

RESULTS

Identified ACAN variants showed perfect cosegregation with phenotype. Adult individuals had mildly disproportionate short stature [median height, -2.8 standard deviation score (SDS); range, -5.9 to -0.9] and a history of early growth cessation. The condition was frequently associated with early-onset osteoarthritis (12 families) and intervertebral disc disease (9 families). No apparent genotype-phenotype correlation was found between the type of ACAN mutation and the presence of joint complaints. Childhood height was less affected (median height, -2.0 SDS; range, -4.2 to -0.6). Most children with ACAN mutations had advanced bone age (bone age - chronologic age; median, +1.3 years; range, +0.0 to +3.7 years). Nineteen individuals had received growth hormone therapy with some evidence of increased growth velocity.

CONCLUSIONS

Heterozygous ACAN mutations result in a phenotypic spectrum ranging from mild and proportionate short stature to a mild skeletal dysplasia with disproportionate short stature and brachydactyly. Many affected individuals developed early-onset osteoarthritis and degenerative disc disease, suggesting dysfunction of the articular cartilage and intervertebral disc cartilage. Additional studies are needed to determine the optimal treatment strategy for these patients.

The role of aggrecan in normal and osteoarthritic cartilage

Aggrecan is a large proteoglycan bearing numerous chondroitin sulfate and keratan sulfate chains that endow articular cartilage with its ability to withstand compressive loads. It is present in the extracellular matrix in the form of proteoglycan aggregates, in which many aggrecan molecules interact with hyaluronan and a link protein stabilizes each interaction. Aggrecan structure is not constant throughout life, but changes due to both synthetic and degradative events. Changes due to synthesis alter the structure of the chondroitin sulfate and keratan sulfate chains, whereas those due to degradation cause cleavage of all components of the aggregate. These latter changes can be viewed as being detrimental to cartilage function and are enhanced in osteoarthritic cartilage, resulting in aggrecan depletion and predisposing to cartilage erosion. Matrix metalloproteinases and aggrecanases play a major role in aggrecan degradation and their production is upregulated by mediators associated with joint inflammation and overloading. The presence of increased levels of aggrecan fragments in synovial fluid has been used as a marker of ongoing cartilage destruction in osteoarthritis. During the early stages of osteoarthritis it may be possible to retard the destructive process by enhancing the production of aggrecan and inhibiting its degradation. Aggrecan production also plays a central role in cartilage repair techniques involving stem cell or chondrocyte implantation into lesions. Thus aggrecan participates in both the demise and survival of articular cartilage.

Association between the expression of aggrecan and the distribution of aggrecan gene variable number of tandem repeats with symptomatic lumbar disc herniation in Chinese Han of Northern China

STUDY DESIGN

Case-control study.

OBJECTIVE

To examine the association between the expression of aggrecan and the aggrecan gene variable number of tandem repeats (VNTR) polymorphism with symptomatic lumbar disc herniation (LDH) in Chinese Han of Northern China.

SUMMARY OF BACKGROUND DATA

Aggrecan fragments have been found in human degenerated discs, and an association between the aggrecan VNTR polymorphism and intervertebral disc degeneration has been previously reported in middle-aged Finnish men. However, the relationship between the munity of symptomatic LDH with the expression of aggrecan and aggrecan gene VNTR has not been well studied.

METHODS

The disease group consisted of 70 patients already diagnosed with symptomatic LDH. The control group consisted of 14 patients restricted to spinal trauma and 113 healthy blood donors without symptoms of LDH who were not diagnosed with LDH. Disc tissue samples were obtained from surgical operations, and blood samples were donated from all participants. The aggrecan expression in isolated tissues was assessed by Western blot using specific antibodies. The aggrecan gene VNTR region was analyzed by polymerase chain reaction.

RESULTS

The aggrecan expression positive rate of control group was statistically and significantly higher (P < 0.001) than that of the disease group. Moreover, there was a statistically significant higher frequency of allele 25 or allele 21 in disease group compared with controls (P(A25) = 0.003416 and P(A21) = 0.000716). Compared with the participants with 2 alleles > 25 repeats, subjects with 1 or 2 alleles < or = 25 repeats statistically and significantly overrepresented the disease group without the expression of aggrecan (P < 0.001).

CONCLUSION

The findings suggest a relation between aggrecan and symptomatic LDH, where symptomatic LDH has a lower tendency of allele repeats. In addition, this study observed an association between the distribution of aggrecan gene VNTR polymorphism and the expression of aggrecan in symptomatic LDH.

A missense mutation in the aggrecan C-type lectin domain disrupts extracellular matrix interactions and causes dominant familial osteochondritis dissecans

Osteochondritis dissecans is a disorder in which fragments of articular cartilage and subchondral bone dislodge from the joint surface. We analyzed a five-generation family in which affected members had autosomal-dominant familial osteochondritis dissecans. A genome-wide linkage analysis identified aggrecan (ACAN) as a prime candidate gene for the disorder. Sequence analysis of ACAN revealed heterozygosity for a missense mutation (c.6907G > A) in affected individuals, resulting in a p.V2303M amino acid substitution in the aggrecan G3 domain C-type lectin, which mediates interactions with other proteins in the cartilage extracellular matrix. Binding studies with recombinant mutated and wild-type G3 proteins showed loss of fibulin-1, fibulin-2, and tenascin-R interactions for the V2303M protein. Mass spectrometric analyses of aggrecan purified from patient cartilage verified that V2303M aggrecan is produced and present in the tissue. Our results provide a molecular mechanism for the etiology of familial osteochondritis dissecans and show the importance of the aggrecan C-type lectin interactions for cartilage function in vivo.

Domain shuffling and the evolution of vertebrates

The evolution of vertebrates has included a number of important events: the development of cartilage, the immune system, and complicated craniofacial structures. Here, we examine domain shuffling as one of the mechanisms that contributes novel genetic material required for vertebrate evolution. We mapped domain-shuffling events during the evolution of deuterostomes with a focus on how domain shuffling contributed to the evolution of vertebrate- and chordate-specific characteristics. We identified approximately 1000 new domain pairs in the vertebrate lineage, including approximately 100 that were shared by all seven of the vertebrate species examined. Some of these pairs occur in the protein components of vertebrate-specific structures, such as cartilage and the inner ear, suggesting that domain shuffling made a marked contribution to the evolution of vertebrate-specific characteristics. The evolutionary history of the domain pairs is traceable; for example, the Xlink domain of aggrecan, one of the major components of cartilage, was originally utilized as a functional domain of a surface molecule of blood cells in protochordate ancestors, and it was recruited by the protein of the matrix component of cartilage in the vertebrate ancestor. We also identified genes that were created as a result of domain shuffling in ancestral chordates. Some of these are involved in the functions of chordate structures, such as the endostyle, Reissner's fiber of the neural tube, and the notochord. Our analyses shed new light on the role of domain shuffling, especially in the evolution of vertebrates and chordates.

An overview of the etiology of otosclerosis

Otosclerosis is the primary disease affecting the homeostasis of otic capsule and is among the most common causes of acquired hearing loss. Otosclerosis is considered as a multifactor disease, caused by both genetic and environmental factors. The aim of the present review is to summarize and analyze the bibliographic data, associated with the etiology of the disease. In some cases, the otosclerosis has an autosomal dominant mode of inheritance with incomplete penetrance. Genetic studies reveal the occurrence of at least nine chromosomal loci as candidate genes of the disease. The localized measles virus infection of the otic capsule has been postulated as a possible etiological theory. The role of hormonal factors, immune and bone-remodeling system in the etiopathogenesis of otosclerosis and the association of the disease with the disorders of the connective tissue are the issues of the present study. Despite the extensive research, many etiological factors and theories have been suggested and the process of development of the otosclerosis remains unclear.

L-Sox5 and Sox6 drive expression of the aggrecan gene in cartilage by securing binding of Sox9 to a far-upstream enhancer

The Sry-related high-mobility-group box transcription factor Sox9 recruits the redundant L-Sox5 and Sox6 proteins to effect chondrogenesis, but the mode of action of the trio remains unclear. We identify here a highly conserved 359-bp sequence 10 kb upstream of the Agc1 gene for aggrecan, a most essential cartilage proteoglycan and key marker of chondrocyte differentiation. This sequence directs expression of a minimal promoter in both embryonic and adult cartilage in transgenic mice, in a manner that matches Agc1 expression. The chondrogenic trio is required and sufficient to mediate the activity of this enhancer. It acts directly, Sox9 binding to a critical cis-acting element and L-Sox5/Sox6 binding to three additional elements, which are cooperatively needed. Upon binding to their specific sites, L-Sox5/Sox6 increases the efficiency of Sox9 binding to its own recognition site and thereby robustly potentiates the ability of Sox9 to activate the enhancer. L-Sox5/Sox6 similarly secures Sox9 binding to Col2a1 (encoding collagen-2) and other cartilage-specific enhancers. This study thus uncovers critical cis-acting elements and transcription factors driving Agc1 expression in cartilage and increases understanding of the mode of action of the chondrogenic Sox trio.

Long-term cyclical in vivo loading increases cartilage proteoglycan content in a spatially specific manner: an infrared microspectroscopic imaging and polarized light microscopy study

Understanding the changes in collagen and proteoglycan content of cartilage due to physical forces is necessary for progress in treating joint disorders, including those due to overuse. Physical forces in the chondrocyte environment can affect the cellular processes involved in the biosynthesis of extracellular matrix. In turn, the biomechanical properties of cartilage depend on its collagen and proteoglycan content. To understand changes due to physical forces, this study examined the effect of 80 cumulative hours of in vivo cyclical joint loading on the cartilage content of proteoglycan and collagen in the rabbit metacarpophalangeal joint. The forepaw digits of six anesthetized New Zealand White adult female rabbits were repetitively flexed at 1 Hz with an estimated joint contact pressure of 1 to 2 MPa. Joints were collected from loaded and contralateral control specimens, fixed, decalcified, embedded, and thin-sectioned. Sections were examined under polarized light microscopy to identify and measure superficial and mid zone thicknesses of cartilage. Fourier Transform Infrared microspectroscopy was used to measure proteoglycan and collagen contents in the superficial, mid, and deep zones. Loading led to an increase in proteoglycan in the cartilage of all six rabbits. Specifically, there was a 46% increase in the cartilage deep zone (p = 0.003). The collagen content did not change with loading. Joint loading did not change the superficial and mid zone mean thicknesses. We conclude that long-term (80 cumulative hours) cyclical in vivo joint loading stimulates proteoglycan synthesis. Furthermore, stimulation is localized to cartilage regions of high hydrostatic pressure. These data may be useful in developing interventions to prevent overuse injuries or in developing therapies to improve joint function.

Aggrecan core protein of a certain length is protective against hand osteoarthritis

OBJECTIVE

To study the contribution of aggrecan VNTR (variable number of tandem repeats) polymorphism to clinically differing manifestations of hand osteoarthritis (OA).

DESIGN

Five hundred thirty Finnish females representing two academically similar occupations with completely diverse exposure to hand load were included. Radiographs of hands were analysed, the OA findings were graded and the subjects were divided into categories. Aggrecan VNTR alleles were identified by Southern hybridization. Statistical analyses were used to compare joint involvement and pathological findings with the prevalences of the alleles and genotypes.

RESULTS

Subjects homozygous for the most common aggrecan VNTR allele, A27 with 27 repeats, had a significantly lower risk of hand OA, with OR 0.46 (95% CI 0.27-0.78) for OA of grade 2 or more. Our results suggest that carrying two copies of the alleles with less than 27 repeats could predispose a subject to a severe hand OA (OR 2.45, 95% CI 1.17-5.12) and carrying two copies of the alleles with more than 27 repeats also increases the risk of the disease (OR 1.73, 95% CI 1.03-2.89).

CONCLUSIONS

These findings indicate that allele A27 provides protection from hand OA and that alleles shorter or longer than this may predispose subjects to the disease. Furthermore, they suggest that a certain number of tandem repeats provide for optimal functioning of the aggrecan molecule and that the contribution of genetic factors to the development of hand OA may be even more important than that of environmental factors.

A mutation in the variable repeat region of the aggrecan gene (AGC1) causes a form of spondyloepiphyseal dysplasia associated with severe, premature osteoarthritis

Spondyloepiphyseal dysplasia (SED) encompasses a heterogeneous group of disorders characterized by shortening of the trunk and limbs. The autosomal dominant SED type Kimberley (SEDK) is associated with premature degenerative arthropathy and has been previously mapped in a multigenerational family to a novel locus on 15q26.1. This locus contains the gene AGC1, which encodes aggrecan, the core protein of the most abundant proteoglycan of cartilage. We screened AGC1 for mutations and identified a single-base-pair insertion, within the variable repeat region of exon 12 in affected individuals from the family with SEDK, that introduces a frameshift of 212 amino acids, including 22 cysteine residues, followed by a premature stop codon. This is the first identification of an AGC1 mutation causing a human disorder. This finding extends the spectrum of mutated genes that may cause SED and thus will aid in the molecular delineation of this complex group of conditions.

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.

Glucosamine sulfate modulates the levels of aggrecan and matrix metalloproteinase-3 synthesized by cultured human osteoarthritis articular chondrocytes

OBJECTIVE

The functional integrity of articular cartilage is determined by a balance between chondrocyte biosynthesis of extracellular matrix and its degradation. In osteoarthritis (OA), the balance is disturbed by an increase in matrix degradative enzymes and a decrease in biosynthesis of constitutive extracellular matrix molecules, such as collagen type II and aggrecan. In this study, we examined the effects of the sulfate salt of glucosamine (GS) on the mRNA and protein levels of the proteoglycan aggrecan and on the activity of matrix metalloproteinase (MMP)-3 in cultured human OA articular chondrocytes.

DESIGN

Freshly isolated chondrocytes were obtained from knee cartilage of patients with OA. Levels of aggrecan and MMP-3 were determined in culture media by employing Western blots after incubation with GS at concentrations ranging from 0.2 to 200 microM. Zymography (casein) was performed to confirm that effects observed at the protein level were reflected at the level of enzymatic activity. Northern hybridizations were used to examine effects of GS on levels of aggrecan and MMP-3 mRNA. Glycosaminoglycan (GAG) assays were performed on the cell layers to determine levels of cell-associated GAG component of proteoglycans.

RESULTS

Treatment of OA chondrocytes with GS (1.0-150 microM) resulted in a dose-dependent increase in aggrecan core protein levels, which reached 120% at 150 microM GS. These effects appeared to be due to increased expression of the corresponding gene as indicated by an increase in aggrecan mRNA levels in response to GS. MMP-3 levels decreased (18-65%) as determined by Western blots. Reduction of MMP-3 protein was accompanied by a parallel reduction in enzymatic activity. GS caused a dose-dependent increase (25-140%) in cell-associated GAG content. Chondrocytes obtained from 40% of OA patients failed to respond to GS.

CONCLUSIONS

The results indicate that GS can stimulate mRNA and protein levels of aggrecan core protein and, at the same time, inhibit production and enzymatic activity of matrix-degrading MMP-3 in chondrocytes from OA articular cartilage. These results provide a cogent molecular mechanism to support clinical observations suggesting that GS may have a beneficial effect in the prevention of articular cartilage loss in some patients with OA.

Identification of a locus for a form of spondyloepiphyseal dysplasia on chromosome 15q26.1: exclusion of aggrecan as a candidate gene

We have investigated a family with an autosomal dominant form of spondyloepiphyseal dysplasia (SED) characterised by short stature and severe premature degenerative arthropathy. Previous studies have excluded linkage between this condition and the locus for the type II collagen gene. Here we report the identification of linkage between this disorder and a locus on the long arm of chromosome 15 between markers D15S979 and D15S1004. According to current linkage maps and sequence data, this locus includes that of the aggrecan gene (AGC1). Our linkage data from the SED family show, however, that AGC1 maps to a locus that is proximal to D15S979. This proximal location for AGC1 is further supported by linkage data from a second family with an autosomal recessive form of multiple epiphyseal dysplasia that also maps to the SED locus. In both families AGC1 is therefore excluded as a candidate gene.

A remote upstream element regulates tissue-specific expression of the rat aggrecan gene

The regulation of chondrogenesis and of the genes expressed as markers of chondrocyte differentiation is poorly understood. The hyaluronan-binding proteoglycan aggrecan is an essential and specific component of cartilage, but the aggrecan proximal promoter is expressed in an unregulated fashion in vitro. DNA comprising the rat aggrecan gene (83 kb including the 30-kb first intron) was surveyed for active elements, which would impart selective expression to the aggrecan promoter in transfection assays in vitro. A 4.7-kb DNA fragment (P3) with cell-specific enhancer activity was discovered approximately 12 kb upstream of the transcription start site; this active DNA fragment is position- and orientation-independent, and strongly stimulates aggrecan promoter expression in chondrocytes, while weakly suppressing transcription in fibroblasts. Most of this activity has been localized to P3-7, a 2.3-kb internal fragment of P3. Another enhancer element (A23), which is not tissue-specific, was discovered about 70 kb downstream of the transcription start site. Several lines of transgenic mice were created using combinations of these DNA elements to drive the lacZ reporter gene. Neither a short (900 bp) nor a long (3.7 kb) promoter alone showed detectable expression in 14.5-day embryos, whereas placing the P3 tissue-specific enhancer together with P0 gave strong expression restricted to embryonic cartilage of transgenic mice. The A23 downstream enhancer in conjunction with P0 did not confer expression. This is the first report of a gene control region which confers authentic tissue-specific regulation of aggrecan in vitro or in vivo and should greatly facilitate understanding the coordinate regulation of chondrocytic genes.

Mapping of a new autosomal dominant nonsyndromic hearing loss locus (DFNA30) to chromosome 15q25-26

Hearing impairment is the most common inherited human sensory defect. Nonsyndromic Hearing Impairment (NSHI) is the most genetically heterogeneous trait known. Over 70 loci have been mapped and a total of 19 genes have been identified. We report here a novel locus (DFNA 30) for autosomal dominant NSHI that we mapped to chromosome 15q25-26 in an Italian four-generation family. The haplotype analysis has identified a critical interval of 18 cM between markers D15S151 and D15S130. This region does not overlap with DFNB16 locus but partially coincides with the otosclerosis (OTS) locus. Localisation of the locus DFNA30 is a first step towards the identification of the gene.

Domain organization, genomic structure, evolution, and regulation of expression of the aggrecan gene family

Proteoglycans are complex macromolecules, consisting of a polypeptide backbone to which are covalently attached one or more glycosaminoglycan chains. Molecular cloning has allowed identification of the genes encoding the core proteins of various proteoglycans, leading to a better understanding of the diversity of proteoglycan structure and function, as well as to the evolution of a classification of proteoglycans on the basis of emerging gene families that encode the different core proteins. One such family includes several proteoglycans that have been grouped with aggrecan, the large aggregating chondroitin sulfate proteoglycan of cartilage, based on a high number of sequence similarities within the N- and C-terminal domains. Thus far these proteoglycans include versican, neurocan, and brevican. It is now apparent that these proteins, as a group, are truly a gene family with shared structural motifs on the protein and nucleotide (mRNA) levels, and with nearly identical genomic organizations. Clearly a common ancestral origin is indicated for the members of the aggrecan family of proteoglycans. However, differing patterns of amplification and divergence have also occurred within certain exons across species and family members, leading to the class-characteristic protein motifs in the central carbohydrate-rich region exclusively. Thus the overall domain organization strongly suggests that sequence conservation in the terminal globular domains underlies common functions, whereas differences in the central portions of the genes account for functional specialization among the members of this gene family.

Aggrecan: A Target Molecule of Autoimmune Reactions

Aggrecan in cartilage forms aggregates with hyaluronan and link protein, embedded in a collagen network. It accounts for the compressive stiffness and resilience of the hyaline cartilage. Many forms of inflammatory arthritis were shown to be accompanied with aggrecan degradation and loss from the cartilage. The loss of this major component of cartilage renders the tissue more vulnerable when exposed to abrasive forces. Therefore, aggrecan degradation may significantly contribute to cartilage destruction in arthritis. Furthermore, fragments of degraded aggrecan are released during joint inflammation. Thus, molecules of an avascular, immune-privileged tissue (hyaline cartilage) may become accessible to the cells of the immune system. Similarly, there is a "leakage" of aggrecan fragments from cartilage during aging and after joint injury, which may also lead to autosensibilisation. Autoimmune reactivity to aggrecan can be detected in human joint diseases, as well as in animal models of arthritis. The epitopes involved in these processes are currently being identified. Recent data from work with mice suggest a strong immune response focused to the N-terminal G1 domain of aggrecan that leads to arthritis and spondylitis.

The folded protein modules of the C-terminal G3 domain of aggrecan can each facilitate the translocation and secretion of the extended chondroitin sulfate attachment sequence

Aggrecan is a multidomain proteoglycan containing both extended and folded protein modules. The C-terminal G3 domain contains a lectin-like, complement regulatory protein-like, and two alternatively spliced epidermal growth factor-like modules. It has been proposed that the lectin module alone has a necessary role in the intracellular translocation and secretion of proteins expressed containing G3. Constructs containing human aggrecan G3 together with 1155 bases of the adjacent chondroitin sulfate attachment region (CS-2) were prepared with different combinations and deletions of the protein modules and transfected into mammalian cells of monkey or hamster origin. The results showed that the products containing only the unfolded protein sequences (CS-2 with or without the C-terminal tail sequence) were translated and accumulated intracellularly but were not secreted. In contrast the constructs containing any of the folded protein modules and the extended CS-2 region were translated and secreted from the cells. The results show that the lectin module was not unique in facilitating the intracellular translocation and secretion of the G3 domain. The conservation of G3-like domains within the aggrecan family of proteoglycans may therefore result from their participation in other extracellular functions.

Recombinant human aggrecan G1-G2 exhibits native binding properties and substrate specificity for matrix metalloproteinases and aggrecanase

A recombinant human aggrecan G1-G2 fragment comprising amino acids Val(1)-Arg(656) has been expressed in Sf21 cells using a baculovirus expression system. The recombinant G1-G2 (rG1-G2) was purified to homogeneity by hyaluronan-Sepharose affinity chromatography followed by high performance liquid chromatography gel filtration, and gave a single band of M(r) 90,000-95,000 by silver stain or immunoblotting with monoclonal antibody 1-C-6. The expressed G1-G2 bound to both hyaluronan and link protein indicating that the immunoglobulin-fold motif and proteoglycan tandem repeat loops of the G1 domain were correctly folded. Further analysis of secondary structure by rotary shadowing electron microscopy confirmed a double globe appearance, but revealed that the rG1-G2 was more compact than its native counterpart. The size of rG1-G2 by SDS-polyacrylamide gel electorphoresis was unchanged following digestion with keratanase and keratanase II and reduced by only 2-5 kDa following digestion with either O-glycosidase or N-glycosidase F. Recombinant G1-G2 was digested with purified matrix metalloproteinases (MMP), isolated aggrecanase, purified atrolysin C, or proteinases present in conditioned medium from cartilage explant cultures, and the products analyzed on SDS gels by silver stain and immunoblotting. Neoepitope antibodies recognizing the N-terminal F(342)FGVG or C-terminal DIPEN(341) sequences were used to confirm MMP cleavage at the Asn(341) downward arrow Phe bond, while neoepitope antibodies recognizing the N-terminal A(374)RGSV or C-terminal ITEGE(373) sequences were used to confirm aggrecanase cleavage at the Glu(373) downward arrow Ala bond. Cleavage at the authentic MMP and aggrecanase sites revealed that these proteinases have the same specificity for rG1-G2 as for native aggrecan. Incubation of rG1-G2 with conditioned medium from porcine cartilage cultures revealed that active soluble aggrecanase but no active MMPs, was released following stimulation with interleukin-1alpha or retinoic acid. Atrolysin C, which cleaves native bovine aggrecan at both the aggrecanase and MMP sites, efficiently cleaved rG1-G2 at the aggrecanase site but failed to cleave at the MMP site. In contrast, native glycosylated G1-G2 with or without keratanase treatment was cleaved by atrolysin C at both the aggrecanase and MMP sites. The results suggest that the presence or absence per se of keratan sulfate on native G1-G2 does not affect the activity of atrolysin C toward the two sites.

Sulphation heterogeneity in the trisaccharide (GalNAcSbeta1, 4GlcAbeta1,3GalNAcS) isolated from the non-reducing terminal of human aggrecan chondroitin sulphate

We report here the isolation and sulphation isomer analyses of trisaccharides GalNAcS(beta1,4)GlcA(beta1,3)GalNAcS (in which S indicates sulphate) derived from the non-reducing termini of aggrecan chondroitin sulphate. Rat chondrosarcoma and human aggrecans were digested for 1 h at 37 degrees C with 30 micro-units of endo-chondroitinase ABC per microgram of chondroitin sulphate, and trisaccharides were isolated from the digests by ToyoPearl HW40S gel-filtration chromatography. Four trisaccharide species were identified; their sulphation isomer compositions, as determined by digestion with chondroitinase ACII and fluorescence-based ion-exchange HPLC, were GalNAc4Sbeta1,4GlcAbeta1,3GalNAc4S, GalNAc4Sbeta1,4GlcAbeta1,3GalNAc6S, GalNAc4,6Sbeta1,4GlcAbeta1, 3GalNAc4S and GalNAc4,6Sbeta1,4GlcAbeta1,3GalNAc6S. The abundances of such sequences in chondroitin sulphate on aggrecan from normal (foetal to 72 years of age) and from osteoarthritic human knee cartilages were also established. The results showed that non-reducing terminal GalNAc4S or GalNAc4,6S can be linked to either a 4-sulphated or a 6-sulphated disaccharide, suggesting that the sulphation of the last disaccharide might not have a direct effect on the specificity of chondroitin sulphate terminal GalNAc sulphotransferases. Furthermore, for each aggrecan preparation examined, the 4S-to-6S ratio of all chain interior disaccharides was equivalent to that in the last repeating disaccharides at the non-reducing terminus, suggesting that neither chondroitin 4-sulphotransferase nor chondroitin 6-sulphotransferase shows preferential activity near the chain terminus.

Structure of the human beta2-glycoprotein I (apolipoprotein H) gene

The gene encoding the human plasma protein beta2-glycoprotein I or apolipoprotein H was cloned and its structure determined. The gene which consists of eight exons was shown to span 18 kb and was localized to chromosome 17q23-24. The transcriptional initiation site was assigned to a position 31 bp upstream of the start codon. Several consensus sequence elements relevant for regulation of transcription in liver were seen in the 5'-upstream region of the gene. Exon 1 contains the 5'-UTR together with the signal peptide coding sequences. Short consensus repeats (SCRs) 1, 3, 4, and 5 are encoded by single exons each while SCR2 is encoded by two exons. Exon 8 comprises the region encoding the C-terminal end of beta2-glycoprotein I (from His-310), the stop codon and the 3'-UTR.

Matrix proteoglycans: from molecular design to cellular function

The proteoglycan superfamily now contains more than 30 full-time molecules that fulfill a variety of biological functions. Proteoglycans act as tissue organizers, influence cell growth and the maturation of specialized tissues, play a role as biological filters and modulate growth-factor activities, regulate collagen fibrillogenesis and skin tensile strength, affect tumor cell growth and invasion, and influence corneal transparency and neurite outgrowth. Additional roles, derived from studies of mutant animals, indicate that certain proteoglycans are essential to life whereas others might be redundant. The review focuses on the most recent genetic and molecular biological studies of the matrix proteoglycans, broadly defined as proteoglycans secreted into the pericellular matrix. Special emphasis is placed on the molecular organization of the protein core, the utilization of protein modules, the gene structure and transcriptional control, and the functional roles of the various proteoglycans. When possible, proteoglycans have been grouped into distinct gene families and subfamilies offering a simplified nomenclature based on their protein core design. The structure-function relationship of some paradigmatic proteoglycans is discussed in depth and novel aspects of their biology are examined.

Membrane type 1 matrix metalloproteinase (MT1-MMP) cleaves the recombinant aggrecan substrate rAgg1mut at the 'aggrecanase' and the MMP sites. Characterization of MT1-MMP catabolic activities on the interglobular domain of aggrecan

The recent detection of membrane type 1 matrix metalloproteinase (MT1-MMP) expression in human articular cartilage [Büttner, Chubinskaya, Margerie, Huch, Flechtenmacher, Cole, Kuettner, and Bartnik (1997) Arthritis Rheum. 40, 704-709] prompted our investigation of MT1-MMP's catabolic activity within the interglobular domain of aggrecan. For these studies we used rAgg1mut, a mutated form of the recombinant fusion protein (rAgg1) that has been used as a substrate to monitor 'aggrecanase' catabolism in vitro [Hughes, Büttner, Eidenmüller, Caterson and Bartnik (1997) J. Biol. Chem. 272, 20269-20274]. The rAgg1 was mutated (G332 to A) to avoid the generation of a splice variant seen with the original genetic construct, which gave rise to heterogeneous glycoprotein products. This mutation yielded a homogeneous recombinant product. Studies in vitro with retinoic acid-stimulated rat chondrosarcoma cells indicated that the rAgg1mut substrate was cleaved at the 'aggrecanase' site equivalent to Glu373-Ala374 (human aggrecan sequence enumeration) in its interglobular domain sequence segment. The differential catabolic activities of the recombinant catalytic domain (cd) of MT1-MMP and matrix metalloproteinases (MMPs) 3 and 8 were then compared by using this rAgg1mut as a substrate. Coomassie staining of rAgg1mut catabolites separated by SDS/PAGE showed similar patterns of degradation with all three recombinant enzymes. However, comparative immunodetection analysis, with neoepitope antibodies BC-3 (anti-ARGS...) and BC-14 (anti-FFGV...) to distinguish between 'aggrecanase' and MMP-generated catabolites, indicated that the catalytic domain of MT1-MMP exhibited strong 'aggrecanase' activity, cdMMP-8 weak activity and cdMMP-3 no activity. In contrast, cdMMP-3 and cdMMP-8 led to strongly BC-14-reactive catabolic fragments, whereas cdMT1-MMP resulted in weak BC-14 reactivity. N-terminal sequence analyses of the catabolites confirmed these results and also identified other potential minor cleavage sites within the interglobular domain of aggrecan. These results indicate that MT1-MMP is yet another candidate for 'aggrecanase' activity in vivo.

Connective tissue markers of rheumatoid arthritis

Rheumatoid arthritis (RA) is a common systemic autoimmune disorder of unknown aetiology. The most common outcome of RA is a progressive development of joint destruction and deformity. Early introduction of disease-modifying antirheumatic drugs seems important for prevention of the long term injuries of articular cartilage and bone. Early diagnosis and selection of patients with rapidly progressive disease therefore is of clinical significance. Routine laboratory tests are valuable in monitoring for renal, hepatic and haematological side effects of medical treatment. Determination of rheumatoid factor contributes to the classification of arthritis as RA, and acute phase reactants are useful for quantifying and comparing the level of inflammatory activity in the course of a given patient. There is, however, a lack of sensitive and specific biochemical markers for RA, and frontline biochemical research is devoted to characterizing molecules which are of diagnostic and prognostic value, as well as molecules which are indicators of the degree of joint cartilage and bone destruction. The present survey summarizes current knowledge concerning possible tissue-specific marker molecules of RA.

Load-controlled compression of articular cartilage induces a transient stimulation of aggrecan gene expression

The effects of short- and long-term load-controlled compression on the levels of aggrecan mRNA have been determined. Results show that a compressive stress of 0.1 MPa on bovine articular cartilage explants for 1, 4, 12, and 24 h produces a transient up-regulation of aggrecan mRNA synthesis. At 1 h, aggrecan mRNA levels in loaded explants were increased 3.2-fold compared to control explants. At longer times (>/=4 h), the levels of aggrecan mRNA returned to baseline values or stayed slightly higher. There is a dose dependence in the response of the explant to increasing levels of compressive stress (0-0.5 MPa) for 1 h. Aggrecan mRNA levels increased 2- to 3-fold at 0-0.25 MPa. At 0.5 MPa, the level of aggrecan mRNA was lower than those at 0.1 and 0.25 MPa. This dose-dependent effect suggests a reversal of the stimulatory effects of compression on aggrecan gene expression at higher loads. After 24 h of compression, the levels of aggrecan mRNA in explants subjected to any of the stress levels were not significantly different from those in control explants. The stimulatory effect of 0.1 MPa compressive stress on aggrecan mRNA levels was blocked by Rp-cAMP and U-73122, indicating the involvement of the classical signal transduction pathways in the mechanical modulation of aggrecan gene expression. The responses of link protein mRNA to compression paralleled those of aggrecan, while there was no significant change in expression of the gene for the housekeeping protein elongation factor-1 alpha. The results indicate that articular cartilage chondrocytes can respond to short-term compressive loads by transiently up-regulating expression of the aggrecan gene. The fact that long-term compression did not significantly alter aggrecan mRNA levels suggests that previously observed inhibitory effects of prolonged static compression on proteoglycan synthesis in articular cartilage may be, for the most part, mediated through mechanisms other than suppression of aggrecan mRNA levels.

Knee joint immobilization decreases aggrecan gene expression in the meniscus

Aggrecan is the major proteoglycan of the meniscus, and its primary function is to give the meniscus its viscoelastic compressive properties. The objective of this study was to determine the effect of joint immobilization on aggrecan gene expression in the meniscus. The right hindlimbs of six mature beagles were knee cast-immobilized in 90 degrees of flexion and supported by a sling to prevent weightbearing, while the contralateral limb was left free to bear weight. The animals were sacrificed at 4 weeks, and the anterior and posterior halves of the medial and lateral menisci were analyzed separately. Analysis of aggrecan gene expression by quantitative polymerase chain reaction showed decreased aggrecan gene expression in menisci from immobilized knees (P < 0.01, two-way analysis of variance). Aggrecan gene expression decreased by a factor of 2 to 5.5 in the different regions examined. Analysis of the composition of the meniscus also showed decreased proteoglycan content and increased water content with immobilization (P < 0.05, two-way analysis of variance). These results show that joint immobilization can significantly affect meniscal cellular activity and composition and can therefore potentially affect meniscal function.

Regulatory activities of the 5'- and 3'-untranslated regions and promoter of the human aggrecan gene

Identification and characterization of the regulatory elements of the human aggrecan gene are necessary first steps in addressing the molecular mechanisms through which the gene is regulated. Using luciferase reporter constructs driven by the human aggrecan promoter or the cytomegalovirus promoter, the 5'- and 3'-untranslated regions of the human aggrecan gene were found to regulate gene expression transcriptionally in a promoter- and/or cell type-specific manner. Independent of cell type, the 5'-untranslated region was inhibitory with respect to the cytomegalovirus promoter, but it was stimulatory to the human aggrecan promoter. The 5'-untranslated region inhibited the cytomegalovirus promoter by approximately 60% in both chondrocytes and NIH 3T3 cells, but it stimulated the activity of the human aggrecan promoter about 8-fold in chondrocytes and 40-fold in NIH 3T3 cells. In contrast, the 3'-untranslated region inhibited the activities of the human aggrecan promoter by 40-70% in both cell types, but it stimulated the cytomegalovirus promoter activities by 50-60% in NIH 3T3 cells and inhibited its activity by 70% in chondrocytes. The differential effects of the untranslated regions on the two types of promoters may be a reflection of differences in regulation of TATA-less promoters, such as the human aggrecan promoter, and TATA-containing promoters, such as the cytomegalovirus promoter.

Complete coding sequence of bovine aggrecan: comparative structural analysis

The previously available sequence for bovine aggrecan included only the KS domain, the C-terminal portion of the CS-2 domain, and the entire CS-3 and G3 domains. We have isolated cDNA clones for previously uncharacterized portions of the bovine aggrecan sequence, and, when we combined them with previously published incomplete sequences, have obtained a complete sequence for the entire core protein. The bovine aggrecan sequence, which is a composite of new sequence data and previously published incomplete sequences, is 2327 residues in length. Although there is significant conservation of G1, G2, and G3 globular domains between species, there are differences in the length of the interglobular domain, in the number of KS domain hexapeptide repeats and CS domain repeats, and in alternative splicing within the G3 domain. The bovine aggrecan KS domain contains 24 repeats of a hexapeptide motif. The largely uncharacterized CS-1 domain of bovine aggrecan was found to contain 27 variable repeats of a 21-residue consensus sequence. A notable feature of the bovine CS-1 domain is in the distribution of single Ser-Gly dipeptides, the majority of which are separated by 7 or 8 amino acids, compared to the human, where discrete pairs of Ser-Gly dipeptides are separated by 13 amino acids. The CS-2 domain contains a total of six "homology domains" with 4 complete and 2 partial approximately 100-residue repeats. Each "homology domain" contains a "nodal" region with few sites for CS chain addition that is highly conserved between species, suggesting a possible role in aggrecan biosynthesis or catabolism.

Sequence and chromosomal localization of the mouse brevican gene

Brevican is a brain-specific proteoglycan belonging to the aggrecan family. Phage clones containing the complete mouse brevican open reading frame of 2649 bp and the complete 3'-untranslated region of 341 bp were isolated from a mouse brain cDNA library, and cosmid clones containing the mouse brevican gene were isolated from a genomic library using a PCR-generated DNA fragment as probe. The obtained genomic sequence of 13,700 nucleotides revealed that the murine gene has a size of approximately 13 kb and contains the sequence of the mRNA for the secreted brevican isoform on 14 exons. The exon-intron structure reflected the structural organization of the multidomain protein brevican. No consensus TATA sequence was found upstream of the first exon, and RNase protection experiments revealed multiple transcriptional start sites for the brevican gene. The first part of the sequence of intron 8 corresponded to an alternative brevican cDNA, coding for a GPI-linked isoform. Single strand conformation polymorphism analysis mapped the brevican gene (Bcan) to chromosome 3 between the microsatellite markers D3Mit22 and D3Mit11.

Utilization of a recombinant substrate rAgg1 to study the biochemical properties of aggrecanase in cell culture systems

This paper describes the first report of the production and use of an artificial recombinant protein substrate to study "aggrecanase" activity. The substrate (rAgg1) is composed of the complete interglobular domain (IGD) of human aggrecan flanked by the "marker" sequences FLAGTM at the amino terminus and the human immunoglobulin G1 constant region at the carboxyl terminus. The expressed protein occurs as large multimolecular aggregates (>120 kDa) that, upon reduction, consist of a major isoform of 72 kDa (containing the IGD) and a minor 39-kDa species that through alternative splicing has had the IGD deleted. Using this recombinant substrate we developed a novel agarose cell culture system containing either rat chondrosarcoma or bovine chondrocytes that could be used in studies of the biochemical characterization of aggrecanase activities. These studies showed the following. (i) rAgg1 is a suitable substrate for aggrecanase proteolysis. (ii) Aggrecanase activity was specifically induced by exposing chondrocytes to retinoic acid. (iii) A considerable time period was required to synthesize and/or activate aggrecanase, with considerable differences in that found in rat chondrosarcoma versus bovine chondrocyte culture systems. (iv) Aggrecanase cleavage of the aggrecan IGD does not require the presence of the G1 or G2 globular domains or keratan sulfate post-translational modification in the IGD. (v) Aggrecanase is a diffusible activity that does not require association with the chondrocyte plasma membrane or immediate pericellular matrix for its action.

A human-specific polymorphism in the coding region of the aggrecan gene. Variable number of tandem repeats produce a range of core protein sizes in the general population

Aggrecan, one of the major structural genes of cartilage, encodes a proteoglycan core protein composed of an extended central glycosaminoglycan-bearing domain, flanked by globular domains at each end. The central region consists of long stretches of repeating amino acids that serve as attachment sites for glycosaminoglycans such as chondroitin and keratan sulfate; the terminal globular domains interact with other cartilage components. The glycosaminoglycan attachment region is encoded in several species by a single large exon, within which are several different types of repeating sequences. Several species show within this exon a similar block of conserved repeats for attachment of chondroitin sulfate, but in humans this group of repeats is particularly well conserved. Examination of genomic DNA from a population of unrelated individuals by polymerase chain reaction or Southern blot assays shows this block of repeat sequences exists in multiple allelic forms, which differ by the number of repeats at this site in each allele. Thirteen different alleles have been identified, with repeat numbers ranging from 13 to 33. This is an unusual example of an expressed variable number of tandem repeat polymorphism. This polymorphism is apparently restricted to humans, of several species examined. This polymorphism results in individuals with differing length aggrecan core proteins, bearing different numbers of potential attachment sites for chondroitin sulfate. The possibility exists for a molecular understanding of biological variation in cartilage functional properties.

Structural and functional analysis of the chick chondroitin sulfate proteoglycan (aggrecan) promoter and enhancer region

Aggrecan is a large chondroitin sulfate proteoglycan, the expression of which is both tissue-specific and developmentally regulated. Here we report the cloning and sequencing of the 1.8-kilobase genomic 5' flanking sequence of the chick aggrecan gene and provide a functional and structural characterization of its promoter and enhancer region. Sequence analysis reveals potential Sp1, AP2, and NF-I related sites, as well as several putative transcription factor binding sites, including the cartilage-associated silencers CIIS1 and CIIS2. A number of these transcription factor binding motifs are embedded in a sequence flanked by prominent inverted repeats. Although lacking a classic TATA box, there are two instances in the 1.8-kb genomic fragment of TATA-like TCTAA sequences, as have been defined previously in other promoter regions. Primer extension and S1 protection analyses reveal three major transcription start sites, also located between the inverted repeats. Transient transfections of chick sternal chondrocytes and fibroblasts with reporter plasmids bearing progressively reduced portions of the aggrecan promoter region allowed mapping of chondrocyte-specific transcription enhancer and silencer elements that are consistent with the sequence analysis. These findings suggest the importance of this regulatory region in the tissue-specific expression of the chick aggrecan gene.

Alternative splicing of the unique "PLUS" domain of chicken PG-M/versican is developmentally regulated

We investigated the occurrence of alternatively spliced forms (V0, V1, V2, and V3) of PG-M/versican, a large chondroitin sulfate proteoglycan in developing chicken retinas, using the reverse transcription-polymerase chain reaction. We characterized the PLUS domain, which is apparently unique to the chicken molecule and is regulated by alternative splicing. PG-M in chicken retinas consisted of four forms with (V0, V1, V2, and V3) and two forms without (V1 and V3) the PLUS domain (PG-M+ and PG-M-, respectively). The four forms of PG-M+ were found in all samples examined, but the occurrence of the two PG-M- forms was regulated developmentally. Genomic analysis has revealed that the PLUS and CS-alpha domains are encoded by a single exon, and this exon has an internal alternative 5'-splice donor site, allowing alternative spliced forms that do not include the 3'-end of the exon. Sequences corresponding to the chicken PLUS domain (plus) were not found in mouse and human and may have disappeared during evolution. Sequence similarity suggests that the PLUS domain corresponds to the keratan sulfate attachment domain of aggrecan and that it has a distinct function in the chicken eye.

Analysis of proteoglycan messages in human articular cartilage by a competitive PCR technique

A competitive PCR technique has been established to allow quantitation of message levels within tissues without the need for cell isolation. The method utilizes an internal RNA standard that uses the same oligonucleotide primers as the authentic message for both reverse transcription and DNA polymerization. While the technique does not give absolute message levels when applied to intact tissues, because of incomplete extraction yields, it can be used to give values relative to any reference message level. The technique has been applied to the analysis of the message levels for aggrecan, versican, link protein, decorin, biglycan, fibromodulin and lumican in human articular cartilage isolated from individuals ranging in age from the neonate to the mature adult. The data indicate that the messages for versican and link protein are always present in lesser abundance than that for aggrecan, and while the aggrecan message levels tend to increase in the adult, those for versican and link protein do not. With respect to the family of leucine-rich repeat proteoglycans, the message for decorin is by far the most abundant at all ages and shows a marked increase in abundance in the adult relative to the juvenile. The messages for fibromodulin and lumican also show increased abundance in the adult, whereas that for biglycan shows no marked age-related trend. The message levels for decorin were also higher than those for aggrecan at all ages.

Species-specific alternative splicing of the epidermal growth factor-like domain 1 of cartilage aggrecan

Aggrecan transcripts contain two alternatively spliced exons that code for two epidermal growth factor-like domains (EGF1 and EGF2). Whereas the EGF2 sequence is expressed at a uniform level among different species, the EGF1 sequence has been detected only in human aggrecan transcripts. In this study we have used the nested primer reverse transcription-PCR (RT-PCR) method to compare the expression of the EGF1 exon in human, bovine and dog aggrecan transcripts. Our results indicate that this exon is expressed in a species-specific manner. In addition to its significant expression level in human transcripts, the EGF1 sequence can be detected in a small portion of bovine aggrecan transcripts as shown with nested primer RT-PCR. In contrast, the same module is not detectable in dog aggrecan transcripts, although an EGF1 sequence is present in the dog aggrecan gene. The expression level of the EGF1 exon in the aggrecan transcripts correlates with the strength of the polypyrimidine tract upstream of the exon. The EGF1 sequence also shows much less conservation between the species than the EGF2 sequence. The species-specific expression and high sequence variation of the EGF1 exon imply that this sequence is likely to code for an aggrecan domain having no cartilage-specific function.

Quantification of aggrecan and link-protein mRNA in human articular cartilage of different ages by competitive reverse transcriptase-PCR

A competitive reverse transcriptase-PCR (RT-PCR) assay has been developed for the quantification of particular mRNA species in human articular cartilage. Competitor RNA species were synthesized that differed from the amplified target sequence only by the central insertion of an EcoRI restriction site. By using known amounts of synthetic target and competitor RNA, it was shown that competitor RNA molecules designed in this way are reverse-transcribed and amplified with equal efficiency to the target of interest. Furthermore quantification could be performed during the plateau phase of the PCR, which was necessary when using ethidium bromide fluorescence as a detection system. The inhibition of aggrecan and link-protein mRNA expression by interleukin 1 or tumour necrosis factor in monolayers of human articular chondrocytes quantified by this competitive RT-PCR method compared favourably with Northern hybridization studies. The main advantage of this technique is that it can be used to quantify levels of mRNA with RNA extracted directly from 100 mg wet weight of human articular cartilage. Age-related changes in aggrecan and link-protein mRNA were therefore quantified in human articular cartilage directly after dissection from the joint. The concentration of link-protein mRNA was higher in immature cartilage than in mature cartilage when expressed relative to the amount of glyceraldehyde-3-phosphate dehydrogenase mRNA, but no age-related changes were observed in aggrecan mRNA expression. The ratio of aggrecan to link-protein mRNA was higher in mature cartilage than in immature tissue. These age-related differences in the molecular stoichiometry of aggrecan and link-protein mRNA might have implications with respect to the regulation of the formation and the stability of the proteoglycan aggregates in cartilage.

The protein fold of the hyaluronate-binding proteoglycan tandem repeat domain of link protein, aggrecan and CD44 is similar to that of the C-type lectin superfamily

Link protein and aggrecan of the extracellular matrix each contain two proteoglycan tandem repeat (PTR) domains that interact with hyaluronate. Consensus secondary structure predictions for 59 PTR sequences and 129 C-type lectin sequences give similar patterns of two alpha-helices and up to seven beta-strands. Protein fold recognition analyses show that the 59 PTR sequences are highly compatible with the C-type lectin crystal structure. The predicted fold consists of a conserved motif formed from an antiparallel beta-sheet flanked by two alpha-helices, the motif being attached to two distinct types of beta-sheet region in the two superfamilies. Arg9 or Lys11 on an exposed loop and up to three other Arg residues in the beta-sheet region are conserved and may form part of a hyaluronate binding site.

Age-related changes in the content of the C-terminal region of aggrecan in human articular cartilage

The content of the C-terminal region of aggrecan was investigated in samples of articular cartilage from individuals ranging in age from newborn to 65 years. This region contains the globular G3 domain which is known to be removed from aggrecan in mature cartilage, probably by proteolytic cleavage, but the age-related changes in its abundance in human cartilage have not been described previously. The analysis was performed by immunosorbant assay using an antiserum (JD5) against recombinant amino acid residues of human aggrecan, on crude extracts of cartilage without further purification of aggrecan. The results showed that the content of the C-terminal region decreased with age relative to the G1 domain content (correlation coefficient = 0.463). This represented a 92% fall in the content of this region of the molecule from newborn to 65 years of age. furthermore, when the G1 content of the cartilage extracts was corrected to only include the G1 attached to aggrecan and to exclude the G1 fragments which accumulate as a by-product of normal aggrecan turnover (free G1), the age-related decrease in the C-terminal region remained very pronounced. Analysis by composite agarose/PAGE showed that the number of subpopulations of aggrecan resolved increased from one in newborn to three in adult cartilage. All of these reacted with an antiserum to the human G1 domain, but only the slowest migrating species reacted with the C-terminal region antiserum (JD5). Similar analysis by SDS/PAGE confirmed the presence of high-molecular-mass (200 kDa) proteins reactive with JD5, but no reactive fragments of lower electrophoretic mobility were detected. In contrast, when probed with the antiserum to the human G1 domain, the immunoblots showed protein species corresponding to the free G1 and G1-G2 fragments, which were present at high concentrations in adult cartilage. The results suggest that the loss of the C-terminal region is not directly part of the process of aggrecan turnover, but it is a slow independent matrix process that occurs more extensively with aging as turnover rates become slower. Young cartilage with the fastest turnover contains least molecules lacking the C-terminal region, whereas in old tissue with slow turnover few molecules retain this region. An increase in the cleavage of this region with age may also contribute to this change. The content of the C-terminal region may thus give a measure of the abundance of newly synthesized aggrecan.

The ins and outs of aggrecan

Aggrecan is a large and highly complex macromolecule, uniquely structured to fill space in the extracellular matrix (ECM) of cartilage. Lethal chondrodystrophies resulting from mutations in the structural gene for aggrecan demonstrate the serious consequences of the absence of aggrecan. Other chondrodystrophies are testimony to the importance of post-translational modifications. Here, Barbara Vertel reviews the role of aggrecan in the ECM of cartilage, discusses genetic mutations affecting aggrecan and highlights intracellular features of its synthesis and processing.