Cytoskeletal abnormalities in chondrocytes with EXT1 and EXT2 mutations

Advanced colon cancer coexisting with multiple Osteochondromatosis in a child; coincidence or causality? - A case report

INTRODUCTION AND IMPORTANCE

Childhood colorectal cancers are extremely rare and so is Osteochondromatosis. Both diseases do not have epidemiological records in African countries. The aim of this report is to present a rare coexistence of CRC and multiple enchondromas in a child.

PRESENTATION OF CASE

A case of a 12-year-old boy who presented with a large bowel obstruction secondary to an advanced tumor of the descending colon. He was also diagnosed with multiple osteochondromas affecting legs, arms, ribs, scapula, clavicle and pelvis. No positive family history was recorded. An urgent left hemicolectomy and diverting transverse colostomy was done. The colon can as stage IIIB and the patient received adjuvant chemotherapy. After 8 months of follow up, the colostomy was successfully reversed without any endoscopic signs of tumor growth or distant metastasis.

CLINICAL DISCUSSION

Colorectal cancer in childhood is rare. It may present with aggressive histological subtypes in children as compared to adults. There is little to no reports on the coexistence of colorectal cancer and multiple Osteochondromatosis. Microsatellite instability in DNA tumor is common in Colon Cancer and variety of mutations of EXT-1 and EXT-2 genes goes with Enchondromatosis.

CONCLUSION

The coexistence of two rare conditions is the remarkable issue in this case report. There are no prior reports in literature. Further genomic sequencing maybe required to better understand this coexistence.

Glomerular permeability is not affected by heparan sulfate glycosaminoglycan deficiency in zebrafish embryos

Proteinuria develops when specific components in the glomerular filtration barrier have impaired function. Although the precise components involved in maintaining this barrier have not been fully identified, heparan sulfate proteoglycans are believed to play an essential role in maintaining glomerular filtration. Although in situ studies have shown that a loss of heparan sulfate glycosaminoglycans increases the permeability of the glomerular filtration barrier, recent studies using experimental models have shown that podocyte-specific deletion of heparan sulfate glycosaminoglycan assembly does not lead to proteinuria. However, tubular reabsorption of leaked proteins might have masked an increase in glomerular permeability in these models. Furthermore, not only podocytes but also glomerular endothelial cells are involved in heparan sulfate synthesis in the glomerular filtration barrier. Therefore, we investigated the effect of a global heparan sulfate glycosaminoglycan deficiency on glomerular permeability. We used a zebrafish embryo model carrying a homozygous germline mutation in the ext2 gene. Glomerular permeability was assessed with a quantitative dextran tracer injection method. In this model, we accounted for tubular reabsorption. Loss of anionic sites in the glomerular basement membrane was measured using polyethyleneimine staining. Although mutant animals had significantly fewer negatively charged areas in the glomerular basement membrane, glomerular permeability was unaffected. Moreover, heparan sulfate glycosaminoglycan-deficient embryos had morphologically intact podocyte foot processes. Glomerular filtration remains fully functional despite a global reduction of heparan sulfate.

Glycosaminoglycans in the blood of hereditary multiple exostoses patients: Half reduction of heparan sulfate to chondroitin sulfate ratio and the possible diagnostic application

Hereditary multiple exostoses (HME) is an autosomal dominant skeletal disorder with wide variation in clinical phenotype and is caused by heterogeneous germline mutations in two of the Ext genes, EXT-1 and EXT-2, which encode ubiquitously expressed glycosyltransferases involved in the polymerization of heparan sulfate (HS) chains. To examine whether the Ext mutation could affect HS structures and amounts in HME patients being heterozygous for the Ext genes, we collected blood from patients and healthy individuals, separated it into plasma and cellular fractions and then isolated glycosaminoglycans (GAGs) from those fractions. A newly established method consisting of a combination of selective ethanol precipitation of GAGs, digestion of GAGs recovered on the filter-cup by direct addition of heparitinase or chondroitinase reaction solution and subsequent high-performance liquid chromatography of the unsaturated disaccharide products enabled the analysis using the least amount of blood (200 µL). We found that HS structures of HME patients were almost similar to those of controls in both plasma and cellular fractions. However, interestingly, although both the amounts of HS and chondroitin sulfate (CS) varied depending on the different individuals, the amounts of HS in both the plasma and cellular fractions of HME patient samples were decreased and the ratios of HS to CS (HS/CS) of HME patient samples were almost half those of healthy individuals. The results suggest that HME patients' blood exhibited reduced HS amounts and HS/CS ratios, which could be used as a diagnostic biomarker for HME.

Chondrogenesis on sulfonate-coated hydrogels is regulated by their mechanical properties

Many studies have demonstrated that sulfur-containing acidic groups induce chondrogenesis in vitro and in vivo. Recently, it is increasingly clear that mechanical properties of cell substrates largely influence cell differentiation. Thus, the present study investigated how mechanical properties of sulfonate-coated hydrogels influences chondrogenesis of mesenchymal stem cells (MSCs). Sulfonate-coated polyacrylamide gels (S-PAAm gels) which have the elastic modulus, E, of about 1, 15 and 150 kPa, were used in this study. MSCs cultured on the high stiffness S-PAAm gels (E=∼150 kPa) spread out with strong expression of stress fibers, while MSCs cultured on the low stiffness S-PAAm gels (E=∼1 kPa) had round shapes with less stress fibers but more cortical actins. Importantly, even in the absence of differentiation supplements, the lower stiffness S-PAAm gels led to the higher mRNA levels of chondrogenic markers such as Col2a1, Agc and Sox9 and the lower mRNA levels of an undifferentiation marker Sca1, indicating that the mechanical properties of S-PAAm gels strongly influence chondrogenesis. Blebbistatin which blocks myosin II-mediated mechanical sensing suppressed chondrogenesis induced by the low stiffness S-PAAm gels. The present study demonstrates that the soft S-PAAm gels effectively drive MSC chondrogenesis even in the absence of soluble differentiation factors and thus suggests that sulfonate-containing hydrogels with low stiffness could be a powerful tool for cartilage regeneration.

No haploinsufficiency but loss of heterozygosity for EXT in multiple osteochondromas

Multiple osteochondromas (MO) is an autosomal dominant disorder caused by germline mutations in EXT1 and/or EXT2. In contrast, solitary osteochondroma (SO) is nonhereditary. Products of the EXT gene are involved in heparan sulfate (HS) biosynthesis. In this study, we investigated whether osteochondromas arise via either loss of heterozygosity (2 hits) or haploinsufficiency. An in vitro three-dimensional chondrogenic pellet model was used to compare heterozygous bone marrow-derived mesenchymal stem cells (MSCs EXT(wt/-)) of MO patients with normal MSCs and the corresponding tumor specimens (presumed EXT(-/-)). We demonstrated a second hit in EXT in five of eight osteochondromas. HS chain length and structure, in vitro chondrogenesis, and EXT expression levels were identical in both EXT(wt/-) and normal MSCs. Immunohistochemistry for HS, HS proteoglycans, and HS-dependent signaling pathways (eg, TGF-β/BMP, Wnt, and PTHLH) also showed no differences. The cartilaginous cap of osteochondroma contained a mixture of HS-positive and HS-negative cells. Because a heterozygous EXT mutation does not affect chondrogenesis, EXT, HS, or downstream signaling pathways in MSCs, our results refute the haploinsufficiency theory. We found a second hit in 63% of analyzed osteochondromas, supporting the hypothesis that osteochondromas arise via loss of heterozygosity. The detection of the second hit may depend on the ratio of HS-positive (normal) versus HS-negative (mutated) cells in the cartilaginous cap of the osteochondroma.

Clinical outcome and genotype in patients with hereditary multiple exostoses

Hereditary multiple exostoses (HME) is an autosomal dominant skeletal disorder with a wide spectrum of clinical manifestations. In 52 out of 60 individuals from HME+ families, exostoses became clinically apparent. In this study, the clinical and radiological outcome of these 52 HME patients (19 families) was investigated by medical history, clinical examination, and radiographs. In addition to correlating phenotype with genotype, a linkage/exclusion analysis was performed in 35 HME patients. We found several correlations between HME genes (EXT1, EXT2) and phenotype. Compared to EXT2-linkage, female individuals with EXT1-linkage were smaller in stature. Patients with EXT1-linkage and patients with undetermined linkage (EXT?) were more severely affected, underwent more surgeries, and showed a higher number of exostoses at follow-up. Moreover, we found an increased phenotype risk for limb shortening for EXT1- and EXT?-linkage. This study corresponds to data of other investigators who showed that EXT1 mutations are associated with a more severe phenotype than other EXT forms. (c) 2007 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 25:1541-1551, 2007.

Regulation of chondrocyte differentiation by the actin cytoskeleton and adhesive interactions

Chondrocyte differentiation is a multi-step process characterized by successive changes in cell morphology and gene expression. In addition to tight regulation by numerous soluble factors, these processes are controlled by adhesive events. During the early phase of the chondrocyte life cycle, cell-cell adhesion through molecules such as N-cadherin and neural cell adhesion molecule (N-CAM) is required for differentiation of mesenchymal precursor cells to chondrocytes. At later stages, for example in growth plate chondrocytes, adhesion signaling from extracellular matrix (ECM) proteins through integrins and other ECM receptors such as the discoidin domain receptor (DDR) 2 (a collagen receptor) and Annexin V is necessary for normal chondrocyte proliferation and hypertrophy. Cell-matrix interactions are also important for chondrogenesis, for example through the activity of CD44, a receptor for Hyaluronan and collagens. The roles of several signaling molecules involved in adhesive signaling, such as integrin-linked kinase (ILK) and Rho GTPases, during chondrocyte differentiation are beginning to be understood, and the actin cytoskeleton has been identified as a common target of these adhesive pathways. Complete elucidation of the pathways connecting adhesion receptors to downstream effectors and the mechanisms integrating adhesion signaling with growth factor- and hormone-induced pathways is required for a better understanding of physiological and pathological skeletal development.

The role of EXT1 in nonhereditary osteochondroma: identification of homozygous deletions

BACKGROUND

Multiple osteochondromas is a hereditary syndrome that is characterized by the formation of cartilage-capped bony neoplasms (osteochondromas), for which exostosis (multiple)-1 (EXT1) has been identified as a causative gene. However, 85% of all osteochondromas present as solitary (nonhereditary) lesions in which somatic mutations in EXT1 are extremely rare, but loss of heterozygosity and clonal rearrangement of 8q24 (the chromosomal locus of EXT1) are common. We examined whether EXT1 might act as a classical tumor suppressor gene for nonhereditary osteochondromas.

METHODS

Eight nonhereditary osteochondromas were subjected to high-resolution array-based comparative genomic hybridization (array-CGH) analysis for chromosome 8q. The array-CGH results were validated by subjecting tumor DNA to multiple ligation-dependent probe amplification (MLPA) analysis for EXT1. EXT1 locus-specific fluorescent in situ hybridization (FISH) was performed on nuclei isolated from the three tissue components of osteochondroma (cartilage cap, perichondrium, bony stalk) to examine which parts of the tumor are of clonal origin.

RESULTS

Array-CGH analysis of tumor DNA revealed that all eight osteochondromas had a large deletion of 8q; five tumors had an additional small deletion of the other allele of 8q that contained the EXT1 gene. MLPA analysis of tumor DNA confirmed these findings and identified two additional deletions that were smaller than the limit of resolution of array-CGH. FISH analysis of the cartilage cap, perichondrium, and bony stalk showed that these homozygous EXT1 deletions were present only in the cartilage cap of osteochondroma.

CONCLUSION

EXT1 functions as a classical tumor suppressor gene in the cartilage cap of nonhereditary osteochondromas.

Spatiotemporal distribution of heparan sulfate epitopes during murine cartilage growth plate development

Heparan sulfate proteoglycans (HSPGs) are abundant in the pericellular matrix of both developing and mature cartilage. Increasing evidence suggests the action of numerous chondroregulatory molecules depends on HSPGs. In addition to specific functions attributed to their core protein, the complexity of heparan sulfate (HS) synthesis provides extraordinary structural and functional heterogeneity. Understanding the interactions of chondroregulatory molecules with HSPGs and their subsequent outcomes has been limited by the absence of a detailed analysis of HS species in cartilage. In this study, we characterize the distribution and variety of HS species in developing cartilage of normal mice. Cryo-sections of femur and tibia from normal mouse embryos were evaluated using immunostaining techniques. A panel of unique phage display antibodies specific to particular HS species were employed and visualized with secondary antibodies conjugated to Alexa-fluor dyes. Confocal microscopy demonstrates that HS species are dynamic structures within developing growth plate cartilage and the perichondrium. GlcNS6S-IdoUA2S-GlcNS6S species are down regulated and localization of GlcNS6S-IdoUA-GlcNS6S species within the hypertrophic zone of the growth plate is lost during normal development. Regional differences in HS structures are present within developing growth plates, implying that interactions with and responses to HS-binding proteins also may display regional specialization.

Differentiation-induced loss of heparan sulfate in human exostosis derived chondrocytes

An exostosis or osteochondroma is an aberrant bony growth occurring next to the growth plate either as an isolated growth abnormality or as part of the Hereditary Multiple Exostosis (HME) syndrome. Mutations in either exostosin 1 (EXT1) or exostosin 2 (EXT2) gene cause the HME syndrome and also some isolated osteochondromas. The EXT1 and EXT2 genes are glycosyltransferases that function as hetero-oligomers in the Golgi to add repeating glycosaminoglycans (GAGs) to heparan sulfate (HS) chains. Previously, we demonstrated that HS is markedly diminished in the exostosis cartilage cap and that the HS proteoglycan, perlecan, has an abnormal distribution in these caps. The present studies were undertaken to evaluate which chondrocyte-specific functions are associated with diminished HS synthesis in human chondrocytes harboring either EXT1 or EXT2 mutations. Systematic evaluation of exostosis cartilage caps and chondrocytes, both in vitro and in vivo, suggests that chondrocyte-specific cell functions account for diminished HS levels. In addition, we provide evidence that perichondrial cells give rise to chondrocytes that clonally expand and develop into an exostosis. Undifferentiated EXT chondrocytes synthesized amounts of HS similar to control chondrocytes; however, EXT chondrocytes displayed very poor survival in vitro under conditions that promote normal chondrocyte differentiation with high efficiency. Collectively, these observations suggest that loss of one copy of either the EXT1 or EXT2 gene product compromises the perichondrial chondrocytes' ability to differentiate normally and to survive in a differentiated state in vitro. In vivo, these compromised responses may lead to abnormal chondrocyte growth, perhaps from a perichondrial stem cell reserve.

Heparan sulfate proteoglycans: Coordinators of multiple signaling pathways during chondrogenesis

Heparan sulfate proteoglycans are abundantly expressed in the pericellular matrix of both developing and mature cartilage. Increasing evidence indicates that the action of numerous chondroregulatory molecules depends on these proteoglycans. This review summarizes the current understanding of the interactions of heparan sulfate chains of cartilage proteoglycans with both soluble and nonsoluble ligands during the process of chondrogenesis. In addition, the consequences of mutating genes encoding heparan sulfate biosynthetic enzymes or heparan sulfate proteoglycan core proteins on cartilage development are discussed.

Physiology and pathophysiology of the growth plate

Longitudinal growth of the skeleton is a result of endochondral ossification that occurs at the growth plate. Through a sequential process of cell proliferation, extracellular matrix synthesis, cellular hypertrophy, matrix mineralization, vascular invasion, and eventually apoptosis, the cartilage model is continually replaced by bone as length increases. The regulation of longitudinal growth at the growth plate occurs generally through the intimate interaction of circulating systemic hormones and locally produced peptide growth factors, the net result of which is to trigger changes in gene expression by growth plate chondrocytes. This review highlights recent advances in genetics and cell biology that are illuminating the important regulatory mechanisms governing the structure and biology of the growth plate, and provides selected examples of how studies of human mutations have yielded a wealth of new knowledge regarding the normal biology and pathophysiology of growth plate cartilage.

Reevaluation of a genetic model for the development of exostosis in hereditary multiple exostosis

EXT1 and EXT2 are genes that have been shown to cause hereditary multiple exostosis (HME), a syndrome marked by the formation of bony growths juxtaposed to the growth plate. These genes are members of a growing family of proteins with glycosyltransferase activity required for the synthesis of heparan sulfate chains. This protein activity is predicted to play a role in the expression of proteoglycans on the cell surface and in the extracellular matrix. We and others have previously suggested that a two-hit mutational model applies to the development of an exostosis where a germline mutation coupled with a somatic mutation results in the loss of EXT1 or EXT2 function and subsequent tumor formation. We report the direct sequencing and loss of heterozygosity (LOH) analysis of 12 exostoses from 10 HME families, 4 solitary exostoses, and their corresponding constitutional DNA. Of the 16 exostoses screened, we find only one solitary case in which two somatic mutations, a deletion and an LOH, are present. This provides limited support for the two-hit hypothesis involving the EXT1 and EXT2 genes for the development of an exostosis. Alternative models are developed based on the functional significance of EXT proteins in heparan sulfate biosynthesis.

Heparan sulfate abnormalities in exostosis growth plates

Hereditary multiple exostoses (HME), a condition associated with development and growth of bony exostoses at the ends of the long bones, is caused by germline mutations in the EXT genes. EXT1 and EXT2 function as glycosyltransferases that participate in the biosynthesis of heparan sulfate (HS) to modify proteoglycans. HS proteoglycans, synthesized by chondrocytes and secreted to the extracellular matrix of the growth plate, play critical roles in growth plate signaling and remodeling. As part of studies to delineate the mechanism(s) by which an exostosis develops, we have systematically evaluated four growth plates from two HME and two solitary exostoses. Mutational events were correlated with the presence/absence and distribution of HS and the normally abundant proteoglycan, perlecan (PLN). DNA from the HME exostoses demonstrated heterozygous germline EXT1 or EXT2 mutations, and DNA from one solitary exostosis demonstrated a somatic EXT1 mutation. No loss of heterozygosity was observed in any of these samples. The chondrocyte zones of four exostosis growth plates showed absence of HS, as well as diminished and abnormal distribution of PLN. These results indicate that, although multiple mutational events do not occur in the EXT1 or EXT2 genes, a complete loss of HS was found in the exostosis growth plates. This functional knockout of the exostosis chondrocytes' ability to synthesize HS chains further supports the observations of cytoskeletal abnormalities and chondrocyte disorganization associated with abnormal cell signaling.

Alpha-melanocyte-stimulating hormone protects against mesenteric ischemia-reperfusion injury

Mesenteric ischemia-reperfusion (I/R) injury to the intestine is a common and often devastating clinical occurrence for which there are few therapeutic options. alpha-Melanocyte-stimulating hormone (alpha-MSH) is a tridecapeptide released by the pituitary gland and immunocompetent cells that exerts anti-inflammatory actions and abrogates postischemic injury to the kidneys and brainstem of rodents. To test the hypothesis that alpha-MSH would afford similar protection in the postischemic small intestine, we analyzed the effects of this peptide on intestinal transit, histology, myeloperoxidase activity, and nuclear factor-kappaB (NF-kappaB) activation after 45 min of superior mesenteric artery occlusion and <or=6 h of reperfusion. Rats subjected to I/R exhibited markedly depressed intestinal transit, histological evidence of severe injury to the ileum, increased myeloperoxidase activity in ileal cytoplasmic extracts, and biphasic activation of NF-kappaB in ileal nuclear extracts. In contrast, rats treated with alpha-MSH before I/R exhibited intestinal transit and histological injury scores comparable to those of sham-operated controls. In addition, the alpha-MSH-treated rats demonstrated less I/R-induced activation of intestinal NF-kappaB and myeloperoxidase activity after prolonged (6 h) reperfusion. We conclude that alpha-MSH significantly limits postischemic injury to the rat small intestine.

Chondrogenic activity of the heparan sulfate proteoglycan perlecan maps to the N-terminal domain I

C3H10T1/2 cells differentiate along a chondrogenic pathway when plated onto the extracellular matrix (ECM) protein perlecan (Pln). To identify the region(s) within the large Pln molecule that provides a differentiation signal, recombinant Pln-sequence-based polypeptides representing distinct structural domains were assayed for their ability to promote chondrogenesis in C3H10T1/2 cells. Five distinct domains, along with structural variations, were tested. The N-terminal domain I was tested in two forms (IA and IB) that contain only heparan sulfate (HS) chains or both HS and chondroitin sulfate (CS) chains, respectively. A mutant form of domain I lacking attachment sites for both HS and CS (Pln I(mut)) was tested also. Other constructs consecutively designated Pln domains II, III(A-C), IV(A,B), and V(A,B) were used to complete the structure-function analysis. Cells plated onto Pln IA or Pln IB but no other domain rapidly assembled into cellular aggregates of 40-120 microm on average. Aggregate formation was dependent on the presence of glycosaminoglycan (GAG) chains, because Pln I-based polypeptides lacking GAG chains either by enzymatic removal or mutation of HS/CS attachment sites were inactive. Aggregates formed on GAG-bearing Pln IA stained with Alcian Blue and were recognized by antibodies to collagen type II and aggrecan but were not recognized by an antibody to collagen type X, a marker of chondrocyte hypertrophy. Collectively, these studies indicate that the GAG-bearing domain I of Pln provides a sufficient signal to trigger C3H10T1/2 cells to enter a chondrogenic differentiation pathway. Thus, this matrix proteoglycan (PG) found at sites of cartilage formation in vivo is likely to enhance early stage differentiation induced by soluble chondrogenic factors.

High quality RNA isolation from tumours with low cellularity and high extracellular matrix component for cDNA microarrays: application to chondrosarcoma

AIMS

High quality RNA isolation from cartilaginous tissue is considered difficult because of relatively low cellularity and the abundance of extracellular matrix rich in glycosaminoglycans and collagens. Given the growing interest and technical possibilities to study RNA expression at a high throughput level, research on tissue with these characteristics is hampered by the lack of an efficient method for obtaining sufficient amounts of high quality RNA.

METHODS

This paper presents a robust protocol combining two RNA isolation procedures, based on a combination of Trizol and RNA specific columns, which has been developed to obtain high molecular weight RNA from fresh frozen and stored tissue of normal cartilage and cartilaginous tumours. Using this method, RNA was isolated from normal cartilage, peripheral chondrosarcoma, and central chondrosarcoma.

RESULTS

The yields ranged from 0.1 to 0.5 microg RNA/mg tissue. RNA isolated with this method was stable and of high molecular weight. RNA samples from normal cartilage and from two chondrosarcomas isolated using this method were applied successfully in cDNA microarray experiments. The number of genes that give interpretable results was in the range of what would be expected from microarray results obtained on chondrosarcoma cell line RNA. Signal to noise ratios were good and differential expression between tumour and normal cartilage was detectable for a large number of genes.

CONCLUSION

With this newly developed isolation method, high quality RNA can be obtained from low cellular tissue with a high extracellular matrix component. These procedures can also be applied to other tumour material.

Etiological point mutations in the hereditary multiple exostoses gene EXT1: a functional analysis of heparan sulfate polymerase activity

Hereditary multiple exostoses (HME), a dominantly inherited genetic disorder characterized by multiple cartilaginous tumors, is caused by mutations in members of the EXT gene family, EXT1 or EXT2. The corresponding gene products, exostosin-1 (EXT1) and exostosin-2 (EXT2), are type II transmembrane glycoproteins which form a Golgi-localized heterooligomeric complex that catalyzes the polymerization of heparan sulfate (HS). Although the majority of the etiological mutations in EXT are splice-site, frameshift, or nonsense mutations that result in premature termination, 12 missense mutations have also been identified. Furthermore, two of the reported etiological missense mutations (G339D and R340C) have been previously shown to abrogate HS biosynthesis (McCormick et al. 1998). Here, a functional assay that detects HS expression on the cell surface of an EXT1-deficient cell line was used to test the remaining missense mutant exostosin proteins for their ability to rescue HS biosynthesis in vivo. Our results show that EXT1 mutants bearing six of these missense mutations (D164H, R280G/S, and R340S/H/L) are also defective in HS expression, but surprisingly, four (Q27K, N316S, A486V, and P496L) are phenotypically indistinguishable from wild-type EXT1. Three of these four "active" mutations affect amino acids that are not conserved among vertebrates and invertebrates, whereas all of the HS-biosynthesis null mutations affect only conserved amino acids. Further, substitution or deletion of each of these four residues does not abrogate HS biosynthesis. Taken together, these results indicate that several of the reported etiological mutant EXT forms retain the ability to synthesize and express HS on the cell surface. The corresponding missense mutations may therefore represent rare genetic polymorphisms in the EXT1 gene or may interfere with as yet undefined functions of EXT1 that are involved in HME pathogenesis.

Diminished levels of the putative tumor suppressor proteins EXT1 and EXT2 in exostosis chondrocytes

The EXT family of putative tumor suppressor genes affect endochondral bone growth, and mutations in EXT1 and EXT2 genes cause the autosomal dominant disorder Hereditary Multiple Exostoses (HME). Loss of heterozygosity (LOH) of these genes plays a role in the development of exostoses and chondrosarcomas. In this study, we characterized EXT genes in 11 exostosis chondrocyte strains using LOH and mutational analyses. We also determined subcellular localization and quantitation of EXT1 and EXT2 proteins by immunocytochemistry using antibodies raised against unique peptide epitopes. In an isolated non-HME exostosis, we detected three genetic hits: deletion of one EXT1 gene, a net 21-bp deletion within the other EXT1 gene and a deletion in intron 1 causing loss of gene product. Diminished levels of EXT1 and EXT2 protein were found in 9 (82%) and 5 (45%) exostosis chondrocyte strains, respectively, and 4 (36%) were deficient in levels of both proteins. Although we found mutations in exostosis chondrocytes, mutational analysis alone did not predict all the observed decreases in EXT gene products in exostosis chondrocytes, suggesting additional genetic mutations. Moreover, exostosis chondrocytes exhibit an unusual cellular phenotype characterized by abnormal actin bundles in the cytoplasm. These results suggest that multiple mutational steps are involved in exostosis development and that EXT genes play a role in cell signaling related to chondrocyte cytoskeleton regulation.

Recent advances in the study of the biosynthesis and functions of sulfated glycosaminoglycans

Recent cDNA cloning of the glycosyltransferases involved in the synthesis of the sulfated glycosaminoglycan sidechains of proteoglycans has provided important clues to answering long-standing questions concerning the mechanisms of both chain polymerization and the biosynthetic sorting of glucosaminoglycans (heparin/heparan sulfate) and galactosaminoglycans (chondroitin/dermatan sulfate). These biosynthetic mechanisms are crucial to the expression and regulation of the biological functions of glycosaminoglycans in development and pathophysiology.