IRF6 mutation screening in non-syndromic orofacial clefting: analysis of 1521 families.

Van der Woude syndrome (VWS) is an autosomal dominant malformation syndrome characterized by orofacial clefting (OFC) and lower lip pits. The clinical presentation of VWS is variable and can present as an isolated OFC, making it difficult to distinguish VWS cases from individuals with non-syndromic OFCs. About 70% of causal VWS mutations occur in IRF6, a gene that is also associated with non-syndromic OFCs. Screening for IRF6 mutations in apparently non-syndromic cases has been performed in several modestly sized cohorts with mixed results. In this study, we screened 1521 trios with presumed non-syndromic OFCs to determine the frequency of causal IRF6 mutations. We identified seven likely causal IRF6 mutations, although a posteriori review identified two misdiagnosed VWS families based on the presence of lip pits. We found no evidence for association between rare IRF6 polymorphisms and non-syndromic OFCs. We combined our results with other similar studies (totaling 2472 families) and conclude that causal IRF6 mutations are found in 0.24-0.44% of apparently non-syndromic OFC families. We suggest that clinical mutation screening for IRF6 be considered for certain family patterns such as families with mixed types of OFCs and/or autosomal dominant transmission.

Functional Significance of MMP3 and TIMP2 Polymorphisms in Cleft Lip/Palate.

Evidence from biological and human studies strongly supports a role for MMP and TIMP genes as candidate genes for non-syndromic cleft lip with or without cleft palate (NSCL/P). We previously showed the association of promoter polymorphisms in MMP3 (rs3025058 and rs522616) and TIMP2 (rs8179096) with NSCL/P. In this study, we examined the functional significance of these polymorphisms. A specific DNA-protein complex for MMP3 rs522616 A was detected, and this allele by itself showed greater promoter activity than the G allele. However, the effect of rs522616 was ultimately regulated by the rs3025058 allele on the background. For TIMP2 rs8179096, the T allele showed a 2.5-fold increase in promoter activity when compared with allele C, whereas both C and T alleles were found to bind to nuclear factor kappa B. Our results provide new evidence that promoter polymorphisms in MMP3 and TIMP2 are functional and may affect gene transcription with possible effects on craniofacial development leading to NSCL/P.

Comparative analysis with collagen type II distinguishes cartilage oligomeric matrix protein as a primary TGFß-responsive gene.

OBJECTIVE: This study aims to investigate the regulation of expression of Cartilage oligomeric matrix protein (COMP), which is predominately expressed by chondrocytes and functions to organize the extracellular matrix. Mutations in COMP cause two skeletal dysplasias: pseudoachondroplasia and multiple epiphyseal dysplasia. The mechanism controlling COMP expression during chondrocyte differentiation is still poorly understood. DESIGN: Primary human bone marrow-derived stem cells were induced to differentiate into chondrocyte by pellet cultures. We then compared the temporal expression of COMP with the well-characterized cartilage-specific Type II collagen (Col2a1), and their response to transforming growth factor (TGF)ß and Sox trio (Sox5, 6, and 9) stimulation. RESULTS: COMP and Col2a1 expression are differentially regulated by three distinct mechanisms. First, upregulation of COMP mRNA precedes Col2a1 by several days during chondrogenesis. Second, COMP expression is independent of high cell density but requires TGF-ß1. Induction of COMP mRNA by TGF-ß1 is detected within 2h in the absence of protein synthesis and is blocked by specific inhibitors of the TGFß signaling pathway; and therefore, COMP is a primary TFGß-response gene. Lastly, while Col2a1 expression is intimately controlled by the Sox trio, overexpression of Sox trio fails to activate the COMP promoter. CONCLUSION: COMP and Col2a1 expression are regulated differently during chondrogenesis. COMP is a primary response gene of TGFß and its fast induction during chondrogenesis suggests that COMP is suitable for rapidly accessing the chondrogenic potential of stem cells.

The role of cartilage oligomeric matrix protein (COMP) in skeletal disease.

Cartilage oligomeric matrix protein is a non-collagenous extracellular matrix protein expressed primarily in cartilage, ligament, and tendon. Cartilage oligomeric matrix protein has been studied extensively because mutations in the gene cause two skeletal dysplasias, pseudoachondroplasia and multiple epiphyseal dysplasia. Pseudoachondroplasia is a disproportionate dwarfing condition associated with joint abnormalities, while multiple epiphyseal dysplasia is less severe. Both of these skeletal dysplasias have a characteristic chondrocyte pathology that consists of intracellular retention of cartilage oligomeric matrix protein and other extracellular matrix proteins in an enlarged rough endoplasmic reticulum. This toxic intracellular retention of extracellular matrix proteins causes chondrocyte cell death thereby decreasing linear bone growth. Additionally, when cartilage oligomeric matrix protein and the other co-retained proteins are not exported to the extracellular matrix, the resulting matrix is abnormal and easily erodes with normal physical activity. Cartilage oligomeric matrix protein is also a marker for joint destruction associated osteoarthritis, rheumatoid arthritis, joint trauma, and intense activity. Serum cartilage oligomeric matrix protein levels are higher in aggressive cases of arthritis and levels are used to predict future disease progression. Recent studies have identified molecular functions of cartilage oligomeric matrix protein that may contribute to its role in skeletal disease. These molecular functions include: binding other ECM proteins, catalyzing polymerization of type II collagen fibrils, and regulation of chondrocyte proliferation. Here, we review cartilage oligomeric matrix protein's role in skeletal disease and potential molecular mechanisms.

The population-based prevalence of achondroplasia and thanatophoric dysplasia in selected regions of the US.

There have been no large population-based studies of the prevalence of achondroplasia and thanatophroic dysplasia in the United States. This study compared data from seven population-based birth defects monitoring programs in the United States. We also present data on the association between older paternal age and these birth defects, which has been described in earlier studies. The prevalence of achondroplasia ranged from 0.36 to 0.60 per 10,000 livebirths (1/27,780-1/16,670 livebirths). The prevalence of thanatophoric dysplasia ranged from 0.21 to 0.30 per 10,000 livebirths (1/33,330-1/47,620 livebirths). In Texas, fathers that were 25-29, 30-34, 35-39, and > or =40 years of age had significantly increased rates of de novo achondroplasia among their offspring compared with younger fathers. The adjusted prevalence odds ratios were 2.8 (95% CI; 1.2, 6.7), 2.8 (95% CI; 1.0, 7.6), 4.9 (95% CI; 1.7, 14.3), and 5.0 (95% CI; 1.5, 16.1), respectively. Using the same age categories, the crude prevalence odds ratios for de novo cases of thanatophoric dysplasia in Texas were 5.8 (95% CI; 1.7, 9.8), 3.9 (95% CI; 1.1, 6.7), 6.1 (95% CI; 1.6, 10.6), and 10.2 (95% CI; 2.6, 17.8), respectively. These data suggest that thanatophoric dysplasia is one-third to one-half as frequent as achondroplasia. The differences in the prevalence of these conditions across monitoring programs were consistent with random fluctuation. Birth defects monitoring programs may be a good source of ascertainment for population-based studies of achondroplasia and thanatophoric dysplasia, provided that diagnoses are confirmed by review of medical records.

Model systems for studying skeletal dysplasias caused by TSP-5/COMP mutations.

Cartilage oligomeric matrix protein, also known as thrombospondin-5 (TSP-5), is an extracellular matrix protein found primarily in cartilage and musculoskeletal tissues. TSP-5 is of interest because mutations in the gene cause two skeletal dysplasias, pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED/EDM1). Both PSACH and EDM1 have a characteristic chondrocyte phenotype distinguished by giant rough endoplasmic reticulum (rER) cisternae containing TSP-5 and other extracellular matrix proteins such as type IX collagen and matrilin-3. The accumulation of proteinaceous material in the rER compromises cellular function and leads to premature chondrocyte death. Both in vitro and in vivo models have been generated with varying degrees of success to study the cellular mechanisms of the disease process. Here we review and discuss in vitro and in vivo PSACH and MED model systems and describe two transgenic mouse lines expressing human mutant TSP-5 protein. These model systems have revealed several important features of the PSACH cellular pathology: unfolded protein response activation, upregulation of apoptosis and inappropriate assembly of matrix network in the rER. Some of these models are valuable reagents that may be of use in testing therapeutic interventions. (Part of a Multiauthor Review).

Apoptosis staining in cultured pseudoachondroplasia chondrocytes.

Pseudoachondroplasia (PSACH) is a skeletal dysplasia caused by a mutation in cartilage oligomeric matrix protein (COMP), a glycoprotein of normal cartilage matrix. PSACH chondrocytes have a distinctive phenotype with enlarged rER cisternae containing COMP, aggrecan, type IX collagen, and chaperone proteins. Ultrastructural studies suggested that this accumulation compromises cell function, hastening cell death, and consequently reducing the number of cells in the growth plate contributing to linear bone growth. Using the alginate bead system, we cultured control and PSACH chondrocytes for twenty weeks and one year to determine the effect of the mutation on size and number of cartilage nodules; and the presence of apoptotic cell death (TUNEL assay). At 20 weeks, beads containing PSACH or control chondrocytes did not differ in size and number of cartilage nodules or number of TUNEL-positive cells. After one year, nodule number, size and percent cartilage per bead were significantly less in PSACH nodules, and the number of cells staining positive for apoptosis was significantly greater than in controls (71.8% vs. 44.6%). The increase in apoptosis in PSACH nodules correlates with a decrease in growth of cartilage, supporting our hypothesis that death of damaged cells contributes to the growth plate defects in PSACH.

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.

A natural history of cleidocranial dysplasia.

Cleidocranial dysplasia (CCD) is an autosomal dominant skeletal dysplasia associated with clavicle hypoplasia and dental abnormalities. The condition is caused by mutations in the CBFA1 gene, a transcription factor that activates osteoblast differentiation. Clinical characteristics associated with CCD have previously been described in case reports and small case series. This study was undertaken to gain a more complete delineation of clinical complications associated with CCD. The study population was composed of 90 CCD individuals and 56 relative controls ascertained from genetic and dental practices in the United States, Canada, Europe, and Australia. A number of previously unrecognized complications were significantly increased including: genua valga, scoliosis, pes planus, sinus infections, upper respiratory complications, recurrent otitis media, and hearing loss. Primary Cesarean section rate was significantly increased compared to relative controls and the general population rate. Finally, dental abnormalities, including supernumerary teeth, failure of exfoliation of the primary dentition, and malocclusion, are serious and complex problems that require intervention. Clinical recommendations based on the results of this study are included.

Calreticulin, PDI, Grp94 and BiP chaperone proteins are associated with retained COMP in pseudoachondroplasia chondrocytes.

Cartilage oligomeric matrix protein (COMP), a large pentameric glycoprotein and member of the thrombospondin (TSP) group of extracellular proteins, is found in the territorial matrix surrounding chondrocytes. More than 50 unique COMP mutations have been identified as causing two skeletal dysplasias: pseudoachondroplasia (PSACH); and multiple epiphyseal dysplasia (EDM1). Recent studies suggest that calcium-binding and calcium-induced protein folding differ between wild type and mutant proteins, and abnormal processing of the mutant COMP protein contributes to the characteristic enlarged lamellar appearing rER cisternae in PSACH and EDMI chondrocytes in vivo and in vitro. Towards the goal of delineating the pathogenesis of PSACH and EDM1, in-vivo PSACH growth plate and in-vitro PSACH chondrocytes cultured in alginate beads were examined to identify and localize the chaperone proteins participating in the processing of the retained extracellular matrix proteins in the PSACH rER. Aggrecan was localized to both the rER cisternae and matrix while COMP and type IX collagen were only found in the rER. Type II collagen was solely found in the ECM suggesting that it is processed and transported differently from other retained ECM proteins. Five chaperone proteins: BiP (Grp78); calreticulin (CRT); protein disulfide (PDI); ERp72; and Grp94, demonstrated immunoreactivity in the enlarged PSACH cisternae and the short rER channels of chondrocytes from both in-vivo and in-vitro samples. The chaperone proteins cluster around the electron dense material within the enlarged rER cisternae. CRT, PDI and GRP94 AB-gold particles appear to be closely associated with COMP. Immunoprecipitation and Western blot, and Fluorescence Resonance Energy Transfer (FRET) analyses indicate that CRT, PDI and GRP94 are in close proximity to normal and mutant COMP and BiP to mutant COMP. These results suggest that these proteins play a role in the processing and transport of wild type COMP in normal chondrocytes and in the retention of mutant COMP in PSACH chondrocytes.

Analysis of the promoter region of human cartilage oligomeric matrix protein (COMP).

Cartilage oligomeric matrix protein (COMP) is an extracellular matrix protein expressed in cartilage, ligament, and tendon. The importance of COMP in the matrix of these cells is underscored by the discovery that mutations in COMP cause the skeletal dysplasias, pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (EDM1). Here, we present the first report on the analysis of the human COMP promoter region in cartilage, ligament, and tendon cells. A 1.7-kb region of the COMP promoter has been cloned and sequenced and no TATA or CAAT boxes were found. Primer extension identified multiple transcription start sites. All four transcription start sites were utilized in chondrocytes with only three of them utilized in tendon and ligament cells. Differential regulation was observed for different parts of this 1.7-kb region with the 370-bp proximal region conveying the strongest promoter activity. The highest activity was observed in tendon and ligament. Finally, we provide evidence that the DNA binding protein SP1 plays a role in the regulation of COMP expression. These results indicate that COMP expression within these cells is regulated in a unique manner that differs from the expression of other extracellular matrix genes.

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.

Pseudoachondroplasia and multiple epiphyseal dysplasia: New etiologic developments.

Pseudoachondroplasia (PSACH) (OMIM#177170) and multiple epiphyseal dysplasia (MED) are separate but overlapping osteochondrodysplasias. PSACH is a dominantly inherited disorder characterized by short-limb short stature, loose joints, and early-onset osteoarthropathy. The diagnosis is based on characteristic clinical and radiographic findings. Only mutations in the cartilage oligomeric matrix protein (COMP) gene have been reported in PSACH, and all family studies have been consistent with linkage to the COMP locus on chromosome 19. Multiple epiphyseal dysplasia (MED) is a relatively mild chondrodysplasia but like PSACH, MED causes early-onset joint degeneration, particularly of the large weight-bearing joints. Given the clinical similarity between PSACH and MED, it was not surprising that the first MED locus identified was the COMP gene (EDM1). Mutations causing MED have now been identified in five other genes (COL9A1, COL9A2, COL9A3, DTDST, and MATN3), making MED one of the most genetically heterogeneous disorders. This article reviews the clinical features of PSACH and MED, the known mutations, and the pathogenetic effect of COMP mutations on the cartilage extracellular matrix.

Familial case of Potocki-Shaffer syndrome associated with microdeletion of EXT2 and ALX4.

Multiple exostosis, biparietal foramina, minor craniofacial abnormalities, and mental retardation are characteristic of the syndrome associated with a proximal deletion of 11p (MIM # 601224), which has been shown to be a true contiguous gene deletion syndrome. The presence of multiple exostosis is associated with deletion of the EXT2 gene. Similarly, the presence of biparietal foramina has been shown to be associated with the deletion of ALX4 located proximally to EXT2. Specific genes related to mental retardation and craniofacial abnormalities, however, have yet to be identified. We report on a family with a microdeletion of 11(pll.2p11.2) with multiple exostosis and biparietal foramina without mental retardation or craniofacial abnormalities. Our results suggest that genes related to mental retardation and craniofacial development must be located outside of the D11S1785-D11S1385 region.

Delta 469 mutation in the type 3 repeat calcium binding domain of cartilage oligomeric matrix protein (COMP) disrupts calcium binding.

Cartilage oligomeric matrix protein (COMP/TSP5), a large glycoprotein found in the territorial matrix surrounding chondrocytes, is the fifth member of the thrombospondin (TSP) gene family. While the function of COMP is unknown, its importance is underscored by the finding that mutations in the highly conserved type 3 repeat domain causes two skeletal dysplasias. Pseudoachondroplasia (PSACH) and Multiple Epiphyseal Dysplasia, Fairbanks type (EDM1). The type 3 repeats are highly conserved low-affinity Ca(2+)binding domains that are found in all TSP genes. This study was undertaken to determine the effects of mutations on calcium binding and structure of the type 3 repeat domains. Wild-type (WT) and Delta469 recombinant COMP (rCOMP) proteins containing the entire calcium-binding domain were expressed in E. coli and purified. Equilibrium dialysis demonstrated that WT bound 10-12 Ca(2+)ions/molecule while Delta469 bound approximately half the Ca(2+)ions. Circular dichroism (CD) spectrometry had striking spectral changes for the WT in response to increasing concentrations of Ca(2+). These CD spectral changes were cooperative and reversible. In contrast, a large CD spectral change was not observed at any Ca(2+)concentration for Delta469. Moreover, both WT and Delta469 proteins produced similar CD spectral changes when titrated with Zn(2+), Cu(2+)and Ni(2+)indicating that the Delta469 mutation specifically affects only calcium binding. These results suggest that the Delta469 mutation, in the type 3 repeat region, interferes with Ca(2+)binding and that filling of all Ca(2+)binding loops may be critical for correct COMP protein conformation.

Cartilage oligomeric matrix protein is a calcium-binding protein, and a mutation in its type 3 repeats causes conformational changes.

Mutations in residues in the type 3 calcium-binding repeats and COOH-terminal globular region of cartilage oligomeric matrix protein (COMP) lead to two skeletal dysplasias, pseudoachondroplasia and multiple epiphyseal dysplasia. It has been hypothesized that these mutations cause COMP to misfold and to be retained in the endoplasmic reticulum. However, this hypothesis is not supported by previous reports that COMP, when purified in the presence of EDTA, shows no obvious difference in electron microscopic appearance in the presence or absence of calcium ions. Since this discrepancy may be due to the removal of calcium during purification, we have expressed wild-type COMP and the most common mutant form found in pseudoachondroplasia, MUT3, using a mammalian expression system and have purified both proteins in the presence of calcium. Both proteins are expressed as pentamers. Direct calcium binding experiments demonstrate that wild-type COMP, when purified in the presence of calcium, is a calcium-binding protein. Rotary shadowing electron microscopy and limited trypsin digestion at various calcium concentrations show that there are conformational changes associated with calcium binding to COMP. Whereas COMP exists in a more compact conformation in the presence of calcium, it shows a more extended conformation when calcium is removed. MUT3, with a single aspartic acid deletion in the type 3 repeats, binds less calcium and presents an intermediate conformation between the calcium-replete and calcium-depleted forms of COMP. In conclusion, we show that a single mutation in the type 3 repeats of COMP causes the mutant protein to misfold. Our data demonstrate the importance of calcium binding to the structure of COMP and provide a plausible explanation for the observation that mutations in the type 3 repeats and COOH-terminal globular region lead to pseudoachondroplasia.

Confirmation of the mapping of the Camurati-Englemann locus to 19q13. 2 and refinement to a 3.2-cM region.

Camurati-Englemann syndrome (DPD1) is an autosomal dominant condition associated with progressive cortical sclerosis of the diaphyses of all the long bones. Clinical features include abnormal gait, muscle weakness and wasting, and generalized fatigue. The DPD1 gene was recently mapped to a 15.1-cM region on chromosome 19q13.2. We have narrowed the region containing the DPD1 gene to a 3.2-cM region flanked by short tandem repeat markers, D19S881 and D19S718. TGFB1, a candidate gene mapped within this region, was excluded.

Cytoskeletal abnormalities in chondrocytes with EXT1 and EXT2 mutations.

The EXT genes are a group of putative tumor suppressor genes that previously have been shown to participate in the development of hereditary multiple exostoses (HME), HME-associated and isolated chondrosarcomas. Two HME disease genes, EXT1 and EXT2, have been identified and are expressed ubiquitously. However, the only known effect of mutations in the EXT genes is on chondrocyte function as evidenced by aberrant proliferation of chondrocytes leading to formation of bony, cartilage-capped projections (exostoses). In this study, we have characterized exostosis chondrocytes from three patients with HME (one with EXT1 and two with EXT2 germline mutations) and from one individual with a non-HME, isolated exostosis. At the light microscopic level, exostosis chondrocytes have a stellate appearance with elongated inclusions in the cytoplasm. Confocal and immunofluorescence of in vitro and in vivo chondrocytes showed that these massive accumulations are composed of actin bundled by 1.5-microm repeat cross-bridges of alpha-actinin. Western blot analysis shows that exostosis chondrocytes from two out of three patients aberrantly produce high levels of muscle-specific alpha-actin, whereas beta-actin levels are similar to normal chondrocytes. These findings suggest that mutations in the EXT genes cause abnormal processing of cytoskeleton proteins in chondrocytes.