Jacqueline T. Hecht

Apnea and sudden unexpected death in infants with achondroplasia

Thirteen infants with achondroplasia and sudden unexpected death or unexplained apnea were discovered through nonsystematic retrospective case collection. Most were initially thought to have died from sudden infant death syndrome. However, historical and pathologic findings suggest that many of these infants had apnea and sudden unexpected death secondary to acute or chronic compression of the lower brainstem or cervical spinal cord. Infants with achondroplasia evidently are at considerably increased risk for such deaths between 1 month and 1 year of age. Appropriate intervention, given these previously unrecognized risks, may include cervical restraint, polysomnographic evaluation, and apnea monitoring.

Genetic epidemiology study of idiopathic talipes equinovarus.

Previous genetic studies of idiopathic talipes equinovarus (ITEV) suggest an environmental and genetic component to the etiology of ITEV. The present study was undertaken to assess the role of causal factors in the development of ITEV. A total of 285 propositi were ascertained, with detailed family history information available in 173 cases and medical records on the remaining 112 propositi. Information was collected on specific prenatal, parental, and demographic factors. No racial heterogeneity was noted among any of the factors. The overall ratio of affected males to females was 2.5:1. The incidence of twinning among all propositi was significantly increased (P=0.006) above the expected population frequency. A family history of ITEV was noted in 24.4% of all propositi studied. These findings, in addition to the detailed analysis of 53 pedigrees with ITEV history, suggest the potential role of a gene or genes operating in high-risk families to produce this foot deformity.

The Fate of Cartilage Oligomeric Matrix Protein Is Determined by the Cell Type in the Case of a Novel Mutation in Pseudoachondroplasia

We have identified a novel missense mutation in a pseudoachondroplasia (PSACH) patient in one of the type III repeats of cartilage oligomeric matrix protein (COMP). Enlarged lamellar rough endoplasmic reticulum vesicles were shown to contain accumulated COMP along with type IX collagen, a cartilage-specific component. COMP was secreted and assembled normally into the extracellular matrix of tendon, demonstrating that the accumulation of COMP in chondrocytes was a cell-specific phenomenon. We believe that the intracellular storage of COMP causes a nonspecific aggregation of cartilage-specific molecules and results in a cartilage matrix deficient in required structural components leading to impaired cartilage growth and maintenance. These data support a common pathogenetic mechanism behind two clinically related chondrodysplasias, PSACH and multiple epiphyseal dysplasia.

Variation in IRF6 contributes to nonsyndromic cleft lip and palate

Nonsyndromic cleft lip with or without cleft palate (NSCLP) is a common craniofacial birth defect which results in lifelong medical and social consequences. While there have been a number of attempts to identify the genes responsible for this disorder, the results have not been consistent among populations and no single gene has been identified as playing a major susceptibility role. Van der Woude syndrome, a disorder characterized by lower‐lip pits with or without cleft lip/palate, results in many cases from mutations in the interferon regulatory factor 6 (IRF6) gene. Recently, Zucchero et al. [2004: N Engl J Med 351:769–780] detected an association between SNPs in IRF6 and NSCLP in a number of different populations. A subsequent study by Scapoli et al. [2005: Am J Hum Genet 76:180–183] confirmed this association in an Italian population. We examined the same SNPs as Scapoli et al. [2005] in our large, well‐characterized sample of NSCLP families and trios, and also detected an altered transmission of IRF6 alleles. This additional confirmation further strengthens the IRF6 association and suggests that IRF6 plays a role in NSCLP susceptibility.

Variation in WNT genes is associated with non-syndromic cleft lip with or without cleft palate

Non-syndromic cleft lip with or without cleft palate (NSCLP) is a common birth defect. Genetic and environmental factors have been causally implicated and studies have begun to delineate genetic contributions. The Wnt genes are involved in regulating mid-face development and upper lip fusion and are therefore strong candidates for an etiological role in NSCLP. Furthermore, the clf1 region in A/WyN clefting susceptible mice contains the Wnt3 and Wnt9B genes. To assess the role of the Wnt family of genes in NSCLP, we interrogated seven Wnt genes (Wnt3, Wnt3A, Wnt5A, Wnt7A, Wnt8A, Wnt9B and Wnt11) in our well-defined NSCLP dataset. Thirty-eight single nucleotide polymorphisms were genotyped in 132 multiplex NSCLP families and 354 simplex parent-child trios. In the entire dataset, single-nucleotide polymorphisms (SNPs) in three genes, Wnt3A (P = 0.006), Wnt 5A (P = 0.002) and Wnt11 (P = 0.0001) were significantly associated with NSCLP after correction for multiple testing. When stratified by ethnicity, the strongest associations were found for SNPs in Wnt3A (P = 0.0007), Wnt11 (P = 0.0012) and Wnt8A (P = 0.0013). Multiple haplotypes in Wnt genes were associated with NSCLP, and gene-gene interactions were observed between Wnt3A and both Wnt3 and Wnt5A (P = 0.004 and P = 0.039, respectively). This data suggests that alteration in Wnt gene function may perturb formation and/or fusion of the facial processes and predispose to NSCLP.

CHARACTERIZATION OF CARTILAGE OLIGOMERIC MATRIX PROTEIN (COMP) IN HUMAN NORMAL AND PSEUDOACHONDROPLASIA MUSCULOSKELETAL TISSUES

Cartilage oligomeric matrix protein (COMP), the fifth member of the -thrombospondin gene family, is an extracellular matrix calcium-binding protein. The importance of COMP is underscored by the finding that mutations in COMP cause the human dwarfing condition, pseudoachondroplasia (PSACH). Here, we report the results of human tissue distribution and cell secretion studies of human COMP. COMP is expressed and secreted by cultured monolayer chondrocyte, tendon and ligament cells, and COMP secretion is not restricted to a differentiated chondrocyte phenotype. Whereas COMP is retained in the endoplasmic reticulum that accumulates within PSACH chondrocytes in vivo, COMP is not retained intracellularly in the dedifferentiated PSACH chondrocytes in cultures. These results lend further support to the hypothesis that retention of COMP is related to the terminal PSACH chondrocyte phenotype, processing of proteins related to extracellular matrix formation, and maintenance in cartilage.

Interaction of cartilage oligomeric matrix protein/thrombospondin 5 with aggrecan.

Cartilage oligomeric matrix protein/thrombospondin 5 (COMP/TSP5) is a major component of the extracellular matrix (ECM) of the musculoskeletal system. Its importance is underscored by its association with several growth disorders. In this report, we investigated its interaction with aggrecan, a major component of cartilage ECM. We also tested a COMP/TSP5 mutant, designated MUT3 that accounts for 30% of human pseudoachondroplasia cases, to determine if the mutation affects function. Using a solid-phase binding assay, we have shown that COMP/TSP5 can bind aggrecan. This binding was decreased with MUT3, or when COMP/TSP5 was treated with EDTA, indicating the presence of a conformation-dependent aggrecan binding site. Soluble glycosaminoglycans (GAGs) partially inhibited binding, suggesting that the interaction was mediated in part through aggrecan GAG side chains. Using affinity co-electrophoresis, we showed that COMP/TSP5, in its calcium-replete conformation, bound to heparin, chondroitin sulfates, and heparan sulfate; this binding was reduced with EDTA treatment of COMP/TSP5. MUT3 showed weaker binding than calcium-repleted COMP/TSP5. Using recombinant COMP/TSP5 fragments, we found that the “signature domain” could bind to aggrecan, suggesting that this domain can mediate the interaction of COMP/TSP5 and aggrecan. In summary, our data indicate that COMP/TSP5 is an aggrecan-binding protein, and this interaction is regulated by the calcium-sensitive conformation of COMP/TSP5; interaction of COMP with aggrecan can be mediated through the GAG side chains on aggrecan and the “signature domain” of COMP/TSP5. Our results suggest that COMP/TSP5 may function to support matrix interactions in cartilage ECM.

Asymmetric Lower-Limb Malformations in Individuals with Homeobox PITX1 Gene Mutation

Clubfoot is one of the most common severe musculoskeletal birth defects, with a worldwide incidence of 1 in 1000 live births. In the present study, we describe a five-generation family with asymmetric right-sided predominant idiopathic clubfoot segregating as an autosomal-dominant condition with incomplete penetrance. Other lower-limb malformations, including patellar hypoplasia, oblique talus, tibial hemimelia, developmental hip dysplasia, and preaxial polydactyly, were also present in some family members. Genome-wide linkage analysis with Affymetrix GeneChip Mapping 10K mapping data from 13 members of this family revealed a multipoint LOD(max) of 3.31 on chromosome 5q31. A single missense mutation (c.388G-->A) was identified in PITX1, a bicoid-related homeodomain transcription factor critical for hindlimb development, and segregated with disease in this family. The PITX1 E130K mutation is located in the highly conserved homeodomain and reduces the ability of PITX1 to transactivate a luciferase reporter. The PITX1 E130K mutation also suppresses wild-type PITX1 activity in a dose-dependent manner, suggesting dominant-negative effects on transcription. The propensity for right-sided involvement in tibial hemimelia and clubfoot suggests that PITX1, or pathways involving PITX1, may be involved in their etiology. Implication of a gene involved in early limb development in clubfoot pathogenesis also suggests additional pathways for future investigations of idiopathic clubfoot etiology in humans.

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.

Retention of cartilage oligomeric matrix protein (COMP) and cell death in redifferentiated pseudoachondroplasia chondrocytes.

Cartilage oligomeric matrix protein (COMP) is a large extracellular glycoprotein that is found in the territorial matrix surrounding chondrocytes. Two skeletal dysplasias, pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (EDM1) are caused by mutations in the calcium binding domains of COMP. In this study, we identified two PSACH mutations and assessed the effect of these mutations on redifferentiated chondrocyte structure and function. We confirmed, in vitro, that COMP is retained in enormous cisternae of the rough endoplasmic reticulum (rER) and relatively absent in the PSACH matrix. The rER accumulation may compromise chondrocyte function, leading to chondrocyte death. Moreover, while COMP appears to be deficient in the PSACH matrix, the matrix appeared to be normal but the over-all quantity was reduced. These results suggest that the abnormality in linear growth in PSACH may result from decreased chondrocyte numbers which would also affect the amount of matrix produced.