A position effect on TRPS1 is associated with Ambras syndrome in humans and the Koala phenotype in mice

Ambras syndrome (AS) is a rare form of congenital hypertrichosis with excessive hair on the shoulders, face and ears. Cytogenetic studies have previously implicated an association with rearrangements of chromosome 8. Here we define an 11.5 Mb candidate interval for AS on chromosome 8q based on cytogenetic breakpoints in three patients. TRPS1, a gene within this interval, was deleted in a patient with an 8q23 chromosomal rearrangement, while its expression was significantly downregulated in another patient with an inversion breakpoint 7.3 Mb downstream of TRPS1. Here, we describe the first potential long-range position effect on the expression of TRPS1. To gain insight into the mechanisms by which Trps1 affects the hair follicle, we performed a detailed analysis of the hair abnormalities in Koa mice, a mouse model of hypertrichosis. We found that the proximal breakpoint of the Koa inversion is located 791 kb upstream of Trps1. Quantitative real-time polymerase chain reaction, in situ hybridization and immunofluorescence analysis revealed that Trps1 expression levels are reduced in Koa mutant mice at the sites of pathology for the phenotype. We determined that the Koa inversion creates a new Sp1 binding site and translocates additional Sp1 binding sites within a highly conserved stretch spanning the proximal breakpoint, providing a potential mechanism for the position effect. Collectively, these results describe a position effect that downregulates TRPS1 expression as the probable cause of hypertrichosis in AS in humans and the Koa phenotype in mice.     

Runx2 and Runx3 are essential for chondrocyte maturation, and Runx2 regulates limb growth through induction of Indian hedgehog

The differentiation of mesenchymal cells into chondrocytes and chondrocyte proliferation and maturation are fundamental steps in skeletal development. Runx2 is essential for osteoblast differentiation and is involved in chondrocyte maturation. Although chondrocyte maturation is delayed in Runx2-deficient (Runx2(-/-)) mice, terminal differentiation of chondrocytes does occur, indicating that additional factors are involved in chondrocyte maturation. We investigated the involvement of Runx3 in chondrocyte differentiation by generating Runx2-and-Runx3-deficient (Runx2(-/-)3(-/-)) mice. We found that chondrocyte differentiation was inhibited depending on the dosages of Runx2 and Runx3, and Runx2(-/-)3(-/-) mice showed a complete absence of chondrocyte maturation. Further, the length of the limbs was reduced depending on the dosages of Runx2 and Runx3, due to reduced and disorganized chondrocyte proliferation and reduced cell size in the diaphyses. Runx2(-/-)3(-/-) mice did not express Ihh, which regulates chondrocyte proliferation and maturation. Adenoviral introduction of Runx2 in Runx2(-/-) chondrocyte cultures strongly induced Ihh expression. Moreover, Runx2 directly bound to the promoter region of the Ihh gene and strongly induced expression of the reporter gene driven by the Ihh promoter. These findings demonstrate that Runx2 and Runx3 are essential for chondrocyte maturation and that Runx2 regulates limb growth by organizing chondrocyte maturation and proliferation through the induction of Ihh expression.     

Conditional deletion of Indian hedgehog from collagen type 2alpha1-expressing cells results in abnormal endochondral bone formation

Indian hedgehog (Ihh) is actively involved in endochondral bone formation. Although expression of Ihh is mostly restricted to pre-hypertrophic chondrocytes, the role of chondrocyte-derived Ihh in endochondral bone formation is not completely understood. To address such unresolved issues, we used the Cre/loxP approach to generate mice (Col2alpha1Cre; Ihhd/Ihhd) in which the Ihh gene was selectively ablated from collagen type II expressing cells. Mutant mice were born with the expected ratio of Mendelian inheritance, but died shortly after birth and were smaller in size, exhibiting malformed and retarded growth of limbs with severe skeletal deformities. Alizarin red S staining showed abnormal mineralization of axial and appendicular bones. Histological analysis of mutant long bones revealed abnormal endochondral bone formation with loss of a normal growth plate. In addition, in vivo bromo-deoxyuridine (BrdU) labelling showed a marked decrease in chondrocyte proliferation. A delay in chondrocyte hypertrophy in Col2alpha1Cre; Ihhd/Ihhd mice was detected by the expression of collagen type X and osteopontin, using in situ hybridization. Furthermore, there was no expression of bone markers such as collagen type I, bone Gla protein, Runx2/Cbfa1 or PTH-R in the perichondrium of mutant mice, indicating the absence of osteoblasts from endochondral bones. Thus, selective loss of chondrocyte-derived Ihh recapitulated the defects in Ihh(-/-) animals, providing direct in vivo evidence that Ihh not only regulates chondrocyte proliferation and differentiation but also exerts effects on osteoblast differentiation. Understanding the exact functions of the molecules involved in endochondral bone formation will form the basis for further study to determine the molecular mechanisms of skeletal diseases involving various cellular components of bone.     

Constitutive activation of MEK1 in chondrocytes causes Stat1-independent achondroplasia-like dwarfism and rescues the Fgfr3-deficient mouse phenotype

We generated transgenic mice that express a constitutively active mutant of MEK1 in chondrocytes. These mice showed a dwarf phenotype similar to achondroplasia, the most common human dwarfism, caused by activating mutations in FGFR3. These mice displayed incomplete hypertrophy of chondrocytes in the growth plates and a general delay in endochondral ossification, whereas chondrocyte proliferation was unaffected. Immunohistochemical analysis of the cranial base in transgenic embryos showed reduced staining for collagen type X and persistent expression of Sox9 in chondrocytes. These observations indicate that the MAPK pathway inhibits hypertrophic differentiation of chondrocytes and negatively regulates bone growth without inhibiting chondrocyte proliferation. Expression of a constitutively active mutant of MEK1 in chondrocytes of Fgfr3-deficient mice inhibited skeletal overgrowth, strongly suggesting that regulation of bone growth by FGFR3 is mediated at least in part by the MAPK pathway. Although loss of Stat1 restored the reduced chondrocyte proliferation in mice expressing an achondroplasia mutant of Fgfr3, it did not rescue the reduced hypertrophic zone, the delay in formation of secondary ossification centers, and the achondroplasia-like phenotype. These observations suggest a model in which Fgfr3 signaling inhibits bone growth by inhibiting chondrocyte differentiation through the MAPK pathway and by inhibiting chondrocyte proliferation through Stat1.     

Trps1 plays a pivotal role downstream of Gdf5 signaling in promoting chondrogenesis and apoptosis of ATDC5 cells

Tricho-rhino-phalangeal syndrome (TRPS) is an autosomal dominant skeletal disorder caused by mutations of TRPS1 . Based on the similar expression patterns of Trps1 and Gdf5, we hypothesized a possible functional interaction between these two molecules. Using a chondrogenic cell line (ATDC5), we investigated the association of Gdf5-mediated signaling pathways with Trps1 and the phenotypic changes of ATDC5 cells due to over-expression or suppression of Trps1. Treatment of cells with Gdf5 enhanced Trps1 protein levels and phosphorylation of p38 mitogen-activated protein kinase (MAPK) in a dose-dependent manner. Nuclear translocation of Trps1 was also induced by Gdf5. These effects were blocked by a dominant negative form of activin-linked kinase 6 (dn-Alk6) and by SB203580, an inhibitor of the p38 MAPK pathway. Conversely, Gdf5 expression was suppressed by the over-expression of Trps1. Trps1-overexpressing ATDC5 (O/E) cells differentiated into chondrocytes more quickly than mock-infected control cells, whereas cells transfected with dn-Alk6 showed slower differentiation. On the other hand, O/E cells showed an increase of apoptosis along with the up-regulation of cleaved caspase 3 and down-regulation of Bcl-2, whereas dn-Alk6 cells showed suppression of apoptosis. In conclusion, Trps1 acts downstream of the Gdf5 signaling pathway and promotes the differentiation and apoptosis of ATDC5 cells.     

A diastrophic dysplasia sulfate transporter (SLC26A2) mutant mouse: morphological and biochemical characterization of the resulting chondrodysplasia phenotype

Mutations in the diastrophic dysplasia sulfate transporter (DTDST or SLC26A2) cause a family of recessively inherited chondrodysplasias including, in order of decreasing severity, achondrogenesis 1B, atelosteogenesis 2, diastrophic dysplasia (DTD) and recessive multiple epiphyseal dysplasia. The gene encodes a widely distributed sulfate/chloride antiporter of the cell membrane whose function is crucial for the uptake of inorganic sulfate, which is needed for proteoglycan sulfation. To provide new insights in the pathogenetic mechanisms leading to skeletal and connective tissue dysplasia and to obtain an in vivo model for therapeutic approaches to DTD, we generated a Dtdst knock-in mouse with a partial loss of function of the sulfate transporter. In addition, the intronic neomycine cassette in the mutant allele contributed to the hypomorphic phenotype by inducing abnormal splicing. Homozygous mutant mice were characterized by growth retardation, skeletal dysplasia and joint contractures, thereby recapitulating essential aspects of the DTD phenotype in man. The skeletal phenotype included reduced toluidine blue staining of cartilage, chondrocytes of irregular size, delay in the formation of the secondary ossification center and osteoporosis of long bones. Impaired sulfate uptake was demonstrated in chondrocytes, osteoblasts and fibroblasts. In spite of the generalized nature of the sulfate uptake defect, significant proteoglycan undersulfation was detected only in cartilage. Chondrocyte proliferation and apoptosis studies suggested that reduced proliferation and/or lack of terminal chondrocyte differentiation might contribute to reduced bone growth. The similarity with human DTD makes this mouse strain a useful model to explore pathogenetic and therapeutic aspects of DTDST-related disorders.     

Third case of 8q23.3-q24.13 deletion in a patient with Langer-Giedion syndrome phenotype without TRPS1 gene deletion

Langer-Giedion syndrome (LGS) is a contiguous gene syndrome caused by a hemizygous deletion on chromosome 8q23.3-q24.11 involving TRPS1 and EXT1 genes. We report on a girl with LGS phenotype and a 7.5?Mb interstitial deletion at chromosome 8q23.3-q24.13. Array-comparative genomic hybridization (a-CGH) revealed a deletion encompassing only the EXT1 and not the TRPS1 gene. Even though the deletion of TRPS1 and EXT1 genes is responsible for craniofacial and skeletal features of LGS, there have been previous reports of patients with LGS phenotype and 8q24 deletions leaving the TRPS1 gene intact. To our knowledge, this is the third such case. Our patient differs from previously reported LGS patients without TRPS1 gene deletion in that she has the typical LGS facial dysmorphism and skeletal abnormalities. However, the girl is of normal height and has only a mild developmental delay. Additionally, she has dyslalia and premature adrenarche classified as Tanner stage 3 premature pubarche which have not yet been described as features of LGS. We examine the molecular breakpoints and phenotypes of our patient and previously reported cases.     

Molecular dissection of a contiguous gene syndrome: localization of the genes involved in the Langer--Giedion syndrome

The Langer-Giedion syndrome (tricho-rhino-phalangeal syndrometype II, TRPS II) is characterized by craniofacial dysmorphismand skeletal abnormalities. It combines the clinical featuresof TRPS I and multiple cartilaginous exostoses (EXT). We haveused YAC cloning, Southern blotting, PCR analysis, and fluorescencein situ hybridization to study chromosome 8 deletions, translocatlons,an inversion, and an Insertion in patients with TRPS I, TRPSII or EXT. Our results indicate that the TRPS gene maps morethan 1,000 kb proximal to the EXT1 gene and that both genesare affected in TRPS II. We conclude that TRPS II is not dueto pleiotropic effects of mutations in a single gene, but thatit is a true contiguous gene syndrome.     

Clinical, biochemical, and genetic analysis of two korean patients with trichorhinophalangeal syndrome type I and growth hormone deficiency

Tricho-rhino-phalangeal syndrome type I (TRPSI) is a rare autosomal dominant hereditary disorder characterized by sparse hair, bulbous nose, long philtrum, thin upper lip, and skeletal abnormalities including cone-shaped epiphyses, shortening of the phalanges, and short stature. TRPSI is caused by mutations in the TRPS1 gene. Herein, we report two Korean cases of TRPSI. Although both patients (a 17-year-old-female and a 14-year-old male) had typical clinical findings, Patient 1 had an additional growth hormone (GH) deficiency. Treatment with recombinant human growth hormone (rhGH) 0.7 IU/kg/week led to an increase in growth velocity. Over 10 years of GH therapy, the mean growth velocity was 5.7±0.9 cm/year. However, the patient 2 did not show apparent GH deficiency by GH stimulation test, had a poor response with rhGH therapy and GH therapy was discontinued after 6 months. Upon genetic analysis of the TRPS1 gene, two mutations were found. Patient 1 had a heterozygous mutation c.2520dupT (p.Arg841LysfsX3) which had not been previously reported. Patient 2 had a known nonsense mutation c.1630C>T (p.Arg544X). In summary, we were the first to report Korean patients with mutation of TRPS1.