Perichondrium phenotype and border function are regulated by Ext1 and heparan sulfate in developing long bones: a mechanism likely deranged in Hereditary Multiple Exostoses.

During limb skeletogenesis the cartilaginous long bone anlagen and their growth plates become delimited by perichondrium with which they interact functionally. Yet, little is known about how, despite being so intimately associated with cartilage, perichondrium acquires and maintains its distinct phenotype and exerts its border function. Because perichondrium becomes deranged and interrupted by cartilaginous outgrowths in Hereditary Multiple Exostoses (HME), a pediatric disorder caused by EXT mutations and consequent heparan sulfate (HS) deficiency, we asked whether EXT genes and HS normally have roles in establishing its phenotype and function. Indeed, conditional Ext1 ablation in perichondrium and lateral chondrocytes flanking the epiphyseal region of mouse embryo long bone anlagen - a region encompassing the groove of Ranvier - caused ectopic cartilage formation. A similar response was observed when HS function was disrupted in long bone anlagen explants by genetic, pharmacological or enzymatic means, a response preceded by ectopic BMP signaling within perichondrium. These treatments also triggered excess chondrogenesis and cartilage nodule formation and overexpression of chondrogenic and matrix genes in limb bud mesenchymal cells in micromass culture. Interestingly, the treatments disrupted the peripheral definition and border of the cartilage nodules in such a way that many nodules overgrew and fused with each other into large amorphous cartilaginous masses. Interference with HS function reduced the physical association and interactions of BMP2 with HS and increased the cell responsiveness to endogenous and exogenous BMP proteins. In sum, Ext genes and HS are needed to establish and maintain perichondrium's phenotype and border function, restrain pro-chondrogenic signaling proteins including BMPs, and restrict chondrogenesis. Alterations in these mechanisms may contribute to exostosis formation in HME, particularly at the expense of regions rich in progenitor cells including the groove of Ranvier.

Stuve-Wiedemann syndrome: a skeletal dysplasia characterized by bowed long bones.

OBJECTIVE: To describe the prenatal sonographic features of Stuve-Wiedemann syndrome (SWS). METHODS: A retrospective review of all cases of confirmed SWS during an 8-year period was conducted. Clinical and historical data and outcome of the pregnancies were noted. Fetal biometry, skeletal survey, amniotic fluid volume and associated anomalies were recorded. A sonographic algorithm was proposed to distinguish SWS from other bent bone disorders. RESULTS: In total, there were 10 cases, six of which were diagnosed prenatally. The main prenatal features of SWS were mild-to-moderate micromelia and bowing of the lower limb bones, affecting the tibia more than the femur. There was relative sparing of fibula and upper limb bones, with normal scapulae and clavicles. Camptodactyly was the main associated anomaly. All fetuses developed growth restriction in the late second trimester with oligohydramnios in half of the cases. These features could appear late in pregnancy. Although the thoracic dimensions were normal in the majority of fetuses, respiratory insufficiency, as a result of myotonia, was a leading cause for mortality. CONCLUSIONS: It is possible to diagnose SWS prenatally. SWS is associated with high mortality during the first year of life, and those who survive have high morbidity.

Displasia tanatoforica: reporte de un caso / Thanatophoric dysplasia: a case report

Este caso concierne a paciente de 15 años quien fue referida del medio rural por presentar en su control pre-natal hallazgo ecográfico de malformación congénita: Polihidramnios severo, edema fetal generalizado (anasarca) hidrops no inmune con antecedente de hermano con malformación esuquelética congénita acondroplasia. Fue estudiada en el Servicio de ARO, observándose durante su hospitalización ausencia de frecuencia cardiaca fetal razón por la cual, se realiza inducción de trabajo de parto con obtención de producto con signos de maceración además de malformaciones esqueléticas en miembros superiores e inferiores, edema generalizado, desprendimiento de placenta 20 por ciento. Posteriormente se realiza curetaje uterino para verificar presencia o no de restos placentarios, después de 2 días de hospitalización la paciente se egresa debido a buena evolución clínica.

ColVa1 and ColXIa1 are required for myocardial morphogenesis and heart valve development.

Genetic mutations in minor fibrillar collagen types Va1 (ColVa1) and XIa1 (ColXI) have been identified in connective tissue disorders including Ehlers-Danlos syndrome and chondrodysplasias. ColVa1+/- and ColXIa1-/- mutant mice recapitulate these human disorders and show aberrations in collagen fiber organization in connective tissue of the skin, cornea, cartilage, and tendon. In the heart, fibrous networks of collagen fibers form throughout the ventricular myocardium and heart valves, and alterations in collagen fiber homeostasis are apparent in many forms of cardiac disease associated with myocardial dysfunction and valvular insufficiency. There is increasing evidence for cardiac dysfunction in connective tissue disorders, but the mechanisms have not been addressed. ColVa1+/- and ColXIa1-/- mutant mice were used to identify roles for ColVa1 and ColXIa1 in ventricular myocardial morphogenesis and heart valve development. These affected cardiac structures show a compensatory increase in type I collagen deposition, similar to that previously described in valvular and cardiomyopathic disease. Morphological cardiac defects associated with changes in collagen fiber homeostasis identified in ColVa1+/- and ColXIa1-/- mice provide an insight into previously unappreciated forms of cardiac dysfunction associated with connective tissue disorders.

Transgenic expression of the EXT2 gene in developing chondrocytes enhances the synthesis of heparan sulfate and bone formation in mice.

Hereditary multiple exostoses (HME), a dominantly inherited disorder characterized by multiple cartilaginous tumors, is caused by mutations in the gene for, EXT1 or EXT2. Recent studies have revealed that EXT1 and EXT2 are required for the biosynthesis of heparan sulfate and exert maximal transferase activity as a complex. The Drosophila homologue of EXT1 (tout-velu) regulates the movement and signaling of Hedgehog protein, which plays an important role in the regulation of chondrocyte differentiation and bone development. In this study, to investigate the biological role of EXT2 in bone development in vivo and the pathological role of HME mutations in the development of exostoses, we generated transgenic mice expressing EXT2 or mutant EXT2 in developing chondrocytes. Histological analyses and micro-CT scanning showed that the biosynthesis of heparan sulfate and the formation of trabeculae were upregulated in EXT2-transgenic mice, but not in mutant EXT2-transgenic mice. The expression of EXT1 is concomitantly upregulated in EXT2-transgenic and even mutant EXT2-transgenic mice, suggesting an interactive regulation of EXT1 and EXT2 expression. These findings support that the EXT2 gene encodes an essential component of the glycosyltransferase complex required for the biosynthesis of heparan sulfate, which may eventually modulate the signaling involved in bone formation.

Evidence for regulation of cartilage differentiation by the homeobox gene Hoxc-8.

Homeobox genes of the Hox class are required for proper patterning of skeletal elements, but how they regulate the differentiation of specific tissues is unclear. We show here that overexpression of a Hoxc-8 transgene causes cartilage defects whose severity depends on transgene dosage. The abnormal cartilage is characterized by an accumulation of proliferating chondrocytes and reduced maturation. Since Hoxc-8 is normally expressed in chondrocytes, these results suggest that Hoxc-8 continues to regulate skeletal development well beyond pattern formation in a tissue-specific manner, presumably by controlling the progression of cells along the chondrocyte differentiation pathway. The comparison to Hoxd-4 and Isl-1 indicates that this role in chondrogenesis is specific to proteins of the Hox class. Their capacity for regulation of cartilage differentiation suggests that Hox genes could also be involved in human chondrodysplasias or other cartilage disorders.

Scanning electron microscopy of cartilage in mice with hereditary chondrodysplasia.

Mice born with hereditary, recessive chondrodysplasia (cho/cho) are dwarfed because the cartilage model upon which the endochondral osseous skeleton develops is defective. The mutant's cartilage matrix lacks cohesiveness which apparently contributes to the absence of columnar alignment of proliferating epiphyseal chondrocytes in developing tubular (long) bones. The present communication reviews our current understanding of skeletal dysplasia as it relates to defective chondrogenesis, and presents observations made with the scanning electron microscope of cellular disarray and nonuniform size and distribution of collagen fibrils which confirm the existence of a matrix defect. Autoradiographic experiments on tibial cartilage, similar to those performed on sternal cartilage, confirm the normal pattern of sulfate labeling by mutant epiphyses.