Steven G. Brodie

Physiological and Pathological Secretion of Cartilage Oligomeric Matrix Protein by Cells in Culture

Abnormalities in cartilage oligomeric matrix protein (COMP), a pentameric structural protein of the cartilage extracellular matrix, have been identified in pseudoachondroplasia and multiple epiphyseal dysplasia, two human autosomal dominant osteochondrodysplasias. However, the function of the protein remains unknown. With the goal of establishing a model to study the mechanisms by which COMP mutations cause disease, we have analyzed synthesis and secretion of COMP in cultured chondrocytes, tendon, and ligament cells. Pentameric protein detected inside of control cells suggested that pentamerization is an intracellular process. Patient cells expressed mutant and normal RNA and secreted COMP at levels similar to controls, suggesting that abnormal pentamers are likely to be found in the extracellular matrix. Inclusions within patient cartilage stained with anti-COMP antibodies, and cultured cells presented similar inclusions, indicating that presumably abnormal COMP pentamers are less efficiently secreted than normal molecules. We conclude that the COMP disorders are likely to result from a combination of a decreased amount of COMP in the matrix and a dominant negative effect due to the presence of abnormal pentamers in cartilage.

Platyspondylic lethal skeletal dysplasia, San Diego type, is caused by FGFR3 mutations.

The platyspondylic lethal skeletal dysplasias (PLSDs) are a heterogeneous group of short-limb dwarfing conditions. The most common form of PLSD is thanatophoric dysplasia (TD), which has been divided into two types (TD1 and TD2). Three other types of PLSD, or TD variants (San Diego, Torrance, and Luton), have been distinguished from TD. The most notable difference between TD and the variants is the presence of large rough endoplasmic reticulum (rER) inclusion bodies within chondrocytes of the variants. We examined 22 cases of TD variants for the presence of missense mutations in the fibroblast growth factor receptor 3 (FGFR3) gene. All 17 cases of the San Diego type (PLSD-SD) were heterozygous for the same FGFR3 mutations found in TD1. No mutations were identified in the Torrance and Luton types. Large inclusion bodies were found in all 14 cases of PLSD-SD. Similar inclusion bodies were present in two of 72 TD1 cases, but not in 39 controls. The material retained within the rER stained only with antibody to the FGFR3 protein. The radiographic and morphologic differences between TD and PLSD-SD may be a consequence of other genetic factors, perhaps in the processing of mutant FGFR3 molecules within the rER. The presence of rER inclusion bodies cannot reliably discriminate between closely related skeletal dysplasias.

Thanatophoric dysplasia type I with syndactyly

We report on a case of thanatophoric dysplasia type 1 (TD1) due to a Tyr373Cys mutation in the fibroblast growth factor receptor 3 (FGFR3) gene with soft tissue syndactyly of the fingers and toes. Syndactyly has not been previously described in TD or other conditions with FGFR3 mutations, but occurs in several craniosynostosis syndromes due to mutations in FGFR2. We conclude that mutations in FGFR3 may also be associated with developmental abnormalities due to interference with programmed cell death.

Radiographic and morphologic findings in a previously undescribed type of mesomelic dysplasia resembling atelosteogenesis type II

The mesomelic chondrodysplasias are a heterogeneous group of dwarfing disorders characterized by shortness of the middle segments of limbs. We report on a 25-week fetus with disproportionate shortness of limbs with an apparently distinct form of mesomelic dysplasia. Radiographic findings at necropsy included ulnar deviation of hands, talipes equinovarus, distal tapering of the humeri, and hypoplastic fibulae, radii, and ulnae. Chondro-osseous morphology showed mild shortness of the physeal columns, overgrowth of perichondral bone, peripheral ingrowth of mesenchymal cells into the physis, and numerous areas of fibrillar degeneration with rings of collagen surrounding the chondrocytes. Ultrastructural findings included a degenerated territorial matrix, pericellular halos of collagen, and dilated loops of rough endoplasmic reticulum in chondrocytes. The radiographic appearance of the long bones is distinct from that of previously described mesomelic dysplasias. The chondro-osseous morphologic findings and the distal tapering of the humerus are somewhat reminiscent of atelosteogenesis type II, but the pattern of matrix degeneration and the presence of inclusion bodies in the chondrocytes distinguish it from disorders of sulfate transport.

Extra pelvic ossification centers in thanatophoric dysplasia and platyspondylic lethal skeletal dysplasia-San Diego type.

Abstract The platyspondylic lethal skeletal dysplasias (PLSD) are a group of heterogeneous disorders including thanatophoric dysplasia (TD) and the TD variants (San Diego, Torrance, and Luton types). TD is the most common form and has been divided into two subtypes (TD1 and TD2) based on clinical and radiologic criteria and analysis of mutations in the fibroblast growth factor receptor 3 (FGFR3) gene. The variants are distinguished from TD by characteristic radiographic and chondro-osseous morphologic features. We have recently identified FGFR3 mutations in PLSD-San Diego type (PLSD-SD) which are identical to those found in TD1, but the known TD FGFR3 mutations were not found in the other PLSD variants. After reviewing radiographs from 32 cases of PLSD-SD and 47 cases of TD with gestational ages under 24 weeks, we noted novel accessory ossification centers in the ischia of 18 cases of PLSD-SD and 44 of TD, and the ilia in 18 cases of PLSD-SD and 20 of TD. Only three cases of TD and five cases of PLSD-SD did not have extra pelvic ossification centers. At a gestational age greater than 24 weeks, the extra centers are fused with the main bone. The radiographic appearance and chondro-osseous morphology of cases with and without accessory pelvic ossification centers were otherwise indistinguishable. Morphologically, the accessory pelvic ossification centers resulted from membranous ossification. Extra pelvic ossifications are a common radiographic finding in TD and PLSD-SD.

Lethal osteosclerotic skeletal dysplasia with intracellular inclusion bodies

We report an apparently previously undescribed form of lethal osteosclerotic skeletal dysplasia in a 30-week male fetus with micromelic shortness of the limbs. Radiographic findings at necropsy included increased density in all bones, most marked in the skull, mandible, and pubis. The ribs were very short, abnormally modeled, and wide anteriorly. The vertebrae were posteriorly hypoplastic and wedged, particularly in the cervical and lumbar regions. The femora and tibiae were short with wide distal metaphyses, undermodeled diaphyses, and coxa vara. The humeri, radii, and ulnae were also short and undermodeled with proximal and distal flare. Chondro-osseous morphology showed short chondrocyte columns, extension of hypertrophic cells into the metaphysis, and overgrowth of perichondral bone. In the resting cartilage there were large chondrocytes containing a homogeneous material staining pink with von Kossa trichrome, gray with toluidine blue, and black with silver methenamine. The cortical bone was lacking and the trabecular bone was hypercellular, thick, and coarse. Ultrastructurally, the resting zone chondrocytes were large and round with condensed chromatin and dilated loops of rough endoplasmic reticulum. The radiographic and histopathologic findings in this case are unique and differ from those seen in other reported lethal osteosclerotic skeletal dysplasias.

Lethal osteosclerotic osteochondrodysplasia with platyspondyly, metaphyseal widening, and intracellular inclusions in sibs.

We report on a previously undescribed form of lethal osteosclerotic skeletal dysplasia in sibs from nonconsanguineous parents. Radiographic findings included increased density in the base of the skull, clavicles, vertebrae, ribs, and the metaphyseal regions of the long bones. There was midface hypoplasia, a large anterior fontanel, micrognathia, and hypoplastic, wafer-thin vertebrae. The clavicles, ribs, metacarpals, metatarsals, and phalanges were especially thickened and widened. The long bones were shortened with flared metaphyses. Chondroosseous morphology of resting cartilage and growth plate was relatively normal, but there was hypercellular cortical and trabecular bone, and marrow fibrosis. Ultrastructurally, the resting chondrocytes, osteoblasts, and nonhematopoietic marrow cells had dilated rough endoplasmic reticulum (inclusion bodies). The radiographic and morphologic characteristics in this case are unique and differ from those seen in other previously reported lethal osteosclerotic skeletal dysplasias.