Thanatophoric dysplasia. Correlation among bone X-ray morphometry, histopathology, and gene analysis.

OBJECTIVE: Documentation through X-ray morphometry and histology of the steady phenotype expressed by FGFR3 gene mutation and interpolation of mechanical factors on spine and long bones dysmorphism. MATERIALS AND METHODS: Long bones and spine of eight thanatophoric dysplasia and three age-matched controls without skeletal dysplasia were studied after pregnancy termination between the 18th and the 22nd week with X-ray morphometry, histology, and molecular analysis. Statistical analysis with comparison between TD cases and controls and intraobserver/interobserver variation were applied to X-ray morphometric data. RESULTS: Generalized shortening of long bones was observed in TD. A variable distribution of axial deformities was correlated with chondrocyte proliferation inhibition, defective seriate cell columns organization, and final formation of the primary metaphyseal trabeculae. The periosteal longitudinal growth was not equally inhibited, so that decoupling with the cartilage growth pattern produced the typical lateral spurs around the metaphyseal growth plates. In spine, platyspondyly was due to a reduced height of the vertebral body anterior ossification center, while its enlargement in the transversal plane was not restricted. The peculiar radiographic and histopathological features of TD bones support the hypothesis of interpolation of mechanical factors with FGFR3 gene mutations. CONCLUSIONS: The correlated observations of X-ray morphometry, histopathology, and gene analysis prompted the following diagnostic workup for TD: (1) prenatal sonography suspicion of skeletal dysplasia; (2) post-mortem X-ray morphometry for provisional diagnosis; (3) confirmation by genetic tests (hot-spot exons 7, 10, 15, and 19 analysis with 80-90% sensibility); (4) in negative cases if histopathology confirms TD diagnosis, research of rare mutations through sequential analysis of FGFR3 gene.

Brain and bone abnormalities of thanatophoric dwarfism.

OBJECTIVE: The purpose of this article is to present the imaging findings of skeletal and brain abnormalities in thanatophoric dwarfism, a lethal form of dysplastic dwarfism. CONCLUSION: The bony abnormalities associated with thanatophoric dwarfism include marked shortening of the tubular bones and ribs. Abnormal temporal lobe development is a common associated feature and can be visualized as early as the second trimester. It is important to assess the brains of fetuses with suspected thanatophoric dwarfism because the presence of associated brain malformations can assist in the antenatal diagnosis of thanatophoric dwarfism.

Parathyroid hormone receptor type 1/Indian hedgehog expression is preserved in the growth plate of human fetuses affected with fibroblast growth factor receptor type 3 activating mutations.

The fibroblast growth factor receptor type 3 (FGFR3) and Indian hedgehog (IHH)/parathyroid hormone (PTH)/PTH-related peptide receptor type 1 (PTHR1) systems are both essential regulators of endochondral ossification. Based on mouse models, activation of the FGFR3 system is suggested to regulate the IHH/PTHR1 pathway. To challenge this possible interaction in humans, we analyzed the femoral growth plates from fetuses carrying activating FGFR3 mutations (9 achondroplasia, 21 and 8 thanatophoric dysplasia types 1 and 2, respectively) and 14 age-matched controls by histological techniques and in situ hybridization using riboprobes for human IHH, PTHR1, type 10 and type 1 collagen transcripts. We show that bone-perichondrial ring enlargement and growth plate increased vascularization in FGFR3-mutated fetuses correlate with the phenotypic severity of the disease. PTHR1 and IHH expression in growth plates, bone-perichondrial rings and vascular canals is not affected by FGFR3 mutations, irrespective of the mutant genotype and age, and is in keeping with cell phenotypes. These results indicate that in humans, FGFR3 signaling does not down-regulate the main players of the IHH/PTHR1 pathway. Furthermore, we show that cells within the bone-perichondrial ring in controls and patients express IHH, PTHR1, and type 10 and type 1 collagen transcripts, suggesting that bone-perichondrial ring formation involves cells of both chondrocytic and osteoblastic phenotypes.

Sonographic and molecular diagnosis of thanatophoric dysplasia type I at 18 weeks of gestation.

Thanatophoric dysplasia is the most common type of lethal skeletal dysplasia. It can usually be diagnosed with ultrasound, but differential diagnosis with other osteochondrodysplasias is not always possible. Mutations in the fibroblast growth factor receptor 3 (FGFR3) gene have been demonstrated to cause two distinct subtypes of the disorder. We describe a case of thanatophoric dysplasia type I diagnosed at 18 weeks of gestation by ultrasonography. Genomic DNA obtained by chorionic villus sampling showed a C to G substitution at position 746 in the FGFR3 gene, resulting in a Ser249Cys substitution already known to be associated with type I disease. Implications for perinatal management are discussed.

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: new radiologic, morphologic, and histologic aspects toward the exact definition of the disorder.

A comprehensive seven-step study on THANATOPHORIC DYSPLASIA without cloverleaf skull (TD Type I) was carried out postmortem on three aborted fetuses of 19, 26-27, and 34 weeks and one preterm neonate of 35 weeks gestation, respectively. The characteristic x-ray configuration of the spine in TD Type I presenting with H-, U- or reversed U-shape vertebrae were shown to correlate with the inclination or reclination of the vertebral bodies within a kyphotic or lordotic segment. The bowing of the tubular bones in TD Type I is explained by a diminished mechanical stability that is causally related to a specific cartilage structure. The perichondral spurs are defined by their morphologic structure, and their origin is attributed to a normal perichondral ossification in the presence of an impaired enchondral ossification. Impairment of enchondral ossification was more evident in the periphery than in the center of the metaphyses leading to a tongue-shaped osseous cone directed toward the epiphysis. The perichondral spurs and the linguiform enchondral growth plate resulted in a three-phase maple leaf-like contour of the metaphyses of tubular bones and acetabular roof. The nature of the perichondral fibrous bands, the fibrovascular bundles, and the fibrovascular bands of the growth plate and their significance in atypical ossification processes are discussed in detail. It is suggested that the diminished longitudinal growth of the skeleton is caused by a reduced mitotic activity of cartilage cells in the proliferative zone leading to a reduction of cell numbers in the columnar zone transversely oriented spongiosa bars resulting from desmal ossification of the metaphyseal fibrovascular structures.

Thanatophoric dysplasia type II: new entity?

There is some question about whether the two forms of thanatophoric dysplasia (TD), Type II with and Type I without cloverleaf skull, belong to the same entity. Thus, we investigated one 6-day-old TD with cloverleaf skull using examination of the external phenotype, radiology, autopsy, skeleton preparation, large section histology, detailed section histology, and ultrastructure. The loss of the three-phase contours-characteristics for Type I (54)--in certain metaphyses, the absence of the perichondral spurs to some extent, and their substitution by a structure similar to the perichondral "ring of Lacroix" have a suggested origin in normal cartilage-bone tissue. The same mechanism is postulated (a) for the appearance of less bent or normally shaped tubular bones compared with TD Type I, and their corresponding increased mechanical stability, and (b) for the less amount of platyspondyly in Type II than in Type I. We suggest that the malformation of the cloverleaf skull has its origin in the promontory growth of the relatively normal cartilage-bone tissues at the skull base resulting in an early synostosis and a consecutive fusion of the cranial sutures. The ultrastructural analysis of chondrocytes demonstrates the significant contribution of electron microscopy for TD studies. We suggest that pathologically altered light chondrocytes accounts for plump cross-striated collagen fibrils, the reduced cellular proliferation, and the impaired formation of columnar and hypertrophic zones. It is clear that normal cartilage-bone tissue distributed among "thanatophoric" tissue is the reason for the differences between Type I and Type II. Hypotheses are presented that explain this tissue mosaicism. Thus, TD Type I and TD Type II do not represent two different entities but the same entity with varying features due to mutational events occurring at different times.

Abnormal FGFR 3 expression in cartilage of thanatophoric dysplasia fetuses.

Thanatophoric dysplasia (TD), the commonest lethal skeletal dysplasia in humans, is accounted for by recurrent mutations in the fibroblast growth factor receptor 3 gene (FGFR 3), causing its constitutive activation in vitro. Taking advantage of medical abortion of 18 TD fetuses, cartilage sections were studied for FGFR 3 gene expression by in situ hybridization and immunohistochemistry. Specific antibodies revealed high amounts of FGFR 3 in cartilage of TD fetuses with no increased level of the corresponding mRNA. The specific signal was mainly detected in the nucleus of proliferative and hypertrophic chondrocytes. Based on this observation and the abnormal expression of collagen type X in hypertrophic TD chondrocytes, we suggest that constitutive activation of the receptor through formation of a stable dimer increases its stability and promotes its translocation into the nucleus, where it might interfere with terminal chondrocyte differentiation.

Quantitation of craniofacial anomalies in utero: fetal alcohol and Crouzon syndromes and thanatophoric dysplasia.

The study of fetal growth and development by ultrasound has been greatly facilitated in the past few years by the availability of anthropometric standards for the fetal body. Thus, the obstetrician is able to discern between normal and grossly abnormal, and even to quantitate certain fine fetal structures such as the face. This paper presents results obtained from a group of 5 patients referred to the Medical Center from private practices in Indianapolis, Indiana. Prenatal cephalometric analyses by ultrasound suggested the presence of craniofacial anomalies in all 5 cases. However, such defects were not detectable by routine ultrasonographic examination. A clinical examination after birth of each of these 5 patients suggested the following diagnoses: Fetal Alcohol Syndrome (FAS) in 2 individuals, Fetal Alcohol Effects (FAE) in one individual, Crouzon Syndrome (CS) in one patient, and Thanatophoric Dysplasia (TD) in one patient. In order to compare the craniofacial measurement values for each patient to normal standards, we developed Z-Score profiles and Pattern Variability Indexes (PVI) as described by Garn et al. [1984, 1985]. The values presented here support the idea that even mildly abnormal fetal craniofacial patterns are detectable by this relatively new application of ultrasound. At the present time, no conclusions can be made regarding the diagnostic accuracy of these patterns and profiles. However, the potential value of fetal cephalometry for documenting craniofacial dysmorphology is clearly indicated.

Thanatophoric dysplasia: fetal manifestations and prenatal diagnosis.

This paper describes two fetuses with thanatophoric dysplasia (TD) diagnosed in utero by ultrasonography. The fetuses were found to have severely short (less than 3rd centile), mildly bowed bones in one of them at 20 weeks and straight bones in the other at 34 weeks; bell-shaped chest; abnormal ribs (broadened and flattened at their ends); severe lung hypoplasia; hypoplastic, round-shaped vertebral bodies with hypoplastic arches; abnormally small pelvic bones, phalanges, metacarpals, and metatarsals. There was also an incipient "cloverleaf" skull deformity produced by fused posterior sagittal and lambdoidal sutures in the 20-week fetus and a definitive cloverleaf skull with communicating hydrocephaly in the 34-week fetus. The autopsies did not show any other abnormality. By xeroradiography after delivery, marked abnormalities of the endochondral and perichondral bone structures could be demonstrated in the 20-week fetus but not in the 34-week fetus. They appear to constitute two different conditions. These cases are good examples of the possibilities brought by ultrasound to the analysis of the fetal phenotype in utero.

The role of mesenchyme-like tissue in the pathogenesis of thanatophoric dysplasia.

We have studied the light microscopic, transmission, and scanning electron microscopic (SEM) findings in 13 cases of thanathophoric dysplasia (TD) and 4 control infants. In the TD growth plate, areas with less abnormal cartilage and bone alternated with areas of severely abnormal cartilage and bone. These latter abnormal areas were always found around tongues of apparent mesenchymal tissue that appeared to penetrate from the investing perichondrium and periosteum. The ultrastructure of the less abnormal areas was similar to that of the control infants, including cell and matrix structure as well as mineralization. The abnormal cartilage and bone had many ultrastructural abnormalities that were also found in the adjacent mesenchymal tissue. The mesenchymal cells, adjacent chondrocytes, and osteoblasts contained dilated endoplasmic reticulum and moderately large intracytoplasmic vacuoles. In the area adjacent to the cartilage, the matrix of the apparent mesenchyme contained thin collagen fibers and proteoglycan granules, whereas the matrix adjacent to the bone contained thick bundles of short collagen fibers. The matrix of the surrounding cartilage and bone resembled the adjacent matrix in the mesenchyme. In addition, many vesicular structures or osmiophilic particles were found in the matrix of the mesenchyme and adjacent cartilage and bone. SEM examination showed normal and abnormal bone trabeculae adjacent to each other. In the abnormal trabeculae, there were large, densely packed osteoblastic and osteocytic lacunae. The calcified collagen fibers had a random orientation, in contrast to the longitudinal orientation in the relatively normal bone. Chemical studies of collagen in the metaphyses of bones from five infants with TD showed a small amount of collagen type III (less than 5%), which was not found in three control infants. Thus, a basic pathogenetic mechanism in the skeletal abnormalities of TD appears to be the focal replacement of the growth plate and periosteum by persisting abnormal mesenchymal-like tissue from which the abnormal bone originates.