Molecular-pathogenetic classification of genetic disorders of the skeleton

Skeletal dysplasias and the osteoarthritic phenotype

In contrast to late-onset osteoarthritis (OA), the appearance of precocious OA has historically been recognized as a particularly aggressive form of the disorder that is frequently inherited as a Mendelian trait. In general, precocious OA appears as a consequence of many skeletal dysplasias, which, although individually rare, comprise a sizable population of patients when viewed in toto. In these patients the disease is often rapidly progressive and includes features of articular and extra-articular involvement that are not typical of classic OA. The molecular pathology of the chondro-osseous disorders has been the focus of intense study in recent years, with the promise of providing insight into skeletal development and homeostasis, as well as the aetiology and pathogenesis of degenerative joint disease.

Macrocephaly and sclerosis of the tubular bones in an isolated patient: a mild case of craniodiaphyseal dysplasia?

We report a 56-year-old woman, mainly suffering from painful legs and the inability to run. Radiologically, marked sclerosis and hyperostosis of the skull bones is present resulting in macrocephaly. Most tubular bones of the limbs, as well as the clavicles, are affected by sclerosis. By mutation analysis of the TGFB1, SOST and LRP5 genes, we were able to exclude the diagnoses of Camurati-Engelmann disease, Van Buchem disease, sclerosteosis, high-bone-mass trait and endosteal hyperostosis (Worth type). We believe this patient represents one of the very few examples of adult craniodiaphyseal dysplasia with a mild form of the disease and moderate facial changes.

Genetic approaches to stature, pubertal timing, and other complex traits

The factors that regulate the timing of puberty remain largely elusive, as do the factors that modulate childhood growth and adult height. However, it is clear that these developmental processes are highly heritable--much of the natural variation in growth and timing of puberty is due to genetic variation within the population. In this review, we discuss how recent genetic and genomic advances can be exploited to help understand the genetic regulation of these processes. In particular, we describe how genome-wide linkage scans and association studies, in conjunction with haplotype-based approaches, are potentially useful tools to increase our understanding of these two complex traits. Discovery of the genetic variants that regulate these two traits would expand our understanding of human neuroendocrinology, postnatal development, and the general architecture of complex genetic traits.

Genetic disorders of the skeleton: a developmental approach

Although disorders of the skeleton are individually rare, they are of clinical relevance because of their overall frequency. Many attempts have been made in the past to identify disease groups in order to facilitate diagnosis and to draw conclusions about possible underlying pathomechanisms. Traditionally, skeletal disorders have been subdivided into dysostoses, defined as malformations of individual bones or groups of bones, and osteochondrodysplasias, defined as developmental disorders of chondro-osseous tissue. In light of the recent advances in molecular genetics, however, many phenotypically similar skeletal diseases comprising the classical categories turned out not to be based on defects in common genes or physiological pathways. In this article, we present a classification based on a combination of molecular pathology and embryology, taking into account the importance of development for the understanding of bone diseases.

HIP/RPL29 down-regulation accompanies terminal chondrocyte differentiation

HIP is a heparin/heparan sulfate (Hp/HS) binding protein identical to ribosomal protein L29 that displays diverse biological functions. There is strong evidence that abnormal expression and quantitative deficiencies of essential molecules such as extracellular matrix (ECM) proteins, transcription factors, and ribosomal proteins can seriously impair embryonic development. As observed for HS-bearing molecules, high levels of HIP/RPL29 are found in proliferating chondrocytic precursors and chondrocytes of developing growth plate. Here, we demonstrate both in vitro and in developing mouse embryos that HIP/RPL29 is down-regulated in terminally differentiated chondrocytes corresponding to the late hypertrophic zone of the growth plate. Because cartilage serves as a template for endochondral bone formation, we hypothesize that the presence of HIP/RPL29 during early chondrogenesis is essential for normal skeletal growth and patterning. In particular, we believe that HIP/RPL29 expression is required to maintain proliferation of chondrocytes and avoid skeletal shortening. Increasing evidence suggests that multifunctional ribosomal proteins of eukaryotic cells are important regulators of cell growth and differentiation, not simply structural parts of translational machinery. To investigate the role of HIP/RPL29 normal expression during cartilage formation, we designed a ribozyme-mediated knock-down approach to partially down-regulate HIP/RPL29 expression in the multipotent mouse embryonic skin fibroblast cell line C3H/10T (1/2). This technology permitted us to avoid the insufficient expression associated with more severe consequences, such as lethality, and provided advantages similar to those obtained with mutations generating hypomorphic phenotypes. Our results show that partial reduction of HIP/RPL29 levels accelerates differentiation of C3H/10T(1/2) into cartilage-like cells. In conclusion, our data indicate that HIP/RPL29 constitutes an important novel regulator of chondrocytic growth and differentiation.

One gene, two phenotypes: ROR2 mutations in autosomal recessive Robinow syndrome and autosomal dominant brachydactyly type B

Autosomal recessive Robinow syndrome (RRS) is a severe skeletal dysplasia with short stature, generalized limb shortening, segmental defects of the spine, brachydactyly, and a dysmorphic facial appearance. The gene encoding receptor orphan receptor tyrosine kinase 2 (ROR2) is located on chromosome 9q22 and homozygous loss-of-function mutations in this gene are responsible for RRS. Moreover, knocking out the mouse Ror2 gene causes mesomelic dwarfism in the homozygous state, with almost identical features to recessive Robinow syndrome. The protein product of this gene is a cell membrane receptor, containing distinct motifs including an immunoglobulin-like (Ig) domain, a Frizzled-like cysteine-rich domain (FRZ or CRD), and a kringle domain (KD) in the extracellular region; and an intracellular region with tyrosine kinase (TK), serine/threonine-rich, and proline-rich structures. The extracellular motifs of the ROR2 protein are known to be involved in protein-protein interactions. The tyrosine kinase domain is involved in an as yet uncharacterized signaling pathway. Interestingly, heterozygous mutations in ROR2 have recently been shown to give rise to autosomal dominant brachydactyly type B1 (BDB1). This condition is characterized by terminal deficiency of fingers and toes. A variety of mutations have been reported in ROR2. Here, these genetic defects are compiled and possible genotype-phenotype correlations are discussed.

Analysis of clones from a human cartilage cDNA library provides insight into chondrocyte gene expression and identifies novel candidate genes for the osteochondrodysplasias

To begin to define the gene expression pattern in fetal cartilage and to identify uncharacterized candidate genes for the osteochondrodysplasias, we analyzed clones from a fetal cartilage cDNA library. Sequence analysis of 420 cDNA clones identified 210 clones derived from established genes but, for many of them, expression in cartilage had not been previously reported. Among the established genes were 14 genes known to produce skeletal abnormalities in either humans or mice when mutated. Thirty-two uncharacterized genes and their respective chromosomal positions were also identified. To further understand the expression profile of these genes in fetal cartilage, we constructed a cDNA microarray utilizing the clones. The microarray was used to determine which genes had higher expression in cartilage as compared with dedifferentiated, cultured chondrocytes. Many of the established genes, as well as five of the uncharacterized genes, had increased expression in cartilage, suggesting an important role for these genes in the differentiated state of chondrocytes. These data provide new candidate genes for the osteochondrodysplasias and demonstrate the usefulness of cartilage cDNA microarrays in expanding our understanding of the complexity of fetal cartilage gene expression.

Developmental skeletal anomalies

A genetic and molecular revolution is taking place in medicine today. Led by the Human Genome Project, genetic information and concepts are changing the way diseases are defined, diagnoses are made, and treatment strategies are developed. The profound implications of actually understanding the molecular abnormalities of many clinical problems are affecting virtually all medical and surgical disciplines. The ability to apply knowledge gleaned from the laboratory is our best hope for developing strategies to modify the pathologic effects of genes (by drug therapy), repair genes (gene therapy), and restore lost or affected tissues (tissue engineering). Instead of an empiric trial-and-error approach to therapy, it may become feasible to tailor treatment to the specific molecular malfunction. In this review we have chosen to emphasize a few selected musculoskeletal disorders, including skeletal dysplasias, spinal deformities, developmental dislocation of the hip, and idiopathic clubfoot. The logical extension of our understanding of the molecular players in many of these disorders is to establish precisely what the products of the affected genes do during skeletal development, and how mutations disturb these functions to produce the characteristic phenotype. Despite the many hypotheses generated from the work in human genetics, and the knowledge that has been gained from animal models, there remains a relatively poor understanding of how these genes interfere with skeletal development. Unraveling these mysteries and defining them in molecular and cellular terms will be the challenges for the near future.

Skeletal dysplasias caused by a disruption of skeletal patterning and endochondral ossification

Identification of a number of the genes that cause skeletal dysplasias has helped clinicians to provide accurate diagnoses, genetic counseling, and pre-natal diagnosis for this complex group of disorders. This review considers how some of the recent advances in human and murine genetics have led to an increased understanding of normal bone development and, in particular, the processes of skeletal patterning and endochondral ossification.

Birth prevalence and pattern of osteochondrodysplasias in an inbred high risk population

BACKGROUND

Define the pattern and birth prevalence of the different types of osteochondrodysplasias in newborn infants in the United Arab Emirates (UAE) population, which is highly inbred and where termination of pregnancy is not accepted.

METHODS

All infants with a birth weight of 500 gm and above in the three hospitals in Al Ain Medical District of the UAE were studied prospectively over a period of 5 years. For each live birth or stillbirth with suspected skeletal dysplasia, a detailed clinical and radiological examination was carried out. Pregnancy history and information regarding parental age, ethnic origin, family history, and level of consanguinity were obtained and a pedigree was constructed.

RESULTS

Among the 38,048 births during the study period, 36 (9.46/10,000 births) had some type of skeletal dysplasia. Eighteen cases were attributed to autosomal recessive genes (4.7/10,000 births), 10 were due to apparent new dominant mutations (2.62/10,000), five were autosomal dominant type (1.3/10,000) and one was X-linked dominant type (0.26/10,000). In three cases, inheritance was unknown. The most common recessive type of skeletal dysplasia in our series was fibrochondrogenesis (1.05/10,000), followed by chondrodysplasia punctata (0.78/10,000). The birth prevalence rate of skeletal dysplasia doubled in the last 2 years of the 5-year observation period (6.74/10,000 in 1996 vs. 12.86/10,000 in 1999, and 13.45/10,000 in 2000). This increase involved cases caused by new dominant mutations, and occurred mainly in the first half of 1999.

CONCLUSION

This prospective study has identified a high birth prevalence of skeletal dysplasia, and risk factors are postulated. These findings represent an accurate birthprevalence figure and a useful baseline for this group of birth defects in the UAE.