Genetic disorders of the skeleton: a developmental approach

Abstract

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.

Figure 1

Patterning of the axial and appendicular skeleton. A, Somitogenesis. Presomitic mesoderm on both sides of the neural tube becomes subdivided into somites in craniocaudal direction. To illustrate the spatial and temporal coordination of somite formation, a time difference of 90 min between the left and right portion of the presomitic mesoderm is depicted here. Waves of notch expression originating from the caudal pole of the embryo time the somite border formation so that a new somite is formed every 90 min. Spacial coordination is conveyed by a gradient of FGF8 expression. Gray shading of newly formed somites represents polarized expression of Dll3. Somites give rise to several structures, among them the axial skeleton. B, Dorsoventral patterning of the axial skeleton. Each somite subdivides into a dermatomyotome, from which the muscles of the extremities are derived, and a sclerotome, which develops into the axial skeleton. This process is regulated by Shh, which is expressed by the notochord. C, Patterning of the limb buds. The AER is an important signaling center for proximodistal outgrowth of limb structures. p63 expression is crucial for sustaining the AER. The ZPA directs anteroposterior patterning by expression of Shh. Under the influence of Shh, the repressor form of Gli3, Gli3R, is converted to the activating form Gli3A, and a gradient of Shh and Gli3 expression is established. Expression of the Hoxd-cluster genes occurs in a highly ordered fashion as a result of the Shh/Gli3 gradient created by the ZPA. Wnt7a shows a polarized expression, which is strongest in the dorsal ectoderm of the limb bud. It induces Lmx1 in the underlying mesenchyme, which has a key role in dorsoventral patterning. D, The result of the patterning process becomes visible as mesenchymal condensations at later developmental stages. These condensations either develop into cartilaginous bone precursors in endochondral ossification or are directly transformed into bone in desmal ossification.

Figure 2

SHFM caused by mutations in p63. Whereas the middle toes of the left foot show the typical features of the disorder, the right foot appears to be only mildly affected, illustrating the very variable and often asymmetric manifestation. The phenotype is thought to arise from an insufficiency of the AER during patterning of the distal limb structures.

Figure 3

Campomelic dysplasia in a stillborn fetus. Note the hallmarks of the disorder: bowing of femora and tibiae, hypoplasia of scapulae and pelvis, and a small, bell-shaped thorax with a decreased number of ribs. SOX9 mutations cause a defective formation of early condensations that precede the formation of a cartilaginous skeleton.

Figure 4

Chondrocyte differentiation and proliferation in the growth plate. Chondrocytes are arranged in four layers, representing their stages of differentiation: (i) resting, (ii) proliferating, (iii) prehypertrophic, and (iv) hypertrophic/terminal hypertrophic chondrocytes. The latter are replaced by bone by the joint action of osteoblasts (green) and multinuclear osteoclasts (blue). Genes that are characteristically expressed within each layer are listed on the right side. A complex signaling network regulates proliferation and differentiation of chondrocytes. Prehypertrophic chondrocytes express Ihh, which regulates PTHrP, which is expressed in the joint region, via the perichondrium. PTHrP, in turn, inhibits differentiation of proliferating chondrocytes. FGFs inhibit chondrocyte proliferation and stimulate their differentiation. BMPs act antagonistically to the FGFs. The intensity of gray shading indicates the degree of calcification of the extracellular matrix.

Figure 5

Function and regulation of the osteoclast. Osteoblasts/stromal cells (yellow) regulate proliferation, differentiation, and activity of the osteoclast via RANKL and M-CSF. M-CSF binds to its receptor, c-fms, on mononuclear osteoclast precursors and stimulates their proliferation. RANKL stimulates differentiation into multinuclear resorbing osteoclasts via RANK, which transduces the signal with the help of TRAF6 and NFκB. As soon as the mature osteoclast attaches to the bone surface, it forms a ruffled border membrane containing large amounts of H⁺-ATPase complexes (green) and the chloride channel ClC-7 (red), which together transport HCl and acidify the resorption lacuna. Protons are provided by the cytosolic carbonic anhydrase type II (CAII). Acidic proteases like cathepsin k (blue circles) become active and degrade the bone matrix proteins (mainly collagen) and release bound growth factors, above all Tgfβ. Tgfβ is able to signal back to the osteoblast establishing a negative feedback loop by stimulating OPG expression. Degradation products are transported out of the cell by transcytosis. K= cathepsin k.

Figure 6

Sandwich vertebra in ADOII. ADOII can be diagnosed by characteristic radiological findings. The radiograph shows the typical sandwichlike appearance of the vertebra by thickening of the endplates.