The Role of Sonic Hedgehog in Human Holoprosencephaly and Short-Rib Polydactyly Syndromes.

The Hedgehog (HH) signalling pathway is one of the major pathways controlling cell differentiation and proliferation during human development. This pathway is complex, with HH function influenced by inhibitors, promotors, interactions with other signalling pathways, and non-genetic and cellular factors. Many aspects of this pathway are not yet clarified. The main features of Sonic Hedgehog (SHH) signalling are discussed in relation to its function in human development. The possible role of SHH will be considered using examples of holoprosencephaly and short-rib polydactyly (SRP) syndromes. In these syndromes, there is wide variability in phenotype even with the same genetic mutation, so that other factors must influence the outcome. SHH mutations were the first identified genetic causes of holoprosencephaly, but many other genes and environmental factors can cause malformations in the holoprosencephaly spectrum. Many patients with SRP have genetic defects affecting primary cilia, structures found on most mammalian cells which are thought to be necessary for canonical HH signal transduction. Although SHH signalling is affected in both these genetic conditions, there is little overlap in phenotype. Possible explanations will be canvassed, using data from published human and animal studies. Implications for the understanding of SHH signalling in humans will be discussed.

Short rib polydactyly syndrome type III: histopathogenesis of the skeletal phenotype.

A morphological study of the skeletal system in a case of short rib polydactyly syndrome type III (SRPS-III) documented a "bajonet" deformity of the ribs for misalignment and overlap of cartilaginous and bony ends. This deformity resulted from a 'tandem' change in endochondral bone formation that is, arrested orthotopic cartilage maturation and etherotopic perichondral cartilage differentiation and ossification. At the cartilaginous end, cartilage maturation and vascular invasion were absent. At the bony end, longitudinal bone growth occurred by a perichondral ectopic growth plate. 'Miniature' versions of this 'tandem' change were also demonstrated in the long bones of the limbs and included focally arrested orthotopic cartilage maturation at the growth plates, perichondral cartilage differentiation, and ossification within cartilage canals. Our morphological study indicates that a generalized loss of syncrony in cartilage removal and osteogenic differentiation occurs in all growth plates, albeit with varied expressivity, and represents, at tissue level, the mechanism by which the SRPS-III skeletal phenotype develops.

Proliferation and collagen biosynthesis of osteoblasts and chondrocytes in short rib syndrome type beemer.

We report on a case of lethal short-limbed skeletal dysplasia with extremely short ribs, median cleft upper lip and palate, malrotation of intestine, lung hypoplasia with bilateral segmentation defect, atrial septum defect, union of distal urethra and vagina, and complex brain malformations. Based on radiological criteria and the pattern of associated abnormalities a short rib syndrome without polydactyly (Type Beemer) was diagnosed. Morphologically, the growth plate showed a reduced proliferation zone and an enlarged zone of hypertrophic cartilage. In addition, islands of persistent hypertrophic cartilage were present even in the metaphysis. In monolayer cell cultures supplemented with 10% fetal calf serum proliferation was normal in articular chondrocytes, reduced in costal chondrocytes, and elevated in osteoblasts from the patient. Clonal growth of costal and articular chondrocytes in methylcellulose could be stimulated normally by insulin-like growth factor-I (IGF-I), IGF-II, and human growth hormone (hGH). However, the response to transforming growth factor beta 1 (TGF-beta 1) was markedly elevated in articular chondrocytes of the patient compared to those of 3 fetal controls. Quantitative collagen synthesis in both osteoblasts and chondrocytes from the patient did not differ significantly from that of controls. Osteoblasts synthesized predominantly collagen I and minor amounts of collagen III, chondrocytes synthesized primarily collagen II. All collagen chains including CNBr-peptides of collagen II showed normal migration in PAA gel electrophoresis.