The short-rib polydactyly (SRP) syndromes are a heterogeneous group of perinatal lethal skeletal disorders with polydactyly and multisystem organ abnormalities. Homozygosity by descent mapping in a consanguineous SRP family identified a genomic region that contained DYNC2H1, a cytoplasmic dynein involved in retrograde transport in the cilium. Affected individuals in the family were homozygous for an exon 12 missense mutation that predicted the amino acid substitution R587C. Compound heterozygosity for one missense and one null mutation was identified in two additional nonconsanguineous SRP families. Cultured chondrocytes from affected individuals showed morphologically abnormal, shortened cilia. In addition, the chondrocytes showed abnormal cytoskeletal microtubule architecture, implicating an altered microtubule network as part of the disease process. These findings establish SRP as a cilia disorder and demonstrate that DYNC2H1 is essential for skeletogenesis and growth.

The clinical signs most frequently associated with a FLNB mutation are the presence of supernumerary carpal and tarsal bones and short, broad, spatulate distal phalanges, particularly of the thumb. All individuals with Larsen syndrome-associated FLNB mutations are heterozygous for either missense or small inframe deletions. Three mutations are recurrent, with one mutation, 5071G-->A, observed in 6 of 20 subjects. The distribution of mutations within the FLNB gene is non-random, with clusters of mutations leading to substitutions in the actin-binding domain and filamin repeats 13-17 being the most common cause of Larsen syndrome. These findings collectively define autosomal dominant Larsen syndrome and demonstrate clustering of causative mutations in FLNB.

Spondylocarpotarsal synostosis syndrome (SCT) is an autosomal recessive disease that is characterized by short stature, and fusions of the vertebrae and carpal and tarsal bones. SCT results from homozygosity or compound heterozygosity for nonsense mutations in FLNB. FLNB encodes filamin B, a multifunctional cytoplasmic protein that plays a critical role in skeletal development. Protein extracts derived from cells of SCT patients with nonsense mutations in FLNB did not contain filamin B, demonstrating that SCT results from absence of filamin B. To understand the role of filamin B in skeletal development, an Flnb-/- mouse model was generated. The Flnb-/- mice were phenotypically similar to individuals with SCT as they exhibited short stature and similar skeletal abnormalities. Newborn Flnb-/- mice had fusions between the neural arches of the vertebrae in the cervical and thoracic spine. At postnatal day 60, the vertebral fusions were more widespread and involved the vertebral bodies as well as the neural arches. In addition, fusions were seen in sternum and carpal bones. Analysis of the Flnb-/- mice phenotype showed that an absence of filamin B causes progressive vertebral fusions, which is contrary to the previous hypothesis that SCT results from failure of normal spinal segmentation. These findings suggest that spinal segmentation can occur normally in the absence of filamin B, but the protein is required for maintenance of intervertebral, carpal and sternal joints, and the joint fusion process commences antenatally.

The filamins are a family of cytoplasmic proteins that bind to and organize actin filaments, link membrane proteins to the cytoskeleton, and provide a scaffold for signaling molecules. Mutations in the gene encoding filamin B (FLNB) cause a spectrum of osteochondrodysplasias, including atelosteogenesis type I (AOI) and atelosteogenesis type III (AOIII). AOI and AOIII are autosomal dominant lethal skeletal dysplasias characterized by overlapping clinical findings that include vertebral abnormalities, disharmonious skeletal maturation, hypoplastic long bones, and joint dislocations. Previous studies have shown that heterozygosity for missense mutations that alter the CH2 domain and repeat 6 region of filamin B produce AOI and AOIII. In this study, 14 novel missense mutations in FLNB were found in 15 unrelated patients with AOI and AOIII. The majority of the mutations resided in exon 2 and exon 3, which encode the CH2 domain of the actin-binding region of filamin B. The remaining mutations were found in exon 28 and exon 29, which encode repeats 14 and 15 of filamin B. These results show that clustering of mutations in two regions of FLNB produce AOI/AOIII, and highlight the important role of this cytoskeletal protein in normal skeletogenesis.

Activating mutations in fibroblast growth factor receptor 3 (FGFR3) cause several human dwarfisms characterized by diminished long bone growth (1). In cartilage, FGFR3 alters chondrocyte proliferation and differentiation by up-regulation of cell cycle inhibitors and stimulation of cartilage matrix degradation (2-5). The anti-proliferative action of FGF 2 signaling in cartilage contrasts with the usual mitogenic response of cells to FGF stimulus (6), but the molecular basis of this paradox remains unclear. Recently, Erk MAP kinase was found as a candidate for FGFR3-mediated inhibition of chondrocyte proliferation and differentiation (7-10).Protein kinase C (PKC) comprises a family of serine/threonine kinases that phosphorylate the consensus motif RXX(S/T)XR (11). The PKCs are further divided into three subfamilies based on sequence similarities and modes of activation. The conventional PKCs (PKC␣, -␤I, -␤II, and -␥) are activated by phosphatidylserine, diacylglycerol, and Ca 2ϩ , the novel PKCs (PKC␦, -⑀, -, and -) require only phosphatidylserine and diacylglycerol, and the atypical PKCs (aPKC; PKC and -) respond to phosphatidylserine alone (12). The PKC phosphorylation motif is present in many proteins (13), implicating PKCs as broad specificity protein kinases. PKCs are involved in numerous signaling events including activation of the Erk MAP kinase pathway. This is evident by potent Erk activation in cells treated with phorbol esters, such as phorbol-12-myristate-13-acetate (PMA), which activates both conventional PKCs and novel PKCs through binding of their diacylglycerol site (14). In PMA-treated cells, PKCs target the Erk module at the level of both Raf-1 and MEK, through direct activatory phosphorylation or indirectly (15-21). Apart from PMA-mediated Erk activation, PKCs appear to be crucial for long term Erk activation by growth factors, including FGFs (20,[22][23][24][25], as well as for oncogenic .FGF signaling in chondrocytes leads to long term Ras/Erk activation, which appears to account for the growth inhibitory outcome of FGF treatment (8, 9). To date, little is known about chondrocyte properties of FGF signaling permitting prolonged Erk activity, although slow down-regulation of mutated FGFR3 appears to be involved (30,31).