TGFß and BMP Dependent Cell Fate Changes Due to Loss of Filamin B Produces Disc Degeneration and Progressive Vertebral Fusions.

Spondylocarpotarsal synostosis (SCT) is an autosomal recessive disorder characterized by progressive vertebral fusions and caused by loss of function mutations in Filamin B (FLNB). FLNB acts as a signaling scaffold by linking the actin cytoskleteon to signal transduction systems, yet the disease mechanisms for SCT remain unclear. Employing a Flnb knockout mouse, we found morphologic and molecular evidence that the intervertebral discs (IVDs) of Flnb-/-mice undergo rapid and progressive degeneration during postnatal development as a result of abnormal cell fate changes in the IVD, particularly the annulus fibrosus (AF). In Flnb-/-mice, the AF cells lose their typical fibroblast-like characteristics and acquire the molecular and phenotypic signature of hypertrophic chondrocytes. This change is characterized by hallmarks of endochondral-like ossification including alterations in collagen matrix, expression of Collagen X, increased apoptosis, and inappropriate ossification of the disc tissue. We show that conversion of the AF cells into chondrocytes is coincident with upregulated TGFß signaling via Smad2/3 and BMP induced p38 signaling as well as sustained activation of canonical and noncanonical target genes p21 and Ctgf. These findings indicate that FLNB is involved in attenuation of TGFß/BMP signaling and influences AF cell fate. Furthermore, we demonstrate that the IVD disruptions in Flnb-/-mice resemble aging degenerative discs and reveal new insights into the molecular causes of vertebral fusions and disc degeneration.

NRP1 haploinsufficiency predisposes to the development of Tetralogy of Fallot.

Tetralogy of Fallot (TOF) is the most common cyanotic congenital heart defect. It involves anatomical abnormalities that change the normal flow of blood through the heart resulting in low oxygenation. Although not all of the underlying causes of TOF are completely understood, the disease has been associated with varying genetic etiologies including chromosomal abnormalities and Mendelian disorders, but can also occur as an isolated defect. In this report, we describe a familial case of TOF associated with a 1.8 Mb deletion of chromosome 10p11. Among the three genes in the region one is Neuropilin1 (NRP1), a membrane co-receptor of VEGF that modulates vasculogenesis. Hemizygous levels of NRP1 resulted in a reduced expression at the transcriptional and protein levels in patient-derived cells. Reduction of NRP1 also lead to decreased function of its activity as a co-receptor in intermolecular VEGF signaling. These findings support that diminished levels of NRP1 contribute to the development of TOF, likely through its function in mediating VEGF signal and vasculogenesis.

HSP47 and FKBP65 cooperate in the synthesis of type I procollagen.

Osteogenesis imperfecta (OI) is a genetic disorder that results in low bone mineral density and brittle bones. Most cases result from dominant mutations in the type I procollagen genes, but mutations in a growing number of genes have been identified that produce autosomal recessive forms of the disease. Among these include mutations in the genes SERPINH1 and FKBP10, which encode the type I procollagen chaperones HSP47 and FKBP65, respectively, and predominantly produce a moderately severe form of OI. Little is known about the biochemical consequences of the mutations and how they produce OI. We have identified a new OI mutation in SERPINH1 that results in destabilization and mislocalization of HSP47 and secondarily has similar effects on FKBP65. We found evidence that HSP47 and FKBP65 act cooperatively during posttranslational maturation of type I procollagen and that FKBP65 and HSP47 but fail to properly interact in mutant HSP47 cells. These results thus reveal a common cellular pathway in cases of OI caused by HSP47 and FKBP65 deficiency.

WDR34 mutations that cause short-rib polydactyly syndrome type III/severe asphyxiating thoracic dysplasia reveal a role for the NF-κB pathway in cilia.

Short-rib polydactyly (SRP) syndrome type III, or Verma-Naumoff syndrome, is an autosomal-recessive chondrodysplasia characterized by short ribs, a narrow thorax, short long bones, an abnormal acetabulum, and numerous extraskeletal malformations and is lethal in the perinatal period. Presently, mutations in two genes, IFT80 and DYNC2H1, have been identified as being responsible for SRP type III. Via homozygosity mapping in three affected siblings, a locus for the disease was identified on chromosome 9q34.11, and homozygosity for three missense mutations in WDR34 were found in three independent families, as well as compound heterozygosity for mutations in one family. WDR34 encodes a member of the WD repeat protein family with five WD40 domains, which acts as a TAK1-associated suppressor of the IL-1R/TLR3/TLR4-induced NF-κB activation pathway. We showed, through structural modeling, that two of the three mutations altered specific structural domains of WDR34. We found that primary cilia in WDR34 mutant fibroblasts were significantly shorter than normal and had a bulbous tip. This report expands on the pathogenesis of SRP type III and demonstrates that a regulator of the NF-κB activation pathway is involved in the pathogenesis of the skeletal ciliopathies.

Clinical and radiographic delineation of Bent Bone Dysplasia-FGFR2 type or Bent Bone Dysplasia with Distinctive Clavicles and Angel-shaped Phalanges.

Bent Bone Dysplasia-FGFR2 type is a relatively recently described bent bone phenotype with diagnostic clinical, radiographic, and molecular characteristics. Here we report on 11 individuals, including the original four patients plus seven new individuals with three longer-term survivors. The prenatal phenotype included stillbirth, bending of the femora, and a high incidence of polyhydramnios, prematurity, and perinatal death in three of 11 patients in the series. The survivors presented with characteristic radiographic findings that were observed among those with lethality, including bent bones, distinctive (moustache-shaped) small clavicles, angel-shaped metacarpals and phalanges, poor mineralization of the calvarium, and craniosynostosis. Craniofacial abnormalities, hirsutism, hepatic abnormalities, and genitourinary abnormalities were noted as well. Longer-term survivors all needed ventilator support. Heterozygosity for mutations in the gene that encodes Fibroblast Growth Factor Receptor 2 (FGFR2) was identified in the nine individuals with available DNA. Description of these patients expands the prenatal and postnatal findings of Bent Bone Dysplasia-FGFR2 type and adds to the phenotypic spectrum among all FGFR2 disorders. © 2016 Wiley Periodicals, Inc.

Bent bone dysplasia-FGFR2 type, a distinct skeletal disorder, has deficient canonical FGF signaling.

Fibroblast growth factor receptor 2 (FGFR2) is a crucial regulator of bone formation during embryonic development. Both gain and loss-of-function studies in mice have shown that FGFR2 maintains a critical balance between the proliferation and differentiation of osteoprogenitor cells. We have identified de novo FGFR2 mutations in a sporadically occurring perinatal lethal skeletal dysplasia characterized by poor mineralization of the calvarium, craniosynostosis, dysmorphic facial features, prenatal teeth, hypoplastic pubis and clavicles, osteopenia, and bent long bones. Histological analysis of the long bones revealed that the growth plate contained smaller hypertrophic chondrocytes and a thickened hypercellular periosteum. Four unrelated affected individuals were found to be heterozygous for missense mutations that introduce a polar amino acid into the hydrophobic transmembrane domain of FGFR2. Using diseased chondrocytes and a cell-based assay, we determined that these mutations selectively reduced plasma-membrane levels of FGFR2 and markedly diminished the receptor's responsiveness to extracellular FGF. All together, these clinical and molecular findings are separate from previously characterized FGFR2 disorders and represent a distinct skeletal dysplasia.

Prenatal and postnatal findings in serpentine fibula polycystic kidney syndrome and a review of the NOTCH2 spectrum disorders.

Serpentine fibula polycystic kidney syndrome (SFPKS; OMIM600330) is a rare skeletal dysplasia with a characteristic phenotype that includes polycystic kidneys, S-shaped fibulas, and abnormal craniofacial features. SFPKS shares features with Alagille (AGS; OMIM) and Hajdu-Cheney (HCS; OMIM10250) syndromes. All three syndromes result from mutations in the gene that encodes NOTCH2, one of the receptors involved in Notch signaling. Notch signaling is a major developmental signaling pathway, as well as a key regulator of numerous cellular processes. In this report, we present the prenatal ultrasound and postnatal findings in a 23-week fetus with severe manifestations of SPKS and heterozygosity for a de novo mutation in exon 34 of NOTCH2. These findings expand the phenotypic spectrum of NOTCH2 mutations and demonstrate the findings in the prenatal period.