Chromosomal localization, genomic characterization, and mapping to the Noonan syndrome critical region of the human Deltex (DTX1) gene

Regulation of Notch signaling by E3 ubiquitin ligases

Notch signaling is an evolutionarily conserved pathway that is widely used for multiple cellular events during development. Activation of the Notch pathway occurs when the ligand from a neighboring cell binds to the Notch receptor and induces cleavage of the intracellular domain of Notch, which further translocates into the nucleus to activate its downstream genes. The involvement of the Notch pathway in diverse biological events is possible due to the complexity in its regulation. In order to maintain tight spatiotemporal regulation, the Notch receptor, as well as its ligand, undergoes a series of physical and biochemical modifications that, in turn, helps in proper maintenance and fine-tuning of the signaling outcome. Ubiquitination is the post-translational addition of a ubiquitin molecule to a substrate protein, and the process is regulated by E3 ubiquitin ligases. The present review describes the involvement of different E3 ubiquitin ligases that play an important role in the regulation and maintenance of proper Notch signaling and how perturbation in ubiquitination results in abnormal Notch signaling leading to a number of human diseases.

Imbalance of plasminogen activator inhibitor type-1 (PAI-1) and tissue plasminogen activator (t-PA) activity in patients with Noonan syndrome

BACKGROUND

Patients with Noonan syndrome have been reported to have an increased incidence of bleeding with trauma and surgery. Although various abnormalities of hemostasis have been described in reported series, there has been no consistent abnormality described and none have investigated fibrinolytic pathways.

METHODS

Fibrinolytic parameters including tissue plasminogen activator (t-PA; activity and antigen), plasminogen activator inhibitor type-1 (PAI-1; activity and antigen), plasminogen (activity and antigen), thrombin-antithrombin complex, prothrombin time (PT), and activated partial thromboplastin time where studied in 12 patients with Noonan syndrome (total of 15 data sets). PT and activated partial thromboplastin time were performed locally whereas the fibrinolytic parameters were performed in a single national reference laboratory. Individual results were compared with published age-appropriate norms and the mean values of the study patients were compared with the midpoint of normal range for each component measured. The ratio of PAI-1/t-PA activity was calculated for each set of results and compared with a "normal" value calculated from the midpoint normal range value for each component.

RESULTS

Patients with Noonan syndrome consistently demonstrated an elevation of t-PA activity in relationship to PAI-1 activity and a decreased PAI-1/t-PA ratio (P<0.01 for each comparison).

CONCLUSIONS

Patients with Noonan syndrome seem to have an imbalance in fibrinolytic components favoring fibrinolysis. This may be an important contributor to the bleeding noted in these patients.

Genetic regions that interact with loss- and gain-of-function phenotypes of deltex implicate novel genes in Drosophila Notch signaling

The Notch signaling pathway is an evolutionarily conserved mechanism that regulates many cell fate decisions. The deltex (dx) gene encodes an E3-ubiquitin ligase that binds to the intracellular domain of the Notch protein and regulates Notch signaling in a positive manner. However, it is still not clear how Dx does this. We generated a transgenic line, GMR-dx, which overexpresses dx in the developing Drosophila eye disc. The GMR-dx line showed a rough-eye phenotype, specific transformation of a photoreceptor cell (R3 to R4), and a rotation defect in the ommatidia. This phenotype was suppressed in combination with a dx loss-of-function mutant, indicating that it was due to a dx gain-of-function. We previously reported that overexpression of Dx results in the stabilization of Notch in late endosomes. Here, we found that three motifs in Dx, a region that binds to Notch, a proline-rich motif and a RING-H2 finger, were required for this stabilization, although the relative activity of these variants in this assay did not always correspond to the severity of the rough-eye phenotype. In an attempt to identify novel genes of the Notch pathway, we tested a large collection of chromosomal deficiencies for the ability to modify the eye phenotypes of the GMR-dx line. Twelve genomic segments that enhanced the rough-eye phenotype of GMR-dx were identified. To evaluate the specificity of these interactions, we then determined whether the deletions also interacted with the wing phenotypes associated with a loss-of-function mutation of dx, dx24. Analyses based on whole-genome information allowed us to conclude that we have identified two novel loci that probably include uncharacterized genes involved in Dx-mediated Notch signaling.

Isolation and characterization of chicken TaxREB107, a putative DNA binding protein abundantly expressed in muscle

Myogenic regulatory factors (MRFs) are vital transcription factors that act at multiple points during development to establish the skeletal muscle phenotype. This class of muscle-restricted, basic helix-loop-helix (bHLH) proteins acts in concert with additional transcriptional modulators to precisely control muscle gene expression. We have isolated the chicken homologue of Tax responsive element binding protein 107 (TaxREB107). The cDNA is 83% homologous at the amino acid level to human and mouse TaxREB107 and contains a centrally located leucine zipper motif. Northern analysis demonstrated that the gene is expressed in multiple tissues including skeletal muscle. Immunofluorescent staining revealed that the cTaxREB107 protein is located in both the nuclear and cytoplasmic compartments. Distinct localization to the nucleoli supports the evidence that TaxREB107 is a ribosomal protein. Because TaxREB proteins also are implicated in transcriptional regulation, we overexpressed cTaxREB107 in embryonic myoblasts. cTaxREB107 increased troponin I reporter gene activity as well as MRF-directed transcription from a multimerized skeletal muscle E-box reporter gene (4Rtk-luc). However, cotransfection of expression plasmids coding for MyoD and cTaxREB107 did not produce an increase in 4Rtk-luc suggesting that cTaxREB107 enhances myogenic gene transcription through a means independent of a physical association with MyoD. In conclusion, our results define a role for cTaxREB107 during avian myogenesis as a positive modulator of skeletal muscle gene expression.

Temporal bone histopathological study of Noonan syndrome

Four temporal bone specimens, obtained from three individuals 1--6 years of age with Noonan syndrome (NS), were studied histopathologically. All four specimens were accompanied by similar inner ear abnormalities including the reduced number of spiral ganglion cells, enlarged lateral semicircular canal, and dislocated endolymphatic sac and vestibular aqueduct. The mean population of spiral ganglion cells (15,699 cells) was approximately half of those (32,978 cells) in four age-matched control cases. In addition, they had several middle ear abnormalities including the remaining mesenchyme and dehiscence of the facial canal. To our knowledge, this is the first report to describe the histopathological temporal bone findings in patients with NS. We discuss the implications of the observed abnormalities with regard to clinical issues.