Venous malformations (VMs), the most common errors of vascular morphogenesis in humans, are composed of dilated, serpiginous channels. The walls of the channels have a variable thickness of smooth muscle; some mural regions lack smooth muscle altogether. A missense mutation resulting in an arginine-to-tryptophan substitution at position 849 in the kinase domain of the receptor tyrosine kinase TIE2 segregates with dominantly inherited VM in two unrelated families. Using proteins expressed in insect cells, we demonstrate that the mutation results in increased activity of TIE2. We conclude that an activating mutation in TIE2 causes inherited VMs in the two families and that the TIE2 signaling pathway is critical for endothelial cell-smooth muscle cell communication in venous morphogenesis.

The human opportunistic pathogen Pseudomonas aeruginosa strain PA14 infects both plants and animals. Previously, using plants to screen directly for P. aeruginosa virulence-attenuated mutants, we identified a locus, pho34B12, relevant in mammalian pathogenesis. Here, nonsense point mutations in the two opposing ORFs identified in the pho34B12 locus revealed that one of them, mvfR (multiple virulence factor Regulator), is able to control all of the phenotypes that mutant phoA34B12 displays. Both genetic and biochemical evidence demonstrate that the mvfR gene encodes a LysR-like transcriptional factor that positively regulates the production of elastase, phospholipase, and of the autoinducers, 3-oxo-dodecanoyl homoserine lactone (PAI I) and 2-heptyl-3-hydroxy-4-quinolone (PQS), as well as the expression of the phnAB operon, involved in phenazine biosynthesis. We demonstrate that the MvfR protein is membrane-associated and acts as a transcriptional activator until cells reach stationary phase, when a unique negative feedback mechanism is activated to signal the downregulation of the MvfR protein. This work reveals an unprecedented virulence mechanism of P. aeruginosa and identifies a unique indispensable player in the P. aeruginosa quorum-sensing cascade.P seudomonas aeruginosa is a well known opportunistic pathogen that primarily causes infections in immunocompromised individuals and patients with cystic fibrosis (1, 2). Its rather impermeable membrane (1), the presence of ␤-lactamase (2, 3), and its various efflux systems (4-6) all make P. aeruginosa the leading cause of nosocomial infections and hospital-acquired pneumonia (7).Despite detailed knowledge about some of the extracellular proteins and several surface-associated components identified in P. aeruginosa, many virulence factors essential for pathogenicity and the mechanisms by which they coordinately function during pathogenesis remain to be elucidated. By using a multihost system consisting of plants, nematodes, and animals, and a systematic genetic approach, we have identified a large number of virulence-associated genes encoding either unidentified or previously identified virulence factors (reviewed in refs. 8 and 9). Uncovering loci involved in pathogenesis is facilitated by our high-throughput approach; thus, the challenge lies in the identification of functions.The pho34B12 locus is one of the novel virulence genes of P. aeruginosa identified by using the strategy outlined previously. Previous studies have shown that mutation at this locus reduced the ability of P. aeruginosa strain PA14 to cause disease in plants and animals. Partial characterization of the pho34B12 locus indicated that one of the potential genes contained in this locus might encode a novel quorum-sensing (QS) transcriptional regulator controlling the production of the virulence-associated factors (elastase, phospholipase, and pyocyanin) relevant to mammalian pathogenesis (10). Pyocyanin seems to contribute to the persistence of P. aeruginosa in the lungs of patients with cystic ...