Platelet-derived growth factor receptor beta is a protein that in humans is encoded by the PDGFRB gene. Mutations in PDGFRB are mainly associated with the clonal eosinophilia class of malignancies.
Gene The PDGFRB gene is located on human chromosome 5 at position q32 (designated as 5q32) and contains 25 exons. The gene is flanked by the genes for granulocyte-macrophage colony-stimulating factor and Colony stimulating factor 1 receptor (also termed macrophage-colony stimulating factor receptor), all three of which may be lost together by a single deletional mutation thereby causing development of the 5q-syndrome. Other genetic abnormalities in PDGFRB lead to various forms of potentially malignant bone marrow disorders: small deletions in and chromosome translocations causing fusions between PDGFRB and any one of at least 30 genes can cause Myeloproliferative neoplasms that commonly involve eosinophilia, eosinophil-induced organ injury, and possible progression to aggressive leukemia (see below).
Structure The PDGFRB gene encodes a typical receptor tyrosine kinase, which belongs to the type III tyrosine kinase receptor (RTK) family and is structurally characterized by five extracellular immunoglobulin-like domains, a single membrane-spanning helix domain, an intracellular juxtamembrane domain, a split tyrosine kinase domain and a carboxylic tail. In the absence of ligand, PDGFRβ adopts an inactive conformation in which the activation loop folds over the catalytic site, the juxtamembrane region over a loop occluding the active site and the carboxy-terminal tail over the kinase domain. Upon PDGF binding the dimerization of receptor releases the inhibitory conformations due to auto-phosphorylation of regulatory tyrosine residues in trans fashion. Tyrosine residues 857 and 751 are major phosphorylation sites for the activation of PDGFRβ. The molecular mass of the mature, glycosylated PDGFRβ protein is approximately 180 kDa.
Modes of activation Activation of PDGFRβ requires de-repression of the receptor's kinase activity. The ligand for PDGFRβ (PDGF) accomplishes this in the course of assembling a PDGFRβ dimer. Two of the five PDGF isoforms activate PDGFRβ (PDGF-B and PDGF-D). The activated receptor phosphorylates itself and other proteins, and thereby engages intracellular signaling pathways that trigger cellular responses such as migration and proliferation. There are also PDGF-independent modes of de-repressing the PDGFRβ's kinase activity and hence activating it. For instance, forcing PDGFRβ into close proximity of each other by overexpression or with antibodies directed against the extracellular domain. Alternatively, mutations in the kinase domain that stabilize a kinase active conformation result in constitutive activation. Unlike PDGFRα, PDGFRβ cannot be indirectly activated. This is because PDGFRβ recruits RasGAP and thereby attenuates Ras/PI3K activity, which is required to engage a feed-forward loop that is responsible for this mode of activation.
Role in physiology/pathology The phenotype of knock out mice demonstrates that PDGFRB is essential for vascular development, and that PDGFRB is responsible for activating PDGFRβ during embryogenesis. Eliminating either PDGFRB, or PDGF-B reduces the number of pericytes and vascular smooth muscle cells, and thereby compromises the integrity and/or functionality of the vasculature in multiple organs, including the brain, heart, kidney, skin and eye. In vitro studies using cultured cells indicate that endothelial cells secrete PDGF, which recruits PDGFRβ-expressing pericytes that stabilize nascent blood vessels. Mice harboring a single activated allele of PDGFRB show a number of postnatal phenotypes including reduced differentiation of aortic vascular smooth muscle cells and brain pericytes. Similarly, differentiation of adipose from pericytes and mesenchymal cells is suppressed. Misregulation of the PDGFRβ's kinase activity (typically activation) contributes to endemic diseases such as cancer and cardiovascular disease.
PDGFRB mutations
5q- Syndrome Human chromosome 5 deletions that remove three adjacent genes, those for granulocyte-macrophage colony-stimulating factor, PDGFRB, and colony stimulating factor 1 receptor, cause the chromosome 5q deletion syndrome (5q- syndrome). This syndrome is a unique type of myelodysplastic syndrome characterized by a prolonged disease course, a low rate of transformation to an aggressive form of leukemia, and an anemia which in many patients is profound, refractory to traditional therapies (e.g. iron supplements, Erythropoietin), and requiring maintenance red blood cell transfusions. The disease is treated with a chemotherapy drug, lenalidomide.
PDGFRB Translocations Human chromosome translocations between the PDGFRB gene and at least any one of 30 genes on other chromosomes lead to myeloid and/or lymphoid neoplasms that are many ways similar to the neoplasm caused by the fusion of the PDGFRA (i.e. platelet derived growth factor receptor A or alpha-type-platelet derived growth factor receptor) gene with the FIP1L1 gene (see FIP1L1-PDGFRA fusion gene. The most common of these rare mutations is the translocation of PDGFRB gene with the ETV6 gene (also termed ETS variant gene 6).
… excerpt ends here. Continue reading the full article.
