GENE THERAPY Gene delivery of PDGF for wound healing therapy Nicola C Petrie*, Jan J Vranckx, Daniela Hoeller, Feng Yao, Elof Eriksson Laboratory oflVound Repair and Gene Transfer, Division of Plastic Surgery, Brigham and ltVomen's Hospital, Boston, MA, US *and Department of Surgery, University of Bristol, Bristol, UK Reproduced by permission of the European Tissue Repair Society Key words: Gene therapy, PDGF, wound healing Introduction Gene therapy is an approach to modifying host cells to express a therapeutic protein by transfer of genetic material through a variety of techniques. Early studies of candidate disease states amenable to such therapy focused on congenital conditions involving the permanent mutation of a gene, However, gene th'rapy techniques may also be of benefit in temporary conditions, such as impaired healing, where the shortage or lack of a specific protein contributes to the aetiology of the problem. Plate!et-derived growth factor (POGF) is one of the first cytokines released in response to injury and is therefore an obvious candidate for development as a therapeutic agent. Advances in cloning technology have enabled expression of a recombinant POGF, which has subsequently been applied as an exogenous treatment in a number of animal models. Oespite the inefficiency of exogenous growth factor delivery, these studies demonstrated proof of concept that POGF was able to influence wound healing favourably and provided a platform from which to launch strategies to deliver POGF more efficiently by gene therapy. Background The existence of a platelet-derived mitogen, which would later become known as POGF, was first suspected in 1974 follOWing the observations that serum prepared from platelet-free plasma displayed significantly less mitogenic potency than serum prepared from whole bloodl. 1 The responsible factor was later purified in several laboratories and shown to be a glycoprotein of 27,000-31,0000a composed of two peptide chains linked by disulphide bonds"". These initial studies established the existence of two different chains termed A and B that associated to form three isoforms - homodimers AA and BB and the heterodimer AB. More recently two further isoforms have been described © 2005 Tissue Viability Society existing as homodimers of two new chains termed C' and D''l, POGF is normally sequestered within the alpha granules of circulating platelets'lJ.". When the platelet is induced to degranulate, for e..xample, as a result of contact with suben lorhclial sUlfaces exposed as a consequence of injury to the vascular endothelial cells POGF is released into the serum and therefore into the wound micro- environment'·'·12.". Once released, POGr has been shown to exert its biological effects through interaction with specific, high affinity receptors""'< demonstrated by purification studie· to be protein tyrosine kinase receptors""". Cross- competition studies have revealed the existence of two diA'erent POGF receptor c1asses 2D , which are activated by the formation of receptor dimers following the binding of one subunit of PDGF to each receptor'lJ.,..". The type A POGF receptor binds both the POGF-A and POGF-B chains (and therefore POGF dimers AA, AB and BB), whereas the B type receptor binds only the POGF-B chain (and therefore only the BB dimer). The diverse locations at which this receptor has been demonstrated and also the numerous cell types which have been shown to respond to POGF in culture highlight the many varied functions of the PDGF protein ll '"'''.3''. Within the conte..xt of wound healing PDGF has a number of roles. Platelet aggregation and degranulation are the initial cellular events in response to tissue injury and mediate the localised delivery of POGF to the wound bed. The presence of POGF then initiates a chemotactic cascade attracting fibroblasts, macrophages, neutrophils and smooth muscle cells"- 11 . The initial supply of POGF from platelets is rapidly exhausted following the formation of a fibrin plug at the site of endothelial damage. However, since PDGF is also produced by fibro-blasts 1K , macrophages"""", smooth a.nd skeletal muscle cells·"·41 and endothelial cells';·". the initial chemotaxis induced by POGF derived from platelets initiates a positive feedba k loop that stimulates continued expression of POGF from alternate sources. VOL 15 NO.4 NOVEMBER 2005 16