Platelet-derived growth factor (PDGF) in primary brain tumours of neuroglial origin.

It has become clear that disruptions in the genome of somatic cells play a causative role in tumour development. We know that the ultimate formation of a malignancy is the result of a multistep process in which the functional loss and/or the altered or increased expression of genes play important roles. One such family of genes are the oncogenes, encoding protein products with mainly growth stimulating effects. Platelet-derived growth factor (PDGF) belongs to the family of oncogenes. It is likely that PDGF plays an essential role in the development of at least a subgroup of malignant astrocytic tumours that do not contain amplification of the EGF-receptor. The expression of PDGF alpha-receptors is related to tumour progression in these tumours, and some of the most malignant tumours were shown to contain amplification of the PDGF alpha-receptor. It is also clear now from several experimental studies that PDGF can drive the transformed phenotype, and that PDGF antagonists, by blocking the PDGF autocrine pathway revert the transformed phenotype of certain tumour cells. Because of the findings that receptor protein tyrosine kinases such as the EGF- and the PDGF-receptor play a crucial role in the development of gliomas, it is possible that inhibitors of the phosphorylation of the protein tyrosine kinases will be future candidates for glioma therapy. They might be able to at least delay the development of a fully malignant glioma. The role of PDGF in other tumours of neuroglial origin in the central nervous system has not been studied as extensively as its role in gliomas. Recent data suggest that also for the primitive neuroectodermal tumours overexpression of the PDGF alpha-receptor is related to malignancy of the tumours. For other tumours, such as neuroblastomas, PDGF exerts a differentiating rather than a mitogenic function and is an important survival factor. Further studies are needed to elucidate the role of PDGF in these non-glial primary brain tumours. Moreover, for a complete understanding of the role of PDGF in malignancies of the CNS, it is important to explore its function in the development of the normal CNS further.