Formation of intracranial tumors by genetically modified human astrocytes defines four pathways critical in the development of human anaplastic astrocytoma

The formation of human malignant gliomas is thought to involve the accumula tion of multiple genetic alterations, To define the function of specific al terations in glioma formation, we serially introduced genetic alterations f unctionally equivalent to those noted in human malignant gliomas into norma l human astrocytes (NHAs). We then monitored the ability of each of these a lterations to contribute to the growth of otherwise genetically stable NHAs into intracranial malignant gliomas. Using this model, we show that expres sion of human telomerase catalytic component (hTERT), but not E7-mediated i nactivation of pRb or E6/E7-mediated inactivation of p53/pRb, was sufficien t to initiate the tumorigenic process by circumventing cellular senescence in astrocytes, hTERT expression, even in combination with inactivation of p 53/pRb, did not transform astrocytes. These alterations together, however, cooperated with ras pathway activation (initiated by expression of mutant H -Ras), but not with phosphatidylinositol 3-kinase pathway activation (initi ated by expression of myristoylated Akt) or epidermal growth factor recepto r activation, to allow for the formation of intracranial turners strongly r esembling p53/pRb pathway-deficient, telomerase-positive, ras-activated hum an grade m anaplastic astrocytomas. These results identify four pathways as key in the development of human anaplastic astrocytomas.