Sandra H. Bigner

Mapping of homozygous deletions on human chromosome 10q23 has led to the isolation of a candidate tumor suppressor gene, PTEN, that appears to be mutated at considerable frequency in human cancers. In preliminary screens, mutations of PTEN were detected in 31% (13/42) of glioblastoma cell lines and xenografts, 100% (4/4) of prostate cancer cell lines, 6% (4/65) of breast cancer cell lines and xenografts, and 17% (3/18) of primary glioblastomas. The predicted PTEN product has a protein tyrosine phosphatase domain and extensive homology to tensin, a protein that interacts with actin filaments at focal adhesions. These homologies suggest that PTEN may suppress tumor cell growth by antagonizing protein tyrosine kinases and may regulate tumor cell invasion and metastasis through interactions at focal adhesions.

The epidermal growth factor receptor (EGFR) gene is amplified in 40% of malignant gliomas, and the amplified genes are frequently rearranged. We have characterized the genetic alterations associated with these rearrangements in five malignant gliomas. In one tumor the rearrangement resulted in the deletion of most of the extracytoplasmic domain of the receptor, resulting in a hybrid mRNA between new sequences and the truncated EGFR sequence. The predicted amino acid sequence of the protein from this tumor was remarkably similar to that described for several viral erbB oncogenes. Four other tumors were noted to have internal deletions of the EGFR gene. These rearrangements brought about in-frame deletions affecting either of two cysteine-rich domains in the extracytoplasmic portion of the molecule. The clonal nature of these alterations, and the fact that identical alterations were seen in more than one tumor, suggests a role for these mutant receptor proteins in tumorigenesis. Further, these studies document the existence of tumor-specific cell surface molecules resulting from somatic mutation.

Primary malignant gliomas from 63 patients were analyzed to determine the relationship between amplification of the gene encoding the epidermal growth factor receptor (EGFR) and expression of the corresponding mRNA. Twenty-four tumors were found to have amplified the EGFR gene and amplification of other genes occurred in three additional tumors. Hybridization with synthetic RNA probes was used to quantitate mRNA levels in situ. All 24 tumors with amplification of the EGFR gene had high levels of expression of this gene, while none of the 39 tumors without amplification had increased levels. This shows that, in human gliomas, large increases in the expression of the EGFR gene are invariably associated with alterations in gene structure.

A gene, termed gli, was identified that is amplified more than 50-fold in a malignant glioma. The gene is expressed at high levels in the original tumor and its derived cell line and is located at chromosome 12 position (q13 to q14.3). The gli gene is a member of a select group of cellular genes that are genetically altered in primary human tumors.

Six new permanent cell lines were established from human gliomas and compared to nine other cell lines from human gliomas. All fifteen lines had individually distinct HLA phenotypes and all but two, which were from a black patient, had type B glucose-6-phosphate-de;hydrogenase isoenzymes. Morphologically, the lines could be classified into four patterns descriptively designated as fibroblastic, fascicular, epithelial, or glial. Four of the lines grew progressively and could be serially transplanted when injected into athymic mice; two others grew initially and then regressed. From none to 100% of cells developed elongated tapering processes and showed reduction in nuclear-cytoplasmic ratio in the presence of 1 mM cyclic AMP and theophylline. Levels of 2'-3' cyclic nucleotide 3'-phosphohydrolase activity ranged from nondetectable to 12.78 +/- 1.49 micromoles 2' AMP formed per hr mgm total protein. None of the lines had detectable S-100 protein, but two had readily demonstrable glial fibrillary acidic protein in indirect immunofluorescence. Fibronectin levels in spent culture supernatants ranged from undetectable levels to 21.4 micrograms/ml/10(5) cells. All but one line shared surface antigens with normal human adult or fetal brain, as detected in absorption analyses with nonhuman primate antiserum raised against glioblastoma multiforme tissue or cell line U-251 MG. Although there were many common properties of the lines, each line had a unique profile of the parameters evaluated. This heterogeneity most likely reflects the individuality of the tumors of origin and individual genotypes and capacity for a range of phenotypic expression of cells.