Early passages of the human teratocarcinoma cell line PA1 are not tumorigenic in nude mice, while late passages are. A transforming gene present in late passages of PA1 cells was isolated as a biologically active molecular clone and is a new isolate of the human rasN locus. Its transforming activity is due to a single G---A (G, guanine; A, adenine) point mutation at the codon for amino acid 12 which changes the codon for glycine so that an aspartic acid residue is expressed. In contrast to late passage PA1 cells (passages 106, 330, and 338), DNA from the PA1 cell line at early passages (passage 36) does not yield rasN foci in DNA transfection assays. Thus, the presence of an activated rasN in PA1 cells correlates with enhanced tumorigenicity of the cell line and, more importantly, may have arisen during cell culture in vitro.

We have been using PA-1 human teratocarcinoma cells to study mechanisms by which oncogenes induce transformation. Tumorigenic PA-1 cells at passages greater than 100 (greater than P100) contain a spontaneously activated N-ras oncogene, while earlier-passage preneoplastic cells contain only the germ-line protooncogene and are nontumorigenic. One preneoplastic cell clone of PA-1 cells can be transformed by introduction of the cloned PA-1 N-ras in gene-transfer experiments, while another earlier-passage clonal cell line cannot be transformed. The goal of this investigation was to determine how human cells progress from resistance to susceptibility to ras oncogene-induced transformation. Somatic cell hybridization experiments described in this report indicate that the resistance of the low-passage cells to transformation is a dominant trait suppressing transformation. Loss of chromosomes from hybrid segregants suggested that tumor suppressors exist on chromosomes 1, 4, and 11. Extended in vitro passaging of somatic cell hybrids also resulted in the loss of chromosomes. Chromosome 1 was lost in these populations of cells, implying that reduction of this chromosome may promote proliferation and not specifically affect tumor formation.

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