We have analyzed, in vivo and in vitro, the behavior of two retroviral vectors carrying the genomic or cDNA N-ras oncogene to study the role of N-ras in the initiation and development of thymic lymphomas. The vector bearing the genomic gene produced an array of transcripts originating from the LTR and the oncogene promoter. The majority of the transcripts initiated at the LTR did not carry the packaging signal producing low titer clones. The cDNA vector produced two transcripts correctly spliced and the titers obtained were as high as 10(6) pfu/ml. Bone marrow cell infection and grafting of lethally irradiated mice was performed. The integrated vector in blood cells was followed at different times, observing that the provirus can disappear and reappear in peripheral blood cells during the course of the experiment. This observation fits with the hypothesis of clonal contribution of small number of stem cells in the renewal of blood cells. No tumors were detected in the infected animals, probably due to low expression of the integrated provirus. These experiments provide information on the advantages and disadvantages of genomic versus cDNA constructs in retroviral vectors.

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