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Persistent and stable expression of foreign genes has been achieved in mammalian cells by integrating the genes into the host chromosomes. However, this approach has several shortcomings in practical applications. For example, large scale production of protein pharmaceutics frequently requires laborious amplification of the inserted genes to optimize the gene expression. The random chromosomal insertion of exogenous DNA also results occasionally in malignant transformation of normal tissue cells, raising safety concerns in medical applications. Here we report a novel cytoplasmic RNA replicon capable of expressing installed genes stably without chromosome insertion. This system is based on the RNA genome of a noncytopathic variant Sendai virus strain, Cl.151. We found that this variant virus establishes stable symbiosis with host cells by escaping from retinoic acid-inducible gene I-interferon regulatory factor 3-mediated antiviral machinery. Using a cloned genome cDNA of Sendai virus Cl.151, we developed a recombinant RNA installed with exogenous marker genes that was maintained stably in the cytoplasm as a high copy replicon (about 4 x 10(4) copies/cell) without interfering with normal cellular function. Strong expression of the marker genes persisted for more than 6 months in various types of cultured cells and for at least two months in rat colonic mucosa without any apparent side effects. This stable RNA replicon is a potentially valuable genetic platform for various biological applications.

In two lines of transgenic rats (pX rats) from WKAH and F344 strains and carrying the HTLV-I pX gene under control of the mouse H-2Kd promoter, mammary carcinomas developed predominantly in females starting at about 5 months of age. The incidence of the tumor reached about 40% when the rats were 12 months old. Histology of the tumor was undifferentiated carcinoma with massive infiltration of granulocytes into the tumor tissue. Systemic granulocytosis and hepato-splenomegaly due to extramedullary granulocytopoiesis were seen in pX rats and nude mice bearing pX mammary tumor. mRNAs of both pX and host genes, Gro and MIP-2, which are granulocyte chemoattractants of the IL-8 family, were highly expressed in the tumor tissue. Since expression and point mutation of several oncogenes and anti-oncogene, related with mammary carcinomas, were not demonstrated, hitherto unidentified novel oncogenic pathways may be transactivated by the pX transgene in these pX rats. pX mammary carcinoma cell lines, which have similar characteristics to the primary tumor, were established and the cells underwent apoptosis under the serum deprived conditions. The pX rats and the pX mammary carcinomas appear to be suitable models for analyses of HTLV-I pX oncogenesis and immune pathogenesis in vivo and in vitro.

We have intended to improve gene-transfer technique into hematopoietic stem cells for somatic gene therapy. 1) We have developed a new packaging cell line, ampGPE for retroviral production. LTR-less gag, pol or env genes from Moloney murine leukemia virus were separately inserted into BMGNeo vector. Packaging cell lines containing 20-50 copies of these two kinds of plasmid were obtained. Retrovirus stock for gene-transfer have been produced at a high titer (10(5)-10(6) cfu/ml) and without replication-competent viruses by using ampGPE. 2) Retrovirus-transduced murine CFU-GM have been found to selectively proliferate (5-10 fold/week) in liquid capture with recombinant murine IL-3, human IL-6 and G418 to consequently obtain enough amount of, highly concentrated (70-100%), and gene-transferred murine CFU-GM for gene-delivery system.