INFLUENCE OF EXPRESSION AND CIS-ACTING SE QUENCES FROM AVIAN-LEUKOSISVIRUSES (ALVS) ON STABILITY OF (ALV)-BASED RETROVIRUS VECTORS

Defective avian leukosis virus (ALV)-based vectors expressing the neo and LacZ genes were constructed under the control of cis-acting elemen ts originated from 4 avian retroviruses: avian erythroblastosis virus (AEV), Rous associated viruses 1 (RAV-1) and 2 (RAV-2) and the Schmidt Ruppin strain of Rous sarcoma virus subgroup D (SR-RSV-D). We used th ese vectors to study the long-term stability of beta-galactosidase exp ression (encoded by the LacZ gene) in a permanent cell line from quail fibroblasts (QT6). Infection of the immortalized QT6 cell line with t hese vectors resulted in unstable beta-galactosidase expression. We de termined whether this instability of provirus expression war correlate d with: (1) presence of G418 selection; (2) deletion in the proviral g enome; (3) hypermethylation of the proviral genome; (4) position of th e neo and LacZ genes in the proviral genome; and (5) the transcription al activity of the long terminal repeat (LTP) elements of proviral vec tors. We observed that G418 selection pressure applied to infected QT6 cells lead to a more stable LacZ gene expression. Moreover, our resul ts suggest a correlation between the stability of proviral gene expres sion and the level of gene expression driven by the LTR elements and d epending on the strain origin of these.