Stable and unstable transgene integration sites in the human genome: extinction of the Green Fluorescent Protein transgene in K562 cells

In gene transfer experiments including gene therapy studies, expression of the integrated transgenes in host cells often declines with time. The molec ular basis of this phenomenon is not clearly understood. We have used the G reen Fluorescent Protein (GFP) gene as both a selectable marker and a repor ter to study long-term transgene integration and expression in K562 cells. Cells transfected with plasmids containing the GFP gene coupled to the HS2 or HS3 enhancer of the human beta -globin Locus Control Region (LCR) or the cytomegalovirus (CMV) enhancer were sorted by either fluorescence-activate d-cell-sorting (FACS) alone or FAGS combined with drug selection based on a co-integrated drug resistance gene. The two groups of selected cells were subsequently cultured for long periods up to 250 cell generations. Comparis on of long-term GFP transgene integration and expression in these two group s of cells revealed that the K562 genome contains two types of transgene in tegration sites: i) abundant unstable sites that permit transcription but n ot long-term integration of the transgenes and thus eliminate the transgene s in 60-250 cell generations and ii) rare stable sites that permit both eff icient transcription and long-term stable integration of the transgenes for at least 200 cell generations. Our results indicate that extinction of GFP expression with time is due at least in part to elimination of the gene fr om the host genome and not entirely to transcriptional silencing of the gen e. However, long-term, stable expression of the transgene can be achieved i n cells containing the transgene integrated into the rare, stable host site s. (C) 2000 Elsevier Science B.V. All rights reserved.