Differences of globin transgene expression in stably transfected cell lines and transgenic mice

Previous studies demonstrated that DNase I hypersensitive site -40 (HS-40) of the alpha-globin locus is capable of greatly enhancing expression of a hybrid beta/gamma-globin transcriptional unit in plasmid-transfected murine erythroleukemia (MEL) cells. However, as reported here, this same gamma-globin gene expression cassette was only transcribed at trace amounts in erythroid cells of transgenic mice. This lack of expression was not directly attributable to the beta/gamma-globin transcriptional unit, since this same unit linked to a composite beta-globin locus control region was expressed at high levels in transgenic mice. This lack of expression was also not directly attributable to chromosomal position effects, since addition of chromatin insulators failed to increase the frequency of expression. DNase I hypersensitivity and chromatin immunoprecipitation assays demonstrated that the lack of expression was correlated with a closed chromatin structure. We hypothesize that transgenes undergo dynamic changes in chromatin conformation following chromosomal integration and that the discrepant results reported here can be attributed to the relatively high level of chromatin remodeling that occurs in the transgenic mouse model, coupled with the relative inability of the HS-40 element to maintain an open chromatin state under such conditions.