Garrick D, Fiering S, Martin D I, Whitelaw E
Department of Biochemistry, University of Sydney, New South Wales, Australia.
Nat Genet. 1998 Jan;18(1):56-9. doi: 10.1038/ng0198-56.
In both plants and Drosophila melanogaster, expression from a transgenic locus may be silenced when repeated transgene copies are arranged as a concatameric array. This repeat-induced gene silencing is frequently manifested as a decrease in the proportion of cells that express the transgene, resulting in a variegated pattern of expression. There is also some indication that, in transgenic mammals, the number of transgene copies within an array can exert a repressive influence on expression, with several mouse studies reporting a decrease in the level of expression per copy as copy number increases. However, because these studies compare different sites of transgene integration as well as arrays with different numbers of copies, the expression levels observed may be subject to varying position effects as well as the influence of the multicopy array. Here we describe use of the lox/Cre system of site-specific recombination to generate transgenic mouse lines in which different numbers of a transgene are present at the same chromosomal location, thereby eliminating the contribution of position effects and allowing analysis of the effect of copy number alone on transgene silencing. Reduction in copy number results in a marked increase in expression of the transgene and is accompanied by decreased chromatin compaction and decreased methylation at the transgene locus. These findings establish that the presence of multiple homologous copies of a transgene within a concatameric array can have a repressive effect upon gene expression in mammalian systems.
在植物和黑腹果蝇中，当转基因拷贝以串联体阵列形式排列时，转基因位点的表达可能会被沉默。这种重复诱导的基因沉默通常表现为表达转基因的细胞比例下降，导致表达呈现斑驳状模式。也有一些迹象表明，在转基因哺乳动物中，阵列内转基因拷贝的数量可对表达产生抑制作用，多项小鼠研究报告称，随着拷贝数增加，每个拷贝的表达水平会下降。然而，由于这些研究比较了转基因整合的不同位点以及具有不同拷贝数的阵列，所观察到的表达水平可能受到不同的位置效应以及多拷贝阵列的影响。在此，我们描述了使用位点特异性重组的lox/Cre系统来生成转基因小鼠品系，其中在同一染色体位置存在不同数量的转基因，从而消除位置效应的影响，并能够单独分析拷贝数对转基因沉默的作用。拷贝数减少导致转基因表达显著增加，并伴随着转基因位点处染色质压缩程度降低和甲基化减少。这些发现表明，串联体阵列中多个同源转基因拷贝的存在可对哺乳动物系统中的基因表达产生抑制作用。
