Position-independent expression of transgenes in zebrafish

The variability in expression patterns of transgenes, caused by the influence of neighboring chromatin, is called 'position effect'. Border elements are DNA sequences, which have the ability to alleviate position effects. The abilities of two types of border elements, scs/scs' from the D. melanogaster 87A7 heat shock locus and the A-element from the chicken lysozyme gene, to protect transgenes from position effects were quantified in developing zebrafish embryos. The transgenic construct used was FV3CAT, which consists of the carp beta-actin transcriptional regulatory region, the chloramphenicol acetyltransferase (CAT) gene and the 3'-untranslated region from the Chinook salmon growth hormone gene. FV3CAT constructs flanked by either scs/scs'-elements or A-elements were introduced into zebrafish chromosomes and the spatial and temporal expression patterns of the transgenes were quantified in multiple generations of transgenic zebrafish. Levels of transgene expression were uniform in the pre-differentiated and fully differentiated populations of cells present during embryonic development. Levels of transgene expression were proportional to the numbers of integrated transgenes. Expression of transgenes per cell varied less than two-fold in different transgenic lines. Both types of border elements were able to prevent the influences of neighboring chromatin on transgene expression through three generations of fish. The results are consistent with the ability of border elements to function with equal efficiencies in the many cell types found in vertebrates. Thus, inclusion of border elements in genetic constructs can provide reliable and reproducible levels of gene expression in multiple lines of fish.