EXPRESSION OF THY-1 LACZ FUSION GENES IN THE CNS OF TRANSGENIC MICE

Thy-1 is a cell surface glycoprotein of unknown function that is found on nerve cells and mature T-lymphocytes. To study the regulation of T hy-1 gene expression, mouse Thy-1.2 genomic sequences were joined to v arious marker sequences and the resulting chimeric constructs were use d to produce nearly three dozen independent lines of transgenic mice. The starting point for our studies was an 8.2 kb EcoRI fragment that b egins 1.7 kb 5' to the transcription start site and ends with 1.3 kb o f 3' flanking sequences. Addition of a small marker oligonucleotide to the 3' untranslated region of this fragment had little or no effect o n gene regulation. All of the lines derived from injection of this con struct expressed the transgene in the appropriate tissues. Thus, as ex pected, the Thy-1.2 genomic fragment contains all of the information n ecessary for tissue-specific, position-independent expression of the m odified transgene. Unexpectedly, Thy-1/lacZ hybrid genes did not mimic this behavior. Using either mRNA or histochemical detection of lacZ p rotein, these constructs were expressed in patterns that varied dramat ically from line to line. This behavior suggests that integration site -specific effects dominate the cis-active Thy-1 regulatory elements le ading to wide variability of expression. This is further emphasized by the observation that the bacterial reporter protein was found in a fe w non-neuronal cell-types, in contrast to the known pattern of native Thy-1 expression. These results suggest that either the Thy-1.2 sequen ces which are necessary for appropriate brain-specific expression are not contained solely within the proposed CNS enhancer in the first int ron, or that fusion of the Thy-1.2 sequences with the lacZ coding regi on may disrupt normal Thy-1 regulatory signals (or result in the creat ion of new regulatory elements).