Role of the Q10 class I regulatory element region 1 in controlling tissue-specific expression in vivo

The MHC class I regulatory element (CRE) region 1 has been previously described as a positive cis-acting regulatory element essential for class I gene expression. We have generated transgenic mice (CBA x C57BL/6) with the MHC class I gene H-2Dd driven by two different 400-bp promoter regions of Q10, a nonpolymorphic MHC class I gene expressed in the liver, kidney, and fetal yolk sac. One transgene contained the wild-type Q10 promoter (Q10WT/Dd). The second construct (Q10M3/Dd) had 2 bp substitutions introduced in region 1 of the CRE that reconstituted the CRE inverted repeat present in classical class I genes. Mice containing the wild-type Q10/Dd gene expressed membrane-bound H-2Dd molecules in a tissue-restricted expression pattern similar to that observed for endogenous Q10. In mice containing the mutant construct (Q10M3/Dd), H-2Dd was also expressed in the thymus, a tissue not normally associated with Q10 expression but, surprisingly, the Dd was not expressed in other lymphoid tissues. Furthermore, thymic expression was greatest on double positive (CD4+ CD8+) thymocytes. Thymic Dd expression was correlated with the presence of what appears to be a previously unidentified transcription factor in thymocytes that is capable of interacting with the CRE-inverted repeat. These results show that the mutations in region 1 altered the tissue-specific regulation of the Q10 promoter in vivo, although an intact inverted repeat did not restore the ubiquitous pattern of expression characteristic of classical class I genes. Thus, these results indicate that elements in addition to CRE region 1 in the Q10 promoter region serve to limit ubiquitous tissue expression.