Transgenic analysis of the thyroid-responsive elements in the alpha-cardiac myosin heavy chain gene promoter

The role of two putative, cis-acting thyroid hormone-responsive elements, TRE1 and TRE2, located at -129 to -149 and -102 to -120, respectively, on the murine alpha-myosin heavy chain (MHC) gene, has been investigated in transgenic mice. These motifs are present in a 4.5-kilobase fragment lying upstream of the transcriptional start site of the mouse alpha-MHC gene: this fragment directs appropriate expression of a reporter gene in transgenic mice (Subramaniam, A., Jones, W. K., Gulick, J., Wert, S., Neumann, J., and Robbins, J. (1991) J. Biol. Chem. 266, 24613-24620). Here, we independently mutate the TRE1 and TRE2 elements by base substitution. The mice were analyzed for transgene expression in different muscle and non-muscle tissues including the atria and ventricles. Normal levels of transgene expression were observed in euthyroid mice carrying a mutation in TRE1. In contrast to these results, mice in which TRE2 was mutated showed reduced levels of CAT activity in both the atria and ventricles, suggesting a previously undefined role for this element in the constitutive up-regulation of the alpha-MHC gene. In hypothyroid mice carrying either of these mutations, the complete cessation of ventricular expression of the chloramphenicol acetyltransferase transcripts that takes place in the alpha-5.5 (wild type) animals did not occur.