Muscle gene transfer studies of a 27-BP segment of the troponin I fast gene IRE enhancer

The fast-skeletal-muscle-fiber-specific expression of the troponin I(fast) (TnIfast) gene is driven by an Intronic Regulatory Element (IRE) located within the first intron of the gene. The IRE is a 148 bp transcriptional enhancer that contains several known and suspected cis-regulatory elements. These include the E-box, the closely-spaced MEF2 site and CACT box, the CACC site, and the CAGG element. Previous loss-of-function studies performed using the quail TnIfast IRE suggest that its activity depended on the MEF2 and CACT elements. The goal of my thesis research was to determine whether the MEF2 and CACT sites were not only necessary, but also sufficient, to support IRE activity. I prepared head-to-tail multimers of a 27-bp IRE segment that consisted largely of the near-adjacent MEF2 and CACT elements and did not contain any other known/suspected elements. These multimers were cloned upstream of a reporter gene consisting of the minimal promoter of the quail TnIfast gene linked to sequences encoding human placental alkaline phosphatase. The transcriptional capabilities of the constructs were assessed by gene transfer into the mouse soleus muscle in vivo by intramuscular injection/electroporation, and histochemical analysis of reporter enzyme plap expression including quantitative microdensitometry. I found that expression of these constructs was readily detectable and that it was markedly reduced by prior mutation of the CACT and, especially, of the MEF2 sites. These data indicate that the short DNA segment containing MEF2 and CACT elements is sufficient to drive expression in skeletal muscle and confirms the functional importance of these specific elements.Although constructs containing the wild-type IRE 27-bp region were expressed, there was little preferential expression in fast fibers, in contrast to expression driven by the complete 148-bp IRE. Thus my results indicate that the MEF2 and CACT elements are not sufficient to drive fast fiber-type-specific expression, and suggest that additional elements outside of the 27-bp region tested are also necessary for fiber-type-specificity