Intergenic transcription and developmental regulation of cardiac myosin heavy chain genes.

Cardiac myosin heavy chain (MHC) gene expression undergoes a rapid transition from beta- to alpha-MHC during early rodent neonatal development (0-21 days of age). Thyroid hormone (3,5,3'-triiodothyronine, T(3)) is a major player in this developmental shift; however, the exact mechanism underlying this transition is poorly understood. The goal of this study was to conduct a more thorough analysis of transcriptional activity of the cardiac MHC gene locus during the early postnatal period in the rodent, in order to gain further insight on the regulation of cardiac MHC genes. We analyzed the expression of alpha- and beta-MHC at protein, mRNA, and pre-mRNA levels at birth and 7, 10, 15, and 21 days after birth in euthyroid and hypothyroid rodents. Using novel technology, we also analyzed RNA expression across the cardiac gene locus, and we discovered that the intergenic (IG) region between the two cardiac genes possesses bidirectional transcriptional activity. This IG transcription results in an antisense RNA product as described previously, which is thought to exert an inhibitory effect on beta-MHC gene transcription. On the second half of the IG region, sense transcription occurs, resulting in expression of a sense IG RNA that merges with the alpha-MHC pre-mRNA. This sense IG RNA transcription was detected in the alpha-MHC gene promoter, approximately -1.8 kb relative to the alpha-MHC transcription start site. Both sense and antisense IG RNAs were developmentally regulated and responsive to a hypothyroid state (11, 14). This novel observation provides more complexity to the cooperative regulation of the two genes, suggesting the involvement of epigenetic processes in the regulation of cardiac MHC gene locus.