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1 Physiology and Biophysics, University of California Irvine, Irvine, CA, USA
* To whom correspondence should be addressed. E-mail: fhaddad{at}uci.edu.
Hypertension has been shown to cause cardiac hypertrophy and a shift in myosin heavy chain (MHC) gene expression from the faster 
to slower 
MHC isoform. The expression of the and MHC pre-mRNAs, mRNAs, as well as the newly discovered antisense [beta] RNA were analyzed in three regions of the normal control (NC) and 12-day pressure overloaded (AbCon) hearts: the left ventricle apex, left ventricle base, and the septum. The RNA analyses in the AbCon heart targeted both the 5' and the 3' ends of each RNA molecule. MHC mRNA expression significantly increased relative to control in all three regions, regardless of the target site (5'or 3' end). In contrast, MHC pre-mRNA expression in the AbCon heart depended on the site of the measurement (5' vs. 3' end). For example, while the pre-mRNA did not change when targeted at the 3' end (last intron), it increased significantly in the AbCon heart when measurement targeted the 5' end (2nd intron) of the 25kb molecule. Analyses of the antisense RNA revealed that its expression in the AbCon heart was significantly decreased relative to control regardless of its measurement site. A negative correlation was observed between the mRNA expression and the antisense RNA (P<0.05) suggesting an inhibitory role of antisense RNA on the sense MHC gene expression. In contrast, a positive correlation was observed between the antisense [beta]RNA and the MHC pre-mRNA (P<0.05). This latter observation along with the MHC gene position relative to that of the antisense suggest that the MHC and antisense transcription is co-regulated likely via common intergenic regulatory sequences. Our results suggest that the increased MHC expression in the AbCon heart is not only the result of increased MHC transcription but also involves an antisense RNA regulation scheme. Although the exact mechanism concerning antisense regulation is not clear, it could involve modulation of both transcriptional activity of the MHC gene and post-transcriptional processing.
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