Rescue of high expression beta-tropomyosin transgenic mice by 5-propyl-2-thiouracil - Regulating the alpha-myosin heavy chain promotor

Tropomyosin is an essential component of the sarcomeric thin filament in st riated muscle that participates in the regulation of muscle contraction thr ough Ca2+- mediated activation. The two predominant tropomyosin isoforms ex pressed in striated muscle are alpha- and beta-tropomyosin, which exhibit a n 86% amino acid identity between themselves. Previous studies by our labor atory utilized a transgenic mouse system to overexpress beta-tropomyosin in the heart to address the functional differences between these two tropomyo sin isoforms, Interestingly, when a high percentage of beta-tropomyosin rep laces alpha-tropomyosin in the hearts of transgenic mice, the mice die due to severe cardiac abnormalities. In this study, we have rescued these high expression beta-tropomyosin mice by turning off the alpha-myosin heavy chai n promoter, which is driving the beta-tropomyosin transgene. This down-regu lation of the ru-myosin heavy chain promoter was accomplished by the admini stration of 5-propyl-2-thiouracil, which disrupts thyroid hormone synthesis and inhibits promoter activity through thyroid regulatory elements located in the 5'-flanking region of the promoter. Results show that as beta-tropo myosin expression is down-regulated, alpha-tropomyosin expression is increa sed, Also, alpha- and beta-myosin heavy chain expression is modified in res ponse to the changes in thyroid hormone expression. Morphological analysis of these rescued mice show a moderate pathological phenotype, characterized by atrial myocytolysis; echocardiographic analyses demonstrate altered ven tricular functions, such as peak filling rates and left ventricular fractio nal shortening. This is the first report demonstrating that transcriptional regulatory elements located within the alpha-myosin heavy chain promoter c an be manipulated to rescue potentially lethal phenotypes, such as high exp ression beta-tropomyosin transgenic mice.