Expression of endogenous and exogenous growth hormone (GH) messenger (m) RNA in a GH-transgenic tilapia (Oreochromis niloticus) Antje Caelers 1 , Norman Maclean 2, *, Gyulin Hwang 2 , Elisabeth Eppler 1 & Manfred Reinecke 1 1 Division of Neuroendocrinology, Institute of Anatomy, University of Zu ¨rich, Zu ¨rich, Switzerland 2 Division of Cell Science, School of Biological Sciences, University of Southampton, Hampshire, SO16 7PX, UK Received 7 May 2004; revised 31 August 2004; accepted 6 September 2004 Key words: absolute quantification, growth hormone, mRNA, ocean pout antifreeze promoter, transgene tilapia Abstract We have previously produced transgenic fish from crosses between a wild-type female tilapia (Oreochr- omis niloticus) and a G1 transgenic male. This line of growth-enhanced tilapia carries a single copy of a chinook salmon (s) growth hormone (GH) gene spliced to an ocean pout antifreeze promoter (OPA- FPcsGH) co-ligated to a carp b-actin/lacZ reporter gene construct, integrated into the tilapia genome. Because little is known about the expression sites of transgenes, we have characterised the gene expres- sion patterns of sGH and tilapia (t)GH in transgenic tilapia using a newly established real-time PCR to measure the absolute mRNA amounts of both hormones. The sGH gene, which was expected to be expressed mainly in liver, was also found to be expressed in other organs, such as gills, heart, brain, skeletal muscle, kidney, spleen, intestine and testes. However, in pituitary no sGH mRNA but only tGH mRNA was found. Tilapia GH mRNA in wild-type pituitary amounted to 226 ± 30 pg/lg total RNA but in transgenics only to 187 ± 43 pg/lg total RNA. Liver exhibited the highest level of sGH mRNA (8.3 ± 2.5 pg/lg total RNA) but the extrahepatic sites expressed considerable amounts of sGH mRNA ranging from 4.1 ± 2.0 pg/lg total RNA in gills to 0.2 ± 0.08 pg/lg total RNA in kidney. The widespread expression of the sGH gene is assumed to be due to the tissue specificity of the type III AFP gene promoter. It is assumed that our transgenic experiments, which in contrast to some other approaches caused no obvious organ abnormalities, mimick the GH expression during ontogeny. Because sGH mRNA is expressed both in liver and in extrahepatic sites it may not only promote secre- tion and release of liver-derived (endocrine) IGF-I leading to an overall growth enhancement but also stimulate IGF-I expression within the different organs in a paracrine/autocrine manner and, thus, fur- ther promote organ growth. Introduction Transgenic fish are of value both as model spe- cies in fundamental research and as potentially genetically superior brood-stock for commercial food production (see Fletcher & Davies, 1991; Hew et al., 1995; Iyengar et al., 1996). The first transgenic fish were produced about 20 years ago (Maclean and Talwar, 1984; Zhu et al., 1985), but considerable progress has been made since that time. Now, fish of many species have been successfully transformed. One attractive commer- cially relevant scenario regarding transgenic fish lines is the production of growth-enhanced fish following the introduction of novel growth hor- mone (GH) coding genes. Growth enhancement of transgenic fish using novel piscine GH sequences has been achieved by a few researchers *Author for correspondence E-mail: nm4@soton.ac.uk Transgenic Research 14: 95–104, 2005. Ó Springer 2005