Cloning and characterization of new transcript variants of insulin-like growth factor-I in Sika deer (Cervus elaphus)

Insulin-like growth factor I (IGF-I) is a multi-promoter gene that has complex biological functions and plays an important role in cell differentiation and proliferation, animal growth and metabolism. IGF-I function is of particular importance in embryonic development. In this study, 5' and 3' RACE techniques were used to clone full-length cDNA of the IGF-I gene from the deer liver for the first time. Six transcript variants were identified: Class-1-Ea, Class-1-Eb, Class-1-Ec, Class-2-Ea, Class-2-Eb and Class-2-Ec. Analysis of amino acid sequence alignments indicated that different transcript variants of IGF-I from deer are highly conserved with other mammals and show high homology with Ovis aries, Capra hircus and Bos taurus. The IGF-I transcript variants show low homology with Mus musculus, Rattus norvegicuc and Canis familiaris. A rooted phylogenetic tree was constructed and suggested that the evolutionary molecular relationship of deer was closest to O. aries and farthest from M. musculus and R. norvegicuc. The results from semi-quantitative RT-PCR showed that in adult deer, the tissue expression level of IGF-I was high in liver and spleen and low in heart, lung, kidney and duodenum. The tissue expression level of Class-1-Ea IGF-I mRNA was higher than that of other transcript variants, while class-1 IGF-I mRNA tissue expression was higher than class-2 IGF-I mRNA tissue expression in liver and spleen. In fetal deer, only Class-1-Ea IGF-I mRNA was expressed, and the tissue expression level in the liver, lung, kidney and duodenum was higher than the tissue expression level in the heart and spleen. These results implied that transcript variants of the IGF-I gene have different roles in embryonic development and animal growth in deer.