Transcriptional regulation of the viral telomerase RNA

Abstract

Marek’s disease virus (MDV) is an avian alphaherpesvirus which causes T-lymphomas in chicken. The identification of the sequence encoding the viral RNA subunit of telomerase (vTR) in the MDV genome focuses a new viral element potentially implicated on MDV lymphomagenesis. Indeed, the telomerase ribonucleoprotein is implicated in cell immortalization process. We previously reported that the viral TR reconstitutes a functional telomerase activity in vitro. Thus, we investigated telomerase regulation during MDV lymphomagenesis by studying the telomerase activity level and the expression of viral and cellular telomerase related genes during in vivo infection of chickens by the MDV-RB1B oncogenic strain. The up-regulation of telomerase activity seems to be directly correlated to tumor formation in chickens. Furthermore, the transcriptional analysis of telomerase related genes revealed that only the vTR expression is up-regulated concomitantly with telomerase activity during tumorigenesis. In vitro assays using the luciferase reporter gene corroborated those results. Indeed, the vTR promoter is up to two-fold more efficient than the chTR promoter in avian cells. By a directed mutagenesis promoters study, we determined that the TATA box like element present in both vTR and chTR promoters is necessary to their expression. Then, we investigated which specific cis-elements of the vTR promoter are involved in its increased activity. We showed that the regulation of vTR expression in vitro is especially mediated by the E-box located 2 nucleotides downstream of the transcriptional start site of the gene. Moreover, overexpression assays of the c-Myc oncoprotein indicate its involvement in the transcription process.