Epstein-Barr virus latent gene transcription in nasopharyngeal carcinoma cells: coexpression of EBNA1, LMP1, and LMP2 transcripts

Abstract

Epstein-Barr virus (EBV) genome-positive nasopharyngeal carcinomas (NPCs) regularly express the virus-coded nuclear antigen EBNA1, but not other EBNAs, and a subset of tumors also appear to be latent membrane protein LMP1 positive; the status of NPCs with respect to a second virus-coded latent membrane protein LMP2 is unknown. In the present work the EBV-NPC cell interaction has been analyzed at the RNA level with reverse transcription and polymerase chain reaction-based amplification to detect specific latent viral mRNAs. All four transplantable NPC cell lines studied and 17 of 18 fresh snap-frozen NPC biopsy specimens expressed an EBNA1 mRNA with a BamHI Q/U/K splice structure exactly like that recently identified in group I Burkitt's lymphoma (BL) cell lines and shown to be driven from a novel viral promoter, Fp. The BamHI Y3/U/K-spliced EBNA1 mRNA characteristic of virus-transformed B-lymphoblastoid cell lines (LCLs) was never found in NPCs. These same NPC biopsy specimens were then analyzed for evidence of the various LMP transcripts which are constitutively expressed in LCLs but down-regulated in BL cells. While only 3 of 18 tumors gave a clear LMP1 mRNA-specific signal after first-round amplification with either of two sets of polymerase chain reaction primers, the majority proved to be LMP1 mRNA positive after second-round amplification with nested primers. A rather similar pattern of results was obtained with respect to LMP2B mRNA expression, such transcripts being detectable only in a subset of tumors, and then at apparently low levels. In contrast, clear evidence of LMP2A mRNA expression was obtained in 17 of 17 fresh biopsies. The predominant form of EBV infection in NPCs, with coexpression of EBNA1 and LMP mRNAs, is therefore quite distinct from that seen in BL cells (in which EBNA1 is the only expressed mRNA) and in LCL cells (in which all six EBNA and three LMP transcripts are present). This third form of EBV latency may not be restricted to NPC but may have more general relevance in the context of EBV infection in vivo.