Nasopharyngeal carcinoma and Epstein-Barr virus

Abstract

Nasopharyngeal carcinoma (NPC) is the most common tumor in southern China and also quite frequent among the Eskimos and in Alaska. NPC is invariably associated with Epstein-Barr virus (EBV). Our primary aim was to investigate the potential contribution of the EBV-encoded latent membrane protein (LMP) in the development of NPC, particularly its tumorigenic and immunogenic properties and the possible prognostic value of LMPI expression in NPC. Analysis of EBV gene expression in 125 NPC biopsies has revealed that viral gene expression was limited to one nuclear protein (EBNAI), three latent membrane proteins (LMPI, LMP2A and LMP2B) and three additional transcripts (EBER 1, EBER2 and BamHlA). EBNAI, required for the maintenance of viral episomes, was consistently detected in all tumor biopsies and the transcription of this gene originated exclusively from the F/Q promoter whereas the C and W promoters were inactive. None of the NPC biopsies or control tissues expressed detectable amounts of EBNA2-6.In 65% of the biopsies, LMPI protein was detected by immunoblotting and a Xhol restriction site polymorphismlocated in the coding sequence of the LMPI gene was found (36/37 positive cases).

LMP2A and BamHlA transcripts were detected in the majority of NPC specimens and expression of LMP2B and LMPI were coupled. This pattern suggested that expression of the EBV genome is regulated in a tissue specific fashion. Sequence analysis of the B95-8 (EBV prototype) virus-derived LMPI and a Chinese NPC-derived LMPI (C-LMPI) indicated that a variant virus strain is prevalent in China. We found that the expression of C-LMPI was inversely correlated with the degree of CpG methylation within its transcription control region whereas all EBV coding genes including the LMPI region were highly methylated, suggesting that disease-associated hypo-methylation plays a role in regulating the expression of this gene. A clinical study of 74 NPC cases suggested that LMPI positive tumors grow faster and more invasively than LMPI negative tumors, and functional studies showed that the C-LMPI gene had higher clonability and tumorigenicity than B95-8-derived LMPI. Using a murine model in vivo system, C-LMPI was non-immunogenic, in contrast to a B-cell derived homologue, B-LMPI. In addition, we also explored the possible contribution of inactivation of tumor suppressor genes in NPC development. Using CA repeat polymorphic markers, allelic loss was observed in 11 of 17 patients. Two separate regions of allelic deletion were found on the short arm of chromosome 3: 3p21.1-pl4.3 and 3pl4.2-pl4.1. Taken together, our findings demonstrate that a substrain of EBV is prevalent in China and its LMPI may be more tumorigenic and less immunogenic than the corresponding gene isolated from B-lymphocytes. Multiple additional factors, such as inactivation of suppressor gene function by chromosomal deletion are likely to underlie the many steps that lead to oncogenic transformation, and the development of NPC.