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Quantitative analysis of plasma Epstein-Barr virus (EBV) DNA has been shown to be useful for the detection, monitoring and prognostication of NPC. However, it is unclear whether these circulating EBV DNA molecules are originated from the tumor or are released as a result of the expansion of latently EBV-infected B-lymphocytes due to immunosuppression. Therefore, in this study, we investigated the clonality of plasma EBV DNA in NPC patients and patients receiving immunosuppressive therapy by studying a highly polymorphic region, the BNLF1 region, of the viral genome. EBV DNA encoding the BNLF1 region was amplified by PCR from the plasma of 24 NPC patients, 3 patients suffering from systemic lupus erythematosis (SLE) and 5 renal transplant (RT) recipients. The PCR products were cloned into plasmid vectors and sequenced. The sequences were compared to the sequence of B95-8 cell line (accession no. v01555). The sequencing results are shown in the table. In 21 of the 24 NPC patients, all clones showed identical sequence. In the remaining three patients, two showed an additional nucleotide change in one clone, and one showed an additional nucleotide change in two clones. In the eight patients receiving immunosuppressive therapy, three showed identical sequence for all clones and four showed an additional nucleotide change in two or three clones. In the remaining patient, three additional nucleotides change was observed in two clones. Patients with more than one sequence detected are in bold. There is a significant difference in the frequency of detecting sequence variations in plasma EBV DNA between NPC patients and patients receiving immunosuppressive treatment (p=0.012, fisher exact test). To determine the origin of plasma EBV DNA, we have cloned and sequenced microdissected tumor tissues from 15 NPC patients. The BNLF1 sequence of tumor was identical to that of the plasma EBV DNA in all patients. Our results have provided direct evidence that plasma EBV DNA in NPC patients is likely to be derived from the tumor instead of latently EBV-infected B-lymphocytes due to immunosuppression. This work is supported by the Research Grant Council of Hong Kong SAR (CUHK 4086/02M) and CUHK strategic research area (Asian cancer studies).

[Proc Amer Assoc Cancer Res, Volume 46, 2005]