Delta Np63 alpha promotes Epstein-Barr virus latency in undifferentiated epithelial cells

Author summary Epstein-Barr virus (EBV) is an important cause of both epithelial cell and B cell human cancers. EBV-infected tumors have predominantly latent viral infection, allowing them to escape the cell killing that occurs during lytic viral infection. EBV is highly lytic in normal differentiated oral epithelial cells. Thus, an important question is how the virus maintains the latent form of viral infection in EBV-associated epithelial cell tumors such as undifferentiated nasopharyngeal carcinoma (NPC). This study demonstrates that the cellular transcription factor Delta Np63alpha, which is specifically expressed in undifferentiated basal epithelial cells and is over-expressed in NPC tumors, maintains EBV latency by inhibiting the activity of the viral immediate-early (IE) promoter (Zp) that drives expression of the BZLF1 IE protein. A related splice variant, TAp63 alpha, found in some EBV+ lymphomas, has a similar inhibitory effect. Our findings reveal that Delta Np63alpha and TAp63 alpha contribute to EBV latency in both epithelial and B cell tumors. Furthermore, since differentiation results in loss of Delta Np63alpha expression, our results help to explain why lytic EBV reactivation is promoted by epithelial cell differentiation. Epstein-Barr virus (EBV) is a human herpesvirus that causes infectious mononucleosis and contributes to both B-cell and epithelial-cell malignancies. EBV-infected epithelial cell tumors, including nasopharyngeal carcinoma (NPC), are largely composed of latently infected cells, but the mechanism(s) maintaining viral latency are poorly understood. Expression of the EBV BZLF1 (Z) and BRLF1 (R) encoded immediate-early (IE) proteins induces lytic infection, and these IE proteins activate each other's promoters. Delta Np63 alpha (a p53 family member) is required for proliferation and survival of basal epithelial cells and is over-expressed in NPC tumors. Here we show that Delta Np63 alpha promotes EBV latency by inhibiting activation of the BZLF1 IE promoter (Zp). Furthermore, we find that another p63 gene splice variant, TAp63 alpha, which is expressed in some Burkitt and diffuse large B cell lymphomas, also represses EBV lytic reactivation. We demonstrate that Delta Np63 alpha inhibits the Zp promoter indirectly by preventing the ability of other transcription factors, including the viral IE R protein and the cellular KLF4 protein, to activate Zp. Mechanistically, we show that Delta Np63 alpha promotes viral latency in undifferentiated epithelial cells both by enhancing expression of a known Zp repressor protein, c-myc, and by decreasing cellular p38 kinase activity. Furthermore, we find that the ability of cis-platinum chemotherapy to degrade Delta Np63 alpha contributes to the lytic-inducing effect of this agent in EBV-infected epithelial cells. Together these findings demonstrate that the loss of Delta Np63 alpha expression, in conjunction with enhanced expression of differentiation-dependent transcription factors such as BLIMP1 and KLF4, induces lytic EBV reactivation during normal epithelial cell differentiation. Conversely, expression of Delta Np63 alpha in undifferentiated nasopharyngeal carcinoma cells and TAp63 alpha in Burkitt lymphoma promotes EBV latency in these malignancies.