THE E6/E7 PROMOTER OF EXTRACHROMOSOMAL HPV16 DNA IN CERVICAL CANCERS ESCAPES FROM CELLULAR REPRESSION BY MUTATION OF TARGET SEQUENCES FOR YY1

Human papillomavirus type 16 (HPV16) induces squamous intraepithelial lesions of the cervical mucosa which may develop into invasive cancer. The expression of viral oncogenes in advanced neoplasias appears increased relative to the proliferating cell layers of low grade lesions raising questions about molecular mechanisms of deregulation of transcription. In a lymph node metastasis of a cervical cancer, we observed full-length HPV16 plasmids and molecules with a small deletion, which was mapped to the long control region (LCR). Both wild type and shortened LCR were amplified by PCR, cloned into the promoter test plasmid pBLCAT6 and sequenced to identify a 107 bp deletion from position 7794 to 7901 in the short LCR. CAT expression in cervical cancer-derived HT3, SiHa and CaSki cells appeared 5- to 6-fold increased under the control of the short LCR. This could be traced back to elevated levels of mRNA initiated at the viral oncogene promoter. A slight further increase in CAT expression was noted in the presence of the HPV16 E2 protein which is probably due to the deletion of one E2 binding site and consequent relief from E2 repression. Computer-assisted sequence analysis and band-shift experiments with purified YY1 protein and wild type or mutated oligonucleotides identified four binding sites for this cellular transcriptional repressor within the promoter-proximal segment of the HPV16 LCR, three of which were removed by the deletion. A LCR fragment comprising these YY1 binding sites was cloned in front of the heterologous thymidine kinase gene promoter and suppressed CAT expression 3- to 4-fold. This silencer activity was abolished by a mutation in the first YY1 binding site affected by the deletion. The LCRs of episomal HPV16 DNAs from three additional cases of cervical cancer were cloned and sequenced which revealed a naturally occurring point mutation in this very binding site in one case. The corresponding LCR showed a 4-fold enhanced promoter activity when compared with the HPV16 wild type LCR. This suggests that deletion or mutation of target sequences for YY1 represents a newly identified, repeatedly used strategy of HPV16 to escape from cellular repression.