THE INTERACTION OF TRANSCRIPTION FACTORS WITH THE LONG CONTROL REGION OF HPV 16

Abstract

Human papillomavirus type 16 (HPV 16) infects mucosal epithelia and causes neoplastic lesions that may progress towards cancer. A segment of 850 base pairs (bp) of the 7904 bp circular DNA genome of HPV 16 termed the long control region (LCR) does not contain genes but contains the control elements for transcriptional activation and probably replication of the virus. Research done in our lab previous to the work towards this thesis showed that transcriptional activation of HPV 16 is cell-type-specific, i.e. it was observed in cells derived from a cervical carcinoma (HeLa) but not in fibroblast cells or the breast turner cell line MCF7. Transcriptional specificity of HPV 16 may therefore contribute to the epitheliotropism of the virus. A 400 bp segment of the LCR was sufficient to mediate this enhancer activity, and it was suspected that 7 binding sites for the factor NF1 and 3 for AP1 are involved in enhancer function. The work towards this thesis addressed detailed aspects of the activation through these 2 types of factors, and was aimed towards identification of additional factors involved in enhancer function. It was previously found and in parts demonstrated in this thesis that the NF1 and AP1 sites of HPV 16 are occupied by factors with binding specificities indistinguisl1able from "bona fide" NF1 and AP1 sites. Numerous deletion and point mutation studies on the 400 bp LCR fragment that contains the enhancer activity certified the functional contribution of most if not all of these sites to enhancer function. Two additional factors were found to activate the HPV 16 enhancer. One of them, originally termed PVF, was identified as the binding site for the transcription factor TEF-2, which also binds the SV40 GT-1 motif and the glob in CAC box. Another site was shown to bind in HPV 16 the factor Oct-1 as well as a likely novel factor, termed NFA. This latter factor may be particularly functionally relevant since only NFA but not Oct1 binds a related motif in different papillomaviruses. A comparison of t11ese findings with the genomes and transcription factor binding sites of HPV 6, 11, 18, 31 and 33 led to the proposal that - in spite of a lack of a significant amount of sequence homology, all genital HPV enhancers have similar compositions of transcription factor binding sites, namely a cluster of NF1 sites complemented by singular or few binding sites for AP1, NFA, TEF-2 and certain steroid receptors. This highly specific pattern may hold a clue to understanding the cell-type specificity of these enhancers. Since cell-type specific footprints have yet to be identified, and most if not all of the enhancer activity is eliminated by the removal of binding sites of seemingly ubiquitous factors, potential mechanisms are discussed on how ubiquitous binding transcription factors may still bring about cell-type specific function.