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Characterization of rare human papillomavirus type 11 mRNAs coding for regulatory and structural proteins, using the
M O Rotenberg1, L T Chow, T R Broker
1Biochemistry Department, University of Rochester School of Medicine, New York 14642.
Abstract:
Certain human papillomavirus (HPV) types cause warts, dysplasias, and carcinomas of the ano-genital and oral mucosa. Because of the inability to propagate HPVs in cultured cells, the paucity of viral mRNAs in human lesions, and the complexity of alternatively spliced transcripts derived from different promoters, it has not been possible to ascertain the exact structures of the majority of the mRNA species and the proteins encoded. We have adapted the recently developed polymerase chain reaction to amplify cDNAs of rare, type 11 HPV mRNAs isolated from a productively infected human foreskin xenograft in an athymic mouse. The oligonucleotide primers were designed to flank each of the mRNA splice sites previously mapped by electron microscopic analysis of heteroduplexes formed between cloned HPV-11 DNA and viral mRNAs isolated from genital warts. The splice junctions were determined by direct sequencing of the PCR-amplified cDNA products or after the cDNA was cloned into a plasmid vector. We provide the first direct evidence for the existence of rare mRNAs with the potential to encode regulatory proteins that have been hypothesized to exist for HPVs. Depending on the lengths of the upstream exons, the translation frame used and the possibility of internal reinitiation during translation, one pair of mRNAs with the same splice junction could encode the viral DNA copy number modulating protein E1-M, the enhancer repression protein E2-C, or both. A second pair of mRNAs, also with identical splice junctions, encode the enhancer-regulating protein E2; the longer of the two could also encode, in its 5' exon, either or both of the E6 and E7 proteins. Finally, we demonstrate that the doubly spliced late message for the major virion capsid protein L1 also contains the entire coding region for the early E1 E4 protein in the first two exons, with the initiation codon for the L1 protein located precisely at the splice acceptor of the third exon. The potential of this late mRNA to encode both the E1 E4 protein and the capsid protein could contribute to the preponderance of the E4 protein in the lesion.
Insights
This study uses PCR to identify rare human papillomavirus (HPV) mRNAs, revealing new protein-coding possibilities for HPV regulatory proteins and capsid production. This advances understanding of HPV gene expression and disease mechanisms.
Area of Science:
- Virology
- Molecular Biology
- Oncology
Background:
- Certain human papillomavirus (HPV) types are oncogenic, causing ano-genital and oral cancers.
- Previous research faced challenges in characterizing HPV mRNA structures and encoded proteins due to difficulties in cell culture propagation and analysis of rare transcripts in lesions.
Purpose of the Study:
- To elucidate the precise structures of rare HPV type 11 mRNAs and identify the proteins they encode.
- To provide direct evidence for the existence and function of hypothesized regulatory proteins in HPV infection.
Main Methods:
- Adaptation of the polymerase chain reaction (PCR) to amplify cDNAs of rare HPV type 11 mRNAs from xenograft models.
- Design of oligonucleotide primers flanking previously mapped mRNA splice sites.
- Determination of splice junctions via direct sequencing of PCR products or after cloning into a plasmid vector.
Main Results:
- Direct evidence for rare HPV mRNAs encoding hypothesized regulatory proteins, including E1-M, E2-C, and E2.
- Identification of a longer mRNA transcript potentially encoding E2, E6, and E7 proteins.
- Demonstration that the late mRNA for the L1 capsid protein also encodes the E1 E4 protein, explaining E4 protein abundance in lesions.
Conclusions:
- The study successfully characterized previously elusive HPV mRNA structures and their encoded proteins.
- Findings reveal novel insights into HPV gene expression, including the dual coding potential of late mRNA, which may contribute to disease pathogenesis.
- This research provides a foundation for understanding HPV regulatory mechanisms and developing targeted therapeutic strategies.