Extrachromosomal Amplification of Human Papillomavirus Episomes Is a Mechanism of Cervical Carcinogenesis

Abstract

Abstract HPV16 is the most oncogenic type of human papillomaviruses (HPV). Integration of HPV into the human genome is an important mechanism of carcinogenesis but is absent in at least 30% of HPV16+ tumors. We applied long-read whole-genome sequencing (WGS) to cervical cancer cell lines and tumors to characterize HPV16 carcinogenesis in the absence of integration. Large tandem arrays of full-length and unique truncated viral genomes integrated into multiple chromosomes were identified in two HPV16+ cell lines. The dispersion of characteristic viral variants to multiple integration sites indicates that viral deletions formed as extrachromosomal DNA (a phenomenon we term HPV superspreading). In addition, we identified an HPV16+ cell line with unintegrated (episomal) DNA that has tandem arrays of full-length, truncated, and rearranged HPV16 genomes (multimer episomes). Cytogenetic analysis of this cell line shows intense extrachromosomal HPV staining, including structures resembling double-minute chromosomes. WGS of HPV16+ cervical tumor samples from Latin America revealed that 11 of 20 tumors with only episomal HPV (EP) had intact monomer episomes. The remaining nine EP tumors had multimer and rearranged HPV genomes. The majority (80%) of HPV rearrangements and deletions disrupted the E1 and E2 genes, and EP tumors overexpressed the E6 and E7 viral oncogenes, a similar profile to tumors with HPV integration. Tumors with putative multimer HPV integrations display HPV multimers and concatemers of human and viral sequences. Our data uncovered a novel mechanism for HPV16 to cause cancer without integration through aberrant episomal replication, forming rearranged, mutated, and multimer episomes. Significance: Multimers of the HPV genome are generated in cervical tumors replicating as extrachromosomal episomes, which is associated with deletion and rearrangement of the HPV genome and provides a mechanism for oncogenesis without integration.