Physical state & copy number of high risk human papillomavirus type 16 DNA in progression of cervical cancer

Despite effective screening programmes and lately introduction of two successful prophylactic human papillomavirus (HPV) vaccines for cancer prevention, cervical cancer is still a leading gynaecological malignancy of the developing world1. Globally, there are approximately 530,000 new cases of cervical cancer with an annual mortality rate of 275,0002. Interestingly, of the total global cervical cancer burden, more than 85 per cent of the total cases and 88 per cent of the total mortality are contributed by developing countries. Among the high risk HPV types established as aetiological factors for cervical cancer, infection of HPV16 or HPV18 has collectively been implicated in causation of more than 70 per cent of global cervical cancer burden. In contrast to other regions of the world, upto 90 per cent of the cervical cancer lesions in Indian women harbour specifically the HPV type 16; whereas, presence of other HPVs including HPV18 is relatively low3,4,5. The dominance of HPV16 in comparison to other high risk HPV types particularly in global and Indian scenario indicates toward a major role of HPV16 infection. It is likely that factors such as viral copy number and physical state of viral genome (episomal vs. integrated or mix) may have important clinical implications in viral persistence and progression of cervical neoplasia as suggested in some earlier studies6,7. Integration is considered as a key event in cervical carcinogenesis which results in loss of episomal viral DNA as it is well-documented that E2 gene product derived from episomal DNA have an inhibitory effect on viral oncogene expression8, and integrants are spontaneously selected during cancer progression due to selective growth advantage and endogeneous antiviral response9. Therefore, apart from the expression of viral oncogenes E6 and E7 that lead to tumorigenic transformation, physical state of virus and viral copy number have been demonstrated to be the major risk factors for development of high-risk HPV-mediated cervical cancer6,10. On the contrary, integration of HPV16 viral DNA into the host genome and high viral copy number within infected epithelial cells have been associated with an increased persistence of HPV infection and an increased risk of developing cervical intraepithelial neoplasia 2/3 (CIN2/3) or cancer6,7,11. Unlike HPV18 which shows high frequency of integrated viral genomes in cervical carcinomas, only a proportion of cases ranging from 28 to 67 per cent, depending on the techniques used, demonstrate presence of integrated HPV16 in invasive cervical carcinoma11,12. Some studies have demonstrated that integration of the HPV genome has also been found in low-grade lesions and even in normal cervices13,14,15 whereas others have shown that not all invasive cancers carry the integrated HPV genome14,16. However, besides a few sporadic reports4,17, data related to viral load and its physical status particularly with reference to HPV16- are lacking. The potential reason for higher pathogenicity of Indian HPV16 subtype may be related to its biological behaviour with respect to its viral load and integration events during progression of the early infection to cervical carcinogenesis. Moreover, investigations on the viral load and integration of HPV16 independently or in combination have been attempted in the past with variable results. The reasons for such variability may partly be ascribed to multiple infections which could have confounding effect. In the present study, we examined copy number and physical state of infecting monotypic high risk HPV16 in cervical cancerous and pre cancerous tissues.