High Risk Hpv

Outline the importance of the so called high risk human papillomavirus types in cervical cancer

Human Papilloma Virus (HPV) consists of DNA and a protein coat. HPV strains are classified into high and low- risk types. 85% of HPV strains are of low risk. Low risk strains cause condylomas (genital warts) and laryngeal papilloma, most common types are 6 and 11. High risk HPV causes cervical, oral and anogenital cancer, the most common types being 16 and 18.

High risk HPV strains are transmitted through skin contact although transmission through sexual contact is more common. This can be prevented by using condoms. People with weaker immune systems are more susceptible to infection. Smoking, age and multiple sexual partners are other risk factors increasing persistent infection.

HPV cannot bind to squamous epithelial cells. It uses micro abrasion to destroy cell membranes and interact with integrins to enter the cell through endocytosis. The viral genome then replicates 20- 200 times.¹ Carcinogenesis involves viral oncogenes being integrated into the host cells genome. The E2 protein prevents the transcri ption of the E6 and E7 oncoproteins. When the viral genome is integrated into the host genome the E2 protein is not transcribed, producing excess E6 and E7 oncoproteins. E6 binds to p53 forming a trimeric complex with enzyme EAP targeting it for proteolysis. The E7 protein binds to the underphosphorylated form of pRb. As both proteins are degraded, their tumour suppressor function is lost, and the cervical epithelial cells have no DNA regulation. The binding of E7 to pRb displaces the E2 transcri ption factor (from the host cell) which is normally bound to pRb. The binding of the E2F to RbF ensures that it is in inactive form. When free from RbF, E2F is activated, allowing transcri ption of genes encoding proteins which are important in the progression of the cell cycle and DNA replication. These include cyclins and cyclin dependent kinases. ²

High risk E6 and E7 oncoproteins have a greater affinity for the tumour suppressor proteins compared to low-risk HPV A study conducted by Roman A and Armstrong DJ in virology 1999³ confirmed this by comparing the ability of HPV -6 and HPV -16 to transactivate the E2 promoter. This study was conducted upon baby rat kidney cells and human keratinocytes. HPV type 16 has seen to cause 45 % of all cervical cancers. High- risk HPV strains integrate their genome more efficiently into host s cell genome. The E6 oncoprotein has functions that promote carcinogenesis independent of its action on p53. These cellular targets include proteins that are involved in immune evasion and in apoptosis such as BAK and TNF receptor 1 (TNF R1), FADD and C-myc proteins. A recent study indicated that tumour cells have an increased expression of BCL2 and a lower expression of BAK, supressing apoptosis. ²

E7 has also been shown to interact and downregulate chaperone proteins, which may result in structural differences between tumour cells and cells of the epithelial lining.

A study indicated that cells infected with HPV had higher rates of phosphorylation of serine residues. These hyperphosphorylated proteins influence deregulating certain signalling pathways.

HPV E6 and E7 binding to P53, with Rb interfering with the normal functions of the tumour suppressors

A micronuclei count can also be used as a marker of HPV infected cells. Micronuclei arise when chromosomes are not accurately incorporated into daughter nuclei during mitosis. Cells which were infected had a higher micro nuclei account compared to those that did not. ⁴

HPV infection causes cervical intraepithelial neoplasia. CIN is classified into three grades: CIN 1 (mild), CIN 2 (moderate) and CIN3 (severe). Not all cases of CIN progress to cervical cancer as most HPV infections are cleared by the immune system. CIN 1 and CIN 2 often clear however can progress to CIN3 and then to cervical cancer. ⁵

Koilocytes are HPV infected cervical squamous cells and show a distinct clearing between the nucleus and cytoplasm called the perinuclear halo, a greater nuclear to cytoplasmic ratio and hyperchromatic nuclei.

Cervical Cancer is the 14th most common cancer in females in the UK, accounting for 2% of all new cancer cases in females (2017). Screening can prevent around 75 percent of cervical cancers. The UK Cervical Screening Program was introduced in 1988. ⁶

Cervical screening is amongst the most successful screening programmes in the UK and is offered to all women between the ages of 25 and 64. However in some cases abnormal cells cannot be identified due to them being morphologically indistinct and arising higher up in the cervix .

The Pap test is a method of cervical screening that involves taking a smear of cells from the cervix. This smear was examined under a microscope. Patients with abnormal cells (CIN) were eligible for HPV DNA testing. From December 2019, the cervical screening program introduced primary HPV testing. This means the Pap smear is first tested for the presence of HPV DNA. Only those cases with HPV DNA are examined under a microscope for the abnormality or grading of CIN. High grade CIN is referred to a colposcopy. Colposcopy (visual examination of the cervix) is used to detect lesions and biopsy them. A biopsy is removal of tissue for histological examination.⁷

In 2008, the national HPV immunisation program for girls aged 12-13 was started. From September 2019, the NHS offers all 12-13-year olds the HPV vaccine. NHS implemented the quadrivalent vaccine Gardasil.

HPV vaccines can be divided into bi-valent (16 and 18 strains) or quadri-valent strains (6,11,16,18) targeting 2-4 types of HPV. The vaccines work by mimicking the L1 capsid protein which can reassemble into virus like particles and insert viral DNA into cells. The vaccine however does not contain viral DNA and therefore poses no risk of infection and still triggers an immune response. ^8,9

All HPV vaccines have been found to have high efficacy (close to 100%) for prevention of CIN, therefore hoping to reduce the number of cervical cancers even further.¹⁰

References

1. Schiller JT, Day PM, Kines RC (June 2010). "Current understanding of the mechanism of HPV infection". Gynecologic Oncology. 118 (1 Suppl): S12-7. doi: 10.1016/j.ygyno.2010.04.004. PMC 3493113. PMID 20494219.

2. Cancer Res Treat. 2005 Dec 37(6): 319 324.

The Role of HPV E6 and E7 Oncoproteins in HPV-associated Cervical Carcinogenesis

Eun-Kyoung Yim, Ph.D. and Jong-Sup Park, M.D., Ph.D

3. Armstrong DJ, Roman A. The relative ability of human papillomavirus type 6 and human papillomavirus type 16 E7 proteins to transactivate E2F responsive elements is promoter- and cell-dependent. Virology. 1997 239:238 246.

4. Genet Mol Biol. 2014 Jun 37(2): 360 363. Association between human papillomavirus (HPV) DNA and micronuclei in normal cervical cytology

Ana Paula Rebelo Cassel,¹ Regina Bones Barcellos,² Cl udia Maria Dornelles da Silva,² Sabrina Esteves de Matos Almeida,^2,3 and Maria Lucia Rosa Rossetti¹

5. Robbins Pathologic Basis of Disease - 6th edition by Ramzi S. Cotran, Vinay Kumar and Tucker Collins

6. CancerResearchUK.org

7. Cervical screening programe overview GOV.UK

8. National Cancer Institute

9. www.nhs.uk- HPV Vaccine Overview

10. CDC.gov (Centre for Disease control and Prevention)