Molecular Mechenism of Human Papillommvirus Diseases

Ⅰ.Human papillomavirus:

A DNA tumor virus with a doule-strand close circular DNA genome of 7900 bases. More than 70 types identified with most of them associated with the skun infection andabout 27 types infect the ano-genital mucosa.It encodes 8 genes which express 14 protin products.

Function of HPV transcripts:

E1: act on origin of replication to induce episomal DNA replication.

E2: together with E1 forms origin binding complex. Also a transcription regulator, Particularly a repressor of the E6 promoter.

E4: Act with intermediate filaments of host cell, associate wih viral particle release.

E5a: Anoncoprotein related to the signal transduction of host cell. Act on the ER,  Golgi, related to the endocellular trafficking.

E6: Anoncogene, inactivation of p53 tumor suppresser gene.

E7: An oncogene, inactivation of Rb tumor suppresser gene.

E7 gene alone of either low or high risk HPV transactivate host DNA replication  In the differention of squamouse epithelium.  Differentiation of host cell is required to turn on the E6/E7 promoter and hence Induction of host DNA replication.

L1: External capsid protein of the hexahedral capsid.

L2: Internal capsid protein of the hexahedral capsid.

A productive life cycle of HPV depends on the differention of squamous epithelium. So far it is very difficult to culture HPV. The identification of HPV relies on DNA detection. The weak systemic antigenicity makes serological test both insensitive and nonspecific.

Ⅱ.Human Papillomavirus and Cervical Cancer:

1. Epidemiology:

There are 5000,000 new cases of cervical or anal cancer each year around the world, which are related with the HPV infection. Cervical cancer has been the leading prevalent cancer in Taiwan since 1985.

HPV 16, 18 are related to the development of squamous, adeno-and small cell carcinoma of cervij, avd metastasis of the cervical cancer.

PHYLOGENETIC TREE OF HPV BASED ON AMINO ACID SEQUENCES

About 50-80% of CIN patient have HPV infection, 90% or more human cervical cancers contain HPV DNA. Those remaining HPV(-) cervical cell lines have p53 or RB gene mutation. The most prevalent HPV type in cervical cancer is type 16, folloewd by type 18, 31, 33. In Taiwan and fareast countries, type58, next to type 16 is the second most prevalent type. The prevalence of HPV infection in genital tract is about 25% of female less than 55 years old. The lifelong risk of HPV(+) women to develop cancer is about 3.7%. It usually takes 20 to 50 years.

Patholphysiology:

(A) HPV infection

The primary infection site of HPV is at the squamous-columnar(S-C) junction of Cervical epithelium, where the cells are actively proliferating. The columnar epithelial cells at the S-C junction toeard the endocervix with age. Most cervical cancer occurs at the region between the new and old S-C junctions, which is therefor named “transforming zone”. The original S-C junction of a teenage girl is more exposed at the exocervix and cells within are more vulnerable to HPV infection if she has early sexual exposure. Other estrogen-excess status that leads to cervical extropian and vulnerability include pregnancy and use of oral pills. The way of HPV entry into the epithelium is unknown, probably is through injured (erotic) or inflamatory epithelium and get into the basal (stem) cells.

HPV Transmission

•Cutaneous HPVs are transmitted casually.

•Genital HPVs are transmitted sexually.

Important variables for acquisition:

－ Age at first intercourse

－ Number of sex partners

－ Coincident STDs

－ Immunc status

(B)Integration of HPV DNA into host genome

HPV DNA is usually extrachromosomal in CIN lesions. In malignment lesion Viral DNA is usually found to be integrated. The integration disrupt HPV circular genome at the E2 gene which is a repressor for E6/E7 promoter. This results in an over-expression of E6 and E7 oncogenes. No specificity has been observed for intrachromosomal localizations of HPV integration. One report disclose HPV 16 integrated near the c-src, c-raf and c-myc oncogenes. A small percentage of malignant tumors harbors only nonintegrated copies of viral DNA. The integration is clonal in nature indicating the HPV infection precedes the tumor growth.

The infection of HPV is tissue (i. e. keratinocyte) specific. The abundant AP-1 in Keratinocyte but not fibroblast may be responsible, since there is a crucial AP-1 binding site at the E6 promoter.

(C) E6 and E7 oncogenes

The vast majority of HPV-positive cancer cells express high level of E6 and E7. E6, E7 genes from high risk HPV together are capable of immortalizing primary human keratinocytes, subsequent transfection with v-ras, or after long-time culture, may lead to malignant clones.

Protein products of deregulated E6 and E7 genes bind to tumor repressor gene products,p53 and pRb, and leads to loss of function of these tumor suppressor proteins.p53 and pRb, are also bound to other viral oncoproteins such as E1b (p53) and E1a (pRb) of adenovirus, and large T antigen (both) of SV40 virus.

Expression of the high risk HPVs may lead to aneuploidy (change of chromosome number) and contribute to the other cellular events necessary for a full cancer development.

E7 protein only is sufficient to turn on the DNA replication machinery. E7 binds  

Rb protein, ieading to its sequestration to an inactive form. The pRb normally activate the transfection factor E2F, which is a major machinery to activate the expression of a lot of replication related enzymes and entry into the cell cycle. The major role of E2F is the transcriptional activation of cellular genes that encod proteins important for creating the S phase environment and allowing DNA replication to occur. E6 targeted at the cell cycle check point controlled by p53 which is bound and degraded by E6.

2. Natural history of HPV infection

It generally takes 5 to 10 years for LSIL to progress to HSIL and another 10 to 20 years to invasive Ca. About 1/3 of HPV infection persisted, 1/10 progress to CIN and less than 1/100 ti Ca.

3. Histological Change of HPV infection:

(A) Condyloma acuminata: Papillomatosis, acanthosis with hyperkeratosis, parakeratosis, presence of nuclear atypia and periunclear hals (koilocytosis).

(B)Low grade squamous intraepithelial lesion (LSIL): Aberrant differentiation (mild dysplasia), perinuclear atypia (Koilocytes), absence of abnormal mitotic figure. Comparable to CIN Ⅰ.

(C) High grade squamous intraepithelial lesion (HSIL): Plus the presence of abnormal mitotic figure. Comparable to CIN II,III and CIS.

*Flat cervical condyloma and low grade CIN have the same histology picture.

*Nuclear atycality: enlarged hyperchromatis smudged nuclei, empty or fragmenting nuclei.

*Abnormal mitotic figure: multinucleation, abnormal metaphase, bizarre forms,empty or fragmenting.

III. Perspective of HPV Infection Disease

Other Etiologic Factors of Cervical Carcinogenesis:

Infection of HPVis essential but both insefficient and insufficient for cervical carcinogenesis. In thedevelopment of cancer, it usually takes sij or more “hit” on critical growth control genes (oncogenes or tumor suppresser genes). Infection of HPV leads to inactivation of p53 and Rb, but other etiologic factors are required for the other 4 or more “hits”. Smoking, estrogen and other chronic infections are proven risk factors. All are genotoxic which may lead to genomic instability (or microsatellite instability) and rapid accumulation of genetic hits.