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.
B)
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.
3.
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.
2.
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.