cancer, and low-risk types, such as 6 and

Transmission and transformation:
Reviewing HPV’s lifecycle
Betty M. Chung and
Tyler C. Cymet, DO
Although the currently accepted understanding of human papillomavirus (HPV)
transmission is incomplete, reviewing the
known information about the HPV lifecycle is a useful exercise to provide physicians with a foundation from which to
question how HPV is transmitted and how
it may be transformed into a carcinogenic
infection. As HPV transmission comes to
be better understood through greater physician awareness and further research, we can
expect the development of more efficient
and more patient population-inclusive
vaccines.
The human papillomaviruses belong to
the Papillomaviridae family, which consists
of more than 100 nonenveloped, icosahedral capsid, double-stranded circular DNA
viruses. These viruses are associated with
benign cutaneous and mucosal lesions
(papillomas, verrucae, warts and condylomas); neoplasms and malignant tumors of
the head and neck (oropharyngeal and
esophageal), vulva, penis, perianal and anal
regions, cervix; and nonmelanoma skin
cancers.
HPV is a ubiquitous virus, and productive infection with subsequent virion synthesis requires differentiation of the basal
epithelial host cell.1 However, based on
clinical data that takes note of recurrence of
HPV lesions in immunosuppressed renal
transplant patients and children with recurrent laryngeal papillomatosis infected with
certain types of HPV, there may also exist
a latent viral state. This viral state can be
reactivated, in which HPV DNA may
be detectable in the absence of active virion synthesis and assembly.1
At least 40 of these viruses are known to
infect the genital tract and include those
high-risk types, such as 16, 18, 31, 33 and
45, that have been associated with cervical
4
cancer, and low-risk types, such as 6 and
11, that have not been shown to be correlated with malignancies.2 HPV is thought
to gain entry into the human body via
microtrauma or abrasions to the skin or
mucosa that expose the basal cells of the epithelium to HPV infection. HPV demonstrates tropism toward the keratinocytes in
the basal layer of the stratified squamous
epithelia of these structures.3 Although
most recent literature has focused on the
sexual mode of HPV transmission, researchers also have observed cases of nonsexual transmission, such as in virgin females, children and neonates.4, 5
The virus then uncoats and delivers its
genome of eight genes to the host cell nucleus to be expressed as autonomous replicating episomal or extrachromosomal elements. Using host cell machinery, viral
DNA is replicated and then segregated into
progeny cells. Subsequently, one daughter
cell remains in the basal layer to serve as a
reservoir for the replication of more viral
DNA while the other daughter cell migrates suprabasally, where HPV inhibits the
cell from its normal exit from the cell cycle.
Differentiation of infected cells is necessary for productive infection of the host
cell and is coupled with expression of late
viral capsid genes for completion of the
viral lifecycle.10
HPV infection and lifecycle
HPV infects squamous epithelial cells in
the cervix, glans of the penis, penile shaft,
scrotum and anal verge by interacting with
putative host cell surface receptors such as
heparan sulfate proteoglycans6 and alpha 6
integrins.7 The virus enters the cell via
clathrin-mediated8 or caveolin-mediated
endocytosis,9 depending on the HPV type.
HPV genome and replication
The HPV genome contains eight genes
with multiple promoters and a number of
splice variants that are expressed either
early (E) or late (L) in the HPV lifecycle.
The early genes encode nonstructural proteins that participate in DNA replication,
transcriptional regulation, cell transforma-
tion and viral assembly and release. The late
(L) genes, L1 and L2, encode viral capsid
proteins. An additional 1000bp noncoding
region of the 8000bp HPV genome contains transcriptional regulatory sequences
and the viral origin of replication.11
Papillomaviruses are thought to have
two modes of replication: stable and vegetative. The stable replicative form exists as
a circular episomal genome in the cells of
the basal and parabasal layers of low-risk
HPV infected squamous epithelial cells
while the vegetative form occurs in highly
differentiated cells at the epithelial surface.
The hallmark of malignant transformation
by high-risk HPV types is the integration
of the viral genome into the host cell
genome.12
The first viral genes to be expressed, E6
and E7, are involved in cell transformation
in HPV high-risk types. E6 complexes with
the cellular ubiquitin ligase. The E6AP and
the E6AP/E6 complex acts as a p53 specific ubiquitin protein ligase to increase the
rate of p53 degradation, thereby negating
the anti-proliferative and pro-apoptotic
functions of p53 in cases of DNA damage
and cellular stress.13 E7, similar to E6, is
also an oncoprotein. E7 complexes with
hypophosphorylated pocket proteins pRb,
p107 or p130, facilitating the release of the
transcription factor E2F, which then constitutively activates DNA synthesis and cell
proliferation.14 The Rb tumor suppressor
protein normally binds to and inactivates
E2F. E7 has also been shown to inactivate
the cyclin dependent kinase (Cdk) inhibitors, p21/Cip1 and p27/Kip1 and
complex with cyclins A and E.1, 15
However, in low-risk HPV types, such
as HPV6 and HPV11, E6 may or may not
bind to p53. In either case, it does not stimulate p53 degradation and E7 does not
bind pRb with high affinity.16 In some
HPV types, E6 has been shown to enhance
telomerase activity, allowing for continuous
cell proliferation.17 The other early genes
are expressed as the cell differentiates.
E1 and E2 are both DNA binding proteins. E1 is a DNA helicase/ATPase involved in viral genome replication while E2
is a transcription factor that binds to four
sites within the viral noncoding region and
recruits E1 to the viral origin of replication.18 E1 and E2 have been shown to be
sufficient for transient replication of the
HPV genome. In addition, increased levels
of active E1 and E2 have been correlated
with increased viral copy number.1,19 E2
also binds to the early promoter and decreases the expression of E6 and E7. There-
concentrations of E2, displacement of basal
transcription factors are thought to be responsible for the inactivation of the promoter and repression of E6 and E7 expression.7 Additionally, a switch in promoter
usage to the differentiation-dependent late
promoter, which is not repressible by E2,
during differentiation is due to changes in
fore, loss of E2 during integration of viral
DNA into the host genome is the first stage
in transformation.20
E2 exists in a full-length form that acts
as a transcriptional activator of the early
promoter at low levels of E2 expression and
a truncated form at high levels of E2 expression that acts as a transcriptional repressor of the early promoter production of oncoproteins E6 and E7.7, 21, 22 At higher
cellular signaling and not genome amplication. This switch results in increased levels
of E1 and E2 and subsequent replication of
the viral genome.1, 7
E4 gene sequences are not highly conserved among the various HPV types. Although E4 has been shown to associate
with cytokeratins to destabilize the cytokeratin network and aid in viral release at the
epithelial cell layer surface, its primary
5
function is still unknown.23 However, E4
has been shown to cause G2 cell cycle arrest and antagonize E7-mediated cell proliferation. Researchers also have hypothesized that E4 associates with E2.7
E5 proteins are hydrophobic transmembrane proteins that seem to be weakly involved in cellular transformation and modulate the activity of the epidermal growth
factor (EGF) receptor tyrosine kinase signaling pathway.24 E5 is present on plasma,
Golgi and ER membranes. Researchers also
believe that E5 associates with the vacuolar ATPase and delays endosomal acidification in addition to inhibiting gap junctionmediated cell-cell communication in
keratinocytes. This results in an increase in
EGF receptor recycling to the cell membrane and an inability to respond to
paracrine growth inhibitory signals.7, 25
E5 expression in high-risk HPV types is
thought to occur early during carcinogen-
enough––enough that viral particles are
sloughed off at the skin surface to infect
other cells and hosts. L2 expression pre-
Most HPV infections are
latent or asymptomatic in
immunocompetent hosts but
can become reactivated in
cases of cell-mediated
immunosuppression.
cedes that of L1 and the presence of E2 is
thought to increase the efficiency of this
process.27 The lytic release of viral progeny
requires terminal differentiation of stratified epithelium, a prerequisite for the activation of differentiation-dependent late
viral promoters.28 Most HPV infections are
latent or asymptomatic in immunocompetent hosts but can become reactivated in
cases of cell-mediated immunosuppression.
diated S phase entry and cell proliferation
do result in the accumulation of multiple
mutations in the host genome.7, 29 However, cases of rapid onset of carcinogenesis
after initial infection also have been
noted.30 The highest incidences of HPVrelated cancers are associated with cancer of
the cervix in females and of the anus in
homosexual men.31 Research has suggested that HPV can more easily access the
basal epithelial cells at both of these sites
due to the presence of reserve cells in the
transformation zone that will eventually
take on the basal cell phenotype.7
Integration of HPV DNA at specific
sites in the host cell genome has been suggested to be an early and critical step in cancer progression.31 This also suggests the
participation of certain flanking sequences
in the integration of viral DNA as a step in
carcinogenesis.32
Final notes
While we can describe the groups of patients that are more likely than others to develop cancer from HPV infection, we still
do not understand very well why homosexual men, women who have sex with uncircumcised males and women with multiple
sexual partners are more likely to develop
cancer.
Although research has made great
strides in putting together the pieces of the
puzzle with respect to HPV—and we currently vaccinate women against the four
most virulent types of HPV associated with
cervical cancer—we still have more to discover before we can protect all individuals
who are susceptible to the development of
HPV-related cancers. ❙ ww
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Betty M. Chung is a DO/PhD candidate at the
University of Medicine and Dentistry of New
Jersey-School of Osteopathic Medicine.
Tyler C. Cymet, DO, is the associate vice president
for medical education at the American Association
of Colleges of Osteopathic Medicine. He is the president of the Maryland Association of Osteopathic
Physicians and is on the Board of Trustees for
DOCARE International. He can be reached at
[email protected]
BPV1 pseudovirions. J Virol. 2000;272:382-393.
7