Viral carcinogenesis

2.  Introduction
 RNA viruses
 History
 RNA Genome
 Reverse Transcription
 Human T-Lymphotropic virus type I and II
 HIV
 Hepatitis C virus
 DNA Viruses
 Hepadna virus
 HPV
 Epstein Barr virus
 Polyoma virus
 Conclusion
 References

3.  A large number of DNA and RNA viruses
have proved to be oncogenic in a wide
variety of animals, ranging from amphibia to
primates, and the evidence grows stronger
that certain forms of human cancer are of
viral origin.

4.  Members of two RNA virus families -
Retroviridae and Flaviviridae.
 Retroviruses - human T-lymphotropic virus
(HTLV) and human immunodeficiency virus
(HIV).
 Flavivirus hepatitis C virus (HCV).

5.  Classified on the basis of pathogenesis -
Oncoretrovirus, Lentivirus, and Spumavirus
groups.
 Basis of virus particle morphology-virus
types A-D.

6.  Now, organized into seven genera based on
molecular genetic analysis: Alpha-, Beta-,
Gamma-, Delta-, and Epsilon-retroviruses,
Lentiviruses, and Spumaviruses.
 Deltaretroviruses HTLV-1 and HTLV-2.
 Lentiviruses HIV-1 and HIV-2.

7.  Human T-lymphotropic virus type 1 (HTLV-1)
contribute directly to the development of
adult T-cell leukemia (ATL).
 HIV and HCV are associated with human
malignancy, but likely contribute to its
development in an indirect manner.

8.  5 untranslated region, the three genes
common to all retroviruses gag, pol, and env,
and a 3 untranslated region and
polyadenylated tail.

9.  gag gene encodes viral structural proteins.
 pol encodes viral enzymatic proteins.
 env encodes viral envelope glycoproteins.

10.  Following entry into a
cell - single-stranded
viral genome is
converted to a double-
stranded DNA copy by
reverse transcriptase,
an RNA-dependent
DNA polymerase.

11.  Then, the retroviral
integrase protein
inserts the double-
stranded DNA viral
genome into a host cell
chromosome where it
permanently resides
as a provirus.

12.  It is duplicated along
with the cell's genome,
passed on to daughter
cells during mitosis,
and subsequently
transcribed and
processed into mRNA.

13.  Reverse transcription and integration of the
viral genome into the cell favors the three
major mechanisms by which oncogenic
retroviruses may participate in the malignant
transformation process.

14.  1. Slowly transforming viruses-alter
cellular gene expression by random
integration of a provirus within or adjacent to
cellular protooncogenes (insertional
mutagenesis).
 Direct physical disruption of a gene or effects
of viral promoters and enhancers on cellular
gene expression can lead to a malignant
phenotype in infected cells.

15.  2. Acutely transforming retroviruses have
incorporated into their genomes viral
oncogenes derived from cellular
protooncogenes (protooncogene capture).
 Subsequently transfer these altered or
deregulated oncogenes into newly infected
cells, thus leading to development of a
malignant phenotype.

16.  3. Trans-acting retroviruses alter cellular
gene expression and function and,
consequently, the control of cell growth via
viral proteins that act in trans.

17.  Adult T cell Leukemia, an
aggressive malignancy of
CD4+ T cells.
 Transmission of HTLV-1 - blood
transfusion, needle sharing,
breast feeding, and from male
to female (rarely the reverse)
by sexual intercourse.

18.  In addition to gag, pol, and env, the virus
genome contains additional open reading
frames (ORF I-IV) located in the pX region of
the genome.
 The two best characterized are the trans-
regulating proteins Tax and Rex.

19.  Rex promotes the cytoplasmic accumulation
of singly-spliced (env) and unspliced
(genomic) mRNAs.
 Tax activates transcription from the HTLV-1
LTR by associating with a number of cellular
transcription factors.
 Additional HTLV-1 proteins - p12I, p13II, p30II.

20.  Virus spreads to
uninfected cells by cell-
to-cell transmission of
the virus.
 This cell-to-cell spread of
HTLV-1 appears to
involve polarization of
the cytoskeleton of
infected cells to a cell-
cell junction, promoting
spread of virus to new
cells.

21.  In addition to cell-to-cell virus transmission,
the number of HTLV-1 infected cells within
an individual increases by simple mitosis of
provirus-containing T-cells - amplifying, the
number of infected T cells.
 In individuals infected with HTLV-1,
significant viremia is not detected.

22.  Outcome of HTLV-1 infection is an
asymptomatic carrier state.
 The genome of HTLV-1 contains, in addition
to the usual retroviral genes, a unique region
called pX.
 This region encodes protein called TAX.
 The secrets of its transforming activity are
locked in the TAX gene.

23.  The TAX protein can activate the
transcription of several host cell genes,
including genes encoding the cytokine IL-2
and its receptor and the gene for GM-CSF.
 TAX can repress the function of several
tumor suppressor genes that control the cell
cycle including the CDKIs CDKN2A/p16 and
TP53.

24.  In acute ATL, tumor cells aggressively
infiltrate multiple organs, commonly involving
lymph nodes, liver, spleen, skin, and lung.
 The age of onset averages 58 years (range,
24 to 85 years), with a male-to-female ratio
of 1.4:1

25.  ATL has been classified into four stages: acute,
chronic, smoldering, and lymphomatous.
 Characteristics of the malignancy may include
hypercalcemia, elevated lactate dehydrogenase
levels, cutaneous leukemic infiltrates, lytic bone
lesions, lymphadenopathy, and liver or spleen
lesions.
 Death is often the result of opportunistic
infection.

26.  The virus was isolated from a patient with
atypical T-cell variant hairy cell leukemia.
 HTLV-2 is transmitted by: contaminated
blood, breast feeding, and sexual
intercourse.

27.  HIV-1 and HIV-2 are
members of the
Lentivirus genus of
retroviruses.
 HIV-2 can also cause
AIDS in humans and
monkeys, the majority of
AIDS cases worldwide
are the result of HIV-1
infection

28.  HIV replicates actively following initial
infection, which results in high levels of
viremia.
 The high rate of viral replication, combined
with a high mutation rate due to lack of
proof-reading ability of reverse transcriptase
during reverse transcription, results in the
extreme genetic variability-HIV-1.

29.  Highly cytopathic for CD4-positive T cells.
 HIV encodes two trans-acting proteins, Tat
and Rev.
 In HIV-infected persons, non-Hodgkin's
lymphoma - Burkitt's, immunoblastic, and
primary CNS, Kaposi's sarcoma, anal
squamous cell carcinoma and cervical
cancer are all AIDS-defining illnesses.

30.  Many of the neoplasms common to AIDS
patients are associated with infection by
DNA viruses. These viruses include Kaposi's
sarcoma-associated herpesvirus/human
herpes virus-8, Epstein-Barr virus, and
human papilloma virus.

31.  HCV infection is a well-
established risk factor
for the development of
hepatocellular
carcinoma .
 HCV belongs to the
Hepacivirus genus of
the Flaviviridae family
of viruses

32.  HCV infection is strongly associated with the
development of hepatic cirrhosis and HCC.
 Following initial infection by HCV, about 25%
of people develop acute clinical hepatitis
while others are asymptomatic.
 HCV infection is chronic in 50% to 80% of
cases. Of these cases, 60% to 70% will
develop chronic hepatitis, with about 20% of
this group progressing to cirrhosis.

33.  The estimated proportion of individuals
chronically infected with HCV who develop
HCC is estimated to be 1% to 5%.
 Surgical resection of tumors or liver
transplantation are the only two curative
therapies for HCC; therefore, early detection
of HCC is important

34.  General characteristics that have emerged for
many human cancer viruses are:
 (1) the viruses that have been implicated in
human carcinogenesis are frequently ubiquitous
(e.g., Epstein-Barr virus [EBV], human
papillomavirus [HPV], hepatitis viruses);
 (2) cancer is a rare outcome of virus infection
and only a small percentage of infected
individuals develops cancer;

35.  (3) the time intervals between the initial
infection and cancer development is long
(usually decades);
 (4) the cancers are usually clonal;
 (5) chemical or physical agents are often
implicated as playing cofactor roles.

36.  Hepatocellular
carcinoma (HCC) is
one of the world's
commonest
malignancies.
 HBV is a small DNA
virus classified as a
member of the
hepadnavirus family .

37.  Primary HBV infection produces either a
subclinical infection or acute liver injury-95% of
such infections resolve.
 5% of patients develop persistent hepatic
infection and viremia, and most of the
demonstrated HCC risk falls within this
subgroup of infections.
 Another factor that adds to risk is the severity of
chronic liver injury: asymptomatic carriers have
lower HCC risk than those with chronic active
hepatitis or cirrhosis.

38.  The induction of hepatocellular injury is
thought to be important in HCC
pathogenesis because it triggers in the liver
a stereotyped proliferative response.
 Proliferation increases opportunities for
replicative errors (mutations) that over time
can contribute to the loss of normal cellular
growth control.

39.  The papillomaviruses
are nonenveloped
DNA viruses that
induce squamous
epithelial and
fibroepithelial tumors in
their natural hosts.

40.  In 1951, a Canadian cytologist Ernest Ayre
demonstrated squamous epithelial cells with
a ‘perinuclear halo’ in smears from the
uterine cervix - precancer cells and some
long standing infection or inflammation.
 Koss and Durfee, in 1956, named these
squamous cells as “koilocytes”, from the
Greek word ‘koilos’ meaning ‘hollow cell’.

41.  It was in 1968 - the ultrastructural finding of
koilocytes and viral particles within them in
genital condylomas.
 The year 1977 - Human Papilloma Viruses
(HPV) and they play a role in the etiology of
Squamous Cell Carcinoma and its
precursors.

42.  Koilocytes are actually virus-infected
squamous epithelial cells and the virus found
in the nuclei of koilocytes is consistent with
HPV.

43.  It can be classified to α – HPV predominate
in mucosal sites such as genital and oral
mucosa;
 β – HPV and γ – HPV predominate at skin
sites.
 Further categorized as high –
risk(oncogenic) strains – cervical cancer.
 Low risk(non oncogenic) – benign diseases.

44.  HPV-6 and -11 are the two most common
low-risk types; they account for the majority
of genital warts, which rarely progress to
malignancy.
 HPV-16 and -18 are the major high-risk
types and predominate in invasive
anogenital cancers.

45.  The buccal mucosa, being the site that is
most exposed to chemical carcinogens,
infections, and trauma, is most vulnerable to
carcinogenesis.
 Abrasions caused due to this continuous
exposure might make this mucosal surface
more susceptible to HPV by making it easier
for the virus to gain entry into the basal cells.

46.  Of the HPV family, more than 12 types have
been found in oral lesions, including HPVs 1,
2, 4, 6, 7, 11, 13, 16, 18, 30, 32, and 57.
 HPV-13 and -32 appears to be restricted to
oral lesions.

47.  A higher risk of oral cancer has been found
to be associated with,
 Number of sexual partners.
 Younger age at first sexual intercourse.
 Practice of oral sex.
 History of genital warts.

48.  Following the HPV infection of the host
tissue, The HPV genome is integrated into
the host genome and two products are
formed – E6 and E7 protein.

49.  ‘E6 protein’ that forms a complex leading to
the degradation of p53 gene thereby
inhibiting apoptosis.
 ‘E7 protein’ that disturbs the retinoblastoma
tumor suppressor gene thereby causing
increased DNA synthesis and proliferation

50.  A breakpoint in the E1/ E2 sequence allows
integration of HPV into the host genome and
significantly increases its tumorigenicity
through upregulation of E6 and E7 encoded
in the early open reading frame of the virus.
 Expression of E6 and E7 is negatively
regulated by E2 protein, which is also
encoded in the early open reading frame of
the virus.

51.  By altering host genome functions, HPV E6
and E7 disrupts the p53 and pRb tumor
suppressor genes, as well as numerous
cellular proteins involved in carcinogenesis.
 Subsequently, infected cells develop defects
in gene expression controlling apoptosis,
DNA repair, and cell cycle, thus paving the
way for cellular transformation.

52.  Nearly 100 distinct types of HPV have been
identified, only a few are seen to be
associated with OSCC.
 Balram et al have reported high prevalence
of HPV-16 and HPV-18 (42% & 47%
respectively) in their study on oral cancer
from Indian betel quid chewers.

53.  Chang et al, Shroyer & Greer, Mork et al and
Charfi et al have, in separate studies
demonstrated the high association rate of
HPV-16 in OSCC cases as between 62%
and 95%.
 Cabibi et al shows that the sensitivity of
detection of HPV infection by identification of
koilocytes was 74% and the specificity was
72%.

54.  Allen et al diagnosed severe epithelial
dysplasia of oral mucosa, concluded that
‘Oral Koilocytic Dysplasia’ (OKD)
represented a unique pathological entity and
the presence of HPV could be predicted
under light microscopy with 80% accuracy.

55.  Studies by Fornatora et al have also
reported OKD in association with OSCC,
which exhibits features of both HPV infection
(koilocytes) and oral epithelial dysplasia.

56.  Balram et al, suggested that the high
prevalence of HPV in OSCC point to the
continuous viral infection being an etiologic
factor with the betel quid (tobacco) causing
additional mutagenic steps in the
carcinogenic process.

57.  Recent studies - increasing incidence of OSCC
of the tongue in patients below 40 years of age,
in the absence of any known habit of using
tobacco.
 It has been reported to have increased over the
past decade and is thought to be the second
most common site for malignancy oral cavity.
 Microscopic evidence of koilocytes and HPV
DNA have been demonstrated in around 75% of
these tongue cancer patients, suggesting a
direct etiologic role for HPV in these cases, in
the absence of any other known factor.

58.  HPV infection of the mouth and of the
oropharynx, like HPV infection of the uterine
cervix, is associated with high-risk sexual
behaviour, in particular with orogenital sex.

59.  EBV is a remarkably
unusual virus that
potentially induces
malignant-infected B-
lymphocyte
proliferation in primary
human infection.
 EBV-infected B cells
are the source of
reactivated lytic virus
infection.

60.  EBV infects epithelial cells of the oropharynx and
B lymphocytes.
 It gains entry into B cells via the CD21 molecule,
which is expressed on all B cells.
 Within B lymphocytes, the linear genome of EBV
circularizes to form an episome in the cell
nucleus.

61.  The infection of B cells is latent; that is, there
is no replication of the virus and the cells are
not killed, but the latently infected B cells are
immortalized and acquire the ability to
propagate indefinitely in vitro.

62.  The latent membrane protein-1 (LMP-1) binds to
and activates a signaling molecule that is normally
activated by the CD40 receptor in B cells.
 This receptor is the key recipient of helper T-cell
signals, which are normally required for full B-cell
responses .

63.  Mimicking CD40, LMP-1 activates the NFκB
and JAK/STAT signaling pathways and
promotes B-cell survival and proliferation, all
of which are helper T cell-induced responses
that occur in the absence of T cells (or any
other signals) in EBV-infected B cells.

64.  Thus, the virus has efficiently co-opted a
normal pathway of B-cell activation in order
to increase the number of cells it can infect
and inhabit.

65.  The EBV-encoded EBNA-2 gene
transactivates several host genes, including
CYCLIN D and members of the SRC family,
promoting the transition of resting B cells
from G0 to G1.
 EBNA-2 also activates the transcription of
LMP-1 and is a key regulator of viral gene
expression.
 Thus, several viral genes collaborate to
render B cells immortal.

66.  EBV has been implicated in the
pathogenesis of several human tumors:
 Burkitt lymphoma, post-transplant
lymphoproliferative disease, primary central
nervous system lymphoma in AIDS patients.

67.  SV40 DNA or antigens
in different human
cancers -
osteosarcomas,
mesotheliomas, brain
tumors, and non-
Hodgkin's lymphomas.
 SV40 is a nonhuman
primate virus that
naturally infects Asian
macaques.

68.  Source - contaminated poliovirus vaccines
given between 1955 and 1963.
 SV40 is a highly oncogenic virus in rodent
cells.
 Valuable model for determining the various
mechanisms by which DNA tumor viruses
contribute to tumor formation.

69.  Understanding the role of virus in the etiology of
oral cancer may be essential in determining
prognosis and treatment and for disease
prevention.
 Over the past few years, there have been
significant advances in the understanding of the
tumor pattern, and various options are now
available as far as treatment modalities and
techniques of surgical reconstruction are
concerned.

70.  DeVita, Hellman and Rosenberg’s; Cancer
Principles And Practices In Oncology; 8th
edition.
 Kumar ,Abbas ,Fasusto ,Robbins And Cotran
,Pathologic Basics Of Diseases ,Elsevier,
Seventh Edition.
 Kumar, Robbins And Cotran , Basic
Pathology, Seventh Edition.

71.  Rakesh S et al; ASSOCIATION OF HUMAN
PAPILLOMA VIRUS WITH ORAL SQUAMOUS
CELL CARCINOMA – A BRIEF REVIEW; Oral &
Maxillofacial Pathology Journal; Vol 1, No 2 Jul-
Dec 2010.
 Liviu Feller et al, Human papillomavirus-
mediated carcinogenesis and HPV-associated
oral and oropharyngeal squamous cell
carcinoma. Part 2: Human papillomavirus
associated oral and oropharyngeal squamous
cell carcinoma, Head & Face Medicine 2010,
6:15

72.  Chocolatewala NM, Chaturvedi P, Role of
human papilloma virus in the oral
carcinogenesis: an Indian perspective, J Cancer
Res Ther. 2009 Apr-Jun;5(2):71-7.
 Kari Syrjänen et al, Morphological and
immunohistochemical evidence suggesting
human papillomavirus (HPV) involvement in
oral squamous cell carcinogenesis,
International Journal of Oral Surgery,Volume
12, Issue 6, December 1983, Pages 418–424.