Human Papillomaviruses Types 16 and 18 
LaimonisA. Laimins, Ph.D. — Assistant Investigator 
Dr. Laimins is also Assistant Professor of Molecular Genetics and Cell Biology at the University of Chicago. 
He received his Ph.D. degree in biophysics and theoretical biology from the University of Chicago. His 
postdoctoral research was done with George Khoury at the National Cancer Institute of the NIH. 
MY laboratory is studying the molecular biol- 
ogy of human papillomaviruses types 16 
and 18 and their relationship to neoplastic dis- 
ease. HPV-16 and -18 are the probable causative 
agents of the many cancers of the urogenital re- 
gion, in particular those of the cervix of the 
uterus. Infection by HPVs is, however, not suffi- 
cient by itself to induce cervical cancer; other 
cellular events, such as the activation of onco- 
genes, may also be required. We are currently ana- 
lyzing the molecular mechanisms by which HPVs 
contribute to the development of malignancy. 
Human papillomaviruses are classified into 
various subtypes, depending on sequence homol- 
ogy. If an isolate shares less than 50 percent DNA 
homology with other subtypes, it is classified as a 
new viral type. More than 66 distinct HPV sub- 
types have been identified. The majority of papil- 
lomaviruses (numbered according to chronologi- 
cal order of discovery) cause benign skin warts; 
only a limited subset is associated with cervical 
cancer (HPV-16, -18,-31, and -33). HPV-6 is re- 
sponsible for many genital warts; HPV- 1 6 and - 1 8 
are found in only a small proportion of benign 
lesions. In contrast, more than 95 percent of all 
malignant lesions contain HPV-16, -18, and -31 
sequences. An important question that our labora- 
tory is studying is what factors differentiate a 
virus that causes benign lesions from one capable 
of inducing malignancies. Although the number 
of infected individuals has increased 10-fold in 
the last 10 years, effective monitoring proce- 
dures such as the PAP smear have limited the in- 
crease in cervical cancers. 
HPV- 1 6 and - 1 8 have a latent period between 
the time of infection and the appearance of a neo- 
plastic state. In the benign state the virus is 
usually found as an episome; in the malignant 
state the genome is usually found integrated into 
the host chromosome. One model suggests that 
this integration process is important in the devel- 
opment of the cancerous state, perhaps through 
the removal of a dominant inhibitor of transfor- 
mation. Thus viral integration into the chromo- 
some may be the first step of a multistep process 
in the development of neoplasia. 
We have been studying how HPV viral genes 
contribute to the transformation process. Using 
in vitro protocols, we have transformed both hu- 
man and mouse cells in tissue culture. In one set 
of experiments, we introduced HPV DNA into the 
immortalized rodent cell line NIH 3T3 and found 
that these cells could induce tumors after injec- 
tion into mice. Since NIH 3T3 cells by themselves 
do not induce tumors, this demonstrates the tu- 
morigenic potential of HPVs. 
We have also investigated the ability of HPV to 
transform primary human epithelial cells, the nat- 
ural host cell for HPV infection. HPV can immor- 
talize these cells, which normally only have a lim- 
ited life span in vitro. In addition, using a system 
that mimics the differentiation properties of epi- 
thelial cells in vitro, we have shown that HPV 
sequences alter the ability of epithelial cells to 
differentiate. Although cells transfected by HPV- 
16 and -18 continue to stratify, the expression of 
viral genes disrupts the normal differentiation 
pattern. 
The morphological changes that we observe in 
this tissue culture system are similar to those seen 
in low-grade cervical neoplasias. In tissue cul- 
ture, our HPV cell lines quickly lose the ability to 
differentiate and develop the appearance of high- 
grade neoplasias. Two HPV genes, E6 and E7, 
seem to be required for this transformation pro- 
cess. This system may be an in vitro model for the 
development of cervical cancer; we hope to uti- 
lize it to identify important factors involved in 
controlling the development of malignancy. 
These observations support the etiological role of 
HPV in the development of cervical cancer. 
Although it is still not possible to propagate 
HPVs in tissue culture, we have made significant 
progress in this area. We have duplicated two fea- 
tures of a productive infection in vitro-, the dif- 
ferentiation-specific amplification of viral ge- 
nomes and the expression of late transcripts. A 
block still remains at the level of synthesis of cap- 
sid proteins, but we hope to overcome this obsta- 
cle in the near future. 
To understand the life cycle of papillomavi- 
ruses, we are examining the factors involved in 
the tissue-specific regulation of viral gene ex- 
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