Regulation of Human Retroviral 
Gene Expression 
Bryan R. Cullen, Ph.D. — Associate Investigator 
Dr. Cullen is also Associate Professor in the Section of Genetics and the Department of Microbiology and 
Immunology and Associate Medical Research Professor in the Department of Medicine at Duke University 
Medical Center. He received his master's degree in virology from the University of Birmingham, England. 
After emigrating to the United States, Dr. Cullen worked as a research technician for several years before 
reentering graduate school at the University of Medicine and Dentistry of New Jersey, where he received 
his Ph.D. degree in microbiology. Before accepting his current position at Duke, Dr. Cullen studied gene 
regulation in higher eukaryotes, as a laboratory head in the Department of Molecular Genetics 
at Hoffmann- la Roche. 
RETROVIRUSES derive their name from their 
ability to reverse the normal flow of genetic 
information from DNA to RNA. They possess the 
remarkable ability to synthesize a double- 
stranded DNA copy of their single-stranded RNA 
genome and then to integrate this DNA copy into 
the genome of the infected host cell. Once the 
genome of a retrovirus is integrated into a host 
chromosome, it is indistinguishable from a host 
gene and may be actively transcribed by the host's 
transcriptional machinery. 
The infection of an animal by retroviruses can 
result in a number of disease states, of which the 
most common is leukemia. Indeed, the avian leu- 
kemia virus (ALV) was the first oncogenic virus to 
be defined experimentally. ALV and the some- 
what similar murine leukemia viruses continue to 
be studied extensively as models for this virus 
group. This research has not only helped to delin- 
eate the retroviral replication cycle but has also 
greatly advanced our understanding of retroviral 
oncogenesis. Most importantly, this research 
has allowed the definition of a number of cellular 
genes, the oncogenes, whose inappropriate ex- 
pression can contribute to cellular transformation. 
Although animal retroviruses have been the 
subject of scientific research for some time, the 
discovery of human retroviruses occurred only 
within the last decade. Two major groups of 
pathogenic human retroviruses have been identi- 
fied thus far. Human T cell leukemia viruses 
(HTLV-I and HTLV-II) are known to be causative 
agents of human leukemias, including adult T 
cell leukemia, and are significant disease agents 
in several parts of the world, including Japan and 
the Caribbean basin. Of even more concern are 
the human immunodeficiency viruses (HIV-1 and 
HIV-2), which are a leading cause of disease and 
death in parts of Africa and in the United States. A 
third group of retroviruses, the human foamy vi- 
ruses (HFV), has recently been detected in hu- 
man populations in both Africa and Oceania but 
has not yet been clearly associated with any 
disease. 
A striking feature of all three classes of human 
retroviruses is that they each encode regulatory 
proteins that control both the quantity and qual- 
ity of viral gene expression. This regulatory com- 
plexity is not observed in many animal retrovi- 
ruses, including the avian and murine leukemia 
viruses, and may strongly influence the patho- 
genic potential of these "complex retroviruses." 
The major focus of this laboratory has been the 
determination of the role and mechanism of ac- 
tion of these viral trans-activators, with a concen- 
tration particularly on the Tat and Rev regulatory 
proteins of HIV- 1 . 
In the past we demonstrated that the Tat pro- 
tein of HIV-1 acts on sequences located within 
the HIV-1 promoter element (the long terminal 
repeat or LTR) to increase the level of expression 
of linked genes. This increased viral gene expres- 
sion occurs via a bimodal mechanism that in- 
volves an increase in the rate of transcription of 
HIV-1 mRNAs and in the efficiency of transla- 
tional utilization of those RNAs. The target se- 
quence for Tat is a 59-nucleotide RNA stem-loop 
structure located at the very 5' end of all viral 
mRNA molecules. The direct interaction of Tat 
with this RNA structure leads to an enhancement 
of viral transcription initiation and elongation. 
This mechanism, which may be unique to Tat, 
remains poorly understood but is likely to in- 
volve the interaction of Tat with cellular proteins 
that are currently poorly defined. The identifica- 
tion and characterization of these cellular pro- 
teins is a major research aim of this laboratory. 
A second HIV- 1 protein, Rev, is required for the 
expression of viral structural proteins but is dis- 
pensable for the expression of viral regulatory 
proteins. Our research has demonstrated that Rev 
acts post-transcriptionally to induce the cytoplas- 
mic expression of the unspliced or incompletely 
spliced RNAs that encode the viral structural pro- 
teins Gag and Env, while simultaneously repress- 
ing the expression of the fully spliced RNAs that 
encode the viral regulatory proteins, including 
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