Regulation of Human Retroviral 
Gene Expression 
Bryan R. Cullen, Ph.D. — Associate Investigator 
Dr. Cullen is also Associate Professor in 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 fersey, 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 Inc. 
RETROVIRUSES derive their name from their 
ability to reverse the normal flow of genetic 
information from DNA to RNA. They have the 
unique ability to synthesize a double-stranded 
DNA copy of their single-stranded RNA genome 
and then to integrate this DNA copy into the ge- 
nome of the infected host cell. Once the genome 
of a retrovirus is integrated into a host chromo- 
some, it is indistinguishable from a host gene and 
may be actively transcribed by the host transcrip- 
tional machinery. 
The infection of an animal by retroviruses can 
result in a number of disease states, of which the 
most common is leukemia. The avian leukemia 
virus (ALV), discovered by Ellerman and Bang in 
1908, was the first oncogenic virus to be defined 
experimentally. ALV and the somewhat similar 
murine leukemia viruses continue to be studied 
extensively as models for this virus group. This 
research has not only helped to delineate the ret- 
roviral replication cycle but has also greatly ad- 
vanced our understanding of retroviral oncogen- 
esis. Most importantly, this research has allowed 
the definition of a number of cellular genes, the 
oncogenes, whose inappropriate expression 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. 
In the past I worked extensively on the regula- 
tion of gene expression by the avian leukemia 
viruses. This background has greatly facilitated 
the current research of this laboratory, which 
focuses on the regulation of gene expression 
within the human retroviruses and particularly 
on HIV-1. The genome of HIV-1, like the ge- 
nomes of the avian and murine leukemia viruses, 
contains sequences encoding the viral structural 
genes env (envelope glycoprotein), gag (capsid 
protein) , and pol (RNA-dependent DNA polymer- 
ase) . However, the complexity of the HIV- 1 ge- 
nome, despite its similar size, is far greater than 
observed for these animal retroviruses. In particu- 
lar, HIV- 1 is now known to encode six additional 
gene products that have no equivalents in the 
avian and murine leukemia viruses. Two of these 
viral proteins. Tat and Rev, are nuclear regulatory 
proteins that are essential for HIV-1 replication. 
Both of these viral trans-activators appear to pos- 
sess unprecedented mechanisms of action that in- 
volve highly specific RNA-protein interactions. 
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, particularly, 
elongation. This mechanism, which appears 
unique to Tat, remains poorly understood but is 
likely to involve the interaction of Tat with cellu- 
lar proteins that are currently poorly defined. The 
identification and characterization of these cellu- 
lar proteins 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- 
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