Replication and Pathogenesis of RNA Viruses 
Michael M.-C. Lai, M.D., Ph.D. — Investigator 
Dr. Lai is also Professor of Microbiology and Neurology at the University of Southern California School of 
Medicine. He obtained his M.D. degree from National Taiwan University. He studied retroviruses with 
Peter Duesberg at the University of California, Berkeley, where he obtained his Ph.D. degree in molecular 
biology and continued for postdoctoral work. 
MEDICAL history is marked by extraordinary 
successes against viral infections, but it is 
also punctuated by the continual emergence of 
new viruses. Since viruses, in general, contain 
very limited genetic information, they must rely 
upon host cells for their own growth. How they 
cause diseases and how they continue to flourish 
in nature are not only interesting subjects in 
themselves, but offer a lesson in the everyday 
workings of normal cells. Our laboratory is inter- 
ested in RNA viruses, replicating entities in 
which RNA, in contrast to DNA, is the genetic 
material. We are exploring how these viruses rep- 
licate and cause diseases. 
One of those we are studying is the corona- 
virus, named for its similar appearance to the co- 
rona of the sun. The virus causes the common 
cold in humans and a variety of gastrointestinal 
and respiratory problems in animals. It also 
causes symptoms very similar to those of multiple 
sclerosis, thus providing a model system for study- 
ing this disease. It has an RNA genome of 3 1 ,000 
nucleotides, which is the longest known stable 
RNA. We are interested in learning how this un- 
usually large RNA expresses its genes and main- 
tains its genetic stability, despite an overwhelm- 
ingly high frequency of error in RNA synthesis. 
We have recently determined the complete se- 
quence of the genome, giving us a glimpse of 
how the viral genes express themselves. 
The virus utilizes a novel RNA synthesis mecha- 
nism, a discontinuous process that fuses a leader 
RNA to a gene located some distance from it. This 
unusual mechanism allows the leader RNA to 
control the expression of viral genes and change 
the biological properties and pathogenicity of 
the virus as a result. This means that the enzyme 
catalyzing coronaviral RNA synthesis is also un- 
usual, which is, indeed, suggested from the se- 
quence of the gene encoding the enzyme. Our 
laboratory is investigating this novel RNA synthe- 
sis mechanism. 
Another unusual characteristic of coronavirus 
RNA has been revealed in our findings: it can un- 
dergo genetic exchange (RNA-RNA recombina- 
tion) at an extraordinarily high rate. RNA-RNA re- 
combination was previously thought to be a rare 
event in nature. We demonstrated, however, that 
it occurs readily between coronaviruses. This re- 
combination can take place almost anywhere in 
the RNA genome, both in tissue culture cells and 
during animal infections. 
Furthermore, we demonstrated that recombi- 
nant viruses could become a predominant virus 
population under certain conditions, replacing 
the parental viruses by a simple process of natural 
selection. Thus recombination represents a pow- 
erful evolutionary tool for RNA viruses. From the 
standpoint of viral biology, RNA recombination 
may be the genetic mechanism by which corona- 
viruses weed out defective RNA sequences gener- 
ated by errors in RNA synthesis. Coronavirus is 
thus able to maintain an RNA genome larger than 
was thought theoretically possible. RNA recombi- 
nation has now been demonstrated in many dif- 
ferent viruses, suggesting its important role in 
virus evolution. 
This genetic phenomenon also has an impor- 
tant implication in vaccine development for dis- 
eases such as AIDS (acquired immune deficiency 
syndrome) , since genetic exchanges may lead to 
genetic instability of attenuated virus vaccines. 
We are continuing to study the RNA recombina- 
tion mechanism and attempting to use it as a ge- 
netic tool in determining how viruses cause 
disease. 
Another virus we are studying is hepatitis delta 
virus (HDV) , a human hepatitis virus commonly 
associated with a severe form of hepatitis. HDV, 
by itself, does not infect humans because it is de- 
fective and requires another viral agent, hepatitis 
B virus (HBV), to supply an essential envelope 
protein in order to infect liver cells. The HDV has 
been shown to cause epidemics of fulminant hep- 
atitis in many parts of the world. In the United 
States, it is prevalent among intravenous drug 
abusers. 
The virus contains a circular, single-stranded 
RNA genome of only 17,000 nucleotides. It is the 
only animal virus with a circular RNA. This ge- 
nome structure is reminiscent of a group of plant 
pathogens, viroids or virusoids, which cause a va- 
riety of plant diseases. Indeed, the similarity be- 
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