Genetic analysis offers insights into the workings of a
notorious virus
If one were to rank the world's most gruesome ways to
die, Ebola infection would surely sit near the top of the
list. It begins with a sudden fever and then kills by
liquefying peoples' insides. Fifty to 90 percent of those
who become ill, die, making Ebola among the most
lethal viruses known. Other diseases kill far larger
numbers of people, but Ebola's mystery and ferocity has
come to symbolize the growing risk from emerging and
re-emerging pathogens.
The two worst Ebola outbreaks occurred in Zaire in
1976 and 1995, with almost no cases reported in
between. (Some reserchers have even speculated that the
"Plague of Athens" that ended in 425 B.C. was an
isolated Ebola epidemic.) No one knows where the
virus hides during its respites.
In the April 16, issue of the Proceedings of the National
Academy of Sciences, U.S.A., Anthony Sanchez and his
colleagues at the Centers for Disease Control and
Prevention in Atlanta report some notable progress in
uncovering the genetic mechanisms that make this
microscopic monster tick.
Virologists have classified the four known variants of
the Ebola virus (the Zaire, Sudan, Ivory Coast and
Reston strains) along with Marburg, a genetically related
monkey virus, into the genus Filovirus. Sanchez's team
analyzed certain key viral genes; using that information,
the researchers placed the filoviruses along a
phylogenetic tree, a branching diagram that shows the
evolutionary relationship between organisms. Each of
the five viruses occupies its own, genetically distinct
branch. Sprouting from these are evolutionary
"twigs"--mini-strains into which the Ebola variants are
further divided.
Such minor divergences are an inevitable result of
mutations, natural errors in the replication process. The
big surprise revealed in the Sanchez paper is just how
tiny those twigs are. Scientists had expected to see far
more genetic heterogeneity in each of the Ebola strains,
given the way that the viruses reproduce.
In most higher organisms, the genetic material (DNA)
proofreads each replica to ensure that the genetic
material in daughter cells is exactly the same as in the
parents. The story is somewhat different for viruses.
Take for example HIV, the virus that causes AIDS. HIV
is a retrovirus: the genetic material within the virus is
RNA, a simpler molecule that lacks the double-helix
structure of DNA. Once inside a host cell, HIV's RNA is
converted into DNA before the virus reproduces. In this
way, retroviruses too have proofreading abilities, but
not as effective as those in higher organisms.
Filoviruses, however, reproduce as RNA; unlike HIV,
they do not have the ability to check each copy.
Consequently, "the error rate is one million fold greater
than that of DNA based systems," says Timothy Nichol,
one of Sanchez's co-authors. To find out how Ebola's
penchant for mutation plays out in the real world, the
CDC researchers compared the Ebola strain captured
from the 1976 outbreak in Kikwit, Zaire, to one taken
from the 1995 outbreak in Yambuku, Zaire. Even though,
the two epidemics occurred more than 1,000 kilometers
apart, and the virus had 18 years to mutate, the genetic
sequences of the two isolates of Ebola-Zaire are
virtually identical. In addition, Bernard LeGuenno at the
Pasteur Institute found that the recent 1996 outbreak of
the Ebola-Zaire strain in Gabon is also nearly identical
to the 1976 isolate. Something seems to be restraining
the natural tendency of filoviruses toward genetic
divergence.
Sanchez and his colleagues posit that, for the past 20
years, Ebola-Zaire has essentially been most
comfortable in its original form--in other words, natural
selection pressures have favored the survival of the
original strain over any mutants. This penchant for the
status quo lies in marked contrast to the behavior of HIV.
HIV also mutates as it reproduces (proofreading is not
foolproof), though not as readily as do filoviruses. But
because humans apply a selective force by attacking HIV
with drugs, the mutants that are, by chance, resistant will
proliferate.
To determine the genetic distance between strains,
Sanchez's team focused their attention on the genes in
which the viruses differ the most. Since all Ebola strains
are known to have jumped species--from their presumed
natural hosts to monkeys and humans--the researchers
predicted that the greatest variations would occur among
the genes which give the virus its ability to recognize
different cell species. In the case of Ebola, the most
relevant gene turned out to be the glycoprotein gene,
which produces proteins that sit on the virus's surface
and are thought to shuttle the virus inside the host cell.
The CDC group has since looked at other parts of the
Ebola genome; so far, the glycoprotein gene does in fact
seem the most variable, Nichol reports.
Choosing to analyze the glycoprotein gene allowed an
added insight: it revealed that filoviruses, in addition to
killing their victims by destroying the cells they infect,
might work by suppressing the immune system. This may
be one of the reasons they are so deadly. Sanchez's team
found that a section of the filovirus glycoprotein gene is
very similar to the corresponding sections of the
glycoprotein genes in other viruses whose function is to
subdue the immune system. Indeed, Ebola victims
usually die without evidence of an effective immune
response.
A related line of research concerns the search for an
animal "reservoir," the hiding place where the Ebola
resides during the long stretches between human
outbreaks. Scientists don't yet know in which creature or
creatures Ebola lies dormant, but groups from the CDC,
the World Health Organization and the U.S. Army are all
currently screening hundreds of African animal species
in search of the reservoir. Whatever it is, the high degree
of similarity between the 1976 and 1995 Ebola strains in
Zaire (less than 1.6 percent change in the studied RNA
segments) suggests to the CDC team that the reservoir is
the same in both locations and that the creature is either
widespread in Zaire, or else is a migratory species.
On the other hand, the four Ebola strains, along with the
Marburg virus, show genetic divergences as great as 45
percent. Such marked differences hint that the various
filoviruses are carried by more than one species. Each
strain may have slowly co-evolved to live comfortably
with their own as yet unknown hosts.
Compared to other less spectacular but more widely
distributed illnesses such as tuberculosis and hepatitis,
Ebola actually poses a rather limited threat: precisely
because it kills its victims so quickly, it cannot easily
spread. Yet all pathogens mutate, sometimes leading to
the appearance of more pernicious versions (as
demonstrated by the emergence of drug-resistant strains
of TB). Virologists are continuing their studies of the
genetics of filoviruses, hoping to find a treatment or a
vaccine. But even if they ever do, the threat that Ebola
will reappear, like so many other pathogens, will always
remain.
Gunjan Sinha and Corey S. Powell news writers
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