TSRI Scientists Show that Rare Genetic Mutations Increase
Susceptibility to Sepsis
By Jason Socrates
Bardi
A group of researchers from The Scripps Research Institute
(TSRI) have discovered rare genetic mutations in a subset
of people who come down with a particular kind of severe sepsis,
an acute and often deadly disease.
These rare mutations in a human gene called TLR4
lend susceptibility to meningococcal sepsis, which strikes
over 2,500 people a year in the United States. About half
of those who contract meningococcal sepsis are younger than
the age of two, and the disease has an overall case fatality
rate of 12 percent.
"It's a very fast-moving, dramatic, and often fatal disease,"
says TSRI Immunology Professor Bruce Beutler, who led the
research, which will be published in an upcoming issue of
the journal Proceedings of the National Academy of Sciences.
"We took a large number of people with meningococcal sepsis
and compared them to normal controls who were ethnically and
geographically matched," says Beutler. "We found that the
TLR4 gene had more mutations in the sepsis populations."
Besides demonstrating that the risk of severe sepsis increases
with these mutations, which can be passed from parent to child,
the study also suggests that it may be possible to protect
people who are at risk. While not practical at the moment,
eventually patients with mutations to their TLR4 genes
might be given prophylactic treatment, for instance, before
they undergo surgery or travel somewhere they are likely to
be exposed to meningococcal bacteria.
Blinding the Immune System
Scientists had suspected that genetic factors determine
who gets the severe form of the disease and who does not.
Evidence for this was seen during outbreaks of this disease,
when the severe and deadly form of sepsis was more likely
to strike related individuals.
Further evidence was seen in mice, and it was known for
years that certain strains of mice are more susceptible to
infections with bacteria like N. meningitides—but
nobody had ever shown this to be true in humans. Now, the
TSRI team found a number of rare mutations in one essential
gene in the innate immune system called Toll-like receptor
4 (TLR4), which is important in endotoxin recognition,
giving people a higher probability of contracting meningococcal
sepsis.
TLR4 is part of the mammalian endotoxin receptor and is
an important gene because it detects the early stages of a
bacterial infection. It is a powerful pro-inflammatory receptor,
responsible for activating the immune system to attack invading
gram-negative bacteria like N. meningitidis. During
a mammalian innate immune response, TLR4 recognizes endotoxins
from the bacteria and activates macrophages, which then ingest
and destroy the foreign pathogens.
"The TLRs are the eyes of the innate immune system," says
Beutler, who had suspected that these eyes may be myopic for
some people and that mutations in the TLR4 gene may
underlie the genetic component of severe sepsis.
In order to address this, Beutler looked for extremely rare
mutations in the gene by obtaining hundreds of samples of
DNA from his collaborator Martin L. Hibberd, then a doctor
at Imperial College in London (Hibberd has since taken a position
at the Genome Institute of Singapore). He then amplified and
sequenced the entire TLR4 gene from each patient, looking
for all the rare mutations that are present in the gene—rather
than taking the traditional approach of looking at one common
mutation within that gene.
After the researchers had analyzed all the genetic information,
they found a significant excess of low frequency mutations
in the TLR4 genes of the people who had contracted
sepsis versus the control group.
Instead of asking how many people have one given mutation,
Beutler and his colleagues simply asked how many people have
any genetic variation. A few individuals had the same
mutations, but by and large they were single, isolated mutations.
"However," warns Beutler, "we can only account for a certain
fraction of the [genetic] risk."
Nevertheless, this is the first time that a comparison of
the collective mutations at a given genetic locus has been
made in any infectious disease. Infectious diseases are, after
all, primarily caused by an invading organism, and only in
recent times has a concerted effort been made to find genetic
determinants of susceptibility in the patient. Beutler and
his team had to write special software to make the comparison
of the thousand different sequences possible and find the
individual mutations therein. Significantly, the technique
of measuring the genetic variation "load" of the entire gene
locus could be applied to other sorts of diseases as well—particularly
diseases in which both genes and environment play a role.
The article, "Assay Of Locus-Specific Genetic Load Implicates
Rare Tlr4 Mutations in Meningococcal Susceptibility," is authored
by Irina Smirnova, Navjiwan Mann, Annemiek Dols, H. H. Derkx,
Martin L. Hibberd, Michael Levin, and Bruce Beutler. The article
is available online at: http://www.pnas.org/cgi/content/abstract/1031605100v1,
and will be published in an upcoming issue of the journal
Proceedings of the National Academy of Sciences.
This work was supported by The National Institutes of Health
and also by the Meningitis Research Foundation, a United Kingdom
charity.
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"[Sepsis is] a very fast-moving, dramatic,
and often fatal disease," says Professor Bruce Beutler.
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