Published in Volume
119, Issue 9
(September 1, 2009)J Clin Invest.
Copyright © 2009, American Society for Clinical
Tales from the gene pool: a genomic view of infectious
The Journal of Clinical Investigation
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Published September 1, 2009
Research into the pathogenesis, prevention, and control of infectious and parasitic
diseases remains a global priority, as these scourges continue to be a substantial
cause of mortality and morbidity. The plethora of molecular tools that are now
readily available has facilitated a genome-wide approach to studying the pathogenesis
of such diseases, with direct implications for disease prevention and treatment. The
articles in this Review Series describe how genome-wide approaches have provided
insight into a range of human pathogens, leading to greater understanding of the
human diseases that they cause, and highlight some of the challenges that must be
overcome if we are to maximize what we learn from the wealth of genomic information
Infectious agents, including bacteria, viruses, fungi, and parasites, are a cause of
substantial mortality and morbidity throughout the world. Indeed, the WHO has
established that in 2004, the most recent year for which such information is currently
available, infectious and parasitic diseases were the second leading cause of death
worldwide (Figure 1) (1). They were also the leading cause of burden of disease, as
determined by disability-adjusted life years, which are defined as the sum of the years
of life lost due to premature mortality and the years of health and productivity lost
due to disability (1). Although low-income
countries carry the majority of the burden of infectious and parasitic diseases,
research into the pathogenesis, prevention, and control of these diseases remains a
global priority, with the immense economic benefits of controlling these diseases likely
to be felt worldwide. The emergence of drug-resistant strains of bacteria, viruses, and
other parasites means that diseases once believed to be under control have reemerged as
global health concerns. Examples of this include the emergence of strains of
Mycobacterium tuberculosis (the bacterium that causes tuberculosis)
that are resistant to the drugs used as first-line treatment and strains of
Staphylococcus aureus that are resistant to many commonly used
antibiotics such as penicillin, methicillin, and vancomycin. Emerging diseases, in
particular those caused by newly identified infectious agents or newly identified
strains or forms of an infectious agent, also provide an ongoing global health concern.
SARS and a potential influenza A H5N1 pandemic (avian flu) are usually the examples
mentioned in relation to this point, but the ongoing influenza A H1N1 pandemic (swine
flu) has highlighted how rapidly and unexpectedly such diseases can emerge.
The 2004 worldwide ten leading causes of death and ten leading causes of death
from infectious and parasitic diseases. The global burden of disease: 2004 update, published by the WHO
in 2008 (1), provides estimates of mortality
and burden of disease by cause for all regions of the world in 2004. The data in
this publication, which were used to generate this figure, indicate that
infectious and parasitic diseases were the second leading cause of death in the
world in 2004, after cardiovascular diseases. Specifically, approximately 17
million and 9.5 million deaths were a result of cardiovascular diseases and
infectious and parasitic diseases, respectively. Among those who died of
infectious and parasitic diseases, diarrheal diseases were the leading cause of
death, closely followed by HIV/AIDS. *These numbers exclude deaths from liver
cancer and cirrhosis resulting from chronic HBV infection.
Recent technological advances mean that many new, high-throughput molecular tools are
now available to those studying infectious and parasitic diseases at a reasonable price.
Among these, genome sequencing and microarray technologies have enabled researchers to
take a genome-wide approach to investigate pathogenesis and pathogen-host interactions.
This approach has been termed by some “pathogenomics” (2).
The first complete bacterial genome sequence, that of Haemophilus
influenzae, was reported in 1995 (3).
Since then, the ability to rapidly sequence many millions of nucleotides has enabled
researchers to generate a wealth of genomic data, with complete genomes of many
eukaryotes and their pathogens (including each major human pathogen) now available. For
example, at the time of writing (July 2009), the influenza genome sequencing project had
made available through GenBank the complete sequences of 3,733 human and avian influenza
isolates (National Institute of Allergy and Infectious Diseases;
more than 880 bacterial genomes had been completed (Genomes OnLine Database, version
http://www.genomesonline.org/gold.cgi). Although analysis of individual
genomes, in particular the first complete genome for a given pathogen, can provide
important new information about pathogenesis, many pathogenomic studies involve
comparison of multiple strains and/or isolates of a single pathogen, as researchers seek
to gain insight into specific disease phenotypes and genotype-phenotype relationships.
The authors of the articles in this Review Series on genomic approaches to infectious
disease seek to highlight the advances made in understanding the pathogenesis of a
select number of important human pathogens using genomic technologies and indicate how
such techniques can lead to greater understanding of human diseases. We hope that these
Reviews will highlight the utility and current limitations of the pathogenomics
Using pathogenomics and more
As indicated above, more than 880 bacterial genomes have been completely sequenced
(Genomes OnLine Database, version 2.0;
http://www.genomesonline.org/gold.cgi). Among these are full genome
sequences for 13 strains of group A Streptococcus (GAS), a
Gram-positive bacterium responsible for several diseases in humans, ranging from mild
conditions, such as pharyngitis, tonsillitis, and impetigo, to the more nefarious, such
as toxic shock–like syndrome and necrotizing fasciitis (often known as
flesh-eating disease). In the first article in this Review Series (4), James Musser and Samuel Shelburne III discuss how these genome
sequences, together with microarray technology, high-throughput proteomics, and enhanced
bioinformatics, have been used to provide molecular insight into GAS virulence, clone
emergence, and disease specificity.
There are also at least 14 complete genome sequences for S. aureus,
another Gram-positive bacterium that is a leading cause of bacterial infections of the
bloodstream, lower respiratory tract, and skin and soft tissue in the United States.
These infections can give rise to diseases that range from mild conditions, such as
impetigo and cellulitis, to those that are life threatening, such as pneumonia,
meningitis, toxic shock syndrome, and septicemia. Methicillin-resistant S.
aureus (MRSA) is a growing threat worldwide, and an increasing number of
cases of MRSA infection occur outside healthcare facilities, in otherwise healthy
people, and are known as community-associated MRSA (CA-MRSA) infections. In their Review
(5), Frank DeLeo and Henry Chambers focus on
the growing threat of CA-MRSA in the United States and highlight how genome-wide
approaches are beginning to provide insight into the emergence and virulence of this
pathogen. However, they note that more work needs to be done, and their hope is that the
complete sequencing of many more S. aureus genomes might help firmly
establish how new, more virulent strains emerge.
When the first complete genome sequence of Helicobacter pylori was
published in 1997 (6), it was the seventh
completely sequenced bacterial genome. There are currently at least 7 complete genome
sequences for this Gram-negative bacterium that colonizes the human stomach, causing
peptic ulceration, gastric lymphoma, and gastric adenocarcinoma. In the third article in
this Review Series (7), John Atherton and Martin
Blaser discuss, from a genetic and molecular perspective, how H. pylori
has adapted to humans (a species they have colonized for over 50,000 years) and how
H. pylori–human interactions shape disease pathogenesis.
As a corollary to this, they suggest that the increasing absence of H.
pylori from the stomach throughout the life of many individuals might have led
to human physiological changes and contributed to recent increases in esophageal
As with bacterial pathogens, complete genomic sequence data has been generated in large
amounts for many RNA viruses, probably because their genomes are quite small
(approximately 10,000 nucleotides), making the process relatively easy and cheap. In his
Review (8), Edward Holmes uses three very
different RNA viruses that infect humans — influenza virus, HIV, and dengue
virus — as examples to put forward the case that while the abundance of
genomic data has taught us much about the evolution and epidemiology of these viruses,
it has yet to provide insight into disease pathogenesis, prevention, and control. He
argues that, at least in the case of RNA viruses, the potential of genomics has yet to
be fully harnessed, because much sequence data is often collected and stored out of
context of other key data, including epidemiological, clinical, and immunological data.
He proffers the hope that future integration of these variables and increasing use of
metagenomics (analysis of all the DNA of all the microbes recovered in an environmental
sample) will help pathogenomic studies provide crucial insight into viral disease
pathogenesis, prevention, and control.
While complete genome sequences for bacteria and RNA viruses that infect humans have
been generated in abundance, genomic approaches to studying parasitic diseases have
lagged behind. For example, the complete sequence of the genome of Plasmodium
falciparum, the parasite that causes the most deadly form of malaria, was
not published until 2002 (9). That same year, a
full genome sequence for Anopheles gambiae, a particularly important
mosquito vector for the Plasmodium species, was also published (10). As Thomas Wellems, Karen Hayton, and Rick
Fairhurst note in the fifth article in this Review Series (11), it is hoped that these genomic advances will provide insight
into the molecular processes underlying P.falciparum transmission and infection and new avenues to explore to
overcome the difficult challenges of malaria control. However, they also devote
substantial discussion to human genetic polymorphisms, such as that responsible for
sickle-cell hemoglobin, that have been selected for by the life-threatening
complications of infection with P.falciparum.
How human genetics affects the outcome of infection with pathogenic agents is the focus
of the Review by Alexandre Alcaïs, Laurent Abel, and Jean-Laurent Casanova
(12). As Casanova and colleagues point out,
although infectious diseases are thought by many to be solely environmental diseases,
variability in susceptibility to and the clinical manifestations of disease among
individuals in a population who are infected with the same infectious agent indicates
other factors are probably at play. Substantial evidence now indicates that one of these
factors is human genetics and that there are numerous forms of genetic susceptibility to
infectious disease, from those inherited in a monogenic manner to those inherited in a
multigenic fashion. The authors even speculate that “infectious diseases are
largely genetically determined, probably more so than most other human
In the final article in this Review Series (13),
Rino Rappuoli, Kate Seib, and their colleagues discuss how genomic approaches can be
harnessed for vaccine development. Most vaccines currently in use in humans were
developed using conventional culture-based methods. However, the authors argue that the
use of large-scale high-throughput genomic analyses to generate vaccines, an approach
termed reverse vaccinology, will open up the possibility of developing vaccines for
infectious agents that could not be targeted using conventional vaccinology approaches
(13). This approach has been used to develop a
vaccine that is currently in phase III clinical trials against serogroup B
Neisseria meningitidis (MenB), the most common cause of
meningococcal disease in the developed world. The use of other technologies, such as
transcriptomics, proteomics, and structural vaccinology, to complement the genomic
approaches to vaccine development is also highlighted.
Despite the brief amount of time since the sequencing of the complete genome of
H. influenzae (3), it is
already becoming difficult to imagine approaching issues related to infectious diseases
without considering the genomic data now available in abundance, and the articles in
this Review Series highlight some of the questions that have been answered by such data.
However, this is an ongoing process, and the wealth of pathogenomic data has also raised
an immense number of new questions, many of which researchers would not even have been
able to formulate before the ready availability of high-throughput genomic and
microarray technologies. Further technological advancement, such as the recent use of
massively parallel sequencing in picoliter-size reaction vessels to sequence the
complete diploid genome of a single individual, James D. Watson, (14) is likely to produce even more genomic information in the
future, facilitating yet more questions.
How do we move forward? In his Review (8), Holmes
suggests that genomic data must be integrated with other relevant variables to provide
clues to disease pathogenesis, prevention, and control. Assimilating all relevant
information for an individual infectious agent and disease will require enormous
cooperation, and it is hoped that readers working in any discipline will be stimulated
Conflict of interest: The author has declared that no conflict of
Citation for this article:J. Clin. Invest.119:2452–2454 (2009). doi:10.1172/JCI40662
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