Simon J. Foote
is the Joint Head of the
Genetics and Bioinformatics
Division at The Walter and
Eliza Hall Institute of Medical
Research and Director of the
Menzies Research Institute,
Hobart.
Controlling malaria and
African trypanosomiasis: The
role of the mouse
Simon J. Foote, Fuad Iraqi and Stephen J. Kemp
Fuad Iraqi
is a scientist at the International
Livestock Research Institute,
Nairobi, Kenya.
Date received (in revised form): 20th July 2005
Stephen J. Kemp
is Professor of Molecular
Genetics at the University of
Liverpool, UK, and visiting
scientist at the International
Livestock Research Institute,
Nairobi, Kenya.
Malaria and trypanosomiasis are vector-borne protozoal diseases which disproportionately
affect the poor. Both give rise to immense human suffering; malaria exerts its effect directly on
human health, while trypanosomiasis causes damage largely though its effect on the health and
productivity of the livestock on which so many poor people depend. These diseases both have
multifaceted and poorly understood mechanisms of pathogenesis, combined with relatively
complex life cycles characterised by multiple stages in both insect vector and mammalian host.
In both cases, there is a dramatic effect of host genotype on disease progression. This effect is
apparent in both the human and cattle hosts and among inbred mouse strains. This provides an
opportunity to use the mouse to probe the mechanisms underlying resistance or susceptibility
to pathology. The availability of high-density linkage maps, the genome sequence and
transcriptomics tools has transformed the power of the mouse to illuminate such fundamental
aspects of the host–parasite interaction.
Keywords: trypanosomiasis
trypanosomosis, malaria,
trypanotolerance, genetic
resistance, susceptibility,
plasmodium, trypanosoma
Abstract
INTRODUCTION
Simon J. Foote,
Menzies Research Institute,
17 Liverpool St,
Hobart, Tasmania 7000,
Australia.
Tel: +61 3 6226 7703
Fax: +61 3 6226 7704
E-mail: [email protected]
The mouse has become an essential tool
for studying biological processes in
mammals. Of particular importance in the
study of the host response to infectious
disease is understanding why some
naturally occurring genotypes are
dramatically more sensitive to disease than
are others. How far the existing mouse
strains are true models of resistance and
sensitivity in humans or cattle is
debatable. The ways in which these strains
differ in terms of disease progression will
be discussed below; however, they
unquestionably provide an opportunity to
identify host factors that influence disease
progression. In this respect, the naturally
occurring variants are quite different from
N-ethyl-N-nitrosourea (ENU) mutants,
knockouts or transgenics, which
invariably involve single genes with
catastrophic loss (or gain) of function.
Malaria
The genetics of the malaria parasite and its
host have been extensively studied in
murine systems. A mapping experiment
using the human parasite requires the use
of mosquitoes to infect chimpanzees with
Plasmodium falciparum gametocytes.
Murine malarias mean that it is possible to
work entirely in the mouse — a far more
tractable and cost-effective model. Such
studies have focused on increasing our
understanding of the parasite, including
the development of drug resistance,
antigenic variation and erythrocytic
invasion pathways. Mouse models have
also been extensively used to dissect the
host response to disease, including the
innate and adaptive immune responses.
Malaria is endemic in more than 90
countries, comprising most of the
developing world. It is estimated that
malaria, caused by the haemoprotozoan
parasite Plasmodium, infects 300–500
million people worldwide, and kills an
estimated 2–3 million people annually —
mainly young children and pregnant
women. In spite of the one-time hope
that malaria would be eradicated, this
disease is still the world’s most common
and severe parasitic disease of man. Vector
control and drug treatment have been the
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Controlling malaria and African trypanosomiasis: The role of the mouse
Genetic variations in
humans protect against
severe malaria
Mutations in the Dhfr
gene protect both
human and mouse
malarias from the antimalarial drug
pyrimethamine
major defences against the disease, but
environmental concerns and vector and
parasite resistance have strongly curtailed
the effectiveness of this strategy. Despite a
large international effort, antimalarial
vaccines have not been produced. It is
therefore critical that a vaccine is
developed and ‘resistance-proof’
antimalarial drugs are found. Such an
effort would be greatly facilitated by
improvement of our understanding of the
mechanisms of host resistance and
immunity to malaria. It is still not known
why children die from malaria, when
their siblings and peers are infected by the
same strains and yet exhibit milder
symptoms and survive. There are obvious
genetic differences in populations from
endemic areas, and perhaps these
mutations — plus others yet to be found
— offer protection to the 99 per cent of
children who survive childhood malaria.
Mouse strains offer a valid model of
human malaria. There are a number of
different parasites infecting the laboratory
mouse, and while none of these behaves
like P. falciparum, the deadly strain of
human malaria, they do model parts of the
disease. There are also significant interstrain variations which have been
exploited to study host responses.
Therefore, the mouse model offers a
powerful tool for dissecting and
understanding the host response
mechanism to malaria, and through
comparative genomics it will be possible to
identify the orthologous genes in humans.
Murine malaria parasites
The unequivocal malaria parasites of
murine rodents fall into four groups:
P. berghei, P. yoelii, P. vinckei and
P. chabaudi. These four species are the
most commonly used in the mouse
model. The blood stages of the berghei
group are morphologically
indistinguishable and are notable for their
marked predilection for immature
erythrocytes in the blood of
experimentally infected laboratory mice.
This characteristic allows parasites of this
group (P. berghei and P. yoelii sspp.) to be
readily distinguished from P. vinckei sspp.
and P. chabaudi sspp.1,2 The P. berghei
parasite is thought to be a monomorphic
species.3 The P.c. chabaudi (P.c.) species
comprises two subspecies, P.c. chabaudi
and P.c. adami. P.c. chabadi has been found
in only two locations in Africa.1,2 A full
comparison of all of the stages of the life
cycles of the two subspecies has not been
made, and it is therefore not known if
there are morphological differences
between P.c. chabaudi and P.c. adami. The
P. vinckei species comprises four
subspecies with minor morphological
differences.1,4
The genetics of drug resistance
Some of the early work on drug resistance
was carried out in murine parasites.
Pyrimethamine-resistant parasites have
been selected from all murine malaria
species — that is, P. berghei,5,6 P. yoelii,7
P. chabaudi8 and P. vinckei.9 Genomewide scan studies in crosses between
pyrimethamine-sensitive and -resistant
lines of P. yoelii and P. chabaudi,
respectively, showed that the resistant
phenotype appeared to be controlled by a
single locus.7,8 Subsequently, a mutation
in the Dhfr gene was identified as a
probable candidate gene underlying
pyrimethamine resistance in
P. falciparum.10
Given that all P. yoelii isolates examined
so far are innately chloroquine resistant,11
P. chabaudi parasites have been selected for
resistance to chloroquine — until recently
the most widely used of all antimalarial
compounds. It has been shown that the
level of resistance in the chloroquineresistant clone is probably due to an
accumulation of mutations at different
loci, each contributing a low level of
resistance.12 No mutation was found in
the murine parasite, however; the
mechanism of resistance to this drug was
elucidated through studying the human
parasite itself. With current technology
and tools, and with the availability of the
genome sequence,13 it will be possible to
identify the drug-resistant genes in
P. falciparum.
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Foote, Iraqi and Kemp
Different murine
malarias show different
diseases in the mouse
P. chabaudi has also been used to study
resistance to sulphonamides.
Sulphonamides are a family of drugs
including sulfadoxine, which inhibits the
enzyme dihydropteroate synthase. The
combination of sulphadoxine and
pyrimethamine (known as S/P or
Fansidar) has been shown to be more
effective than either drug used alone,
and this has become the most widely
used treatment for malaria after
chloroquine. Resistance to S/P appeared
in P. falciparum within a few years of its
introduction into the field.
Sulphonamide-resistant parasites have
also been identified in P. berghei.14 In
general, sulfadiazine-resistant parasites
have a reduced requirement for paminobenzoic acid (PABA), which
suggests that resistant parasites could
have acquired the ability to synthesise
PABA de novo.14,15 Again, however, the
mutations giving rise to sulphonamide
resistance were first identified in the
human malarias.
Host innate immune response
to murine malarias
Inbred mouse strains also differ markedly
in their susceptibility to Plasmodium
infection.16 Parasites such as P. berghei and
P. vinckei, and some P. yoelii and P.
chabaudi strains, can cause lethal infections
in mice. There are, however, substantial
differences in the timing of death. For
example, mice susceptible to the cerebral
malarial syndrome caused by P. berghei die
early on in infection and mice resistant to
the cerebral effects die later from severe
anaemia. Other isolates, including
P. yoelii, P.c. chabaudi and P. vinckei petteri,
cause infections in most strains of mice
and may kill the mouse through severe
acute anaemia or may resolve after an
initial parasitaemia and are then either
completely eliminated (in the case of
P. yoelii) or have smaller patent
recrudescences for several months (as in
the cases of P.c. chabaudi and P. vinckei
petteri).17
For P.c. chabaudi it has been shown that
the genetic background of the host
controls the outcome of infection, with
blood-stage infection in inbred mice
following either a lethal or a non-lethal
course in a host strain-dependent
manner.18,19 P. chabaudi is similar to
P. falciparum in that it usual infects
normocytes, undergoes partial
sequestration (although predominantly in
the liver and not in the brain) and, in
resistant strains of laboratory mice,
recovery from the acute primary
parasitaemia is followed by one or more
patent recrudescences.19–21
Table 1 summarises the current
collective information about the
susceptibility of different inbred mouse
strains after challenge with P. chabaudi
(http://www.informatics.jax.org/).
Table 1: Susceptibility of different inbred mouse to malaria22 and
trypanosome infection.23
The genetic background
dictates either survival
or death in the P.
chabaudi model of
murine malaria
Mouse strain
Plasmodium
chabaudi
Plasmodium
berghei
Trypanosoma
congolense
C57BL/6
CBA/J
DBA/2J
C57L/J
SJL/J
DBA/1J
A/HeJ
AKR/J
BALB/c
A/J
C3H/HeJ
C58/J
ST/Bj
Resistant
Resistant
Resistant
Susceptible
Susceptible
Resistance
Susceptible
Resistant
Resistant
Susceptible
Susceptible
Susceptible
Susceptible
Susceptible
Susceptible
Susceptible
Susceptible
Susceptible
Susceptible
Susceptible
Susceptible
Intermediate
Susceptible
Susceptible
Intermediate
Resistant
Susceptible
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Controlling malaria and African trypanosomiasis: The role of the mouse
Many murine genetic
loci control outcome to
infection in the P.
chabaudi model
Mutations in a murine
pyruvate kinase allele
protect mice from
death by the P. chabaudi
parasite
Differences in susceptibility are
enormous. For example, infection of Mus
spretus by P. chabaudi has virtually no
identifiable clinical effect and the
parasitaemias are extremely low; however,
infecting the molossinus strain MOLF/Ei
kills the mice within one week of
infection, about three to four days before
other susceptible strains.
Strain variations in clinical outcome
have been used to map quantitative trait
loci (QTL) for host response in
P. chabaudi. The first reports were
published in 1997 by two separate teams
— Foote and colleagues23 and Fortin and
colleagues.24 Foote and colleagues23 used
two F2 resource populations in their
genome-wide search for resistance QTL,
which were developed by crossing one
resistant strain, C57Bl/6, with two
susceptible strains, SJL and C3H/He. This
study found a locus on chromosome 9
(Mmu9) in both crosses, while an
additional locus mapped to chromosome
8 (Mmu8) in the C57BL/6 3 C3H/He
cross only. The two loci were shown to
be associated with control of both host
survival and peak parasitaemia. These loci
have been named char1 (Mmu9) and char2
(Mmu8) for P. chabaudi resistance loci.
Fortin and colleagues24 used a
backcross (BC) strategy to map host
response QTL and, in a C57Bl/6
(resistant) by A/J (susceptible) cross,
identified two loci, mapping to
chromosomes 8 (Mmu8) and 5 (Mmu5).
The Mmu8 QTL overlapped with that
reported by Foote and colleagues.23
Furthermore, suggestive QTL mapped to
chromosomes 15 (Mmu15) and 17
(Mmu17).
An additional study by Burt and
colleagues25 reported a significant QTL
on chromosome 17 (Mmu17) which they
named char3. This QTL was confirmed to
be associated with clearance of parasites
from the circulation later in infection.
Female mice were used in these crosses
because they are more resistant to P.
chabaudi infection than are males.26 Char3
mapped to the proximal end of Mmu17.
The most well-known candidate region
underlying this locus is the mouse major
histocompatibility (MHC) locus, the H2
complex. The MHC locus is involved in
many, if not all, host response loci to
infectious disease and has been implicated
in a differential response to malaria
through association of the tumour
necrosis factor-alpha gene with the
response to malaria in humans.27
Recently, Fortin and colleagues28 have
reported a host response QTL in
P. chabaudi infection. This QTL mapped
to chromosome 3 (Mmu3) and was
designated char4. It was found to be
associated with peak parasitaemia. A
second suggestive linkage on
chromosome 10 (Mmu10) shows an
additive effect with char4 on peak
parasitaemia on a segment of
approximately 10 centimorgans on
chromosome 3. The founders of the F2
resource population in this study were of
a recombinant congenic strain (AcB55),
which was shown to be highly resistant to
malaria infection, and the susceptible
(A/J) strain. The AcB55 strain was
derived from AcB/BcA recombinant
congenic strains, bred from malariasusceptible A/J and resistant C57BL/6
mice. Despite lower parasitaemia, AcB55
and another congenic strain, AcB61,
showed significant pathology
(splenomegaly, a low number of
erythrocytes and a high reticulocyte
concentration) when compared with the
parental strains.29,30 Sequence analyses of
char4 candidate genes revealed a mutant
liver and red cell-specific pyruvate kinase
allele (Pklr 269A ). Mutations in the gene
encoding pyruvate kinase in humans can
lead to haemolytic anaemia. It is not
obvious how this mutation in mice leads
to resistance to malaria, and work is
ongoing to solve this issue.
Two major studies have been carried
out to fine-map the location of char2: the
production of char2-congenic mice25 and
the use of advanced intercross lines;31
however, none of these studies have
achieved the high resolution necessary to
clone and identify the genetic factors that
underline the variant. This might be due
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P. berghei produces a
cerebral-like malaria
syndrome in mice
Response to berghei and
yoelii malarias is also
under multigenic
control
to the existence of several genes that
interact with each other to control
parasitic levels. An AIL F11 population31
was also used to resolve char3 into two
linked loci (char3 and char7) and to
confirm the char5 and char6 loci,
previously introduced as suggestive loci,
controlling parasitaemia on chromosome
5. Finally, a new locus identified after
genetic analyses of the AIL F11
population, char8, was linked to
parasitaemia control and was located on
chromosome 11.32 This region shares
homology with human chromosome
5q31-q33, which was linked to the
control of P. falciparum blood infection
levels in an independent study.33 Thus,
the same locus seemed to play a role in
the control of parasite levels in both hosts
and it is possible that a common resistance
gene exists in both species.
Cerebral malaria is a major cause of
morbidity and mortality in Africa,
especially in children.34 It involves an
immunopathophysiological process
caused by P. falciparum in humans and
P. berghei ANKA in rodents. It is
characterised pathologically in humans as
a dense sequestration of parasitised
erythrocytes in cerebral blood vessels.35–37
Inbred mouse strains respond differently
to cerebral malaria after infection with P.
berghei.38–40 This is summarised in Table 1
(from http://www.informatics.jax.org/).
Furthermore, Bagot and colleagues41 have
recently reported the development of new
inbred mouse strains derived from wild
stock and found to be resistant to P.
berghei infection.
Recently, Nagayasu and colleagues42
published the first results of a genomewide scan for resistant QTL to P. berghei
infection using an F2 intercross
population generated from moderate
resistant, DBA/2, and susceptible,
C57BL/6, mouse strains. They showed a
single significant QTL associated with
resistance on chromosome 18 (Mmu18).
Additional suggestive QTL were also
reported on chromosomes 5 (Mmu5), 8
(Mmu8) and 14 (Mmu14). Sex-specific
QTL were seen when the sexes were
analysed separately. Male mice showed a
significant linkage on chromosome 18
(Mmu18), and suggestive linkage on
chromosomes 11 (Mmu11) and 19
(Mmu19). Female mice showed a
suggestive QTL on chromosomes 18
(Mmu18) and 5 (Mmu5). This report
demonstrated that female mice were more
susceptible than males. This is in contrast
to results with P .chabaudi infection. Four
loci have been reported, in three different
studies, to be involved in the
development of murine cerebral malaria
induced by P. berghei ANKA.42–44
Infection with the erythrocytic stage of
the reticulocytotropic rodent malaria
P. yoelii leads to generally non-lethal
infection from which mice recover and
are subsequently resistant to rechallenge.45
Accordingly, levels of susceptibility are
evaluated by the degree of parasitaemia
and anaemia, rather than lethality. Mouse
strains C57BL/6, BALB/c and SWR/J
develop greater parasitaemia and anaemia
after infection than do AKR/J or DBA/
2J.46 Several studies have suggested that
susceptibility to infection by P. yoelii is
under a multigenic control mechanisms
and that genes which influence P. yoelii
infection do not influence the outcome of
infection by other parasite strains.46,47 In
an early study, it was shown that the
BALB/c.D2 congenic mouse strain
(congenic for the mtv-7 locus, located on
the distal region of chromosome 1)
exhibits an altered susceptibility
phenotype to the parental BALB/c line,
suggesting that this locus is involved in
resistance to P .yoelii infection.48
Ohno and colleagues49 have reported
the results of a genome-wide search for
resistance QTL to P. yoelii infection.
Their study was conducted in a BC
resource population. The BC population
was generated by crossing the F1 of the
susceptible mouse strain NC/Jic and the
resistant strain 129/SvJ, with NC/Jic
([NC/Jic 3 129/SvJ] 3 NC/Jic). The
results showed a single QTL, controlling
host survival and peak parasitaemia,
mapping to chromosome 9 (Mmu9),
within the locus of char1. These authors
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Controlling malaria and African trypanosomiasis: The role of the mouse
Resistance to disease by
T. congolense in cattle is
strain dependent
proposed that this QTL may represent a
ubiquitous locus controlling susceptibility
to murine malaria.
Table 2 summarises all the significant
genetic loci linked to the host response to
P. chabaudi, P. yoelii and P. berghei
parasites.
TRYPANOSOMIASIS
Trypanosomiasis is endemic in a 7 million
km2 area of Africa and is one of the major
constraints to livestock productivity, with
60 million cattle at risk of infection.50
Although humans are directly affected by
some species of trypanosome, it may be
argued that people are most severely
affected by the trypanosomes which infect
their livestock, and it is here that research
on innate resistance mechanisms has been
focused. The disease in cattle is largely
due to the tsetse fly-transmitted
Trypanosoma congolense and costs an
estimated $1.3bn per annum.51 The
biology of trypanosomiasis has been
studied in some detail in mice and cattle;
however, the systemic nature of this
extracellular pathogen has made it difficult
to identify the mechanisms responsible for
the control of the infection. Critically,
although the parasite can be maintained in
culture, there is no satisfactory in vitro
model of the response to infection.
While most breeds of cattle are highly
susceptible to trypanosomiasis, it has been
known for many years that certain breeds
of domestic ruminants show a remarkable
resistance to the consequences of
infection. This has been termed
‘trypanotolerance’ because the host
tolerates the presence of parasites while
apparently controlling levels of
parasitaemia and, critically, without
showing the severe anaemia and reduced
growth rate characteristic of infection in
susceptible hosts.52 A unique F2 resource
population derived from trypanotolerant
N’Dama bulls crossed with susceptible
Boran cows revealed nine QTL
influencing the outcome of infection with
T. congolense, with a confidence such that
more than 90 per cent will have real
effects.53
This cattle-based study was extremely
expensive, however, and identified QTL
with a relatively low resolution.
Nevertheless, trypanotolerance research
Table 2: Significant genetic loci linked to P. chabaudi, P. yoelii and P. berghei resistance. Adapted from Hernandez et al.64
Murine parasites
Phenotype
Locus
Murine
chromosome
Candidate genes
References
Plasmodium chabaudi adami DS
Plasmodium yoelii 17XL
P. c. adami DS
Plasmodium chabaudi chabaudi AS
P. c. chabaudi 54X
P. c. adami DS
P. c. chabaudi 54X
P. c. chabaudi AS
Parasitaemia
9
(9)
8
Transferrin (Trf),
retinol-binding proteins (Rbp1 and Rbp2)
Glycophorin A (GypA), chondroitin sulphate
proteoglycan 3 (Cspg3)
23,44
Parasitaemia
char1
(pymr)
char2
Parasitaemia
char3
17
24,31
Parasitaemia
char4
3
P. c. chabaudi 54X
P. c. chabaudi 54X
Parasitaemia
Parasitaemia
char5
char6
5
5
P. c. chabaudi 54X
Parasitaemia
char7
17
P. c. chabaudi 54X
Parasitaemia
char8
11
Plasmodium berghei ANKA
Cerebral malaria
18
P. berghei ANKA
P. berghei ANKA
P. berghei ANKA
Cerebral malaria berr1
Cerebral malaria berr2
Cerebral malariac cmsc
1
11
17
Tumour necrosis factor-alpha (Tnf), lymphotoxinalpha (Lta), lymphotoxin-beta (Ltb)
Lymphoid enhancer binding factor 1 (Lef1),
complement component factor I (Cfi)
Acetylcholinesterase (Ache), erythropoietin (Epo)
Correlation in cytokine production 1 (Cora1),
NADPH oxidase subunit (Ncf1)
Complement component 3 (C3), interleukin 25
(Il25)
Interleukin 4 (Il4), granulocyte macrophage
colony-stimulating factor 2 (Csf2)
Colony-stimulating factor 1 receptor (Csf1r),
Cd14 antigen (Cd14)
Transforming growth factor beta2 (Tgfb2)
Tumour necrosis factor-alpha (Tnf), lymphotoxinalpha (Lta)
23,24,31,65
28
31
31
31
32
42
40
40
44
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Mice show strain
dependent responses to
trypanosomiasis
Advanced intercross
lines can accurately
map response loci
has, from the start, gained value by
comparison with the mouse system, and
this synergy continues to grow as the
mouse becomes an increasingly powerful
model for functional genomics. The
various inbred strains of mice show a
range of responses to trypanosome
infection.54
Trypanotolerance resistance QTL were
first identified in mice by Kemp et al.,55
and this is probably the first reported
localisation of QTL determining resistance
to an infectious disease by genome
scanning. This study was conducted in two
separate F2 intercross populations. One
was a cross between resistant C57BL/6J
(B6) and susceptible BALB/c (B) strains
the other between A/J (A) and B6 strains.
Mice were challenged with T. congolense
(which is the most important trypanosome
infecting livestock) and QTL mapping was
based on a single trait — survival time.
This study revealed three QTL influencing
survival time in both crosses, which were
termed Tir1, -2 and -3.
Subsequently, the use of advanced
intercross generations of mice56 allowed
significantly improved mapping
resolution due to the increased number of
recombination events. Again, B6 3 A and
B6 3 B intercrosses were used
independently, but this time as an F6.
This led to a minimum 95 per cent
confidence interval of around 1
centimorgan for the QTL of largest effect
(Tir1) and also resolved Tir 3 into three
separate QTL.57,58
Clapcott et al.59 also showed evidence
that Tir1 (on Chr17) shows a parent-oforigin effect. In this study, four backcross
populations derived from the C57BL/6J
and BALB/c parental strains were
challenged with T. congolense. The two
populations with F1 fathers and BALB/c
mothers had a significantly higher overall
survival rate than the two populations
with BALB/c fathers and F1 mothers.
Analysis of these populations revealed all
the expected QTL, except that the Tir1
QTL was essentially absent in the
backcross in which the resistant allele was
inherited from the female F1.
The murine mapping data may now be
integrated with the bovine mapping data
in order to illuminate both systems. In
doing this, however, the critical unknown
factor is how far the mouse is a ‘model’ of
the cow, or indeed human, system. There
are important differences between murine
and bovine tolerance; resistant cattle
normally show little, if any, disease and
remain healthy and productive despite the
presence of parasites. By contrast, even
‘resistant’ mouse strains usually succumb
eventually, although they have mean
survival times very much longer than
‘susceptible’ strains. It is clear, therefore,
that there are important differences
between the systems; however, the
assumption is that any mechanism which
allows one mouse to survive twice as long
as another mouse is worthy of
investigation. It was certainly not
expected that the same genes would
underlie QTL in different species, but it is
expected that an understanding of
mechanisms modulating disease response
in one system will be informative in
another species.
WHERE ARE WE NOW, AND
WHERE ARE WE HEADING,
WITH THE USE OF MURINE
MODELS?
At face value, the genetic use of mouse
models of malaria and trypanosomes have
not told us an enormous amount about the
biology of the host response to malaria.
Only one gene has been described which
mediates the host response to malaria,
although there have been a number of
response QTL identified and there is hope
that these will teach us something about
the host response to disease. This
statement must, however, be seen in the
more general context of mapping QTL in
the mouse. There has been a very low rate
of conversions of QTL identification to
gene identification. This is because,
despite the publication of the sequence of
the mouse genome,60 narrowing the
intervals is still arduous and many
identified QTL are often genetically
complex.
2 2 0 & HENRY STEWART PUBLICATIONS 1473-9550. B R I E F I N G S I N F U N C T I O N A L G E N O M I C S A N D P R O T E O M I C S . VOL 4. NO 3. 214–224. NOVEMBER 2005
Controlling malaria and African trypanosomiasis: The role of the mouse
Congenic lines and
microarray analysis may
aid in the identification
of host-response genes
Host-directed therapies
may mimic the effect of
genetic resistance in
future therapeutics
There is, however, hope on the
horizon. There seems to be only a limited
number of haplotypes in the laboratory
strains of mice and, in each strain, these
haplotypes occupy a reasonably sizeable
stretch of DNA. It is therefore possible to
deduce the resistant and susceptible
haplotypes by examining multiple mouse
strains. The question of locus
heterogeneity will still complicate this
approach. Other approaches to mapping
QTL include large numbers of advanced
recombinant congenic lines in which
recombinant intervals will be small and
the large numbers of lines will enable
accurate localisation of the QTL.
In the more immediate future, many
groups which have been generating
congenic lines for the known QTL will
be presenting their results, and the results
from an examination of the biology of
these lines will also become available.
These could give some interesting insights
into associated phenotypes ahead of the
identification of the underlying genes.
The development of microarray
technology, with the spotting of mouse
genes onto chips, is presently being used
to follow changes in the expression of all
mouse genes as a response to infection. In
particular, it is possible to contrast the
response of resistant and susceptible
genotypes. It is hoped that this will
illuminate the differences between the
cascade of events in the two systems and
determine where genes within the QTL
participate in such pathways.
Following completion of the
P. falciparum and T. brucei genome
sequence and their imminent annotation
and publication,13,61 now seems an
appropriate time to study the switching
on and off of genes in host and parasite
following an infection. This information
will open the doors for understanding the
communication which must occur
between host and parasite. Without
doubt, the new era of functional
genomics and proteomics will provide a
great opportunity for researchers to
understand and rescue these diseases.
Another interesting technique which
has received much interest recently is
the use of ENU mutagenesis62 to
generate genome-wide mutations and to
screen for these mutations using a
phenotype-based screen. This has not
yet been published for an infectious
disease, but a slight change to the
normal protocol of looking for new
phenotypes would be to find suppressor
mutations using a lethal malarial
infection as the index phenotype. This
would identify novel mutations which
would be able to prevent the lethality of
infection. The genes mutated in such a
screen would then be ideal candidates
for host-directed therapy. Such therapy
would mimic nature’s own response, the
generation of mutations in the human
genome which are common in endemic
regions and which protect against
disease, and which, in many cases, have
done so successfully for many millennia.
Genes identified through the ENU
mutagenesis would be targets for
pharmacological intervention and would
be refractory to the development of
resistance by the parasite because they
would not be under the genetic control
of the parasite.
Finally, the steady rise in the number
of mapped QTL in mice, and in other
species, is beginning to provide useful
additional information. There are
frequent examples of QTL affecting a
range of traits which have overlapping
locations. For example, a QTL
associated with resistance to
Leishmania63 overlaps the trypanosometolerance QTL Tir1 on Chr17. QTL
associated with many physiological and
immune functions are also being mapped
and many are likely to represent the
same underlying polymorphism. There
remains a great deal of difficulty in the
analysis of this information, however,
because there is no consistency in the
way in which it is reported and stored
on databases, although a number of
studies are now addressing this issue and
it is likely that tools for systematic
alignment of multiple QTL are
imminent.
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Foote, Iraqi and Kemp
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