Journal of APPLIED
BIOMEDICINEISSN 1214-0287 (on-line)
ISSN 1214-021X
(printed)
Volume 6 (2008), No 2
Genetically altered mice, man and medicine
Nirmala Bhogal
Address: N. Bhogal, FRAME, Russell and Burch House,96-98 North Sherwood Street, Nottingham NG14EE, UK
nirmala@frame.org.uk
Received 6th February 2008.
Revised 10th March 2008.
Published online 12th March 2008.
Full text
article (pdf)
Abstract in xml formatSUMMARY
Several million genetically altered mice are used worldwide each year for research, toxicity testing, or
simply to create or sustain mutant models. In fact, as our understanding of the genetic differences between
mice and men improves, so does the drive to create mouse models of human disease and toxicity.
However, while some models have proven to be useful and confer tangible benefits in terms of clinical
management of disease, many others add little value to clinical medicine or human safety emphasising
the need for a thorough investigation of the actual or future value of such models to human medicine.
KEYWORDS
disease; genetically altered; mouse; mutant; welfare
INTRODUCTION
The mouse is the most frequently used laboratory
animal. Historically, mice were considered vermin
and even today countries such as the US actively
exclude mice from the legal protection afforded other
laboratory mammals. If it is also considered that mice
are relatively easy to breed and inexpensive to
maintain in captivity and that most genetic
engineering techniques have been perfected for mice,
it is perhaps not surprising that research and testing in
these animals outstrips that on any other mammalian
species. Couple the latter with the fact mice have
short life spans and produce large numbers of
offspring, means they are suited for genetic
experimentation and toxicity testing whilst avoiding
issues with long term, post-experimental care.
However, the biggest selling point for mouse models
is the fact that the mouse (Mouse Genome
Sequencing Consortium 2002) and human genomes
(International Human Genome Sequencing
Consortium 2001) are of similar sizes, apparently
around 95% syntenic and display reputedly high
functional equivalence.
While the short life-spans of mice and their size
are advantageous, these characteristics equally
confound extrapolation of information from studies
on mice to human disease and medicine. Equally,
although the mouse and human genomes appear at
first glance to be strikingly similar, since a single
nucleotide difference can dramatically alter protein
expression, trafficking, structure or function, the fact
that mouse and human genes are on average only
85% similar is at best problematic and at worst leads
to ill-informed assumptions about drug activity and
safety.
These issues are explored further in the context of
specific diseases to determine where genetically
altered (GA) mouse models have and have not
benefited the study of human disease and human
medicine. Throughout this article, human genes are
given in uppercase and italics and mouse homologues
in lower case and italics.
MOUSE MODELS OF SICKLE CELL
ANAEMIA
Sickle cell anaemia (SCA) is an inherited blood
disorder that affects the shape, haemoglobin content,
half-life and blood vessel traffic of red blood cells.
The symptoms of SCA include pain and
complications such as stroke that can currently be
managed medically using a number of generic
therapeutics. The prospects for developing gene
therapies and enhancing stem cell therapies are being
investigated in mouse models. The latter show
greatest promise although, at present, problems with
reprogramming stem-ness with oncogenic and
retroviral DNA remain unresolved (Hanna et al.
2007).
Ordinarily, 95-98% of the haemoglobin content
is haemoglobin A, which is composed of two alpha and
two beta chains with a smaller proportion of
haemoglobin composed of two alpha with two delta chains
(Haemoglobin A2) or two gamma chains (fetal
haemoglobin). In SCA, the majority of red blood
cells contain haemoglobin S, because of mutations in
the á chain gene (betas mutations). Early mouse models
of SCA were created by replacing the mouse
haemoglobin genes (hbA and hbB) with the
equivalent human genes (HbA and HbB) with betas
mutations. Although expressing mainly haemoglobin
S (Greaves et al. 1990), these mice displayed little
red blood cell sickling that was increased in SAD
mice that carried three point mutations, namely the ás
Antilles beta23Ile and D Punjab beta121Gln mutations in
the HbB gene (Trudel et al. 1991). Although useful
for examining the effects of potential anti-sickling
agents, these mice do not display haemolytic anaemia
that is seen in SCA humans.
The Berkeley mouse has deactivated mouse hbA
and hbB genes and, instead, expresses the full
complement of human SCA globin alpha and beta genes,
including the mutant betas genes (genotype:
Tg[Hu-miniLCRalpha1ggammaAgammadeltabetaS]Hbao//HbaoHbbo//Hbbo).
Even this model only displays some features of the
human disease (Paszty et al. 1997). For instance,
contrary to the increased red blood cell haemoglobin
levels seen in patients, the Berkley mouse exhibits
reduced red blood cell haemoglobin levels. This may
be a result of the fact that mice express very low
levels of fetal haemoglobin, a protein that protects
against depleted haemoglobin levels in human SCA
sufferers. The latter is linked to the different
severities of SCA-related symptoms seen in patients.
Together with the fact that red blood cell volumes are
much smaller in mice than in humans, this
presumably explains why Berkeley mice are prone to
more severe SCA than many patients. Differences in
the main site of haematopoiesis (the spleen in the
Berkeley mouse and the bone-marrow in SCA
patients) might also contribute to species differences
since splenic hypertrophy is apparent in 1-6 month
old mice, whereas in humans, initial hypertrophy is
followed by almost total loss of spleen function
(Manci et al. 2006). Despite these problems, studies
in mouse SCA models have resulted in promising
new therapies, including a new hemoglobin-based
oxygen carrier (HBOC), HRC 101 (Hemosol in
Shichor et al. 2007). This is because the same organs
are affected in mice and humans, albeit to different
extents, and because the therapeutic efficacies of
HBOCs are largely dependent on ADME and being
able to reach the target site.
SPECIES DIFFERENCES ARISING FROM
ALTERNATIVE PATHWAYS AND
FUNCTIONAL NON-EQUIVALENCE:
DIABETES AND CYSTIC FIBROSIS
Human diabetes embraces a cluster of diseases
characterised by an inability to regulate blood
glucose levels. The link between obesity and the
incidence of diabetes has spurred the use of some
mouse models. However, it is clear that models such
as the non-obese diabetic (NOD) mice, the product of
selective breeding and which spontaneously develop
insulin-dependent diabetes, can be used to decipher
the complex pathways associated with the
autoimmune component of type I diabetes. Indeed,
there appears to be considerable similarity between
the roles of the components of the major
histocompatibility complex II and cytokines in the
mouse model and human conditions (Wicker et al.
2005).
A species analogue of Diapep277TM (DeveloGen,
Germany) was first assessed in NOD mice. It was
found to react with heat shock protein 60 on
autoimmune T cells in NOD mice and, as a result,
promote insulin secretion. The underlying mechanism
is proposed to be a result of changes in the types of
cytokines release from T cells from those that
promote to those that counter inflammation (Elias et
al. 1997, Ablamunits et al. 1998). According to the
outcome of phase I and phase II clinical trials,
Daipep277TM promises to be the first peptide
immunomodulatory therapeutic able to tackle the
autoimmune problems presented by diabetes. The
phase III clinical trials data are expected to be
published later this year.
Other alternatives to insulin therapy, such as
immune cell reprogramming therapies are being
investigated in such mouse models. Should these
restorative therapies become available, the need for
long term use of insulin by certain patients might be
dispensed with. However, the cost of such
personalised medicines might be prohibitively
expensive such that research into this area must be
carefully appraised.
Maturity onset diabetes of the young (MODY)
and adult type 2 diabetes are, likewise, multigenic
disorders. As such, several GA mouse models have
been used to studies on individual components of
type 2 diabetes. However, functional non-equivalence
between mice and humans creates problems with
extrapolating between studies in mouse models to
drug development and clinical medicine. For
instance, loss-of-function mutations in the insulin
receptor gene (resulting in poor insulin sensitivity),
combined with the absence of a KATP channel (which
regulates insulin secretion), were insufficient to
trigger diabetes in mice (Kanezaki et al. 2004)
whereas equivalent mutations lead to diabetes in
humans. Similarly, Sur1 knock-out mice carrying
loss-of-function mutations in the SUR1 receptor for
sulphonylureas that trigger closing of KATP channels
(thus reducing insulin secretion) are able to regulate
insulin secretion via a second insulin regulatory
mechanism that is not found in humans (Hiota et al.
2002). This limits the use of GA mice for developing
therapeutics that target KATP channel activity.
Functional non-equivalence is not restricted to
diabetes. In the 1980s, cystic fibrosis (CF) was linked
to mutations in the gene encoding the CF
transmembrane conductance regulator (CFTR; Kerem
et al. 1989) that normally facilitates chloride ion
movement into and out of cells in the lungs and other
organs. Recent evidence suggests that the so-called
mouse CFTR is not a true homologue for the human
protein (Rozmahel et al. 1997). This may go some
way to explaining why studies in mouse models that
attempt to capture features of CF have failed to
generate any tangible improvements in medical
management of this disease.
Around 70% of CF sufferers carry a single
phenylalanine residue in the encoded protein but
about 1300 frameshift and site-specific mutations in
the human CFTR gene or the upstream promoter
regions that have similar health consequences but
with varied onset and severity. In humans, CFTR
gene mutations result in the accumulation of thick,
sticky mucus and other secretions that increase the
individual's susceptibility to infections, impair
cellular secretion and/or transport, and increase
morbidity. Since CF is autosomal and recessive, two
defective copies of the CFTR gene are needed before
the disease manifests itself. Since not all CFTR gene
mutations have the same phenotypic consequences,
and the likelihood that a person would inherit
identical mutant alleles is low whereas mouse models
are created within homozygous mutations and
deletions in mind. Furthermore, knock out mice such
as cftr (-/-) mice do not develop mucus-congested
lungs because mice but instead tend to die mainly as
a result of gastrointestinal obstruction, which is not a
feature of the human disease (Ameen et al. 2000).
Pancreatic disorders and loss of fertility, seen in a
high proportion of CF patients, are also rarely seen in
cftr (-/-) mice.
It now seems CF is likely to be a multigenic
disorder, dependent on the expression and function of
other proteins (Kunzelmann et al. 2002). Hence, after
decades of CFTR-related research newer, more useful
model of airway mucus hypersecretion include a
mouse model overexpressing an epithelial sodium ion
channel under the control of an airway-specific
promoter. This model exhibits increased sodium ion
absorption and higher lung mucous concentrations, as
seen in human CF lung disease (Mall et al. 2004).
Perhaps such recent models hold greater potential for
developing gene therapy and small molecule
interventions but this remains to be seen.
IMMUNODEFICIENCY MODELS
GA mice models of infections caused by pathogens
such as the human poliovirus (Boot et al. 2003) and
used to develop and test vaccines and to bioassay
pathogens, such as BSE (bovine spongiform
encephalopathy; Jenkins and Combes 1999) are
commonly engineered so that they possess humanised
immune systems. The objective is to overcome
problems with various immune cell niches that are
significantly different in mice and humans. Models
include severe combined immune deficiency (SCID)
mice that carry two mutated copies of the SCID gene
(SCID/SCID mice) lack T-cells and B-cells and
SCID/beige mice that also lack natural killer activity.
Total body irradiation of BALB/c mice to destroy the
haematopoietic system, followed by its replacement
by engrafting bone-marrow samples from SCID mice
results in the generation of mouse models capable of
mounting primary and secondary cellular and
humoral immune responses specific to a number of
infections, including hepatitis B, hepatitis C and HIV
(Ayash-Rashkovsky et al. 2005). Such mouse models
are additionally used as recipients for human tumour
cell xenografts (Chang and Zhang 1995). For
example, SCID mice also homozygous for the
urokinase plasminogen activator (uPA-SCID mice)
transplanted with primary human hepatocytes and
infected with hepatitis B virus or hepatitis C virus can
be used as models of human hepatitis infection
(Meuleman et al. 2005).
MOUSE MODELS OF SENSORY FUNCTION
A number of GA mice have been used to study
sensorineural or conductive deafness. The Shaker1
mouse that contains mutations in myosin VIIA gene
(Gibson et al. 1995) displays profound hearing loss,
hyperactivity, head-tossing and circling activity.
Mutations in the equivalent human myosin VIIA
gene (MYO7A) lead to non-syndromic deafness and
Usher's syndrome-associated deafness. However, half
of all Usher's syndrome sufferers also exhibit retinitis
pigmentosa which is not seen in even aged mice. This
discordance may be because of the short lifespan of
the mouse, which prevents the retina from receiving
sufficient exposure to light to elicit pathological
changes (El Amraoui et al. 1996).
Specific mouse models of autosomal dominant
retinitis pigmentosa (RP), nevertheless, exist
including rhodopsin knock-out rho (-/-) mice and
transgenic mice that carry equivalent rhodopsin
mutations to those related to human forms of the
disorder (Sung et al. 1994). Although these models
have proven informative about the visual process
(Dalke and Graw 2005), differences between the
organisation of the mouse and human visual systems,
that are linked to the fact that mice are nocturnal
animals whereas humans are not, complicate
extrapolation of information from studies with these
mouse models to human medicine. For instance, the
retinas of mice possess fewer cone cells (for colour
vision) but a higher density of rod cells (for night
vision) than human retinas and lack fovea that affords
visual acuity to humans. Mice also possess two,
rather than three, visual pigments with one of the two
pigments possessing absorption spectra overlap with
rhodopsin such that this colour pigment can partly
compensate for visual defects that result from
loss-of-function mutations in rhodopsin in a way that
is not possible in the human visual system. This may
explain why mice carrying rhodopsin mutations do
not develop severe retinopathies, as seen in humans
(Jacobs et al. 1991). Nevertheless, research in mouse
models has led to an initial assessment of stem cell
therapies for RP and the realisation that the retina has
regenerative capacity (MacLaren et al. 2006). Such
treatments offer greater hope for patients who at
present are almost exclusively reliant on the use of
vitamin A to delay the onset of RP.
MOUSE MODELS OF DEGENERATIVE
DISEASES
Duchenne Muscular Dystrophy (DMD) is
characterised by early onset muscle degeneration and
affects mainly male children. Not only is this not so
in the DMD mouse model dy/dy2j but also most of
these mice exhibits muscle with rapid onset and die
within 6 months. Even taking into account
differences between the life-spans of mice (2 3
years) and humans (70 90 years), this is too rapid
and, indeed, the muscle membrane appears to be
more extensively damaged in dy/dy2j mice than in
DMD patients. Furthermore, human DMD is thought
to be myogenic in origin while DMD in mice is
believed to have a neurogenic origin. The mdx mouse
mimics the X-linked recessive nature of human DMD
and although this is an improvement over the dy/dy2j
model, this model fails to exhibit many of the
features of the human disease including the fact that
muscle necrosis in young mdx mice is totally reversed
and the mice are essentially disease-free by the age of
5 weeks (Daingain and Vrbova 1984). Studies on the
dko mouse model that lacks expression of dystrophin
and another muscle architectural protein, utrophin,
not only revealed potential problems of using large
DNA for dystrophin gene therapy (Gardner et al.
2006) but have also triggered efforts to develop
alternative therapies. One such treatment option
might include morphilino anti-sense oliionucleotides
that reverse DMD in the mdx mouse (Fletcher et al.
2007). These treatments are currently going through
clinical trials in Europe.
Age-related neurodegenerative disorders, such as
Alzheimer's disease (AD), are generally of slower
onset than DMD. For instance, AD sufferers exhibit
progressive loss of cognitive function and memory
that is thought to relate to the development of
amyloid plaques (APs) and neurofibrillary tangles
(NFTs) in their brains and a loss of neuronal synaptic
density and this can take several decades to manifest.
Traditionally, mouse models were used to examine
the roles of individual AD components, each
displaying some, but not all, of the features of the
human disease. The 3xTg-AD model captures all the
main features of the human disease. Genes encoding
human K670-N/M671-L APP (APPswe) and human
P301L tau proteins were inserted into Thy1.2
cassettes, to drive CNS expression of mutant
preselinin 1 and tau proteins and these constructs
were injected into single cell embryos from PS1-KI
mice containing the homozygous presenilin 1
mutation (PS1M146V). This presenilin 1 mutation
predisposes mice to rapidly developing APs that also
carry the amyloid precursor protein (APP) double
mutant K670-N/M671-L (APPswe) gene (Holcomb
et al. 1998). However, there were no signs of NFTs
in the mice. By contrast, only NFTs were evident in
a transgenic mouse model expressing the human
P301L tau protein (Gotz et al. 2001). In this model
neuron-specific CNS expression was achieved by
placing the expression of P301L tau protein under the
control of a neuron-specific mouse promoter. The
3xTg-AD mice captured features of both these
models to exhibit progressive, brain region-specific
neuropathological features characteristic of the
human disease, with synaptic dysfunction preceding
AP formation, which, in turn, preceded NFT
formation. Furthermore, these mice are not
behaviourally abnormal from birth thus a more
suitable model for what is an age-related disease in
humans. One of the most promising new treatments
of AD, AF267B, was developed using this model and
is currently in phase 1 investigative clinical trials in
the US. Early results show the compound was well
tolerated at tested doses in a group of young, healthy
men although clinical efficacy remains to be
established.
TANGIBLE BENEFITS TO CLINICAL
MEDICINE
Mouse models, as all animal studies, should only be
developed only when there is a realistic prospect for
the development of new medical treatments and
where alternative studies on lower order species and
in vitro systems cannot provide the type of
information that is needed to advance the field of
research in question. In this respect, use of mouse
models to study human congenital and developmental
disorders must be carefully evaluated. Take for
instance, cleft palate. It is the most common form of
congenital bone dysmorphia in humans and develops
within the first trimester of pregnancy. It is not only
multigenic, but also dependent on non-genetic
factors, thus a reliable mouse model is difficult to
develop. Furthermore, while many GA mouse models
have been identified, cleft palate is often seen in
addition to other phenotypes. Homeobox and
regulatory genes have been heavily implicated in
developmental disorders and it is presumably, in part,
due to random and unidentified mutations in these
genes that cleft palate develops. Thus, while such
studies add weight to the supposition that cleft palate
has a complex aetiology, it adds little to in utero
diagnosis or surgical correction of the disorder.
Furthermore, there are severe animal welfare
implications of studying cleft palate in mice, with GA
offspring left unable to feed properly, or survive to
point at which the underlying mechanisms of cleft
deformation can be deciphered. The homozygous
MSX-1 null mutant mouse, for instance lacks
MSX-1 a homeobox gene that is expressed in a
number of developing organs in vertebrates results
in neonates failing to survive, because of severe
craniofacial abnormalities (Satokata and Maas 1994).
Thus, studying the aetiology of the disorder is almost
impossible in the context of a complex mammalian
species and in many respects monitoring general
homeobox gene function in simpler organisms, such
as nematode worms, might be significantly
advantageous as the prelude to studies in mice.
Modelling human Down's syndrome (DS) in
mice might be challenged on a similar basis. DS
exists in three main forms that result in variable
levels of mental retardation and physically evident
features. The most common form results in an extra
copy of chromosome 21 in all the cells of the affected
individual's body (trisomy 21). Mosaic trisomy 21 is
similar, except that the extra chromosome 21 is only
present in some of the cells of the affected individual.
The third form of DS is caused by translocational
errors that result in repeats within chromosome 21.
Trisomy (Ts) mouse models, including the naturally
occurring Ts16 mouse model that carries an extra
copy of mouse chromosome 16 (Epstein et al. 1985),
were first described in the 1970s. However, since the
human chromosome 21 and mouse chromosome 16
are not entirely syntenic and human and mouse
chromosomal numbers and gene arrangements differ,
it is not surprising that Ts16 mice display
developmental cardiovascular defects that are not
evident in human DS (Bacchus et al. 1987). The fact
that Ts mice do not survive beyond birth, prohibits
the study of CNS development and of the age-related
diseases, such as AD, associated with DS. The
Ts65Dn mouse carries an extra copy of the part of
mouse chromosome 16 equivalent to the critical
region of human chromosome 21 (Akeson et al.
2001). These mice survive to adulthood, and display
some of the characteristics of human DS, including
delayed postnatal development, muscular trembling,
male sterility, skeletal malformations and abnormal
cholinergic function but still do not develop AD
(Holtzman et al. 1996, Hunter et al. 2004). Other
attempts to create a mouse model of DS involved the
addition of an almost entire copy of human
chromosome 21 to the mouse genome, to give the
Tc1 mosaic mouse (70) which more closely models
mosaic trisomy 21 in humans in that these mice
exhibit neurodegeneration and premature ageing. The
prospects for treating DS are challenged on the basis
of 1) the ethics of altering the genetic make-up of an
individual 2) human rights-related issues, such as the
right not to be discriminated against on the basis of
being a DS sufferers 3) the lack of technology.
Perhaps in view of these problems, focusing on the
meeting the healthcare needs of DS patients would be
a more prudent investment.
CAN WE MODEL PSYCHIATRIC
DISORDERS IN MICE?
Attempts to model a number of psychiatric disorders
in mice have been made. However, most models
focus mutating or deleting single genes that have a
tentative link to a psychiatric disorder. Indeed, with
clear differences in the organisation of the mouse and
human brains, the relevance of these models is far
from certain. This entire field of research is very
much in its infancy.
A prime example of this is the use of GA mouse
models to understand the etiology of schizophrenia.
Schizophrenia in humans is believed to be a
multigenic, its development can be triggered by
external factors and it can result in a variety of
dysfunctional social behaviours. Relating the
outcome of studies in GM mouse models has not
been particularly successful for these reasons.
Similarly, the failure of the NOTCH 3 gene knock-in
mouse model for stroke and dementia syndrome
(Lundkvist et al. 2004) to demonstrate clinical signs
such as recurring strokes and dementia in adult
patients could be attributed to differences in the
spatial or temporal expression of species gene
homologues, the organisation of the mouse and
human brains or species longevity.
Depression affects a growing number of people
and is one of the most prevalent illnesses in the
world. It is also clear that whilst the first
antidepressants were, in essence, a serendipitous find
in the 1950s, many patients remain unresponsive or
exhibit unacceptable side effects even to more refined
treatments. Mice with knockout or site-specific
mutations of single genes resulting in altered
serotonergic, adrenergic, peptide, glutamate or
immunological central nervous function have been
developed (reviewed in Cryan and Mombereau
2004). A variety of behavioural tests (but note not
tests that measure dependency) form part of the
regulatory requirements for testing new psychoactive
therapeutics. These are often tests conducted in mice
in a piece meal fashion, and it is clear that some
symptoms of human depression, such as weight
changes, are much more easily modelled and
measured in mice than other symptoms, such as
suicidal tendencies. In fact, the high-profile
withdrawals of and post-market restrictions place on
a number of selective serotonin reuptake inhibitors
highlight two major problems with mouse models:
Firstly, Seroxat increases suicidal tendencies in
adolescence but traditional mouse tests for signs of
depression such as the forced swim test fail to
pick this up. Secondly, other common side effects
such as dependency and liver toxicity appear to be
strain and age dependent and the outcome of
preclinical studies, as in the case of Zelmid, can,
therefore, be inconclusive.
CARCINOGENICITY AND MUTAGENICITY
TESTING
A number of GA mouse models now exist for
carcinogenicity testing, including the E -pim-1
transgenic mouse, the rasH2 transgenic mouse, the
p53+/- heterozygous knock-out mouse, and the
Tg.AC transgenic mouse. These models are often
able to detect carcinogens much more sensitively
than genotypically normal strains and, as a
consequence, fewer mice might be required for each
chemical than would ordinarily be required if
genetically normal strains were used. For instance,
the E -pim-1 transgenic mouse, which contains the
activated pim-1 oncogene, develops cancers when
exposed to low levels of mild genotoxic carcinogens
(Kroese et al. 1997) and the rasH2 transgenic mouse
that carries a mutated p21 Ha-ras oncogene, displays
enhanced susceptibility to neoplastic induction
(MacDonald 2004). This often means that genotoxins
and carcinogens can be identified in a shorter study
time frame than by traditional assays (Yamamoto et
al. 1996). Other models, such as the p53+/-
heterozygous knock-out mice (reviewed by
Jacobson-Kram et al. 2004), are useful for detecting
specific types of carcinogens whereas models such as
the Tg.AC (that carries an activated v-Ha-ras
oncogene fused to a mouse globin promoter) is suited
to detecting topically applied chemicals that cause
skin cancers. Although these models have yet to be
formally accepted into the regulatory testing
framework, the US Food and Drug Administration
have suggested how several of these models could be
used in this context.
Similarly, the Organisation for Economic
Cooperation and Development (OECD) is
considering the benefits of using the lacI BigBlueTM
mouse and LacZ Muta mouse transgenic models for
mutagenicity testing (Wahnschaffe et al. 2005). The
mutagenic target of Muta mouse is a bacterial LacZ
gene, and the target of Big Blue is the LacI repressor
gene. Both LacZ and LacI serve as reporter genes:
The LacI repressor binds to the lac operon and
prevents the expression of beta-galactosidase from the
LacZ gene. Hence, mutations in the LacI repressor
gene result in changes in the levels of beta-galactosidase
expression, whereas mutations in the LacZ gene
result in the expression of mutated forms of the
enzyme. This means that exposure to a mutagen is, in
theory, results in mutations which can be detected by
analysing beta-galactosidase activity by expressing the
enzyme from DNA extracted from the tissues of
exposed mice. Tests in these transgenic mice are not,
however, able to detect mutagens that cause
chromosomal aberrations. Hence, a strategy that takes
into account the information which can be reasonably
obtained by using ex vivo and cell-based assays
should be applied before tests are conducted in mice,
in order to minimise in vivo studies, whether or not
they employ GA animals.
The incidence of cancer in humans is linked to
the accumulation or induction of genetic damage or
mutations or with dysfunctional DNA repair. Hence,
the risk of cancer amplified with each round of cell
division. This relationship is less obvious in
laboratory mice with around 30% of mice developing
cancers during their 2-3 year life-spans. One reason
for this could be that mice are relatively deficient in
antineoplastic defence mechanisms (Hanahan and
Weinberg 2000), although higher metabolic rates in
mice than in humans, and species differences in
detoxification undoubtedly contribute to the higher
frequency of cancers in mice than in humans.
Furthermore, most human cancers arise in epithelial
cell layers but this is not usually the case in mice.
Models such as p53 mutant mice do, however,
develop tumours that more closely resemble the
epithelial cancers found in humans (Artandi et al.
2000), presumably because cells from these mice
contain chromosomes with more similar telomeres to
human telomeres, the latter of which appear to be
more susceptible to end-to-end chromatid fusion and
non-reciprocal translocation of chromatid DNA and
the abnormal karyotypes common to human but not
most mouse cancers (Artardi and Jacks 1999).
Indeed, p53 +/- heterozygous mouse and transgenic
have been used to establish a number of cancer
models for drug development and cancer research.
DISCUSSION
There are strong arguments for and against the use of
mouse models in medical research. Indeed, whereas
some models have contributed significantly to the
protection of human health, many other models have
led to a false sense of hope for patients worldwide. In
particular, the inability of many GA mouse models to
recapitulate all the features of a human disease has
often resulted in several mouse models being created
for studies on different aspects of the disease in
question, with the interpretation of the information
from such studies confounded by differences in the
genetic backgrounds of the mice used. This has
potentially slowed, rather than expedited, the
development of new medical treatments. Hence, there
is a desperate need to formally validate or invalidate
GA mouse models in very much the same way as is
required for many non-animal methods used for
regulatory testing.
GA mice might provide a more sensitive way of
detecting chemical toxicants than genetically normal
mice, reducing the number of animals needed per
study. They may also provide a scientifically valid
alternative to studies with severe consequences for
the welfare of higher order laboratory mammals, such
as non-human primates, for instance, where
transgenic TgPVR 21 mice expressing the human
polio virus receptor are used to neurovirulence test
the oral polio vaccine (OPV; Dragunsky et al. 2003).
In these instances, a case for using mouse models can
be made on the basis of animal welfare. However, in
some cases, decades of research in GA mice have
failed to provide any tangible benefit to human
health. The first knockout, transgenic and trisomy
mice were all produced in the 1980s. These included
HPRT (hypoxanthine-guanine phosphoribosyl
transferase) knock-out mouse. HPRT gene mutations
in humans have been associated with Lesch-Nyhan
syndrome (LNS; Mansour et al. 1988). Despite the
early link between the gene and the corresponding
human disease, only some of the symptoms of the
disease can be treated and there is currently no
specific treatment for the neurological symptoms of
LNS. Similarly, the first transgenic mouse contained
a mutated form of the metallothionein-1 (MT-1) gene
homologous to the human gene associated with
Menkes disease (Cox and Palmiter 1892) but despite
over 20 years of research, the main way of managing
this disease is using copper supplements. Equally, in
the case of disorders such as cleft palate and DS,
although GA mouse models have permitted some of
the features of the diseases to be studied, it is not
clear whether this will assist with the management or
prevention of the diseases themselves.
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