Mouse Model: Pros and Cons

Ankita Das

Chetana Tamadaddi

Prashant Waiker

Indian Institute of Science Education & Research

Bhopal

MOUSE : Mammalian model organism

GenomeMammalian20 chromosomes2.6 Gb~25000 genes99% have human counterpart

StrainsInbredOutbredRecombinant inbredConsomicFluorescent

Life Cycle4-day oestrus20-day gestation4-8 pups per litter2-8 litters per female7 weeks to sexual maturity2-3 year lifespan

Reverse geneticsKnockoutsTransfenicsConditional expressionInducible expressionRetroviral vectorssiRNA

The Mouse as an Experimental System

Assisted reproductionCryopreservationEmbryo rederivationIn vitro fertilizationIntracytoplasmic sperm injectionCloning

ToolsGenome sequenceEmbryonic stem cellsExpression arraysGene-trap libraries Insertional vector librariesBAC libraries

Basic questions to be asked

1. The mouse model is the one most commonly used for studies of human physiology

and disease, but is it the best one?

2. How useful and valid are these models for mimicking human disease?

3. What characterises mouse models that have proven useful for basic science,

clinical research and drug discovery?

4. What measures are needed to improve new and existing mouse models?

Out of the model organisms which may be

genetically modified, the mouse is: •Best model for mammalian development

•Closely related to humans (mammals)

•10-15 offspring per litter and approximately one litter every month.

•Genome sequenced

•Many inbred strains characterized (450 available)

•Genetic manipulations well developed

•Mice are small, have a short generation time and an accelerated

lifespan (one mouse year equals about 30 human years)

Phylogenetic analysis

Not so far Indeed they are

Genomics

99% of mouse genes

have an equivalent in

humans making mice

ideal for the studying

the functions human

genes in health as

well as in diseases.

For significant genetic

changes, mouse

orthologs were close

to random in

matching to their

human counterparts

i.E the E-Value is

higher for the

alignment

Human

Mouse

Life cycleFast Slow

EmbryologyHUMAN MOUSE

• Gestation: Mouse: 18 days; Human:

270 Days

• Different hormonal regulation in

development of embryos

• Human: hCG; mouse: mPRL

• Human : Pituitary gland secretion not

required

Hum. Reprod. Update (2003) 9 (6):531-

539.

Development

Manipulations at the embryo level

• Allows genetic manipulation of embryo possible

• Early embryo can be split to yield two “twins”

• Two morulas can be combined to form a chimera

• Cells from an embryo can be injected into another blastocyst to form a chimera

• Embryonic manipulation difficult inside the mother

• Extra-uterine embryo culture is difficult.

• Need to cut open the uterus - ethical issue

• Similarity

Genes responsible for building and

operating both organs are 90% identical--

which means that the mouse brain can be

a powerful tool for unravelling the mystery

of human mental disorders, memory and

intelligence related phenomenon.

• Difference

Human Vs Mouse Brain

Smooth surface-less memory

More convulsions –higher order

memory

Glia appear to play a more significant role in various brain functions

70% neurons +30% glia 30% neuron+70% glia

Proportion of Neuron and Glial cells varies between Mouse and Humans

Transgenic Mouse

• Advantages :

• Contain extra genetic material integrated into the genome in every cell.

• Generated to carry cloned oncogenes provide good model for human cancer

• Good models for muscle growth study by overexpressing the transgene insulin-like Growth factor in differentiated muscle fibers.

• Disadvantages:

• Transgene integration is apparently random ;

• Many experiments reveal that the genetic surrounding of the inserted transgenic construct is modulating the expression pattern of the transgene itself both qualitatively and quantitatively.

• Transgenic rescue of knockout mice is time-consuming, expensive and labour intensive.

Knock-out mouse

Disadvantages:

• About 15% of Knock-outs are developmentally lethal. This limits studies to only embryonic development.

• It is difficult to determine a gene’s function in relation to human diseases.

• Custom knock-out mice is very expensive ($3000 -$30,000)

Advantages :

• Very specific endogenous gene has been altered in such a way that interferes with normal expression.

• To study effects of gene products, biochemical pathways, alternative (compensatory) pathways, and developmental pathways

• Models to test the beneficial effects of drugs or gene therapy

Behavioural studies

Disadvantages :• Lab Environment Vs Natural environment

• Animals lack self- consciousness, self-reflection and

consideration

• Hallmarks of behavioural disorder such as

Depressed mood, Low-self esteem, suicidal

tendency are hardly accessible in mice

• Hence behaviours like Depression, Autism, suicidal

tendency can not be studied in mouse models.

(wikipedia)

Advantages :• Mouse model shares many Features with

human brain functions like Anxiety, Hunger,

circadian rhythm, memory and other emotional

responses (Peter van Meer and Jacob Raber)

• Robust phenotypes in mouse models -effective

treatments for components of Autism spectuam

disorders (Jill L. Silverman et.al 2010)

• Effects of alcoholism

Neurodegenerative disorders

1. Mice live only about two years, while people

can live for 80 years or more.

2. Neurodegenrative disorders are normally late

onset disorders.

3. Hence not a good model to study parkinson’s

and alzheimer’s disease and other diseases of

aging.

4. Incapable of expressing some cognitive

human disease symptoms.

Cancer Model

Drug Target Studies

Few more Limitations:1. Early acting mutant phenotype difficult to study

2. Poor models to study inflammation in humans, a condition present in many

humans

3. Research mice are inbred and do not capture the genetic variation existing in

human population.

4.The research works have found that the responses in mice correlated poorly not

with those in humans but also with one another.

5.Drugs that have shown promise in mice have done poorly in humans.

6.Forward genetics : To identify novel gees involved in embryogenesis is difficult as

it is prohibitively expensive.

Poor models to study inflammation in humans, a condition present in many

humans

• Mice are resistant to infections and inflammations.

• Mice have lived for millennia in environments

teaming with microbes and they short gestation

periods and large litters. This shows they have

evolved with different strategies for dealing with

infections

Research mice are inbred and do not capture the genetic variation existing in

human population

• The research works have found that the responses in mice correlated poorly not with those in humans but also with one another.

Proc Natl Acad Sci U S A. Feb 26, 2013; 110(9): 3507–3512.

Mice trials did not translated to human!!!

Cost-ineffectiveness of mouse models:

http://www.ohsu.edu/xd/research/research-cores/transgenics/services-costs.cfm

Humanised mouse

A mouse's liver reacts differently to drugs as compared to the liver of

humans.

This makes it difficult to predict whether or not the potential drug being

tested will be toxic in humans.

Various types of human cells and tissues are

engrafted and function, as they would in

humans are considered extremely useful in

basic and applied human disease research

DRUG FACTORY FOR FUTURE

Few more advantages of mouse model

Brown Norway Rat

• Similarities to human sensitization

• Serotonergic bronchoconstriction

• IgE mediated

• Early and late phase reactions

• Airway hyperreactivity

• Tissue + BAL accumulation of neutrophils, eosinophils,

lymphocytes

• Good inflammatory responses

Concluding remarks

Just as no one human is representative of an entire population of humans, no one

strain of mouse is representative of all mice or much less, humans.

Although each of these systems can provide valuable insight, as researchers, we

know that the models are just that: models.

The only perfect model of human disease would be humans themselves, but

performing many kinds of research on humans is an ethical and practical

impossibility.

But, Thanks to animal research, primarily in mice, cancer survival rates have continued

to rise.

Herceptin – a humanised mouse protein – has helped to increase the survival rate

of those with breast cancer; it could not have been attained without animal

research in mice.

While Fleming discovered penicillin without using animals, he shared the Nobel

Prize with Florey and Chain who, by testing it on mice, discovered how penicillin

could be used to fight infections inside the body.

http://www.understandinganimalresearch.org.uk/about-us/the-science-action-network/forty-reasons-why-we-need-animals-in-research/

Alternatives for Human models other than mouse/ any

model organism

Human, inducible pluripotent stem cells can also elucidate new

mechanisms for understanding cancer and cell regeneration.

Imaging studies (such as MRI or PET scans) enable non-invasive study of

human subjects.

Recent advances in genetics and genomics can identify disease-associated

genes, which can be targeted for therapies.

THANK YOU