1

Mouse Genetics

Mutagenesis and introduction to obesity mutants

Characteristics

Mutagenesis

Obesity

Mouse CharacteristicsPowerful models of human disease

Large litter size (8)

High re-mating frequency for males (females = 3 weeks)

Hardy and easy to raise

Generation time 2-3 months

Store frozen embryos

Genome sequence completed February 2001

Internal fertilisation and embryos develop in utero

Mouse MutantsOver 1200 mutations catalogued

Mutants obtained through:-

• Mouse fancy societies

• Large scale inbreeding programmes

• Mutagen risk assessment

• F1 screens for dominant mutations

• Directed screens using deletions

Haplo-insufficiency

T/+ - short tail

T/T -no tail, mesodermal defects

Sey/+ - small eyes

Sey/Sey - no eye, no nose, brain defects

In humans mutations in PAX6 are associated with an autosomal dominant disease (ANIRIDIA)

New large scale F1 screens for dominant effects exploitbehavioural assays or medical technology

G0

F1

Females from C3H inbred strain

Mutagenised males fromBALB inbred strain

Screen 40 000 F1 mice for behavioural and other defects

Confirm genetic origin of defects by backcrossing to C3H

Map using polymorphisms between parental strains

2

Screening for eye defects by ophthalmoscope

6000 mice screened - 25 mutants

Genes affected include the collagen Col4a1 the growth factor receptor Egfr and

the homeobox gene Phox2b

Radiation induced mutations often involve deletions

Deletions can be mapped by cytogenetics or by loss of strain specific DNA markers

A 3 MB visible deletion covers a region of Chr 4 containing52 protein coding genes 39 with human homologues

10-15% of the mouse genome is covered by deletions

ENUDeletion stock

Screen F1

F1 screen for directed recessive mutations

Mutations outside the region covered by the deletion willonly cause an F1 phenotype is their effects are dominant

Back crossing to wild type mice allows dominant and recessive effects in the F1 to be distinguished non-agouti

Recessive loss of function alleles(ae) have black coats

Dominant gain of function alleles(Ay) have yellow coats

Ay/+ mice also show obesityImmunological defects and sterility

Obesity mutants

Obesity mutantsObese (ob) and diabetic (db)are two fully recessive mutantswith almost identical phenotypes

Early onset obesityDiabetes

HyperphagiaIncreased metabolic efficiencyDefective thermoregulation

Female sterility

Parabiosis - shared circulationob/ob with wild type

ob/ob lose weight db/db with wild type

Wild type starvedb/db with ob/ob

ob/ob starve

3

The lipostat model

metabolismappetite

circulating factor

Fat secretes a circulating factor that acts on the brain

Fatter mice produce more circulating factor

High levels induce increased metabolic rate and reduced appetite

Fat reserves reduced causing amount of factor produced to fall

ob = factor and db =receptor?ob/ob with wild type

ob/ob lose weight db/db with wild type

Wild type starvedb/db with ob/ob

ob/ob starve

Summary

Mice can model human diseases are amenable to genetic analysis, a complete genome sequence is available for two inbred strains and extensivemutant collections

Systematic F1 screening for mutants is underway

Mutant mice can be used to model weight regulationand obesity in humans

ReferencesMice as modelshttp://genome.wellcome.ac.uk/doc_WTD023552.html

History of mouse geneticshttp://www.informatics.jax.org/silver/chapters/1-2.shtml

Bradley A (2002) Mining the mouse genome Nature 420 pp512-514http://www.nature.com/nature/journal/v420/n6915/full/420512a.html