Mouse mutagenesisMouse mutagenesis

Roberta Rivi, MDRoberta Rivi, MDLaboratory of Molecular Embryology Laboratory of Molecular Embryology

Mutagenesis?Mutagenesis?

• Goal: determination of the function of every Goal: determination of the function of every gene (gene (functional genomicsfunctional genomics))

• Gene-drivenGene-driven– also called also called reverse geneticsreverse genetics

• Phenotype-drivenPhenotype-driven– also called also called forward geneticsforward genetics (classical (classical

approach)approach)

Mutagenesis strategiesMutagenesis strategies

Mutagenesisstrategy (type)

Mutagenesisfrequency Type of mutation induced Primary advantages Primary disadvantages

Spontaneous 5 x 10-6 per locus Point mutations, small deletions, chromosomalrearrangements, and insertions of endogenousretrovirus-like sequences.

Vis ible phenotypes; only requirementis observant mouse handlers.

Only visible phenotypes detected, atvery low frequency.

X-ray 13Š50 x 10-5 perlocus

Chromosomal rearrangements: ranging fromsimple deletions, inversions and translocations, tocomplex rearrangements.

Rearrangements act as a molecularlandmark for cloning.

Multiple genes affected, hard todissect individual gene function.

Chlorambucil (chemicalmutagen)

127 x10-5 perlocus

Chromosomal rearrangements, especially smallerdeletions (100Š500 kb) and translocations.

Same as X-ray, but highermutagenesis frequency.

Multiple genes affected, hard todissect individual gene function.

Ethylnitrosourea(chemical mutagen)

150 x 10-5 perlocus

Primarily generates point mutations, occasionallyvery small deletions (20Š50 bp).

Single-gene mutations, amenable tohigh throughput.

No molecular landmarks for cloning.

Transgene/retroviral (insertionalmutagen)

5Š10% oftransgenicanimals

Disrupts endogenous gene expression or codingsequence. Sometimes causes chromosomalrearrangements.

Provides a molecular landmark forcloning.

Labour-intensive, not applicable tohigh-throughput approaches.

Gene targeting(insertional mutagen)

Almost 100% oftransgenicanimals*

Generates insertions or deletions, as designed. Can design type of mutation asrequired.

Requires knowledge of gene and itsstructure, labour-intensive,unpredictable phenotypes.

Trapping (insertionalmutagen)

Almost 100% oftransgenicanimals*

Disrupts endogenous coding sequence. Forward-genetic strategy, easy toclone mutated gene, reportsendogenous gene-expression pattern.

Unpredictable phenotypes.

* Requires pre-screening ofembryonic stem cells in vitro.

Insertional mutagensInsertional mutagens

Gene targeting

Generates insertions or deletions, as designed.

Can design type of mutation as required.

Requires knowledge of gene and its structure, labor-intensive, unpredictable phenotypes.

Trapping Disrupts endogenous coding sequence.

Forward-genetic strategy, easy to clone mutated gene, reports endogenous gene-expression pattern.

Unpredictable phenotypes.

Homologous recombination

HSV-tk

Homology Homology armarm

Homology Homology armarm

Modified from http://cancer.ucsd.edu/tgm/genetargeting.asp

Conditional?

• Early lethality does not allow to study the role of a gene in organogenesis

Conditional?

• Early lethality does not allow to study the role of a gene in organogenesis

• Tissue-specificity of gene ablation is dependent upon the expression of Cre

Conditional?

• Early lethality does not allow to study the role of a gene in organogenesis

• Tissue-specificity of gene ablation is dependent upon the expression of Cre– One contruct x many Cre lines

Phage P1: Cre-loxP system

http://bioweb.wku.edu/courses/biol22000/16Recombination/Fig.html

Insertional mutagensInsertional mutagens

Gene targeting

Generates insertions or deletions, as designed.

Can design type of mutation as required.

Requires knowledge of gene and its structure, labour-intensive, unpredictable phenotypes.

Trapping Disrupts endogenous coding sequence.

Forward-genetic strategy, easy to clone mutated gene, reports endogenous gene-expression pattern.

Unpredictable phenotypes.

Gene-trapGene-trap

Gene-trapGene-trap

Gene-trapGene-trap

Phenotype driven mutagenesis: chemical Phenotype driven mutagenesis: chemical mutagenesismutagenesis

Spontaneous 5 x 10-6 per locus

Point mutations, small deletions, chromosomal rearrangements, and insertions of endogenous retrovirus-like sequences.

Visible phenotypes; only requirement is observant mouse handlers.

Only visible phenotypes detected, at very low frequency.

X-ray 13–50 x 10-5 per locus

Chromosomal rearrangements: ranging from simple deletions, inversions and translocations, to complex rearrangements.

Rearrangements act as a molecular landmark for cloning.

Multiple genes affected, hard to dissect individual gene function.

Chlorambucil 127 x10-5 per locus

Chromosomal rearrangements, especially smaller deletions (100–500 kb) and translocations.

Same as X-ray, but higher mutagenesis frequency.

Multiple genes affected, hard to dissect individual gene function.

Ethyl-Nitroso-Urea

150 x 10-5 per locus

Primarily generates point mutations, occasionally very small deletions (20–50 bp).

Single-gene mutations, amenable to high throughput.

No molecular landmarks for cloning.

N-ethyl-N-nitrosourea (ENU)N-ethyl-N-nitrosourea (ENU)

• We are using the chemical N-ethyl-N-nitrosourea (ENU) to saturate wild type chromosomes with point mutations.

• By determining the function of genes on a mouse chromosome, we can extrapolate to predict function on a human chromosome.

• We expect many of the new mutants to represent models of human diseases such as birth defects, patterning defects, growth and endocrine defects, neurological anomalies, and blood defects.

Chemical mutagenesisChemical mutagenesis

Modified from RIKEN (The Institute of Physical and Chemical Research)Modified from RIKEN (The Institute of Physical and Chemical Research)http://www.gsc.riken.go.jp/Mouse/AboutUs/overview.htmhttp://www.gsc.riken.go.jp/Mouse/AboutUs/overview.htm

Why Use ENU as a Mutagen Why Use ENU as a Mutagen 

• ENU is an alkylating agent that is a powerful mutagen in mouse spermatogonial stem cells, producing single locus mutation frequencies of 6 X 10-3 to 1.5 x 10-3, equivalent to obtaining a mutation in a single gene of choice in one obtaining a mutation in a single gene of choice in one out of every 175 to 655 gametes screenedout of every 175 to 655 gametes screened.

• Because it is a point mutagen, ENU can induce many different types of alleles. Loss of function mutations, viable hypomorphs of lethal complementation groups, antimorphs, and gain-of function mutations have been isolated in mouse mutagenesis screens.

• Missense changes are a common finding in many human disease mutations, therefore the ENU mutations will complement and extend the information provided by targeted gene disruptions.

Screening for phenotypes Screening for phenotypes

MappingMapping

446.4 unaffected 44.6 affected

Line 04/004 LICIA - MATT

(e16.5)

Smaller embryoSevere microcephaly and micrognathiaAbnormal limb morphology & orientation exhibits also a general defect in Endochondral Ossification (all long bones affecteD)

Line 04/004Line 04/004 LICIA - MATTLICIA - MATT 131.5 131.6

Alizarin Red: stains BoneAlcian Blue: stains Cartilage

Line 04/014Line 04/014 LICIA - GIUSEPPINA - BETTALICIA - GIUSEPPINA - BETTA

mx

mb

tc

unaffected affected

429.1

430.3

mx

mb

tchy

Rudimentary or hypoplastic mx (maxilla) and mb (mandible - jaw)Absent hyoidRudimentary tracheal cartilage

Alizarin Red: stains BoneAlcian Blue: stains Cartilage

Mermaid (merm)Mermaid (merm)

Pax3

Sna

ilPa

x3

S

nail