B6.Cg-Fxn^tm1Mkn Tg(FXN)YG22Pook/J

Stock number: 012910
Product name: YG22R
Research areas: Mouse/Human Gene Homologs, Neurobiology Research, Sensorineural Research

Description

This human FXN YAC transgenic mouse model has the mouse frataxin knockout allele (Fxn^-) and the FXN YAC transgene from founder YG22 (carrying a single copy of the human FXN gene with ~190 GAA trinucleotide sequence repeats). Mice homozygous for the knockout and hemizygous for the YG22 transgene, called YG22R mice, are rescued from knockout lethality and have transgene expression that models the phenotype of Friedreich's Ataxia (FRDA).

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Detailed description

Importation of this model was supported by the Friedreich's Ataxia Research Alliance (FARA).

The YG22 transgenic founder line carries a single copy of the human FXN gene with one GAA trinucleotide repeat sequence of 190 repeats. High levels of human FXN gene product (mRNA and protein) are detected by RT-PCR and Western blot analysis. Approximately 40-50% of the endogenous mouse Fxn gene product (protein) is detected by Western blot analysis in mice heterozygous for the Fxn^- knockout allele alone. Mice homozygous for the knockout allele and hemizygous for the YG22 transgene, called YG22R mice, are rescued from knockout lethality and have transgene expression that results in an age-dependent, tissue-specific expansion of the GAA repeat, with expansion accumulation observed in the CNS (particularly cerebellum), similar to the human pathology of Friedreich's Ataxia. The GAA triplet repeat exhibits intergenerational instability. These bi-allelic mice also exhibit progressive retinal degeneration, impaired and decreased locomotor activity and coordination, and an increase in body weight. At 9 months of age, muscle strength is decreased. Although, histological analysis reveals progressive vacuolation of the dorsal root ganglia, no abnormalities or loss of cerebellar Purkinje cells were detected. Iron deposition in cardiac tissue of 14-18 month old mice is observed. Dorsal root ganglia neurons contain lipofuscin deposits. Large axons are swollen and exhibit secondary demyelination. Cardiomyocytes contain lipofuscin deposits, free glycogen accumulation and disrupted mitochondria. Mitochondrial respiratory chain function is decreased and oxidized protein levels are increased.

In an attempt to offer alleles on well-characterized or multiple genetic backgrounds, alleles are frequently moved to a genetic background different from that on which an allele was first characterized. This is the case for the strain above. It should be noted that the phenotype could vary from that originally described. We will modify the strain description if necessary as published results become available.

It should be noted that the YAC transgenic FRDA mouse models available from The Jackson Laboratory (YG22R, YG8R and YG8sR) may have different GAA repeat sizes than the animals used in the publications described below.
Publications from Dr. Mark A. Pook (Brunel University London) in 2014-2015 examine the correlation between the FRDA-like pathological phenotype and frataxin-deficiency in the YAC transgenic FRDA mouse models YG22R (two copies of the FRDA transgene; 190 GAA repeats expanded to ~170-260), YG8R (two copies of the FRDA transgene; 90-190 GAA repeats expanded to ~120-220) and YG8sR (a contraction of the transgene to a single copy; ~120-150 GAA repeats) compared to Y47R control mice (one copy of the FRDA transgene with a stable 9 GAA repeat) and C57BL6/J wildtype mice. The results described below are all in comparison to control mice.
The three YAC transgenic FRDA mouse models (YG8R, YG22R and YG8sR) had a progressive decrease in the motor coordination; the degree of impairment was most significant in YG8R mice. All three models exhibited GAA repeat somatic instability in the brain, cerebellum and liver, as well as exhibited glucose intolerance and insulin hypersensitivity. The greatest FXN deficiency of the three models tested was in YG8sR. Expression levels of the transgenes in various tissues are reported in the Pook publications.

Development

These double mutant mice, heterozygous for the targeted mutation and hemizygous for the transgene, were generated by crossing transgenic mice (founder line YG22) with Fxn^tm1Mkn targeted mutant mice.

Drs. Michel Koenig and Helene Puccio generated the Fxn targeted mutation allele by disrupting exon 4 and flanking sequences with a loxP site flanked PGK-neo cassette. The construct was electroporated into 129/Sv derived embryonic stem (ES) cells. Correctly targeted ES cells were injected into C57BL/6J blastocysts. The resulting chimeric animals were crossed to C57BL/6J mice, and then backcrossed to C57BL/6J for five generations. The mice carry the floxed selection cassette.

The transgenic construct consisting of a 370kb YAC insert that includes the entire human FXN gene with GAA triplet repeat sequences, was injected into fertilized CBA X C57BL/6 hybrid fertilized oocytes. Founder line YG22 was established, carrying a single copy of the human FXN gene with one GAA trinucleotide repeat sequence of 190 repeats. Founder animals were bred to wildtype C57BL/6J mice.

The two lines were then crossed to generate the double mutant. The Donating Investigator breeds mice that are heterozygous for the Fxn^tm1Mkn targeted mutation and hemizygous for the YG22 transgene to mice that are heterozygous for the Fxn^tm1Mkn targeted mutation to obtain mice homozygous for the Fxn^tm1Mkn targeted mutation and hemizygous for the YG22 transgene.
The mice were then backcrossed to C57BL/6J for at least five generations.

Allele

Symbol: Tg(FXN)YG22Pook
Name: transgene insertion YG22, Mark A Pook
MGI accession ID: MGI:4412181
Generation method: Transgenic
Attributes: Inserted expressed sequence; Humanized sequence
Marker symbol: Tg(FXN)YG22Pook
Marker name: transgene insertion YG22, Mark A Pook
Chromosome: UN
Strain of origin: CBA x C57BL/6
Expressed genes: FXN,frataxin,human
Site of expression: The YG22 human FXN YAC transgene contains a single copy of the entire human FXN gene with one GAA trinucleotide repeat sequence of 190 repeats. High levels of human FXN gene product (mRNA or protein) are detected by RT-PCR and western blot analysis.
Molecular note: The transgenic construct consists of a 370kb YAC insert that includes the entire human FXN gene with GAA triplet repeat sequences, and was injected into fertilized CBA X C57BL/6 hybrid fertilized oocytes. Founder line YG22 was generated, showing a reduced transmission rate of 21%, indicating germ-line mosaicism of the transgene, compared to 52% transmission to offspring for founder line YG8. One copy of the FXN gene was detected, and contains a single 190 GAA repeat sequence.
General note: Line YG22 exhibits slightly greater age-related repeat instability than line YG8, with a bias toward repeat expansion.

Allele

Symbol: Fxn^tm1Mkn
Name: targeted mutation 1, Michel Koenig
MGI accession ID: MGI:2177162
Generation method: Targeted
Attributes: Null/Knockout
Marker symbol: Fxn
Marker name: frataxin
Chromosome: 19
Strain of origin: 129/Sv
Expressed genes: Fxn,frataxin,mouse, laboratory
Molecular note: A loxP-flanked PGK-neomycin resistance cassette replaced a genomic DNA fragment containing exon 4, which is highly conserved and often mutated in humans. An additional line was also produced in which the loxP flanked neomycin cassette was removed by Cre mediated recombination, but no distinction was made between these alleles in the original reference. From J:90098: The presence of a human FRDA transgene in hemizygous form in a Frda^tm1Mkn homozygous null background rescues the embryonic lethal phenotype and complements for the loss of endogenous mouse frataxin.