STOCK Fxn^tm1Mkn Tg(FXN)YG8Pook/2J

Stock number: 024113
Product name: YG8sR
Research areas: Neurobiology Research

Description

This human FXN YAC transgenic mouse model has the mouse frataxin knockout allele (Fxn^-) and the human FXN YAC transgene small repeat YG8s (contracted integration to a single copy of the human FXN gene with GAA trinucleotide repeats [~250-300]). Mice homozygous for the knockout and hemizygous for the YG8s transgene, called YG8sR mice, are rescued from knockout lethality and have transgene expression that models the phenotype of Friedreich's Ataxia (FRDA). Compared to other human FXN YAC transgenic rescue mouse models, the YG8sR model expresses a contracted human frataxin, resulting in much greater FXN deficiency. As such, the donating investigator concludes that YG8sR can be considered the most suitable YAC transgenic GAA repeat-based mouse model for the investigation of potential therapies for FRDA.

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Detailed description

This human FXN YAC transgenic mouse model has the mouse frataxin knockout allele (Fxn^-) and the YG8 small repeat human FXN YAC transgene (YG8s). The YG8s transgene has a contraction of the original YG8 transgene integration to a single copy. Like other GAA repeat mouse models, the YG8s transgene exhibits somatic GAA repeat instability. Upon arrival at The Jackson Laboratory in 2014, the transgene contained ~200 GAA repeats. Repeats are now in the 250-300 range (Jan 2017).

Compared to other human FXN YAC transgenic rescue mouse models, the YG8sR model expresses a contracted human frataxin, resulting in much greater FXN deficiency. As such, the donating investigator concludes that YG8sR can be considered the most suitable YAC transgenic GAA repeat-based mouse model for the investigation of potential therapies for Friedreich's Ataxia (FRDA).

Mice that are heterozygous for Fxn^- and hemizygous for the YG8s transgene are viable and fertile with normal lifespan. When singly homozygous for the frataxin knockout allele, mice are embryonic lethal. Mice homozygous for the knockout (Fxn^-/-) and hemizygous for the YG8s transgene, called YG8 small repeat human FXN YAC transgenic rescue mice (or YG8sR mice), exhibit an age-dependent GAA repeat expansion accumulation in the CNS (particularly cerebellum), progressive decrease in motor coordination and significant functional locomotor deficits; all similar to the human pathology of FRDA. YG8sR mice also exhibit glucose intolerance and insulin hypersensitivity. Of note, YG8sR mice have a progressive gain in weight compared to C57BL6/J mice (which may affect rotarod performance).

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 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

This human FXN YAC transgenic mouse model has the mouse frataxin knockout allele (Fxn^+/-) and the YG8 small repeat human FXN YAC transgene (YG8s). When homozygous for the knockout and hemizygous for the transgene, these mice are called YG8 small repeat transgenic rescue mice (or YG8sR). Specific details below.

The frataxin knockout allele (Fxn^tm1Mkn) was designed by Drs. Michel Koenig and Helene Puccio to disrupt exon 4 and flanking sequences of the frataxin locus on chromosome 19 with a loxP-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 wildtype C57BL/6J mice. Heterozygous mice (Fxn^+/-) were used as described below. C57BL/6J-congenic frataxin knockout mice are described and available from The Jackson Laboratory as Stock No. 016842.

The YG8 human FXN YAC transgene was created by Dr. Mark A. Pook by first obtaining the 370 kbp YAC (Y37FA12) that includes the entire human FXN gene with nine GAA repeats [(GAA)₉] at the mutation locus within intron 1 of FXN. The GAA repeat expansion was modified to contain (GAA)₁₉₀. The modified YAC transgene was injected into the pronuclei of (C57BL/6 x CBA)F1 fertilized oocytes. Founder animals were bred to wildtype C57BL/6J mice. Founder line YG8 was established and backcrossed to wildtype C57BL/6J mice. YG8 mice initially contained an expansion of (GAA)_90-190, but was subsequently bred to contain (GAA)_120-220. YG8 mice have two tandem copies of the transgene integrated on mouse chromosome 16. Dr. Pook bred YG8 and Fxn^+/- mice together to generate the YG8R human FXN YAC transgenic mouse model (see Stock No. 012253). In 2012, Dr. Pook bred his colony of YG8R with Y47R control transgenic mice (Fxn^+/- mice containing the same human FXN YAC transgene with normal-sized (GAA)₉ repeats; see Stock No. 024097). The resulting animals contained a contraction of the transgene integration to a single copy with (GAA)₁₂₀; these animals were called YG8 small repeat human FXN YAC transgenic rescue mice (YG8sR). Like other GAA repeat mouse models, the YG8sR mice exhibit somatic GAA repeat instability. The GAA repeat size was expanded by selective breeding. In 2014, Dr. Pook sent YG8sR animals with ~200 GAA repeats on a "mixed-but-predominantly C57BL/6J" genetic background to The Jackson Laboratory as Stock No. 024113. Upon arrival, sperm was cryopreserved. To establish our living colony, an aliquot of the frozen sperm was used to fertilize oocytes from Fxn^+/- females (Stock No. 016842). In December 2014, a high density genome-wide scan showed that our live colony was 91-94% congenic on the C57BL/6J background. Please inquire about the current GAA repeat values.

Allele

Symbol: Tg(FXN)YG8Pook
Name: transgene insertion YG8, Mark A Pook
MGI accession ID: MGI:3797739
Generation method: Transgenic
Attributes: Inserted expressed sequence; Humanized sequence
Marker symbol: Tg(FXN)YG8Pook
Marker name: transgene insertion YG8, Mark A Pook
Chromosome: UN
Strain of origin: CBA x C57BL/6
Expressed genes: FXN,frataxin,human
Site of expression: The human FXN YAC transgene contains the promoter and coding regions of human FXN with GAA trinucleotide sequence repeats. Individual strains at The Jackson Laboratory have specific variants of this this transgene. See Detailed Description section for greater detail of transgene insertion site, copy number, GAA repeat values and expression levels.
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 YG8 was established, carrying 2 tandem copies of the human FXN gene with two GAA trinucleotide repeat sequences of 82 and 190 repeats. This line displayed a transmission rate of 52% to offspring.
General note: Line YG22 exhibits slightly greater age-related repeat instability, 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.