Regional FMRP deficits and large repeat expansions into the full mutation range in a new Fragile X premutation mouse model
Introduction
The 5' UTR of the human Fragile X mental retardation 1 (FMR1) gene contains a CGG·CCG-repeat tract whose size varies from 5 to > 2000 repeats. Individuals with 55–200 repeats are said to be carriers of Fragile X premutation alleles. They are at risk for Fragile X associated tremor and ataxia syndrome (FXTAS) (Hagerman and Hagerman, 2004) and premature ovarian failure (Sherman, 2000). Premutation alleles produce elevated levels of FMR1 mRNA and this RNA is thought to have toxic effects (Jin et al., 2003, Handa et al., 2005). Premutation alleles are also prone to expand, with female carriers being at risk of having children with alleles with > 200 repeats. These carriers of so-called full mutation alleles are likely to have Fragile X mental retardation syndrome (FXS) (Verkerk et al., 1991, Yu et al., 1991). The symptoms of FXS result from a deficiency of the FMR1 gene product, Fragile X mental retardation protein (FMRP), which arises due to a combination of repeat-induced promoter silencing and difficulties in translating mRNA with large numbers of repeats (Feng et al., 1995). There is some overlap of symptoms in premutation and full mutation carriers probably because the negative effect of the repeat on translation is apparent even in the premutation range and results in a significant decrease in the level of FMRP (Kenneson et al., 2001, Primerano et al., 2002, Tassone et al., 2004).
The mechanism of expansion is not known and large expansions from a premutation-sized allele to an allele in the full mutation size range have not been previously described in mice. This has led to the idea that expansions do not occur in these animals and thus that they are not good models for studying the expansion mechanism. We describe here Knock-in (KI) mice we have generated that share key features of human premutation carriers not seen in a previous KI mouse model (Bontekoe et al., 2001, Willemsen et al., 2003). These animals also provide the first examples in mice of large repeat expansions that transform a premutation-sized allele into a full mutation sized allele in a single generation.
Section snippets
Generation of the Fragile X premutation KI mice
Mice were maintained in accordance with the guidelines of the NIDDK Animal Care and Use Committee and with the Guide for the Care and Use of Laboratory Animals (NIH publication no. 85-23, revised 1996). The outline of the strategy used to generate the targeting vector is shown in Fig. 1 and described in more detail in the figure legend. Briefly, the mouse Fmr1 gene was identified in a BAC clone (BAC 16411) derived from 129/SvJ mouse embryonic stem (ES) cells (Genome Systems, Inc, St. Louis, MO,
Generation of Fragile X premutation mice
Unlike the previous fragile X premutation mouse model which was generated by replacement of a region including the endogenous murine repeat tract with one derived from a yeast artificial chromosome (YAC) containing a human premutation allele (Bontekoe et al., 2001), the repeat tract in the premutation mice described here were generated by serial ligation of short, stable CGG·CCG-repeat tracts as previously described (Grabczyk and Usdin, 1999). The premutation-sized repeat tract was added as the
Acknowledgements
We would like to thank Lisa Garrett in the NHGRI Embryonic Stem Cell and Transgenic Core Facility, and Sonia Farmer and Maria Jorge in the NIDDK mouse facility. This research was supported in part by the Intramural Research Program of the NIDDK and NHGRI (NIH).
References (30)
- et al.
Elevated Fmr1 mRNA levels and reduced protein expression in a mouse model with an unmethylated Fragile X full mutation Experimental
Cell Research
(2007) Microarray identification of FMRP-associated brain mRNAs and altered mRNA translational profiles in fragile X syndrome
Cell
(2001)- et al.
Fibroblast growth factor receptor 3 is a negative regulator of bone growth
Cell
(1996) Variation of the CGG repeat at the fragile X site results in genetic instability: resolution of the Sherman paradox
Cell
(1991)- et al.
Generation of microgram quantities of trinucleotide repeat tracts of defined length, interspersion pattern, and orientation
Anal. Biochem.
(1999) Long CGG-repeat tracts are toxic to human cells: implications for carriers of Fragile X premutation alleles
FEBS Lett.
(2005)- et al.
Detecting steroidal effects on immediate early gene expression in the hypothalamus
Neuroprotocols
(1992) RNA-mediated neurodegeneration caused by the fragile X premutation rCGG repeats in Drosophila
Neuron
(2003)- et al.
Long uninterrupted CGG repeats within the first exon of the human FMR1 gene are not intrinsically unstable in transgenic mice
Genomics
(1998) - et al.
Epigenetic and genotype-specific effects on the stability of de novo imposed methylation patterns in transgenic mice
J. Biol. Chem.
(2000)
Elevated levels of FMR1 mRNA in carrier males: a new mechanism of involvement in the fragile-X syndrome
Am. J. Hum. Genet.
Identification of a gene (FMR-1) containing a CGG repeat coincident with a breakpoint cluster region exhibiting length variation in fragile X syndrome
Cell
Clinical features of boys with fragile X premutations and intermediate alleles
Am. J. Med. Genet. B Neuropsychiatr. Genet.
Instability of a (CGG)98 repeat in the Fmr1 promoter
Hum. Mol. Genet.
Fmr1 knockout mice: a model to study fragile X mental retardation
Cell
Cited by (0)
- 1
Current address: Department of Surgery, Georgetown University, Washington DC, 20057, United States.
- 2
Current address: Department of Genetics, Louisiana State University Health Sciences Center, New Orleans, LA 70112, United States.
- 3
Current address: Department of Medicine, UCSF, San Francisco, CA 94943-0794, United States.