RESUMO
The human FMR1 gene contains a CGG repeat in its 5' untranslated region. The repeat length in the normal population is polymorphic (5-55 CGG repeats). Lengths beyond 200 CGGs (full mutation) result in the absence of the FMR1 gene product, FMRP, through abnormal methylation and gene silencing. This causes Fragile X syndrome, the most common inherited form of mental retardation. Elderly carriers of the premutation, defined as a repeat length between 55 and 200 CGGs, can develop a progressive neurodegenerative syndrome: Fragile X-associated tremor/ataxia syndrome (FXTAS). In FXTAS, FMR1 mRNA levels are elevated and it has been hypothesised that FXTAS is caused by a pathogenic RNA gain-of-function mechanism. We have developed a knock in mouse model carrying an expanded CGG repeat (98 repeats), which shows repeat instability and displays biochemical, phenotypic and neuropathological characteristics of FXTAS. Here, we report further repeat instability, up to 230 CGGs. An expansion bias was observed, with the largest expansion being 43 CGG units and the largest contraction 80 CGG repeats. In humans, this length would be considered a full mutation and would be expected to result in gene silencing. Mice carrying long repeats ( approximately 230 CGGs) display elevated mRNA levels and decreased FMRP levels, but absence of abnormal methylation, suggesting that modelling the Fragile X full mutation in mice requires additional repeats or other genetic manipulation.
Assuntos
Modelos Animais de Doenças , Proteína do X Frágil da Deficiência Intelectual/genética , Proteína do X Frágil da Deficiência Intelectual/metabolismo , Síndrome do Cromossomo X Frágil/genética , Camundongos/genética , Sequências Repetidas Terminais/genética , Alelos , Animais , Química Encefálica , Metilação de DNA , Proteína do X Frágil da Deficiência Intelectual/análise , Humanos , Masculino , Camundongos Knockout , Mutação , Biossíntese de Proteínas , RNA Mensageiro/análise , RNA Mensageiro/metabolismo , Transcrição GênicaRESUMO
Fragile X syndrome is one of 14 trinucleotide repeat diseases. It arises due to expansion of a CGG repeat which is present in the 5'-untranslated region of the FMR1 gene, disruption of which leads to mental retardation. The mechanisms involved in trinucleotide repeat expansion are poorly understood and to date, transgenic mouse models containing transgenic expanded CGG repeats have failed to reproduce the instability seen in humans. As both cis-acting factors and the genomic context of the CGG repeat are thought to play a role in expansion, we have now generated a knock-in mouse Fmr1 gene in which the murine (CGG)8 repeat has been exchanged with a human (CGG)98 repeat. Unlike other CGG transgenic models, this model shows moderate CGG repeat instability upon both in maternal and paternal transmission. This model will now enable us to study the timing and the mechanism of repeat expansion in mice.
Assuntos
Síndrome do Cromossomo X Frágil/genética , Proteínas do Tecido Nervoso/genética , Regiões Promotoras Genéticas , Proteínas de Ligação a RNA , Repetições de Trinucleotídeos , Alelos , Animais , Modelos Animais de Doenças , Eletroporação , Feminino , Proteína do X Frágil da Deficiência Intelectual , Amplificação de Genes , Humanos , Masculino , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Knockout , Camundongos Transgênicos , Polimorfismo Genético , Células-TroncoAssuntos
Mutação , Polidactilia/genética , Sindactilia/genética , Dedos do Pé/anormalidades , Animais , Cruzamentos Genéticos , Feminino , Heterozigoto , Homozigoto , Deformidades Congênitas dos Membros/genética , Masculino , Camundongos , Camundongos Endogâmicos C3H , Fenótipo , Recombinação GenéticaRESUMO
Fragile X syndrome is caused by an expansion of the CGG repeat present in the 5' UTR of the FMR1 gene. A lot has been elucidated about the genetics of the disease, but not much is known about the mechanisms involved in repeat instability. Transgenic animals with a premutation allele [(CGG)11AGG(CGG)60CAG(CGG)8] in the human FMR1 promoter were generated to study the inheritance of this repeat in mice. Three independent lines, B6, B7 and B29, in total 263 transgenic animals, were tested for repeat instability. In all meiosis and mitosis tested, the repeat inherited stably. This suggests that other factors might be important in repeat (in)stability.
