Regional FMRP deficits and large repeat expansions into the full mutation range in a new Fragile X premutation mouse model

Abstract

Carriers of FMR1 alleles with 55-200 repeats in the 5' UTR are at risk for Fragile X associated tremor and ataxia syndrome. The cause of the neuropathology is unknown but is thought to be RNAmediated. Maternally transmitted premutation alleles are also at risk of expansion of the repeat tract into the "full mutation" range (>200 repeats). The mechanism responsible for expansion is unknown. Full mutation alleles produce reduced amounts of the FMR1 gene product, FMRP, which leads to Fragile X mental retardation syndrome. We have developed a murine model for Fragile X premutation carriers that recapitulates key features seen in humans including a direct relationship between repeat number and Fmr1 mRNA levels, an inverse relationship with FMRP levels and Purkinje cell dropout that have not been seen in a previously described knock-in mouse model. In addition, these mice also show a differential deficit of FMRP in different parts of the brain that might account for symptoms of the full mutation that are seen in premutation carriers. As in humans, repeat instability is high with expansions predominating and, for the first time in a mouse model, large expansions into the full mutation range are seen that occur within a single generation. Thus, contrary to what was previously thought, mice may be good models not only for the symptoms seen in human carriers of FMR1 premutation alleles but also for understanding the mechanism responsible for repeat expansion, a phenomenon that is responsible for a number of neurological and neurodevelopmental disorders.

Reduced Hippocampal Activation During Recall is Associated with Elevated FMR1 mRNA and Psychiatric Symptoms in Men with the Fragile X Premutation

“…Although it is not immediately clear how this connects to possible dysfunction of the hippocampus in premutation carriers, it is an indication that even young boys show some effect of their premutation status. Such developmental consequences could be caused either by elevated FMR1 mRNA (Tassone et al 2000a, b, c;Jacquemont et al 2003;Oostra and Willemsen 2003;Allen et al 2004;Hagerman and Hagerman 2004) or a mild reduction in FMRP as is known to occur in individuals with the premutation, especially with high CGG repeat numbers (Tassone et al 2000a,b,c;Kenneson et al 2001;Entezam et al 2007). We would expect lower FMRP to result in abnormally reduced pruning during development (Irwin et al 2000;Bagni and Greenough 2005;McKinney et al 2005;Grossman et al 2006) which would in theory result in larger and potentially more active hippocampi.…”

“…During normal fetal development, the hippocampus is one of the areas in which FMR1 transcription is the highest (Abitbol et al 1993) and also demonstrates one of the highest expression rates of FMR1 mRNA in the human brain in adults . In a study of the knock-in mouse model of the premutation (Entezam et al 2007), FMRP expression was significantly reduced in several brain regions, including the hippocampus. In those brain areas sampled in post mortem studies of brain tissue from older premutation males with FXTAS, the hippocampus shows the largest percentage of cells with intranuclear inclusions, again suggesting that this brain region may be particularly affected in FXTAS (Greco et al 2002(Greco et al , 2006.…”

The cyclic AMP phenotype of fragile X and autism

“…An FX knock-in premutation mouse engineered using serial ligation (SL) methodology (Grabczyk and Usdin 1999), rather than with a YAC, has reduced FMRP expression that is associated with repeat length and varies in its extent across the brain. Unlike the YAC knock-in mouse (Brouwer, Mientjes et al 2007), the SL knock-in mouse can also expand its repeat length to a full mutation in one generation (Entezam, Biacsi et al 2007). Premutation knock-in mice could prove useful to model the cognitive decline of memory in the FX premutation and full mutation phenotype.…”

Epigenetics and Neural Developmental Disorders

“…The murine premutation transcript is also poorly translated even though the TSS remains unchanged. 32 It may be that the translation difficulties in both humans and mice result from the stable RNA hairpins that the Usdin group have shown to be formed by the CGG-repeat 38. Dr Usdin's group has also shown that the translation difficulties lead to regional deficiencies of FMRP in the premutation mouse brain.…”

“…Dr Usdin's group has also shown that the translation difficulties lead to regional deficiencies of FMRP in the premutation mouse brain. 32 This may explain why human premutation carriers show some but not all of the symptoms of Fragile X mental retardation syndrome seen in full mutation carriers. Dr. Usdin's group has also shown that CGG-RNA hairpins are substrates for the human Dicer enzyme 38 raising the possibility that the gene silencing seen in full mutation carriers is triggered by the expanded CGG-repeat via some sort of RNA interference-type mechanism.…”