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Altered glutamatergic and dopaminergic signaling has been proposed as contributing to the specific striatal cell death observed in Huntington's disease (HD). However, the precise mechanisms by which mutant huntingtin sensitize striatal cells to dopamine and glutamate inputs remain unclear. Here, we demonstrate in knock-in HD striatal cells that mutant huntingtin enhances dopaminemediated striatal cell death via dopamine D 1 receptors. Moreover, we show that NMDA receptors specifically potentiate the vulnerability of mutant huntingtin striatal cells to dopamine toxicity as pretreatment with NMDA increased D 1 R-induced cell death in mutant but not wild-type cells. As potential underlying mechanism of increased striatal vulnerability, we identified aberrant cyclin-dependent kinase 5 (Cdk5) activation. We demonstrate that enhanced Cdk5 phosphorylation and increased calpain-mediated conversion of the Cdk5 activator p35 into p25 may account for the deregulation of Cdk5 associated to dopamine and glutamate receptor activation in knock-in HD striatal cells. Moreover, supporting a detrimental role of Cdk5 in striatal cell death, neuronal loss can be widely prevented by roscovitine, a potent Cdk5 inhibitor. Significantly, reduced Cdk5 expression together with enhanced Cdk5 phosphorylation and p25 accumulation also occurs in the striatum of mutant Hdh Q111 mice and HD human brain suggesting the relevance of deregulated Cdk5 pathway in HD pathology. These findings provide new insights into the molecular mechanisms underlying the selective vulnerability of striatal cells in HD and identify p25/Cdk5 as an important mediator of dopamine and glutamate neurotoxicity associated to HD.

Fragile X syndrome (FXS) is the commonest cause of inherited mental retardation in males. Even though this affirmation is repeated in virtually all papers referring to FXS, the precise frequency of this syndrome in the general population is unknown. We present a general population screening analyzing an anonymous series of 5,000 consecutive newborn males from the neonatal screening program of the population of Catalonia in Spain. The aim of the study is to determine the incidence of FXS via a simple and economical methodology based on the nonamplification of the fragment containing the CGG repeats of the FRAXA locus in the samples carrying alleles over 52 repeats. From the initial 5,000 samples, 4,920 were in the normal range, 15 gave rise to bands with more than 52 repeats (11 corresponded to intermediate alleles and four premutated alleles). After further studies, two samples were considered to be carriers of full mutations. According to these results, the incidence of FXS affected newborn males is 1 in 2,466, and 1 in 1,233 males is a carrier of the premutation. We can deduce that 1 in 8,333 is an affected female with clinical manifestations and 1 in 411 will be a premutation carrier woman. Upon reviewing the literature, there seems to be variability in the frequencies found by the different groups. Therefore, given that our study is limited to the Catalan population in Spain, these results should be taken as valid for the Catalan region and should only be extrapolated to other populations with caution.

Fragile X syndrome (FXS) is the most common form of inherited mental retardation. Clinical manifestations are due to the absence of the FMRP protein. Affected patients have widely variable phenotypes which are more variable in females than males, presumable due to X inactivation. We report the expression pattern of FMRP in cerebral cortex and ovary in a control and a full-mutated female fetus. FMRP was expressed in mutated and control fetal tissues, although at different levels and patterns. Control fetal cerebral cortex showed FMRP expression in almost all cells, whereas the full mutation carrier showed FMRP positivity in roughly 50% of cortical cells without any specific pattern. In the ovary samples, FMRP expression was seen in all germ cells surrounded by FMRP-negative paragranulosa and interstitial cells. The Müllerian epithelium of the fetal Fallopian tube was continuously positive in the control case, whereas the full mutation carrier showed a discontinuous patchy pattern. Expression of homologue proteins FXR1P and FXR2P showed no differences between control and full mutation fetuses. The pattern of FMRP expression in full mutation carrier females is in agreement with a random X-inactivation in maturing fetal tissues. Immunohistochemical results on cerebral tissues provide a clue for the variation of mental affection among female carriers, depending not only on the number of cells devoid of FMRP, but also on the ultimate destination of those cells in sensitive or more silent location for a proper cerebral development.

Our data support the hypothesis that the FMR1 gene is one of the genes associated with POF and EM. Analysis of the CGG expansion in the FMR1 gene may be justified in women with POF and EM until the real role of the FMR1 premutation is determined.

Fragile X syndrome is the commonest familial form of inherited mental retardation. The molecular defect is an expansion of the CGG trinucleotide repeats in the 5' untranslated region of the FMR1 gene that is inherited in an unstable fashion in fragile X families. In an attempt to provide more information about the CGG tract intergenerational variation, we have evaluated 642 transmissions in 175 Fragile X families. PCR and Southern blot (StB12.3) was used to analyse the CGG number. Among premutated alleles, 90.2% showed expansion, two-thirds to a full mutation while the rest remained in the premutation range, 5.5% of alleles did not vary and finally 4.3% of them reduced in size. Premutated females showed an increased risk of expansion to the full mutation depending on the CGG tract. The estimated risk for 80 triplets is more than seven times that of a woman carrying 59 CGG, the risk being 100% for alleles of >100 repeats. Fifty-nine repeats was the smallest allele that expanded to full mutation. Contractions were detected more frequently in males than in females, being statistically significant. This study contributes to the literature by increasing the data available regarding transmissions in Fragile X families and it allows us to perform more precise genetic counselling for women with the CGG repeat in the premutation range.

Fragile X syndrome is caused by lack of the protein FMRP. FMRP mediates mRNA binding, dendritic mRNA transport and translational control at spines. We examined the role of functional domains of FMRP in neuronal RNA-granule formation and dendritic transport using different FMRP variants, including the mutant FMRP_I304N and the splice-variant FMRP_Iso12. Both variants are absent from dendritic RNA-granules in Fmr1 knockout neurons. Co-transfection experiments showed that wild-type FMRP recruits both FMRP variants into dendritic RNA-granules. Co-transfection of FXR2, an FMRP homologue, also resulted in redistribution of both variants into dendritic RNAgranules. Furthermore, the capacity of the variants to transport their mRNAs and the mRNA localization of an FMR1 construct containing silent point-mutations affecting only the G-quartetstructure was investigated. In conclusion, we show that wild-type FMRP and FXR2P are able to recruit FMRP variants into RNA-granules and that the G-quartet-structure in FMR1 mRNA is not essential for its incorporation in RNA-granules.

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