Preliminary evidence of an effect of cerebellar volume on postural sway in FMR1 premutation males

Recent evidence suggests that early changes in postural control may be discernible among females with premutation expansions (55–200 CGG repeats) of the fragile X mental retardation 1 (FMR1) gene at risk of developing fragile X‐associated tremor ataxia syndrome (FXTAS). Cerebellar dysfunction is well described in males and females with FXTAS, yet the interrelationships between cerebellar volume, CGG repeat length, FMR1 messenger RNA (mRNA) levels and changes in postural control remain unknown. This study examined postural sway during standing in a cohort of 22 males with the FMR1 premutation (ages 26–80) and 24 matched controls (ages 26–77). The influence of cerebellar volume, CGG repeat length and FMR1 mRNA levels on postural sway was explored using multiple linear regression. The results provide preliminary evidence that increasing CGG repeat length and decreasing cerebellar volume were associated with greater postural sway among premutation males. The relationship between CGG repeat length and postural sway was mediated by a negative association between CGG repeat size and cerebellar volume. While FMR1 mRNA levels were significantly elevated in the premutation group and correlated with CGG repeat length, FMR1 mRNA levels were not significantly associated with postural sway scores. These findings show for the first time that greater postural sway among males with the FMR1 premutation may reflect CGG repeat‐mediated disruption in vulnerable cerebellar circuits implicated in postural control. However, longitudinal studies in larger samples are required to confirm whether the relationships between cerebellar volume, CGG repeat length and postural sway indicate greater risk for neurological decline.     

Cerebral Protein Synthesis in a Knockin Mouse Model of the Fragile X Premutation

The (CGG)n-repeat in the 5′-untranslated region of the fragile X mental retardation gene (FMR1) gene is polymorphic and may become unstable on transmission to the next generation. In fragile X syndrome, CGG repeat lengths exceed 200, resulting in silencing of FMR1 and absence of its protein product, fragile X mental retardation protein (FMRP). CGG repeat lengths between 55 and 200 occur in fragile X premutation (FXPM) carriers and have a high risk of expansion to a full mutation on maternal transmission. FXPM carriers have an increased risk for developing progressive neurodegenerative syndromes and neuropsychological symptoms. FMR1 mRNA levels are elevated in FXPM, and it is thought that clinical symptoms might be caused by a toxic gain of function due to elevated FMR1 mRNA. Paradoxically, FMRP levels decrease moderately with increasing CGG repeat length in FXPM. Lowered FMRP levels may also contribute to the appearance of clinical problems. We previously reported increases in regional rates of cerebral protein synthesis (rCPS) in the absence of FMRP in an Fmr1 knockout mouse model and in a FXPM knockin (KI) mouse model with 120 to 140 CGG repeats in which FMRP levels are profoundly reduced (80%–90%). To explore whether the concentration of FMRP contributes to the rCPS changes, we measured rCPS in another FXPM KI model with a similar CGG repeat length and a 50% reduction in FMRP. In all 24 brain regions examined, rCPS were unaffected. These results suggest that even with 50% reductions in FMRP, normal protein synthesis rates are maintained.     

Apparent regression of the CGG repeat in FMR1 to an allele of normal size

The fragile X syndrome is the result of amplification of a CGG trinucleotide repeat in the FMR1 gene and anticipation in this disease is caused by an intergenerational expansion of this repeat. Although regression of a CGG repeat in the premutation range is not uncommon, regression from a full premutation (>200 repeats) or premutation range (50–200 repeats) to a repeat of normal size (<50 repeats) has not yet been documented. We present here a family in which the number of repeats apparently regressed from approximately 110 in the mother to 44 in her daughter. Although the CGG repeat of the daughter is in the normal range, she is a carrier of the fragile X mutation based upon the segregation pattern of Xq27 markers flanking FMR1. It is unclear, however, whether this allele of 44 repeats will be stably transmitted, as the daughter has as yet no progeny. Nevertheless, the size range between normal alleles and premutation alleles overlap, a factor that complicates genetic counseling.     

Reduced FMR1 mRNA translation efficiency in fragile X patients with premutations

The Fragile X mental retardation gene (FMR1) contains a polymorphic trinucleotide CGG repeat in the 5' untranslated region (UTR) of the FMR1 messenger. We have characterized three lymphoblastoid cell lines derived from unrelated male carriers of a premutation that overexpress FMR1 mRNA and show reduced FMRP level compared to normal cells. The analysis of polysomes/mRNPs distribution of mRNA in the cell lines with a premutation shows that the polysomal association of FMR1 mRNA, which is high in normal cells, becomes progressively lower with increasing CGG repeat expansion. In addition, we could detect a very low level of FMR1 mRNA in a lymphoblastoid cell line from a patient with a full mutation. In this case, FMR1 mRNA is not at all associated with polysomes, in agreement with the complete absence of FMRP. The impairment of FMR1 mRNA translation in patients with the Fragile X syndrome with FMR1 premutation is the cause of the lower FMRP levels that leads to the clinical involvement.     

The fragile X premutation: into the phenotypic fold

Premutation alleles (55-200 CGG repeats) of the fragile X mental retardation 1 gene (FMR1) are known to contribute to the fragile X phenotype through genetic instability and transmission of full mutation alleles (>200 repeats). There is now mounting evidence that the premutation alleles themselves contribute to clinical involvement, including premature ovarian failure among female carriers and a new tremor/ataxia syndrome among older male carriers. Recent observations also provide direct evidence of dysregulation of the FMR1 gene in the premutation range, which may explain many of the clinical observations.     

Expansion of the fragile X CGG repeat in females with premutation or intermediate alleles

The CGG repeat in the 5' untranslated region of the fragile X mental retardation 1 gene (FMR1) exhibits remarkable instability upon transmission from mothers with premutation alleles. A collaboration of 13 laboratories in eight countries was established to examine four issues concerning FMR1 CGG-repeat instability among females with premutation (approximately 55-200 repeats) and intermediate (approximately 46-60 repeats) alleles. Our central findings were as follows: (1) The smallest premutation alleles that expanded to a full mutation (>200 repeats) in one generation contained 59 repeats; sequence analysis of the 59-repeat alleles from these two females revealed no AGG interruptions within the FMR1 CGG repeat. (2) When we corrected for ascertainment and recalculated the risks of expansion to a full mutation, we found that the risks for premutation alleles with <100 repeats were lower than those previously published. (3) When we examined the possible influence of sex of offspring on transmission of a full mutation-by analysis of 567 prenatal fragile X studies of 448 mothers with premutation and full-mutation alleles-we found no significant differences in the proportion of full-mutation alleles in male or female fetuses. (4) When we examined 136 transmissions of intermediate alleles from 92 mothers with no family history of fragile X, we found that, in contrast to the instability observed in families with fragile X, most (99/136 [72.8%]) transmissions of intermediate alleles were stable. The unstable transmissions (37/136 [27.2%]) in these families included both expansions and contractions in repeat size. The instability increased with the larger intermediate alleles (19% for 49-54 repeats, 30.9% for 55-59, and 80% for 60-65 repeats). These studies should allow improved risk assessments for genetic counseling of women with premutation or intermediate-size alleles.     

A cerebellar tremor/ataxia syndrome among fragile X premutation carriers

Fragile X syndrome is a neurodevelopmental disorder that is not known to have any progressive neurological sequelae in adulthood. However, a neurological condition involving intention tremor, ataxia, and cognitive decline has recently been identified among older male carriers of premutation alleles of the FMR1 gene. This condition is clinically distinct from fragile X syndrome and arises through a different molecular mechanism involving the same gene (FMR1). Characteristic findings on magnetic resonance imaging include cerebral and cerebellar volume loss and altered signal intensities of the middle cerebellar peduncles. A striking feature of this fragile X-associated tremor/ataxia syndrome is the presence of ubiquitin-positive neuronal and astroglial intranuclear inclusions. Unlike the CAG repeat expansion diseases, which lead to altered protein products, there is no known protein abnormality among FMR1 premutation carriers. Thus, inclusion formation may reflect a gain-of-function effect of the FMR1 mRNA or the CGG repeat itself. Finally, since this syndrome may represent one of the more common single-gene causes of tremor, ataxia, and dementia among older males, FMR1 DNA testing should be considered when evaluating adult patients with tremor/ataxia.     

Fragile X gene instability: anchoring AGGs and linked microsatellites.

Interspersed AGGs within the FMR1 gene CGG repeat region may anchor the sequence and prevent slippage during replication. In order to detect the AGG position variations, we developed a method employing partial MnlI restriction analysis and analyzed X chromosomes from 187 males, including 133 normal controls (117 with 20-34 and 16 with 35-52 repeats), plus 54 fragile X premutations with 56-180 repeats. Among controls, the interspersed AGG positions were highly polymorphic, with a heterozygosity of 91%. Among the control samples, 1.5% had no AGG positions, 25% had one, 71% had two, and 3% had three. Among the fragile X premutation samples, 63% had no AGG, while 37% had only one AGG. Analysis of premutation samples within fragile X families showed that variation occurred only within the 3' end of the region. Thus, the instability was polar. Controls with > or = 15 pure CGG repeats were associated with the longest alleles of two nearby microsatellites, FRAXAC1 with 20-21 repeats and DXS548 with 202-206 bp and with increased microsatellite heterozygosity. The association of long pure CGG regions, as with fragile X chromosomes, with the longer and more heterozygous microsatellite alleles suggests they may be related mechanistically. Further, our results do not support a recent suggestion that the frequency of fragile X alleles may be increasing. Finally, analysis of a set of nonhuman primate samples showed that long pure CGG tracks are variable in size and are located within the 3' region, which suggests that polar instability within FMR1 is evolutionarily quite old.     

Screening for FXTAS in 95 Spanish patients negative for Huntington disease

Fragile X syndrome is the most common form of hereditary mental retardation. The molecular basis of this syndrome is mainly a CGG expansion in the 5' untranslated region of the FMR1 gene. Expansions with more than 200 CGG repeats abolish gene expression causing the classical fragile X phenotype. Premutation carriers (55-200 CGG) have normal cognitive function with increased risk of developing premature ovarian failure and fragile X-associated tremor-ataxia syndrome (FXTAS). Some clinical features associated with FXTAS, such as tremor, gait ataxia, cognitive decline, and generalized brain atrophy, are also seen in other movement disorders. Ninety-five patients referred for HD, who tested negative for the expansion in the IT15 gene, were screened for FMR1 CGG-repeat expansion. One FMR1 premutation male carrier was detected, giving an FXTAS frequency of 1.6%. Our results highlight that FXTAS is still not well diagnosed; therefore, we recommend FMR1 premutation screenings in all patients with late-onset tremor, ataxia, and cognitive dysfunction.