Phenotypic variation in male-transmitted fragile X: genetic inferences

Emerging pharmacologic treatment options for fragile X syndrome

Fragile X syndrome (FXS) is the most common single gene cause of intellectual disability and autism spectrum disorder. Caused by a silenced fragile X mental retardation 1 gene and the subsequent deficiency in fragile X mental retardation protein, patients with FXS experience a range of physical, behavioral, and intellectual debilitations. The FXS field, as a whole, has recently met with some challenges, as several targeted clinical trials with high expectations of success have failed to elucidate significant improvements in a variety of symptom domains. As new clinical trials in FXS are planned, there has been much discussion about the use of the commonly used clinical outcome measures, as well as study design considerations, patient stratification, and optimal age range for treatment. The evidence that modification of these drug targets and use of these failed compounds would prove to be efficacious in human clinical study were rooted in years of basic and translational research. There are questions arising as to the use of the mouse models for studying FXS treatment development. This issue is twofold: many of the symptom domains and molecular and biochemical changes assessed and indicative of efficacy in mouse model study are not easily amenable to clinical trials in people with FXS because of the intolerability of the testing paradigm or a lack of noninvasive techniques (prepulse inhibition, sensory hypersensitivity, startle reactivity, or electrophysiologic, biochemical, or structural changes in the brain); and capturing subtle yet meaningful changes in symptom domains such as sociability, anxiety, and hyperactivity in human FXS clinical trials is challenging with the currently used measures (typically parent/caregiver rating scales). Clinicians, researchers, and the pharmaceutical industry have all had to take a step back and critically evaluate the way we think about how to best optimize future investigations into pharmacologic FXS treatments. As new clinical trials are coming down the drug discovery pipeline, it is clear that the field is moving in a direction that values the development of molecular biomarkers, less subjective quantitative measures of symptom improvement, and rating scales developed specifically for use in FXS in conjunction with drug safety. While summarizing preclinical evidence, where applicable, and discussing challenges in FXS treatment development, this review details both completed clinical trials for the targeted and symptomatic treatment of FXS and introduces novel projects on the cusp of clinical trial investigation.

Evidence for RNA-mediated toxicity in the fragile X-associated tremor/ataxia syndrome

Fragile X premutation carriers are at risk for developing a late-onset, progressive neurodegenerative disorder termed fragile X-associated tremor/ataxia syndrome (FXTAS). A growing body of evidence suggests the characteristic excess CGG repeat containing FMR1 mRNA observed in premutation carriers is pathogenic and leads to clinical features of FXTAS. The current model suggests premutation mRNA transcripts can induce the formation of intranuclear inclusions by the sequestration of RNA-binding proteins and other proteins. The sequestered proteins are prevented from performing their normal functions, which is thought to lead to the neuropathology-observed FXTAS. This paper discusses the existing evidence that microsatellite expansions at the level of RNA play a role in the disease pathogenesis of FXTAS and some of the approaches that may uncover downstream effects of expanded riboCGG expression.

Behavioural and cognitive phenotypes in idiopathic autism versus autism associated with fragile X syndrome

BACKGROUND

In order to better understand the underlying biological mechanism/s involved in autism, it is important to investigate the cognitive and behavioural phenotypes associated with idiopathic autism (autism without a known cause) and comorbid autism (autism associated with known genetic/biological disorders such as fragile X syndrome). Parental effects associated with each type of autism also serve to cast light on the biological underpinnings of autism.

METHOD

Forty-nine participants with idiopathic autism (AD; Mean age: 11.16; SD: 6.08) and their parents (45 mothers; 34 fathers), and 48 participants with fragile X syndrome and co-morbid autism (FXS/AD; Mean age: 17.30; SD: 10.22) and their parents (32 mothers; 30 fathers) were administered the ADOS-G and the age-appropriate Wechsler test to ascertain autism and cognitive profiles respectively.

RESULTS

The AD and FXS/AD groups showed a similar profile on the ADOS domains, with slightly higher scores on the Communication domain in the FXS/AD group, after adjusting for full-scale IQ. Marked differences between the groups in their cognitive abilities were apparent, with the FXS/AD group showing significantly lower scores on all subtests except Comprehension. While no parental effects were found for the FXS/AD group, a paternal effect was apparent on the combined ADOS score for the AD group. Moreover, midparental effects were found in this group for full-scale IQ (FSIQ) and verbal IQ (VIQ). Analyses also revealed parental effects for the subtests of Similarities, Vocabulary, and Information with predominantly maternal effect, and Digit Span with predominantly paternal effect. Both parents contributed to the midparental effect for Processing Speed.

CONCLUSIONS

The results, together with our previous findings, suggest that the postulated combination of susceptibility genes for autism may primarily involve cognitive rather than behavioural processes.

Neuroanatomical, molecular genetic, and behavioral correlates of fragile X syndrome

Fragile X syndrome (FXS) is a leading cause of inherited mental retardation. In the vast majority of cases, this X-linked disorder is due to a CGG expansion in the 5' untranslated region of the fmr-1 gene and the resulting decreased expression of its associated protein, FMRP. FXS is characterized by a number of cognitive, behavioral, anatomical, and biological abnormalities. FXS provides a unique opportunity to study the consequence of mutation in a single gene on the development and proper functioning of the CNS. The current focus on the role of FMRP in neuronal maturation makes it timely to assemble the extant information on how reduced expression of the fmr-1 gene leads to neuronal dysmorphology. The purpose of this review is to summarize recent genetic, neuroanatomical, and behavioral studies of fragile X syndrome and to offer potential mechanisms to account for the pleiotropic phenotype of this disorder.

Transcription of the FMR1 gene in individuals with fragile X syndrome

Fragile X syndrome generally arises as a consequence of a large expansion of a CGG trinucleotide repeat element that is located in the GC-rich promoter region of the fragile X mental retardation gene (FMR1). In the conventional model for fragile X, clinical involvement arises as a consequence of silencing of the FMR1 gene, with the attendant loss of FMR1 protein (FMRP). However, it has recently been demonstrated that most males with large premutation alleles (100-200 repeats), or with unmethylated full mutation alleles, have FMR1 mRNA levels that are higher than normal, despite reduced levels of FMRP. In the current work, we extend and confirm these observations using quantitative (fluorescent) reverse transcription polymerase chain reaction on larger sample populations, establishing that even for smaller premutation alleles (55-100 repeats) the mRNA levels are significantly elevated (mean 2.1-fold elevation; P = 3.9 x 10(-3)), relative to normal controls. Thus, an abnormal molecular phenotype is established close to the upper end of the normal range. We also demonstrate that the levels of FMR1 mRNA are elevated in females with premutation alleles; however, the mRNA levels are more varied than in the males, and are attenuated in a manner that is consistent with the fraction of normal alleles that are active in any given individual. Finally, we demonstrate that in lymphoblastoid cells derived from a patient with a severe form of fragile X caused by a point mutation in the second KH domain of the gene, but with a normal CGG element (25 repeats), the FMR1 mRNA level is normal. Thus, although models in which FMRP level (or level of function) modulates transcriptional activity remain viable, other explanations for the elevated message levels, including direct (cis) effects of the CGG element on transcription, must also be considered.

Clinical involvement and protein expression in individuals with the FMR1 premutation

Most individuals with the fragile X premutation are clinically unaffected; however, some show clinical manifestations, including learning difficulties, emotional problems, or even mental retardation. The basis of clinical involvement in these individuals is unknown. Premutation alleles are reportedly associated with normal levels of mRNA and protein (FMRP). To examine this issue in more detail, we studied six individuals with a premutation. We are reporting these cases to demonstrate a spectrum of phenotypic involvement which can be seen clinically. These cases include one individual with the premutation who has no evidence of FMR1 gene dysfunction but has mental retardation from other causes. Other cases presented here show varying degrees of FMR1 gene dysfunction as assessed by FMRP and FMR1 mRNA levels and various clinical features of fragile X. In two cases we observed a significant reduction in FMRP expression and an elevated FMR1 mRNA expression level associated with moderate cognitive deficit. Thus, the utilization of FMRP measures can be helpful in understanding for which premutation patients clinical involvement is caused by dysfunction of the FMR1 gene.

Elevated levels of FMR1 mRNA in carrier males: a new mechanism of involvement in the fragile-X syndrome

Fragile-X syndrome is a trinucleotide-repeat-expansion disorder in which the clinical phenotype is believed to result from transcriptional silencing of the fragile-X mental retardation 1 (FMR1) gene as the number of CGG repeats exceeds approximately 200. For premutation alleles ( approximately 55-200 repeats), no abnormalities in FMR1-gene expression have been described, despite growing evidence of clinical involvement in premutation carriers. To address this (apparent) paradox, we have determined, for 16 carrier males (55-192 repeats), the relative levels of leukocyte FMR1 mRNA, by use of automated fluorescence-detection reverse transcriptase-PCR, and the percent of lymphocytes that are immunoreactive for FMR1 protein (FMRP). For some alleles with>100 repeats, there was a reduction in the number of FMRP-positive cells. Unexpectedly, FMR1 mRNA levels were elevated at least fivefold within this same range. No significant increase in FMR1 mRNA stability was observed in a lymphoblastoid cell line (160 repeats) derived from one of the carrier males, suggesting that the increased message levels are due to an increased rate of transcription. Current results support a mechanism of involvement in premutation carriers, in which reduced translational efficiency is at least partially compensated through increased transcriptional activity. Thus, diminished translational efficiency may be important throughout much of the premutation range, with a mechanistic switch occurring in the full-mutation range as the FMR1 gene is silenced.

Tissue-specific methylation differences in a fragile X premutation carrier

Methylation of a premutation was found in a small percentage of blood cells in a male premutation carrier for the FMR1 mutation. To investigate the inter-tissue heterogeneity and possible clinical implications of this finding, fibroblast cells from the subject were also studied. Although the premutation size was found to be the same in leukocytes and fibroblasts, the methylation pattern was different. In cultured fibroblasts, the premutation was completely unmethylated, as is typical of premutations, whereas methylation of the premutation was detected in a small percentage of lymphocytes. However, the change in methylation did not affect the FMR1 protein (FMRP) expression, as immunocytochemical analysis of FMRP performed on cultured skin fibroblasts and a blood smear revealed normal levels of expression in both tissues.

FRAXE and mental retardation

Mental impairment and instability of the CCG repeat at FRAXE is described in six kindreds. Cosegregation of FRAXA and FRAXE was found within one of these kindreds. Cytogenetic expression of FRAXE was shown to skip a generation when associated with a reduction in size of the CCG expansion when transmitted through a male; however, in general, transmission occurred through females and a copy number increased from one generation to the next. In these respects the behaviour of FRAXE paralleled that of FRAXA. A relationship between FRAXE and non-specific mental impairment is strongly suggested by the occurrence in these families of more mentally impaired male and female carriers, after removal of index cases, than could reasonably be expected by chance.

Use of robust statistical methods to determine the effect of fragile X on means and variance components of a quantitative trait

Owing to the presence of outliers, an estimated 3.5% in the ridge breadth data and 1.7% in the height data, the effect of fragile X on height and ridge breadth was examined using robust statistical techniques for data collected from 54 families afflicted with this disorder. It is shown that fragile X affects ridge breadth and height in a different manner. Fragile X women had a greater mean ridge breadth than normal women, whereas there was a similar trend, but no significant difference, between normal and fragile X men. Fragile X men were shorter than normal men, but no significant difference between the mean height of normal and fragile X women was observed. However, fragile X girls were shown to grow more quickly and to stop growing earlier than normal girls. An examination of the covariance between relatives classified according to fragile X status showed that for both traits the effect of fragile X was to reduce the covariance between parents and offspring, which produced the effect of departure from an additive polygenic model of inheritance.

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.

Fragile X syndrome in a normal IQ male with learning and emotional problems

The present case study features an adult male who was diagnosed with fragile X syndrome after the identification of this syndrome in his more affected brother. The patient presented with a Full Scale IQ within the broad range of normal and has been diagnosed with a schizotypal personality disorder. He shows significant deficits in the social and emotional aspects of daily life, but has striking cognitive strengths relating to reading and vocabulary as compared to most males affected with fragile X syndrome. DNA testing of blood leukocytes revealed that he has a fully expanded FMR1 CGG repeat mutation associated with almost complete lack of methylation. Protein studies demonstrate a limited production of FMRP, the protein produced by the FMR1 gene. It is believed that the near absence of methylation of the fully expanded mutation and the resultant expression of the FMR1 protein is responsible for the strong cognitive abilities of this fragile X patient.

High functioning fragile X males: demonstration of an unmethylated fully expanded FMR-1 mutation associated with protein expression

Fragile X (fra(X)) males with a standardized IQ score of 70 or higher represent a high functioning (HF) or nonretarded fra(X) male group. This group, which does not include nonpenetrant males, has received little research attention to date. Of 221 fra(X) males who had been evaluated through The Children's Hospital in Denver since 1981 and had completed cognitive or developmental testing, 29 (13%) were high functioning by the above definition. We found that HF males on the whole had a lower cytogenetic score and were younger than retarded fra(X) males, but there was no difference between these two groups in the number of typical fra(X) physical manifestations present. FMR-1 DNA testing was performed on 134 fra(X) males and methylation status was determined for 51 of these. A greater percentage of HF males had a mosaic pattern or an incompletely methylated full mutation than did retarded males. A unique DNA pattern, an unmethylated fully expanded mutation, was discovered in 3 of the highest functioning fra(X) males. Protein studies performed on 2 of these males demonstrated the presence of FMR-1 protein, albeit at lower levels than normal. FMR-1 protein was not present in retarded fra(X) males. Significant FMR-1 protein expression may be responsible for higher cognitive functioning in the 2 males with unmethylated fully expanded mutations compared to retarded fra(X) males.

Transmitting males and carrier females in fragile X--revisited

Fragile X "transmitting males" have customarily been defined as phenotypically normal hemizygotes, who show very few or no fragile sites, and who transmit the fragile X premutation to phenotypically normal daughters. However, an objective justification of this definition was lacking. The discovery of an unstable CCG repeat as the genetic basis of fragile X further emphasized the apparent distinction between the "normal transmitting males" with short repeat and expression of the FMR1 gene, and the affected males with larger repeats (delta > 0.6 kb) and a complete lack of FMR1 transcription. We have recently shown that the transition between these two groups in phenotypic expression of fragile X is gradual, mainly on account of methylation mosaicism. However, there were insufficient data on the phenotype within the short repeat (0.0 < delta < 0.6) range. In this paper we approach this problem by comparing some clinical, anthropometric, and psychometric data from a sample of normal transmitting males with those from their non-fragile X male relatives. Moreover, female carriers with short repeat are compared for the same traits with their non-fragile X female relatives. The results have shown that both males and females with a short repeat differed significantly from normal on several psychometric and physical measurements, and males only showed differences in typical facial traits. Further studies of genotype-phenotype correlations within the short repeat range, including the estimate of FMR1 gene function and a more exact estimate of repeat size, is required before genetic explanation for the clinical findings can be provided.(ABSTRACT TRUNCATED AT 250 WORDS)

Asymmetry of methylation with FMR-1 full mutation in two 45,X/46,XX mosaic females associated with normal intellect

The full FMR-1 mutation is known to cause the fragile X syndrome [Fra(X)], but variable expression in females, including normal to deficient intellect, may be related to random X-inactivation (lyonization). We have evaluated 2 mosaic 45,X/46,XX females who are cytogenetically fra(X) positive, have an FMR-1 full mutation, and are of normal intellect. There were 50% fra(X) chromosomes in the 45,X cells of one of the females; this has not been reported previously. In both patients, there was a strong asymmetry of FMR-1 methylation with the normal allele being totally or 90% unmethylated and the mutant allele being similarly methylated. Thus, the apparent selective inactivation of the full mutant FMR-1 allele appears to have resulted in limited expression with normal intellect. The presence of the fra(X) chromosome in 45,X cells is unique; however, there may be no relationship to the asymmetric inactivation of the mutant allele which could be due to chance or a mechanism yet to be delineated.

Between-generation differences in ascertainment and penetrance: relevance to genetic hypotheses in fragile X

Between-generation differences in ascertainment were examined in 54 extended fragile X pedigrees, where all available members were clinically, psychometrically, and cytogenetically investigated. In 24 families a diagnosis was verified by molecular characterization using the pfxa3 fragile X-specific probe. We found considerable differences between generations in relative proportions of affected fragile X subjects versus 'non-penetrant' carriers. We also found deviation in the segregation ratio in unbiased samples of relatives in pedigrees. We claim that these irregularities are influenced by different rates of ascertainment, depending on the clinical expression of the condition (penetrance) and the fertility of fragile X individuals in a pedigree, as well as by the thoroughness of clinical investigation in individual families. Penetrance and fertility were estimated in fragile X females assessed by psychometric tests, and they were compared with earlier estimates based on a subjective judgement of their intellectual status. We suggest that the standard correction for ascertainment bias, such as has been applied in segregation analysis of this condition, is not sufficient to adjust for all types of bias.

The full mutation in the FMR-1 gene of male fragile X patients is absent in their sperm

Fragile X syndrome is characterized at the molecular level by amplification of a (CGG)n repeat and hypermethylation of a CpG island preceeding the open reading frame of the fragile X gene (FMR-1) located in Xq27.3. Anticipation in this syndrome is associated with progressive amplification of the (CGG)n repeat from a premutation to a full mutation through consecutive generations. Remarkably, expansion of the premutation to the full mutation is strictly maternal. To clarify this parental influence we studied FMR-1 in sperm of four male fragile X patients. This showed that only the premutation was present in their sperm, although they had a full mutation in peripheral lymphocytes. This might suggest that expansion of the premutation to the full mutation in FMR-1 does not occur in meiosis but in a postzygotic stage.

Fragile X family with unusual digital and facial abnormalities, cleft lip and palate, and epilepsy

We present a fragile X family with unusual clinical manifestations. These findings, which often occur in the X-linked FG syndrome, include minor limb anomalies, cleft lip and palate, characteristic facial appearance, gastrointestinal problems and epilepsy, and intellectual disability. In a total sample of 54 fra(X) families, the frequency of minor limb anomalies was estimated to be 32% in the affected males and 19% in the female heterozygotes. These anomalies tend to occur in several members of the same family, where some craniofacial abnormalities reported as characteristic of the FG syndrome have also been encountered. Possible mechanisms for the occurrence of these unusual manifestations in the fra(X) families are discussed.

Premutation for the Martin-Bell syndrome analyzed in a large Sardinian family: II. Neuropsychological and behavioral data

We describe the neuropsychological and behavioral profiles of 48 critical members of a previously reported Sardinian pedigree [Filippi et al., 1991], in which the fully manifested Martin-Bell syndrome (MBS), observed among males of the latest generations, is clearly the result of step-wise mutational events occurred repeatedly along the X-chromosome pathway linking all of them to a common ancestress, who must have been heterozygous for a fragile X (FRAX) premutation. We found that the unquestionable presence in the family of normal transmitting males and females could not be determined on the basis of neuropsychological and behavioral data alone. However, we think that the large variation observed in the expression of most diagnostic parameters among the MBS patients and their close female relatives in this family, could by itself be a connotation of the genome instability which characterizes the FRAX region in pedigrees segregating for the FRAX premutation(s) and mutation(s).

The fragile-X syndrome. On the way to a behavioural phenotype

The fragile-X syndrome accounts for up to 10% of individuals with mental handicap, and 50% of cases of X-linked mental retardation. Knowledge of the genetic basis of mental functioning, psychopathology, and neuropsychology is being furthered by this recently recognised condition. The disorder has considerable significance for psychiatrists, particularly, but by no means exclusively, those working in the field of mental handicap and with children. This review outlines the slow clarification of this complex and important behavioural phenotype and the implications of these advances for identification, diagnosis, genetic counselling and a wide range of management interventions.

Problems in ascertainment of transmitting males in Martin-Bell syndrome

The difficulty of assigning families affected with the Martin-Bell syndrome (MBS) into the category of male transmission is emphasised and illustrated by examples of 3 MBS families. These examples demonstrate how the ability to detect transmitting males depends on the number of generations available for investigation, and also on the "spread" of clinical investigation across many branches of the family regardless of what appears to be an unremarkable family history. Some unusual properties of male transmission are shown, and the problem of selective ascertainment of the particular MBS male individuals in different generations in a set of pedigrees is discussed.

Premutation for the Martin-Bell syndrome analyzed in a large pedigree segregating also for G6PD-deficiency. I: A working hypothesis on the nature of the FRAX-mutations

A large Sardinian family including 13 Martin-Bell syndrome (MBS) patients, several instances of normal transmitting males or females, and the G6PD-Mediterranean mutant segregating in some of its branches, has been thoroughly investigated with the hope of gaining further insight on the nature of the FRAX-mutation. All the MBS patients and the 15 obligate heterozygous women present in the pedigree could be traced back through their X-chromosome lineage to the same ancestress, who must have been heterozygous for a silent premutation at the FRAX-locus. This premutation appears to have turned into a true FRAX-mutation at least 9 times during the gametogenesis of the ancestress' X-related descendants of whom four are males. This finding alone suggests that the transition from the FRAX premutation to the true mutation can be the result of intra- as well as interchromosomal events. This conclusion is supported by the additional observation that the genetic phase between the FRAX and the G6PD loci remained unaltered when the transition occurred in a repulsion double heterozygote for the premutation and the G6PD-Mediterranean mutant. The data described are compatible with the hypothesis that MBS patients and normal transmitting males are, respectively, hemizygous for deletion or duplication products generated by aberrant recombination events at a highly recombinogenic site of the region Xq27-Xqter. The overall message stemming from this report is that no firm conclusion can be drawn on the genetic linkage between the FRAX-locus and other markers of this region until the nature of the FRAX-mutations and the mechanism of their occurrence are fully understood.

Early prenatal diagnosis of the fragile site at Xq27.3 associated with Martin-Bell syndrome

Early prenatal diagnosis of the fragile X was attempted in 44 pregnancies, including one twin pregnancy at risk of Martin-Bell (MB) syndrome. The sex ratio was 24M:21F. The fragile site was reproducibly demonstrated in cultured chorionic villus (CV) cells in eight male and five female fetuses. Six of the male and three of the female fetuses were terminated. Simultaneous RFLP analysis provided confirmative data with flanking DNA markers in 3 of 13 analysed cases. Recombination and/or non-informativeness at available distal and/or proximal loci were found in nine cases. In one male fetus, discordance between the haplotype and cytogenetics (fragile-X-negative) suggested the presence of a normal male transmitter, a double meiotic cross-over within the region, or a false-negative cytogenetic diagnosis. However, discordance between prenatal and post-termination/postnatal cytogenetic findings was not observed in this series. The use of excess thymidine for induction of the fragile X in cultured CV cells provided in the majority of cases a safe and rapid method for cytogenetic diagnosis, with options for early induced termination in fragile-X-positive pregnancies, for simultaneous RFLP analysis, and for subsequent second-trimester analysis of fetal blood in complicated cases.

Short-term memory and cognitive variability in adult fragile X females

We investigated the possibility that adult fragile X [fra(X)] heterozygotes have a distinct or specific cognitive profile, with a particular focus on visuospatial and/or memory deficits. With a sample of 13 adult fra(X) female carriers (2 fra(X) positive) and age-matched control women, we performed 2 tests: Wechsler Adult Intelligence Scale-Revised (WAIS-R) and the Revised Visual Retention Test (RVRT). An identifiable cognitive profile was not evident in the study group, but significant differences were evident in RVRT performance in number correct and number of errors when compared to controls and normative data. The combination of the WAIS-R and RVRT data suggests that the short-term memory component of the tasks may be of more significance than visuospatial performance in the deficits observed.

Possible erasure of the imprint on a fragile X chromosome when transmitted by a male

Although most males with the fragile-X [fra(X)] syndrome do not reproduce, there are 2 published pedigrees that include affected males who have daughters and who thus appear to have transmitted the fragile-X chromosome to their progeny. In addition, one published fra(X) pedigree includes an apparently normal male who expresses cytogenetically the fra(X) site at high frequency and who has 3 daughters. In the 6 daughters of these 3 males, there is little or no cytogenetic expression of the fra(X). I interpret these pedigrees within the context of my X-inactivation imprinting model of the fra(X) syndrome (Genetics 117:587-599): the cytogenetic manifestation of the imprinted state of the mutant fra(X) chromosome [high percentage of cytogenetic expression] is no longer present in daughters of imprinted males. I propose that the imprinted state is erased when an imprinted fragile-X chromosome is passed through a male. Such erasure in the gender opposite to the gender that established the imprint is in accord with other examples of chromosome imprinting in mammals. Additional data from unpublished fra(X) pedigrees are requested.

Fragile X syndrome in an extended family with special reference to an affected male with Klinefelter syndrome

We report on a large family (4 generations), with 77 studied individuals, 9 mentally retarded males, and one affected female with fragile X syndrome [fra(X)]. The analysis of 6 flanking polymorphic DNA markers showed that the affection is transmitted, through the carrier daughters to the grandsons and the greatgrandsons and that the great-grandfather is a transmitting male. This observation led us to question the importance of these clinically normal males, who are nonexpressing carriers and termed transmitting males. One propositus, described as a mentally retarded young man, had inherited identical restriction polymorphisms from his mother. Chromosome analysis showed a Klinefelter syndrome, with a fragile site in 18% of the cells leading to the conclusion that the nondisjunction occurred at the first stage of the maternal meiosis.

Neuropsychological dimensions of the fragile X syndrome: support for a non-dominant hemisphere dysfunction hypothesis

This study contends that males with the fragile X syndrome feature problems in the visuospatial sphere as compared with Down syndrome males matched on vocabulary ability. Fragile X males suffer impairments of constructional functions, as demonstrated by their poor performance on block construction tests and on drawing tasks. These problems exist in association with visuoperceptive impairments, including the inability to reliably estimate angular relationships (Judgement of Line Orientation). They have shortened Digit and Corsi spans, and may feature some deficits in left hand co-ordination. The observation of a pervasive non-verbal deficit may also apply to carrier females, who despite functioning at an overall higher level, feature a similar pattern of deficits. It is possible that the deficit in non-dominant hemisphere functioning may be a pathognomonic feature of the chromosomal abnormality.

Application of the anthropometric discriminant functions in estimation of carrier probabilities in Martin-Bell syndrome

A method of estimating the likelihood ratio and the risk of an individual being affected with Martin Bell syndrome (MBS) from anthropometric measurements is described. The procedure is based on the discriminant functions (one for each sex), generated in our previous study in order to separate the individuals with MBS from the normal individuals. The procedure is illustrated by the examples of estimating the likelihood and the likelihood ratio in four individuals of either sex, belonging to MBS families, where the discriminant score value obtained from each individual is compared with the empirical (normalized) distribution of discriminant scores from the known MBS and normal subjects of a corresponding sex. The ways in which the risk of an individual being MBS is estimated in the general population or in members of the MBS families are indicated. The limitations of the discriminant diagnosis based on body measurements, as well as its particular applications in studies of the Martin-Bell syndrome, are discussed.

Longitudinal IQ changes in fragile X males

Previous studies have reported a drop of IQ in males with Martin-Bell or fragile X syndrome during childhood and adolescence. It is uncertain when and why this drop occurs and whether it affects all fragile X males. We have analyzed longitudinal IQ data on all 24 fragile X males who have been followed for at least 2 years through the Child Development Unit in Denver and who have been evaluated with a Stanford-Binet Form L-M assessment. A significant drop in IQ occurred for the group as a whole, but an individual z analysis demonstrated that only 7 of 24 had a significant decline in IQ. Visual inspection demonstrates an initial decline in IQ beginning in middle childhood and continuing through adolescence. We postulate that this drop occurs because of relatively greater weaknesses with abstract reasoning and higher symbolic language skills that are stressed in the cognitive testing of later childhood and adolescence.

Study of a family with a fragile site of the X chromosome at Xq27-28 without mental retardation

The fragile site Xq27-28 was observed in several individuals of a large family. It is expressed at a high frequency among the carrier females, even as adults, and in one clinically normal male. None of the members of this family is affected with the mental retardation normally linked to this fragile site. Cytogenetic and flanking DNA marker polymorphism studies suggest a possible dissociation between the fragile site and clinical expression of the disease.

Dissociation between mental retardation and fragile site expression in a family with fragile X-linked mental retardation

We report an extended family in which two brothers with a fragile X chromosome are mentally retarded while a third brother with the fragile site is both phenotypically and mentally normal. The study of six probes detecting restriction fragment length polymorphisms on either sides of the fragile site Xq27 confirmed that the fragile X regions inherited by these three brothers were identical from DXS102 to the telomere. These data highlight the heterogeneity of the fragile X syndrome, which is discussed in the framework of the different hypotheses previously proposed.

Linkage analysis using multiple DNA polymorphic markers in normal families and in families with fragile X syndrome

Linkage data, using the polymorphic markers 52A (DXS51), F9, 4D-8 (DXS98), and St14 (DXS52), are presented from 14 fragile X pedigrees and from 7 normal pedigrees derived from the collection of the Centre d'Etude du Polymorphisme Humaine. A multipoint linkage analysis indicates that the most probable order of these four loci in normal families is DXS51-F9-DXS98-DXS52. Recombination frequencies (theta) corresponding to maximum LOD scores (Z) were obtained by two-point linkage analysis for a number of linkage groups, including: DXS51-F9 (Z = 5.94, theta = 0.03), F9-DXS98 (Z = 0.51, theta = 0.26), F9-DXS52 (Z = 0.84, theta = 0.27), and DXS98-DXS52 (Z = 0.32, theta = 0.20). A multipoint linkage analysis of these loci, including the fragile X locus, was also performed for the fragile X population and the data support the relative order (DSX51, F9, DXS98)-FRAXA-DXS52. Recombination frequencies and maximum LOD scores, which again were derived from two-point linkage analyses, were obtained for the linkage groups DXS51-F9 (Z = 9.96, theta = 0) and F9-DXS52 (Z = 0.07, theta = 0.45) as well as for the groups DXS51-FRAXA (Z = 2.42, theta = 0.15), F9-FRAXA (Z = 1.30, theta = 0.18), DXS98-FRAXA (Z = 0.05, theta = 0.36), and DXS52-FRAXA (Z = 2.42, theta = 0.15). The linkage data was further tested for the presence of genetic heterogeneity both within and between the fragile X and normal families for the intervals DXS51-F9, F9-DXS52, F9-FRAXA, and DXS52-FRAXA using a modification of the A test. Except for the interval F9-FRAXA (P less than 0.10) there was no evidence of genetic heterogeneity for each of the various linkage groups examined. The heterogeneity detected for the interval F9-FRAXA, however, was most likely due to one family (Fx-28) that displayed very tight linkage between these two loci.

Clinical features and reproductive patterns in fragile X female heterozygotes

Clinical findings are presented on 113 fragile X female heterozygotes from 44 families, based on physical examination, behavioural assessment, and reproductive history. In 85% of a subsample of 92 adult females non-verbal IQ score derived from the Block Design test was 85 or less. Verbal ability deficits were much less common. Typical facial characteristics, irregular teeth, and hypermobility of finger joints occurred in approximately 40% of adult females, but facial abnormalities were less common in children. Some physical anomalies and the level of intellectual impairment were, in adult carriers, associated with the presence of fragile X sites. The commoner physical anomalies or typical facial characteristics and intellectual abilities differed significantly between the known female heterozygotes and their 40 presumed normal relatives. Frequency of miscarriages was increased in fragile X females; in spite of this, a moderate increase in the number of children has been encountered in female carriers with borderline intellectual impairment. This important problem has genetical implications and needs further investigation. The importance is emphasised of a more detailed clinical examination of the females at risk in fragile X families.

Anthropometry in Martin-Bell syndrome

Thirty anthropometric measurements were analyzed in 147 adults with Martin-Bell syndrome (MBS) (56 men and 91 women) and in a random sample of 108 normal women and 111 men. Results of the univariate comparison of the age, height, or weight-adjusted variables between MBS and normal individuals of either sex indicated that a decrease in stature, in upper limb length, and in upper face height, and an increase in jaw length, chest circumference, and waist width occurred in both affected men and in heterozygous women. While the increase in ear height and breadth and in hypermobility of finger joints and decrease in palm width and bigonial diameter occurred only in affected men, increased bispinal and bitrochanteric diameters, upper arm circumference, and palm and wrist widths were characteristic deviations in heterozygous women. Multivariate analysis in the form of principal components showed some differences in the pattern of interrelationships in individual measures between MBS and normal individuals. In particular, and in contrast with both normal groups, height and weight tended to load on separate components (as did head and midfacial measures) in MBS individuals. A discriminant function based on all body measurements included in this study resulted in almost complete separation of discriminant scores of normal from those of MBS men and in good separation of the scores from normal and heterozygous women. Classification rates based on these functions were from 95% in men to 85% in women. These already high classification rates may be further improved mainly by enlarging the samples and including some other category of traits such as dermatoglyphic measurements.

Discriminant analysis of dermatoglyphic measurements in fragile X males and females

Two hundred and eight fragile X subjects (92 males and 116 females) and matched Australian (60 males and 32 females) and British (122 males and 118 females) normal samples were used to calculate 4 discriminant functions, based on dermatoglyphic measurements. The most efficient discriminating variables between fragile X and normal males, selected by means of the Wilk's stepwise method, included: ridge counts on fingers 1-3, the hallucal (f) count on soles, the atd angle, and pattern intensities in palmar areas 2, 4 and 5 as well as on fingers 4 and 5. In females, the ridge breadth, the hallucal (e) count, the atd angle and pattern intensities in palmar areas 3-5 as well as on fingers 1, 3 and 5 comprised the final discriminant. The misclassification rate based on distributions of individual discriminant scores in each pair of samples, and on prior probabilities, was lowest (16.8%) in fragile X males compared with the Australian normal subjects. In both female comparisons, this rate approached 44%. A bias to misclassification rates resulting from various analytical procedures and some properties of the data are discussed. We conclude that the discriminant function based on dermatoglyphic measured variables alone is not good enough for assessing carrier probabilities for fragile X, especially in females. However, we have been able to select the best discriminators which may be used, together with other measured body characteristics, to obtain a more powerful discriminant function. Moreover, a consideration of discriminant scores based on dermatoglyphic traits only may help in estimating carrier probabilities.(ABSTRACT TRUNCATED AT 250 WORDS)

Individual variation and specific cognitive deficits in the fra(X) syndrome

Mental retardation has been associated with fra(X) but comprehensive psychological evaluation has rarely been applied to 2 major behavioral questions 1) the extent of individual variation in IQ among fra(X) males and the possibility of some fra(X) males being of normal IQ; and 2) whether there is a depression in general IQ or whether specific abilities are impaired. The problems of developing an effective battery of tests for assessing fra(X) are discussed. These questions were examined in 54 individuals, comprising fra(X) males, their obligate carrier mothers and those sisters shown to have the fra(X). Among noninstitutionalised males nonverbal IQ as measured on a Block Design test ranged from 100 to 0, and vocabulary scores while generally higher, ranged from 79-33. The males scored low on a digit span memory task, while performance on a memory of objects task was adequate. Despite lower overall scores, a similar pattern and variability emerged in institutionalised males. Daughters were extremely variable in performance and the mothers performed much better, supporting the view that women who have children are a selected subset of fra(X) syndrome individuals. The performance of one male is discussed in detail. His vocabulary and nonverbal IQ scores were normal, despite his having other specific cognitive deficits. The pattern of abilities and behavior seen in fra(X) may result in an overestimation of intelligence and underestimation of penetrance when based on clinical impressions rather than formal psychological assessment. The implications of this for molecular and for population genetic approaches to fra(X) are discussed.

Dermatoglyphic findings in fragile X syndrome: a causal hypothesis points to X-Y interchange

Dermatoglyphic analysis which included topological pattern elements, pattern intensities and dermatoglyphic measurements was performed in 90 male and 110 female fragile X subjects. Data on cytogenetic and intellectual assessments were also available in these individuals. The following conclusions have been drawn from the obtained results. 1. The fragile X syndrome is associated with an appreciable deviation in dermatoglyphic patterns and measurements, especially in males, which may be utilized for diagnostic purposes. 2. The variability in patterns and measurements is considerably increased which, together with irregular distributions, indicates that the fragile X condition may not be homogeneous. 3. The type of deviation in either sex is not inconsistent with the effect of an excessive dosage of the Y, combined with a deficit of the X chromosome and is, therefore, indicative of the X--Y chromosomal rearrangement as an initial lesion in fragile X. Since the deviation may not be specific, direct evidence for such rearrangement should be sought by means of molecular techniques.