Prevalence of the fragile-X syndrome in mentally retarded boys in a Swedish county

In an etiological study of an unselected series of mentally retarded children (IQ less than 70) born 1959-1970 in a northern Swedish county, 12 of 205 boys (5.9%) were found to have a fragile site on the distal end of the C-chromosome (fra (X) (q27]. The incidence of the fra (X) syndrome was calculated to be 1:1500 boys in this county. If this is true for the whole of Sweden, 30-40 new cases of the fra (X) syndrome should be born yearly in Sweden. This must be considered a minimum figure, since a certain proportion of individuals with fra (X) are not observed in groups of mentally retarded patients. Next to trisomy 21, the fragile X syndrome is the most common specific cause of mental retardation among mentally retarded boys in Sweden.     

Fragile X screening program in New York State

Most fragile X [fra(X)] males in New York State have not been identified. Hence, a large number of female relatives are unaware of their risks for having an affected child. A program was established in New York State in 1987 to screen for the fra(X) syndrome in mentally retarded males with living relatives. The goal of the program is to identify affected males and inform their families about the diagnosis. In this way relatives would be able to assess their risks for having a fra(X) male. In order to identify the males a screening form was developed to assess 10 features which included physical characteristics, behavior, and family history. Males who exhibited at least 5 of these manifestations were selected for cytogenetic analysis. Any male who had macroorchidism or a family history of mental retardation was also included. A total of 995 males have been screened of which 352 (35%) were selected for cytogenetic analyses. Seventeen (10.5%) of the 161 completed studies were positive for fra(X). A large number of possible female carriers were identified in the families of the propositi. This program identifies fra(X) males in a population of the mentally retarded for whom there had been no previous diagnosis. By using a two-step procedure, it is possible to screen a large population of the mentally retarded for fra(X) without testing each male cytogenetically.     

Fragile X frequency in a mentally retarded population in Brazil

Seventy-five male and 50 female students from 2 special schools for mildly, moderately retarded, or borderline individuals were screened clinically and cytogenetically in order to estimate the contribution of fragile X [fra(X)] syndrome to the cause of mental retardation in Brazil. We found 6 males (8%) from 4 families and 2 unrelated females (4%) with fra(X) chromosomes. One male and one female were isolated cases. The estimated frequency of Martin-Bell [fra(X)] syndrome among mentally impaired individuals in Brazil was similar to that previously reported in other countries.     

Fragile site X chromosomes in mentally retarded boys.

The fragile X syndrome is a common X-linked mental retardation and autism, affecting females as well as males. The fragile site X chromosomes were studied in a series of 153 mentally retarded boys of unknown etiology to determine the frequency of fragile X syndrome, and to assess the feasibility of making a clinical diagnosis of the fragile X syndrome in young boys before cytogenetic results were known. The 10 boys (6.4%) were positive for fra (X) (q27). The phenotype of fra (X) (q27) positive patients were typical except one who also had sex chromosomal mosaicism. There were three pairs of siblings among the fra (X) (q27) positive patients. Frequency of expression of the fragile site was in 10 to 47 per cent of cells. In addition, 19 boys showed a previously unsuspected chromosomal abnormality. The frequency of the fragile X syndrome in the present study is not significantly different from those in Caucasians and Japanese population. The fragile X syndrome can be recognized by noting key aspects of family history as well as the clinical features in mentally retarded boys.     

Martin-Bell syndrome in Greece,with report of another 47,XXY fragile X patient

A cytogenetic investigation was carried out among 200 mentally retarded boys in Greece for the detection of the fragile X [fra(X)] syndrome. Thirteen patients were found to carry fra(X) (6.5%). Of those, six boys had a history of familial X-linked mental retardation, two had the phenotype of the Martin-Bell syndrome, four had only mental retardation of unknown etiology, and one was a mentally retarded patient with Klinefelter syndrome. The remaining 187 boys were fra(X) negative. Our findings emphasize the importance of early identification of this syndrome in the diagnosis and prevention, through proper genetic counselling, of mental retardation.     

Fragile X syndrome: incidence, clinical and cytogenetic findings in the black and white populations of South Carolina

Individuals in South Carolina with the Fragile X [fra(X)] or Martin-Bell syndrome have been ascertained by referral for evaluation of facial abnormalities, macroorchidism or mental deficit; by screening patients in residential and day programs for the mentally retarded; and by family follow up after an index case has been identified. Between 1982 and 1987, 100 positive fra(X) males were diagnosed. Of these, 35 were residents of residential facilities for the mentally retarded representing 2.5% of the population of institutionalized males. Another 23 were found in community day programs for the mentally retarded. Of these 58 cases, 28 (48%) were ascertained by screening for the craniofacial characteristics of the Martin-Bell syndrome, namely long face, midface hypoplasia, prominent forehead, large mandible and large simple pinnae. Although this screening procedure proved to be productive, it was found that the craniofacial traits of long face, midface hypoplasia, large jaw and simple pinnae were found less frequently in black fra(X) positive males and in prepubertal boys of both races.     

The fragile X syndrome in a large family. II. Psychological investigations.

Intelligence levels and intelligence profiles were investigated in 52 members of a large family with the fragile X syndrome. The mental abilities were evaluated by the three Wechsler intelligence tests (WAIS, WISC-R, and WPPSI). Chromosomal and psychological data were then compared. In 22 non-retarded fra(X) negative family members, a mean IQ of 102 was found (males 97, females 106). Eleven non-retarded carrier females had IQs between 70 and 110 (mean 86.8), which is 20 points below the mean of normal women (alpha = 0.01). One non-retarded male with 6% fra(X) positive cells had an IQ of 98. His intelligence profile closely resembled the profile in the non-retarded female carriers. The highest IQ in the group of retarded males was 31. The mentally retarded females scored IQs between 26 and 41. In male and female patients verbal intelligence substantially exceeded performance abilities. There was a considerable gap between the highest IQ in the group of retarded females and the lowest IQ in the group of non-retarded carriers (41 and 71 respectively) and a considerable overlap was found between the IQ levels of the non-retarded carriers and normal women.     

Fragile Xq27.3 in female heterozygotes for the Martin-Bell syndrome.

X inactivation studies have been carried out on lymphocytes from eight unrelated females heterozygous for the Martin-Bell syndrome. Four of these carriers were of normal IQ and four were mentally handicapped. When BrdU was used to differentiate between the active and inactive X chromosome an average of 55% of fra(X) were active in the retarded subjects, but only 27% were active in those of normal IQ. When 3H thymidine was used to differentiate between the active and inactive X chromosome, an average of 58% of mitoses from handicapped subjects and 33% of mitoses from normal subjects showed an active fra(X) in informative cells. These results are compared with previously published studies and it is concluded that the number of inactive fra(X) chromosomes calculated as a proportion of all cells scored is the same in mentally normal and mentally retarded subjects. However, the number of active fra(X) chromosomes is consistently higher in the retarded than in the normal females.     

Fragile (X) syndrome: a study of the psychological profile in 23 prepubertal patients

In this study a further analysis of the psychological profile in the prepubertal fragile (X) (fra(X] male was performed. The results of the psycho-diagnostic examination of 23 fra(X) boys were compared to a control group of 17 males of the same age with 'non-specific' mental retardation. A number of important quantitative and qualitative differences were observed between the two groups. In the preschool age group the majority of fra(X) boys was mildly mentally retarded. In the school-age group, however, most boys were moderately to severely mentally retarded. This indication of a decline in intellectual performance with age in the fra(X) syndrome was confirmed by a longitudinal individual follow-up of seven fra(X) boys in this age group. In contrast to intellectual performance, appearance of the attention deficit disorder (or hyperkinesis), with its attendent overactivity and impulsiveness, decreases with age, and is independent of the intellectual level. Autistic behaviour was more frequently observed in the youngest fra(X) males, and was more pronounced in the moderately mentally retarded. In more than 50% of the boys of preschool age the association of hyperkinesis and autistic features was found. Language and speech development in the fra(X) syndrome is both symptomatic and specific. Beside a severe, global speech retardation, there are some distinct speech characteristics in the young fra(X) males such as rapid speech rhythm, speech impulsiveness and perseverative speech.     

Fragile-X mutation and Klinefelter syndrome: a reappraisal

To date the concurrent presence of the fragile-X and the Klinefelter syndromes in the same individual has been found at least 8 times either in the course of screening for the fra(X) condition in mentally retarded males or among the relatives of fra(X) propositi. Given the high frequency of both events in the general population and the heterogeneous approaches with which the above cases were ascertained, it has not been possible to determine unequivocally so far whether the finding is purely coincidental or the expression of some underlying biological relationship. To evaluate the issue, we have screened a large population of institutionalized mentally retarded males for microorchidism, and submitted to a full karyotype analysis and fra(X) testing the patients that were found to have marked bilateral microorchidism. Thus, in a total of 32 microorchidism patients identified among 1115 mentally retarded males, we found 6 to have a 47,XXY chromosome complement in all (or in most) of their cells, with one of them having also the fra(X) marker in 9% of the metaphases examined. In addition, another bearer of the fra(X) marker (but only in 4% of his metaphases) was found among 26 47,XXY mentally normal males ascertained throughout routine cytogenetic analysis of males with microorchidism referred to our genetic counseling unit during the last 10 years. In our laboratory the fra(X) marker has never been observed with such a frequency in a total of several hundred normal XY males and XX females studied as control cases in the course of previously reported family and population studies.(ABSTRACT TRUNCATED AT 250 WORDS)     

Three families with high expression of a fragile site at Xq27.3, lack of anomalies at the FMR-1 CpG island, and no clear phenotypic association.

We report on 3 families where the presence and segregation at high frequency of a fragile Xq27.3 site is not associated with the mutations and methylation anomalies typically seen in the fragile X [Fra(X)] syndrome. In one family, a folate insensitive fragile site was associated with Robin sequence in the propositus. In a second family a fra(X) negative mother has two fra(X) positive sons (one mentally retarded and the other newborn). The third family presents very high expression of a folate sensitive site, unlinked to mental retardation, and was described previously by Voelckel et al. [1989]. The fragile sites in these or similar families recently described must be different from the one associated with the fra(X) syndrome. Their association with a clinical phenotype or with mental retardation is certainly not consistent, and may represent an ascertainment bias. However, the relatively high frequency with which they have been found among previously diagnosed fra(X) families suggests that, at least in some cases, the association with mental impairment may be significant. In two families reported up to now, a male with high expression of such variant fra(X) site failed to transmit it to his daughter, which may reflect an imprinting effect. Previously diagnosed families should be reinvestigated before direct DNA analysis is used for prenatal or carrier diagnosis of the fra(X) syndrome.     

Cytogenetic diagnosis of the fragile X syndrome: efficiency, utilization, and trends

In order to assess the impact of the increasing awareness of the fra(X) syndrome and a broader approach to fra(X) testing, we analyzed our laboratory experience for 1980-1988. In 1981-1986, there was an average of 80 cases/year (62 male; 18 female). The 103 (74 male; 29 female) cases in 1987 represent a 45% increase over the prior 3 years; this sustained in 1988 with 106 cases. The fra(X) positive yield decreased from a high of 49% in 1980 to an average of 20% (range 15-24%) in 1981-1984, 10% (range 9-11%), in 1985-1987 and 7% in 1988. The positive rate for males and females was nearly identical in both time periods. The positive yield for mentally retarded individuals with a family history of mental retardation dropped from an average of 20% for 1981-1984 and 33% for 1985-87 to 13% for 1988; however, the positive fra(X) rate for mentally retarded individuals decreased from an average of 23% in 1981-1984 to 9% in 1985-1987 and 7% in 1988. The decreasing fra(X) yield and increasing case load are directly attributable to the relaxation of criteria for referral and testing related to the referral of all mentally retarded patients, and to the perceived malpractice liability for not doing a "complete" evaluation. Although the burden for cytogenetic laboratories is considerable, the yield of positive fra(X) cases is still worthwhile, and may be maximized by the use of improved screening criteria.     

Mental impairment in cytogenetically positive fragile X females

Prenatal diagnosis is now available to fragile X (fra[X]) syndrome families and has proven reliable when testing male fetuses. It has been reported that approximately one-third of heterozygous fra(X) females demonstrate mental impairment. Based on this, families usually continue pregnancies involving a female fetus. The purpose of this study is to investigate whether a 35% risk for mental impairment is appropriate when counseling heterozygous women carrying fra(X)-positive female fetuses. Forty-three cytogenetically positive (greater than or equal to 2%) daughters of known fra(X) carrier women were ascertained postnatally in an unbiased fashion. Their mother's carrier status was determined on the basis of at least one son with Martin-Bell syndrome. In addition to peripheral blood cytogenetic studies, all daughters had cognitive testing to determine full-scale IQ. In this study, 55.8% (24/43) were mentally impaired (IQ less than 85) compared with the expected 35%. Of these, 42% were mentally retarded (IQ less than 70). Although we do not know the correlation between percent fragility by peripheral blood compared with the percent fragility by amniocentesis or CVS, we assume that they are relatively comparable such that a female who is positive with greater than or equal to 2% fragility would probably be positive by all methods. This report suggests that the penetrance of mental impairment in females with a percent fragility of greater than or equal to 2% may be as high as 55%. Further studies are necessary to clarify this issue so that accurate information can be given in genetic counseling.     

The fragile X syndrome in a large family. III. Investigations on linkage of flanking DNA markers with the fragile site Xq27.

In a large family with the fragile X syndrome, we performed linkage investigations with six probes, detecting RFLPs at both sides of the fragile site Xq27. The nearest flanking markers were cX55.7 (DXS105) on the centromeric side (theta = 0.04, lod 5.0) and St14 (DXS52) on the telomeric side (theta = 0.08, lod 4.0). Non-penetrance could be shown by the presence of the grandpaternal X chromosome in three mentally retarded fra(X) positive males. A second non-penetrant male in this family had inherited an abnormal grandmaternal X chromosome. His carrier mother had two retarded fra(X) positive brothers. Intermediate between the non-penetrant and fully penetrant males was a non-retarded male, who expressed the fragile site in 6% of his cells. His X chromosome showed the same polymorphisms as were found in his seven severely retarded brothers. In five fra(X) negative females the presence of an abnormal X chromosome could be demonstrated. Despite the existence of non-penetrance in this pedigree, there was no close linkage between a factor IX polymorphism and the fragile site (theta = 0.16, lod 1.9). However, in six descendants of a non-penetrant male, the change to penetrance appeared to be accompanied by a low recombination frequency for flanking markers.     

Effect of X inactivation on fragile X frequency and mental retardation

The probability of a heterozygote being affected was estimated from the distribution of frequencies of early-replicating fragile X [fra(X)] chromosome in normal and mentally retarded heterozygotes, taking into account the prior probabilities of 0.35 for mental retardation and 0.65 for normality. The estimated probability of a heterozygote with 100% early-replicating fra(X) being mentally retarded was 78%, which coincides with the value of penetrance in males. Therefore, the manifestation of retardation in females seems to differ from that in males due solely to X inactivation. The frequencies of early-replicating fra(X) were significantly increased among the heterozygotes with the highest frequencies of fra(X) both in the normal group and in the mentally retarded. The mean frequencies of early-replicating fra(X) were 0.42 and 0.68 for normal and mentally retarded heterozygotes, respectively. Considering the overall frequency of retarded heterozygotes as 0.35, the mean frequency of early-replicating fra(X) obtained for all heterozygotes was 0.51, which is in accordance with the hypothesis of random X inactivation. Thus the fragile site appears to have equal chances of being detected when located either on the early- or on the late-replicating X. This leads to the conclusion that the frequency of the fragile site is a consequence of the proportion of cells with the active Martin-Bell syndrome (MBS) gene and not the result of a better visualization of the site on the early-replicating X.     

Fragile (X)(q27) sites in a pedigree with female carriers showing mild to severe mental retardation.

A pedigree showing the fragile site at Xq27 in a severely retarded female and in other less retarded carriers is described. Two of the four moderately retarded males with the fra(X)(q27) show macro-orchidism, and a variety of other features usually used to support the effects of the fra(X)(q27) are also inconsistent. A second fragile site at (10)(q23) is also present and in the two oldest females its frequency is not decreased, whereas the fra(x)(q27) is not detectable in these females although probably present. It is concluded that pedigrees showing mentally retarded females and probable X linkage should be included in studies of the fra(X)(q27).     

Fragile X syndrome with extra microchromosome

A mentally retarded male with Martin-Bell syndrome, who has an extra microchromosome and is fra X negative in cytogenetic study is reported. Because of its small size, the origin of the microchromosome could not be determined. Two other affected males in this family (a cousin and a nephew of the proband) were fragile X positive, 24% and 26%, respectively. Cytogenetic studies and DNA analysis with the probe St B 12.3 were performed on several members of the family. The proband and the two other affected males showed a similar full mutation on the molecular study. This study emphasizes the importance of molecular analysis in the diagnosis of fragile X syndrome, particularly when cytogenetic studies demonstrate fra X negative in individuals in families likely to have X-linked mental retardation.     

Autosomal folate sensitive fragile sites in normal and mentally retarded individuals in Greece

The frequencies of autosomal folate sensitive fragile sites were compared in populations of mentally retarded fra(X) negative (N = 220) and normal children (N = 76) in Greece. In addition, the frequency of autosomal fragile sites was studied in 20 known fra(X) children in order to test if the fra(X) syndrome is associated with general chromosome instability. The frequencies of both common and rare autosomal fragile sites did not differ significantly between the mentally retarded and the normal children, although the rate of expression was considerably higher in the retarded group. Autosomal fragile sites were not increased in the fra(X) patients. Fra(3)(p14) was by far the most frequent one in all groups. The frequency of fra(6)(q26) was found to be considerably higher among the mentally retarded children, this difference being almost statistically significant. Further cytogenetic studies of normal and retarded individuals are required in order to elucidate this point further.     

Anthropometric and craniofacial patterns in mentally retarded males with emphasis on the fragile X syndrome

Butler MG, Pratesi R, Watson MS, Breg WR, Singh DN. Anthropometric and craniofacial patterns in mentally retarded males with emphasis on the fragile X syndrome. Clin Genet 1993: 44: 129–138. © Munksgaard, 1993 Anthropometric and craniofacial profile patterns indicating the percent difference from the overall mean were developed on 34 physical parameters with 31 white, mentally retarded males (23 adults and 8 children) with the fra(X) syndrome matched for age with 31 white, mentally retarded males without a known cause of their retardation. The fra(X) syndrome males consistently showed larger dimensions for all anthropometric variables, with significant differences for height, sitting height, arm span, hand length, middle finger length, hand breadth, foot length, foot breadth, and testicular volume. A craniofacial pattern did emerge between the two groups of mentally retarded males, but with overlap of several variables. Significant differences were noted for head circumference, head breadth, lower face height, bizygomatic diameter, inner canthal distance, ear length and ear width, with the fra(X) syndrome males having larger head dimensions (head circumference, head breadth, head length, face height and lower face height), but smaller measurements for minimal frontal diameter, bizygomatic diameter, bigonial diameter, and inner canthal distance. Several significant correlations were found with the variables for both mentally retarded males with and without the fra(X) syndrome. In a combined anthropometric and craniofacial profile of 19 variables comparing 26 white fra(X) syndrome males (13 with high expression (>30%) and 13 with low expression (<30%), but matched for age), a relatively flat profile was observed with no significant differences for any of the variables. Generally, fra(X) syndrome males with increased fragile X chromosome expression have larger amplifications of the CGG trinucleotide repeat of the FMR-1 gene. No physical differences were detectable in our study between fra(X) males with high expression and apparently larger amplifications of the CGG trinucleotide repeats compared with those patients with low expression. Our research illustrates the use of anthropometry in identifying differences between mentally retarded males with or without the fra(X) syndrome and offers a comprehensive approach for screening males for the fra(X) syndrome and selecting those individuals for cytogenetic and/or molecular genetic testing.     

Fragile X mental retardation: prevalence in a group of institutionalized patients in Italy and description of a novel EEG pattern

We have studied the prevalence of the fra (X) and of the autosomal fragile sites fra (10) (q25) and fra (16) (q22) in patients from an institute for the mentally retarded in Italy. We found six cases (1.9%) of fra (10) (q25) and 9 (2.9%) of fra (16) (q22). The study of the fra (X) was restricted to a subgroup of 91 males who did not have other chromosome anomalies or variants, and led to the discovery of 4 fra (X) cases. These 4 had the Martin-Bell syndrome; 3 of them were epileptic and had a characteristic EEG pattern originating during sleep from the temporal lobe not previously described in fra (X) mental retardation.