Mental retardation, acromegalic face, and megalotestes in two half-brothers: a specific form of X-linked mental retardation without fra(X) (q)?

We describe a family with two half-brothers affected with severe mental retardation. The phenotype in the affected individuals is characterized by apparent acromegaly, profound mental retardation, and hyperactivity. The mother has analogous but less severe facial anomalies and mild mental impairment. Screening for fra(X) (q) was negative in peripheral lymphocytes using methotrexate for fra(X) enhancement. The clinical findings in our patients are similar to those described by Fryns et al. [1986] in two patients with acquired lesions of the central nervous system. CT investigations in one of our patients showed areas of hyperdensity in the pontine region and a small subarachnoid cyst. The pedigree suggests X-linked inheritance. The association of apparent acromegaly, CNS anomalies, megalotestes, and mental retardation in this family supports the hypothesis that a distinct syndrome may exist with phenotype anomalies more severe than those characteristic for the Martin-Bell syndrome but without fragile X.     

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.     

Origin of the supernumerary X chromosome in a patient with fragile X and Klinefelter syndrome.

We report on a 10-year-old patient with the fragile X [fra(X)] syndrome and a 47,XXY karyotype. He had Martin-Bell syndrome, including typical craniofacial findings and mental retardation. The fra(X) was detected on both X chromosomes of the patient in 8% of the metaphases examined. DNA analysis using X-chromosome sequences from the pericentromeric region and from distal Xq suggests that the patient is homozygous at the fra(X) locus due to maternal nondisjunction during meiosis II.     

The fragile X in Sicily: an epidemiological survey

We have studied a group of 349 institutionalized propositi with mental retardation, and found 12 fra(X)-positive cases among 155 males (7.7%) and 8 fra(X)-positive cases among 194 females (4.1%). The males had characteristic manifestations of the Martin-Bell syndrome. Another 7 males, who were initially considered "borderline", having expression of fra(X) less than 4% and a non-characteristic phenotype, were eventually considered negative. Among 5,624 patients (2,764 males and 2,860 females) that were admitted to the Pediatric Department of the University of Catania during the period July 1986 - June 1987, 210 (120 males and 90 females) had mental retardation. Of these, 75 were analyzed for the presence of fra(X) (q27.3); 5 males (0.18% of all males) and 2 females (0.07% of all females) were fra(X)-positive. The males had the Martin Bell syndrome phenotype. The presence of fra(X) (q27) was confirmed in another 4 male propositi that were referred to our outpatient services with a clinical diagnosis of Martin-Bell syndrome.     

The fragile X-chromosome: an evaluation of the results in a routine cytogenetic laboratory in the period 1981–1986

We report on the cytogenetic studies, performed in a routine cytogenetic laboratory between 1981 and 1986, on 428 subjects: 291 probands with non-specific mental retardation, 101 first-degree relatives of fra(X) positive patients and 36 non-retarded patients, referred for other reasons. As a rule 50 cells, cultured in folate-deficient medium were analysed. The results are compared with data collected from the literature and with the data expected from prior genetic risks and fra(X) penetrance. If no more than 50 cells were analysed, the fragile site was found in a lower than expected proportion (69.2%) of the retarded first-degree male relatives. The conclude that in a substantial number of unselected patients with mental retardation the diagnosis fra(X) syndrome will only be confirmed cytogenetically if at least 100 cells are analysed. Five percent of the male and 10% of the female index patients showed a fragile X-like abnormality, probably not associated with the Martin-Bell syndrome.     

Aortic root dilatation and mitral valve prolapse in the fragile X syndrome

Forty patients with fragile X [fra(X)] or Martin-Bell syndrome, confirmed by chromosome analysis, underwent full cardiac evaluation including physical examination, chest film, electrocardiography (ECG), and M-mode and 2-dimensional echocardiography. Thirty-four males and six females were studied. Although all patients were asymptomatic, seven males were found to have mild aortic root dilatation. All seven also had evidence of mitral valve prolapse. Twenty-two (55%) of the study patients had mitral valve prolapse with either a click or murmur heard on physical examination and confirmation by M-mode echocardiography. The frequency of mitral valve prolapse was the same in males and females, but 80% of males older than 18 years had mitral valve prolapse. These findings support the hypothesis of a connective tissue dysplasia in the fra(X) syndrome.     

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.     

A premutation that generates a defect at crossing over explains the inheritance of fragile X mental retardation

The view that the Martin-Bell syndrome (X-linked mental retardation with fragile site at Xq27/8) is inherited in a regular X-linked fashion is becoming untenable with the increasing number of reports of transmission through phenotypically normal males. Analysis of the published pedigrees containing such males shows that their heterozygous daughters are never mentally retarded, and have either no fragile site or very few indeed. By contrast, in the next generation, a third of the female heterozygotes are mentally subnormal with an average of 29% fragile sites. These data suggest a premutation that generates the definitive mutation only when transmitted by a female. We propose an inherited sub-microscopic chromosome rearrangement involving the Xq27/8 region that causes no ill effect per se, but generates a significant genetic imbalance when involved in a recombination event with the other X chromosome. This hypothesis explains many of the puzzling genetic aspects of the Martin-Bell syndrome, but it also complicates the interpretation of linkage analysis with genetic markers.     

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 X syndrome: a hypothesis regarding the molecular mechanism of the phenotype

Among all the human chromosomal fragile sites currently recognized, the fragile site mapping to Xq27.3 is the only one associated with an abnormal phenotype. This phenotype, referred to as the Martin-Bell or fragile X syndrome, has mental retardation as its most important manifestation. We propose that this site is associated with an abnormal phenotype due its location on the X chromosome, particularly it's proximity to the q telomere. Thus, if an in vivo break should occur with loss of Xq28 in the fra(X) male, the cell would be nullisomic for the genes distal to the fragile site. Similarly, a female cell would be functionally nullisomic if the break occurred on the active X. Breakage and loss of genetic material at other fragile sites either would have no impact due to complementation by homologous genes or would be lethal if X-linked with a significant deletion (i.e. fra(Xq22]. This leads to the proposal that the fragile X syndrome is due to mosaic nullisomy of distal genes. We describe below the implications of this model and a means to test this hypothesis.     

Aortic hypoplasia and cardiac valvular abnormalities in a boy with fragile X syndrome

An 18-year-old mentally retarded male with the Martin-Bell syndrome was fragile X positive. He died suddenly with viral pneumonia and myocarditis. At autopsy, generalized tubular hypoplasia of the aorta and a mild coarctation were discovered. The base of the mitral and tricuspid valves showed striking aberrations in elastin distribution and structure by light microscopy. Local collagen alterations were also noted. Comparable changes were seen in the skin elastin as well as a severe depletion of acid mucopolysaccharides. These changes suggest a structural disruption underlying the clinical connective tissue problems in some patients with the fragile X syndrome.     

Prenatal diagnosis of fragile X syndrome by placental (chorionic villi) biopsy culture

Second trimester ultrasound-guided fetal blood sampling and placental biopsy were performed on 10 pregnancies at risk for fra(X)-linked mental retardation (Martin-Bell syndrome). Three cases were diagnosed as affected after cytogenetic analysis of fetal blood and placental cultures. The fra(X)(q27.3) and common fragile sites were shown to be expressed at a lower level in placenta than in fetal blood. Induction methods included methotrexate, 5-fluoro-2-deoxyuridine, and excess thymidine. Excess thymidine may give the best expression of fra(X)(q27.3). Enhancement of fra(X)(q27.3) expression was not shown with caffeine or 5-methoxybenzamide.     

Experience with multiple approaches to the prenatal diagnosis of the fragile X syndrome: amniotic fluid, chorionic villi, fetal blood and molecular methods

We have had experience with 160 prenatal diagnosis cases for the fragile X syndrome [fra(X)] or Martin-Bell Syndrome. In 140, amniotic fluid was utilized; 98 had a documented family history of fra(X). The 94 completed cases included 4 no growth; 56 males of which 7 were fra(X)-positive and 2 false-negative; 38 females of which 5 were fra(X) positive. There was no fra(X) positive result when a family history of mental retardation was not documented as fra(X). Molecular methods (RFLPs) were utilized in 10 amniotic fluid and 5 chorionic villus specimens (CVS). Percutaneous umbilical blood sampling was used in 2 negative cases and 1 fra(X) positive case because of timing, tissue culture failure or confirmation of another method. CVS were received in 13 cases, and RFLPs were utilized in 5 of the CVS cases. There was no positive fra(X) CVS chromosome result in males, 1 positive result in a female, but 2 false negatives were detected by RFLPs. On the basis of the results, it can be concluded that cytogenetic and molecular methods are complementary and best used together and that multiple approaches can enhance the efficiency and reliability of fra(X) prenatal diagnosis.     

How can the frequency of false-negative findings in prenatal diagnoses of fra(X) be reduced: experience with first trimester chorionic villi sampling

We report on 12 prenatal diagnoses performed between weeks 10 and 13 on normal women with a well-documented family history of the Martin-Bell syndrome. Seven were obligate and three were potential carriers. One male and 2 female fetuses were found to be fragile X [fra(X)]-positive. The diagnoses were confirmed in fibroblasts or lymphocytes after interruption or postnatally. In one fra(X)-negative female fetus, the analysis of linked DNA markers indicated that most probably she was a heterozygote. Reexamination after birth gave a fra(X)-positive result. Hence this was a case of a false-negative prenatal fra(X) result. The occurrence of false-negative cytogenetic results represents a common problem that limits the sensitivity of prenatal diagnostics in the Martin-Bell syndrome. A study of linked DNA markers can improve the reliability of negative cytogenetic results in first trimester prenatal diagnosis. In case of doubt, the chromosomes could be reexamined after fetal blood sampling.     

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.     

Abnormal dendritic spine characteristics in the temporal and visual cortices of patients with fragile‐X syndrome: A quantitative examination

Fragile‐X syndrome is a common form of mental retardation resulting from the inability to produce the fragile‐X mental retardation protein. Qualitative examination of human brain autopsy material has shown that fragile‐X patients exhibit abnormal dendritic spine lengths and shapes on parieto‐occipital neocortical pyramidal cells. Similar quantitative results have been obtained in fragile‐X knockout mice, that have been engineered to lack the fragile‐X mental retardation protein. Dendritic spines on layer V pyramidal cells of human temporal and visual cortices stained using the Golgi‐Kopsch method were investigated. Quantitative analysis of dendritic spine length, morphology, and number was carried out on patients with fragile‐X syndrome and normal age‐matched controls. Fragile‐X patients exhibited significantly more long dendritic spines and fewer short dendritic spines than did control subjects in both temporal and visual cortical areas. Similarly, fragile‐X patients exhibited significantly more dendritic spines with an immature morphology and fewer with a more mature type morphology in both cortical areas. In addition, fragile‐X patients had a higher density of dendritic spines than did controls on distal segments of apical and basilar dendrites in both cortical areas. Long dendritic spines with immature morphologies and elevated spine numbers are characteristic of early development or a lack of sensory experience. The fact that these characteristics are found in fragile‐X patients throughout multiple cortical areas may suggest a global failure of normal dendritic spine maturation and or pruning during development that persists throughout adulthood. © 2001 Wiley‐Liss, Inc.     

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.     

The frequency of the fragile X chromosome among schoolchildren in Coventry.

A population study has been carried out among schoolchildren in the City of Coventry in order to ascertain the frequency of mental retardation associated with the fragile X chromosome. The prevalence of the fragile X mental retardation syndrome in the 11 to 16 year age group (the age of greatest ascertainment) was about 1.0 per 1,000 and therefore indicates that the syndrome is a major cause of mental retardation.