Marker X syndrome in an oriental family with probable transmission by a normal male

Instability of a 550-base pair DNA segment and abnormal methylation in fragile X syndrome

The fragile X syndrome, a common cause of inherited mental retardation, is characterized by an unusual mode of inheritance. Phenotypic expression has been linked to abnormal cytosine methylation of a single CpG island, at or very near the fragile site. Probes adjacent to this island detected very localized DNA rearrangements that constituted the fragile X mutations, and whose target was a 550-base pair GC-rich fragment. Normal transmitting males had a 150- to 400-base pair insertion that was inherited by their daughters either unchanged, or with small differences in size. Fragile X-positive individuals in the next generation had much larger fragments that differed among siblings and showed a generally heterogeneous pattern indicating somatic mutation. The mutated allele appeared unmethylated in normal transmitting males, methylated only on the inactive X chromosome in their daughters, and totally methylated in most fragile X males. However, some males had a mosaic pattern. Expression of the fragile X syndrome thus appears to result from a two-step mutation as well as a highly localized methylation. Carriers of the fragile X mutation can easily be detected regardless of sex or phenotypic expression, and rare apparent false negatives may result from genetic heterogeneity or misdiagnosis.

The role of fluorinated pyrimidine analogues in the induction of the in vitro expression of the fragile X chromosome

The modes of action of 5-fluoro-2'-deoxyuridine (FdUrd) and 5-fluoro-2'-deoxycytidine (FdCyd) were studied in PHA-stimulated lymphocytes from normal volunteer donors and a fragile X patient. In both cell types, FdUrd and FdCyd inhibited cell proliferation at concentrations of 3 x 10(-8) M. Thymidylate synthetase was identified as the decisive target for the action of both FdUrd and FdCyd, as judged from the following observations: First, addition of thymidine to the culture medium was able to counteract both FdUrd and FdCyd toxicities, whereas addition of dCyd had no observable effect. Second, inhibition of the in situ thymidylate synthetase activity measured as an increase in the level of [3H]-dThd incorporation coincided with the inhibition of cell proliferation. Third, the inhibition of the thymidylate synthetase-dependent incorporation of [3H]-dUrd into newly synthesized DNA coincided with the inhibition of cell proliferation. The effects of FdUrd and FdCyd on the in vitro expression of fragile site Xq27 of fragile X chromosomes was shown to be based on the depletion of the intracellular pool of thymidine-5'-monophosphate (dTMP), as judged from the following observations: First, both the FdUrd- and FdCyd-dependent induction of site Xq27 coincided with the antiproliferative effects of the respective fluoropyrimidines. Second, addition of thymidine (dThd) to the culture medium both prevented the expression of site Xq27 and neutralized the cytotoxicity of FdUrd and of FdCyd. On the basis of these findings, we provide further evidence for the concept that the fragile X site is located in an AT-rich region.

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.

Fragile X expression in short-term whole blood cultures is affected by cell density

The effect of cell density on expression of the fragile site at (X)(q27.3) in short-term whole-blood cultures from patients with fragile X [fra(X)] or Martin-Bell syndrome was studied. A significant increase in fra(X) frequency was observed in 7 of 8 samples when cell density was decreased. Higher fra(X) frequency was not always noted at below-routine density, but in some cases fra(X) expression was depressed at above-routine density. We conclude that decay of the FUdR effect explains the fact that fra(X) expression is affected by culture density. It is significant that a relationship exists between the two; it suggests that in order to maximize fra(X) expression in cases with low-percentage fra(X) with standard methods, cell density may have to be adjusted. It is possible that in individuals who are normally nonexpressing, such as some obligate female carriers and nonpenetrant males, fra(X) expression may be sensitive to cell density effects.

Fragile X syndrome: growth, development, and intellectual function

We collected data on growth, psychomotor development, speech and language development, and intellectual function on a cohort of 100 males with the fragile X chromosome and 95 carrier females. The data include information on prenatal growth (33 males), growth during the preadult years (32 males), psychomotor development during the first 2 years (25 males), speech and language development (15 males and 5 females), and intellectual function (93 males, 33 females, and 10 obligate carriers who were cytogenetically normal). Birth measurements appeared normal when plotted on the Usher/McLean curves of newborn infants (mean head circumference - OFC - at 40th centile, length at 60th centile and weight at 55th centile). Following birth, OFC rose above the 50th percentile and continued above average throughout the preadult years, whereas average length was above average for the first 5 years only and weight did not deviate from the normal mean. Psychomotor development lagged behind the norm from birth with affected males requiring nearly twice as long as expected to sit alone, walk unassisted, and say first words clearly. All males and females studied had significant language delay; all except one male had abnormalities of articulation. All on whom a clear voice sample was obtained had low voice pitch, and 80% had a hoarse or harsh quality of voice. Five males had word repetitions or perseverative speech during the preadult years. The mean IQ of the 93 males studied was 33 and regression analysis demonstrated a decrease in intellectual performance with age. Four fifths of the female carriers who expressed the fra(X) had intellectual performance in the mentally retarded range and showed similar decrease in performance with age. Obligate female carriers who did not express the fra(X) site had normal IQs (IQ 102 +/- 13.3).

Phenotypic variation in male-transmitted fragile X: genetic inferences

Three families with confirmed and one family with suspected male transmission of the fragile X are presented, with psychological and physical assessment of all available members. The psychological tests used were the Peabody Picture Vocabulary test and Block Design which measured verbal and non-verbal abilities, respectively. Physical status was assessed by recording dysmorphic features and by anthropometric measurements. This study demonstrated that there are appreciable differences in mental and physical status within sibships of daughters of male carriers, as well as recognizable physical alterations and intellectual impairment in the transmitting males. These findings contradict the concept that there are two distinct categories of fragile X carriers: phenotypically normal as opposed to affected. They suggest instead that the defect may be graded and emphasize the importance of intellectual deficits and physical alterations in defining the fragile X phenotype, both in low-penetrant males and female heterozygotes.

Increased sister chromatid exchange frequency at Xq27 site in affected fragile X males

Sister chromatid exchange (SCE) was evaluated in peripheral blood lymphocytes cultured from normal subjects, fragile X carrier females, and fragile X affected males and in general did not differ among all subjects whether cells were grown in thymidine-deficient or complete medium. However, at the Xq27 site, SCE was significantly increased for fragile X affected males, particularly in cells grown in thymidine-deficient medium.

The fragile X syndrome in a large family. I. Cytogenetic and clinical investigations

Cytogenetic and clinical investigations were performed in 85 members of a large family, in which 18 males and seven females were mentally retarded. In the male patients the fragile site Xq27 was found in 6 to 44% (mean 22.5%) of peripheral blood lymphocytes. One non-retarded male expressed the cytogenetic abnormality in 6% of his cells. In 21 females the fra(X) was found in 3 to 28% (mean 8.7%) of their cells. Two obligate carriers did not express the fragile site. A significant difference in expression between the seven retarded (mean 16.7%) and seven non-retarded female carriers of corresponding age (mean 6.3%) was found (alpha = 0.01). No significant correlation between expression and age could be established, either in males or in females. The cytogenetic results appeared to be consistent. To avoid false positives, a cut-off point was chosen: males were considered to be fra(X) negative if no more than one in 100 cells showed the abnormality; for females the cut-off point was two in 100 cells. Segregation analysis did not detect significant deviations from the expected ratios. The putative presence of a transmitting male is discussed. The results of recombinant DNA analysis will be published elsewhere. Clinical investigations confirmed the findings of others. CT scans showed an enlargement of the ventricular system that exceeded the expected age changes.

Implications of fragile X expression in normal males for the nature of the mutation

The fragile site at Xq27, associated with a common form of X-linked mental retardation (XLMR), is expressed in a variable proportion of the peripheral lymphocytes of affected males when the cells are cultured under thymidylate stress (Td stress) produced by folate or thymidylate deprivation. Some clinically normal males--transmitting males--are known to carry and transmit the fragile X mutation and yet show no cytogenetic expression in lymphocytes. Normal males with no family history of X-linked mental retardation express the site only rarely. When the fragile X chromosome from affected males is isolated in a rodent genetic background by somatic cell hybridization, the level of expression is similar to that seen in lymphocytes under Td stress. Here we show that X chromosomes from two transmitting males and two normal control males, all of which were fragile X negative in lymphocytes or lymphoblasts, could be made to express the fragile site in hybrids, although at levels that were below those seen in hybrids from affected males. Furthermore, transmitting males could be differentiated from normal males by their significantly higher expression rates when hybrids were exposed to caffeine before cytogenetic harvest. One male chimpanzee also showed low level expression in hybrid cells. These data suggest that the hybrid system lowers the threshold for fragile X expression, a fragile site at Xq27 may be present on all human and chimpanzee X chromosomes and constitutes a previously unrecognized common fragile site and the hybrid system with caffeine post-treatment can distinguish between the common Xq27 fragile site of control males, the occult mutant fragile site of a transmitting male, and the fully expressed fragile site of an affected male with XLMR. Thus the mutation producing XLMR may represent a multi-step alteration of a naturally occurring DNA sequence producing a continuum of cytogenetic expression and a threshold for clinical manifestation.

Twelve families with fragile X(q27)

Through a community study of boys requiring special education for the severely mentally retarded, 12 families were ascertained in which the fragile X was found to be segregating. By assiduous follow up of these families, it was found that in only four of them could male transmission be ruled out from the grandparents' or great grandparents' generation and that the segregation ratios are disturbed.

Frequency of rare fragile sites among mentally subnormal schoolchildren

The frequency of rare fragile sites was studied among 240 children in special schools for subnormal intelligence (IQ 52-85). 1/130 boys studied (0.8%) had the fragile site at Xq27.3 while it was not found in any girl (0/110). In two children an autosomal rare fragile site at 2q11.2 (2/240, 0.8%) was seen. In addition a constitutional chromosome abnormality was found in a further seven children (7/240, 2.9%).

Mental impairment in Martin-Bell syndrome is probably determined by interaction of several genes: simple explanation of phenotypic differences between unaffected and affected males with the same X chromosome

A family with Martin-Bell syndrome (MBS) is described with transmission of this X-linked trait by a normal male who manifested the fragile site at Xq27. This family shows features apparently typical for all families with a normal male transmitter. The daughters of this male are mentally normal and their fragile site is difficult or impossible to detect but detection of the heterozygous genotype is much easier among the granddaughters. This can be explained by a model assuming that mental deficiency in patients with MBS is determined by several genes, i.e. the X-linked MBS-gene as "major gene" undergoing X-inactivation and interacting with at least one modifying gene. The model assuming one autosomal modifier segregating independently from the MBS-gene is tested using the results of segregation analysis performed by Sherman et al. (1984, 1985). No significant differences have been found between the predictions of this model and the findings of the segregation analysis. Nearly all of the segregation data are exactly predicted by the model. Possible differences are discussed either to be due to biased data or to require slight modification of the model to get a better fit of the data. The apparent phenotypic differences between a normal carrier grandfather and his affected grandsons as well as between his daughters and his heterozygous granddaughters are also simply explained on the basis of this model. Several modifier loci may exist each of them related to one of the various phenotypic effects of the X-linked major gene (MBS-gene) leading to a syndrome that does not include any obligate feature.

Genetic analysis of the fragile-X mental retardation syndrome with two flanking polymorphic DNA markers

The fragile-X mental retardation syndrome, one of the most prevalent chromosome X-linked diseases (approximately equal to 1 of 2000 newborn males), is characterized by the presence in affected males and in a portion of carrier females of a fragile site at chromosomes band Xq27. We have performed a linkage analysis in 16 families between the locus for the fragile-X syndrome, FRAXQ27, and two polymorphic DNA markers that correspond to the anonymous probe St14 and to the coagulation factor IX gene F9. Our results indicate that the order of loci is centromere-F9-FRAXQ27-St14-Xqter. The estimate of the recombination fraction for the linkage F9-FRAXQ27 is 0.12 (90% confidence limits: 0.044-0.225) and 0.10 for FRAXQ27-St14 (90% confidence limits: 0.040-0.185). Recombination between St14 and F9 does not appear to be significantly different in normal and fragile-X families. The two flanking probes were used for diagnosis of the carrier state and for detection of transmission of the disease through phenotypically normal males. They should also allow first-trimester diagnosis with a reliability of about 98% in 40% of the families. Used in conjunction with the cytogenetic analysis, the segregation studies with both probes should improve the genetic counseling for the fragile-X syndrome and should be useful for the formal genetic analysis of this unique disease.

Caffeine enhances fragile (X) expression in somatic cell hybrids

Yunis and Soreng recently demonstrated enhanced expression of common fragile sites as and of fra(X) when 2.2 mM caffeine is added to FUdR treated lymphocyte cultures 6 hours before harvest. We failed to replicate this finding for fra(X) expression in lymphocytes. However, we do find a consistent increase in expression levels in somatic cell hybrids between a Chinese hamster cell line and 3 unrelated individuals with the fra(X) mutation when caffeine is present for the last 2 hours before harvest. This was particularly true for "low-expressing" cell lines, in which a 4-6 fold increase was observed. Using a thymidylate synthase deficient hybrid which could be blocked in the S phase by thymidine deprivation, we found that caffeine significantly reduced the recovery time from thymidine release to mitosis. This produced the highest level of fra(X) expression (48%) only one hour after release from thymidine deprivation. These results show that caffeine does enhance fra(X) expression in at least some cell types. The effect is probably indirect, inhibiting the mitotic delay usually associated with DNA damage.

Maternal effect on intelligence in fragile X males and females

If the mother is the fragile X gene carrier, her daughters (and sons) with the mutation are at high risk of mental retardation. If the father is the (clinically unaffected) carrier, his daughters are normal. This is strong evidence for a maternal effect. The decreased penetrance and variable expressivity in fra(X) offspring of carriers could be related, at least in part, to variabile expression or availability of some maternal factor between pregnancies. We hypothesize a maternal effect in fra(X), with variability in intelligence of heterozygotes and hemizygotes mediated mainly by the maternal uterus or placenta by virtue of different patterns of lyonization in those tissues between pregnancies. If the mother is a carrier, the maternal placenta could develop with a skewed proportion of the normal or the fra(X) genetically active. Each female or male embryo could be exposed to very different environments with respect to genetic activity of the fra(X) chromosome, depending on the site of implantation within the uterus. If the father contributes the fra(X), the intrauterine environment is invariably normal and so are the daughters. Modifiers of the intrauterine effect could include lyonization patterns in tissues of the carrier fetus, and preferential inactivation of the paternal X in extra-embryonic tissues. The ultimate phenotype of the developing heterozygote and hemizygote may be determined by a threshold effect and interaction between the maternal genotype, the placental genotype, and the fetal genotype. The possibility of maternal effect is testable and has implications for treatment.

Normal male carriers in the fra(X) form of X-linked mental retardation (Martin-Bell syndrome)

Evidence for the transmission of X-linked mental retardation through normal male carriers is reviewed in 6 kindreds. In these pedigrees we identified 15 unaffected males who likely had passed the gene on through their daughters. Fifty-one mentally retarded grandsons or great grandsons descended from these male carriers. In total, these males had 50 daughters with only 2 of them being of low intelligence. Two of the male carriers were recently identified through fra(X)- positive results in their mentally normal daughters. Among the sibs of these males, mentally retarded brothers were found in 3 families. This was unexpected since earlier observations suggested that the risk for mental retardation among sibs of nonmanifesting carriers is exceedingly low.

Somatic cell hybrid studies of fragile (X) expression in a carrier female and transmitting male

We have extended our previous studies of fra(X) expression in somatic cell hybrids to the analysis of a carrier female with a low level of expression and to an unaffected, transmitting male who shows no expression in lymphocytes or lymphoblasts. Optimum conditions for fra(X) expression was treatment with 10(-8) M FUdR for 16 hours. In recent experiments, addition of 2.2 mM caffeine 2 hours before harvest was found to increase expression consistently. Two clones from the carrier female containing the fra(X) chromosome but not the normal X showed expression of 2-4%, indicating that expression in heterozygous females is not influenced by the presence or absence of the normal X. Expression rate was increased to 20% by exposure to FUdR plus caffeine. Analysis of hybrids containing only the fra(X) in an inactive state, and after reactivation by 5-azacytidine, showed no change in the frequency of expression. A hybrid clone from the nonexpressing, transmitting male containing only the X and chromosome 13, showed expression ranging from 2% without caffeine to 12% with caffeine in three different experiments. The ability to induce fra(X) expression in hybrid from this nonexpressing male may be explained in one of several ways: 1) a second mutation has occurred, 2) an autosomal suppressor locus was lost, or 3) the hamster genome or unusually short cell cycle lowers the threshold for expression, particularly in the presence of caffeine.

Inheritance of fragile X syndrome: an hypothesis

The fragile X (fra(X), or Martin Bell-MB) syndrome is considered an X-linked recessive trait. However, clinically normal male transmitters of the condition have been observed occasionally. The occurrence of "carrier" males and the observation of other unusual genetic characteristics in the MBS suggest that this condition is not a standard X-linked recessive trait. We propose that the MBS is due to a transposable genetic element which can exist in 3 different chromosomal states and effect 2 different extrachromosomal environments. This model can account for the peculiar genetic behavior of the fragile X syndrome.

Genetic linkage heterogeneity in the fragile X syndrome

Genetic linkage between a factor IX DNA restriction fragment length polymorphism (RFLP) and the fragile X chromosome marker was analyzed in eight fragile X pedigrees and compared to eight previously reported pedigrees. A large pedigree with apparently full penetrance in all male members showed a high frequency of recombination. A lod score of -7.39 at theta = 0 and a maximum score of 0.26 at theta = 0.32 were calculated. A second large pedigree with a nonpenetrant male showed tight linkage with a maximum lod score of 3.13 at theta = 0, a result similar to one large pedigree with a nonpenetrant male previously reported. The differences in lod scores seen in these large pedigrees suggested there was genetic heterogeneity in linkage between families which appeared to relate to the presence of nonpenetrant males. The combined lod score for the three pedigrees with nonpenetrant males was 6.84 at theta = 0. For the 13 other pedigrees without nonpenetrant males the combined lod score was -21.81 at theta = 0, with a peak of 0.98 at theta = 0.28. When lod scores from all 16 families were combined, the value was -15.14 at theta = 0 and the overall maximum was 5.13 at theta = 0.17. To determine whether genetic heterogeneity was present, three statistical tests for heterogeneity were employed. First, a "predivided-sample" test was used. The 16 pedigrees were divided into two classes, NP and P, based upon whether or not any nonpenetrant males were detected in the pedigree. This test gave evidence for significant genetic heterogeneity whether the three large pedigrees with seven or more informative males (P less than 0.005), the eight pedigrees with three informative males (P less than 0.001), or all 16 pedigrees (P less than 0.001) were included in the analysis. Second, Morton's large sample test was employed. Significant heterogeneity was present when the analysis was restricted to the three large pedigrees (P less than 0.025), or to the eight pedigrees with informative males (P less than 0.05) but not when smaller, less informative pedigrees were also included. Third, an "admixture" test for heterogeneity was employed which tests for linkage versus no linkage. A trend toward significance was seen (0.05 less than P less than 0.10) which increased when the analysis was restricted to the larger, more informative pedigrees.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Further segregation analysis of the fragile X syndrome with special reference to transmitting males

A new series of 96 pedigrees with the fra(X) syndrome was analysed using complex segregation analysis with pointers, defining affection as any degree of mental impairment. These families were found to exhibit the same segregation pattern as the first series of 110 pedigrees (Sherman et al. 1984). The best estimate for penetrance of mental impairment in males was 79% and in females was 35% for the combined data. Again, there was little evidence for sporadic cases among affected males. Many more intellectually normal transmitting males have been observed since the existence of such males and the concomitant need to investigate the paternal side of pedigrees was recognized. On further investigation of all 206 pedigrees from the old and new data sets, the sibships of nonexpressing males appeared to be different from those of expressing males. Our analysis, using mental impairment as the phenotype, suggested that obligate carrier mothers and daughters of intellectually normal transmitting males are rarely, if ever, mentally impaired and that the sibs of transmitting males are much less likely to be retarded than the sibs of mentally impaired males. Though mothers and daughters of transmitting males are similar in phenotype, the expression of the gene in their offspring appears to be different: the penetrance of mental impairment is higher in offspring of intellectually normal daughters of transmitting males than in offspring of intellectually normal mothers of transmitting males. The implications of these observations for genetic counseling and for genetic models of the fra(X) syndrome are discussed.

Developmental and behavioural disturbances in 13 boys with fragile X syndrome

Developmental and behavioural aspects were studied in 13 boys aged 2.6-12.5 years from three families with the fragile X syndrome. The following observations were made. Moderate to severe retardation was present in all boys; non-verbal IQs ranged between 25 and 67 (mean 46 +/- 14); IQ and age were negatively correlated (P less than 0.01). Language development was grossly delayed in all boys; most had severe articulation problems. Imitative and symbolic play (e.g. doll play) were strikingly retarded as compared to abstract play (e.g. block design). Autistic features such as no use of eye contact, stereotyped movements and echolalia were found in 9/13 boys; the same number showed aggressive behaviour. General activity was reduced during the 1st year of life; most boys became very hyperactive during the second year; and short attention span and increased distractability were observed in all. Motor development was mildly delayed; all boys were clumsy and moderately hypotonic. The fragile X syndrome ought to be considered in retarded boys with a dissociated developmental pattern, in particular a striking delay in language and play development, and autistic features.

The fragile X syndrome. A study of 83 families

The present report summarizes the experience on the mar(X) syndrome in a total of 157 male patients (44 prepubertal and 113 postpubertal) ascertained through 83 index patients from 83 families under investigation. In one third of the families pedigree data were consistent with X-linked recessive inheritance. In the further two thirds of the families the presenting symptom was familial mental retardation with a mentally retarded mother, or mental subnormality with hyperkinetic behaviour in the male patient. No more than 60% of the adult males presented the typical clinical triad (mental retardation-long face-megalotestes). The most characteristic finding in the mar(X) boy is the psychological profile with severe hyperkinetism, hypersensitivity, handbiting and autistic features in some of them. In 4 of the 27 large mar(X) pedigrees strong evidence was present of a possible transmission of the mar(X) through normal males. The high incidence of mental subnormality in the female offspring of heterozygote carriers, and the relationship between mental status, phenotype, age and expression of the mar(X) in different culture conditions is discussed.

Transmission of the marker X syndrome trait by unaffected males: conclusions from studies of large families

It is well established that apparently unaffected males can be transmitters of the marker X syndrome trait. Cytogenetic and clinical investigations of these male transmitters are only rarely reported for most of these male transmitters are dead by the time the syndrome is diagnosed in their families. We report on cytogenetic and clinical investigations of two unaffected male carriers of the disorder from two large families. Pedigree analysis of these families revealed six other cases of possible male transmission of the marker X syndrome trait. Mental impairment was not reported from the siblings of these unaffected male carriers and could not be observed in their daughters. The mode of transmission of the disorder cannot be fully explained by X-linked inheritance. The phenomenon of unaffected males transmitting the disorder could be due to an autosomal suppressor systeme. Our findings indicate that male transmission may be important for the frequency of the disorder.

Recombinant DNA approach to X-linked mental retardation

Currently, approximately 115 X-linked diseases have been documented. Many of these defects are neurological in nature and about 25% result in a broad spectrum of mental impairments which can be distinguished by biochemical or clinical means. It is not known how many of the retarded population carry an X-linked defect due to lack of a distinct marker. By constructing a molecular linkage map of the X chromosome using DNA polymorphisms, it will be possible to identify these individuals, classify their disorder by the chromosomal region in which their polymorphism is detected, offer genetic counseling and prenatal diagnosis to their family members and try to determine which gene the polymorphism is linked to in order to identify the defect and devise therapy.

Fragile (X) expression: relationship to the cell cycle

The fragile (X) chromosome demonstrable in individuals with one type of X-linked mental retardation is seldom, if ever, seen in more than 50% of cells of affected individuals. We have devised a model to explain this apparent 50% maximum, one essential feature of which is that the fragile (X) will not be seen in cells in their first division in thymidine-depleted media. The validity of our model was tested on lymphoblastoid cell lines from affected males by treating the cells with fluorodeoxyuridine (FUdR) to induce the marker and/or bromodeoxyuridine (BrdU) to determine the cell cycle. We have evidence that the fragile (X) is present in cells in the first and subsequent cell divisions in thymidine-depleted media. In light of these observations our model is not valid and the 50% expression of the fragile site at Xq(28) and other unusual properties of this region of the X chromosome remain unexplained.

Cytogenetic investigations in mentally retarded and normal males from 14 families with the fragile site at Xq28. Results of folic acid treatment on fra(X) expression

Lymphocyte cultures from 27 mentally retarded males aged 1 year to 77 years, and from 11 normal brothers from a total of 14 families with the fragile X segregating have been examined cytogenetically employing three different culture methods including methods for induction of fra(X) by FUdR (flourodeoxyuridine) or MTX (methotrexate). All mentally retarded males showed unequivocal fra(X) expression. No statistically significant correlation between fra(X) expression and age could be demonstrated. No enhancement with FUdR was observed. Fibroblast cultures from 10 retarded males expressed fra(X) in a dose-response relationship to increasing concentrations of FUdR. None of the normal males showed fra(X). In vivo folic acid treatment of seven mentally retarded males resulted in marked reduction in fra(X) expression in lymphocyte cultures grown in medium 199. However, reinduction was achieved by FUdR or MTX, except in one case who temporarily received very high doses of folic acid.

The fragile X syndrome: the patients and their chromosomes

We have reviewed recent publications, mostly from 1980 onwards, concerned with the problem of identifying patients with the fragile X chromosome and mental retardation, considering the two practical sides of the problem, that is, identification by their external appearance and by chromosomal studies. We conclude that this condition covers a large range of physical findings which occur in varying degrees in people with the chromosome marker. We have tried to clarify the existent criteria that have to be considered for an accurate cytogenetic diagnosis.

In vitro reversal of fragile-X expression by exogenous thymidine

A virtually thymidine free medium was used to test the effects of exogenous thymidine on lymphocyte expression of the fragile-X. De novo pathway thymidylic acid synthesis in the cells was blocked by FUdR. Our results suggest that in vitro exogenous thymidine is directly responsible for suppressing expression of the fragile-X. More importantly, delayed addition of exogenous thymidine can negate fragile-X expression after it has first been induced by FUdR.

The fragile X syndrome I: familial variation in the proportion of lymphocytes with the fragile site in males

A review of 61 males with the fragile X positive form of the Martin Bell syndrome from 30 families seen in the past 4 years suggests that the number of lymphocytes with the fragile site on the X chromosome (fra(X) ) in retarded males tends to be characteristic for the individual and similar to that found in other retarded males in the same family. The number of lymphocytes with fra(X) was not correlated with height, weight, occipitofrontal circumference, ear length or mean testicular volume in adults nor with age over the whole series.

The marker (X) syndrome: a cytogenetic and genetic analysis

The results of a cytogenetic and segregation analysis of 110 pedigrees of the mar (X) syndrome are reported. The cytogenetic study indicated an inverse relationship between IQ and the mar(X) frequency in females but not in males. A small but significant effect of age on mar(X) frequency was observed in both males and females, but in females it was restricted to those of normal intelligence, retarded females showing no significant effect. Classical segregation analysis was performed using the program SEGRAN, analysing sexes separately. A 20% deficit of affected males was observed, the most plausible explanation for the majority of these cases being incomplete penetrance. Since this was an unexpected result, the data were scrutinized for possible biases; however, correction of these had little effect on the estimate. The penetrance of mental impairment in carrier females was estimated to be 30% and of mental impairment and/or mar(X) expression to be 56%. Thus 44% of carriers cannot be detected with our definition of affection. No evidence for sporadic cases among affected males was found. Complex segregation analysis was performed using the sex-linked version of POINTER, analysing sexes together. This was done in order to test the results from classical segregation analysis, to test for family resemblance and to estimate mutation rates. It was confirmed that there was a 20% deficit of affected males, that penetrance of mental impairment in females was approximately 30% and that there was no evidence for sporadic males. Thus all males with the gene appear to have received it from their carrier mothers and all mutations must occur in sperm. The mutation rate in sperm was estimated to be as high as 7.2 X 10(-4), implying that over one-half of random carrier females are fresh mutants. Our results have important implications for genetic counseling as they imply that all mothers of isolated affected males are carriers, that normal brothers of affected males have a 17% chance of carrying the gene and transmitting it to all their daughters, and that normal sisters of affected males have, at most, a 30% chance of being carriers. Since there are biases in the data due to the testing of particular individuals, these probabilities must be considered approximations until they are independently confirmed.

Diagnosis of the fragile X syndrome (Martin-Bell syndrome). Clinical findings in 27 males with the fragile site at Xq28

The results of a clinical investigation of 27 males with the fragile X are reported; the age range was from 1 to 77 years. The medical history in pre-, peri- and early post-natal life was unremarkable. Birth weights tended to be above average. In infancy hypotonia and a large head were often found, together with retarded development. Macroorchidism was almost uniformly found after puberty, but apparently not often before. The facial features in the grown-up males were characteristic, confirming previous reports. Minor abnormalities of feet and hands were seen. Mental retardation was often in the moderate range, but all degrees were seen. Psychiatric symptoms were frequently seen, and one child was diagnosed as autistic. A developmental profile is outlined.

Fragile X trait in a large kindred: transmission also through normal males

In the pedigree reported here two apparently normal males may have transmitted the fragile X chromosome. Eighteen family members were examined cytogenetically. The fragile X was detected in a high percentage of cells from nine mentally retarded members of this family (six males and three females) and in one female obligate carrier. Four other obligate carriers showed no or only a few cells with the fragile X.

The strength of association between fragile (X) chromosome presence and mental retardation

In order to obtain a quantitative estimate of the degree of association between presence of fragile X chromosome (fra(X] and mental retardation (MR), existing data from nonretarded males were analyzed. Clearly, fra(X) occurs less frequently among nonretarded compared to MR males. However, incidence estimates for fra(X) based upon existing data hold open the possibility that there may be significant numbers of nonretarded males with fra(X). Additional analyses of data from families with a pattern of fra(X) linked MR showed: (a) the probability that nonretarded male offspring will have fra(X) is very small, and (b) the probability that MR male offspring will have fra(X) is very large. Thus, accurate prognostic decisions can be based upon prenatal diagnosis of fra(X) presence, especially in families with a pattern of fra(X) linked MR.

A cytogenetic study of a population of mentally retarded males with special reference to the marker (X) syndrome

A cytogenetic survey of a population of 274 mentally retarded males on community placement is described. Thirty-five had an aneuploid chromosome constitution and five had the mar(X) syndrome. The range of clinical variation among the mar(X) probands and their affected relatives is described. Family studies were possible for four of the five mar(X) probands and in two families the mar(X) gene was apparently transmitted through a clinically normal male, suggesting that this type of male transmission may be a not uncommon phenomenon.

The fragile X chromosome in a large Indian kindred

A large Indian kindred in which the fragile X chromosome is segregating has been investigated in Cape Town. Eight male hemizygotes and four female heterozygotes were mentally retarded. There is suggestive evidence that one deceased male of reportedly normal intelligence may have been a hemizygote. The existence of the fragile X syndrome in a number of different ethnic groups supports the contention that the gene controlling the phenotype and the fragile site are the same, or at least overlap.

Screening for fra(X)(q) in a population of mentally retarded males

Among 242 institutionalized mentally retarded males in Northern Germany screened for fra(X)(q), 15 (6.2%) with severe mental retardation expressing fra(X)(q) were detected. One patient displayed Klinefelter's syndrome in addition. All fra(X)(q) males showed the typical facial signs, but three of the adults did not express macroorchidism. A preliminary estimation of an overall frequency of 1:2000 males for the fra(X)(q) condition is suggested.

A study of mental retardation in children in the Island of Hawaii

Eighty-one probands from an initial population of 223 school-aged retarded individuals were assessed by history, clinical examination and, where appropriate, cytogenetic analysis. In 51 individuals, the retardation occurred as an isolated event within the family, whereas 30 patients had a family history of retardation. In 39 of the isolated individuals, the retardation was either related to environmental factors or associated with a major neurological abnormality. The remaining 12 patients were phenotypically normal with no cytogenetic abnormality. Of the 30 probands from 15 families with a history of retardation, 3 families had X-linked syndromes. One, with 4 proband daughters, had the mar(X) syndrome and two families were considered to have the phenotypically similar syndrome but without demonstrating the mar(X). In an additional 5 families, the distribution and clinical features of the affected individuals were compatible with nonspecific X-linked mental retardation.

Martin-Bell syndrome fra(X) (q28) in a Sri Lankan family

The Martin-Bell syndrome was observed in a family with six mentally retarded males, born to three sisters, who were of more than average intelligence. The males showed large ears, macrotestes, they were delayed in their development and had speech retardation. Five were pleasant and co-operative, while one had severe temper tantrums. The fra(X)(q28) was observed in the three males cytogenetically examined and in two of the three female potential carriers, who could be studied. It was expressed in 17-36% of the cells in the males cultured in 199 and in 5% and 39% in the two female carriers, studied under addition of methotrexate.

Non-specific X-linked mental retardation: background, types, diagnosis and prevalence

This paper reviews non-specific X-linked mental retardation (X-linked MR), a heterogeneous group of X-linked recessive disorders which are controlled by one or more genes located on the X chromosome. Presently, there are at least three major subdivisions of X-linked MR. The IQ of affected males is variable. Carrier females usually have normal intelligence but some do show partial expression resulting in retardation. Genetics units can diagnose affected males utilizing pedigree analysis, clinical evaluation and cytogenetic testing (fragile X-linked MR type only). Projections indicate X-linked MR may be as common in males as Down syndrome and could account for 25% of the retarded male population.