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

Advancing genomic technologies and clinical awareness accelerates discovery of disease-associated tandem repeat sequences

Expansions of gene-specific DNA tandem repeats (TRs), first described in 1991 as a disease-causing mutation in humans, are now known to cause >60 phenotypes, not just disease, and not only in humans. TRs are a common form of genetic variation with biological consequences, observed, so far, in humans, dogs, plants, oysters, and yeast. Repeat diseases show atypical clinical features, genetic anticipation, and multiple and partially penetrant phenotypes among family members. Discovery of disease-causing repeat expansion loci accelerated through technological advances in DNA sequencing and computational analyses. Between 2019 and 2021, 17 new disease-causing TR expansions were reported, totaling 63 TR loci (>69 diseases), with a likelihood of more discoveries, and in more organisms. Recent and historical lessons reveal that properly assessed clinical presentations, coupled with genetic and biological awareness, can guide discovery of disease-causing unstable TRs. We highlight critical but underrecognized aspects of TR mutations. Repeat motifs may not be present in current reference genomes but will be in forthcoming gapless long-read references. Repeat motif size can be a single nucleotide to kilobases/unit. At a given locus, repeat motif sequence purity can vary with consequence. Pathogenic repeats can be "insertions" within nonpathogenic TRs. Expansions, contractions, and somatic length variations of TRs can have clinical/biological consequences. TR instabilities occur in humans and other organisms. TRs can be epigenetically modified and/or chromosomal fragile sites. We discuss the expanding field of disease-associated TR instabilities, highlighting prospects, clinical and genetic clues, tools, and challenges for further discoveries of disease-causing TR instabilities and understanding their biological and pathological impacts-a vista that is about to expand.

Effects of hyperoxia and caffeine on the expression of fragile site at Xq27.3

To enhance the cytogenetic expression of the fragile X chromosome, we studied the effects of hyperoxia and caffeine on the induction of fragile Xq27.3. A lymphoblastoid cell line (GM 06912) derived from a fragile X male proband was cultured in RPMI 1640 containing 16% dialyzed fetal calf serum. The cells were synchronously subjected to one of 3 different atmospheric oxygen tensions (21%, 21.3 kPa, normoxic; 40%, 40.5 kPa, hyperoxic; or 60%, 60.8 kPa, hyperoxic) during the last 24 hours of the 72 hour culture, immediately after the addition of 2'-deoxy-5-fluorouridine (FUdR) at 25 ng/ml. To study the enhancing effect of caffeine, with or without hyperoxia, a second set of cultures was additionally subjected to caffeine (2.5 mM) during the last 6 hours of the culture. When the fragility of hyperoxic cells (38.1 kPa dissolved oxygen) was compared to that of normoxic control cells (13.3 kPa dissolved oxygen), the difference was significant (P < 0.05). These data suggest that there is a mean increase in the fragile Xq27.3 expressivity as the dissolved oxygen tension increases. Additionally, we observed that caffeine, with or without hyperoxia, significantly (P < 0.05) suppressed the expression of the fragile X site in this lymphoblastoid cell line.

Chromosomal fragility associated with familial Alzheimer's disease

To test whether chromosomal instability is associated with familial Alzheimer's disease, we examined breakage on X chromosomes of fibroblasts derived from patients with familial Alzheimer's disease, using gene cotransfer methodology. The X chromosome is a convenient target for analyzing DNA breakage because of its numerous markers and ease of selection in rodent-human hybrid cells. Patients with familial Alzheimer's disease, including the large Nova Scotia Alzheimer's kindred, show a significantly lower cotransfer of the X-linked glucose-6-phosphate dehydrogenase (G6PD) gene with the selected HPRT gene in hybrid cells, indicating breakage between the markers. Lower cotransfer of the more distant X-linked gene, MIC-2, was statistically significant in this kindred, but not in other patients with familial Alzheimer's disease. The distance between MIC2 and HPRT is sixfold to ninefold greater than that between HPRT and G6PD, suggesting that there may be a "hot spot" for breakage in the latter interval on the X chromosome of patients with familial Alzheimer's disease. The somatic cell hybrid model provides insights into underlying mechanisms for chromosomal breakage induced by the Alzheimer defect. A hypothesis implicating a candidate gene, C1-THF synthase, in the generation of chromosome instability in the pathogenesis of familial Alzheimer's disease, is presented.

A panel of deleted mouse X chromosome somatic cell hybrids derived from the embryonic stem cell line HD3 shows preferential breakage in the Hprt-DXHX254E region

A panel of 91 somatic cell hybrids containing deleted mouse X chromosomes and falling into seven nested intervals has been isolated and characterized from fusions involving the murine embryonic stem cell HD3. Many of the X chromosome breakpoints present in these hybrids fall within regions in which few or no other hybrids were previously available. The apparent enrichment for breakpoints lying within the Hprt-DXHX254E region is discussed in relation to both the nature of the embryonic stem cell fusions and the presence of the Fmr1 gene associated with FRAXA in man within this span.

Fragile X mental retardation and the iduronate sulphatase locus: testing Laird's model of fra(X) inheritance

Fragile X [fra(X)] mental retardation syndrome is the most frequent familial cause of mental handicap. The clinical phenotype is associated with a rare fragile site at Xq27.3. The mutation underlying the disorder, an insertion into the FMR-1 gene, has been characterized, but the pathogenesis of the condition is obscure and the pattern of inheritance is still not fully understood. One model of fra(X) pathogenesis was proposed by Laird in 1987, suggesting that the fra(X) mutation acts as a cis-acting, local block to the pre-oogenesis reactivation of the inactivated X chromosome. To test this model, we examined the activity of the F8, F9 and iduronate sulphatase (IDS) loci. The level of IDS in the serum of fra(X) males was found to be very significantly reduced in the fra(X) group when compared to that of control males: this lends support to Laird's model of fra(X) pathogenesis. However, we detected no methylation differences between fra(X) and control samples at the IDS locus, although such changes are known in fra(X) males at sites closer to the fragile site. Thus the mechanism of the reduction in IDS activity has not been identified.

Frequency of the common fragile site at Xq27.2 under conditions of thymidylate stress: implications for cytogenetic diagnosis of the fragile-X syndrome

The distal long arm of the X chromosome contains at least 2 fragile sites, the well known rare site at Xq27.3 (FRAXA), associated with the fragile-X syndrome, and a common fragile site at Xq27.2 (FRAXD), inducible by high doses of aphidicolin. Lesions at Xq26 have also been reported in lymphocytes of mentally retarded individuals cultured under folate deprivation or thymidylate stress. This study determines the frequency of the fragile site at Xq27.2 and lesions at Xq26 in individuals referred to our laboratory to rule out the fragile-X syndrome and in control individuals using our routine culture system for the diagnosis of the syndrome. FRAXD was expressed in 1/20 (5%) individuals in each of the study groups, in 1-2% of cells. Lesions at Xq26 were found in 1-2% of the lymphocytes of 5/166 (3%) patients referred for fragile-X analysis who were FRAXA negative, and in 1% of cells of 1/20 (5%) control individuals. We conclude (1) the fragile site at Xq27.2 can be demonstrated in normal individuals under conditions of thymidylate stress routinely used for cytogenetic diagnosis of the fragile-X syndrome, (2) this fragile site is present at low levels (1-2%) in all individuals who express it and, therefore, its expression is unlikely to cause false positive diagnoses of the syndrome, (3) lesions at Xq26 are also seen at low levels in lymphocytes of individuals without the syndrome, and (4) accurate differentiation of the rare site at Xq27.3 from other distal Xq fragile sites or lesions will lead to avoidance of unnecessary repeat studies.

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.

Simultaneous expression of the rare and common fragile sites on the X chromosome

The fragile X [fra(X)] syndrome is the most common inherited form of X-linked mental retardation and is associated with a rare folate sensitive fragile site on the X chromosome at band Xq27.3. Recently, a common fragile site located at chromosome band Xq27.2 was delineated (Sutherland & Baker 1990). In order to confirm the previous findings and to further investigate the conditions required for induction of both types of fragile sites, we studied the use of four experimental protocols. Samples from a control male, two fra(X) males and a fra(X) carrier female were studied. Both common and rare fragile sites were seen in the samples from the fra(X) subjects. Up to 4% of cells showed both common and rare fragile sites on the same X chromosome at the 500 band level. The rare and common fragile sites on the X chromosome could be clearly distinguished. From 1 to 3% of the control cells exhibited the common fragile site, while none exhibited the rare fragile site. These protocols should be useful in resolving questionable fra(X) syndrome diagnoses.

Identification of a gene (FMR-1) containing a CGG repeat coincident with a breakpoint cluster region exhibiting length variation in fragile X syndrome

Fragile X syndrome is the most frequent form of inherited mental retardation and is associated with a fragile site at Xq27.3. We identified human YAC clones that span fragile X site-induced translocation breakpoints coincident with the fragile X site. A gene (FMR-1) was identified within a four cosmid contig of YAC DNA that expresses a 4.8 kb message in human brain. Within a 7.4 kb EcoRI genomic fragment, containing FMR-1 exonic sequences distal to a CpG island previously shown to be hypermethylated in fragile X patients, is a fragile X site-induced breakpoint cluster region that exhibits length variation in fragile X chromosomes. This fragment contains a lengthy CGG repeat that is 250 bp distal of the CpG island and maps within a FMR-1 exon. Localization of the brain-expressed FMR-1 gene to this EcoRI fragment suggests the involvement of this gene in the phenotypic expression of the fragile X syndrome.

Fragile X-linked mental retardation and the difficulties of reverse genetics

Fragile X-linked mental retardation is an enigmatic inheritable syndrome in which severe mental retardation, a cytogenetically detectable fragile site at Xq27.3 (FraX) and a number of dysmorphic features are associated. Genetic analysis shows that the mode of inheritance is more complex than a straightforward X-linked recessive trait and probably involves a two-step process for which several models have been proposed. Early attempts at 'cloning the fragile site' provided several DNA segments lying in its general vicinity, and large scale DNA mapping methods were extensively applied in an effort to generate maps including this region. These efforts were complemented by more focussed methods such as microdissection; together these approaches have now provided a number of DNA segments within a 5 cM interval around FraX, and with the help of these new probes the site is indeed being cloned. Unravelling the nature of the sequence(s) responsible for the mental retardation syndrome will probably take some time, however.

Abnormal pattern detected in fragile-X patients by pulsed-field gel electrophoresis

The fragile-X syndrome is the most frequent inherited form of mental retardation, with an incidence of 1 in 1,500 males. It is characterized by the presence of a fragile site at Xq27.3 induced in vitro by folate deprivation or by inhibitors of deoxynucleotide synthesis. Its mode of inheritance is unusual for an X-linked trait, with incomplete penetrance in both males and females. Some phenotypically normal males transmit the mutation to all their daughters who rarely express any symptoms, but penetrance is high in sons and daughters of these carrier women. Genetic and physical mapping of the Xq27-q28 region has confirmed that the disease locus is located at or very near the fragile site. Hypotheses proposed to account for the abnormalities in the inheritance of the disease include sequence rearrangements by meiotic recombination or a mutation that affects reactivation of an inactive X chromosome during differentiation of female germ cells. To detect such rearrangements, or methylation changes that may reflect a locally inactive X chromosome, we used pulsed-field gel analysis of DNA from fragile-X patients with probes close to the fragile-X locus. The probe Do33 (DXS465) detected abnormal patterns in fragile-X patients, but not in normal controls or in non-expressing male transmitters.

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.

Chromosome lesions which could be interpreted as "fragile sites" on the distal end of Xq

Chromosome lesions which could be interpreted as "fragile sites" on the distal end of the long arm of the X chromosome were identified during a cytogenetic study of 160 mentally retarded adult males with no apparent cause of their mental retardation and one normal adult female with a family history of fra (X) syndrome. Peripheral blood samples were cultured in either M199 or RPMI 1640 medium with FUdR or BrdU. Metaphases were examined for chromosome lesions or fragile sites on the distal end of Xq and 3 distinct sites were observed: Xq26, Xq27.2, and Xq27.3. Other chromosome lesions at Xq28 were observed and interpreted as nonspecific telomeric structural changes. Chromosome lesions were observed in cells from 14 of the 161 individuals. These included: 5 patients with an Xq26 site, 2 with the recently reported Xq27.2 site, 4 with the Xq27.3 site (characteristic of the fra (X) syndrome), 2 with nonspecific telomeric structural changes, and one individual with 2 lesions (a nonspecific telomeric structural change and an Xq26 site). Additional research is necessary to determine the frequency and clinical significance, if any, of lesions occurring in this region of the X chromosome and to distinguish among heritable fragile sites, constitutive fragile sites, and nonspecific telomeric structural changes.

No significant increase in spontaneous and ethyl methane sulfonate-induced sister chromatid exchanges at the Xq27.3 fragile site

The present study was performed to determine if the fra(X)(q27.3) site is inherently a high sister chromatid exchange (SCE) site independent of fragility. Therefore, we studied baseline and ethyl methane sulfonate (EMS)-induced SCEs in peripheral blood lymphocytes from 10 retarded fragile-X male patients and eight retarded nonfragile-X male controls. The distributions of SCE scores per chromosome within each experimental condition showed significant interindividual variability in response to EMS treatment in the fragile-X group. Each fragile-X patient was therefore compared with a matched control. No significant differences were found in the distribution of SCE scores per chromosome. In addition, at the Xq27 site, whatever the degree of expressed fragility, no significant differences between fragile-X and control groups were evident in the spontaneous or EMS-induced SCEs. The fra(X)(q27.3) site therefore is not a hot spot for spontaneous or EMS-induced SCEs. Because fluorodeoxyuridine (FUdR) treatment has been shown to induce SCE at this site, our results indicate that the Xq27.3 site must be structurally different from other nonfragile SCE sites.

The common fragile site in band q27 of the human X chromosome is not coincident with the fragile X

The common fragile site on the end of the long arm of the human X chromosome has been shown to be at a different location from the rare fragile site which produces the fragile X syndrome of intellectual handicap. The different locations can be clearly seen in chromosomes at about the 550 band level of resolution. This finding should help resolve difficulties in fragile X cytogenetics where expression of the common fragile site can lead to false positive diagnoses.

A highly polymorphic locus cloned from the breakpoint of a chromosome 11p13 deletion associated with the WAGR syndrome

Children with constitutional deletions of chromosome 11p13 suffer from aniridia, genitourinary malformations, and mental retardation and are predisposed to develop bilateral Wilms tumor (the WAGR syndrome). The critical region for these defects has been narrowed to a segment of band 11p13 between the catalase and the beta-follicle-stimulating hormone genes. In this report, we have cloned the endpoints from a WAGR patient whose large cytogenetic deletion, del(11)(p14.3::p13), does not include the catalase gene. The deletion was characterized using DNA polymorphisms and found to originate in the paternally derived chromosome 11. The distal endpoint was identified as a rearrangement of locus D11S21 in conventional Southern blots of the patient's genomic DNA, but was not detected in leukocyte DNA from either parent or in sperm DNA from the father. The proximal endpoint was isolated by cloning the junction fragment and was mapped in relation to other markers and breakpoints. It defines a new locus in 11p13-delta J, which is close to the Wilms tumor gene and the breakpoint cluster region (TCL2) of the frequent t(11;14)(p13;q11) translocation in acute T-cell leukemia. An unusual concentration of base pair substitutions was discovered at delta J, in which 9 of 44 restriction sites tested (greater than 20%) vary in the population. This property makes delta J one of the most polymorphic loci on chromosome 11 and may reflect an underlying instability that contributed to the original mutation. The breakpoint extends the genetic map of this region and provides a useful marker for linkage studies and the analysis of allelic segregation in tumor cells.

Multivariate analysis of body shape in fragile X (Martin-Bell) syndrome

Human body shape measures can be more informative in studies of developmental abnormalities than distances between body landmarks. Such measures were obtained by an appropriate transformation of 34 distances between trunk/limbs and head/face landmarks in 43 men and 72 women with the Martin-Bell Syndrome (MBS), and in 99 and 103 normal men and women, respectively. The transformation of the original distances was performed by adjusting individual measures for an overall size measure using regression analysis. Thus obtained body shape variables were used in discriminant analysis in order to obtain unbiased classification probabilities of individuals having the MBS or being normal. The average percent correctly classified male and female individuals was high (93 and 87, respectively). Moreover, the body shape variables were used to obtain shape dimensions by means of principal component analysis. There was no difference between the MBS and normal individuals in the first (most important) principal component (shape dimension). This component represents the relative proportions between trunk and limb lengths and widths, or between midfacial lengths and widths. However, there were appreciable differences in some succeeding components. The problem of interpretation of shape dimensions as derived from principal component analysis and of their relevance to abnormal development in the MBS individuals is discussed.

Toward a physical map of the Xq28 region in man: linking color vision, G6PD, and coagulation factor VIII genes to an X-Y homology region

We are using pulsed-field gel electrophoresis (PFGE) to establish a physical map of the human Xq28 region. We have identified a new probe 35.239 (DXYS64), localized in Xq28 by somatic hybrid mapping and belonging to a region of greater than 99% homology between the X and the Y chromosomes. PFGE data show that probes 35.239 and the polymorphic locus DXS115 (probe 767) map within a common 300-kb BssHII fragment. Both probes, in addition, hybridize to 575-kb BssHII and 590-kb ClaI fragments that contain the gene coding for coagulation factor VIII (F8C). The order F8C-DXS115-DXYS64 could be determined. Our results also provide evidence for linkage between the red/green color vision locus (RCP,GCP) and probes MD13 and T1.7 (GdX, DXS254) within a 750-kb ClaI fragment. Although the latter two probes are located within 50 kb of the 3' end of the G6PD gene, a G6PD cDNA probe did not hybridize to this fragment. G6PD, on the other hand, could be linked to F8C on a 290-kb BssHII fragment. All these data allow us to propose the order (RCP,GCP)-MD13-GdX-G6PD-F8C-DXS115-DXYS 64. We also linked probes St14 (DXS52), MN12 (DXS33), and DX13 (DXS15) to a member of a small family of X-linked dispersed sequences (DNF22S3) within a 575-kb BssHII fragment. The preliminary physical map presented here should be useful for further fine mapping of disease genes in the Xq28 region and should be helpful in orientating efforts toward the cloning of sequences close to the fragile X syndrome.

Fragile X syndrome

Fragile X syndrome is one of the most intriguing genetic conditions now being studied. As the most common inherited form of mental retardation, it has an incidence of approximately 1 in 1000 male infants and boys. Because the cytogenetic diagnosis has only recently been available, many affected boys and female carriers have not yet been identified. This article reviews the characteristic, clinical features of fragile X syndrome and discusses treatment and intervention.

Nature of distamycin A-inducible fragile sites

Five rare distamycin A-inducible fragile sites have been identified on human chromosomes: fra(8)(q24.1), fra(11)(p15.1), fra(16)(p12.1), fra(16)(q22), and fra(17)(p12). All of these fragile sites are located at the junction of Giemsa-positive (G) and negative (R) bands and their expression can be induced by a variety of AT specific DNA ligands. Analysis of family data indicate that the distamycin A-inducible fragile sites segregate as a simple codominant trait with complete penetrance, and probands receive these fragile site genes equally from mothers and fathers. Based on current knowledge of chromosome instability, the nature of distamycin A-inducible fragile sites is discussed. Distamycin A-inducible fragile sites appear to be unique chromosomal regions particularly susceptible to fragility under certain stress conditions. They may also be hot spots for recombination, gene amplification, and integration of foreign genomes.

The role of nucleotides in human fragile site expression

Fragile sites are points on chromosomes which tend to break non-randomly when exposed to specific chemical agents or conditions of tissue culture. There are 3 groups of rare fragile sites, and carriers of these range in incidence from about 1 in 20 to 1 in several thousand individuals. Rare fragile sites are essentially chromosome variants with no known phenotypic consequence, except for the fragile X which is associated with the commonest inherited form of mental retardation in man. There are also 3 groups of common fragile sites, carried by all or most individuals. These are part of normal chromosomal architecture. Expression of most of the groups of fragile sites is mediated by perturbations of the nucleotide pool and these, as they relate to each group of fragile sites, are discussed. The rare folate-sensitive fragile sites are expressed when thymidylate or deoxycytidine are in limited supply during DNA synthesis. Other rare fragile sites are induced by bromodeoxyuridine (BrdU). Sets of common fragile sites are induced by BrdU, 5-azacytidine and aphidicolin. Various hypotheses on the molecular nature of fragile sites are considered.

Chromosome breaks and fragile sites in leukemic bone marrow cells

Bone marrow cells from leukemic and nonleukemic patients were examined for chromosome breakage in cultures treated with fluorodeoxyuridine (FUdR) and FUdR plus caffeine. The results indicate that the leukemic cells have more chromosome breakage than the nonleukemic cells when thymidylic synthetase is inhibited by FUdR. Addition of caffeine did not enhance this chromosome breakage. These findings of enhanced breakage by FUdR exposure in vitro, nevertheless, may suggest that leukemic cells in general are more susceptible to breakage than normal cells, thereby predisposing the former to secondary chromosome rearrangements.

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.

Chromosomal localization of the genes for the vitronectin and fibronectin receptors alpha subunits and for platelet glycoproteins IIb and IIIa

The integrins, a family of related membrane receptors involved in cell-cell and cell-matrix interactions, are heterodimeric complexes of alpha and beta subunits. To begin to understand the evolution of these complexes, we studied the genomic organization of several alpha and beta integrin subunits. Using both somatic cell hybrids and an in situ hybridization technique, we have determined the chromosomal location of the genes for the alpha subunits of the vitronectin receptor (VNR alpha), the fibronectin receptor (FNR alpha), and for the alpha subunit of the platelet glycoprotein IIb/IIIa complex, GPIIb. In addition, we have determined the chromosomal location of the gene for the beta subunit of the GPIIb/IIIa heterodimer, GPIIIa. Our studies indicate that the alpha subunits do not localize to a single locus, but that each is found on a different chromosome. The gene for VNR alpha is located on chromosome 2, the gene for FNR alpha is on chromosome 12q11----13, and the gene for GPIIb is on chromosome 17q21----23. In contrast to the chromosomal dispersion of the alpha subunits, the genes for GPIIb and GPIIIa are physically close, with the gene for GPIIIa also located on chromosome 17q21----23. These studies indicate that the genes for the alpha subunits of the integrin family have been dispersed during evolution while GPIIb and GPIIIa are in close physical proximity. This physical proximity of GPIIb and GPIIIa may be involved in the concurrent expression of these proteins by megakaryocytes, and may result in linkage disequilibrium between these two genes, which would limit the use of restriction length polymorphisms in linkage studies of GPIIb/IIIa abnormalities in small kindreds.

Linkage between the fragile X and F9, DXS52 (St14), DXS98 (4D-8) and DXS105 (cX55.7)

Linkage data using the markers F9, DXS105 (cX55.7), DXS98 (4D-8) and DXS52 (St14) are presented from 22 kindreds segregating with the fragile X. Two-point linkage analysis was carried out taking into account cytogenetic results and penetrance classes defined by mental impairment status of mothers. Recombination frequencies (theta) corresponding to the maximum z scores (z) were obtained between F9 (z = 3.48, theta = 0.18), DXS105 (z = 5.06, theta = 0.07), DXS98 (z = 4.79, theta = 0.01) and DXS52 (z = 6.44, theta = 0.09) and the fragile X. Recombination frequencies between marker loci in fragile X families are also presented. These recombination frequencies need to be combined with those from other studies in order to determine the best estimates of map distances for use in genetic counselling, until markers closer to the fragile X, or at the fragile X, can be used. Most potential fra(X) heterozygotes were informative for flanking markers using the above 4 probes. Carrier risks were determined by 3-point analysis using informative flanking markers, taking into account cytogenetic results. Low level fra(X) expression occurred in 2 probable non-carriers; emphasising the need for extreme caution in the interpretation of low rates of expression.

Constitutive fragile sites in fra(X) individuals

Recently, it was proposed that the constitutive fragile site at 3p14 be used as an "internal control" to indicate the effectiveness of the FUdR fragile site induction system. We have tested this hypothesis by determining the frequency of constitutive fragile sites at 1p31, 3p14, and 16q23 in cultures from 42 known fra(X) individuals. At least 50 cells were analyzed from each case. Seventy-four percent (31/42), 95% (40/42) and 90% (38/42) of the fra(X) individuals exhibited frequencies of less than 4% at constitutive fragile sites 3p14, 1p31 and 16q23, respectively. Of the 42 individuals tested, 12 or 28.6% showed no fragility at any of the 3 sites studied. On the other hand, at least one constitutive fragile site was observed in 50 cells studied from over 70% of the 42 people studied. It is suggested that "positive controls" continue to be used, while at the same time recording all fragile sites to identify a combination of constitutive fragile sites that may serve as an internal control indicator, and that DNA marker studies be used to complement cytogenetic testing.

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.

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.

Fragile X expression in normal and mentally retarded subjects: effect of treatment with an antifolic agent

Expression of the fragile site fra(X) (q27.3) in peripheral lymphocytes was evaluated in mentally retarded patients and in normal control individuals before and after administration of the antifolic agent trimethoprim for 7 days. This treatment was effective in converting the status of some individuals from fra(X)-negative to fra(X)-positive. However, the induced level of fra(X) expression was very low and not significantly different in patients and in control subjects and did not increase in those individuals where it was already present before treatment. These data support the contention that fra(X)(q27.3) is a common fragile site and that treatment in vivo with an antifolic agent is not effective in enhancing its degree of expression in vitro. Therefore, such treatment seems to be of no diagnostic value in those cases where the fra(X) syndrome is suspected clinically, but where there is no or very low cytogenetic expression of the fra(X).

Translocation t(3;8)(p14.2;q24.1) in renal cell carcinoma affects expression of the common fragile site at 3p14(FRA3B) in lymphocytes

The common fragile site at 3p14(FRA3B) is cytogenetically close to the positions of translocation and deletion breakpoints frequently observed in renal cell carcinoma (RCC) and small cell carcinoma of the lung. Possible involvement of this fragile site in the familial RCC t(3;8)(p14.2;q24.1) was investigated. Expression of FRA3B, induced by treatment of lymphocytes with aphidicolin, is altered by the translocation. These results suggest that the fragile site is very close to, if not coincident with, the translocation breakpoint.

Fragile X syndrome

The fragile X syndrome is the most common inherited form of mental retardation known. Its phenotype includes large or prominent ears, macroorchidism, and characteristic behavioral problems. It has attracted the interest of cytogeneticists and molecular biologists because of its characteristic fragile site on the X chromosome. It has puzzled geneticists because of its unusual inheritance pattern involving nonpenetrant males. This syndrome has also spearheaded an appreciation of cytogenetic abnormalities in the etiology of all degrees of developmental delay.

The fragile X site in somatic cell hybrids: an approach for molecular cloning of fragile sites

Fragile X syndrome is a common form of mental retardation associated with a fragile site on the human X chromosome. Although fragility at this site is usually evident as a nonstaining chromatid gap, it remains unclear whether or not actual chromosomal breakage occurs. By means of somatic cell hybrids containing either a normal human X or a fragile X chromosome and utilizing two genes that flank the fragile site as markers of chromosome integrity, segregation of these markers was shown to be more frequent if they encompass the fragile site under appropriate culture conditions. Hybrid cells that reveal marker segregation were found to contain rearranged X chromosomes involving the region at or near the fragile site, thus demonstrating true chromosomal breakage within this area. Two independent translocation chromosomes were identified involving a rodent chromosome joined to the human X at the location of the fragile site. DNA analysis of closely linked, flanking loci was consistent with the position of the breakpoint being at or very near the fragile X site. Fragility at the translocation junctions was observed in both hybrids, but at significantly lower frequencies than that seen in the intact X of the parental hybrid. This observation suggests that the human portion of the junctional DNA may contain part of a repeated fragility sequence. Since the translocation junctions join heterologous DNA, the molecular cloning of the fragile X sequence should now be possible.

Genetic heterogeneity of X-linked mental retardation with fragile X. Association of tight linkage to factor IX and incomplete penetrance in males

X-linked mental retardation with fragile X or Martin-Bell Syndrome (MBS) is a frequent cause of mental retardation. So far segregation analysis of MBS in pedigrees ascertained by different, incomplete criteria has produced results, difficult to interpret, which suggest genetic complexity (Sherman et al. 1985). Biochemical and cell biological studies have failed to provide an assay for genetic heterogeneity in MBS and linkage analysis is the only available method. Such analysis, however, is complicated by the incomplete penetrance of the disease in males and the variable penetrance and expression of the defect in heterozygous females. We have used a new approach to test the heterogeneity of recombination between MBS and the coagulation factor IX gene or the anonymous probe 52A in a group of nine families who have sought genetic counselling at Guy's Hospital. We find that both our families alone and our families plus apparently complete samples of pedigrees reported in the literature, separate into two groups: one tightly and one loosely linked to factor IX. In the combined family sample these represent respectively 0.3 and 0.7 of the total and show recombination fractions of 0.0-0.15 and 0.25-0.5. Furthermore, the families with non-penetrant carrier males show tighter linkage to factor IX than the others, thus confirming the suggestion of a systematic difference among MBS families in the recombination between the disease and the factor IX locus. By contrast, no significant differences were found in the recombination between 52A and factor IX in the two groups of MBS families or in these families versus those with Hunter syndrome examined in our laboratory. The causes of the linkage heterogeneity we describe are not known. At least two alternatives can be considered: The existence of two MBS loci or differences in the recombination between a single MBS locus and the factor IX gene. The association between incomplete penetrance and tight linkage to factor IX as well as the discontinuous variation in recombination fraction we have observed seem to favour the former alternative.