Counseling needs and attitudes toward prenatal diagnosis and abortion in fragile-X families

The genetic counseling need of 32 women of normal intelligence at-risk for having children with the fragile-X syndrome (FXS) were determined by a questionnaire study which included assessment of their attitudes toward prenatal diagnosis and the option of pregnancy termination. Eighteen (56%) of the women had one or more children with the FXS and 14 (44%) had no affected children. Twenty-six (81%) of the subjects stated that they would choose to have prenatal diagnosis and 9 (28%) indicated they would terminate an affected pregnancy. There was no significant difference between women who had affected children and those who did not have affected children, nor between Catholics and non-Catholics regarding acceptance of prenatal diagnosis. Catholic women were less likely to consider pregnancy termination than non-Catholics, but the majority of subjects (56%) were unsure what they would do if a fetus they were carrying was found to be affected. Issues the subjects considered most important for discussion with a genetic counselor included: 1) availability of treatment, 2) risk for having an affected grand child, 3) expectations for future functioning of affected children, and 4) availability of prenatal diagnosis.

In situ nick translation of the fragile X region

At the present time, the molecular nature of the fragile site at Xq27.3 is not well understood. To examine the sensitivity of this region to DNAase I, in situ nick translation was performed on metaphase chromosomes from a fragile X (fra(X] positive individual. In this technique DNAase I is used to nick regions of chromosomal DNA that are in "open" conformation. Biotinylated dUTP was incorporated by nick translation at these sites. The incorporation was identified by double antibody labeling and avidin-horseradish peroxidase staining. Spreads, which had been stained with this technique, were photographed and subsequently trypsin-Giemsa G banded (post-GTG banded) for chromosome identification. In 36 of 44 (82%) fra(X) positive male cells, the region distal to fra(X) (q27.3) was prominently stained in contrast to its light staining appearance in GTG preparations. The fragile site itself was outlined more clearly than can be achieved by GTG or homogeneous staining. When autosomal fragile sites were induced by the addition of 1.5 microM aphidicolin 17 hours prior to harvest, 24 of 27 (89%) fragile sites on the ends of autosomes were prominently stained in regions distal to the break. Because the fra(X) and autosomal fragile regions behaved similarly, this suggests that they have a similar conformation. Thus, while autosomal and Xq27.3 fragile sites are strongly induced by different means, the organization of these sites and the regions distal to them appear to be similar.

Fra(X) frequency on the active X-chromosome and phenotype in heterozygous carriers of the fra(X) form of mental retardation

Female heterozygotes of the fra(X) form of mental retardation show variable degrees of mental impairment and phenotype expression of the disorder. This might be an effect of inactivation of the X-chromosome which carries the fra(X)(q). Prior replication studies in heterozygous carriers gave contradictory results with respect to possible genotype-phenotype correlation. In the interpretation of these studies it is important to understand the effect of BrdU on the fra(X)(q) expression. In a group of 13 hemizygous patients with fra(X)(q) and 7 heterozygous carriers we studied the effect of BrdU on fra(X) expression. In the heterozygous carriers the use of BrdU resulted in a significant suppression of the fra(X)(q), while in hemizygous patients no difference in fra(X)(q) frequency with or without BrdU could be observed. It can be concluded that BrdU suppresses the fra(X)(q) preferentially on the inactive X-chromosome. Thus the fra(X)(q) frequency on the active X-chromosome is of primary importance in phenotype correlation studies among heterozygous carriers. In our group of heterozygous carriers we observed a negative correlation between (IQ) phenotype and fra(X)(q) expression on the active X-chromosome. This suggests that the gene for the fra(X)(q) form of mental retardation is on the X-chromosome and undergoes inactivation.

Origin of sex chromosome aneuploidy

Probabilities applicable to RFLPs are derived for the genotypes of XXY, XXX, and XO children, conditional on their mode of origin and the parental mating type, and the theory is applied to family data on the XG locus. The proportion of XXY males arising in spermatogenesis is shown to be 0.41 +/- 0.09. The large distance of XG from the centromere makes distinction between maternal meiosis I and II unreliable, but if man is like Drosophila, most of the maternal non-disjunctions arise in meiosis I, among tetrads that have undergone one or more exchanges. Data from XG show that 0.78 +/- 0.05 of XO females arise from an error involving the paternal sex chromosomes. The XG locus is virtually uninformative about the origin of XXX. Application of the theory to selected RFLPs will be much more incisive because of their large number, lack of dominance, greater heterozygosity, and distribution along the chromosome. Study of RFLPs will facilitate diagnosis of the parental origin of sex chromosome abnormalities and the comparison of recombination rates in regular and trisomic progeny of maternal mei I and mei II origin. A pseudocentromeric model that estimates map distances from exceptional progeny is applied to non-disjunction of the X chromosomes in D. melanogaster, giving good recovery of the expected map and thereby validating this approach for RFLPs in man.

Fragile X families in a northern Swedish county--a genealogical study demonstrating apparent paternal transmission from the 18th century

Eleven families including 35 cases with fra(X) mental retardation (MR) were traced genealogically using the Research Archives at Umeå University. Seven of the cases were women with fra(X). All of the families originated partly or totally from the county of Västerbotten. It was possible to link 7 of the index families to common ancestors over an 8-11 generation span. The remaining 4 families were not traced to the same ancestors. However, they were linked together pair-wise over a 7-8 generation span. Transmission of the fra(X) mutation was studied in these families. In the pedigree analyses, priority was given to maternal transmission. In 2 families the fra(X) mutation was transmitted solely through females over 7 or 8 generations respectively. Within 9 families the mutation was transmitted by males in 2-5 generations in order to reach common ancestors.

Pulsed-field gradient-gel studies around the fragile site

Using pulsed-field gradient-gel electrophoresis (PFGE) we compared two fragile X (fra(X] chromosomes from individuals in families that exhibit linkage heterogeneity between fra(X) and coagulation factor IX (F9). The analysis of very large restriction fragments indicated that there is a structural difference in the interval between fra(X) and F9 near the locus DXS105. Differences were observed in the Sfi I partial digestion analysis and in the Mlu I pattern of the DXS105 region. Digestion with Nru I and Sst II also showed differences between these alleles. The analyses suggest that the alleles differ in a region of greater than 200 kb. Analysis of other normal and fra(X) chromosomes will be necessary to determine whether the observed difference is a normal population variant or if it may be responsible for the linkage heterogeneity observed between these loci.

Fertility with deletion Xq25: report of three cases; possible exceptions for critical region hypothesis

We report on an Arab family in which a mother and two of her daughters, despite having deletion Xq25, are fertile. So far, only one case of deletion Xq25 associated with fertility has been reported. Consistent inactivation of the deleted X chromosome in the proposita and early menopause in the mother were noted. The effect of Xq deletion on fertility and the CRH is discussed.

Pulsed-field gel mapping studies in the vicinity of the fragile site at Xq27.3

Physical mapping strategies are being employed for an analysis of the fragile X site region. Deletion breakpoints which may be close to the fragile site appear to be at a distance of at least several hundred kb from the nearest DNA probes. Further evidence is found for physical linkage between St14 (DXS52), DX13 (DXS15) and MN12 (DXS33). The data indicate differences in CG content and/or methylation levels between the distal and proximal sides of the fragile site.

[Electroretinography changes in carriers of X-chromosomal retinitis pigmentosa]

Out of five families with pedigrees typical for X-linked retinitis pigmentosa (RP) the authors examined eight definite and eight possible female carriers. Subjective symptoms were mild and variable, consisting mainly of relative nyctalopia. Ophthalmoscopic examination revealed a normal fundus, or mild changes suggestive of RP, or even marked changes. Perimetry (Goldmann) revealed some narrowing of the outer isopter corresponding to a significant reduction in visual field area. In testing dark adaptation, the final rod threshold was slightly elevated in five out of 16 subjects. Ganzfeld-electroretinography using matched band filters in conditions of dark and light adaptation to selectively stimulate rod and cone signals revealed reduced b-wave amplitudes in most cases. White stimuli, applied in dark adaptation, generated ERGs with less abnormality than the selective rod and cone ERGs. Peak times of the cone b-waves were prolonged in most cases. The ERG abnormalities were more pronounced in definite carriers and in advanced age. In some cases remarkable differences were noted between the two eyes as regards fundus morphology and ERG responses. The authors conclude that the possibility of improved genetic and educational counseling in families afflicted by X-linked RP justifies the considerable diagnostic effort involved in such examinations.

A male subject with 3 Y chromosomes (48, XYYY): a case report

We describe a male subject with 3 Y chromosomes who presented with complaints of lack of sexual desire and failure to achieve rigid erection. The patient was tall and had markedly incurved little fingers on both hands as well as small testes. Karyotype was 48, XYYY. The patient's parents had normal karyotypes. Endocrinological profile suggested a primary functional disturbance of the gonads. Testicular biopsy demonstrated atrophic seminiferous tubules without spermatogenesis. To our knowledge this case represents the third instance of a man with an 48, XYYY karyotype without mosaicism.

Strategy for molecular cloning of the fragile X site DNA

Fragile X syndrome is a common form of mental retardation associated with a fragile site on the human X chromosome. We have recently demonstrated that the fragile X chromosome, when isolated within a somatic cell hybrid, often participates in translocations involving rodent chromosome arms. Cytogenetic and molecular evidence strongly suggests that the human breakpoint of these translocations is within the fragile X sequence. Hence, the joining of heterologous DNA (i.e. from two species) may permit the molecular cloning of the fragile X site. We describe here the cloning approach employed to enhance the isolation of interspecific chromosome translocation junctions. The human portion of the translocation junction should be derived from the fragile X site sequence.

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.

Aneuploidy and the fragile X syndrome

The possibility that female carriers of the fragile X gene(s) are at increased risk for nondisjunctional events leading to aneuploid offspring has been suggested by several investigators. To better address this question we analyzed pedigrees of 117 families in which the fragile X syndrome is segregating. The 117 pedigrees, originally collected for segregation analyses, included 236 females with offspring whose carrier status was determined by cytogenetic or pedigree analysis or by analyses using flanking DNA markers. These 236 females have had 931 offspring including one 47,XXY and 6 trisomy 21 individuals (1/155). Statistical analysis suggested that the observed rate of trisomy 21 was significantly higher than expected (Fisher's exact test, p less than or equal to 0.05). Assuming a Poisson distribution to calculate the confidence interval for the observed rate of trisomy 21 individuals, we found that the expected rate of 1.6/1000 in this sample fell outside the 99% confidence limits of our observed rate of 1/155. Additional data from a larger sample are needed to replicate these findings.

Relationships between hemispheric lateralization, sex hormones, quality of parenting and adjustment in 47,XXY males prior to puberty

This paper reports findings of a study examining the degree to which intelligence, levels of sex-steroids and gonadotropins, quality of parenting and laboratory-based measures of left and right hemispheric specialization are associated with individual differences in psychosocial functioning in pre-pubertal 47,XXY males. The results indicate that activity level and freedom from tendencies towards withdrawal in these subjects are best predicted by the normality of their left hemispheric functioning.

Investigation of the twinning rate in families with the fragile X syndrome

An excess of twins in families with the Martin-Bell or fra(X) syndrome was noted previously in one family study [Fryns, 1986]. We tried to confirm this observation in a second large sample of families from a different population. We calculated the number of twin births among the total number of live births of known obligate carriers found in fra(X) families ascertained in New South Wales, Australia. We only included births of known sex and excluded triplets. There were 5 male pairs, 3 female pairs and 9 unlike sex pairs of twins born among 752 live births. Thus the twining rate was 1/44 per live birth. We compared this rate to that found in two different types of individuals: 1) the rate of 1/96 which was obtained from the 1985 vital statistics for New South Wales, and 2) the rate 1/75 obtained from a sample of live births of obligate carriers with hemophilia A. The increase in twinning among heterozygotes with the fra(X) was highly significant when compared to the census data (p less than 0.001). However, it was not significantly different from that in the hemophilia data (p less than 0.05) which were collected in the same way as in the fra(X) families.

Is there a fragile(X) negative Martin-Bell syndrome?

During the course of the preventative screening program for the fra(X) syndrome, we identified 32 men with the phenotype but who were fra(X) negative. These were reviewed and none fitted the full criteria, so we were unable to confirm the existence of the fra(X) negative Martin-Bell syndrome. The literature and 4 families previously thought to have the fra(X) negative Martin-Bell syndrome were also reviewed. We were unable to make a concrete diagnosis of the fra(X) negative Martin-Bell syndrome.

The behaviour of fragile X and other aberrations during recovery from low folate conditions

Whole blood from two mentally retarded fra-X brothers was grown in low folate medium where fra-X expression was enhanced. Bromodeoxyuridine was added to mitigate the low folate conditions and metaphases were sampled sequentially, and stained for replication banding, through one cell cycle of recovery. The replication bands allowed detailed analysis of the cell cycle and the allocation of individual cells to precise sub-phases. Various classes of fra-X and all other types of chromosomal aberrations were scored in these classified cells. The fra-X does not conform in morphology to any of the known simple chromatid intrachange types, which were often present within the same cells, but the subsequent fall in frequency once bromodeoxyuridine was added closely paralleled that of the conventional aberrations. Normal folate level frequencies of fra-X are restored by the time early S-phase cells (sub-phase SkI) reach metaphase. When sub-phased cells are rearranged in true chronological sequence, there is a suggestion of a sudden fall in frequency between SkII-III (about 70% of the transit of S). This suggests that the critical point for low folate enhancement occurs in this region of the S-phase. This is somewhat earlier than the band-appearance distribution curve for Xq27 which lies within sub-phase SkIV.

The fragile X syndrome: variability of expression in carrier females

The fra(X) expression in heterozygotes varies considerably from family to family. In family D23, 5 carriers express the fra(X) regardless of age. DNA studies using 3 markers were inconclusive as to whether cytogenetic testing is more reliable in this family than in others. III-2 and III-5 are the critical individuals in the pedigree; further study with closer probes should resolve this question.

Investigation of the segregation of the fragile X mutation in daughters of obligate carrier women

Two reports have suggested that over 50% of the offspring of obligate carrier women receive the mutation for the fra(X) or the Martin-Bell syndrome [Webb et al, 1986; Fryns, 1984]. Such a segregation distortion is difficult to assess for the fra(X) syndrome because of incomplete penetrance, variable expression and probable ascertainment biases. We have attempted to evaluate this possible segregation distortion in daughters of obligate carriers in a large sample of sibships ascertained in a survey of New South Wales, Australia. We used two definitions of expression: 1) presence of fra(X) positive cells if daughters were tested cytogenetically, and 2) mental impairment if daughters were not tested cytogenetically. The segregation frequency was estimated in different types of sibships of obligate carriers based on the way they were ascertained. This was done in order to have an internal check on possible ascertainment biases. Among the 189 cytogenetically tested daughters, 81 were fra(X) positive. Among the 97 untested daughters, 24 were mentally impaired in some way. Therefore, the segregation frequency as defined by fra(X) expression and/or mental impairment was 37%. Thus, no evidence was detected for segregation distortion. These data were significantly different than those collected by Webb et al [1986] and scored by the same method as the present data set.

Improved DNA markers for efficient analysis of fragile X families

We report the characteristics of two new probes that detect BclI RFLPs useful for analysis of fragile X families. With these two probes and a single blot, 34% of women are heterozygous both for the proximal marker DXS105 (closer to the fragile X locus than the factor IX gene) and for the distal markers DXS52 or the factor VIII gene. Combined with the analysis of previously described polymorphic markers, it is possible to have a majority of families fully informative for flanking markers using a limited number of probes and restriction digests.

Multipoint linkage of 9 anonymous probes to HPRT, factor 9, and fragile X

We have analyzed the segregation of restriction fragment length polymorphisms (RFLPs) associated with 9 anonymous probes detecting loci DXS10, DXS15, DXS19, DXS37, DXS51, DXS52, DXS98, DXS99, and DXS100 and probes for HPRT and F9 in a set of 40 families segregating fragile X (fra(X]. Using two-point and multipoint analysis, we have established their relative genetic locations. The results indicate that DXS99 and DXS10, unlike previous reports, are not tightly linked to F9. A new locus was found to map within the F9 - fra(X) region. DXS98 showed 6% recombination with fra(X) and appeared to be the closest locus to fra(X). These results will be useful for mapping the relative position of newly defined X probes in this region and for future genetic studies of families with fra(X), hemophilia B, or Lesch-Nyhan mutations.

Cytogenetic and physical mapping in the region of the X chromosome surrounding the fragile site

Seven DNA probes have been mapped within the Xq27-Xq28 region using in situ hybridization, in some cases on chromosomes expressing the fragile site to enhance the resolution. To complement these studies and investigate the relationship between genetic, cytogenetic and physical distance some of these probes were used for large scale mapping using pulsed field gels. Physical linkage was demonstrated between two loci, F9 and MCF2, which are separated by less than 270 kb, and a restriction map extending over 1,300 kb has been generated.

Fragile-X mutation proposed to block complete reactivation in females of an inactive X chromosome

I discuss two aspects of my proposal that fra(X) chromosomes exist in two states, imprinted and non-imprinted: why do males not imprint the fra(X); does the "Sherman paradox" rule out my proposal?