Further evidence for genetic heterogeneity in the fragile X syndrome

An estimating function approach to linkage heterogeneity

Testing linkage heterogeneity between two loci is an important issue in genetics. Currently, there are four methods (K-test, A-test, B-test and D-test) for testing linkage heterogeneity in linkage analysis, which are based on the likelihood-ratio test. Among them, the commonly used methods are the K-test and A-test. In this paper, we present a novel test method which is different from the above four tests, called G-test. The new test statistic is based on estimating function, possessing a theoretic asymptotic distribution, and therefore demonstrates its own advantages. The proposed test is applied to analyse a real pedigree dataset. Our simulation results also indicate that the G-test performs well in terms of power of testing linkage heterogeneity and outperforms the current methods to some degree.

Genetics, statistics and human disease: analytical retooling for complexity

Molecular biologists and geneticists alike now acknowledge that most common human diseases with a genetic component are likely to have complex etiologies. Yet despite this belief, many statistical geneticists continue applying, in slightly new and different ways, methodologies that were developed to dissect much simpler etiologies. In this article, we characterize, with examples, the various factors that can complicate genetic analysis and demonstrate their shared features and how they affect genetic analysis. We describe a variety of approaches that are currently available, revealing methodological gaps and suggesting new directions for method development. Finally, we propose a comprehensive two-step approach to analysis that systemically addresses the different genetic factors that are likely to underlie complex diseases.

Penetrance of fra(X) gene: influence of grandparental origin of the gene, mental status of the carrier mother, and presence of a normal transmitting male

Previous studies have indicated that the fragile X [fra(X)] gene does not show full penetrance (mental impairment) in carrier females or "carrier" males. The phenomenon of non-expressing carrier males distinguishes the fra(X) syndrome from all other known X-linked disorders. Moreover, penetrance of the fra(X) gene apparently does not show random distribution within fra(X) families, but seems to be reduced in sibs of normal transmitting males (NTM's). The availability of many large multigeneration fra(X) families, studied by cytogenetic and DNA analyses, enabled us to refine the estimates of the penetrance. From our data we conclude that the penetrance in daughters of carrier females is determined by the mental status of the mother. In sons of carrier females, the observed penetrance appears to be influenced by the grandparental origin of the gene as well as by the mental status of the mother. However, in contrast with the average penetrance, we observed a strongly reduced penetrance of the fra(X) gene in brothers (14%) and sisters (21%) of NTM's. These findings have profound implications for genetic counseling.

Problems in ascertainment of transmitting males in Martin-Bell syndrome

The difficulty of assigning families affected with the Martin-Bell syndrome (MBS) into the category of male transmission is emphasised and illustrated by examples of 3 MBS families. These examples demonstrate how the ability to detect transmitting males depends on the number of generations available for investigation, and also on the "spread" of clinical investigation across many branches of the family regardless of what appears to be an unremarkable family history. Some unusual properties of male transmission are shown, and the problem of selective ascertainment of the particular MBS male individuals in different generations in a set of pedigrees is discussed.

Premutation for the Martin-Bell syndrome analyzed in a large pedigree segregating also for G6PD-deficiency. I: A working hypothesis on the nature of the FRAX-mutations

A large Sardinian family including 13 Martin-Bell syndrome (MBS) patients, several instances of normal transmitting males or females, and the G6PD-Mediterranean mutant segregating in some of its branches, has been thoroughly investigated with the hope of gaining further insight on the nature of the FRAX-mutation. All the MBS patients and the 15 obligate heterozygous women present in the pedigree could be traced back through their X-chromosome lineage to the same ancestress, who must have been heterozygous for a silent premutation at the FRAX-locus. This premutation appears to have turned into a true FRAX-mutation at least 9 times during the gametogenesis of the ancestress' X-related descendants of whom four are males. This finding alone suggests that the transition from the FRAX premutation to the true mutation can be the result of intra- as well as interchromosomal events. This conclusion is supported by the additional observation that the genetic phase between the FRAX and the G6PD loci remained unaltered when the transition occurred in a repulsion double heterozygote for the premutation and the G6PD-Mediterranean mutant. The data described are compatible with the hypothesis that MBS patients and normal transmitting males are, respectively, hemizygous for deletion or duplication products generated by aberrant recombination events at a highly recombinogenic site of the region Xq27-Xqter. The overall message stemming from this report is that no firm conclusion can be drawn on the genetic linkage between the FRAX-locus and other markers of this region until the nature of the FRAX-mutations and the mechanism of their occurrence are fully understood.

Linkage homogeneity near the fragile X locus in normal and fragile X families

The fragile X syndrome locus, FRAXA, is located at Xq27. Until recently, few polymorphic loci had been genetically mapped close to FRAXA. This has been attributed to an increased frequency of recombination at Xq27, possibly associated with the fragile X mutation. In addition, the frequency of recombination around FRAXA has been reported to vary among fragile X families. These observations suggested that the genetic map at Xq27 in normal populations was different from that in fragile X populations and that the genetic map also varied within the fragile X population. Such variability would reduce the reliability of carrier risk estimates based on DNA studies in fragile X families. Five polymorphic loci have now been mapped to within 4 cM of FRAXA--DXS369, DXS297, DXS296, IDS, and DXS304. The frequency of recombination at Xq26-q28 was evaluated using data at these loci and at more distant loci from 112 families with the fragile X syndrome. Two-point and multipoint linkage analyses failed to detect any difference in the recombination fractions in fragile X versus normal families. Two-point and multipoint tests of linkage homogeneity failed to detect any evidence of linkage heterogeneity in the fragile X families. On the basis of this analysis, genetic maps derived from large samples of normal families and those derived from fragile X families are equally valid as the basis for calculating carrier risk estimates in a particular family.

Fragile X screening program in New York State

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

Fragile X families in a northern Swedish county: a genealogical study of possibly affected individuals in the nineteenth century

Most studies of fragile X [fra(X)] families are able to document mental impairment only by family history. Using Swedish historical archives and the unique parish catechetical meeting records it is possible to document qualitative phenomena such as literacy for over 100 years. In this way it was possible to identify 7 individuals with mental retardation living in the nineteenth century in an earlier published fra(X) pedigree. Four of them were female. At the present time another 4 severely mentally retarded females with the fra(X) syndrome have been diagnosed in this family. The high prevalence of mentally retarded females might indicate a variant form of the fra(X) syndrome in this family.

Linkage relationships between DXS105, DXS98, and other polymorphic DNA markers flanking the fragile X locus

We report on linkage data between DXS105, DXS98, the locus for the fragile X syndrome (FRAXA), and 3 other polymorphic loci that flank the FRAXA locus. An analysis was undertaken to determine the relative positions of DXS105 and DXS98 and to test the assignment of DXS105 to a location proximal and closely linked to FRAXA. In this study of fragile X fra(X) syndrome families, the DXS105 locus was calculated to be proximal to FRAXA with a maximum lod score of 10.36 at theta = 0.08. DXS105 was also shown to be closely linked to the gene for factor IX (F9)(Z = 11.84 at theta = 0.08) and to DXS98 (Z = 4.91 at theta = 0.04). The order of the loci proximal to FRAXA is most likely centromere-factor IX-DXS105-DXS98-FRAXA-telomere. The use of DXS105 and DXS98 in clinical investigations should significantly increase the accuracy of risk assessment in informative fragile X families.

Linkage analysis in the fragile X syndrome using multiple distal Xq polymorphic DNA markers

Linkage data using the polymorphic loci F9, DXS105, DXS98, DXS52, DXS15, and F8 and the DNA probe 1A1 are presented from 14 families segregating for fragile X [fra(X)] syndrome. Recombination fractions corresponding to the maximum LOD scores obtained by two-point linkage analysis suggest that DXS98 (Zmax = 3.23, theta = 0.0) and DXS105 (Zmax = 2.09, theta = 0.0) are the closest markers proximal to FRAXA and that DXS52 is the closest distal marker (Zmax = 3.55, theta = 0.16). FRAXA is located within a 25 cM interval between F9 and DXS52, coincident with DXS98, on multipoint linkage analysis. Phase-known three way crossover information places F8 outside the cluster (DXS52, DXS15, 1A1). Confidence limits for the markers DXS98 and DXS52 are relatively wide (0.0-0.15 and 0.06-0.31, respectively), but when used in combination with cytogenetic examination offer improved carrier detection in comparison with cytogenetic analysis alone.

Detection of fragile X non-penetrant males by DNA marker analysis

Segregation analysis of the fragile-X [fra(X)] syndrome uncovered an unexpected 20% excess of normal males among sibships by Sherman et al. (Sherman SL, Morton NE, Jacobs PA, Turner G [1984]. Ann Hum Genet 48:21-37; Sherman SL, Jacobs PA, Morton NE, Froster-Iskenius U, Howard-Peebles PN, Neilsen KB, Partington MW, Sutherland GR, Turner G, Watson M [1985]: Hum Genet 63:289-299). This result predicts that about 17% (1/6) of normal sons of carrier fra(X) females will be non-penetrant. A way to test this prediction is by DNA markers. We analyzed DNA samples from 100 families with a set of flanking DNA markers linked to the fra(X) locus. Ten of 51 (19.6%) normal brothers, doubly informative and non-recombinant for flanking DNA markers, were found to be non-penetrant males. This result closely confirms the predictions of the segregation analysis indicating that about 1/6 of normal brothers are non-penetrant carrier males. The use of DNA markers to identify non-penetrant brothers and grandfathers can help to clarify the inheritance of the fra(X) mutation and be of considerable clinical usefulness. Using DNA markers, it was possible to study grandparental transmission in 71 of the families. In 39 families, DNA analysis confirmed the apparent pattern of inheritance. In 18 families, the grandparents had a single daughter with affected children. Of these, a new mutation at the time of their daughters' conception was possible in 15 and quite likely in 3. In 14 families with 2 or more daughters with affected fra(X) offspring, the grandparents had no affected sons or other relatives known to be positive for fra(X).(ABSTRACT TRUNCATED AT 250 WORDS)

Mapping of a new RFLP marker RN1 (DXS369) close to the fragile site FRAXA on Xq27-q28

A new polymorphic DNA marker RN1, defining locus DXS369, was recently isolated. Using different somatic cell hybrids, RN1 was mapped between markers 4D-8 and U6.2. We have narrowed the localization of RN1 to the region between 4D-8 and FRAXA by genetic mapping in fragile X [fra(X)] families. Combined with information from other reports, the following order of loci on Xq27-q28 is suggested: cen-F9-(DXS105-DXS152)-DXS98-DXS369-FRAXA- DXS304-(DXS52-DXS15-F8)-tel. The locus DXS369 is closely linked to FRAXA, with a peak lodscore of 18.5 at a recombination fraction of 0.05. Therefore, RN1 is a useful probe for carrier detection and prenatal diagnosis in fra(X) families.

Linkage analysis of the fragile X syndrome using a new DNA marker U6.2 defining locus DXS304

A new RFLP marker U6.2 defining the locus DXS304 was recently mapped to the distal long arm of the X chromosome. In the present study we report the results of genetic linkage analysis of 13 fragile X [fra(X)] families that were informative for the new marker. Analysis of the recombinants for F9-FRAXA, DXS105-FRAXA, DXS98-FRAXA, DXS52-FRAXA, DXS15-FRAXA, and F8C-FRAXA, places DXS304 distal and near to the FRAXA locus. Combined with results from previous studies, our results support the order Xcen.-F9-DXS105-DXS98-FRAXA-DXS304-DXS5 2-DXS15-F8C-Xqter. Close linkage was observed between DXS304 and the disease locus with a peak lod score of 5.12 at theta = 0.04 from the present study and, with a peak lod score of 17.45 at theta = 0.035 when our data are combined with published data from 2 other studies. The present study confirms that U6.2 is useful for prenatal diagnosis and carrier testing in families affected by fra(X) syndrome.

Cytogenetically negative, linkage positive "fragile X" syndrome

We investigated the family of a 3-year-old boy with manifestations of the Martin-Bell syndrome (MBS). His 17-year-old cousin had classic manifestations of MBS and was fragile X [fra(X)] positive. The 3-year-old boy was fra(X) negative. Linkage analysis with probes flanking the fra(X) region indicated that these cousins had the same X chromosome inherited from a normal grandfather. The DNA and cytogenetic analyses suggest that limitations in the ability to detect the fra(X) mutation cytogenetically may be responsible for fra(X)-negative MBS; or, alternatively, that a crossover occurred between a locus determining the MBS phenotype and one determining fra(X) expression.

Fragile X syndrome in an extended family with special reference to an affected male with Klinefelter syndrome

We report on a large family (4 generations), with 77 studied individuals, 9 mentally retarded males, and one affected female with fragile X syndrome [fra(X)]. The analysis of 6 flanking polymorphic DNA markers showed that the affection is transmitted, through the carrier daughters to the grandsons and the greatgrandsons and that the great-grandfather is a transmitting male. This observation led us to question the importance of these clinically normal males, who are nonexpressing carriers and termed transmitting males. One propositus, described as a mentally retarded young man, had inherited identical restriction polymorphisms from his mother. Chromosome analysis showed a Klinefelter syndrome, with a fragile site in 18% of the cells leading to the conclusion that the nondisjunction occurred at the first stage of the maternal meiosis.

Genetic mapping of new RFLPs at Xq27-q28

The development of the human gene map in the region of the fragile X mutation (FRAXA) at Xq27 has been hampered by a lack of closely linked polymorphic loci. The polymorphic loci DXS369 (detected by probe RN1), DXS296 (VK21A, VK21C), and DXS304 (U6.2) have recently been mapped to within 5 cM of FRAXA. The order of loci near FRAXA has been defined on the basis of physical mapping studies as cen-F9-DXS105-DXS98-DXS369-DXS297-FRAXA-++ +DXS296-IDS-DXS304-DXS52-qter. The probe VK23B detected HindIII and XmnI restriction fragment length polymorphisms (RFLPs) at DXS297 with heterozygote frequencies of 0.34 and 0.49, respectively. An IDS cDNA probe, pc2S15, detected StuI and TaqI RFLPs at IDS with heterozygote frequencies of 0.50 and 0.08, respectively. Multipoint linkage analysis of these polymorphic loci in normal pedigrees indicated that the locus order was F9-(DXS105, DXS98)-(DXS369, DXS297)-(DXS293,IDS)-DXS304-DXS52. The recombination fractions between adjacent loci were F9-(0.058)-DXS105-(0.039)-DXS98-(0.123)-DXS369-(0.00)- DXS297-(0.057)-DXS296- (0.00)-IDS-(0.012)-DXS304-(0.120)-DXS52. This genetic map will provide the basis for further linkage studies of both the fragile X syndrome and other disorders mapped to Xq27-q28.

The role of recombination in the evolvement of the fragile X mutation

The frequency of recombination in the regions adjacent to the fragile X locus was studied in two groups of carriers: daughters of transmitting males and transmitters of maternally inherited fragile X chromosomes. Approximately one-half of the offspring of the former and one quarter of the offspring of the latter are recombinant. Recombinants and parentals are equally distributed among affected and normal offspring in the two groups. These results indicate that crossing-over at or around the fragile X locus occurs in every meiosis in daughters of transmitting males, although the recombinant chromatids do not necessarily carry the fragile X mutation. Hence, crossing-over is unequivocally associated with, but is not the direct cause of, the transition from the primary genetic lesion to the final mutation.

Study of a family with a fragile site of the X chromosome at Xq27-28 without mental retardation

The fragile site Xq27-28 was observed in several individuals of a large family. It is expressed at a high frequency among the carrier females, even as adults, and in one clinically normal male. None of the members of this family is affected with the mental retardation normally linked to this fragile site. Cytogenetic and flanking DNA marker polymorphism studies suggest a possible dissociation between the fragile site and clinical expression of the disease.

Infantile autism, fragile (X) (q27.3) and RFLP analysis in an extended Swedish family

In an extended family with eight individuals with infantile autism, in association with other developmental disorders and fragile (X) (q27.3), DNA techniques were used to investigate linkage between X chromosomal probes and the disorder. F9 was not informative and recombination was found between fragile X and DXS15, DXS51 and DXS52.

Linkage analysis using multiple DNA polymorphic markers in normal families and in families with fragile X syndrome

Linkage data, using the polymorphic markers 52A (DXS51), F9, 4D-8 (DXS98), and St14 (DXS52), are presented from 14 fragile X pedigrees and from 7 normal pedigrees derived from the collection of the Centre d'Etude du Polymorphisme Humaine. A multipoint linkage analysis indicates that the most probable order of these four loci in normal families is DXS51-F9-DXS98-DXS52. Recombination frequencies (theta) corresponding to maximum LOD scores (Z) were obtained by two-point linkage analysis for a number of linkage groups, including: DXS51-F9 (Z = 5.94, theta = 0.03), F9-DXS98 (Z = 0.51, theta = 0.26), F9-DXS52 (Z = 0.84, theta = 0.27), and DXS98-DXS52 (Z = 0.32, theta = 0.20). A multipoint linkage analysis of these loci, including the fragile X locus, was also performed for the fragile X population and the data support the relative order (DSX51, F9, DXS98)-FRAXA-DXS52. Recombination frequencies and maximum LOD scores, which again were derived from two-point linkage analyses, were obtained for the linkage groups DXS51-F9 (Z = 9.96, theta = 0) and F9-DXS52 (Z = 0.07, theta = 0.45) as well as for the groups DXS51-FRAXA (Z = 2.42, theta = 0.15), F9-FRAXA (Z = 1.30, theta = 0.18), DXS98-FRAXA (Z = 0.05, theta = 0.36), and DXS52-FRAXA (Z = 2.42, theta = 0.15). The linkage data was further tested for the presence of genetic heterogeneity both within and between the fragile X and normal families for the intervals DXS51-F9, F9-DXS52, F9-FRAXA, and DXS52-FRAXA using a modification of the A test. Except for the interval F9-FRAXA (P less than 0.10) there was no evidence of genetic heterogeneity for each of the various linkage groups examined. The heterogeneity detected for the interval F9-FRAXA, however, was most likely due to one family (Fx-28) that displayed very tight linkage between these two loci.

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: linkage analysis in black and white populations

Eleven white families and 10 black families have been studied to detect racial differences in the linkage of DNA markers flanking the fragile X site (FRAXA). The differences in the recombination fractions for F9-FRAXA and DX13-FRAXA were not significant. The pair St14-FRAXA exhibited no difference between the two groups. Although the sample size was small, it would appear that these DNA markers can be used in black persons for prenatal diagnosis and genetic counseling. A larger group of families would be necessary to determine if 4D8 and cX55.7 will be equally useful since these appear to have lower heterozygote frequencies in the black population.

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.

Recent experience in prenatal fra(X) detection

At least 35 cases of prenatal fra(X) diagnosis have been confirmed and reported. Amniotic fluid, fetal blood and chorion -ic villus samples have exhibited fra(X) (q27.3) in cultures from 26 males and 9 females. Here we have detected fra(X) in female and male amniotic fluid specimens, AF1/fra(X),X and AF2/fra(X),Y, respectively, and a male CVS/fra(X),Y using both FUdR and excess thymidine (THY) to demonstrate the marker chromosome. Both FUdR and THY detected fra(X) and usually FUdR was superior to THY with the exception of placental cultures. It was important to examine more than one culture per protocol since no fra(X) was observed in one AF2 FUdR culture while another exhibited 19.2% expression. Similarly, confirmation studies in lung fibroblast cultures for AF2 exhibited 4.3% fra(X) in one lab while another found negative results. A similar observation in whole blood cultures was also made recently by us. In addition, we have recently experienced our first false negative fra(X),X prenatal diagnosis. We have observed another case where only one cell in 300 exhibited fra(X) where the male fetus was 50% at-risk and was referred to us after the 20th week of gestation by sonography. On the basis of our experience we recommend the following: 1) the excess THY fra(X) induction system is effective but not superior to FUdR; 2) at least two duplicate cultures per induction system should be analyzed for the marker chromosome to avoid the possibility of false-negative diagnosis; 3) where fra(X) is not demonstrated or is present in very low frequencies in CVS and/or amniotic fluid cultures, complementary DNA marker studies and/or fetal blood cultures must be made available; 4) gestational age dating by ultrasonography is recommended as early as possible.

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.

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.

Linkage heterogeneity and fragile X

A multipoint test of heterogeneity on published data from 57 families with the fragile X syndrome has been undertaken. The hypothesis being tested was that there are two loci coding for fragile X expression, mutations at either of which can produce the phenotype. No predivision of the families was undertaken, as the test used an admixture parameter. Maximum likelihoods of the hypothesis have been calculated and compared with those produced on assuming a single locus for fragile X. The data do not suggest that there are two such loci within the interval between probes 52a and St14. In particular, the large kindred published by Camerino et al. (1983) does not supply convincing evidence of heterogeneity under this test. It is argued that the observed heterogeneity between factor IX and fragile X must have another explanation. There is some evidence for a second locus for fragile X outside the interval noted above; this locus being most probably proximal to these probes. The majority of the data suggesting this result comes from a family published by Davies et al. (1985).

Multilocus analysis of the fragile X syndrome

A multilocus analysis of the fragile X (fra(X] syndrome was conducted with 147 families. Two proximal loci, DXS51 and F9, and two distal loci, DXS52 and DXS15, were studied. Overall, the best multipoint distances were found to be DXS51-F9, 6.9%, F9-fra(X), 22.4%; fra(X)-DXS52, 12.7%; DXS52-DXS15, 2.2%. These distances can be used for multipoint mapping of new probes, carrier testing and counseling of fra(X) families. Consistent with several previous studies, the families as a whole showed genetic heterogeneity for linkage between F9 and fra(X).

Physical mapping studies on the human X chromosome in the region Xq27-Xqter

We have characterized three terminal deletions of the long arm of the X chromosome. Southern analysis using Xq27/q28 probes suggests that two of the deletions have breakpoints near the fragile site at Xq27.3. Flow karyotype analysis provides an estimate of 12 X 10(6) bp for the size of the deleted region. We have not detected the deletion breakpoints by pulsed-field gel electrophoresis (PFGE) using the closet DNA probes, proximal to the fragile site. The physical distance between the breakpoints and the probes may therefore be several hundred kilobases. The use of the deletion patients has allowed a preliminary physical map of Xq27/28 to be constructed. Our data suggest that the closest probes to the fragile site on the proximal side are 4D-8 (DXS98), cX55.7 (DXS105), and cX33.2 (DXS152). PFGE studies provide evidence for the physical linkage of 4D-8, cX55.7, and cX33.2. We have also found evidence for the physical linkage of F8C, G6PD, and 767 (DXS115), distal to the fragile site.

Hypothesis regarding the nature of the fragile X mutation. A reply to Winter and Pembrey

Pembrey et al. (1985) proposed a hypothesis regarding the nature of the fragile X [fra(X)] mutation. Recently they analyzed DNA linkage data (Winter and Pembrey 1986) that we and others have published on fra(X) pedigrees, found significant linkage heterogeneity, and modified their hypothesis to explain the observations. We would like to point out that their modified hypothesis is not supported by the data available.