Multipoint genetic mapping of the Xq26-q28 region in families with fragile X mental retardation and in normal families reveals tight linkage of markers in q26-q27

Glucose-6-Phosphate Dehydrogenase: Update and Analysis of New Mutations around the World

Glucose-6-phosphate dehydrogenase (G6PD) is a key regulatory enzyme in the pentose phosphate pathway which produces nicotinamide adenine dinucleotide phosphate (NADPH) to maintain an adequate reducing environment in the cells and is especially important in red blood cells (RBC). Given its central role in the regulation of redox state, it is understandable that mutations in the gene encoding G6PD can cause deficiency of the protein activity leading to clinical manifestations such as neonatal jaundice and acute hemolytic anemia. Recently, an extensive review has been published about variants in the g6pd gene; recognizing 186 mutations. In this work, we review the state of the art in G6PD deficiency, describing 217 mutations in the g6pd gene; we also compile information about 31 new mutations, 16 that were not recognized and 15 more that have recently been reported. In order to get a better picture of the effects of new described mutations in g6pd gene, we locate the point mutations in the solved three-dimensional structure of the human G6PD protein. We found that class I mutations have the most deleterious effects on the structure and stability of the protein.

Glucose-6-phosphate dehydrogenase deficiency and malaria: cytochemical detection of heterozygous G6PD deficiency in women

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is a X-chromosomally transmitted disorder of the erythrocyte that affects 400 million people worldwide. Diagnosis of heterozygously-deficient women is complicated: as a result of lyonization, these women have a normal and a G6PD-deficient population of erythrocytes. The cytochemical assay is the only reliable assay to discriminate between heterozygously-deficient women and non-deficient women or homozygously-deficient women. G6PD deficiency is mainly found in areas where malaria is or has been endemic. In these areas, malaria is treated with drugs that can cause (severe) hemolysis in G6PD-deficient individuals. A cheap and reliable test is necessary for diagnosing the deficiency to prevent hemolytic disorders when treating malaria. In this review, it is concluded that the use of two different tests for diagnosing men and women is the ideal approach to detect G6PD deficiency. The fluorescent spot test is inexpensive and easy to perform but only reliable for discriminating hemizygous G6PD-deficient men from non-deficient men. For women, the cytochemical assay is recommended. However, this assay is more expensive and difficult to perform and should be simplified into a kit for use in developing countries.

Deletion of 8.5 Mb, including the FMR1 gene, in a male with the fragile X syndrome phenotype and overgrowth

A four-year-old boy with severe psychomotor retardation, facial appearance consistent with the fragile X syndrome, hypotonia, and overgrowth was found to have a deletion including the fragile X gene (FMR1). The breakpoints of the deletion were established between CDR1 and sWXD2905 (approximately 200 kb apart) at Xq27.1 (centromeric) and between DXS8318 (612-1078L) and DXS7847 (576-291L) (approximately 250 kb apart) at Xq28, about 500 kb telomeric to the FMR1 gene. The total length of the deletion is approximately 8.5 Mb. The propositus's mother, who was found to be a carrier of the deletion, showed very mild mental impairment. Except for mental retardation, which is a common finding in all cases reported with similar deletions of chromosome Xq, this patient had generalized overgrowth, exceeding the 97th centile for height and weight. Obesity and increased growth parameters have been reported in other patients with deletions either overlapping or within a distance of 0.5 Mb from the deletion in the present patient. Thus, it is suggested that a deletion of the 8-Mb fragment centromeric to the FMR1 gene might have an effect on growth.

An analysis of Xq deletions

We characterized by fluorescence in situ hybridization and Southern blotting 14 partial Xq monosomies, 11 due to terminal deletions and 3 secondary to X/autosome translocations. Three cases were mosaics with a XO cell line. In view of the possible role played by telomeres in chromosome segregation, we hypothesize a relationship between the loss of telomeric sequences in terminal deletions and the presence of 45,X cells. A correlation between phenotype and extent of deletion reveal that there is no correspondence between the size of the deletion and impairment of gonadal function. Turner stigmata are absent in patients without an XO cell line, when the breakpoint is distal to Xq24. A low birthweight is present whenever the breakpoint is at q22 or more proximal.

Physical mapping of the human hprt chromosomal region (Xq26)

The human hprt chromosomal region (Xq26) was physical-mapped using pulsed field gel electrophoresis (PFGE). This work involved: (i) the recovery of three new genomic DNA markers (DXS1327, DXS1328, and DXS1329), (ii) the ordering of new markers relative to 11 previously available hprt-linked markers by deletion mapping, and (iii) the completion of human T-lymphocyte PFGE Southern blots using the 14 Xq26 markers. A contiguous 1.5-Mb physical map of the region telomeric to hprt was determined. As this map identifies clusters of in vivo unmethylated rare-cutter restriction sites, potential CpG islands are revealed.

Analysis of large deletions in the HPRT gene of primary human fibroblasts using the polymerase chain reaction

Spontaneous and X-ray-induced mutants of the HPRT gene were isolated from two primary human fibroblast lines. The limited life-span of the mutants restricted the use of methods requiring large quantities of DNA, and the polymerase chain reaction (PCR) was used in particular to check for the presence of multiple genomic sites in mutant analysis. Robust PCR primers were designed to amplify sites of up to 1 kb, mostly with 1-kb spacings between sites, over the entire 56-kb HPRT gene region. Using PCR, large deletions were found in 43% of independent X-ray-induced mutants, and their breakpoints were localized where these fell within the gene. Anonymous DNA sites in the Xq26 chromosomal region containing HPRT (covering > or = 1.5 Mb) were also amplified by PCR to assess codeletion with HPRT; sites up to 1 Mb distal to the gene (DXS86, DXS10) were codeleted in some mutants, but no mutant was found with loss of a proximal site (DXS79).

CEPH maps

There are CEPH genetic maps on each homologous human chromosome pair. Genotypes for these maps have been generated in 88 laboratories that receive DNA from a reference panel of large nuclear pedigrees/families supplied by the Centre d'Etude du Polymorphisme Humain. These maps serve as useful tools for the localization of both disease genes and other genes of interest.

Transmission of the fra(X) haplotype from three nonpenetrant brothers to their affected grandsons

We report on a family showing transmission of the fra(X) gene by 3 nonpenetrant, fra(X) negative, normally intelligent, full and half-brothers to their affected grandsons. The mothers of the affected boys are obligate carriers, fra(X) negative, and of normal intelligence. This family illustrates the "Sherman Paradox" and is compatible with the predictions of the Laird X-inactivation imprinting model. In addition, molecular and/or cytogenetic studies have enabled at-risk relatives to learn more about their carrier fra(X) status and have allowed for more accurate genetic counselling.

Tentative assignment of gene for oto-palato-digital syndrome to distal Xq (Xq26-q28)

Detailed physical mapping of oto-palato-digital (OPD) syndrome gene on the X-chromosome was attempted on a family of 3 generations with 2 affected men. Although the result remains statistically non-significant, it indicates that the OPD-I gene might be located on the distal Xq.

Completion of the physical map of Xq28: the location of the gene for L1CAM on the human X chromosome

The gene for the neural cell adhesion molecule L1 (L1CAM) has been shown to be located close to the color vision pigment genes in mouse and man. This location has been confirmed by a number of different mapping strategies in both species. With pulsed field gel electrophoresis it has been proposed that L1CAM lies between the RCP, GCP, and GDX, G6PD loci. We report here a reinterpretation of the location of this gene, based on the physical linkage of L1CAM to the more proximal locus DXS15. This places L1CAM between this marker and the color vision genes (RCP, GCP), a region very dense in CpG islands, expected to contain a large fraction of the disease genes assigned to the Xq28 region. In combination with the physical mapping data on Xq28 described previously, this closes the last remaining gap in the map of the Xq27-Xq28 region. This removes the last contradiction between the maps of this region in the genomes of man and mouse, and confirms the close similarity of order and distances of markers between these organisms.

Physical map of human Xq27-qter: localizing the region of the fragile X mutation

We describe a physical map of the end of the long arm of the human X chromosome encompassing the region from Xq27.2 to the q telomere, inclusive of the chromosomal band Xq28. This region is of particular interest, since it contains the highest density of genes associated with genetic diseases. The map covers a total of 12 megabases (Mb) of DNA and extends from the telomere to 3 Mb beyond the most likely position of the fragile X mutation, defined by a cluster of translocation breakpoints in somatic cell hybrids. The map determines order and position of loci throughout the Xq28 region and localizes cell line breakpoints marking the fragile X region to an interval of 300-700 kilobases between 8 and 8.7 Mb proximal of the Xq telomere.

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.

Physical mapping of the loci Gabra3, DXPas8, CamL1, and Rsvp in a region of the mouse X chromosome homologous to human Xq28

Using pulsed-field gel electrophoresis, a 3 million-bp physical map containing the X-linked loci Gabra3, DXPas8, CamL1, and Rsvp has been constructed for a segment of the mouse X chromosome homologous to human Xq28. Detailed mapping was performed using single and double digestions with rare-cutter restriction enzymes. Gabra3 and DXPas8 have been shown to be physically linked within a maximal distance of 1600 kb, DXPas8 and CamL1 within 750 kb, and CamL1 and Rsvp within 450 kb. In addition, several CpG islands have been detected in the region encompassing CamL1 and Rsvp. These studies confirm a gene order of cen-Gabra3-DXPas8-CamL1-Rsvp-tel determined by genetic mapping in interspecific backcrosses (A.S. Ryder-Cook et al., 1988, EMBO J. 7: 3017-3021; G.E. Herman et al., 1991, Genomics 9: 670-677). Physical distances for the loci studied agree with the calculated genetic distances. Assuming that there is conserved linkage between man and mouse in the region, the physical mapping data presented here may help to clarify the uncertain gene order for some human Xq28 loci.

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.

Genetic mapping of the mouse X chromosome in the region homologous to human Xq27-Xq28

The four loci Gabra3, DXPas8, CamL1, and Bpa, located near the murine X-linked visual pigment gene (Rsvp), have been ordered using 248 backcross progeny from an interspecific mating of (B6CBA-Aw-J/A-Bpa) and Mus spretus. One hundred twenty backcross progeny have been analyzed at seven anchor loci spanning the X chromosome and form a regional mapping panel. An additional 128 progeny have been screened for recombination events between Cf-9 and Dmd. Eighteen recombinants between these loci have been detected in the 248 animals; all of the recombinants were screened at the other anchor loci to identify any double crossovers. Pedigree analysis using these recombinants strongly favors a gene order of (Cf-9)-Gabra3-(DXPas8, Bpa)-CamL1-(Rsvp, P3, Cf-8)-Dmd for the loci studied. Synteny with human Xq27-Xq28 is retained, although the relative order of some loci may differ between the two species.

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

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

Carrier detection of the fragile X syndrome using flanking loci DXS98, DXS105, and DXS304

Diagnosis of the carrier status of the fragile X [fra(X)] syndrome was made in 2 unrelated women who did not express the fragile site. Both were related to several individuals with a typical fra(X) phenotype and the marker X chromosome. A restriction fragment length polymorphism (RFLP) approach was used with probes that flank the fra(X) locus (FRAXA). The loci used for risk calculations of the fra(X) genotype were DXS98 and DXS105 on the centromeric side and a recently characterized locus, DXS304, on the telomeric side. Coincidence correction for the distances between marker loci and FRAXA was made according to the Kosambi function. The DNA marker test gave the risk for one female to be a carrier of 99.7-99.9%. In another family a female was excluded from being a carrier with a probability of greater than 99.7%. The DNA marker U6.2, defining the locus DXS304, has increased the reliability of DNA based diagnosis of carrier status for females-at-risk. It is concluded that DNA analysis can serve as a valuable complement to chromosome analysis in families informative for the more closely linked flanking markers.

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.

Genetic and physical mapping of Xq24-q26 markers flanking the Lowe oculocerebrorenal syndrome

The Lowe oculocerebrorenal syndrome (OCRL) is characterized by congenital cataract, mental retardation, and renal tubular dysfunction. We are using the approaches of linkage analysis, mapping with somatic cell hybrids, and long-range restriction mapping to determine the order of Xq24-q26 markers with respect to each other and to the OCRL locus. DXS42 and DXS100 are proximal to the translocation breakpoint in a female patient with OCRL and a de novo translocation t(X;3)(q25;q27). DXS10, DXS86, HPRT, and DXS177 are distal to the breakpoint. These flanking markers show tight linkage to the disease locus in 11 families segregating for OCRL. Results from field inversion gel analysis show that DXS86 and DXS10 share a 460-kb BssHII fragment. Multipoint analysis to determine the position of HPRT with respect to (DXS10,DXS86) suggests that HPRT is proximal to (DXS10,DXS86). We propose the following order for markers in Xq24-q26: Xcen-(DXS42,DXS37,DXS100)-OCRL-DXS53 -HPRT-[(DXS10,DXS86),DXS177]-Xqter. The identification of additional tightly linked flanking markers extends the number of markers available for use in genetic counseling and begins to define the physical map of the region containing the gene for OCRL.

Yeast artificial chromosomes containing human Xq24-Xq28 DNA: library construction and representation of probe sequences

A library of yeast artificial chromosomes (YACs) with human DNA inserts has been assembled from a human/hamster somatic cell hybrid containing Xq24-Xqter human DNA. Screening of the agar-embedded transformants for human DNA used a manifold of 3000 stainless-steel pins to transfer colonies onto the surface of media. This facilitated the recovery of the 1 in 300 clones that contained a human DNA insert (the remainder had hamster DNA and were discarded). The library described here consists of about two genomic equivalents (102 Mb) of human DNA in 467 clones: 167 were generated by EcoRI partial digestion and contain 25.5 Mb of human DNA; 252 used partial digestion with TaqI and cover 64.2 Mb; and 48 were from sheared DNA inserts and cover 11.7 Mb. Clones were screened by hybridization with 70 probes previously assigned to Xq24-Xq28. Eleven probes did not hybridize to any YACs in the library, and 16 probes hybridized to one YAC each, 23 to two, 13 to three, and 7 to four. Also, individual YACs large enough to detect features like the clustering of polymorphic sequences in subregions of Xq24-Xqter have been obtained. For example, XY58 contained five probe sequences previously independently isolated. The overall yield of YACs containing probe sequences was indistinguishable from Poisson statistical expectations for random cloning (P = 0.9). Thus, YAC libraries such as the one described here can include most, if not all, of the sequences in the source DNA from which the library is derived. These results support the possibility that YACs may provide a reliable bridge between linkage studies and conventional recombinant DNA analyses in mapping of the human genome.

More precise localization of the gene for Hunter syndrome

A linkage analysis between the Hunter syndrome locus (IDS) and four polymorphic loci of the Xq27-Xq28 region, DXS105, DXS98, DXS304, and DXS52, was performed in large families. A significant lod score was obtained between DXS304 and the Hunter gene (Zmax = 6.57 at theta max = 0.0). The Hunter gene can be localized within 7 cM of this marker. In addition, the translocation breakpoint of the Hunter female case described by J. Mossman et al. (1986, Arch. Dis. Child. 58: 911-915) was localized between DXS98 and DXS304 using somatic cell hybrids. These two results are in agreement and give the following order: DXS105-DXS98-IDS-DXS304-DXS52. Probes for these marker loci can thus be used for carrier detection.

Close linkage of the murine locus bare patches to the X-linked visual pigment gene: implications for mapping human X-linked dominant chondrodysplasia punctata

The murine X-linked dominant mutation bare patches (Bpa) has a phenotype similar to and is likely homologous to human X-linked dominant chondrodysplasia punctata (CDPX2). Classic two-point linkage analysis in the mouse with distant markers suggested that Bpa maps near glucose-6-phosphate dehydrogenase (G6pd). We have confirmed the regional localization using interspecific matings with Mus spretus. We have also detected a restriction fragment length polymorphism (RFLP) at the murine X-linked visual pigment (Rsvp) locus in inbred Bpa females using the restriction enzyme PstI. Cumulative data from segregation of alleles using the PstI RFLP and analysis of interspecific backcross progeny at the Rsvp locus suggest that Bpa is tightly linked to Rsvp. Thus, the human CDPX2 gene probably maps within Xq27-Xq28 and not within Xp22.3-Xpter, where deletions associated with X-linked recessive chondrodysplasia punctata (CDPX) have been noted. This strategy should be applicable to the fine mapping of other dominant murine mutations.

Two brothers with mental retardation discordant for the fragile-X syndrome

We describe two male sibs with mental retardation discordant for the fragile-X syndrome. In the younger sib, chromosome analysis under folate deprivation showed a fragile site at Xq27.3 in 12-46% of mitoses. In the older sib, however, repeated chromosome analyses (six different cultures with analysis of 50 mitoses each) under identical conditions could not detect any fragile-X site. Using DNA probes linked to the fragile-X gene, we found evidence that the two sibs inherited a different maternal X chromosome at Xq27.3. This excluded the presence of the fragile-X syndrome in the older sib with a probability of greater than 99%.

Investigation of factor VIII:C gene restriction fragment length polymorphisms and search for deletions in hemophiliac subjects in Algeria

The frequency of alleles for intragenic (intron 17 and intron 25) and extragenic (DXS15 and DXS52) F8C RFLPs was investigated in the Algerian population. Altogether 287 X chromosomes (97 males and 95 females) were studied. The allele frequencies found with the two intragenic F8C RFLPs were not substantially different from those reported in a Mediterranean population. At the highly polymorphic extragenic DXS52 locus the distribution in Algeria differed from that found in France. A new allele (14 kb), called 1 DZ, was found in 3.1% of the chromosomes. Fifty-one families with hemophilia A were studied with the same probes (374 subjects). Of the females, 94% were informative for at least one intra- or extragenic RFLP. Two recombinations were found between DXS52 and F8C, of which one occurred between the DXS15, DXS52 block and F8C, indicating that the two anonymous loci are on the same side of the F8C gene. Only two obvious gene deletions were observed in 73 unrelated hemophiliacs: one encompassed exons 14-22 (about 4.3 kb of cDNA and 36 kb of genomic DNA); the other removed the last exon (exon 26, representing 2 kb of cDNA).

Diagnostic molecular genetics of the fragile X

The approaches to carrier detection and prenatal diagnosis of the fragile X syndrome using DNA probes are described. Since the definitive diagnosis rests upon the cytogenetic demonstration of the fragile X, molecular diagnoses are essentially confined to fragile X family members in whom the fragile X cannot be demonstrated. Since none of the polymorphic probes available are tightly linked to the fragile X, the preferred approach is to use probes which flank the fragile site. Useful probes are listed and suggested recombination fractions for use in diagnosis are given. A strategy is outlined for obtaining the closest informative flanking markers with the minimum amount of laboratory effort. Methods of risk analysis are discussed. It is concluded that molecular analysis is very useful for carrier detection but of limited use in prenatal diagnosis.

Cytogenetic and molecular analysis of an unbalanced translocation (X;7) (q28;p15) in a dysmorphic girl

A severely retarded and dysmorphic girl, carrying an unbalanced X/7 translocation with breakpoints at Xq28 and 7p14, was analyzed by cytogenetic, biochemical and molecular techniques. The X/7 translocated chromosome was found to replicate consistently late in the 105 metaphases analyzed. In 83 of these cells, late replication was limited to the X portion of the abnormal chromosome, whereas in 22 cells incomplete spreading into the autosomal fragment was observed. Southern blot and in situ hybridization experiments with probe G80 (locus D7S373) (previously localized to 7p13-15) and G98 (localized to 7p14-15) assigns the former to 7p15 and the latter to 7p14, thus suggesting the order 7ter-G80-G98-cen. The activity of the enzyme phosphoserine phosphatase localized to 7pter-p14 was increased. Southern blotting experiments with 19 probes spanning the entire X chromosome demonstrated that the translocated chromosome had lost a portion of Xq28 (locus DXS51) but still retained part of Xq27 (F9 locus). The results confirm that the proband is trisomic for the region 7p15-pter and monosomic for the region Xq28-qter. Comparing her phenotype with those of other cases of partial trisomy or monosomy 7p, we confirm that band 7q21 is probably involved in skull development.

Multipoint linkage mapping of the Xq25-q26 region in a family affected by the X-linked lymphoproliferative syndrome

We have performed, in a large Swiss family, a study of linkage between various DNA markers in the Xq24-27 region and the locus for the X-linked lymphoproliferative syndrome (XLP). Our results indicated that the marker DXS37 in Xq25-q26 is genetically linked to the XLP syndrome. The multipoint linkage analysis showed that the disease locus is distal to DXS11, but proximal to the hypoxanthine phosphoribosyl-transferase gene (HPRT).

Assignment of anonymous DNA probes to specific intervals of human chromosomes 16 and X

Anonymous DNA probes mapping to human chromosome 16 and the distal region of the human X chromosome were isolated from a genomic library constructed using lambda EMBL3 and DNA from a mouse/human hybrid. The hybrid cell contained a der(16)t(X;16)(q26;q24) as the only human chromosome. Fifty clones were isolated using total human DNA as a hybridisation probe. Forty six clones contained single copy DNA in addition to the repetitive DNA. Pre-reassociation with sonicated human DNA was used to map these clones by a combination of Southern blot analysis of a hybrid cell panel containing fragments of chromosomes 16 and X and in situ hybridisation. One clone mapped to 16pter----16p13.11, one clone to 16p13.3----16p13.11, four clones to 16p13.3----16p13.13, two clones to 16p13.13----16p13.11, one clone to 16p13.11, seven clones to 16p13.11----16q12 or 16q13, four clones to 16q12 or 16q13, three clones to 16q13----16q22.1, four clones to 16q22.105----16q24, and nineteen clones to Xq26----Xqter. Two clones mapping to 16p13 detected RFLPs. VK5 (D16S94) detected an MspI RFLP, PIC 0.37. VK20 (D16S96) detected a TaqI RFLP, PIC 0.37 and two MspI RFLPs, PIC 0.30 and 0.50. The adult polycystic kidney disease locus (PKD1) has also been assigned to 16p13. The RFLPs described will be of use for genetic counselling and in the isolation of the PKD1 gene. Similarly, the X clones may be used to isolate RFLPs for genetic counselling and the isolation of genes for the many diseases that map to Xq26----qter.

Mapping the mutation causing the X-linked lymphoproliferative syndrome in relation to restriction fragment length polymorphisms on Xq

The X-linked lymphoproliferative syndrome (XLP) results in fatal infectious mononucleosis, hypogamma-globulinemia, and malignant lymphoma. The mutation has been mapped relative to several restriction fragment length polymorphism (RFLP) markers in the Xq21-Xq27 vicinity. The DXS37 locus was found to be near both the DXS42 and XLP loci.

Isolation of a DNA probe of potential use for diagnosis of the fragile-X syndrome

A new cloned DNA probe (U6.2), which recognizes polymorphisms near the locus for the fragile-X syndrome, was isolated. No recombinations were observed between the probe and the disease locus, although recombinations were observed with several other probes known to be located close to the fragile site. The locus defined by the probe, DXS304, cosegregated with the fragile-X phenotype in 29 informative meioses (zeta = 4.97, tau = 0.00). The degree of polymorphism at this locus and its proximity to the fragile-X locus makes it useful for carrier diagnosis and as a new starting point for attempts to clone the gene responsible for the disease.

Further evidence localising the gene for Hunter's syndrome to the distal region of the X chromosome long arm

Cytogenetic re-evaluation of a fibroblast cell line from a female Hunter's syndrome case with a balanced X;autosome translocation, which had previously been reported to have a breakpoint in Xq26 to Xq27, showed the breakpoint to be either between Xq27 and Xq28 or within Xq28. The normal X chromosome was preferentially inactivated, supporting the view that the translocation had disrupted the Hunter gene. The new localisation is now in full agreement with our previous linkage work and other published data. Results of further linkage studies using probes defining the loci DXS86, DXS144, DXS100, DXS102, DXS105, F8C, and DXS134 are also consistent with our original conclusion that the Hunter locus lies within the distal region of the X chromosome long arm.

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.

Dissociation between mental retardation and fragile site expression in a family with fragile X-linked mental retardation

We report an extended family in which two brothers with a fragile X chromosome are mentally retarded while a third brother with the fragile site is both phenotypically and mentally normal. The study of six probes detecting restriction fragment length polymorphisms on either sides of the fragile site Xq27 confirmed that the fragile X regions inherited by these three brothers were identical from DXS102 to the telomere. These data highlight the heterogeneity of the fragile X syndrome, which is discussed in the framework of the different hypotheses previously proposed.

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.

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).

Genetic mapping of nine DNA markers in the q11----q22 region of the human X chromosome

We have ordered nine polymorphic DNA markers within detailed map of the proximal part of the human X chromosome long arm, extending from band q11 to q22, by use of both physical mapping with a panel of rodent-human somatic hybrids and multipoint linkage analysis. Analysis of 44 families (including 17 families from the Centre d'Etude du Polymorphisme Humain) provided highly significant linkage data for both order and estimation of map distances between loci. We have obtained the following order: DXS1-DXS159-DXYS1-DXYS12-DXS3-(DXS94 , DXS178)-DXYS17. The most probable location of DXYS2 is between DXS159 and DXS3, close to DXYS1 and DXYS12. The high density of markers (nine loci within 30 recombination units) and the improvement in the estimation of recombination frequencies should be very useful for multipoint mapping of disease loci in this region and for diagnostic applications.