An intronic region within the human factor VIII gene is duplicated within Xq28 and is homologous to the polymorphic locus DXS115 (767)

The genomic sequences recognized by the anonymous probe 767 (DXS115) are localized to two sites within Xq28. One site lies within intron 22 of the factor VIII gene (FBC). Physical mapping suggests that the second site lies within 1.2 megabases of the F8C gene. The RFLPs detected by 767 are located within the second site. Genetic data suggest that F8C and DXS115 are tightly linked (theta max = .04; Zmax = 8.30). Recombination events in meioses informative for DXS52 (St14), DXS115, and F8C suggest that DXS115 and F8C lie distal to DXS52.     

Genetic mapping of the Xq27-q28 region: new RFLP markers useful for diagnostic applications in fragile-X and hemophilia-B families.

We have characterized and genetically mapped new polymorphic DNA markers in the q27-q28 region of the X chromosome. New informative RFLPs have been found for DXS105, DXS115, and DXS152. In particular, heterozygosity at the DXS105 locus has been increased from 25% to 52%. We have shown that DXS105 and DXS152 are contained within a 40-kb region. A multipoint linkage analysis was performed in fragile-X families and in large normal families from the Centre d'Etudes du Polymorphisme Humain (CEPH). This has allowed us to establish the order centromere-DXS144-DXS51-DXS102-F9-DXS105-FRAX A-(F8, DXS15, DXS52, DXS115). DXS102 is close to the hemophilia-B locus (z[theta] = 13.6 at theta = .02) and might thus be used as an alternative probe for diagnosis in Hemophila-B families not informative for intragenic RFLPs. DXS105 is 8% recombination closer to the fragile-X locus than F9 (z[theta] = 14.6 at theta = .08 for the F9-DXS105 linkage) and should thus be a better marker for analysis of fragile-X families. However, the DXS105 locus appears to be still loosely linked to the fragile-X locus in some families. The multipoint estimation for recombination between DXS105 and FRAXA is .16 in our set of data. Our data indicate that the region responsible for the heterogeneity in recombination between F9 and the fragile-X locus is within the DXS105-FRAXA interval.     

Mapping of the gene for X-linked amelogenesis imperfecta by linkage analysis.

X-linked Amelogenesis imperfecta (AI) is a genetic disorder affecting the formation of enamel. In the present study two families, one with X-linked dominant and one with X-linked recessive AI, were studied by linkage analysis. Eleven cloned RFLP markers of known regional location were used. Evidence was obtained for linkage between the AI locus and the marker p782, defining the locus DXS85 at Xp22, by using two-point analysis. No recombination was scored between these two loci in 15 informative meioses, and a peak lod score (Zmax) of 4.45 was calculated at zero recombination fraction. Recombination was observed between the more distal locus DXS89 and AI, giving a peak lod score of 3.41 at a recombination fraction of .09. Recombination was also observed between the AI locus and the more proximal loci DXS43 and DXS41 (Zmax = 0.09 at theta max = 0.31 and Zmax = 0.61 at theta max = 0.28, respectively). Absence of linkage was observed between the AI locus and seven other loci, located proximal to DXS41 or on the long arm of the X chromosome. On the basis of two-point linkage analysis and analysis of crossover events, we propose the following order of loci at Xp22: DXS89-(AI, DXS85)-DXS43-DXS41-Xcen.     

Genetic and physical mapping of a novel region close to the fragile X site on the human X chromosome

We report the isolation and characterization of a novel DNA marker (1A1) in Xqter in the region of the fragile X. Genetic studies in families segregating for the fragile X syndrome suggest that 1A1 lies between the disease mutation and the distal locus, DXS52. Studies in normal and fragile X families show that 1A1 is tightly linked to DXS52 (Zmax = 17.20; theta max = 0.03) and F8 (Zmax = 7.01; theta max = 0.08). Multipoint mapping of families supports the order Xcen-DXS105-FRAXA-1A1-DXS52-(F8, DXS115)-Xqter. Pulsed-field gel electrophoresis (PFGE) studies demonstrate that 1A1 defines a new region of at least 2 Mb of DNA not physically linked to DXS52 or F8, thus extending the physical map of Xq27-qter to over 4 Mb. Complex partial digestion PFGE patterns, probably due to differing degrees of methylation, are observed with 1A1 in unrelated normal and fragile-X-positive individuals, whereas other distal markers give uniform digestion profiles. Physical data suggest that 1A1 lies in a region less CpG rich than other distal markers in Xq27-qter.     

Linkage analysis in X-linked congenital stationary night blindness.

X-linked congenital stationary night blindness (XL-CSNB) is a nonprogressive disorder of the retina, characterized by night blindness, reduced visual acuity, and myopia. Previous studies have localized the CSNB1 locus to the region between OTC and TIMP on the short arm of the X chromosome. We have carried out linkage studies in three XL-CSNB families that could not be classified as either complete or incomplete CSNB on the criteria suggested by Miyake et al. (1986. Arch. Ophthalmol. 104: 1013-1020). We used markers for the DXS538, DMD, OTC, MAOA, DXS426, and TIMP loci. Two-point analyses show that there is close linkage between CSNB and MAOA (theta max = 0.05, Zmax = 3.39), DXS426 (theta max = 0.06, Zmax = 2.42), and TIMP (theta max = 0.07, Zmax = 2.04). Two multiply informative crossovers are consistent with CSNB lying proximal to MAOA and distal to DXS426, respectively. Multipoint analysis supports this localization, giving the most likely order as DMD-17 cM-MAOA-7.5 cM-CSNB-7.5 cM-DXS426/TIMP-cen, and thus refines the localization of CSNB.     

Mapping of the gene for X-linked liver glycogenosis due to phosphorylase kinase deficiency to human chromosome region Xp22.

X-linked liver glycogenosis (XLG) is a glycogenosis due to deficient activity of phosphorylase kinase (PHK) in liver. PHK consists of four different subunits, alpha, beta, gamma, and delta. Although it is unknown whether liver and muscle PHK subunits are encoded by the same genes, the muscle alpha subunit (PHKA) gene was a likely candidate gene for the mutation responsible for this X-linked liver glycogenosis as it was assigned to the X chromosome at q12-q13. Linkage analysis with X-chromosomal polymorphic DNA markers was performed in two families segregating XLG. First, multipoint linkage analysis excluded the muscle PHKA region as the site of the XLG mutation. Second, evidence was obtained for linkage between the XLG locus and DXS197, DXS43, DXS16, and DXS9 with two-point peak lod scores Zmax = 6.64, 3.75, 1.30, and 0.88, all at theta max = 0.00, respectively. Multipoint linkage results and analysis of recombinational events indicated that the mutation responsible for XLG is located in Xp22 between DXS143 and DXS41.     

Optic atrophy in Leber hereditary optic neuroretinopathy is probably determined by an X-chromosomal gene closely linked to DXS7.

Leber hereditary optic neuroretinopathy (LHON) is a maternally inherited disease, probably transmitted by mutations in mtDNA. The variation in the clinical expression of the disease among family members has remained unexplained, but pedigree data suggest an involvement of an X-chromosomal factor. We have studied genetic linkage of the liability to develop optic atrophy to 15 polymorphic markers on the X chromosome in six pedigrees with LHON. The results show evidence of linkage to the locus DXS7 on the proximal Xp. Tight linkage to the other marker loci was excluded. Multipoint linkage analysis placed the liability locus at DXS7 with a maximum lod score (Zmax) of 2.48 at a recombination fraction (theta) of .0 and with a Zmax - 1 support interval theta = .09 distal to theta = .07 proximal of DXS7. No evidence of heterogeneity was found among different types of families, with or without a known mtDNA mutation associated with LHON.     

The gene for X-linked hydrocephalus maps to Xq28, distal to DXS52.

We report the study of five independent X-linked hydrocephalus (HSAS1) families with polymorphic DNA markers of the Xq28 region. A total of 58 individuals, including 7 living affected males and 22 obligate carriers, have been studied. Maximum lod score was 7.21 at theta = 2.40% for DXS52 (St14-1). A single recombination event was observed between this marker and the HSAS1 locus. Other markers studied were DXS296 (Z = 2.02 at theta = 2.5%), DXS304 (Z = 4.37 at theta = 7.8%), DXS74 (Z = 3.50 at theta = 0%), DXS15 (Z = 1.96 at theta = 5.7%), DXS134 (Z = 3.31 at theta = 0%), and F8C (Z = 5.79 at theta = 0%). These data confirm the localization of the HSAS1 gene to Xq28 and provide evidence for genetic homogeneity of this syndrome. In addition, examination of two obligate recombinant meioses along with multipoint linkage analysis supports the distal localization of the HSAS1 locus with respect to the DXS52 cluster. These observations are of potential interest for future studies aimed at HSAS1 gene characterization.     

Mapping of the von Hippel-Lindau disease locus to a small region of chromosome 3p by genetic linkage analysis.

Genetic linkage studies were performed in 22 families with von Hippel-Lindau (VHL) disease by using polymorphic DNA markers from distal chromosome 3p. Linkage was detected between VHL disease and the markers D3S18 (Zmax = 6.6 at theta = 0.0, confidence interval (CI) 0.00-0.06), RAF1 (Zmax = 5.9 at theta = 0.06, CI 0.01-0.16), and THRB (Zmax 3.4 at theta = 0.11). Multipoint linkage analysis localized the VHL disease gene within a small region (approximately 8 cM) of 3p25-p26 between RAF1 and (D3S191, D3S225) and close to the D3S18 locus. There was no evidence of locus heterogeneity, and families with and without pheochromocytoma showed linkage to D3S18. The identification of DNA markers flanking the VHL disease gene allows reliable presymptomatic and prenatal diagnosis to be offered to informative families.     

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.     

Mapping of the Emery-Dreifuss gene through reconstruction of crossover points in two Italian pedigrees

Summary Two unrelated pedigrees, which show recurrence of Emery-Dreifuss muscular dystrophy (EDMD) in three generations, have been studied using 13 X-linked DNA polymorphisms and somatic cell hybrids to establish the phase of the corresponding alleles in some obligate carriers. The reconstruction of cross-over points on the X chromosomes carrying the EDMD gene excludes from mapping most regions of the X chromosome except for the terminal portion of Xq. Pooled linkage data from the two pedigrees confirm the linkage previously reported with locus DXS15. A cross-over in a carrier female suggests that the EDMD gene is probably located distally to DXS15. In addition the recombinant meioses from one of the two pedigrees suggest the following order for some Xq polymorphic loci: DXS1 (DXYS1-DXS178) DXS42 (F9-DXS15).     

Physical mapping of the factor VIII gene proximal to two polymorphic DNA probes in human chromosome band Xq28: implications for factor VIII gene segregation analysis

Genomic DNA segments for the coagulation factor VIIIc gene (F8C), which exhibits only limited restriction length polymorphism, map to the proximal region of band Xq28 by somatic cell hybridization analysis and in situ hybridization. Using somatic cell hybrids, we have obtained data which place probes DX13 (used to detect locus DXS15) and St14 (used to detect DXS52) distal to F8C, within band Xq28. Previous studies have mapped the factor IX gene (F9) and probe 52A (used to detect DXS51) proximal to F8C, in Xq26----q27 and Xq27, respectively (Camerino et al., 1984; Drayna et al., 1984; Mattei et al., 1985). Thus, the relative order of genetic marker loci in the Xq27----qter region is most likely cen-F9-DXS51-F8C-(DXS15, DXS52)-Xqter. The collection of these molecular probes is thus potentially useful in three-factor crosses of factor VIII gene segregation.     

Assignment of Emery-Dreifuss muscular dystrophy to the distal region of Xq28: the results of a collaborative study.

Emery-Dreifuss muscular dystrophy (EDMD) is an X-linked humeroperoneal dystrophy associated with cardiomyopathy that is distinct from the Duchenne and Becker forms of X-linked muscular dystrophy. Linkage analysis has assigned EDMD to the terminal region of the human X chromosome long arm. We report here further linkage analysis in two multigenerational EDMD families using seven Xq28 marker loci. Cumulative lod scores suggest that EDMD is approximately 2 cM from DXS52 (lod = 15.67) and very close to the factor VIII (F8C) and the red/green color pigment (R/GCP) loci, with respective lod scores of 9.62 and 10.77, without a single recombinant. Several recombinations between EDMD and three proximal Xq28 markers suggest that the EDMD gene is located in distal Xq28. Multipoint linkage analysis indicates that the odds are 2,000:1 that EDMD lies distal to DXS305. These data substantially refine the ability to perform accurate carrier detection, prenatal diagnosis, and the presymptomatic diagnosis of at-risk males for EDMD by linkage analysis. The positioning of the EDMD locus close to the loci for F8C and R/GCP will assist in future efforts to identify and isolate the disease gene.     

Multipoint linkage analysis in X-linked Alport syndrome

Summary In order to localize the gene for the X-linked form of Alport syndrome (ATS) more precisely, we performed restriction fragment length polymorphism analysis with nine different X-chromosomal DNA markers in 107 members of twelve Danish families segregating for classic ATS or progressive hereditary nephritis without deafness. Two-point linkage analysis confirmed close linkage to the markers DXS17(S21) (Z _max = 4.44 at = 0.04), DXS94(pXG-12) (Z _max=8.07 at =0.04), and DXS101(cX52.5) (Z _max=6.04 at =0.00), and revealed close linkage to two other markers: DXS88(pG3-1) (Z _max =6.36 at =0.00) and DXS11(p22–33) (z _max=3.45 at =0.00). Multipoint linkage analysis has mapped the gene to the region between the markers DXS17 and DXS94, closely linked to DXS101. By taking into account the consensus map and results from other studies, the most probable order of the loci is: DXYS1(pDP34)-DXS3(p19-2)-DXS17-(ATS, DXS101)-DXS94-DXS11-DXS42(p43-15)-DXS51(52A). DXS88 was found to be located between DXS17 and DXS42, but the order in relation to the ATS locus and the other markers used in this study could not be determined.     

Assignment of X-linked hydrocephalus to Xq28 by linkage analysis

X-linked recessive hydrocephalus (HSAS) occurs at a frequency of approximately 1 per 30,000 male births and consists of hydrocephalus, stenosis of the aqueduct of Sylvius, mental retardation, spastic paraparesis, and clasped thumbs. Prenatal diagnosis of affected males by ultrasonographic detection of hydrocephalus is unreliable because hydrocephalus may be absent antenatally. Furthermore, carrier detection in females is not possible because they are asymptomatic. Using four families segregating HSAS, we performed linkage analysis with a panel of X-linked probes that detect restriction fragment length polymorphisms. We report here that HSAS, in all tested families, is closely linked to marker loci mapping in Xq28 (DXS52, lod = 6.52 at theta of 0.03; F8, lod = 4.32 at theta of 0.00; DXS15, lod = 3.40 at theta of 0.00). These data assign HSAS to the gene-dense chromosomal band Xq28 and allow for both prenatal diagnosis and carrier detection by linkage analysis.     

The gene for X-linked progressive mixed deafness with perilymphatic gusher during stapes surgery (DFN3) is linked to PGK

Summary A linkage analysis has been performed in a large Dutch kindred with progressive mixed deafness with perilymphatic gusher during stapes surgery (DFN3) using a panel of X-chromosomal RFLPs. Tight linkage (z_max=3.07 at =0.00) was demonstrated with the locus for phosphoglycerate kinase (PGK), which is located at Xq13. Tight linkage was excluded for DXS9 (probe RC8) and DXS41 (probe 99.6) on Xp and for blood clotting factor 9 (FIX) on distal Xq. Deafness is one of the predominant clinical features in males with deletions of the Xq21 band. Our results suggest that this association may be due to involvement of the DFN3 gene.     

Confirmation of the localization of the human GABAA receptor alpha 1-subunit gene (GABRA1) to distal 5q by linkage analysis.

The GABAA receptor is the major inhibitory neurotransmitter receptor in the mammalian brain. To date, 14 genes that encode subunits of this receptor have been identified; these appear to be scattered throughout the human genome and are under investigation as candidate loci for a number of neurological and psychiatric disorders. We report here a highly polymorphic (dC-dA)n repeat within the human alpha 1-subunit gene (GABRA1). Typing of this marker in the Centre d'Etude du Polymorphisme Humain (CEPH) panel of families confirms the previous assignment of the GABRA1 locus to the distal portion of chromosome 5q by demonstrating linkage to the markers CRI-L45 (D5S61) (Zmax = 11.00, theta max = 0.15), CRI-V1022 (D5S54) (Zmax = 7.25, theta max = 0.20), and CRI-P148 (D5S72) (Zmax = 5.71, theta max = 0.24).     

A linkage study of Emery-Dreifuss muscular dystrophy

Summary We have searched for linkage between polymorphic loci defined by DNA markers on the X chromosome and X-linked Emery-Dreifuss muscular dystrophy (EDMD). There are high recombination rates between EDMD and the Xp loci known to be linked to Becker and Duchenne muscular dystrophy. There is a suggestion of linkage between EDMD and the loci DXS52 and DXS15, defined by probes St 14 and DX13 respectively, located at Xq28. for DXS15=1.14 at =0.15. This is in agreement with the previously reported linkage between a disorder strongly resembling EDMD and colour-blindness (Thomas et al. 1972), suggesting that there is a second locus on the X chromosome concerned with muscle integrity.     

A gene for dominant nonspecific X-linked mental retardation is located in Xq28.

A large family (MRX48) with a nonspecific X-linked mental retardation condition is described. An X-linked semidominant inheritance is suggested by the segregation in three generations of a moderate to severe mental retardation in seven males and by a milder intellectual impairment in two females, without any specific clinical, radiological, or biological feature. Two-point linkage analysis demonstrated significant linkage between the disorder and several markers in Xq28 (maximum LOD score [Zmax] = 2.71 at recombination fraction [theta] = 0); multipoint linkage analyses confirmed the significant linkage with a Zmax of 3.3 at theta = 0, at DXS1684. A recombination event observed with the flanking marker DXS8011 delineates a locus between this marker and the telomere. The approximate length of this locus is 8-9 cM, corresponding to 5.5-6 Mb. In an attempt to explain the variable intellectual impairment in females, we examined X-chromosome inactivation in all females of the family. Inactivation patterns in lymphocytes were random or moderately skewed, and no correlation between the phenotypic status and a specific inactivation pattern was observed. The interval of assignment noted in this family overlaps with five MRX loci previously reported in Xq28.     

Genetic heterogeneity in X-linked amelogenesis imperfecta.

The AMELX gene located at Xp22.1-p22.3 encodes for the enamel protein amelogenin and has been implicated as the gene responsible for the inherited dental abnormality X-linked amelogenesis imperfecta (XAI). Three families with XAI have been investigated using polymorphic DNA markers flanking the position of AMELX. Using two-point linkage analysis, linkage was established between XAI and several of these markers in two families, with a combined lod score of 6.05 for DXS16 at theta = 0.04. This supports the involvement of AMELX, located close to DXS16, in the XAI disease process (AIH1) in those families. Using multipoint linkage analysis, the combined maximum lod score for these two families was 7.30 for a location of AIH1 at 2 cM distal to DXS16. The support interval around this location extended about 8 cM proximal to DXS92, and the AIH1 location could not be precisely defined by multipoint mapping. Study of recombination events indicated that AIH1 lies in the interval between DXS143 and DXS85. There was significant evidence against linkage to this region in the third family, indicating locus heterogeneity in XAI. Further analysis with markers on the long arm of the X chromosome showed evidence of linkage to DXS144E and F9 with no recombination with either of these markers. Two-point analysis gave a peak lod score at DXS144E with a maximum lod score of 2.83 at theta = 0, with a peak lod score in multipoint linkage analysis of 2.84 at theta = 0. The support interval extended 9 cM proximal to DXS144E and 14 cM distal to F9.(ABSTRACT TRUNCATED AT 250 WORDS)