Emery-Dreifuss muscular dystrophy: localisation to Xq27.3----qter confirmed by linkage to the factor VIII gene.

Two families with Emery-Dreifuss muscular dystrophy (EMD) have been studied with DNA markers mapping to Xq27.3----qter. No recombination was observed in 11 phase known meioses informative for the factor VIII gene (F8C) and eight phase known meioses informative for DXS15 (DX13), giving maximum lod scores of 3.50 and 2.50 respectively at a recombination fraction of zero. DXS52 (St14) showed one recombinant in 12 phase known meioses giving a maximum lod score of 2.62 at a recombination fraction of 0.07. These results map EMD to the distal end of the long arm of the X chromosome and are an important step in the development of tests for carrier detection and prenatal diagnosis.     

1.4 Mb candidate gene region for X linked dyskeratosis congenita defined by combined haplotype and X chromosome inactivation analysis.

Dyskeratosis congenita (DC) is a rare inherited disorder characterised by the early onset of reticulate skin pigmentation, nail dystrophy, and mucosal leucoplakia. In over 80% of cases bone marrow failure develops and this is the main cause of early mortality. The DC1 gene responsible for the X linked form (MIM 305000) of dyskeratosis congenita has been mapped to Xq28. In order to narrow the candidate gene region, genetic linkage analysis was performed in eight X linked pedigrees using a set of markers spanning Xq28. A maximum lod score of 5.31 with no recombinations was achieved with marker DXS1073. Two recombination events were identified; one of these uses X chromosome inactivation pattern analysis to determine carrier status and haplotype analysis to fine map the recombination breakpoint. The fine mapping of these recombination events has enabled the candidate gene region for X linked dyskeratosis congenita to be defined as the 1.4 Mb interval between Xq3274 and DXS1108.     

X-linked myotubular myopathy: a linkage study

Two families with the congenital X-linked infantile form of myotubular myopathy have been investigated by linkage analysis using markers from the X-chromosome. Linkage was found at the locus Xq28 (with DXS52). The analysis gave a peak lod score of 2.41 at the recombination fraction zero. Free recombinations (theta = 0.50) were seen using the markers DXS84, DXS14 and DXS146 from the p arm of the X-chromosome. Since the disorder is very rare, it is important to add cumulative linkage data from the few families that do exist.     

Linkage of X-linked myopathy with excessive autophagy (XMEA) to Xq28

X-linked myopathy with excessive autophagy (XMEA, MIM 310440) is a rare inherited mild myopathy. We have used 32 polymorphic markers spanning the entire X chromosome to exclude most of the chromosome except the Xq28 region in a large XMEA family. Using three additional families for linkage analysis, we have obtained a significant two-point lod score with marker DXS1183 (Z = 2.69 at theta = 0). Multipoint linkage analysis confirmed the assignment of the disease locus with a maximal lod score of 2.74 obtained at recombination fraction zero. Linkage of XMEA to the Xq28 region is thus firmly established. In addition, we have ruled out the Emery-Dreifuss muscular dystrophy to be allelic with XMEA by direct sequencing of the emerin gene in three of our families.     

Refining the genetic location of the gene for X linked hydrocephalus within Xq28.

The most common inherited form of hydrocephalus, X linked hydrocephalus (HSAS), is characterised by mental retardation, adducted thumbs, and spastic paraplegia. Genetic analysis has mapped the locus for HSAS to subchromosomal band Xq28 within a region of approximately 2 megabases of DNA. In order to refine the location of the disease gene we have conducted genetic linkage analysis with Xq28 marker loci in four additional HSAS families. A lod score of 4.26 with polymorphic marker DXS52 (St14) confirms the linkage of HSAS to Xq28. Identification of a recombination event between the HSAS gene and Xq28 loci F8C and DXS605 (2-19) reduces the size of the interval likely to contain the disease locus to about 1.5 megabases, the distance between DXS605 and DXS52. The locus for neural cell adhesion molecule, L1CAM, maps within this interval and therefore represents a candidate gene for HSAS.     

Linkage analysis using multiple Xq DNA polymorphisms in normal families, families with the fragile X syndrome, and other families with X linked conditions.

Multipoint linkage analysis was undertaken with eight Xq cloned DNA sequences which identify one or more restriction fragment length polymorphisms in 26 families. These families comprise seven phase known normal families with three or more males in the third generation, seven families segregating for haemophilia B, one large family with dyskeratosis congenita, and 11 families with the fragile X syndrome. Phase known meioses informative for three or more loci supported the order centromere--DXYS1--DXS107--DXS102, DXS51--F9--FRAXA--DXS15, DXS52, F8--Xqter in each group of families studied. One of the normal families was segregating for protan colour blindness and showed a phase known recombination which would support the order centromere--F9--DXS52--CBP--Xqter. With the exception of DXYS1, all of these sequences have been localised to Xq27----qter by in situ hybridisation or hybridisation to Xq fragment panels, and on this basis should lie within 20 cM of one another. No recombination was observed between the sequences localised to Xq28, namely DXS52, F8, and DXS15 (between DXS15 and DXS52 Z = 12.25 at theta = 0 with confidence limits of 0 to 5 cM). However, an excess of recombination was apparent in the region of FRAXA with maximal lod scores as follows: F9 versus FRAXA (Z = 2.05, theta = 0.19), DXS52 versus FRAXA (Z = 1.85, theta = 0.26), and DXS15 versus FRAXA (Z = 1.33, theta = 0.27). No consistent differences were observed in the frequency of recombination when families with the fragile X syndrome were compared with normal families or families segregating for other X linked conditions. These results are compared with other published work and support the conclusion that although measurable linkage exists between these flanking markers and FRAXA, the intervals as measured by the frequency of meiotic recombination will seriously limit their clinical usefulness.     

Close linkage of a gene for X linked deafness to three microsatellite repeats at Xq21 in radiologically normal and abnormal families.

We have used three highly polymorphic microsatellite repeats from Xq21 to type families in whom a gene for X linked deafness with perilymphatic gusher (DFN 3) was segregating. All three markers were tightly linked to the disease in its radiologically normal and abnormal forms, with a maximum lod score of 10.37 with DXS995 and 8.44 with DXS986 at zero recombination, and 14.03 with DXS1002 at theta = 0.01. In an isolated case of deafness of this type, DXS995 indicated either the first recombination observed between the marker and the disease gene or a new mutation in the proband. Southern blotting using a cosmid fragment from the candidate region has confirmed a de novo mutation by showing a deletion in the proband which is not present in his mother as judged by dosage analysis. We also describe a family with a paracentric inversion associated with a microdeletion and discuss how deletion mapping using these and other markers in the region has helped to define a candidate region for the gene.     

Localisation of the MRX3 gene for non-specific X linked mental retardation.

A family is described with five affected males segregating a new gene for non-specific X linked mental retardation (MRX). Linkage analysis localised the gene at Xq28-qter. The maximum lod score was 2.89 with DXS52 (St14) at theta = 0.0. A recombinant was observed with DXS304 (U6.2) defining the proximal limit to the localisation. No evidence for linkage was determined using markers at several points along the remainder of the X chromosome, including the regions known to contain MRX1 and MRX2. This delineates the third gene for non-specific X linked mental retardation, MRX3.     

Linkage analysis of two Canadian families segregating for X linked spondyloepiphyseal dysplasia.

X linked spondyloepiphyseal dysplasia (SED) is caused by a growth defect of the vertebral bodies leading to characteristic changes in the vertebral bodies and a short trunk. The gene responsible for this disorder has previously been mapped to Xp22, with a maximum likelihood location between markers DXS16 and DXS92. We present linkage data using microsatellite markers on two Canadian X linked SED families, one of Norwegian descent and the other from Great Britain. In the Xp22 region, three recombination events have occurred in these families, two between markers DXS996 and DXS1043 and one between DXS999 and DXS989. One family shows a maximal lod score of 3.0 at theta = 0 with marker DXS1043 and the other family has a maximal lod score of 1.2 at theta = 0 with markers DXS1224 and DXS418. Both families therefore support the previously reported gene localisation.     

Fine mapping of the dyskeratosis congenita locus in Xq28.

Dyskeratosis congenita (DC) is characterised by reticulate skin pigmentation, mucosal leucoplakia, and nail dystrophy. Bone marrow failure occurs in 50% of patients and is the principal cause of early mortality. In the majority of families the pattern of inheritance of DC is compatible with an X linked recessive trait. The locus for the X linked recessive form of DC has been linked to Xq28. We have now extended our earlier studies by investigating five families with additional Xq28 polymorphic markers; analysis of recombination events in these families has located the DC1 locus between GABRA3 and DXS1108, an interval of approximately 4 Mb.     

Linkage analysis in Spanish families with nonspecific X-linked mental retardation: Significant linkage at Xq13-q21

Mental retardation (MR) is a genetically heterogeneous, clinically variable condition. Many cases of MR are linked to the X chromosome. The aim of this study was to identify candidate loci for nonspecific MR in Spanish samples. We selected seven families with nonspecific MR and a pattern of inheritance compatible with an X-linked disorder and a group of 26 sib pairs of mentally retarded individuals. We performed linkage analysis with a panel of 15 markers evenly distributed along the X chromosome. The study showed linkage to marker DXS8076, located in Xq21.1, by the lod score method (z = 2.11 at straight theta = 0.155) and the nonparametric extended relative pair analysis method (chi(2) = 5.32; P < 0.03). Genetic heterogeneity was found, with an estimated 75% of the families linked at recombination fraction straight theta = 0.10 to the DXS8076 locus (chi(2) = 9.51; P < 0.009). Xq13-q21 is one of the critical regions for X-linked MR previously reported, and our study supports the idea that this region may contain a locus for MR in Spanish patients.     

X linked myotubular myopathy (MTM1) maps between DXS304 and DXS305, closely linked to the DXS455 VNTR and a new, highly informative microsatellite marker (DXS1684).

The locus for X linked recessive myotubular myopathy (MTM1) has previously been mapped to Xq28 by linkage analysis. We report two new families that show recombination between MTM1 and either DXS304 or DXS52. These families and a third previously described recombinant family were analysed with two highly polymorphic markers in the DXS304-DXS52 interval, the DXS455 VNTR and a newly characterised microsatellite, DXS1684 (82% heterozygosity). These markers did not recombine with MTM1 in the three families. Together with the recent mapping of an interstitial X chromosome deletion in a female patient with moderate signs of myotubular myopathy, our data suggest the following order of loci in Xq28: cen-DXS304-(DXS455, MTM1)-DXS1684-DXS305-DXS52-tel. This considerably refined localisation of the MTM1 locus should facilitate positional cloning of the gene. The availability of highly polymorphic and very closely linked markers will markedly improve carrier and prenatal diagnosis of MTM1.     

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 studies in Duchenne and Becker muscular dystrophies.

We have studied the inheritance of four cloned DNA sequences which recognise restriction fragment length polymorphisms on the short arm of the X chromosome in families with Becker and Duchenne muscular dystrophy. We have confirmed linkage of two probe loci to the disease loci and have combined our results with those previously published to give a maximum lod score of 11.642 at a recombination fraction of 0.15 for DXS41 (probe 99.6), and a maximum lod of 15.84 at a recombination fraction of 0.15 for DXS84 (probe 754). Linkage of these diseases to the loci defined by the pERT87 probes and probe pXJ1.1 has also been studied, giving maximum lod scores of 8.634 and 5.118 at recombination fractions of 0.02 and 0.00 respectively. The information obtained using these polymorphic DNA markers, combined with pedigree and CK data, can be used to give more accurate genetic counselling to women at risk in Becker and Duchenne families.     

Search for linkage to schizophrenia on the X and Y chromosomes

Markers for X chromosome loci were used in linkage studies of a large group of small families (n = 126) with at least two schizophrenic members in one sibship. Based on the hypothesis that a gene for schizophrenia could be X-Y linked, with homologous loci on both X and Y, our analyses included all families regardless of the pattern of familial inheritance. Lod scores were computed with both standard X-linked and a novel X-Y model, and sibpair analyses were performed for all markers examining the sharing of maternal alleles. Small positive lod scores were obtained for loci pericentromeric, from Xp11.4 to Xq12. Lod scores were also computed separately in families selected for evidence of maternal inheritance and absence of male to male transmission of psychosis. The lod score for linkage to the locus DXS7 reached a maximum of 1.83 at 0.08% recombination, assuming dominant inheritance on the X chromosome in these families (n = 34). Further investigation of the X-Y homologous gene hypothesis focussing on this region is warranted. © 1994 Wiley-Liss, Inc.     

X linked neonatal myotubular myopathy: one recombination detected with four polymorphic DNA markers from Xq28.

A three generation family with X linked myotubular myopathy (MTM1) was studied with several polymorphic markers from the distal long arm of the X chromosome. A recombination between the disease gene and four markers (loci DXS52, DXS134, DXS15, F8C) from the Xq28 cluster was detected. A new polymorphic marker (U6.2) defining the locus DXS304 in the Xq27-28 region proximal to the Xq28 cluster did not show any recombination with MTM1. These results suggest the following order of loci in distal Xq: cen-DXS42-DXS105-(DXS304, MTM1)-(DXS52, DXS134, DXS15, F8C)-tel.     

Linkage analysis in Spanish families with nonspecific X‐linked mental retardation: Significant linkage at Xq13–q21

Mental retardation (MR) is a genetically heterogeneous, clinically variable condition. Many cases of MR are linked to the X chromosome. The aim of this study was to identify candidate loci for nonspecific MR in Spanish samples. We selected seven families with nonspecific MR and a pattern of inheritance compatible with an X‐linked disorder and a group of 26 sib pairs of mentally retarded individuals. We performed linkage analysis with a panel of 15 markers evenly distributed along the X chromosome. The study showed linkage to marker DXS8076, located in Xq21.1, by the lod score method (z = 2.11 at θ = 0.155) and the nonparametric extended relative pair analysis method (χ² = 5.32; P < 0.03). Genetic heterogeneity was found, with an estimated 75% of the families linked at recombination fraction θ = 0.10 to the DXS8076 locus (χ² = 9.51; P < 0.009). Xq13–q21 is one of the critical regions for X‐linked MR previously reported, and our study supports the idea that this region may contain a locus for MR in Spanish patients. © 2001 Wiley‐Liss, Inc.     

Localisation of a new gene for non-specific mental retardation to Xq22-q26 (MRX35).

Non-specific mental retardation (MR) is a condition in which MR appears to be the only consistent manifestation. The X linked form (MRX) is genetically heterogeneous. We report clinical, cytogenetic, and linkage data on a family with X linked non-specific MR. Two point and multi-point linkage analysis with 18 polymorphic markers, covering the entire chromosome, showed close linkage to DXS1001 and DXS425 with a maximal lod score of 2.41 at 0% recombination. DXS178 and the gene for hypoxanthine phosphoribosyl-transferase (HPRT), located in Xq22 and Xq26 respectively, flank the mutation. All other chromosomal regions could be excluded with odds of at least 100:1. To our knowledge there is currently no other non-specific MR gene mapped to this region. Therefore, the gene causing MR in this family can be considered to be a new, independent MRX locus (MRX35).     

Linkage analysis of a large pedigree with hereditary sideroblastic anaemia.

A large pedigree showing a history of pyridoxine responsive X linked sideroblastic anaemia was screened with several polymorphic DNA markers from the X chromosome. Linkage analysis between each marker and disease status was performed, giving a maximum two point lod score of 3.64 at zero recombination with the microsatellite marker PGK1P1 at Xq11.2-12. Close linkage to PGK at Xq13.3, one of the candidate regions for X linked sideroblastic anaemia, was excluded. Linkage to DNA markers distal to PGK and at Xp21 was also excluded. Multipoint linkage analysis was performed with markers located between Xq11.2-21. The maximum map specific lod score obtained was 3.56 at PGK1P1 (Xq11.2-12). Linkage remained significant over the interval 20 cM proximal to PGK1P1 and 5 cM distal to PGK1P1, with definite exclusion around the PGK locus. The most likely location of the gene involved in sideroblastic anaemia in this pedigree is therefore within the pericentromeric region of the X chromosome. This region includes the erythroid 5-aminolaevulinate synthetase gene of the haem synthesis pathway, which is a candidate gene for X linked sideroblastic anaemia located at Xp11.21.     

Emery-Dreifuss muscular dystrophy: linkage to markers in distal Xq28.

Emery-Dreifuss muscular dystrophy (EMD) is characterised by (1) early contractures of the Achilles tendons, elbows, and postcervical muscles, (2) slowly progressive muscle wasting and weakness with a predominantly humeroperoneal distribution in the early stages, and (3) cardiomyopathy with conduction defects and risk of sudden death. Inheritance is usually X linked recessive but can be autosomal dominant. Family linkage studies have mapped X linked EMD to the distal long arm of the X chromosome but precise genetic localisation has been hampered by the rarity of this condition. We report three new families with X linked Emery-Dreifuss muscular dystrophy studied with DNA markers from Xq27-qter and three previously published families typed for additional markers. No recombination was observed with the red/green cone pigment locus, RGCP (lod score, Z = 2.46), the factor VIII coagulant gene locus, F8C (Z = 6.39), or with DXS115 (Z = 4.94). Two recombinants were observed which mapped EMD distal to DXS15 (DX13) and DXS52 (St14) respectively. Multipoint linkage analysis gave odds exceeding 200:1 for EMD being distal to these markers. A multipoint analysis incorporating published data gave the map cen-DXS304-9cM-DXS15-3cM-DXS52-2 cM-(RGCP,EMD)-3cM-F8C-2cM-DXS115 with odds of 120:1 in favour of a location for EMD between DXS52 and F8C as compared to the next best position distal to F8C.