Linkage mapping of a large Colombian family segregating for X linked retinoschisis: refinement of the chromosomal location.

Juvenile X linked retinoschisis (RS) is a bilateral vitreoretinal dystrophy that develops early in life. Previous linkage studies have localised the RS gene to Xp22.1-p22.3 between DXS207 and AFM 291Wf5, which represents a genetic distance of approximately 3.7 cM. In an effort to facilitate the eventual cloning of the RS gene, we have analysed a large Colombian family, using 10 microsatellite markers that have been mapped to the region Xp22.1-p22.3. A total of 93 members, including 19 affected and eight unaffected males, two affected females, and six obligate carrier females were analysed. Close linkage was observed between the disease locus and DXS999 (Zmax = 2.27, theta max = 0.05), DXS987 (Zmax = 2.61, theta max = 0.1), DXS443 (Zmax = 4.23, theta max = 0.1), and DXS274 (Zmax = 3.49, theta max = 0.05) markers. Recombination with the RS locus was found for all marker loci except DXS197, DXS43, and DXS1195. These results place the RS locus within an interval of approximately 2 cM between the flanking markers DXS1053 and DXS999, approximately 1.7 cM closer than the previously reported boundary. The results also further confirm the lack of genetic heterogeneity of RS.     

Detailed genetic mapping of the von Hippel-Lindau disease tumour suppressor gene.

Von Hippel-Lindau (VHL) disease is an autosomal dominant inherited familial cancer syndrome characterised by a predisposition to the development of retinal, cerebellar, and spinal haemangioblastomas, renal cell carcinoma, and phaeochromocytoma. The gene for VHL disease has been mapped to chromosome 3p25-p26 and flanking markers identified. We report the detailed genetic mapping of the VHL disease locus in 38 families. Significant linkage was detected between VHL disease and D3S601 (Zmax = 18.86 at theta = 0.0, CI 0.0-0.025), D3S18 (Zmax = 11.42 at theta = 0.03, CI 0.005-0.08), RAF1 (Zmax = 11.02 at theta = 0.04, CI 0.007-0.01), and D3S1250 (Zmax = 4.73 at theta = 0.05, CI 0.005-0.15). Multipoint linkage analysis mapped the VHL disease locus between D3S1250 and D3S18 close to D3S601. There was no evidence of locus heterogeneity. This study has (1) confirmed the tight linkage between VHL disease and D3S601, (2) identified D3S1250 as the first marker telomeric to RAF1 which maps centromeric to the VHL disease gene, and (3) narrowed the target region for isolation of the VHL disease gene by positional cloning techniques to a 4 cM interval between D3S1250 and D3S18. These findings will improve the clinical management of families with VHL disease by improving the accuracy of presymptomatic diagnosis using linked DNA markers, and will enhance progress towards isolating the VHL disease gene.     

Refinement of the chromosomal position of the X linked juvenile retinoschisis gene.

Linkage analysis was carried out in seven X linked juvenile retinoschisis (XLRS) families using four DNA probes and four CA repeat polymorphisms from the Xp22 region. Close linkage was observed between the XLRS locus and DXS207 (theta max = 0.04, Zmax = 3.71), DXS999 (theta max = 0.00, Zmax = 4.59), DXS365 (theta max = 0.07, Zmax = 2.22), and DXS451 (theta max = 0.05, Zmax = 3.26). The analysis of recombination breakpoints and multipoint linkage analysis suggests the order Xpter-DXS16-(DXS43, DXS207)-RS-DXS365-(DXS451, DXS41)-Xcen, thereby refining the position of the XLRS locus to an interval of approximately 3-4 cM. These results improve the feasibility of diagnosis in XLRS considerably, since carriers of this disease cannot be identified clinically.     

Linkage of epidermolysis bullosa simplex to keratin gene loci.

Epidermolysis bullosa simplex (EBS) is an autosomal dominant disorder characterised by intraepidermal blistering of the skin. Two families with Weber-Cockayne EBS have been analysed for linkage to keratin gene loci. In the first family, linkage was found to chromosome 17 markers flanking the keratin 14 gene (D17S74: Zmax = +2.45, theta = 0.10; COL1A1: Zmax = +0.97, theta = 0.00) and markers near the keratin 5 gene on chromosome 12 were excluded (D12S17: Z less than -2.0, theta = 0.08; COL2A1: Z less than -2.0, theta = 0.13). In contrast, the second family showed linkage to the region containing the keratin 5 gene (D12S17: Zmax = +1.37, theta = 0.08; COL2A1: Zmax = +0.33, theta = 0.15) and was not linked to the keratin 14 gene (D17S74: Z less than -2.0, theta = 0.14). The Weber-Cockayne form of EBS is genetically heterogeneous with linkage to different keratin gene loci.     

Localization of X-linked dominant Charcot-Marie-Tooth disease (CMT 2) to Xq13

A family is described in which Charcot-Marie-Tooth disease is inherited as an X-linked dominant mutation (CMT2). Ten DNA marker loci on the X chromosome were used to map the disease locus by linkage analysis. The DXYS1 sequence at Xq13 was found to be linked to the CMT2 locus at an estimated distance of 6 cM (Zmax = 2.87 at theta max = 0.06). The data also suggested close linkage of the CMT2 locus to PGK1 (Zmax = 1.51 at theta max = 0) which has also been mapped to Xq13. Another DNA locus (DXS3), in the Xq21.3----Xq22 region, did not show close linkage (Zmax = -2.231 at theta max = 0.01). We conclude that the CMT2 locus is probably in or close to band Xq13.     

Genetic linkage between Von Hippel--Lindau disease and three microsatellite polymorphisms refines the localisation of the VHL locus

Von Hippel—Lindau (VHL) disease is a dominantly inheritedfamilial cancer syndrome characterised by the development ofretinal and central nervous system haemangioblastomas, renalcell carcinoma and phaeochromocytoma. The gene for VHL diseasehas been mapped to chromosome 3p25–p26 and presymtomaticdiagnosis using linked DNA markers is available. We have previouslymapped the VHL disease gene to a 4 cM interval between D3S1250and D3S18. To increase access to presymptomatic diagnosis andto accelerate progress towards isolating the VHL disease genewe attempted to identify microsatellite DNA markers linked tothe disease gene by genetic linkage analysis in 29 families.We found significant linkage between the VHL disease gene anddinucleotide (CA) repeat polymorphisms at D3S1038 (Zmax = 22.24at     

Improved genetic mapping of X linked retinoschisis.

X linked retinoschisis (RS) causes poor vision in affected males owing to radial cystic changes at the macula. Genetic linkage analysis was carried out in 16 British families with X linked retinoschisis using markers from the Xp22 region. Linkage was confirmed between the RS locus and the markers DXS207 (lod score, Zmax = 17.9 at recombination fraction theta = 0.03; confidence interval for theta = 0.007-0.09), DXS1053 (Zmax = 18.0 at theta = 0.01, CI = 0.001-0.06), DXS43 (Zmax = 12.9 at theta = 0.03, CI = 0.004-0.09), DXS1195 (Zmax = 6.4 at theta = 0.00), DXS418 (Zmax = 8.2 at theta = 0.00), DXS999 (Zmax = 21.2 at theta = 0.01, CI = 0.001-0.05), DXS443 (Zmax = 14.2 at theta = 0.03, CI = 0.004-0.09), DXS365 (Zmax = 24.5 at theta = 0.008, CI = 0.001-0.04). Key recombinants placed RS between DXS43 distally and DXS999 proximally. Multipoint linkage analysis gave odds of 344:1 in favour of this location for RS and supported the map Xpter-(DXS207, DXS1053)-DXS43-1 cM-RS-1 cM-DXS999-DXS443-DXS365-DXS1052-Xcen.     

Linkage disequilibrium in inbred North African families allows fine genetic and physical mapping of triple A syndrome

Triple A syndrome (Allgrove syndrome, MIM No. 231550) is a rare autosomal recessive disorder characterised by ACTH-resistant adrenal insufficiency, achalasia of the cardia, and alacrimia. The triple A gene has been previously mapped to chromosome 12q13 in a maximum interval of 6 cM between loci D12S1629 and D12S312. Using linkage analysis in 12 triple A families, mostly originating from North Africa, we confirm that the disease locus maps to the 12q13 region (Zmax = 10.89 at theta = 0 for D12S1604) and suggest that triple A is a genetically homogeneous disorder. Recombination events as well as homozygosity for polymorphic markers enabled us to reduce the genetic interval to a 3.9 cM region. Moreover, total linkage disequilibrium was found at the D12S1604 locus between a rare allele and the mutant chromosomes in North African patients. Analysis of markers at five contiguous loci showed that most of the triple A chromosomes are derived from a single founder chromosome. As all markers are located in a 0 cM genetic interval and only allele 5 at the D12S1604 locus was conserved in mutant chromosomes, we speculate that the triple A mutation is due to an ancient Arabian founder effect that occurred before migration to North Africa. Since we also found linkage disequilibrium at D12S1604 in two patients from Southern Europe (France and Spain), the founder effect might well extend to other Mediterranean countries. Taking advantage of a YAC contig encompassing the triple A minimal physical region, the triple A gene was mapped to a 1.7 Mb DNA fragment accessible to gene cloning.     

Genetic mapping of the Kallmann syndrome and X linked ocular albinism gene loci.

The X linked form of Kallmann syndrome (KAL) and X linked ocular albinism (OA1) have both been mapped to Xp22.3. We have used a dinucleotide repeat polymorphism at the Kallmann locus to type 17 X linked ocular albinism families which had previously been typed for the Xg blood group (XG) and the DNA markers DXS237 (GMGX9), DXS143 (dic56), and DXS85 (782). Close linkage was found between KAL and OA1 with a maximum lod score (Zmax) of 30.14 at a recombination fraction (theta max) of 0.06 (confidence interval for theta: 0.03-0.10). KAL was also closely linked to DXS237 (Zmax = 15.32; theta max = 0.05; CI 0.02-0.12) and DXS143 (Zmax = 14.57; theta max = 0.05; CI 0.02-0.13). There was looser linkage to the Xg blood group (XG) and to DXS85 (782). Multipoint linkage analysis gave the map: Xpter-XG-0.13-DXS237-0.025-KAL-0.025-DXS143-0.01 5-OA1-0.09-DXS85-Xcen. Placement of OA1 proximal to DXS143 was supported by odds of 2300:1 compared to other orders. This confirms our previous localisation of OA1 and improves the genetic mapping of both disease loci.     

一个视网膜色素变性家系致病基因定位和CA4基因突变分析

目的通过对一个常染色体显性视网膜色素变性(autosomal dominant retinitis pigmentosa,adRP)家系致病基因的定位和基因突变分析,以确定该家系的致病基因及其突变形式。方法对15个已知的常染色体显性视网膜色素变性致病基因所在染色体位点进行连锁分析,以确定该家系与疾病连锁的染色体区域,对该区域附近候选基因进行直接序列分析。结果连锁分析提示在D17S701和D17S1604为正的连锁值(logofodds,LOD),分别为Zmax=2.107和Zmax=1.806。其余14个adRP染色体位点的微卫星标记两点LOD值均为负数。单倍型分析进一步将该家系致病基因定位于微卫星标记D17S916和D17S794之间的RP17位点,该位点adRP候选基因碳酸酐酶Ⅳ(carbonic anhydrase4,CA4)直接序列分析在其编码区未发现基因突变。结论将一个中国人常染色体显性视网膜色素变性家系的致病基因定位于RP17位点,但未发现该位点内的CA4基因突变,该家系是否存在CA4基因复杂突变或RP17位点是否存在新的视网膜色素变性致病基因有待于进一步研究。     

Evidence for two psoriasis susceptibility loci (HLA and 17q) and two novel candidate regions (16q and 20p) by genome-wide scan

In a 12.5 cM genome-wide scan for psoriasis susceptibility loci, recombination-based tests revealed linkage to the HLA region (Zmax = 3.52), as well as suggestive linkage to two novel regions: chromosome 16q (60-83.1 cM from pter, Zmax = 2.50), and chromosome 20p (7.5-25 cM from pter, Zmax = 2.62). All three regions yielded P values < or = 0.01 by non-parametric analysis. Recombination-based and allele sharing methods also confirmed a previous report of a dominant susceptibility locus on distal chromosome 17q (108.2 cM from pter, Zmax = 2.09, GENEHUNTER P = 0.0056). We could not confirm a previously reported locus on distal chromosome 4q; however, a broad region of unclear significance was identified proximal to this proposed locus (153.6- 178.4 cM from pter, Zmax = 1.01). Taken together with our recent results demonstrating linkage to HLA-B and -C, this genome-wide scan identifies a psoriasis susceptibility locus at HLA, confirms linkage to 17q, and recommends two novel genomic regions for further scrutiny. One of these regions (16q) overlaps with a recently-identified susceptibility locus for Crohn's disease. Psoriasis is much more common in patients with Crohn's disease than in controls, suggesting that an immunomodulatory locus capable of influencing both diseases may reside in this region.      

Linkage analysis in the spinal muscular atrophy type of facioscapulohumeral disease.

Facioscapulohumeral disease is probably a heterogeneous disorder. We have ascertained and sampled two multigeneration families with the neurogenic form of this disorder, considered to be a type of spinal muscular atrophy (FSHSMA). The two families have 36 affected members. Linkage studies with 10 expressed and seven DNA restriction fragment length polymorphism (RFLP) markers failed to show significant linkage (Zmax greater than or equal to 3.00). However, two areas of probable linkage were defined on chromosomes 1p and 4q with the markers MNS (Zmax = 1.47 at theta max = 0.10) and PGM1 (Zmax = 0.94 at theta max = 0.001) respectively. We are using additional RFLPs from these and other areas of the human genome to screen these families for linkage to FSHSMA.     

Linkage analysis in Usher syndrome type I (USH1) families from Spain.

Usher syndrome (USH) is an autosomal recessive hereditary disorder characterised by congenital sensorineural hearing loss and gradual visual impairment secondary to retinitis pigmentosa (RP). The disorder is clinically and genetically heterogeneous. With regard to Usher type I (USH1), several subtypes have been described, the most frequent being USH1B located on chromosome 11q13.5. Of 18 USH1 families studied by linkage analysis, 12 (67%) showed significant lod score values for locus D11S527 (Zmax=14.032, theta=0.000) situated on chromosome 11q. Our findings suggest considerable genetic heterogeneity in the Spanish USH1 population. It is important to note that one of our families linked to the USH1B locus shows interesting intrafamilial clinical variability. As regards the remaining six USH1 families, the linkage analysis did not provide conclusive data, although two of them show slight linkage to markers located on chromosome 3q (Zmax=1.880, theta=0.000 for D3S1279), the same location that had previously been assigned to some USH3 families.     

Familial hypomagnesemia maps to chromosome 9q, not to the X chromosome: genetic linkage mapping and analysis of a balanced translocation breakpoint

Familial hypomagnesemia with secondary hypocalcemia (HSH) (MIM 307600) was studied in three inbred Bedouin kindreds from Israel. The three kindreds, one extended and two nuclear families, contained 13 affected individuals, 11 males and two females. Assuming that the individuals affected with hypomagnesemia shared a chromosomal region inherited from a common ancestor, we used a DNA pooling strategy in a genome-wide search for loci which show homozygosity for shared alleles in affected individuals. DNA samples from affected individuals within a single kindred were pooled and used as the template for PCR amplification of short tandem repeat polymorphic markers (STRPs). Pooled DNA from unaffected siblings and parents were used as controls. A shift towards homozygosity was observed in the affected DNA pool compared with the control pools with D9S301 (GATA7D12). Genotyping of individual DNA samples with D9S301 and several flanking markers confirmed linkage to chromosome 9 with maximum LOD scores of 3.4 (theta = 0.05), 3.7 (theta = 0) and 2.3 (theta = 0) for the three families. We have identified a 14 cM interval on chromosome 9 (9q12-9q22.2), flanked by proximal marker D9S1874 and distal marker D9S1807, within which all affected individuals from the three kindreds are homozygous for a shared haplotype. The disease segregates with a common affected haplotype in the three families, suggesting that hypomagnesemia is caused by a common ancestral mutation in these families. Although HSH has been previously reported to be X linked, these linkage data demonstrate that the disorder is an autosomal recessive disease in these kindreds. Mapping of a chromosomal breakpoint in a somatic cell line established from a patient with HSH and a balanced X;9 translocation placed the chromosomal breakpoint in a 500 kb region flanked by D9S1844 and D9S273. Identification of the gene responsible for hypomagnesemia will provide insight into the regulation of this essential cation.      

No evidence of genetic heterogeneity in dominant optic atrophy.

Autosomal dominant optic atrophy (OPA, MIM 165500) is an eye disease causing a variable reduction of visual acuity with an insidious onset in the first six years of life. It is associated with a central scotoma and an acquired blue-yellow dyschromatopsia. A gene for dominant optic atrophy (OPA1) has recently been mapped to chromosome 3q in three large Danish pedigrees. Here, we confirm the mapping of OPA1 to chromosome 3q28-qter by showing close linkage of the disease locus to three recently reported microsatellite DNA markers in the interval defined by loci D3S1314 and D3S1265 in four French families (Zmax = 5.13 at theta = 0 for probe AFM 308yf1 at locus D3S1601). Multipoint analysis supports the mapping of the disease gene to the genetic interval defined by loci D3S1314 and D3S1265. The present study provides three new markers closely linked to the disease gene for future genetic studies in OPA.     

A new autosomal recessive retinitis pigmentosa locus maps on chromosome 2q31-q33.

Autosomal recessive retinitis pigmentosa (ARRP) is a genetically heterogeneous disease. To date, mutations in four members of the phototransduction cascade have been implicated in ARRP. Additionally, linkage of the disease to three loci on 1p, 1q, and 6p has been described. However, the majority of cases are still uncharacterised. We have performed linkage analysis in a large nuclear ARRP family with five affected sibs. After exclusion of several regions of the genome known to contain loci for retinal dystrophies, a genomic search for linkage to ARRP was undertaken. Positive lod scores were obtained with markers on 2q31-q33 (Zmax at theta = 0.00 of 4.03, 4.12, and 4.12 at D2S364, D2S118, and D2S389, respectively) defining an interval of about 7 cM for this new ARRP locus, between D2S148 and D2S161. Forty-four out of 47 additional ARRP families, tested with markers on 2q32, failed to show linkage, providing evidence of further genetic heterogeneity.     

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.     

Second family with hearing impairment linked to 19q13 and refined DFNA4 localisation

Until now, over 30 loci have been identified by linkage analysis of affected families that segregate non-syndromic and dominantly inherited forms of hearing impairment (DFNA). A German family with a non-syndromic progressive hearing impairment transmitted in autosomal dominant mode was linked to 19q13.3-q13.4 by a genome-wide scan. Due to the low lod-score (1.85 at theta=0.05) for APOC2-locus we extended the fine mapping attempt with further markers in the same chromosomal region. This resulted in significant evidence for linkage to the markers D19S246 and D19S553 (two-point lod-score of 4.05 and 3.55 at theta=0.0) and a candidate critical region of 14 cM between markers D19S412 and D19S571. This region shows partial overlap with the previously reported DFNA4 critical region. The human gene BAX is orthologous to the rodent Bcl2-related apoptosis gene that is temporally expressed during the postnatal period in the developing inner ear of the mouse. BAX, mapping at a distance of no more than 0.73 cM distally to marker D19S553 appeared a likely candidate in our pedigree but genomic sequencing of coding regions and exon/intron boundaries excluded disease-related mutations. However, additional ESTs in the same region remain to be tested.     

Mapping of a new autosomal dominant non-syndromic hearing loss locus (DFNA43) to chromosome 2p12

Hearing impairment (HI) is the most frequent sensory defect with wide genetic heterogeneity. Approximately 80% of genetic hearing loss is non-syndromic and 15-25% of exhibit autosomal dominant inheritance. We analysed an Italian three generation family in which non-syndromic hearing impairment is transmitted as an autosomal dominant trait. Onset of HI in all affected subjects occurred in the second decade of life, with subsequent gradual progression from moderate to profound loss. HI was bilateral and symmetrical, involving all frequencies. After exclusion of the known DFNA loci with markers from the Hereditary Hearing Loss Homepage (URL: http://dnalab-www.uia.ac.be/dnalab/hhh), a genome wide scan was carried out using 358 highly informative microsatellite markers. Significant linkage (Zmax=4.21, theta=0) was obtained with chromosome 2p12 markers. The results were confirmed by multipoint analysis (Zmax=4.51), using the location score method. Haplotype analysis defined a 9.6 cM disease gene interval on chromosome 2 without overlap with the other identified loci. Fine mapping and identification of candidate genes are in progress.