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).     

Linkage mapping of the spinal muscular atrophy gene

Spinal muscular atrophy (SMA) is a common autosomal recessive disorder resulting in loss of motor neurons. We have performed linkage analysis on a panel of families using nine markers that are closely linked to the SMA gene. The highest lod score was obtained with the marker D5S351 (Z_max = 10.04 at = 0 excluding two unlinked families, and Z_max = 8.77 at = 0.007 with all families). One type III family did not show linkage to the 5q13 markers, and in one type I consanguineous family the affected individual did not show homozygosity except for the marker D5S435. Three recombinants were identified with the closest centromeric marker, D5S435, which position the gene telomeric of this marker. These recombinants will facilitate finer mapping of the location of the SMA gene. Lastly, two families provide strong evidence for a remarkable variability in presentation of the SMA phenotype, with the age at onset in one family varying from 17 months to 13 years.     

Genetic linkage analysis of Canadian spinal muscular atrophy kindreds using flanking microsatellite 5q13 polymorphisms

The spinal muscular atrophies (SMA) are among the most common autosomal recessive disorders. We have performed linkage analysis using both standard restriction fragment length polymorphisms (RFLPs) as well as microsatellite polymorphisms [Ca_(n)] on 49 Canadian SMA families (types 1, 2, and 3) that both flank and are linked to SMA. The closest SMA linkage was observed with the MAP1B locus (z_max=8.04, _max=0.0). Multipoint linkage analysis gave a high probability of SMA mapping between D5S6 and D5S39. Only one family (type 3) that fulfilled our diagnostic criteria for SMA showed nonlinkage with 5q13 markers. This study shows the feasibility of accurate molecular diagnosis of SMA utilizing 5q13 satellite polymorphisms.     

Linkage analysis of familial Alzheimer disease, using chromosome 21 markers

Chromosome 21 markers were tested for linkage to familial Alzheimer disease (FAD) in 48 kindreds. These families had multiple cases of Alzheimer disease (AD) in 2 or more generations with family age-at-onset means (M) ranging from 41 to 83 years. Included in this group are seven Volga German families which are thought to be genetically homogeneous with respect to FAD. Autopsy documentation of AD was available for 32 families. Linkage to the 21 q11-q21 region was tested using D21S16, D21S13, D21S110, D21S1/S11, and the APP gene as genetic markers. When linkage results for all the families were summed, the LOD scores for these markers were consistently negative and the entire region was formally excluded. Linkage results were also summed for the following family groups; late-onset (M greater than 60), early-onset (M less than or equal to 60), Volga Germans (M = 56), and early-onset non-Volga Germans (M less than or equal to 60). For the first three groups, LOD scores were negative for this region. For the early-onset non-Volga German group (six families), small positive LOD scores of Zmax = 0.78 (recombination fraction theta = .15), Zmax = 0.27 (theta = .15), and Zmax = 0.64 (theta = .0), were observed for D21S13, D21S16, and D21S110, respectively. The remainder of the long arm of chromosome 21 was tested for linkage to FAD using seven markers spanning the q22 region. Results for these markers were also predominantly negative. Thus it is highly unlikely that a chromosome 21 gene is responsible for late-onset FAD and at least some forms of early-onset FAD represented by the Volga German kindreds.     

Genetic linkage of Bietti crystallin corneoretinal dystrophy to chromosome 4q35

Bietti crystalline corneoretinal dystrophy (BCD) is an autosomal recessive retinal degeneration characterized by multiple glistening intraretinal dots scattered over the fundus, degeneration of the retina, and sclerosis of the choroidal vessels, ultimately resulting in progressive night blindness and constriction of the visual field. Although BCD has been associated with abnormalities in fatty-acid metabolism and absence of fatty-acid binding by two cytosolic proteins, the genetic basis of BCD is unknown. We report linkage of the BCD locus to D4S426 (maximum LOD score [Z(max)] 4.81; recombination fraction [straight theta] 0), D4S2688 (Zmax=3.97; straight theta=0), and D4S2299 (Zmax=5.31; straight theta=0), on chromosome 4q35-4qtel. Multipoint analysis confirmed linkage to the region telomeric of D4S1652 with a Z(max) of 5.3 located 4 cM telomeric of marker D4S2930.     

A microsatellite polymorphism associated with the PLC1 (phospholipase C) locus: identification, mapping, and linkage to the MODY locus on chromosome 20.

A highly polymorphic (dC-dA)n.(dG-dT)n dinucleotide repeat at the PLC1 locus on human chromosome 20 has been identified. Primers flanking the dinucleotide repeat were used for PCR amplification of the repeat region in 37 informative kindreds from the Centre d'Etude du Polymorphisme Humain. Two-point linkage analysis indicates that PLC1 is closely linked to several chromosome 20 markers, including D20S16 (Zmax = 41.25; theta = 0.07), D20S17 (Zmax = 42.81; theta = 0.09), and ADA (Zmax = 57.24; theta = 0.05). Multipoint linkage analysis places the PLC1 locus between D20S18 and D20S17, 11.2 and 6.6 cM, respectively, from these loci (sex-averaged distances). In addition, the PLC1 gene shows linkage to the maturity-onset diabetes of the young (MODY) locus on chromosome 20 with a lod score of 4.57 at theta = 0.089.     

Fanconi anemia: evidence for linkage heterogeneity on chromosome 20q

Fanconi anemia is a rare autosomal recessive disorder in which affected individuals are predisposed to acute myelogenous leukemia and other malignancies. We report the results of a genetic linkage study involving 34 families enrolled in the International Fanconi Anemia Registry. A significant lod score was obtained between D20S20, an anonymous DNA segment from chromosome 20q, and Fanconi anemia (Zmax 3.04, theta max = 0.12). However, six other anonymous DNA segments from chromosome 20q, including D20S19, which is highly polymorphic and tightly linked to D20S20, showed no or only weak evidence for linkage to Fanconi anemia. An admixture test revealed significant evidence for linkage heterogeneity (chi 2 = 6.10, P = 0.01) at the D20S19 locus. Lod scores suggestive of linkage between Fanconi anemia and this locus were obtained with two of the largest kindreds studied (lods = 2.6 and 2.1, at theta = 0.001). Thus, our data support the provisional assignment of a Fanconi anemia gene to chromosome 20q.     

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.     

A locus for brachydactyly type A-1 maps to chromosome 2q35-q36

Brachydactyly type A-1 (BDA1) was, in 1903, the first recorded example of a human anomaly with Mendelian autosomal dominant inheritance. Two large families, the affected members of which were radiographed, were recruited in the study we describe here. Two-point linkage analysis for pedigree 1 (maximum LOD score [Zmax] 6.59 at recombination fraction [theta] 0.00) and for pedigree 2 (Zmax=5.53 at straight theta=0.00) mapped the locus for BDA1 in the two families to chromosome 2q. Haplotype analysis of pedigree 1 confined the locus for family 1 within an interval of <8.1 cM flanked by markers D2S2248 and D2S360, which was mapped to chromosome 2q35-q36 on the cytogenetic map. Haplotype analysis of pedigree 2 confined the locus for family 2 within an interval of <28. 8 cM flanked by markers GATA30E06 and D2S427, which was localized to chromosome 2q35-q37. The two families had no identical haplotype within the defined region, which suggests that the two families were not related.     

Linkage analysis in spinal muscular atrophy, by six closely flanking markers on chromosome 5

The proximal spinal muscular atrophies (SMA) represent the second most common autosomal recessive disorder, after cystic fibrosis. The gene responsible for chronic SMA has recently been mapped to chromosome 5q by using genetic linkage studies. Among six markers mapping to this region, five were shown to be linked with the SMA locus in 39 chronic SMA families each containing at least two affected individuals. Multilocus analysis by the method of location score was used to establish the best estimate of the SMA gene location. Our data suggest that the most likely location for SMA is between loci D5S6 and D5S39. The genetic distances between these two markers are estimated to be 6.4 cM in males and 11.9 cM in females. Since meiosis were informative with D5S39 and D5S6 in 92% and 87% of SMA families, respectively, it is hoped that the present study will contribute to the calculation of genetic risk in SMA families.     

Evidence supporting tight linkage of X-linked Emery-Dreifuss muscular dystrophy to the factor VIII:C gene.

In a large German family with Emery-Dreifuss muscular dystrophy (EDMD) linkage analysis was performed using the factor IX gene (F9), the factor VIII:C gene (F8), the anonymous DNA probe DXS52, and DXS15 as markers. Tight linkage was found between the EDMD locus and the F8 probe (Zmax = 1.19; theta max = 0.00), DXS15 (Zmax = 1.75; theta max = 0.00) and DXS52 (Zmax = 2.26; theta max = 0.00). Weak linkage was found to F9 (Zmax = 0.02; theta max = 0.43). The data from the literature and our results suggest that the gene locus of EDMD is close to F8 (confidence interval theta = 0-0.07). The new linkage data are useful for carrier detection and diagnosis of EDMD patients before onset of major clinical signs.     

Segregation of a mutation in CNGB1 encoding the β-subunit of the rod cGMP-gated channel in a family with autosomal recessive retinitis pigmentosa

Retinitis pigmentosa (RP) is a clinically and genetically heterogeneous group of retinal diseases leading to blindness. By performing full genome linkage analysis in a consanguineous French family affected with severe autosomal recessive RP, we have excluded linkage to known loci involved in RP and mapped a novel locus to chromosome 16q13-q21 (Zmax=2.83 at theta=0 at the D16S3089 locus). Two candidate genes KIFC3 and CNGB1 mapping to this critical interval have been screened for mutations. The CNGB1 gene, which encodes the beta-subunit of the rod cGMP-gated channel, is mutated in the family presented in this study.     

A sublocus of the multicopy microsatellite marker CMS1 maps proximal to spinal muscular atrophy (SMA) as shown by recombinant analysis

The critical region containing the spinal muscular atrophy (SMA) gene is flanked by the 5q11–q13 markers, D5S435 and D5S557, as determined by linkage analysis. Here we present the results of an analysis of a Dutch SMA family with the multicopy microsatellite marker CMS1. A crossover is revealed in the critical SMA region. We conclude that at least one of the CMS1 subloci maps proximal to the SMA gene. This reduces the minimal SMA region from approximately 1.4 Mb to 600–700 kb.     

Mapping of facioscapulohumeral muscular dystrophy gene to chromosome 4q35-qter by multipoint linkage analysis and in situ hybridization

We have recently assigned the facioscapulohumeral muscular dystrophy (FSHD) gene to chromome 4 by linkage to the microsatellite marker Mfd 22 (locus D4S171). We now report that D4S139, a VNTR locus, is much more closely linked to FSHD. Two-point linkage analysis between FSHD and D4S139 in nine informative families showed a maximum combined lod score (Zmax) of 17.28 at a recombination fraction theta of 0.027. Multipoint linkage analysis between FSHD and the loci D4S139 and D4S171 resulted in a peak lod score of 20.21 at 2.7 cM from D4S139. Due to the small number of recombinants found with D4S139, the position of the FSHD gene relative to that of D4S139 could not be established with certainty. D4S139 was mapped to chromosome 4q35-qter by in situ hybridization, thus firmly establishing the location of the FSHD gene in the subtelomeric region of chromosome 4q. One small family yielded a negative lod score for D4S139. In the other families no significant evidence for genetic heterogeneity was obtained. Studies of additional markers and new families will improve the map of the FSHD region, reveal possible genetic heterogeneity, and allow better diagnostic reliability.     

A gene for pyridoxine-dependent epilepsy maps to chromosome 5q31

Pyridoxine-dependent epilepsy (PDE) is a rare autosomal recessive disorder characterized by generalized seizures in the first hours of life and responding only to pyridoxine hydrochloride. The pathogenesis of PDE is unknown, but an alteration in the binding of pyridoxal 5-phosphate to glutamic acid decarboxylase (GAD) has been postulated in patients with PDE. Results are reported for genetic linkage analyses in four families with consanguineous parents and in one family with nonconsanguineous parents. The GAD1 (2q31) and GAD2 genes (10p23) were tested and excluded. A genomewide search was subsequently performed, using microsatellite markers at an average distance of 10 cM, and the search revealed linkage of the disease-causing gene to markers on chromosome 5q31.2-q31.3 (maximum LOD score [Z(max)] 8.43 at recombination fraction [theta] 0 and Zmax=7.58 at straight theta=0 at loci D5S2017 and D5S1972, respectively). A recombination event, between loci D5S638 and D5S463, in one family defined the distal boundary, and a second recombination event between loci D5S2011 and D5S2017 in another family defined the proximal boundary of the genetic interval encompassing the PDE gene (5.1 cM). Ongoing studies may lead to the identification of the disease-causing gene.     

Genetic and physical mapping of the human cannabinoid receptor gene to chromosome 6q14-q15

A cDNA encoding a G protein-coupled receptor that appears to mediate the behavioral effects of cannabinoids, the psychoactive ingredients of marijuana, has recently been cloned from rat cerebral cortex and expressed. We have now determined the genomic location of the human cannabinoid receptor gene (CNR) by a combination of genetic linkage mapping and chromosomal in situ hybridization. The segregation pattern of a CNR DNA polymorphism was analyzed in 508 individuals from two or three generations of 40 families. Linkage of CNR to chromosome 6 centromeric loci and to DNA markers on the long and short arms was detected. CNR was tightly linked to D6S27, which is known to be located at 6q (log10 odds ratio [lod score, Zmax] of 10.54 at a recombination fraction [theta] of 0.02). Close linkage was suggested between CNR and CGA, the locus for the alpha subunit of human chorionic gonadotropin (Zmax = 2.71 at theta = 0). Moreover, CNR was linked to the two markers 308/BamHI (theta = 0.14) and 308/TaqI (theta = 0.20) defining locus D6Z1, an extended, highly repetitive, and highly conserved sequence localized exclusively to centromeres of all chromosomes and enriched on chromosome 6. In situ hybridization using a biotinylated cosmid probe localizes the gene to 6q14-q15, thereby confirming the linkage analysis and defining a precise alignment of the genetic and cytogenetic maps.     

Evidence for the localization of a malignant hyperthermia susceptibility locus (MHS2) to human chromosome 17q.

Malignant hyperthermia susceptibility is a lethal autosomal dominant disorder of skeletal muscle metabolism that is triggered by all potent inhalation anesthetic gases. Recent linkage studies suggest a genetic locus for this disorder on 19q13.1. We have previously reported three unrelated families diagnosed with MHS that are unlinked to markers surrounding this locus on 19q13.1. In this report we extend these observations and present linkage studies on 16 MHS families. Four families (25%) were found linked to the region 19q12-q13.2 (Zmax = 2.96 with the ryanodine receptor at theta = 0.0). Five families (31%) were found closely linked to the anonymous marker NME1 (previously designated NM23) on chromosome 17q11.2-q24 (Zmax = 3.26 at theta = 0.0). Two families (13%) were clearly unlinked to either of these chromosomal regions. In five additional families, data were insufficient to determine their linkage status (they were potentially linked to two or more sites). The results of our heterogeneity analyses are consistent with the hypothesis that MHS can be caused in humans by any one of at least three distinct genetic loci. Furthermore, we provide preliminary linkage data suggesting the localization of a gene in human MHS to 17q11.2-q24 (MHS2), with a gene frequency of this putative locus approximately equal to that of the MHS1 locus on 19q.     

Mapping of a First Locus for Autosomal Dominant Myxomatous Mitral-Valve Prolapse to Chromosome 16p11.2-p12.1

Myxomatous mitral-valve prolapse (MMVP), also called Barlow disease, is a common cardiac abnormality and affects up to 5% of the population. It is characterized by an excess of tissue that leads to billowing of the mitral leaflets, sometimes complicated by prolapse. Typical histological findings include myxomatous degeneration and degradation of collagen and elastin. Previous reports have proposed an autosomal dominant inheritance of the trait, with age- and sex-dependent expression. By systematic echocardiographic screening of the first-degree relatives of 17 patients who underwent mitral-valve repair, we have identified four pedigrees showing such an inheritance. Genomewide linkage analysis of the most informative pedigree (24 individuals, three generations) showed a significant linkage for markers mapping to chromosome 16p, with a two-point maximum LOD score for D16S3068 (Zmax=3.30 at straight theta=0). Linkage to D16S3068 was confirmed in a second family (Zmax=2.02 at straight theta=0) but was excluded for the two remaining families, thus demonstrating the genetic heterogeneity of the disease. Multipoint linkage analysis performed, with nine additional markers, on the two families with linkage gave maximum multipoint LOD scores of 5.45 and 5.68 for D16S3133, according to a conservative and a stringent model, respectively. Haplotype analysis defined a 5-cM minimal MMVP-1 locus between D16S3068 (16p11.2) and D16S420 (16p12. 1) and a 34-cM maximal interval between D16S404 and D16S3068 when recombination events were taken into account only in affected individuals. The identification of this locus represents a first step toward a new molecular classification of mitral-valve prolapse.     

Hereditary isolated renal magnesium loss maps to chromosome 11q23.

Hypomagnesemia due to isolated renal magnesium loss has previously been demonstrated in two presumably unrelated Dutch families with autosomal dominant mode of inheritance. Patients with magnesium deficiency may suffer from tetany and convulsions, but the patients with hereditary renal magnesium wasting can also be clinically nonsymptomatic. In a genomewide linkage study, we first excluded a possible candidate region, on chromosome 9q, that encompasses the gene for intestinal hypomagnesemia with secondary hypocalcemia and, subsequently, found linkage to markers on chromosome 11q23. Detailed haplotype analyses identified a common haplotype segregating in both families, suggesting both their relationship through a common ancestor and the existence of a single, hypomagnesemia-causing mutation within them. The maximum two-point LOD score (Zmax) was found for marker D11S4127 (Zmax=6.41 at a recombination fraction of. 00), whereas a multipoint analysis gave a Zmax of 8.24 between markers D11S4142 and D11S4171. Key recombination events define a 5. 6-cM region between these two markers on chromosome 11q23. We conclude that this region encompasses a gene, involved in renal magnesium handling, that is mutated in our patients and is different from the gene involved in intestinal magnesium handling.     

High-resolution genetic map around the spinal muscular atrophy (SMA) locus on chromosome 5.

Although autosomal recessive spinal muscular atrophy (SMA) has been mapped to chromosome 5q12-q13, there is for this region no genetic map based on highly informative markers. In this study we present the mapping of two previously reported microsatellite markers in 40 CEPH and 31 SMA pedigrees. We also describe the isolation of a new microsatellite marker at the D5S112 locus. The most likely order of markers (with recombination fractions given in parentheses) is 5cen-D5S6-(.02)-D5S125-(.04)-(JK53CA1/2,D5S11 2)-(.04)-D5S39-qter. The relative order of D5S6, D5S112, and D5S39 was confirmed by in situ hybridization. Multipoint linkage analysis in 31 SMA families indicates that the SMA locus lies in the 6-cM interval between D5S6 and JK53CA1/2, D5S112.