The tyrosinase-positive oculocutaneous albinism locus maps to chromosome 15q11.2-q12.

Tyrosinase-positive oculocutaneous albinism (ty-pos OCA), an autosomal recessive disorder of the melanin biosynthetic pathway, is the most common type of albinism occurring worldwide. In southern African Bantu-speaking negroids it has an overall prevalence of about 1/3,900. Since the basic biochemical defect is unknown, a linkage study with candidate loci, candidate chromosomal regions, and random loci was undertaken. The ty-pos OCA locus was found to be linked to two arbitrary loci, D15S10 and D15S13, in the Prader-Willi/Angelman chromosomal region on chromosome 15q11.2-q12. The pink-eyed dilute locus, p, on mouse chromosome 7, maps close to a region of homology on human chromosome 15q, and we postulate that the ty-pos OCA and p loci are homologous.     

An intragenic deletion of the P gene is the common mutation causing tyrosinase-positive oculocutaneous albinism in southern African Negroids.

Tyrosinase-positive oculocutaneous albinism (OCA2), an autosomal recessive disorder of the melanin biosynthetic pathway, is the most common recessive disorder occurring in southern African Bantu-speaking Negroids, with an overall prevalence of 1/3,900. The OCA2 gene, P, has been mapped to chromosome 15q11-q13, and recently alterations in the P gene have been identified in OCA2 individuals. An intragenic deletion has been described and proposed to be of African origin because of its occurrence in four unrelated African American OCA2 individuals and in two individuals, one from Zaire and the other from Cameroon. This study shows that the intragenic deletion is a common cause of OCA2 in southern African Negroids (114/146 [.78]; OCA2 chromosomes) and is associated with one common haplotype (43/55 [.78]; OCA2 chromosomes), confirming the African origin of this allele. On the basis of haplotype data, it would appear that at least seven additional, less frequent OCA2 mutations occur in this population.     

Rufous oculocutaneous albinism in southern African Blacks is caused by mutations in the TYRP1 gene.

Oculocutaneous albinism (OCA) is the most common autosomal recessive disorder among southern African Blacks. There are three forms that account for almost all OCA types in this region. Tyrosinase-positive OCA (OCA2), which is the most common, affects approximately 1/3,900 newborns and has a carrier frequency of approximately 1/33. It is caused by mutations in the P gene on chromosome 15. Brown OCA (BOCA) and rufous OCA (ROCA) account for the majority of the remaining phenotypes. The prevalence of BOCA is unknown, but for ROCA it is approximately 1/8,500. Linkage analysis performed on nine ROCA families showed that ROCA was linked to an intragenic marker at the TYRP1 locus (maximum LOD score = 3.80 at straight theta=.00). Mutation analysis of 19 unrelated ROCA individuals revealed a nonsense mutation at codon 166 (S166X) in 17 (45%) of 38 ROCA chromosomes, and a second mutation (368delA) was found in an additional 19 (50%) of 38 chromosomes; mutations were not identified in the remaining 2 ROCA chromosomes. In one family, two siblings with a phenotypically unclassified form of albinism were found to be compound heterozygotes for mutations (S166X/368delA) at the TYRP1 locus and were heterozygous for a common 2.7-kb deletion in the P gene. These findings have highlighted the influence of genetic background on phenotype, in which the genotype at one locus can be influenced by the genotype at a second locus, leading to a modified phenotype. ROCA, which in southern African Blacks is caused by mutations in the TYRP1 gene, therefore should be referred to as "OCA3," since this is the third locus that has been shown to cause an OCA phenotype in humans.     

In Southern Africa, brown oculocutaneous albinism (BOCA) maps to the OCA2 locus on chromosome 15q: P-gene mutations identified

In southern Africa, brown oculocutaneous albinism (BOCA) is a distinct pigmentation phenotype. In at least two cases, it has occurred in the same families as tyrosinase-positive oculocutaneous albinism (OCA2), suggesting that it may be allelic, despite the fact that this phenotype was attributed to mutations in the TYRP1 gene in an American individual of mixed ancestry. Linkage analysis in five families mapped the BOCA locus to the same region as the OCA2 locus (maximum LOD 3.07; theta=0 using a six-marker haplotype). Mutation analysis of the human homologue of the mouse pink-eyed dilution gene (P), in 10 unrelated individuals with BOCA revealed that 9 had one copy of the 2.7-kb deletion. No other mutations were identified. Additional haplotype studies, based on closely linked markers (telomere to centromere: D15S1048, D15S1019, D15S1533, P-gene 2.7-kb deletion, D15S219, and D15S156) revealed several BOCA-associated P haplotypes. These could be divided into two core haplotypes, suggesting that a limited number of P-gene mutations give rise to this phenotype.     

Exclusion of two candidate pigment loci,c and b,part of chromosome 11p,and 33 random polymorphic markers as the locus for tyrosinase-positive oculocutaneous albinism

The locus for Tyrosinase-Positive Oculocutaneous Albinism (ty-pos OCA) has not yet been localised. The search for the ty-pos OCA locus has included a search for linkage to candidate pigment loci and a candidate chromosomal region, as well as a random search using highly polymorphic markers in 42 families, including 271 individuals of whom 79 are affected. The lod scores for the tyrosinase (TYR) locus (11q14–q21), homologous to the albino locus, c, in the mouse and the CAS2/TRP1 locus (9p22-pter), homologous to the brown locus, b, in the mouse were -5.89 and -7.22, respectively, at a recombination fraction of =0.01, thus excluding them from being the ty-pos OCA locus. In the candidate chromosomal region, 11p, four loci (probes) were tested, SAA (pSAA82), CALC (pHC36), HBB (Gamma-globin haplotype) and an AC repeat polymorphism at the Wilm's Tumour locus (WT1). A portion of 11p was excluded with the following lod scores: pSAA82 lod=-2.05 at =0.10; pHC36 lod=-3.87 at =0.05; gamma-globin haplotype lod=-2.80 at =0.10; and WT1 lod=-2.34 at =0.10. Thirty-three polymorphic markers randomly distributed on 13 different chromosomes were all excluded from close linkage to ty-pos OCA.     

Fine mapping of autistic disorder to chromosome 15q11-q13 by use of phenotypic subtypes

Autistic disorder (AutD) is a complex genetic disease. Available evidence suggests that several genes contribute to the underlying genetic risk for the development of AutD. However, both etiologic heterogeneity and genetic heterogeneity confound the discovery of AutD-susceptibility genes. Chromosome 15q11-q13 has been identified as a strong candidate region on the basis of both the frequent occurrence of chromosomal abnormalities in that region and numerous suggestive linkage and association findings. Ordered-subset analysis (OSA) is a novel statistical method to identify a homogeneous subset of families that contribute to overall linkage at a given chromosomal location and thus to potentially help in the fine mapping and localization of the susceptibility gene within a chromosomal area. For the present analysis, a factor that represents insistence on sameness (IS)--derived from a principal-component factor analysis using data on 221 patients with AutD from the repetitive behaviors/stereotyped patterns domain in the Autism Diagnostic Interview-Revised--was used as a covariate in OSA. Analysis of families sharing high scores on the IS factor increased linkage evidence for the 15q11-q13 region, at the GABRB3 locus, from a LOD score of 1.45 to a LOD score of 4.71. These results narrow our region of interest on chromosome 15 to an area surrounding the gamma-aminobutyric acid-receptor subunit genes, in AutD, and support the hypothesis that the analysis of phenotypic homogeneous subtypes may be a powerful tool for the mapping of disease-susceptibility genes in complex traits.     

Autistic disorder and chromosome 15q11-q13: construction and analysis of a BAC/PAC contig

Autistic disorder (AD) is a neurodevelopmental disorder that affects approximately 2-10/10,000 individuals. Chromosome 15q11-q13 has been implicated in the genetic etiology of AD based on (1) cytogenetic abnormalities; (2) increased recombination frequency in this region in AD versus non-AD families; (3) suggested linkage with markers D15S156, D15S219, and D15S217; and (4) evidence for significant association with polymorphisms in the gamma-aminobutyric acid receptor subunit B3 gene (GABRB3). To isolate the putative 15q11-q13 candidate AD gene, a genomic contig and physical map of the approximately 1.2-Mb region from the GABA receptor gene cluster to the OCA2 locus was generated. Twenty-one bacterial artificial chromosome (BAC) clones, 32 P1-derived artificial chromosome (PAC) clones, and 2 P1 clones have been isolated using the markers D15S540, GABRB3, GABRA5, GABRG3, D15S822, and D15S217, as well as 34 novel markers developed from the end sequences of BAC/PAC clones. In contrast to previous findings, the markers D15S822 and D15S975 have been localized within the GABRG3 gene, which we have shown to be approximately 250 kb in size. NotI and numerous EagI restriction enzyme cut sites were identified in this region. The BAC/PAC genomic contig can be utilized for the study of genomic structure and the identification and characterization of genes and their methylation status in this autism candidate gene region on human chromosome 15q11-q13.     

The Friedreich ataxia gene is assigned to chromosome 9q13-q21 by mapping of tightly linked markers and shows linkage disequilibrium with D9S15.

Chamberlain et al. have assigned the gene for Friedreich ataxia (FA), a recessive neurodegenerative disorder, to chromosome 9, and have proposed a regional localization in the proximal short arm (9p22-cen), on the basis of linkage to D9S15 and to interferon-beta (IFNB), the latter being localized in 9p22. We confirmed more recently the close linkage to D9S15 in another set of families but found much looser linkage to IFNB. We also reported another closely linked marker, D9S5. Additional families have now been studied, and our updated lod scores are z = 14.30 at theta = .00 for D9S15-FA linkage and z = 6.30 at theta = .00 for D9S5-FA linkage. Together with the recent data of Chamberlain et al., this shows that D9S15 is very likely within 1 cM of the FA locus. We have found very significant linkage disequilibrium (delta Std = .28, chi 2 = 9.71, P less than .01) between FA and the D9S15 MspI RFLP in French families, which further supports the very close proximity of these two loci. No recombination between D9S5 and D9S15 was found in the FA families or Centre d'Etude du Polymorphisme Humain families (z = 9.30 at theta = .00). Thus D9S5, D9S15, and FA define a cluster of tightly linked loci. We have mapped D9S5 by in situ hybridization to 9q13-q21, and, accordingly, we assign the D9S5, D9S15, and FA cluster to the proximal part of chromosome 9 long arm, close to the heterochromatic region.     

Three different frameshift mutations of the tyrosinase gene in type IA oculocutaneous albinism.

Mutations in the gene for the pigment-producing enzyme tyrosinase are responsible for type IA (tyrosinase-negative) oculocutaneous albinism (OCA). Most reported mutations have been single base substitutions. We now report three different frameshift mutations in three unrelated individuals with type IA OCA. The first individual has a single base deletion within a series of five guanidines, resulting in a premature stop codon in exon I on one allele and a missense mutation at codon 382 in exon III on the homologous allele. The second individual is a genetic compound of two separate frameshift mutations, including both the same exon I single base deletion found in the first individual and a deletion of a thymidine-guanidine pair, within the sequence GTGTG, forming a termination codon (TAG) in exon I on the homologous allele. The third individual has a single base insertion in exon I on one allele and a missense mutation at codon 373 in exon III on the homologous allele. The two missense mutations occur within the copper Bbinding region and may interfere with either copper binding to the enzyme or oxygen binding to the copper. These five different mutations disrupt tyrosinase function and are associated with a total lack of melanin biosynthesis.     

The Marfan syndrome locus: confirmation of assignment to chromosome 15 and identification of tightly linked markers at 15q15-q21.3

The Marfan syndrome is a common autosomal dominant disorder of connective tissue. Despite many years of intensive investigation, the primary genetic defect has not yet been identified. Reverse genetic methods, targeted at mapping this disease gene, have resulted in an initial report of linkage of the genetic locus for the Marfan phenotype in Finnish families to two polymorphic markers on chromosome 15. We have investigated four large multiplex American families with classic Marfan syndrome using standard genetic linkage methods. Our data confirm the assignment of the Marfan syndrome gene to chromosome 15, but establish a more centromeric location (defined by markers D15S25 and D15S1) as the most probable site for the genetic defect (lod score = 12.1, theta = 0.00). These data should facilitate identification and characterization of the Marfan syndrome gene and, in selected families, have immediate application to diagnosis of equivocal cases or prenatal counseling.     

Hereditary geniospasm: linkage to chromosome 9q13-q21 and evidence for genetic heterogeneity.

Hereditary geniospasm is an unusual movement disorder causing episodes of involuntary tremor of the chin and the lower lip. Episodes typically start in early childhood and may be precipitated by stress, concentration, and emotion. Hereditary geniospasm is inherited as an autosomal dominant trait, and its cause is not known. We report the results of a genomewide genetic linkage study in a four-generation British family with hereditary geniospasm. Positive two-point LOD scores were obtained for 15 microsatellite markers on the peri-centromeric region of chromosome 9. A maximum two-point LOD score of 5.24 at theta = .00 was obtained for the marker D9S1837. Construction of haplotypes defined an interval of 2.1 cM between the flanking markers D9S1806 and D9S175, thus assigning one locus for hereditary geniospasm to the proximal long arm of chromosome 9q13-q21. Hereditary geniospasm in a second British family is not linked to this region, indicating genetic heterogeneity. These findings may have implications for other inherited focal movement disorders that as yet remain unmapped.     

Fine mapping of the diabetes-susceptibility locus, IDDM4, on chromosome 11q13.

Genomewide linkage studies of type 1 diabetes (or insulin-dependent diabetes mellitus [IDDM]) indicate that several unlinked susceptibility loci can explain the clustering of the disease in families. One such locus has been mapped to chromosome 11q13 (IDDM4). In the present report we have analyzed 707 affected sib pairs, obtaining a peak multipoint maximum LOD score (MLS) of 2.7 (lambda(s)=1.09) with linkage (MLS>=0.7) extending over a 15-cM region. The problem is, therefore, to fine map the locus to permit structural analysis of positional candidate genes. In a two-stage approach, we first scanned the 15-cM linked region for increased or decreased transmission, from heterozygous parents to affected siblings in 340 families, of the three most common alleles of each of 12 microsatellite loci. One of the 36 alleles showed decreased transmission (50% expected, 45.1% observed [P=.02, corrected P=.72]) at marker D11S1917. Analysis of an additional 1,702 families provided further support for negative transmission (48%) of D11S1917 allele 3 to affected offspring and positive transmission (55%) to unaffected siblings (test of heterogeneity P=3x10-4, corrected P=. 01]). A second polymorphic marker, H0570polyA, was isolated from a cosmid clone containing D11S1917, and genotyping of 2,042 families revealed strong linkage disequilibrium between the two markers (15 kb apart), with a specific haplotype, D11S1917*03-H0570polyA*02, showing decreased transmission (46.4%) to affected offspring and increased transmission (56.6%) to unaffected siblings (test of heterogeneity P=1.5x10-6, corrected P=4.3x10-4). These results not only provide sufficient justification for analysis of the gene content of the D11S1917 region for positional candidates but also show that, in the mapping of genes for common multifactorial diseases, analysis of both affected and unaffected siblings is of value and that both predisposing and nonpredisposing alleles should be anticipated.     

Duplication within chromosome region 15q11-q13 in a patient with similarities to Prader-Willi syndrome confirmed by region-specific and band-specific fish.

We report on a patient presenting with mental retardation and obesity and a proximal duplication of chromosome 15. The patient shared some clinical signs with Prader-Willi syndrome. With a region-specific paint, generated by microdissection, a duplication in region 15q11.2-q13 was shown to be present. Subsequently, FISH with probes localized to chromosome region 15q11.2-q12 and microsatellite analysis was used to characterize this chromosome aberration further and an insertion duplication within the region frequently deleted in Prader-Willi and Angelman syndrome was demonstrated.     

A gene for autosomal dominant hypohidrotic ectodermal dysplasia (EDA3) maps to chromosome 2q11-q13.

Autosomal dominant hypohidrotic ectodermal dysplasia (ADHED) is a disorder characterized by fine, slow-growing scalp and body hair, sparse eyebrows and eyelashes, decreased sweating, hypodontia, and nail anomalies. By genetic linkage analysis of a large ADHED kindred, we have mapped a gene for ADHED (EDA3) to the proximal long arm of chromosome 2 (q11-q13). Obligate recombinations localize EDA3 to an approximately 9-cM interval between D2S1321 and D2S308, with no apparent recombinations with markers D2S1343, D2S436, D2S293, D2S1894, D2S1784, D2S1890, D2S274, and CHLC.GAAT11C03.     

A new susceptibility locus for migraine with aura in the 15q11-q13 genomic region containing three GABA-A receptor genes

Migraine is the most common type of chronic episodic headache. Several population-based family studies have suggested a strong genetic predisposition to migraine, especially migraine with aura (MA). Although several susceptibility loci have been identified, none of the numerous studies performed to date have led to the identification of a gene responsible for the more common forms of migraine. GABA-A receptors and their modulator sites seem to be involved in the pathophysiological events that underlie migraine. We report on clinical and molecular data from a total of 10 families with MA, in which MA segregates as an autosomal dominant trait and presents with homogeneous clinical features. After excluding linkage with the known candidate loci, we used a functional candidate approach and genotyped these families with markers from the 15q11-q13 genomic region, which contains the genes encoding GABA-A receptor subunits. Evidence of linkage was obtained with a parametric two-point linkage analysis (maximum LOD score of 5.56 at a recombination fraction of 0.001 for marker GABRB3) and was supported by multipoint analysis (maximum LOD score of 6.54 between markers D15S113 and D15S1019). The critical region spanned 3.6 Mb. These results provide the basis for further investigation of the hypothesized relationship between a GABA-A receptor dysfunction and migraine.     

High-resolution mapping of the gamma-aminobutyric acid receptor subunit beta 3 and alpha 5 gene cluster on chromosome 15q11-q13, and localization of breakpoints in two Angelman syndrome patients.

The gamma-aminobutyric acid (GABAA) receptors are a family of ligand-gated chloride channels constituting the major inhibitory neurotransmitter receptors in the nervous system. In order to determine the genomic organization of the GABAA receptor beta 3 subunit gene (GABRB3) and alpha 5 subunit gene (GABRA5) in chromosome 15q11-q13, we have constructed a high-resolution physical map using the combined techniques of field-inversion gel electrophoresis and phage genomic library screening. This map, which covers nearly 1.0 Mb, shows that GABRB3 and GABRA5 are separated by less than 100 kb and are arranged in a head-to-head configuration. GABRB3 encompasses approximately 250 kb, while GABRA5 is contained within 70 kb. This difference in size is due in large part to an intron of 150 kb within GABRB3. We have also identified seven putative CpG islands within a 600-kb interval. Chromosomal rearrangement breakpoints--in one Angelman syndrome (AS) patient with an unbalanced translocation and in another patient with a submicroscopic deletion--are located within the large GABRB3 intron. These findings will facilitate chromosomal walking strategies for cloning the regions disrupted by the DNA rearrangements in these AS patients and will be valuable for mapping new genes to the AS chromosomal region.     

Assignment of the gene for neuroendocrine protein 7B2 (SGNE1 locus) to mouse chromosome region 2[E3-F3] and to human chromosome region 15q11-q15

The gene for 7B2, a protein found in the secretory granules of neural and endocrine cells (gene symbol SGNE1) was localized to the E3-F3 region of mouse chromosome 2 and to the q11-q15 region of human chromosome 15. This was determined by in situ hybridization, using a mouse 7B2 cDNA and an intronic fragment of the corresponding human gene as probes. The respective locations of SGNE1 in the two species correlate with the conservation of loci between these subregions of mouse chromosome 2 and human chromosome 15. Clinically, the human SGNE1 DNA fragment may serve as a molecular probe of this locus in both the Prader-Willi and the Angelman syndromes, which are often accompanied by submicroscopic chromosomal deletions in the 15q11-15q13 region.