Trait components provide tools to dissect the genetic susceptibility of migraine

The commonly used "end diagnosis" phenotype that is adopted in linkage and association studies of complex traits is likely to represent an oversimplified model of the genetic background of a disease. This is also likely to be the case for common types of migraine, for which no convincingly associated genetic variants have been reported. In headache disorders, most genetic studies have used end diagnoses of the International Headache Society (IHS) classification as phenotypes. Here, we introduce an alternative strategy; we use trait components--individual clinical symptoms of migraine--to determine affection status in genomewide linkage analyses of migraine-affected families. We identified linkage between several traits and markers on chromosome 4q24 (highest LOD score under locus heterogeneity [HLOD] 4.52), a locus we previously reported to be linked to the end diagnosis migraine with aura. The pulsation trait identified a novel locus on 17p13 (HLOD 4.65). Additionally, a trait combination phenotype (IHS full criteria) revealed a locus on 18q12 (HLOD 3.29), and the age at onset trait revealed a locus on 4q28 (HLOD 2.99). Furthermore, suggestive or nearly suggestive evidence of linkage to four additional loci was observed with the traits phonophobia (10q22) and aggravation by physical exercise (12q21, 15q14, and Xp21), and, interestingly, these loci have been linked to migraine in previous studies. Our findings suggest that the use of symptom components of migraine instead of the end diagnosis provides a useful tool in stratifying the sample for genetic studies.     

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.     

An extension of the admixture test for the study of genetic heterogeneity in hereditary multiple exostoses

Hereditary multiple exostoses (EXT) is an autosomal dominant disorder characterized by the presence of multiple cartilage-capped exostoses in the juxta-epiphyseal regions of the long bones. EXT is heterogeneous with at least three different locations currently having been identified on chromosomes 8, 11 and 19. We have tested a series of 29 EXT families for possible linkage to the three disease loci and estimated the probability of linkage of the disease to each locus in our series, by using an extension of the admixture test, which makes modelling of heterogeneous monogenic disease feasible. The maximum likelihood was obtained for proportions of 44%, 28% and 28% of families being linked to chromosome 8, 11 and 19, respectively. The a posteriori probability of linkage of the disease to EXT1, EXT2 and EXT3 was greater than 80% for 8/29, 5/29 and 3/29 families, respectively, and did not give evidence of a fourth locus for the disease. The present approach can be generalized to the investigation of genetic heterogeneity in other monogenic diseases, as it simultaneously estimates the location of each disease gene and the proportion of families linked to each locus. Received: 28 May 1996 / Revised: 7 October 1996     

Autosomal dominant ataxia: Genetic evidence for locus heterogeneity from a cuban founder-effect population

The locus for autosomal dominant ataxia with a diagnosis of olivo-ponto-cerebellar atrophy at autopsy has been previously assigned to chromosome 6p. However, evidence for two alternative locations has been reported. We have recently described a large potential founder-effect population of such patients in the Holguin province of Cuba. With an estimated 1,000 patients available for analysis, this extensive cluster of families provides a unique opportunity for the definitive localization of the genetic mutation. Linkage analysis between the disease locus in this population and markers within and flanking the HLA region on chromosome 6 were undertaken in 12 families comprising over 100 affected individuals. Despite similarity in the clinical phenotype between those families where the disease locus has been reported to be linked to the HLA locus and the Cuban patients, no evidence of linkage to this region could be demonstrated in the latter. The disease locus was excluded from a 96-cM genetic interval of the short arm of chromosome 6, encompassing the F13A1-HLA-GLO1-MUT/D6S4 loci. These data strongly support the existence of genetic heterogeneity for the disease.     

Homozygosity mapping identifies an additional locus for Wolfram syndrome on chromosome 4q

Wolfram syndrome, which is sometimes referred to as "DIDMOAD" (diabetes insipidus, diabetes mellitus, optic atrophy, and deafness), is an autosomal recessive neurodegenerative disorder for which only insulin-dependent diabetes mellitus and optic atrophy are necessary to make the diagnosis. Researchers have mapped Wolfram syndrome to chromosome 4p16.1, and, recently, a gene encoding a putative transmembrane protein has been cloned and mutations have been identified in patients. To pursue the possibility of locus heterogeneity, 16 patients from four different families were recruited. These patients, who have the Wolfram syndrome phenotype, also have additional features that have not previously been reported. There is an absence of diabetes insipidus in all affected family members. In addition, several patients have profound upper gastrointestinal ulceration and bleeding. With the use of three microsatellite markers (D4S432, D4S3023, and D4S2366) reported to be linked to the chromosome 4p16.1 locus, we significantly excluded linkage in three of the four families. The two affected individuals in one family showed homozygosity for all three markers from the region of linkage on chromosome 4p16.1. For the other three families, genetic heterogeneity for Wolfram syndrome was verified by demonstration of linkage to chromosome 4q22-24. In conclusion, we report the unique clinical findings and linkage-analysis results of 16 patients with Wolfram syndrome and provide further evidence for the genetic heterogeneity of this disorder. We also provide data on a new locus that plays a role in the etiology of insulin-dependent diabetes mellitus.     

Periodic catatonia: confirmation of linkage to chromosome 15 and further evidence for genetic heterogeneity

We earlier reported significant evidence for linkage on chromosome 15q15 in periodic catatonia, a sub-phenotype of schizophrenic psychoses. The disorder is characterized by qualitative hyperkinetic and akinetic psychomotor disturbances through acute psychotic episodes and debilitating symptoms in the long term, with psychomotor weakness, grimacing facial movements and apathy. Here, we confirm mapping of a major gene locus on chromosome 15q15 in a second genome scan in a new set of four multiplex families. Non-parametric multipoint linkage analyses identified a broad region with a maximum peak of Z(all) =3.91 ( P=0.006) and Z(lr) =3.04 at D15S1234 ( P=0.001), satisfying conventional criteria for confirmed linkage. Parametric affected-only analyses under an autosomal dominant model gave a maximum HLOD score of 1.65 (D15S1234) with an estimated 47% of families being linked. Analysis of individual families showed that one large family showed linkage, whereas two others could be clearly excluded, confirming genetic heterogeneity. No other locus reached suggestive levels of significance. Haplotype analysis on chromosome 15 in this and previously linked families placed the susceptibility region to a 11-cM interval between marker D15S1042 and D15S659. Periodic catatonia is the first sub-phenotype of schizophrenic psychoses with confirmed linkage despite the existence of considerable genetic heterogeneity.     

Paget disease of bone: mapping of two loci at 5q35-qter and 5q31

Paget disease of bone is characterized by focal increases of the bone-remodeling process. It is the second most common metabolic bone disease after osteoporosis. Genetic factors play a major role in the etiology of Paget disease of bone, and two loci have been mapped for the disorder: PDB1 and PDB2. The gene(s) causing the typical form of the disorder remains to be characterized. To decipher the molecular basis of Paget disease of bone, we performed genetic linkage analysis in 24 large French Canadian families (479 individuals) in which the disorder was segregating as an autosomal dominant trait. After exclusion of PDB2, a genomewide scan was performed on the three most informative family nuclei. LOD scores >1.0 were observed at seven locations. The 24 families were then used to detect strong evidence for linkage to chromosome 5q35-qter. Under heterogeneity, a maximum LOD score of 8.58 was obtained at D5S2073, at straight theta= .1. The same characteristic haplotype was carried by all patients in eight families, suggesting a founder effect. A recombination event in a key family confined the disease region within a 6-cM interval between D5S469 and the telomere. The 16 other families, with very low conditional probability of linkage to 5q35-qter, were further used, to map a second locus at 5q31. Under heterogeneity, a maximum LOD score of 3.70 was detected at D5S500 with straight theta=.00. Recombination events refined the 5q31 region within 12.2 cM, between D5S642 and D5S1972. These observations demonstrate the mapping of two novel loci for Paget disease of bone and provide further evidence for genetic heterogeneity of this highly prevalent disorder. It is proposed that the 5q35-qter and 5q31 loci be named "PDB3" and "PDB4," respectively.     

Localization of one gene for tuberous sclerosis within 9q32-9q34, and further evidence for heterogeneity.

Tuberous sclerosis (TSC) is an autosomal dominant disorder with both neurological and cutaneous manifestations often resulting in significant disability. Although it has been studied clinically and biochemically for many years, the underlying pathophysiology remains unknown. Genetic linkage analysis provides an alternative strategy for understanding the genetic etiology of this disease. Genetic linkage of a gene for TSC to loci in 9q32-9q34 has been reported but has not been a universal finding, since absence of linkage to 9q loci, as well as linkage to loci on 11q, have also been reported. We present here data on 22 families (21 previously unreported) segregating TSC. Our results strongly support a TSC locus in the 9q32-34 region for approximately one-third of families and provide significant evidence for genetic heterogeneity. Application of newly described highly polymorphic dinucleotide repeat marker loci in TSC greatly enhanced the informativeness of our pedigrees and was vital for detecting the heterogeneity. No clear evidence of linkage to chromosome 11q22 markers was found, suggesting that a still unidentified TSC locus elsewhere in the genome may account for the majority of TSC families.     

A study of genetic linkage heterogeneity in adult polycystic kidney disease

Summary The mutation for adult polycystic kidney disease (APKD) has previously been localised to chromosome 16 by the demonstration of genetic linkage with the loci for the alpha-chain of haemoglobin and phosphoglycolate phosphatase. These studies were carried out, however, on only nine families so that the possibility remained that mutations at other genetic loci might produce the disease. Such genetic heterogeneity of linkage would invalidate the general use of chromosome 16 markers for the purposes of detection of the disease, and complicate the characterisation of APKD at the molecular level. Therefore further families were studied to address this question. A total of 28 northern European pedigrees were analysed, all apparently unrelated, and with origins in England, Scotland, Holland and eastern Finland. No evidence was found to suggest heterogeneity of genetic linkage between alpha-globin and the APKD locus in this population.     

Evaluating genetic heterogeneity in complex disorders

OBJECTIVES: The Admixture test is routinely used in linkage analysis to take account of genetic heterogeneity, and yields an estimate of the proportion of families (alpha) segregating the linked disease gene. In complex disorders, the assumptions of the Admixture test are violated. We therefore explore how the estimate of alpha relates to the true proportion of linked families with a complex disorder in a population or dataset. METHODS: We simulated a two-locus heterogeneity model and varied genetic parameters, ascertainment scheme and phenocopy frequency. RESULTS: In this model, alpha is almost always overestimated, by as little as 5% to as much as 60%. The bias is largely attributable to (1). intrafamilial heterogeneity arising from ascertainment of families with many affected members or from analysis of dense pedigrees; (2). low informativeness, which occurs in the presence of reduced penetrance; and (3). differences in the evidence for linkage in linked and unlinked families. This bias is also affected by the analysis phenocopy frequency, but only if the linked locus is dominant and the unlinked locus is recessive. CONCLUSIONS: We conclude that, in complex diseases, the Admixture test has greater value in detecting linkage than in estimating the proportion of linked families in a dataset.     

Genetic heterogeneity in adult dominant polycystic kidney disease in Cypriot families

Polycystic kidney disease is an inherited heterogeneous disorder that affects approximately 11000 Europeans. It is characterized mainly by the formation of cysts in the kidney that lead to end-stage renal failure with late age of onset. Three loci have been identified, PKD1 on the short arm of chromosome 16, which has recently been isolated and characterized, PKD2 on the long arm of chromosome 4, and a third locus of unknown location, that is apparently much rarer. In families that transmit the PKD2 gene there is a significantly later age of onset of symptoms, compared with families that transmit the PKD1 gene, and in general they present with milder progression of symptomatology. For the first time we attempted molecular genetic analysis in seven Cypriot families using highly polymorphic markers around the PKD1 and PKD2 genes. Our data showed that there is genetic and phenotypic heterogeneity among these families. For four of the families we obtained strong evidence for linkage to the PKD1 locus. In two of these families linkage to PKD1 was strengthened by excluding linkage to PKD2 with the use of marker D4S423. In three other families we showed linkage to the PKD2 locus. In the largest of these families one recombinant placed marker D4S1534 distal to D4S231, thereby rendering it the closest proximal marker known to us to date. The application of molecular methods allowed us to make presymptomatic diagnosis for a number of at-risk individuals.     

Detecting linkage for genetically heterogeneous diseases and detecting heterogeneity with linkage data.

Interest in searching for genetic linkage between diseases and marker loci has been greatly increased by the recent introduction of DNA polymorphisms. However, even for the most well-behaved Mendelian disorders, those with clear-cut mode of inheritance, complete penetrance, and no phenocopies, genetic heterogeneity may exist; that is, in the population there may be more than one locus that can determine the disease, and these loci may not be linked. In such cases, two questions arise: (1) What sample size is necessary to detect linkage for a genetically heterogeneous disease? (2) What sample size is necessary to detect heterogeneity given linkage between a disease and a marker locus? We have answered these questions for the most important types of matings under specified conditions: linkage phase known or unknown, number of alleles involved in the cross at the marker locus, and different numbers of affected and unaffected children. In general, the presence of heterogeneity increases the recombination value at which lod scores peak, by an amount that increases with the degree of heterogeneity. There is a corresponding increase in the number of families necessary to establish linkage. For the specific case of backcrosses between disease and marker loci with two alleles, linkage can be detected at recombination fractions up to 20% with reasonable numbers of families, even if only half the families carry the disease locus linked to the marker. The task is easier if more than two informative children are available or if phase is known. For recessive diseases, highly polymorphic markers with four different alleles in the parents greatly reduce the number of families required.     

Close linkage of probe p212 (DXS178) to X-linked agammaglobulinemia

Summary Segregation analysis was performed in three families affected in X-linked agammaglobulinemia (XLA) with five polymorphic DNA probes linked to the disease locus. In agreement with previous studies, no recombination was observed with either pXG12 (DXS94) or S21 (DXS17). Segregation analysis was also performed with a marker, p212 (DXS178), which has been shown to be closely linked to pXG12 in normal families. No cross-over with XLA was observed in these three families and in five additional families previously analyzed with DXS17 and DXS94 (z = 5.92 at = 0). These data provide evidence against genetic heterogeneity in XLA and indicate the value of probe p212 for carrier detection and prenatal diagnosis of XLA. We were able to estimate the carrier status of six females (out of six) in the three previously unreported families.     

Genetic heterogeneity in tuberous sclerosis

Tuberous sclerosis (TSC) is an autosomal dominant disorder characterized by widespread hamartosis. Preliminary evidence of linkage between the TSC locus and markers on chromosome 9q34 was established, but subsequently disputed. More recently, a putative TSC locus on chromosome 11 has been suggested and genetic heterogeneity seems likely. Here we describe an approach combining multipoint linkage analysis and heterogeneity tests that has enabled us to obtain significant evidence for locus heterogeneity after studying a relatively small number of families. Our results support a model with two different loci independently causing the disease. One locus (TSC1) maps in the vicinity of the Abelson oncogene at 9q34 and a second locus (TSC2) maps in the region of the anonymous DNA marker Lam L7 and the dopamine D2 receptor gene at 11q23.     

Linkage of hereditary motor and sensory neuropathy type I to the pericentromeric region of chromosome 17.

Vance et al. have reported linkage of hereditary motor and sensory neuropathy type I (HMSN I) to the pericentromeric region of chromosome 17. We have studied eight families with HMSN I (also called the hypertrophic form of Charcot-Marie-Tooth disease) for linkage of the disease locus to polymorphic loci in the centromeric region of chromosome 17. Linkage has been confirmed for D17S58 (EW301) with a maximum lod score of 5.89 at theta = 0.08 and for D17S71 (pA10-41) with a maximum lod score of 3.22 at theta = 0.08. EW301 is on 17p, 5.5 centimorgans from the centromere. Two families, previously reported as being linked to the Duffy blood group locus on chromosome 1, were included in this study, and one now provides positive lod scores for chromosome 17 markers. There was no evidence of heterogeneity.     

Genetic analysis of new French X-linked juvenile retinoschisis kindreds using microsatellite markers closely linked to the RS locus: further narrowing of the RS candidate region

The gene involved in juvenile retinoschisis (RS) has previously been localized, by genetic linkage analyses, to Xp22.1-p22.2, between DXS274 and DXS43/ DXS207; it is closely linked to the latter markers. From our recent data, this interval represents a genetic distance of approximately 10 cM. In the present study, we have studied 14 French families with X-linked juvenile RS by using four CA polymorphisms that are closely linked to the RS locus and that have recently been included in an Xp22.1-p22.2 high-resolution map. Complete cosegregation with the disease locus was observed for three of them, DXS207, DXS418, and DXS999, which further confirms the locus homogeneity for RS and the close linkage to this region. One recombinant was found with the most proximal marker, AFM291wf5, thereby defining this marker as the new proximal boundary of the candidate region for RS. Under the assumption that DXS207 and DXS43 constitute the distal boundary, the present study further reduces the region containing the disease gene to a interval of 3–4 cM. The results reported here should facilitate the eventual cloning of the RS gene.     

Detection of linkage for heterogeneous disorders by using multipoint linkage analysis.

We have compared the efficiency of the lod score test which assumes heterogeneity (lod2) to the standard lod score test which assumes homogeneity (lod1) when three-point linkage analysis is used in successive map intervals. If it is assumed that a gene located midway between two linked marker loci is responsible for a proportion of disease cases, then the lod1 test loses power relative to the lod2 test, as the proportion of linked families decreases, as the flanking markers are more closely linked, and as more map intervals are tested. Moreover, when multipoint analysis is used, linkage for a disease gene is more likely to be incorrectly excluded from a complete and dense linkage map if true genetic heterogeneity is ignored. We thus conclude that, in general, the lod2 linkage test is more efficient for detecting a true linkage when a complete genetic marker map is screened for a heterogeneous disorder.     

A gene for Usher syndrome type I (USH1A) maps to chromosome 14q.

Usher syndrome (US) is an autosomal recessive disease characterized by congenital hearing impairment and retinitis pigmentosa. It is the most frequent cause of deaf-blindness in adults and accounts for 3 to 6% of deaf children. Here, we report the genetic mapping of a gene for US type I (USH1A), the most severe form of the disease, to the long arm of chromosome 14, by linkage to probe MLJ14 at the D14S13 locus in 10 families of Western France ancestry (Z = 4.13 at theta = 0). Among them, 8 families originated from a small area of the Poitou-Charentes region (Z = 3.78 at theta = 0), suggesting that a founder effect could be involved. However, since not all US type I families were found to be linked to this locus, the present study provides evidence for genetic heterogeneity of this condition (heterogeneity versus homogeneity test HOMOG, P < 0.05; heterogeneity versus no linkage, P < 0.01).     

Linkage analysis identifies a novel locus for restless legs syndrome on chromosome 2q in a South Tyrolean population isolate

Restless legs syndrome (RLS) is a common neurological condition with three loci (12q, 14q, and 9p) described so far, although none of these genes has yet been identified. We report a genomewide linkage scan of patients with RLS (n=37) assessed in a population isolate (n=530) of South Tyrol (Italy). Using both nonparametric and parametric analyses, we initially obtained suggestive evidence of a novel locus on chromosome 2q, with nominal evidence of linkage on chromosomes 5p and 17p. Follow-up genotyping yielded significant evidence of linkage (nonparametric LOD score 5.5, P相似文献   

Inter- and intrafamilial heterogeneity: effective sampling strategies and comparison of analysis methods.

Heterogeneity, both inter- and intrafamilial, represents a serious problem in linkage studies of common complex diseases. In this study we simulated different scenarios with families who phenotypically have identical diseases but who genotypically have two different forms of the disease (both forms genetic). We examined the proportion of families displaying intrafamilial heterogeneity, as a function of mode of inheritance, gene frequency, penetrance, and sampling strategies. Furthermore, we compared two different ways of analyzing linkage in these data sets: a two-locus (2L) analysis versus a one-locus (SL) analysis combined with an admixture test. Data were simulated with tight linkage between one disease locus and a marker locus; the other disease locus was not linked to a marker. Our findings are as follows: (1) In contrast to what has been proposed elsewhere to minimize heterogeneity, sampling only "high-density" pedigrees will increase the proportion of families with intrafamilial heterogeneity, especially when the two forms are relatively close in frequency. (2) When one form is dominant and one is recessive, this sampling strategy will greatly decrease the proportions of families with a recessive form and may therefore make it more difficult to detect linkage to the recessive form. (3) An SL analysis combined with an admixture test achieves about the same lod scores and estimate of the recombination fraction as does a 2L analysis. Also, a 2L analysis of a sample of families with intrafamilial heterogeneity does not perform significantly better than an SL analysis. (4) Bilineal pedigrees have little effect on the mean maximum lod score and mean maximum recombination fraction, and therefore there is little danger that including these families will lead to a false exclusion of linkage.