Candidate gene discovery procedure after follow-up confirmatory analyses of candidate regions of interests for Alzheimer's disease in the NIMH sibling dataset

The objective of this research was to develop a procedure to identify candidate genes under linkage peaks confirmed in a follow-up of candidate regions of interests (CRIs) identified in our original genome scan in the NIMH Alzheimer's diseases (AD) Initiative families (Blacker et al. [1]). There were six CRIs identified that met the threshold of multipoint lod score (MLS) of >or= 2.0 from the original scan. The most significant peak (MLS = 7.7) was at 19q13, which was attributed to APOE. The remaining CRIs with 'suggestive' evidence for linkage were identified at 9q22, 6q27, 14q22, 11q25, and 3p26. We have followed up and narrowed the 9q22 CRI signal using simple tandem repeat (STR) markers (Perry et al. [2]). In this confirmatory project, we have followed up the 6q27, 14q22, 11q25, and 3p26 CRIs with a total of 24 additional flanking STRs, reducing the mean interval marker distance (MID) in each CRI, and substantially increase in the information content (IC). The linkage signals at 6q27, 14q22 and 11q25 remain 'suggestive', indicating that these CRIs are promising and worthy of detailed fine mapping and assessment of candidate genes associated with AD. We have developed a bioinformatics approach for identifying candidate genes in these confirmed regions based on the Gene Ontology terms that are annotated and enriched among the systematic meta-analyzed genes, confirmed by at least three case-control samples, and cataloged in the "AlzGene database" as potential Alzheimer disease susceptibility genes (http://www.alzgene.org).     

Allelic association but only weak evidence for linkage to the apolipoprotein E locus in late-onset Swedish Alzheimer families

An association between the ϵ4 allele of the apolipoprotein E gene (APOE) and late-onset Alzheimer's disease (AD) was recently demonstrated. In order to confirm the association and to gauge the ability of standard genetic linkage methods to identify susceptibility genes, we investigated 15 Swedish late-onset AD families. We found an association of familial AD to the APOE ϵ4 allele (P = 0.01) but no indication of linkage to the APOE region using 2-point linkage analysis, and only weak evidence using the affected pedigree-member (APM) method. Our results confirm an APOE ϵ4 association with late-onset familial AD and indicate that susceptibility genes can easily be missed when using standard lod score and APM genetic linkage analysis. © 1996 Wiley-Liss, Inc.     

Follow-up mapping supports the evidence for linkage in the candidate region at 9q22 in the NIMH Alzheimer's disease Genetics Initiative cohort.

Other than the APOE peak at 19q13, the 9q22 region was identified in our original genomic scan as the candidate region with the highest multipoint lod score (MLS) in the subset of late onset Alzheimer's Disease (AD) families (MLS = 2.9 at 101 cM) from the NIMH Genetics Initiative sample. We have now genotyped an additional 12 short tandem repeats (STR) in this region. Multipoint analysis shows the region remains significant with an increase in the peak MLS from 2.9 to 3.8 at 95 cM near marker D9S1815, and the 1 LOD interval narrows from 21.5 to 11 cM. HLOD scores also provide evidence for significant linkage (4.5 with an alpha = 31%) with a further narrowing of the region to 6.6 cM (92.2-98.8 cM). Single nucleotide polymorphisms (SNPs) in the Ubiquilin1 gene (UBQLN1), located at 83.3 cM, have been reported to be significantly associated to AD, accounting for a substantial portion of the original linkage signal [Bertram et al., 2005]. Our analyses of the higher resolution genotype data generated here provide further support for the existence of a least one additional locus on chromosome 9q22. In an effort to pinpoint this putative AD susceptibility gene, we have begun to analyze SNPs in other candidate genes in and around this narrowed region to test for additional associations to AD.     

Genome screen for loci influencing age at onset and rate of decline in late onset Alzheimer's disease.

We performed an affected sib-pair (ASP) linkage analysis to test for the effects of age at onset (AAO), rate of decline (ROD), and Apolipoprotein E (APOE) genotype on linkage to late-onset Alzheimer's disease (AD) in a sample comprising 428 sib-pairs. We observed linkage of mean AAO to chromosome 21 in the whole sample (max LOD = 2.57). This came entirely from the NIMH sample (max LOD = 3.62), and was strongest in pairs with high mean AAO (>80). A similar effect was observed on chromosome 2q in the NIMH sample (max LOD = 2.73); this region was not typed in the IADC/UK sample. Suggestive evidence was observed in the combined sample of linkage of AAO difference to chromosome 19q (max LOD = 2.33) in the vicinity of APOE and 12p (max LOD = 2.22), with linkage strongest in sib-pairs with similar AAO. Mean ROD showed suggestive evidence of linkage to chromosome 9q in the whole sample (max LOD = 2.29), with the effect strongest in the NIMH sample (max LOD = 3.58), and in pairs with high mean ROD. Additional suggestive evidence was also observed in the NIMH sample with AAO difference on chromosome 6p (max LOD = 2.44) and 15p (max LOD = 1.87), with linkage strongest in pairs with similar AAO, and in the UK sample with mean ROD on chromosome 1p (max LOD = 2.73, linkage strongest in pairs with high mean ROD). We also observed suggestive evidence of increased identical by descent (IBD) in APOE epsilon4 homozygotes on chromosome 1 (max LOD = 3.08) and chromosome 9 (max LOD = 3.34). The previously reported genome-wide linkage of AD to chromosome 10 was not influenced by any of the covariates studied.     

Susceptibility loci for atopic dermatitis on chromosome 21 in a Swedish population

BACKGROUND: Atopic dermatitis (AD) is a hereditary, pruritic, chronic, relapsing, inflammatory skin disease resulting from multiplex interactions between genes and environmental factors. We have previously found several loci showing suggestive linkage on chromosomes 3q14, 13q14, 15q14-15 and 17q21, and weaker linkage to chromosomes 1p32, 4q24-26 and 21q21 in 109 Swedish families. METHODS: In order to confirm the linkage to chromosome 21, we carried out a replication linkage analysis with additional microsatellite markers on chromosome 21 in another set of 295 families. RESULTS: In the extended material, the Z-score was 2.40 (P < 7.4 x 10(-4)) in the region 21q21 for a semi-quantitative variable measurement; the severity of AD. When combining the two data sets into 404 families and stratifying according to asthma status, suggestive linkage was found only in the group of AD patients who also had asthma (Z-score 2.45, P < 7.4 x 10(-4) and 2.69, P < 7.4 x 10(-4)) in two different regions. CONCLUSIONS: Our results suggest that 21q21 could contain a susceptibility gene modulating the severity of AD especially in combination with asthma.     

Genome-wide association and linkage study in the amish detects a novel candidate late-onset Alzheimer disease gene

To identify novel late-onset Alzheimer disease (LOAD) risk genes, we have analysed Amish populations of Ohio and Indiana. We performed genome-wide SNP linkage and association studies on 798 individuals (109 with LOAD). We tested association using the Modified Quasi-Likelihood Score test and also performed two-point and multipoint linkage analyses. We found that LOAD was significantly associated with APOE (P= 9.0 × 10-6) in all our ascertainment regions except for the Adams County, Indiana, community (P= 0.55). Genome-wide, the most strongly associated SNP was rs12361953 (P= 7.92 × 10-7). A very strong, genome-wide significant multipoint peak [recessive heterogeneity multipoint LOD (HLOD) = 6.14, dominant HLOD = 6.05] was detected on 2p12. Three additional loci with multipoint HLOD scores >3 were detected on 3q26, 9q31 and 18p11. Converging linkage and association results, the most significantly associated SNP under the 2p12 peak was at rs2974151 (P= 1.29 × 10-4). This SNP is located in CTNNA2, which encodes catenin alpha 2, a neuronal-specific catenin known to have function in the developing brain. These results identify CTNNA2 as a novel candidate LOAD gene, and implicate three other regions of the genome as novel LOAD loci. These results underscore the utility of using family-based linkage and association analyses in isolated populations to identify novel loci for traits with complex genetic architecture.     

A full genome scan for late onset Alzheimer's disease

We have genotyped 292 affected sibling pairs (ASPs) with Alzheimer's disease (AD) according to NINCDS-ADRDA diagnostic criteria and with onset ages of >/=65 years using 237 microsatellite markers separated by an average distance of 16.3 cM. Data were analysed by SPLINK and MAPMAKER/SIBS on the whole sample of 292 ASPs and subsets of 162 ASPs where both members possessed an apolipoprotein E (APOE)straightepsilon4 allele and 63 pairs where neither possessed anstraightepsilon4 allele. Sixteen peaks with a multipoint lod score (MLS) >1 either in the whole sample, the straightepsilon4-positive or -negative subgroups were observed on chromosomes 1 (two peaks), 2, 5, 6, 9 (two peaks), 10 (two peaks), 12, 13, 14, 19, 21 and X (two peaks). Simulation studies revealed that these findings exceeded those expected by chance, although many are likely to be false positives. The highest lod scores on chromosomes 1 (MLS 2.67), 9 (MLS 2.38), 10 (MLS 2.27) and 19 (MLS 1.79) fulfilLander and Kruglyak's definition of 'suggestive' in that they would be expected to occur by chance once or less per genome scan. Several other peaks were only marginally less significant than this, in particular those on chromosomes 14 (MLS 2.16), 5 (MLS 2.00), 12, close to alpha2-macroglobulin (MLS 1.91), and 21, close to amyloid precursor protein (MLS 1.77). This is the largest genome scan to date in AD and shows for the first time that this is a genetically complex disorder involving several, perhaps many, genes in addition to APOE. Moreover, our data will be of interest to those hoping to identify positional candidate genes using information emerging from neurobiological studies of AD.      

A genome screen for genes predisposing to bipolar affective disorder detects a new susceptibility locus on 8q.

Bipolar affective disorder (BPAD), also known as manic depressive illness, is a severe psychiatric disorder characterized by episodes of mania and depression. It has a lifetime prevalence of approximately 1% in all human populations. In order to identify chromosomal regions containing genes that play a role in determining susceptibility to this psychiatric condition, we have conducted a complete genome screen with 382 markers (average marker spacing of 9.3 cM) in a sample of 75 BPAD families which were recruited through an explicit ascertainment scheme. Pedigrees were of German, Israeli and Italian origin, respectively. Parametric and non-parametric linkage analysis was performed. The highest two-point LOD score was obtained on 8q24 (D8S514; LOD score = 3.62), in a region that has not attracted much attention in previous linkage studies of BPAD. The second best finding was seen on 10q25-q26 (D10S217; LOD score = 2.86) and has been reported in independent studies of BPAD. Other regions showing 'suggestive' evidence for linkage localized to 1p33-p36, 2q21-q33, 3p14, 3q26-q27, 6q21-q22, 8p21, 13q11 and 14q12-q13. In addition, we aimed at detecting possible susceptibility loci underlying genomic imprinting by analyzing the autosomal genotype data with the recently developed extension of the GENEHUNTER program, GENEHUNTER-IMPRINTING. Putative paternally imprinted loci were identified in chromosomal regions 2p24-p21 and 2q31-q32. Maternally imprinted susceptibility genes may be located on 14q32 and 16q21-q23.     

A genome-wide linkage analysis of dementia in the Amish.

Susceptibility genes for Alzheimer's disease are proving to be highly challenging to detect and verify. Population heterogeneity may be a significant confounding factor contributing to this difficulty. To increase the power for disease susceptibility gene detection, we conducted a genome-wide genetic linkage screen using individuals from the relatively isolated, genetically homogeneous, Amish population. Our genome linkage analysis used a 407-microsatellite-marker map (average density 7 cM) to search for autosomal genes linked to dementia in five Amish families from four Midwestern U.S. counties. Our highest two-point lod score (3.01) was observed at marker D4S1548 on chromosome 4q31. Five other regions (10q22, 3q28, 11p13, 4q28, 19p13) also demonstrated suggestive linkage with markers having two-point lod scores >2.0. While two of these regions are novel (4q31 and 11p13), the other regions lie close to regions identified in previous genome scans in other populations. Our results identify regions of the genome that may harbor genes involved in a subset of dementia patients, in particular the North American Amish community.     

A genome-wide screen for linkage in Nordic sib-pairs with multiple sclerosis

Genetic factors influence susceptibility to multiple sclerosis but the responsible genes remain largely undefined, association with MHC class II alleles being the only established genetic feature of the disease. The Nordic countries have a high prevalence of multiple sclerosis, and to further explore the genetic background of the disease, we have carried out a genome-wide screen for linkage in 136 sibling-pairs with multiple sclerosis from Denmark, Finland, Norway and Sweden by typing 399 microsatellite markers. Seventeen regions where the lod score exceeds the nominal 5% significance threshold (0.7) were identified-1q11-24, 2q24-32, 3p26.3, 3q21.1, 4q12, 6p25.3, 6p21-22, 6q21, 9q34.3, 10p15, 10p12-13, 11p15.5, 12q21.3, 16p13.3, 17q25.3, 22q12-13 and Xp22.3. Although none of these regions reaches the level of genome-wide significance, the number observed exceeds the 10 that would be expected by chance alone. Our results significantly add to the growing body of linkage data relating to multiple sclerosis.     

Progress and pitfalls: bipolar molecular linkage studies

This paper reviews the history of molecular genetic linkage studies of bipolar disorder. The topic is introduced with a brief discussion of various genetic concepts, including linkage, lod scores and non-parametric statistics. It is emphasized that criteria for declaring linkage must include independent confirmation by a second group of investigators. Given that the inherited susceptibility for bipolar disorder is most likely explained by multiple genes of small effect, simulations indicate that universal confirmation of valid linkages cannot be expected. With this background, several valid linkages of BP disorder to genomic regions are reviewed. These valid linkages include 18p11, 18q22, 21q21, Xq26 and 4pter. The issue of anticipation and expanding triplet repeats is discussed. Finally, there is a brief section on recommendations for future genetic linkage studies of bipolar disorder.     

Two-dimensional genome-scan identifies novel epistatic loci for essential hypertension

It is well established that gene interactions influence common human diseases, but to date linkage studies have been constrained to searching for single genes across the genome. We applied a novel approach to uncover significant gene-gene interactions in a systematic two-dimensional (2D) genome-scan of essential hypertension. The study cohort comprised 2076 affected sib-pairs and 66 affected half-sib-pairs of the British Genetics of HyperTension study. Extensive simulations were used to establish significance thresholds in the context of 2D genome-scans. Our analyses found significant and suggestive evidence for loci on chromosomes 5, 9, 11, 15, 16 and 19, which influence hypertension when gene-gene interactions are taken into account (5q13.1 and 11q22.1, two-locus lod score=5.72; 5q13.1 and 19q12, two-locus lod score=5.35; 9q22.3 and 15q12, two-locus lod score=4.80; 16p12.3 and 16q23.1, two-locus lod score=4.50). For each significant and suggestive pairwise interaction, the two-locus genetic model that best fitted the data was determined. Regions that were not detected using single-locus linkage analysis were identified in the 2D scan as contributing significant epistatic effects. This approach has discovered novel loci for hypertension and offers a unique potential to use existing data to uncover novel regions involved in complex human diseases.     

Effect of heterogeneity on the chromosome 10 risk in late-onset Alzheimer disease

With the exception of ApoE (APOE), no universally accepted genetic association has been identified with late-onset Alzheimer disease (AD). A broad region of chromosome 10 has engendered continued interest generated from both preliminary genetic linkage and candidate gene studies. To better examine this region, we combined unbiased genetic linkage with candidate gene association studies. We genotyped 36 SNPs evenly spaced across 80.2 Mb in a family-based data set containing 1,337 discordant sibling pairs in 567 multiplex families to narrow the peak region of linkage using both covariate and subset analyses. Simultaneously, we examined five functional candidate genes (VR22, LRRTM3, PLAU, TNFRSF6, and IDE) that also fell within the broad area of linkage. A total of 50 SNPs were genotyped across the genes in the family-based data set and an independent case-control data set containing 483 cases and 879 controls. Of the 50 SNPs in the five candidate genes, 22 gave nominally significant association results in at least one data set, with at least one positive SNP in each gene. SNPs rs2441718 and rs2456737 in VR22 (67.8 Mb) showed association in both family-based and case-control data sets (both P=0.03). A two-point logarithmic odds (LOD) score of 2.69 was obtained at SNP rs1890739 (45.1 Mb, P=0.03 in 21% of the families) when the families were ordered from low to high by ApoE LOD score using ordered subset analysis (OSA). These data continue to support a role for chromosome 10 loci in AD. However, the candidate gene and linkage analysis results did not converge, suggesting that there is more extensive heterogeneity on chromosome 10 than previously appreciated.     

A follow-up linkage study supports evidence for a bipolar affective disorder locus on chromosome 21q22

Evidence for linkage between bipolar affective disorder (BP) and 21q22 was first reported by our group in a single large pedigree with a lod score of 3.41 with the PFKL locus. In a subsequent study, with denser marker coverage in 40 multiplex BP pedigrees, we reported supporting evidence with a two-point lod score of 2.76 at the D21S1260 locus, about 6 cM proximal to PFKL. For cost-efficiency, the individuals genotyped in that study comprised a subset of our large pedigree sample. To augment our previous analysis, we now report a follow-up study including a larger sample set with an additional 331 typed individuals from the original 40 families, improved marker coverage, and an additional 16 pedigrees. The analysis of all 56 pedigrees (a total of 862 genotyped individuals vs. the 372 genotyped previously), the largest multigenerational BP pedigree sample reportedly analyzed to date, supports our previous results, with a two-point lod score of 3.56 with D21S1260. The 16 new pedigrees analyzed separately gave a maximum two-point lod score of 1.89 at D21S266, less than 1 cM proximal to D21S1260. Our results are consistent with a putative BP locus on 21q22.     

Genome-wide scan of bipolar disorder and investigation of population stratification effects on linkage: support for susceptibility loci at 4q21, 7q36, 9p21, 12q24, 14q24, and 16p13.

Bipolar disorder (BPD) is a complex genetic disorder with cycling symptoms of depression and mania. Despite the extreme complexity of this psychiatric disorder, attempts to localize genes which confer vulnerability to the disorder have had some success. Chromosomal regions including 4p16, 12q24, 18p11, 18q22, and 21q21 have been repeatedly linked to BPD in different populations. Here we present the results of a whole genome scan for linkage to BPD in an Irish population. Our most significant result was at 14q24 which yielded a non-parametric LOD (NPL) score of 3.27 at the D14S588 marker with a nominal P-value of 0.0006 under a narrow (bipolar type I only) model of affection. We previously reported linkage to 14q22-24 in a subset of the families tested in this analysis. We also obtained suggestive evidence for linkage at 4q21, 9p21, 12q24, and 16p13, chromosomal regions that have all been previously linked to BPD. Additionally, we report on a novel approach to linkage analysis, STRUCTURE-Guided Linkage Analysis (SGLA), which is designed to reduce genetic heterogeneity and increase the power to detect linkage. Application of this technique resulted in more highly significant evidence for linkage of BPD to three regions including 16p13, a locus that has been repeatedly linked to numerous psychiatric disorders.     

A follow‐up linkage study supports evidence for a bipolar affective disorder locus on chromosome 21q22

Evidence for linkage between bipolar affective disorder (BP) and 21q22 was first reported by our group in a single large pedigree with a lod score of 3.41 with the PFKL locus. In a subsequent study, with denser marker coverage in 40 multiplex BP pedigrees, we reported supporting evidence with a two‐point lod score of 2.76 at the D21S1260 locus, about 6 cM proximal to PFKL. For cost‐efficiency, the individuals genotyped in that study comprised a subset of our large pedigree sample. To augment our previous analysis, we now report a follow‐up study including a larger sample set with an additional 331 typed individuals from the original 40 families, improved marker coverage, and an additional 16 pedigrees. The analysis of all 56 pedigrees (a total of 862 genotyped individuals vs. the 372 genotyped previously), the largest multigenerational BP pedigree sample reportedly analyzed to date, supports our previous results, with a two‐point lod score of 3.56 with D21S1260. The 16 new pedigrees analyzed separately gave a maximum two‐point lod score of 1.89 at D21S266, less than 1 cM proximal to D21S1260. Our results are consistent with a putative BP locus on 21q22. © 2001 Wiley‐Liss, Inc.     

Exclusion of candidate genes from a role in cleft lip with or without cleft palate: linkage and association studies.

Candidate genes and marker loci for cleft lip/palate (CL/P) were tested using linkage analyses and association studies. Eight British families with apparent autosomal dominant inheritance of non-syndromic CL/P participated in the linkage analyses while the association analyses involved 61 unrelated British white people with CL/P and 60 controls. The report of an association between RARA (17q21) and unrelated Australian persons with CL/P (p = 0.016) was not confirmed in British CL/P persons (chi 2 = 0.954, p > 0.1). There was also no evidence of linkage between RARA and the eight CL/P families (Z = -3.211, theta = 0.001). Linkage was excluded between familial CL/P and F13A1 (map position 6p24-25) with an observed maximum lod score of Z = -2.052 at theta = 0.05. No association was found between alleles at VIM (10p13) and the British CL/P subjects (chi 2 = 0.110, p > 0.5). Multipoint analysis excluded linkage between familial CL/P and the markers D1S65 and D1S58 which flank the Van der Woude syndrome locus with a maximum lod score of Z = -4.0. This suggests that the genetic defect underlying VWS is not the same as in non-syndromic CL/P. There was no evidence of linkage between CRTL1 (5q15) and the eight CL/P families (Z = -3.466, theta = 0.05).     

Association and linkage analysis of candidate chromosomal regions in multiple sclerosis: indication of disease genes in 12q23 and 7ptr-15

Four recent genome-wide screen studies in multiple sclerosis (MS) identified a number of candidate regions for susceptibility genes in addition to the HLA complex in 6p21. However, none of these regions provided formally significant evidence for genome-wide linkage. We have investigated such regions in 46 Swedish multiplex MS families, 28 singleton families, 190 sporadic MS patients and 148 normal controls by parametric and nonparametric linkage and association analysis. One microsatellite marker, in 12q23, provided evidence for association in addition to suggestive transmission distortion and slightly positive linkage. In addition, a marker in 7ptr-15 showed a significant transmission distortion as well as a highly significant score in affected pedigree member analysis, but not quite significant deviations in association analysis. One of three markers in 5p, a region implicated in all four previous studies, showed a weakly positive lod score, but no other evidence of importance. Markers in 2p23, 5q11-13, 6q25, 7q21-22, 11q21-23, 13q33-34, 16p13.2, 18p11.32-23, Xp21.3 provided little or no evidence of importance for MS. In summary, these data support the importance of genome-wide screens in the identification of new candidate loci in polygenic disorders.     

Gender-specific association of ATP-binding cassette transporter 1 (ABCA1) polymorphisms with the risk of late-onset Alzheimer's disease

Alzheimer's disease (AD) is a multifactorial neurodegenerative disorder caused by a complex interaction of genetic and environmental factors. Increasing evidence highlights a potential role for cholesterol in the pathophysiology of AD. The ABCA1 gene, located in close vicinity to the 9q linkage peaks identified by genome-wide AD linkage studies, plays an important role in cellular cholesterol efflux, and is likely a good candidate gene. However, results from published genetic association studies between ABCA1 and AD are ambiguous. In the present study, we examined the role of two ABCA1 polymorphisms, R219K (rs2230806) and G-17C (rs2740483) in modifying the risk of late-onset AD (LOAD) in a large American white cohort of 992 AD cases and 699 controls. We observed significant gender x R219K interaction (p=0.00008). Female carriers of the 219K allele showed a 1.75-fold increased risk of developing AD compared to non-219K carrier females (95% CI 1.34-2.29; p=0.00004). The overall two-site haplotype distribution was also significant between female AD cases and controls (p=0.017). The risk associated with the R219K polymorphism was independent of the recently reported significant association in the ubiquilin (UBQLN1) gene in this region on chromosome 9q. Our data suggest a gender-specific and APOE and UBQLN1 independent association between the ABCA1/R219K polymorphism and LOAD.     

Linkage results on 11Q21-22 in Eastern Quebec pedigrees densely affected by schizophrenia

The 11q21-22 region is of interest for schizophrenia because several candidate genes are located in this section of the genome. The 11q21-22 region, including DRD2, was surveyed by linkage analysis in a sample (N = 242) made of four large multigenerational pedigrees densely affected by schizophrenia (SZ) and eight others by bipolar disorder (BP). These pedigrees were ascertained in a large area of Eastern Quebec and Northern New Brunswick and are still being extended. Family members were administered a “consensus best-estimate diagnosis procedure” (DSM-III-R criteria) blind to probands and relatives' diagnosis and to pedigree assignment (SZ or BP). For linkage analysis, 11 microsatellite polymorphism (CA repeat) markers, located at 11q21-22, and comprising DRD2, were genotyped. Results show no evidence of a major gene for schizophrenia. However, a maximum lod score of 3.41 at the D11S35 locus was observed in an affected-only analysis of one large SZ family, pedigree 255. Whether or not the positive linkage trend in pedigree 255 reflects a true linkage for a small proportion of SZ needs to be confirmed through the extension of this kindred and through replication. © 1995 Wiley-Liss, Inc.