Meconium ileus (MI), a life-threatening intestinal obstruction due to meconium with abnormal protein content, occurs in approximately 15 percent of neonates with cystic fibrosis (CF). Analysis of twins with CF demonstrates that MI is a highly heritable trait, indicating that genetic modifiers are largely responsible for this complication. Here, we performed regional family-based association analysis of a locus that had previously been linked to MI and found that SNP haplotypes 5′ to and within the MSRA gene were associated with MI (P = 1.99×10−5 to 1.08×10−6; Bonferroni P = 0.057 to 3.1×10−3). The haplotype with the lowest P value showed association with MI in an independent sample of 1,335 unrelated CF patients (OR = 0.72, 95% CI [0.53–0.98], P = 0.04). Intestinal obstruction at the time of weaning was decreased in CF mice with Msra null alleles compared to those with wild-type Msra resulting in significant improvement in survival (P = 1.2×10−4). Similar levels of goblet cell hyperplasia were observed in the ilea of the Cftr−/− and Cftr−/−Msra−/− mice. Modulation of MSRA, an antioxidant shown to preserve the activity of enzymes, may influence proteolysis in the developing intestine of the CF fetus, thereby altering the incidence of obstruction in the newborn period. Identification of MSRA as a modifier of MI provides new insight into the biologic mechanism of neonatal intestinal obstruction caused by loss of CFTR function.
Cystic fibrosis (CF) is a monogenic disease with considerable phenotypic variability. About 15% of newborns with CF suffer from an intestinal obstruction called meconium ileus (MI), and studies in CF twins have shown that modifier genes play a substantial role in the development of this complication. We used a family-based study design to enrich for genetic modifiers of MI and found that variations in the MSRA gene, represented by combinations of SNPs, or haplotypes, were protective against this manifestation of CF. We investigated association between one of the MSRA haplotypes and MI in an independent sample of CF patients and showed that it had a similar protective effect. Furthermore, CF mice lacking Msra expression had lower mortality due to intestinal obstruction at the time of transitioning to solid food and lived longer than CF mice with normal Msra, thus supporting the protective effect of the haplotype we observed in human CF subjects. The identification of modifiers of MI such as MSRA offers new insight into the mechanism of this life-threatening complication of CF.
Autism Spectrum Disorders (ASD) are phenotypically heterogeneous, characterized by impairments in the development of communication and social behaviour and the presence of repetitive behaviour and restricted interests. Dissecting the genetic complexity of ASD may require phenotypic data reflecting more detail than is offered by a categorical clinical diagnosis. Such data are available from the Social Responsiveness Scale (SRS) which is a continuous, quantitative measure of social ability giving scores that range from significant impairment to above average ability.
We present genome-wide results for 64 multiplex and extended families ranging from two to nine generations. SRS scores were available from 518 genotyped pedigree subjects, including affected and unaffected relatives. Genotypes from the Illumina 6 k single nucleotide polymorphism panel were provided by the Center for Inherited Disease Research. Quantitative and qualitative analyses were done using MCLINK, a software package that uses Markov chain Monte Carlo (MCMC) methods to perform multilocus linkage analysis on large extended pedigrees.
When analysed as a qualitative trait, linkage occurred in the same locations as in our previous affected-only genome scan of these families, with findings on chromosomes 7q31.1-q32.3 [heterogeneity logarithm of the odds (HLOD) = 2.91], 15q13.3 (HLOD = 3.64), and 13q12.3 (HLOD = 2.23). Additional positive qualitative results were seen on chromosomes 6 and 10 in regions that may be of interest for other neuropsychiatric disorders. When analysed as a quantitative trait, results replicated a peak found in an independent sample using quantitative SRS scores on chromosome 11p15.1-p15.4 (HLOD = 2.77). Additional positive quantitative results were seen on chromosomes 7, 9, and 19.
The SRS linkage peaks reported here substantially overlap with peaks found in our previous affected-only genome scan of clinical diagnosis. In addition, we replicated a previous SRS peak in an independent sample. These results suggest the SRS is a robust and useful phenotype measure for genetic linkage studies of ASD. Finally, analyses of SRS scores revealed linkage peaks overlapping with evidence from other studies of neuropsychiatric diseases. The information available from the SRS itself may, therefore, reveal locations for autism susceptibility genes that would not otherwise be detected.
Genome-wide linkage studies have been used to localize rare and highly penetrant prostate cancer (PRCA) susceptibility genes. Linkage studies performed in different ethnic backgrounds and populations have been somewhat disparate, resulting in multiple, often irreproducible signals because of genetic heterogeneity and high sporadic background of the disease. Our first genome-wide linkage study and subsequent fine-mapping study of Finnish hereditary prostate cancer (HPC) families gave evidence of linkage to one region. Here, we conducted subsequent scans with microsatellites and SNPs in a total of 69 Finnish HPC families. GENEHUNTER-PLUS was used for parametric and non-parametric analyses. Our microsatellite genome-wide linkage study provided evidence of linkage to 17q12-q23, with a heterogeneity LOD (HLOD) score of 3.14 in a total of 54 of the 69 families. Genome-wide SNP analysis of 59 of the 69 families gave a highest HLOD score of 3.40 at 2q37.3 under a dominant high penetrance model. Analyzing all 69 families by combining microsatellite and SNP maps also yielded HLOD scores of > 3.3 in two regions (2q37.3 and 17q12-q21.3). These significant linkage peaks on chromosome 2 and 17 confirm previous linkage evidence of a locus on 17q from other populations and provide a basis for continued research into genetic factors involved in PRCA. Fine-mapping analysis of these regions is ongoing and candidate genes at linked loci are currently under analysis.
Prostate cancer; genome-wide linkage; Finland; 17q; 2q
Adolescent idiopathic scoliosis (AIS) is a common disorder with strong evidence for genetic predisposition. Quantitative trait loci (QTLs) for AIS susceptibility have been identified on chromosomes. We performed a genome-wide genetic linkage scan in 7 multiplex families using 400 marker loci with a mean spacing of 8.6 centiMorgans (cM). We used Genehunter Plus to generate linkage statistics, expressed as homogeneity (HLOD) scores, under dominant and recessive genetic models. We found a significant linkage signal on chromosome 12p, whose support interval extends from near 12pter, spanning approximately 10 million bases or 31 cM. Fine mapping within the region using 20 additional markers reveals maximum HLOD = 3.7 at 5 cM under a dominant inheritance model, and a split peak maximum HLOD = 3.2 at 8 and 18 cM under a recessive inheritance model. The linkage support interval contains 95 known genes. We found evidence suggestive of linkage on chromosomes 1, 6, 7, 8, and 14. This study is the first to find evidence of an AIS susceptibility locus on chromosome 12. Detection of AIS susceptibility QTLs on multiple chromosomes in this and other studies demonstrate that the condition is genetically heterogeneous.
AIS; axial skeletal growth; chromosome 12p; genetic heterogeneity; susceptibility locus
Background and aim
Known colorectal cancer syndromes, such as familial adenomatous polyposis and hereditary non‐polyposis colorectal cancer, have been identified in only a small proportion of cases with a family history of disease. In an attempt to identify loci harbouring novel predisposing genes, we have performed a genome wide linkage analysis in 18 colorectal cancer families recruited from the Department of Clinical Genetics at Karolinska Hospital, Sweden.
Multipoint parametric and non‐parametric linkage analyses were performed using two affected status criteria, stringent and less stringent. Parametric analysis was performed under the assumption of locus homogeneity and locus heterogeneity.
The initial scan performed using the less stringent affected status criteria revealed regions of interest on chromosome 11 (marker D11S1314: heterogeneity logarithm of odds (HLOD) score 1.96, non‐parametric LOD (NPL) score 1.28; and marker D11S908: HLOD score 2.10, NPL score 2.16) and chromosome 14 (marker D14S258: HLOD score 2.61, NPL score 2.88). Using the stringent affected status criteria, a locus on chromosome 22 was suggested in the parametric analysis (marker D22S315: HLOD score 1.26). After finemapping of the regions on chromosomes 11 and 14, HLOD and NPL scores were reduced but still within the range of suggestive linkage. Haplotype analysis revealed overlapping regions between D11S987 and D11S4207 (proximal region), D11S4120 and D11S4090 (distal region), on chromosome 11, and between D14S1038 and D14S1069 on chromosome 14.
Our study provides evidence of genetic heterogeneity among Swedish colorectal cancer families. Three novel regions were suggested to be of interest in a proportion of families analysed. Further studies are needed to confirm this result.
linkage analysis; hereditary non‐polyposis colorectal cancer; familial adenomatous polyposis; colorectal cancer; chromosome 11; chromosome 14; chromosome 22
Refractive error is a highly heritable quantitative trait responsible for considerable morbidity. Following an initial genome-wide linkage study using microsatellite markers, we confirmed evidence for linkage to chromosome 3q26 and then conducted fine-scale association mapping using high-resolution linkage disequilibrium unit (LDU) maps. We used a preliminary discovery marker set across the 30-Mb region with an average SNP density of 1 SNP/15 kb (Map 1). Map 1 was divided into 51 LDU windows and additional SNPs were genotyped for six regions (Map 2) that showed preliminary evidence of multi-marker association using composite likelihood. A total of 575 cases and controls selected from the tails of the trait distribution were genotyped for the discovery sample. Malecot model estimates indicate three loci with putative common functional variants centred on MFN1 (180,566 kb; 95% confidence interval 180,505–180, 655 kb), approximately 156 kb upstream from alternate-splicing SOX2OT (182,595 kb; 95% CI 182,533–182,688 kb) and PSARL (184,386 kb; 95% CI 184,356–184,411 kb), with the loci showing modest to strong evidence of association for the Map 2 discovery samples (p<10−7, p<10−10, and p = 0.01, respectively). Using an unselected independent sample of 1,430 individuals, results replicated for the MFN1 (p = 0.006), SOX2OT (p = 0.0002), and PSARL (p = 0.0005) gene regions. MFN1 and PSARL both interact with OPA1 to regulate mitochondrial fusion and the inhibition of mitochondrial-led apoptosis, respectively. That two mitochondrial regulatory processes in the retina are implicated in the aetiology of myopia is surprising and is likely to provide novel insight into the molecular genetic basis of common myopia.
Successful gene mapping strategies for common disease continue to require careful consideration of basic study design with the advent of genome-wide association studies. Here, we take advantage of prior information that the heritability of the quantitative trait myopia in the general population is high and shows evidence of replicated linkage to chromosome 3q26. Based on this, we conducted a fine map linkage disequilibrium association study for the region, using a high-resolution genetic map derived from population-based HapMap Phase II data. For analysis, we used efficient multi-locus tests of association using single nucleotide polymorphism markers genotyped for our sample data and placed on the genetic map measured in linkage disequilibrium units. We followed up preliminary evidence of association for the discovery samples with further genotyping in the same samples to improve the model location estimates for the common functional variants we identified. Three locations were replicated using an independent sample. Two of the identified genes are likely to play an unexpected role in myopia with both pivotal in the healthy housekeeping metabolism of retinal mitochondria. Both proteins interact with OPA1, with nonsynonymous OPA1 mutations causing the unrelated Mendelian disease Autosomal Dominant Optic Atrophy (ADOA) by triggering mitochondrial-led retinal ganglia cell apoptosis.
Preterm birth is the major cause of neonatal death and serious morbidity. Most preterm births are due to spontaneous onset of labor without a known cause or effective prevention. Both maternal and fetal genomes influence the predisposition to spontaneous preterm birth (SPTB), but the susceptibility loci remain to be defined. We utilized a combination of unique population structures, family-based linkage analysis, and subsequent case-control association to identify a susceptibility haplotype for SPTB. Clinically well-characterized SPTB families from northern Finland, a subisolate founded by a relatively small founder population that has subsequently experienced a number of bottlenecks, were selected for the initial discovery sample. Genome-wide linkage analysis using a high-density single-nucleotide polymorphism (SNP) array in seven large northern Finnish non-consanginous families identified a locus on 15q26.3 (HLOD 4.68). This region contains the IGF1R gene, which encodes the type 1 insulin-like growth factor receptor IGF-1R. Haplotype segregation analysis revealed that a 55 kb 12-SNP core segment within the IGF1R gene was shared identical-by-state (IBS) in five families. A follow-up case-control study in an independent sample representing the more general Finnish population showed an association of a 6-SNP IGF1R haplotype with SPTB in the fetuses, providing further evidence for IGF1R as a SPTB predisposition gene (frequency in cases versus controls 0.11 versus 0.05, P = 0.001, odds ratio 2.3). This study demonstrates the identification of a predisposing, low-frequency haplotype in a multifactorial trait using a well-characterized population and a combination of family and case-control designs. Our findings support the identification of the novel susceptibility gene IGF1R for predisposition by the fetal genome to being born preterm.
Preterm birth is the major cause of infant deaths and life-long neurologic and cardiopulmonary morbidity. More than 10% of births in the United States occur prematurely, and the rate is increasing without known effective prevention. Previous premature birth increases the risk 3-fold in subsequent pregnancies. We report here, for the first time to our knowledge, a genome-wide study on susceptibility to spontaneous preterm birth in singleton pregnancies. To detect novel regions of the genome associated with preterm birth, we performed linkage analysis on seven carefully selected large families with recurrent spontaneous premature births. When we studied the fetuses, evidence was found for linkage of a region on chromosome 15 with spontaneous preterm birth, with the highest linkage signals occurring within a single gene, IGF1R. Evidence of the involvement of this gene in the etiology of preterm birth was further strengthened by subsequent haplotype segregation analysis and case-control analysis of an independent patient population. The IGF1R gene encodes insulin-like growth factor receptor 1 (IGF-1R), an important protein that potentially regulates signaling cascades involved in the onset of labor. Our analyses are unique in providing evidence that fetal IGF1R influences the risk of spontaneous preterm labor, leading to preterm birth.
Obsessive compulsive disorder (OCD) has a complex etiology involving both genetic and environmental factors. However, the genetic causes of OCD are largely unknown, despite the identification of several promising candidate genes and linkage regions.
Our objective was to conduct genetic linkage studies of the type of OCD thought to have the strongest genetic etiology (i.e., childhood-onset OCD), in 33 Caucasian families with ≥2 childhood-onset OCD-affected individuals from the United States (US) (N=245 individuals with genotype data). Parametric and non-parametric genome-wide linkage analyses were conducted with Morgan and Merlin in these families using a selected panel of single nucleotide repeat polymorphisms (SNPs) from the Illumina 610-Quad Bead Chip. The initial analyses were followed by fine-mapping analyses in genomic regions with initial heterogeneity LOD (HLOD) scores of ≥2.0.
We identified five areas of interest (HLOD score ≥2) on chromosomes 1p36, 2p14, 5q13, 6p25, and 10p13. The strongest result was on chromosome 1p36.33-p36.32 (HLOD=3.77, suggestive evidence for linkage after fine-mapping). At this location, several of the families showed haplotypes co-segregating with OCD.
The results of this study represent the strongest linkage finding for OCD in a primary analysis to date, and suggest that chromosome 1p36, and possibly several other genomic regions, may harbor susceptibility loci for OCD. Multiple brain-expressed genes lie under the primary linkage peak (approximately 4 mb in size). Follow-up studies, including replication in additional samples and targeted sequencing of the areas of interest, are needed to confirm these findings and to identify specific OCD risk variants.
linkage; genomewide; pedigree; obsessive-compulsive; genetics; multigenerational
Twin studies indicate that the timing of pubertal onset is under strong genetic control. However, genes controlling pubertal timing in the general population have not yet been identified.
To facilitate the identification of genes influencing the timing of pubertal growth and maturation, we conducted linkage mapping of constitutional delay of growth and puberty (CDGP), an extreme variant of normal pubertal timing, in extended families.
Participants and methods:
Fifty-two families multiply affected with CDGP were genotyped with 383 multiallelic markers. CDGP was defined based on growth charts (the age at onset of growth spurt, peak height velocity, or attaining adult height taking place at least 1.5 SD later than average). Chromosomal regions co-segregating with CDGP were identified with parametric affected only linkage analysis using CDGP as a dichotomized trait.
The genome-wide scan detected linkage of CDGP to a region on chromosome 2p13-2q13. The two-point HLOD-score was 1.62 (α 0.27) and the corresponding multipoint HLOD 2.54 (α 0.31). Fine-mapping the region at 1 cM resolution increased the multipoint HLOD-score to 4.44 (α 0.41). The linkage became weaker if also family members diagnosed with CDGP without growth data were included in the analyses.
The pericentromeric region of chromosome 2 harbors a gene predisposing to pubertal delay in multiply affected pedigrees. Our data suggest that this locus may be a component of the internal clock controlling the timing of the onset of puberty.
puberty timing; genes; linkage mapping
Genetic variants are likely to contribute to a portion of prostate cancer risk. Full elucidation of the genetic etiology of prostate cancer is difficult because of incomplete penetrance and genetic and phenotypic heterogeneity. Current evidence suggests that genetic linkage to prostate cancer has been found on several chromosomes including the X; however, identification of causative genes has been elusive.
Parametric and non-parametric linkage analyses were performed using 26 microsatellite markers in each of 11 groups of multiple-case prostate cancer families from the International Consortium for Prostate Cancer Genetics (ICPCG). Meta-analyses of the resultant family-specific linkage statistics across the entire 1,323 families and in several predefined subsets were then performed.
Meta-analyses of linkage statistics resulted in a maximum parametric heterogeneity lod score (HLOD) of 1.28, and an allele-sharing lod score (LOD) of 2.0 in favor of linkage to Xq27-q28 at 138 cM. In subset analyses, families with average age at onset less than 65 years exhibited a maximum HLOD of 1.8 (at 138 cM) versus a maximum regional HLOD of only 0.32 in families with average age at onset of 65 years or older. Surprisingly, the subset of families with only 2–3 affected men and some evidence of male-to-male transmission of prostate cancer gave the strongest evidence of linkage to the region (HLOD = 3.24, 134 cM). For this subset, the HLOD was slightly increased (HLOD = 3.47 at 134 cM) when families used in the original published report of linkage to Xq27-28 were excluded.
Although there was not strong support for linkage to the Xq27-28 region in the complete set of families, the subset of families with earlier age at onset exhibited more evidence of linkage than families with later onset of disease. A subset of families with 2–3 affected individuals and with some evidence of male to male disease transmission showed stronger linkage signals. Our results suggest that the genetic basis for prostate cancer in our families is much more complex than a single susceptibility locus on the X chromosome, and that future explorations of the Xq27-28 region should focus on the subset of families identified here with the strongest evidence of linkage to this region.
A significant proportion of high-risk breast cancer families are not explained by mutations in known genes. Recent genome-wide searches (GWS) have not revealed any single major locus reminiscent of BRCA1 and BRCA2, indicating that still unidentified genes may explain relatively few families each or interact in a way obscure to linkage analyses. This has drawn attention to possible benefits of studying populations where genetic heterogeneity might be reduced. We thus performed a GWS for linkage on nine Icelandic multiple-case non-BRCA1/2 families of desirable size for mapping highly penetrant loci. To follow up suggestive loci, an additional 13 families from other Nordic countries were genotyped for selected markers.
GWS was performed using 811 microsatellite markers providing about five centiMorgan (cM) resolution. Multipoint logarithm of odds (LOD) scores were calculated using parametric and nonparametric methods. For selected markers and cases, tumour tissue was compared to normal tissue to look for allelic loss indicative of a tumour suppressor gene.
The three highest signals were located at chromosomes 6q, 2p and 14q. One family contributed suggestive LOD scores (LOD 2.63 to 3.03, dominant model) at all these regions, without consistent evidence of a tumour suppressor gene. Haplotypes in nine affected family members mapped the loci to 2p23.2 to p21, 6q14.2 to q23.2 and 14q21.3 to q24.3. No evidence of a highly penetrant locus was found among the remaining families. The heterogeneity LOD (HLOD) at the 6q, 2p and 14q loci in all families was 3.27, 1.66 and 1.24, respectively. The subset of 13 Nordic families showed supportive HLODs at chromosome 6q (ranging from 0.34 to 1.37 by country subset). The 2p and 14q loci overlap with regions indicated by large families in previous GWS studies of breast cancer.
Chromosomes 2p, 6q and 14q are candidate sites for genes contributing together to high breast cancer risk. A polygenic model is supported, suggesting the joint effect of genes in contributing to breast cancer risk to be rather common in non-BRCA1/2 families. For genetic counselling it would seem important to resolve the mode of genetic interaction.
Family history is a major risk factor for colorectal cancer and many families segregate the disease as a seemingly monogenic trait. A minority of familial colorectal cancer could be explained by known monogenic genes and genetic loci. Familial polyposis and Lynch syndrome are two syndromes where the predisposing genes are known but numerous families have been tested without finding the predisposing gene. We performed a genome wide linkage analysis in 121 colorectal families with an increased risk of colorectal cancer. The families were ascertained from the department of clinical genetics at the Karolinska University Hospital in Stockholm, Sweden and were considered negative for Familial Polyposis and Lynch syndrome. In total 600 subjects were genotyped using single nucleotide polymorphism array chips. Parametric- and non-parametric linkage analyses were computed using MERLIN in all and subsets of families. No statistically significant result was seen, however, there were suggestive positive HLODs above two in parametric linkage analysis. This was observed in a recessive model for high-risk families, at locus 9q31.1 (HLOD=2.2, rs1338121) and for moderate-risk families, at locus Xp22.33 (LOD=2.2 and HLOD=2.5, rs2306737). Using families with early-onset, recessive analysis suggested one locus on 4p16.3 (LOD=2.2, rs920683) and one on 17p13.2 (LOD/HLOD=2.0, rs884250). No NPL score above two was seen for any of the families. Our linkage study provided additional support for the previously suggested region on chromosome 9 and suggested additional loci to be involved in colorectal cancer risk. Sequencing of genes in the regions will be done in future studies.
A genetic component to the etiology of leprosy is well recognized but the mechanism of inheritance and the genes involved are yet to be fully established.
A genome-wide single nucleotide polymorphism (SNP) based linkage analysis was carried out using 23 pedigrees, each with 3 to 7 family members affected by leprosy. Multipoint parametric and non-parametric linkage analyses were performed using MERLIN 1.1.1.
Genome-wide significant evidence for linkage was identified on chromosome 2p14 with a heterogeneity logarithm of odds (HLOD) score of 3.51 (rs1106577) under a recessive model of inheritance, while suggestive evidence was identified on chr.4q22 (HLOD 2.92, rs1349350, dominant model), chr. 8q24 (HLOD 2.74, rs1618523, recessive model) and chr.16q24 (HLOD 1.93, rs276990 dominant model). Our study also provided moderate evidence for a linkage locus on chromosome 6q24–26 by non-parametric linkage analysis (rs6570858, LOD 1.54, p = 0.004), overlapping a previously reported linkage region on chromosome 6q25–26.
A genome-wide linkage analysis has identified a new linkage locus on chromosome 2p14 for leprosy in Pedigrees from China.
Background & Aims
Neonatal intestinal obstruction (meconium ileus or MI) occurs in 15% of patients with cystic fibrosis (CF). Our aim was to determine the relative contribution of genetic and non-genetic modifiers to the development of this major complication of CF.
Using clinical data and DNA collected by the CF Twin and Sibling Study, 65 monozygous twin pairs, 23 dizygous twin/triplet sets, and 349 sets of siblings with CF were analyzed for MI status, significant covariates, and genome-wide linkage.
Specific mutations in CFTR, the gene responsible for CF, correlated with MI indicating a role for CFTR genotype. Monozygous twins showed substantially greater concordance for MI than dizygous twins and siblings (p=1×10−5) demonstrating that modifier genes independent of CFTR contribute substantially to this trait. Regression analysis revealed that MI was correlated with distal intestinal obstruction syndrome (DIOS; p=8×10−4). Unlike MI, concordance analysis indicated that the risk for development of DIOS in CF patients is primarily due to non-genetic factors. Regions of suggestive linkage (logarithm of the odds of linkage >2.0) for modifier genes that cause MI (chromosomes 4q35.1, 8p23.1, and 11q25) or protect from MI (chromosomes 20p11.22 and 21q22.3) were identified by genome-wide analyses. These analyses did not support the existence of a major modifier gene within the CFM1 region on chromosome 19 that had previously been linked to MI.
The CFTR gene along with two or more modifier genes are the major determinants of intestinal obstruction in newborn CF patients, while intestinal obstruction in older CF patients is primarily due to non-genetic factors.
Twins; siblings; linkage; association; intestinal obstruction; CFM-1
Age at onset of Huntington's disease (HD) is correlated with the size of the abnormal CAG repeat expansion in the HD gene; however, several studies have indicated that other genetic factors also contribute to the variability in HD age at onset. To identify modifier genes, we recently reported a whole-genome scan in a sample of 629 affected sibling pairs from 295 pedigrees, in which six genomic regions provided suggestive evidence for quantitative trait loci (QTL), modifying age at onset in HD.
In order to test the replication of this finding, eighteen microsatellite markers, three from each of the six genomic regions, were genotyped in 102 newly recruited sibling pairs from 69 pedigrees, and data were analyzed, using a multipoint linkage variance component method, in the follow-up sample and the combined sample of 352 pedigrees with 753 sibling pairs.
Suggestive evidence for linkage at 6q23-24 in the follow-up sample (LOD = 1.87, p = 0.002) increased to genome-wide significance for linkage in the combined sample (LOD = 4.05, p = 0.00001), while suggestive evidence for linkage was observed at 18q22, in both the follow-up sample (LOD = 0.79, p = 0.03) and the combined sample (LOD = 1.78, p = 0.002). Epistatic analysis indicated that there is no interaction between 6q23-24 and other loci.
In this replication study, linkage for modifier of age at onset in HD was confirmed at 6q23-24. Evidence for linkage was also found at 18q22. The demonstration of statistically significant linkage to a potential modifier locus opens the path to location cloning of a gene capable of altering HD pathogenesis, which could provide a validated target for therapeutic development in the human patient.
OBJECTIVE--To assess the effectiveness of a two tier neonatal screening strategy for cystic fibrosis, which combines estimation of immunoreactive trypsinogen followed by direct gene analysis in dried blood spot samples collected at age 5 days. DESIGN--Prospective study of two tier screening strategy. The first tier of testing immunoreactive trypsinogen concentration was measured in dried blood spot samples from neonates aged 4-5 days. In the second tier direct gene analysis to detect cystic fibrosis mutations deltaF508 and deltaI506 was performed in those blood spot samples which produced the highest 1% of immunoreactive trypsinogen values. Direct gene analysis was also performed on blood spot samples from infants with suspected or confirmed meconium ileus, regardless of the immunoreactive trypsinogen value. SETTING--The South Australian Neonatal Screening Programme, operating from the department of chemical pathology at Adelaide Children's Hospital. Subjects--All 12,056 neonates born in South Australia between December 1989 and June 1990. No selection criteria were applied. INTERVENTIONS--All infants found to have two recognised cystic fibrosis mutations on direct gene analysis were referred directly for clinical management, and those with one recognised cystic fibrosis mutation were recalled for a sweat test; their families were given genetic counselling. MAIN OUTCOME MEASURES--Direct or exclusion of cystic fibrosis by sweat testing of neonates identified as being at high risk of cystic fibrosis on screening and of those at minimum risk but whose subsequent clinical history raised suspicion about the disease. RESULTS--Of the 12,056 infants screened, 11,907 (98.8%) were reported as "cystic fibrosis not indicated" on the basis of low immunoreactive trypsinogen values. Of the 148 (1.23%) infants with raised immunoreactive trypsinogen values and one (0.008%) with meconium ileus, 132 (1.09%) were reported as cystic fibrosis not indicated, four (0.033%) were identified as having cystic fibrosis, and 13 (0.108%) were recalled for sweat testing after direct gene analysis for the presence of the deltaF508 and deltaI506 cystic fibrosis mutations. No cases of affected infants are known to have been missed to date. CONCLUSION--The strategy of measurement of immunoreactive trypsinogen followed by direct gene analysis is a highly specific neonatal screen for cystic fibrosis, requiring only 2.8 families to be contacted for every case of cystic fibrosis diagnosed.
Specific language impairment is a neurodevelopmental disorder characterized by impairments essentially restricted to the domain of language and language learning skills. This contrasts with autism, which is a pervasive developmental disorder defined by multiple impairments in language, social reciprocity, narrow interests and/or repetitive behaviors. Genetic linkage studies and family data suggest that the two disorders may have genetic components in common. Two samples, from Canada and the US, selected for specific language impairment were genotyped at loci where such common genes are likely to reside. Significant evidence for linkage was previously observed at chromosome 13q21 in our Canadian sample (HLOD 3.56) and was confirmed in our US sample (HLOD 2.61). Using the posterior probability of linkage (PPL) to combine evidence for linkage across the two samples yielded a PPL over 92%. Two additional loci on chromosome 2 and 7 showed weak evidence for linkage. However, a marker in the cystic fibrosis transmembrane conductance regulator (7q31) showed evidence for association to SLI, confirming results from another group (O’ Brien et al. 2003). Our results indicate that using samples selected for components of the autism phenotype may be a useful adjunct to autism genetics.
Autism; Language impairment; Multiple data sets; Heterogeneity; Linkage analysis
A previous study reported a comprehensive quantitative trait locus (QTL) and genome wide association study (GWAS) of southern leaf blight (SLB) resistance in the maize Nested Association Mapping (NAM) panel. Since that time, the genomic resources available for such analyses have improved substantially. An updated NAM genetic linkage map has a nearly six-fold greater marker density than the previous map and the combined SNPs and read-depth variants (RDVs) from maize HapMaps 1 and 2 provided 28.5 M genomic variants for association analysis, 17 fold more than HapMap 1. In addition, phenotypic values of the NAM RILs were re-estimated to account for environment-specific flowering time covariates and a small proportion of lines were dropped due to genotypic data quality problems. Comparisons of original and updated QTL and GWAS results confound the effects of linkage map density, GWAS marker density, population sample size, and phenotype estimates. Therefore, we evaluated the effects of changing each of these parameters individually and in combination to determine their relative impact on marker-trait associations in original and updated analyses.
Of the four parameters varied, map density caused the largest changes in QTL and GWAS results. The updated QTL model had better cross-validation prediction accuracy than the previous model. Whereas joint linkage QTL positions were relatively stable to input changes, the residual values derived from those QTL models (used as inputs to GWAS) were more sensitive, resulting in substantial differences between GWAS results. The updated NAM GWAS identified several candidate genes consistent with previous QTL fine-mapping results.
The highly polygenic nature of resistance to SLB complicates the identification of causal genes. Joint linkage QTL are relatively stable to perturbations of data inputs, but their resolution is generally on the order of tens or more Mbp. GWAS associations have higher resolution, but lower power due to stringent thresholds designed to minimize false positive associations, resulting in variability of detection across studies. The updated higher density linkage map improves QTL estimation and, along with a much denser SNP HapMap, greatly increases the likelihood of detecting SNPs in linkage with causal variants. We recommend use of the updated genetic resources and results but emphasize the limited repeatability of small-effect associations.
Electronic supplementary material
The online version of this article (doi:10.1186/1471-2164-15-1068) contains supplementary material, which is available to authorized users.
Quantitative trait loci; Nested association mapping; Disease resistance; Genome wide association study; Zea mays
Childhood absence epilepsy (CAE) is an idiopathic generalised epilepsy characterised by absence seizures manifested by transitory loss of awareness with 2.5-4Hz spike-wave complexes on ictal EEG. A genetic component to aetiology is established but the mechanism of inheritance and the genes involved are not fully defined. Available evidence suggests that genes encoding brain expressed voltage-gated calcium channels, including CACNG3 on chromosome 16p12-p13.1, may represent susceptibility loci for CAE. The aim of this work was to further evaluate CACNG3 as a susceptibility locus by linkage and association analysis. Assuming locus heterogeneity, a significant HLOD score (HLOD=3.54, α=0.62) was obtained for markers encompassing CACNG3 in 65 nuclear families with a proband with CAE. The maximum NPL score was 2.87 (p<0.002). Re-sequencing of the coding exons in 59 patients did not identify any putative causal variants. A linkage disequilibrium (LD) map of CACNG3 was constructed using 23 single nucleotide polymorphisms (SNPs). Transmission disequilibrium was sought using individual SNPs and SNP-based haplotypes with the pedigree disequilibrium test in 217 CAE trios and the 65 nuclear pedigrees.
Evidence for transmission disequilibrium (p≤0.01) was found for SNPs within a ∼35kb region of high LD encompassing the 5′UTR, exon 1 and part of intron 1 of CACNG3. Re-sequencing of this interval was undertaken in 24 affected individuals. Seventy-two variants were identified: 45 upstream; two 5′UTR; and 25 intronic SNPs. No coding sequence variants were identified, although four variants are predicted to affect exonic splicing.
This evidence supports CACNG3 as a susceptibility locus in a subset of CAE patients.
Absence epilepsy; linkage; association; CACNG3; genetics; splice variants
A substantial proportion of familial colorectal cancer (CRC) is not a consequence of known susceptibility loci, such as mismatch repair (MMR) genes, supporting the existence of additional loci. To identify novel CRC loci, we conducted a genome-wide linkage scan in 356 white families with no evidence of defective MMR (i.e., no loss of tumor expression of MMR proteins, no microsatellite instability (MSI)-high tumors, or no evidence of linkage to MMR genes). Families were ascertained via the Colon Cancer Family Registry multi-site NCI-supported consortium (Colon CFR), the City of Hope Comprehensive Cancer Center, and Memorial University of Newfoundland. A total of 1,612 individuals (average 5.0 per family including 2.2 affected) were genotyped using genome-wide single nucleotide polymorphism linkage arrays; parametric and non-parametric linkage analysis used MERLIN in a priori-defined family groups. Five lod scores greater than 3.0 were observed assuming heterogeneity. The greatest were among families with mean age of diagnosis less than 50 years at 4q21.1 (dominant HLOD = 4.51, α = 0.84, 145.40 cM, rs10518142) and among all families at 12q24.32 (dominant HLOD = 3.60, α = 0.48, 285.15 cM, rs952093). Among families with four or more affected individuals and among clinic-based families, a common peak was observed at 15q22.31 (101.40 cM, rs1477798; dominant HLOD = 3.07, α = 0.29; dominant HLOD = 3.03, α = 0.32, respectively). Analysis of families with only two affected individuals yielded a peak at 8q13.2 (recessive HLOD = 3.02, α = 0.51, 132.52 cM, rs1319036). These previously unreported linkage peaks demonstrate the continued utility of family-based data in complex traits and suggest that new CRC risk alleles remain to be elucidated.
Clear evidence exists for heritability of human longevity, and much interest is focused on identifying genes associated with longer lives. To identify such longevity alleles, we performed the largest genome-wide linkage scan thus far reported. Linkage analyses included 2118 nonagenarian Caucasian sibling pairs that have been enrolled in fifteen study centers of eleven European countries as part of the Genetics of Healthy Ageing (GEHA) project. In the joint linkage analyses we observed four regions that show linkage with longevity; chromosome 14q11.2 (LOD=3.47), chromosome 17q12-q22 (LOD=2.95), chromosome 19p13.3-p13.11 (LOD=3.76) and chromosome 19q13.11-q13.32 (LOD=3.57). To fine map these regions linked to longevity, we performed association analysis using GWAS data in a subgroup of 1,228 unrelated nonagenarian and 1,907 geographically matched controls. Using a fixed effect meta-analysis approach, rs4420638 at the TOMM40/APOE/APOC1 gene locus showed significant association with longevity (p-value=9.6 × 10−8). By combined modeling of linkage and association we showed that association of longevity with APOEε4 and APOEε2 alleles explain the linkage at 19q13.11-q13.32 with p-value=0.02 and p-value=1.0 × 10−5, respectively. In the largest linkage scan thus far performed for human familial longevity, we confirm that the APOE locus is a longevity gene and that additional longevity loci may be identified at 14q11.2, 17q12-q22 and 19p13.3-p13.11. Since the latter linkage results are not explained by common variants, we suggest that rare variants play an important role in human familial longevity.
Human familial longevity; genome-wide linkage analysis; APOE gene; association analysis; nonagenarian sibling pairs
To identify susceptibility loci for visual migraine aura in migraine families primarily affected with scintillating scotoma type of aura.
We included Finnish migraine families with at least 2 affected family members with scintillating scotoma as defined by the International Criteria for Headache Disorders–II. A total of 36 multigenerational families containing 351 individuals were included, 185 of whom have visual aura and 159 have scintillating scotoma. Parametric and nonparametric linkage analyses were performed with 378 microsatellite markers. The most promising linkage loci found were fine-mapped with additional microsatellite markers.
A novel locus on chromosome 9q22-q31 for migraine aura was identified (HLOD = 4.7 at 104 cM). Fine-mapping identified a shared haplotype segment of 12 cM (9.8 Mb) on 9q21-q22 among the aura affecteds. Four other loci showed linkage to aura: a locus on 12p13 showed significant evidence of linkage, and suggestive evidence of linkage was detected to loci on chromosomes 5q13, 6q25, and 13q14.
A novel visual migraine aura locus has been mapped to chromosome 9q21-q22. Interestingly, this region has previously been linked to occipitotemporal lobe epilepsy with prominent visual symptoms. Our finding further supports a shared genetic background in migraine and epilepsy and suggests that susceptibility variant(s) to visual aura for both of these traits are located in the 9q21-q22 locus.
= affected sibpairs;
= cortical spreading depression;
= familial hemiplegic migraine;
= genome-wide association;
= lod score under locus heterogeneity;
= International Classification of Headache Disorders, second edition;
= migraine with aura;
= migraine without aura;
= nonparametric linkage.
The search for susceptibility loci in hereditary prostate cancer (HPC) has proven challenging due to genetic and disease heterogeneity. Multiple risk loci have been identified to date, however few loci have been replicated across independent linkage studies. In addition, most previous analyses have been hampered by the relatively poor information content provided by microsatellite scans. To overcome these issues, we have performed linkage analyses on members of 301 HPC families genotyped using the Illumina SNP linkage panel IVb. The information content for this panel, averaged over all pedigrees and all chromosomes, was 86% (range 83–87% over chromosomes). Analyses were also stratified on families according to disease aggressiveness, age at diagnosis and number of affected individuals to achieve more genetically homogeneous subsets. Suggestive evidence for linkage was identified at 7q21 (HLOD = 1.87), 8q22 (KCLOD = 1.88) and 15q13–q14 (HLOD = 1.99) in 289 Caucasian families, and nominal evidence for linkage was identified at 2q24 (LOD = 1.73) in 12 African American families. Analysis of more aggressive prostate cancer phenotypes provided evidence for linkage to 11q25 (KCLOD = 2.02), 15q26 (HLOD = 1.99) and 17p12 (HLOD = 2.13). Subset analyses according to age at diagnosis and number of affected individuals also identified several regions with suggestive evidence for linkage, including a KCLOD of 2.82 at 15q13–q14 in 128 Caucasian families with younger ages at diagnosis. The results presented here provide further evidence for a prostate cancer susceptibility locus on chromosome 15q and demonstrate the power of utilizing high information content SNP scans in combination with homogenous collections of large prostate cancer pedigrees.
Technological advances in genetics have made feasible and affordable large studies to identify genetic variants that cause or modify a trait. Genetic studies have been carried out to assess variants in candidate genes, as well as polymorphisms throughout the genome, for their associations with heritable clinical outcomes of cystic fibrosis (CF), such as lung disease, meconium ileus, and CF-related diabetes. The candidate gene approach has identified some predicted relationships, while genome-wide surveys have identified several genes that would not have been obvious disease-modifying candidates, such as a methionine sulfoxide transferase gene that influences intestinal obstruction, or a region on chromosome 11 proximate to genes encoding a transcription factor and an apoptosis controller that associates with lung function. These unforeseen associations thus provide novel insight into disease pathophysiology, as well as suggesting new therapeutic strategies for CF.
Non-CFTR gene modifiers strongly influence the age of onset and the organ systems affected in cystic fibrosis patients. Studies have examined genes associated with lung and liver disease, meconium ileus, and diabetes.
Genetic studies of autism over the past decade suggest a complex landscape of multiple genes. In the face of this heterogeneity, studies that include large extended pedigrees may offer valuable insight, as the relatively few susceptibility genes within single large families may be more easily discerned. This genome-wide screen of 70 families includes 20 large extended pedigrees of 6–9 generations, 6 moderate-sized families of 4–5 generations, and 44 smaller families of 2–3 generations. The Center for Inherited Disease Research (CIDR) provided genotyping using the Illumina Linkage Panel 12, a 6K single nucleotide polymorphism (SNP) platform. Results from 192 subjects with an Autism Spectrum Disorder (ASD), and 461 of their relatives revealed genome-wide significance on chromosome 15q, with three possibly distinct peaks: 15q13.1-q14 (HLOD=4.09 at 29,459,872bp); 15q14-q21.1 (HLOD=3.59 at 36,837,208bp); and 15q21.1-q22.2 (HLOD=5.31 at 55,629,733bp). Two of these peaks replicate previous findings. There were additional suggestive results on chromosomes 2p25.3-p24.1 (HLOD=1.87), 7q31.31-q32.3 (HLOD=1.97), and 13q12.11-q12.3 (HLOD=1.93). Affected subjects in families supporting the linkage peaks found in this study did not reveal strong evidence for distinct phenotypic subgroups.
Autism Spectrum Disorder; genetic linkage; extended pedigrees; chromosome 15