Association of systemic lupus erythematosus clinical features with European population genetic substructure

Systemic Lupus Erythematosus (SLE) is an autoimmune disease with a very varied spectrum of clinical manifestations that could be partly determined by genetic factors. We aimed to determine the relationship between prevalence of 11 clinical features and age of disease onset with European population genetic substructure. Data from 1413 patients of European ancestry recruited in nine countries was tested for association with genotypes of top ancestry informative markers. This analysis was done with logistic regression between phenotypes and genotypes or principal components extracted from them. We used a genetic additive model and adjusted for gender and disease duration. Three clinical features showed association with ancestry informative markers: autoantibody production defined as immunologic disorder (P = 6.8×10⁻⁴), oral ulcers (P = 6.9×10⁻⁴) and photosensitivity (P = 0.002). Immunologic disorder was associated with genotypes more common in Southern European ancestries, whereas the opposite trend was observed for photosensitivity. Oral ulcers were specifically more common in patients of Spanish and Portuguese self-reported ancestry. These results should be taken into account in future research and suggest new hypotheses and possible underlying mechanisms to be investigated. A first hypothesis linking photosensitivity with variation in skin pigmentation is suggested.     

QTL mapping in autotetraploids using SNP dosage information

Key message

Dense linkage maps derived by analysing SNP dosage in autotetraploids provide detailed information about the location of, and genetic model at, quantitative trait loci.

Abstract

Recent developments in sequencing and genotyping technologies enable researchers to generate high-density single nucleotide polymorphism (SNP) genotype data for mapping studies. For polyploid species, the SNP genotypes are informative about allele dosage, and Hackett et al. (PLoS ONE 8:e63939, 2013) presented theory about how dosage information can be used in linkage map construction and quantitative trait locus (QTL) mapping for an F₁ population in an autotetraploid species. Here, QTL mapping using dosage information is explored for simulated phenotypic traits of moderate heritability and possibly non-additive effects. Different mapping strategies are compared, looking at additive and more complicated models, and model fitting as a single step or by iteratively re-weighted modelling. We recommend fitting an additive model without iterative re-weighting, and then exploring non-additive models for the genotype means estimated at the most likely position. We apply this strategy to re-analyse traits of high heritability from a potato population of 190 F₁ individuals: flower colour, maturity, height and resistance to late blight (Phytophthora infestans (Mont.) de Bary) and potato cyst nematode (Globodera pallida), using a map of 3839 SNPs. The approximate confidence intervals for QTL locations have been improved by the detailed linkage map, and more information about the genetic model at each QTL has been revealed. For several of the reported QTLs, candidate SNPs can be identified, and used to propose candidate trait genes. We conclude that the high marker density is informative about the genetic model at loci of large effects, but that larger populations are needed to detect smaller QTLs.     

Rapid SNP discovery and genetic mapping using sequenced RAD markers

Single nucleotide polymorphism (SNP) discovery and genotyping are essential to genetic mapping. There remains a need for a simple, inexpensive platform that allows high-density SNP discovery and genotyping in large populations. Here we describe the sequencing of restriction-site associated DNA (RAD) tags, which identified more than 13,000 SNPs, and mapped three traits in two model organisms, using less than half the capacity of one Illumina sequencing run. We demonstrated that different marker densities can be attained by choice of restriction enzyme. Furthermore, we developed a barcoding system for sample multiplexing and fine mapped the genetic basis of lateral plate armor loss in threespine stickleback by identifying recombinant breakpoints in F(2) individuals. Barcoding also facilitated mapping of a second trait, a reduction of pelvic structure, by in silico re-sorting of individuals. To further demonstrate the ease of the RAD sequencing approach we identified polymorphic markers and mapped an induced mutation in Neurospora crassa. Sequencing of RAD markers is an integrated platform for SNP discovery and genotyping. This approach should be widely applicable to genetic mapping in a variety of organisms.     

Analysis of genetic mapping in a waxy/dent maize RIL population using SSR and SNP markers

A maize genetic linkage map was generated using SSR and SNP markers in a F_7:8 recombinant inbred line (RIL) population derived from a cross of waxy corn (KW7) and dent corn (Mo17). A total of 465 markers, including 459 SSR and 6 SNP markers, were assigned to 10 linkage groups which spanned 2,656.5 cM with an average genetic distance between markers of 5.7 cM, and the number of loci per linkage group ranged from 39 to 55. The SSR (85.4%) and SNP (83.3%) markers showed Mendelian segregation ratios in the RIL population at a 5% significance threshold. In linkage analysis of six SNP loci associated with kernel starch synthesis genes (ae1, bt2, sh1, sh2, su1, and wx1), all six loci were successfully mapped and are closely linked with SSR markers in chromosomes 3 (sh2), 4 (su1 and bt2), 5 (ae1), and 9 (sh1 and wx1). The SSR markers linked with genes in starch synthesis may be utilized in marker assisted breeding programs. The resulting genetic map will be useful in dissection of quantitative traits and the identification of superior QTLs from the waxy hybrid corn. Additionally, these data support further genetic analysis and development of maize breeding programs.     

Dissecting the genetic structure of Korean population using genome-wide SNP arrays

Genome-wide SNP arrays have generated unprecedented quantities of data allow the detection of human evolutionary history and dense genome-wide data also enable the identification of distance ancestry among individuals or ethnic groups. To explain wider aspects of the genetic structure of Koreans and the East Asian population, we analyzed 79 individuals from the Korean HapMap project at 555,352 common single-nucleotide polymorphism loci, and compared this data with the worldwide population groups with the 53 ethnic groups from Human Genome Diversity Panel (HGDP-CEPH). Population differentiation (F_ST), Principal Component Analyses, STRUCTURE and ADMIXTURE are examined. In general, all the individual samples studies here were classified into subset of ethnic groups according to their geographical origins. Korean HapMap individuals were grouped together with East Asian populations from HGDP panel. Recently, a sub-population structure within Korean population has been reported. Our result, however, revealed the genetic homogeneity of Korean population. The ADMIXTURE analysis showed that, overall the Korean populations derive 79 % of their genomic ancestry from southern Asia and have relatively little northern Asian ancestry (21 %). The present work, therefore, provide the evidence that the male-biased southern-to-northern migration influenced not only for the genetic make up of the Y chromosome in the Korean population but also, its autosomal composition.     

Fine genetic characterization of elite maize germplasm using high-throughput SNP genotyping

To investigate the genetic structure of Chinese maize germplasm, the MaizeSNP50 BeadChip with 56,110 single nucleotide polymorphisms (SNPs) was used to genotype a collection of 367 inbred lines widely used in maize breeding of China. A total of 41,819 informative SNPs with minor allele number of more than 0.05 were used to estimate the genetic diversity, relatedness, and linkage disequilibrium (LD) decay. Totally 1,015 SNPs evenly distributed in the genome were selected randomly to evaluate the population structure of these accessions. Results showed that two main groups could be determined i.e., the introduced germplasm and the local germplasm. Further, five subgroups corresponding to different heterotic groups, that is, Reid Yellow Dent (Reid), Lancaster Sure Crop (Lancaster), P group (P), Tang Sipingtou (TSPT), and Tem-tropic I group (Tem-tropic I), were determined. The genetic diversity of within subgroups was highest in the Tem-Tropic I and lowest in the P. Most lines in this panel showed limited relatedness with each other. Comparisons of gene diversity showed that there existed some conservative genetic regions in specific subgroups across the ten chromosomes, i.e., seven in the Lancaster, seven in the Reid, six in the TSPT, five in the P, and two in the Tem-Tropical I. In addition, the results also revealed that there existed fifteen conservative regions transmitted from Huangzaosi, an important foundation parent, to its descendants. These are important for further studies since the outcomes may provide clues to understand why Huangzaosi could become a foundation parent in Chinese maize breeding. For the panel of 367 elite lines, average LD distance was 391 kb and varied among different chromosomes as well as in different genomic regions of one chromosome. This analysis uncovered a high natural genetic diversity in the elite maize inbred set, suggesting that the panel can be used in association study, esp. for temperate regions.     

European population substructure: clustering of northern and southern populations

Using a genome-wide single nucleotide polymorphism (SNP) panel, we observed population structure in a diverse group of Europeans and European Americans. Under a variety of conditions and tests, there is a consistent and reproducible distinction between “northern” and “southern” European population groups: most individual participants with southern European ancestry (Italian, Spanish, Portuguese, and Greek) have >85% membership in the “southern” population; and most northern, western, eastern, and central Europeans have >90% in the “northern” population group. Ashkenazi Jewish as well as Sephardic Jewish origin also showed >85% membership in the “southern” population, consistent with a later Mediterranean origin of these ethnic groups. Based on this work, we have developed a core set of informative SNP markers that can control for this partition in European population structure in a variety of clinical and genetic studies.     

Unraveling the genetic diversity of Belgian Milk Sheep using medium-density SNP genotypes

The present study focuses on the Belgian Milk Sheep in Flanders (Belgium) and compares its genetic diversity and relationship with the Flemish Sheep, the Friesian Milk Sheep, the French Lacaune dairy sheep and other Northern European breeds. For this study, 94 Belgian Milk Sheep, 23 Flemish Sheep and 22 Friesian Milk Sheep were genotyped with the OvineSNP50 array. In addition, 29 unregistered animals phenotypically similar to Belgian Milk Sheep were genotyped using the 15K ISGC chip. Both Belgian and Friesian Milk Sheep as well as the East Friesian Sheep were found to be less diverse than the other seven breeds included in this study. Genomic inbreeding coefficients based on runs of homozygosity (ROH) were estimated at 14.5, 12.4 and 10.2% for Belgian Milk Sheep, Flemish Sheep and Friesian Milk Sheep respectively. Out of 29 unregistered Belgian Milk Sheep, 28 mapped in the registered Belgian Milk Sheep population. Ancestry analysis, PCA and F_ST calculations showed that Belgian Milk Sheep are more related to Friesian Milk Sheep than to Flemish Sheep, which was contrary to the breeders' expectations. Consequently, breeders may prefer to crossbreed Belgian Milk Sheep with Friesian sheep populations (Friesian Milk Sheep or East Friesian Sheep) in order to increase diversity. This research underlines the usefulness of SNP chip genotyping and ROH analyses for monitoring genetic diversity and studying genetic links in small livestock populations, profiting from internationally available genotypes. As assessment of genetic diversity is vital for long-term breed survival, these results will aid flockbooks to preserve genetic diversity.     

Construction of a genetic linkage map in Fenneropenaeus chinensis using SNP markers

Genetic linkage maps of Fenneropenaeus chinensis were constructed using a “double pseudo-testcross” strategy with 200 single nucleotide polymorphisms (SNPs) markers. This study represents the first SNP genetic linkage map for F. chinensis. The parents and F ₁ progeny of 100 individuals were used as mapping populations. 21 genetic linkage groups in the male and female maps were identified. The male linkage map was composed of 115 loci and spanned 879.7 cM, with an average intermarker spacing of 9.4 cM, while the female map was composed of 119 loci and spanned 876.2 cM, with an average intermarker spacing of 8.9 cM. The estimated coverage of the linkage maps was 51.94% for the male and 53.77% for the female, based on two estimates of genome length. The integrated map contains 180 markers distributed in 16 linkage groups, and spans 899.3 cM with an average marker interval of 5.2 cM. This SNP genetic map lays the foundation for future shrimp genomics and genetic breeding studies, especially the discovery of gene or regions for economically important traits in Chinese shrimp.     

Characterization of genetic diversity and population structure in wheat using array based SNP markers

Genetic diversity is crucial for successful adaptation and sustained improvement in crops. India is bestowed with diverse agro-climatic conditions which makes it rich in wheat germplasm adapted to various niches. Germplasm repository consists of local landraces, trait specific genetic stocks including introgressions from wild relatives, exotic collections, released varieties, and improved germplasm. Characterization of genetic diversity is done using morpho-physiological characters as well as by analyzing variations at DNA level. However, there are not many reports on array based high throughput SNP markers having characteristics of genome wide coverage employed in Indian spring wheat germplasm. Amongst wheat SNP arrays, 35K Axiom Wheat Breeder’s Array has the highest SNP polymorphism efficiency suitable for genetic mapping and genetic diversity characterization. Therefore, genotyping was done using 35K in 483 wheat genotypes resulting in 14,650 quality filtered SNPs, that were distributed across the B (~?50%), A (~?39%), and D (~?10%) genomes. The total genetic distance coverage was 4477.85 cM with 3.27 SNP/cM and 0.49 cM/SNP as average marker density and average inter-marker distance, respectively. The PIC ranged from 0.09 to 0.38 with an average of 0.29 across genomes. Population structure and Principal Coordinate Analysis resulted in two subpopulations (SP1 and SP2). The analysis of molecular variance revealed the genetic variation of 2% among and 98% within subpopulations indicating high gene flow between SP1 and SP2. The subpopulation SP2 showed high level of genetic diversity based on genetic diversity indices viz. Shannon’s information index (I)?=?0.648, expected heterozygosity (He)?=?0.456 and unbiased expected heterozygosity (uHe)?=?0.456. To the best of our knowledge, this study is the first to include the largest set of Indian wheat genotypes studied exclusively for genetic diversity. These findings may serve as a potential source for the identification of uncharacterized QTL/gene using genome wide association studies and marker assisted selection in wheat breeding programs.

     

Mutagenic primer design for mismatch PCR-RFLP SNP genotyping using a genetic algorithm

Polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) is useful in small-scale basic research studies of complex genetic diseases that are associated with single nucleotide polymorphism (SNP). Designing a feasible primer pair is an important work before performing PCR-RFLP for SNP genotyping. However, in many cases, restriction enzymes to discriminate the target SNP resulting in the primer design is not applicable. A mutagenic primer is introduced to solve this problem. GA-based Mismatch PCR-RFLP Primers Design (GAMPD) provides a method that uses a genetic algorithm to search for optimal mutagenic primers and available restriction enzymes from REBASE. In order to improve the efficiency of the proposed method, a mutagenic matrix is employed to judge whether a hypothetical mutagenic primer can discriminate the target SNP by digestion with available restriction enzymes. The available restriction enzymes for the target SNP are mined by the updated core of SNP-RFLPing. GAMPD has been used to simulate the SNPs in the human SLC6A4 gene under different parameter settings and compared with SNP Cutter for mismatch PCR-RFLP primer design. The in silico simulation of the proposed GAMPD program showed that it designs mismatch PCR-RFLP primers. The GAMPD program is implemented in JAVA and is freely available at http://bio.kuas.edu.tw/gampd/.     

High-resolution analysis of chromosomal imbalances using the Affymetrix 10K SNP genotyping chip

Array-based comparative genome hybridization is a powerful tool for detecting chromosomal imbalances at high resolution. However, the design and setup of such arrays are time consuming and expensive and thus worthwhile only when large numbers of arrays will be processed. To provide a feasible solution, we have developed an algorithm that renders the publicly available Affymetrix 10K SNP genotyping chip useful for high-resolution analysis of chromosomal imbalances. We have used our newly developed algorithm to analyze data from Affymetrix 10K chips that were hybridized with DNA probes from a variety of different sources, such as primary tumors, cell lines, and blood from patients with unbalanced translocations. In summary, we were able to (i) demonstrate the capability of our method by reproduction of published and unpublished data obtained with alternative methods and (ii) identify novel imbalances that were not shown before.     

Identification of functional genetic variations underlying drought tolerance in maize using SNP markers

Single nucleotide polymorphism (SNP) is a common form of genetic variation and popularly exists in maize genome. An Illumina GoldenGate assay with 1 536 SNP markers was used to genotype maize inbred lines and identified the functional genetic variations underlying drought tolerance by association analysis. Across 80 lines, 1 006 polymorphic SNPs (65.5% of the total) in the assay with good call quality were used to estimate the pattern of genetic diversity, population structure, and familial relatedness. The analysis showed the best number of fixed subgroups was six, which was consistent with their original sources and results using only simple sequence repeat markers. Pairwise linkage disequilibrium (LD) and association mapping with phenotypic traits investigated under water-stressed and well-watered regimes showed rapid LD decline within 100-500 kb along the physical distance of each chromosome, and that 29 SNPs were associated with at least two phenotypic traits in one or more environments, which were related to drought-tolerant or drought-responsive genes. These drought-tolerant SNPs could be converted into functional markers and then used for maize improvement by marker-assisted selection.     

Construction of Agropyron Gaertn. genetic linkage maps using a wheat 660K SNP array reveals a homoeologous relationship with the wheat genome

Agropyron Gaertn. (P genome) is a wild relative of wheat that harbours many genetic variations that could be used to increase the genetic diversity of wheat. To agronomically transfer important genes from the P genome to a wheat chromosome by induced homoeologous pairing and recombination, it is necessary to determine the chromosomal relationships between Agropyron and wheat. Here, we report using the wheat 660K single nucleotide polymorphism (SNP) array to genotype a segregating Agropyron F₁ population derived from an interspecific cross between two cross‐pollinated diploid collections ‘Z1842’ [A. cristatum (L.) Beauv.] (male parent) and ‘Z2098’ [A. mongolicum Keng] (female parent) and 35 wheat–A. cristatum addition/substitution lines. Genetic linkage maps were constructed using 913 SNP markers distributed among seven linkage groups spanning 839.7 cM. The average distance between adjacent markers was 1.8 cM. The maps identified the homoeologous relationship between the P genome and wheat and revealed that the P and wheat genomes are collinear and relatively conserved. In addition, obvious rearrangements and introgression spread were observed throughout the P genome compared with the wheat genome. Combined with genotyping data, the complete set of wheat–A. cristatum addition/substitution lines was characterized according to their homoeologous relationships. In this study, the homoeologous relationship between the P genome and wheat was identified using genetic linkage maps, and the detection mean for wheat–A. cristatum introgressions might significantly accelerate the introgression of genetic variation from Agropyron into wheat for exploitation in wheat improvement programmes.     

ALOHOMORA: a tool for linkage analysis using 10K SNP array data

SUMMARY: ALOHOMORA is a software tool designed to facilitate genome-wide linkage studies performed with high-density single nucleotide polymorphism (SNP) marker panels such as the Affymetrix GeneChip(R) Human Mapping 10K Array. Genotype data are converted into appropriate formats for a number of common linkage programs and subjected to standard quality control routines before linkage runs are started. ALOHOMORA is written in Perl and may be used to perform state-of-the-art linkage scans in small and large families with any genetic model. Options for using different genetic maps or ethnicity-specific allele frequencies are implemented. Graphic outputs of whole-genome multipoint LOD score values are provided for the entire dataset as well as for individual families. AVAILABILITY: ALOHOMORA is available free of charge for non-commercial research institutions. For more details, see http://gmc.mdc-berlin.de/alohomora/     

Copy number variations identified in the chicken using a 60K SNP BeadChip

Copy number variation (CNV) is considered an important genetic variation, contributing to many economically important traits in the chicken. Although CNVs can be detected using a comparative genomic hybridization array, the high‐density SNP array has provided an alternative way to identify CNVs in the chicken. In the current study, a chicken 60K SNP BeadChip was used to identify CNVs in two distinct chicken genetic lines (White Leghorn and dwarf) using the penncnv program. A total of 209 CNV regions were identified, distributing on chromosomes 1–22 and 24–28 and encompassing 13.55 Mb (1.42%) of chicken autosomal genome area. Three of seven selected CNVs (73.2% individuals) were completely validated by quantitative PCR. To our knowledge, this is the first report in the chicken identifying CNVs using a SNP array. Identification of 190 new identified CNVs illustrates the feasibility of the chicken 60K SNP BeadChip to detect CNVs in the chicken, which lays a solid foundation for future analyses of associations of CNVs with economically important phenotypes in chickens.     

SNP-based analysis of genetic substructure in the German population

OBJECTIVE: To evaluate the relevance and necessity to account for the effects of population substructure on association studies under a case-control design in central Europe, we analysed three samples drawn from different geographic areas of Germany. Two of the three samples, POPGEN (n = 720) and SHIP (n = 709), are from north and north-east Germany, respectively, and one sample, KORA (n = 730), is from southern Germany. METHODS: Population genetic differentiation was measured by classical F-statistics for different marker sets, either consisting of genome-wide selected coding SNPs located in functional genes, or consisting of selectively neutral SNPs from 'genomic deserts'. Quantitative estimates of the degree of stratification were performed comparing the genomic control approach [Devlin B, Roeder K: Biometrics 1999;55:997-1004], structured association [Pritchard JK, Stephens M, Donnelly P: Genetics 2000;155:945-959] and sophisticated methods like random forests [Breiman L: Machine Learning 2001;45:5-32]. RESULTS: F-statistics showed that there exists a low genetic differentiation between the samples along a north-south gradient within Germany (F(ST)(KORA/POPGEN): 1.7 . 10(-4); F(ST)(KORA/SHIP): 5.4 . 10(-4); F(ST)(POPGEN/SHIP): -1.3 . 10(-5)). CONCLUSION: Although the F(ST )-values are very small, indicating a minor degree of population structure, and are too low to be detectable from methods without using prior information of subpopulation membership, such as STRUCTURE [Pritchard JK, Stephens M, Donnelly P: Genetics 2000;155:945-959], they may be a possible source for confounding due to population stratification.