Haplotype inference for present-absent genotype data using previously identified haplotypes and haplotype patterns

KIR genotype and haplotype frequencies in the multi-ethnic population of Malaysia

Killer cell immunoglobulin-like receptors (KIR) genotype and haplotype frequencies have been reported to vary distinctly between populations, which in turn contributes to variation in the alloreactivity of natural killer (NK) cells. Utilizing the diverse KIR genes to identify suitable transplant donors would prove challenging in multi-ethnic countries, even more in resource-limited countries where KIR genotyping has not been established. In this study, we determined the KIR genotypes from 124 unrelated Malaysians consisting of the Malays, Chinese, Indians, and aboriginal people through polymerase chain reaction sequence-specific primer (PCR-SSP) genotyping and employing an expectation-maximization (EM) algorithm to assign haplotypes based on pre-established reference haplotypes. A total of 27 distinct KIR haplotypes were discerned with higher frequencies of haplotype A (55.2%) than haplotype B (44.8%). The most frequent haplotypes were cA01:tA01 (55.2%), cB01:tB01 (18.1%), and cB02:tA01 (13.3%), while the least frequent haplotypes were cB03:tB01 (1.2%), cB04:tB03 (0.4%), and cB03:tA01 (0.4%). Several haplotypes were identified to be unique to a specific ethnic group. The genotype with the highest frequency was genotype AB (71.8%), followed by AA (19.4%), and BB (8.9%). The Indians exhibited the lowest genotype AA but the highest genotype BB, whereas genotype BB was absent in the aboriginal people. Despite the limitations, the genotype and haplotypes in the Malaysian population were successfully highlighted. The identification of ethnic-specific KIR genotypes and haplotypes provides the first step to utilizing KIR in identifying suitable transplant donors to further improve the transplant outcome in the Malaysian population.

Killer-Cell Immunoglobulin-like Receptor Diversity in an Admixed South American Population

Natural Killer (NK) cells are innate immune cells that mediate antiviral and antitumor responses. NK cell activation and induction of effector functions are tightly regulated by the integration of activating and inhibitory receptors such as killer immunoglobulin-like receptors (KIR). KIR genes are characterized by a high degree of diversity due to presence or absence, gene copy number and allelic polymorphism. The aim of this study was to establish the distribution of KIR genes and genotypes, to infer the most common haplotypes in an admixed Colombian population and to compare these KIR gene frequencies with some Central and South American populations and worldwide. A total of 161 individuals from Medellin, Colombia were included in the study. Genomic DNA was used for KIR and HLA genotyping. We analyzed only KIR gene-content (presence or absence) based on PCR-SSO. The KIR genotype, most common haplotypes and combinations of KIR and HLA ligands frequencies were estimated according to the presence or absence of KIR and HLA genes. Dendrograms, principal component (PC) analysis and Heatmap analysis based on genetic distance were constructed to compare KIR gene frequencies among Central and South American, worldwide and Amerindian populations. The 16 KIR genes analyzed were distributed in 37 different genotypes and the 7 most frequent KIR inferred haplotypes. Importantly, we found three new genotypes not previously reported in any other ethnic group. Our genetic distance, PC and Heatmap analysis revealed marked differences in the distribution of KIR gene frequencies in the Medellin population compared to worldwide populations. These differences occurred mainly in the activating KIR isoforms, which are more frequent in our population, particularly KIR3DS1. Finally, we observed unique structural patterns of genotypes, which evidences the potential diversity and variability of this gene family in our population, and the need for exhaustive genetic studies to expand our understanding of the KIR gene complex in Colombian populations.

Estimating KIR Haplotype Frequencies on a Cohort of 10,000 Individuals: A Comprehensive Study on Population Variations, Typing Resolutions, and Reference Haplotypes

The killer cell immunoglobulin-like receptors (KIR) mediate human natural killer (NK) cell cytotoxicity via activating or inhibiting signals. Although informative and functional haplotype patterns have been reported, most genotyping has been performed at resolutions that are structurally ambiguous. In order to leverage structural information given low-resolution genotypes, we performed experiments to quantify the effects of population variations, reference haplotypes, and genotyping resolutions on population-level haplotype frequency estimations as well as predictions of individual haplotypes. We genotyped 10,157 unrelated individuals in 5 populations (518 African American[AFA], 258 Asian or Pacific Islander[API], 8,245 European[EUR], 1,073 Hispanic[HIS], and 63 Native American[NAM]) for KIR gene presence/absence (PA), and additionally half of the AFA samples for KIR gene copy number variation (CNV). A custom EM algorithm was used to estimate haplotype frequencies for each population by interpretation in the context of three sets of reference haplotypes. The algorithm also assigns each individual the haplotype pairs of maximum likelihood. Generally, our haplotype frequency estimates agree with similar previous publications to within <5% difference for all haplotypes. The exception is that estimates for NAM from the U.S. showed higher frequency association of cB02 with tA01 (+14%) instead of tB01 (-8.5%) compared to a previous study of NAM from south of the U.S. The higher-resolution CNV genotyping on the AFA samples allowed unambiguous haplotype-pair assignments for the majority of individuals, resulting in a 22% higher median typing resolution score (TRS), which measures likelihood of self-match in the context of population-specific haplo- and geno-types. The use of TRS to quantify reduced ambiguity with CNV data clearly revealed the few individuals with ambiguous genotypes as outliers. It is observed that typing resolution and reference haplotype set influence haplotype frequency estimates. For example, PA resolution may be used with reference haplotype sets up to the point where certain haplotypes are gene-content subsets of others. At that point, CNV must be used for all genes.

A hidden Markov model for haplotype inference for present-absent data of clustered genes using identified haplotypes and haplotype patterns

The majority of killer cell immunoglobin-like receptor (KIR) genes are detected as either present or absent using locus-specific genotyping technology. Ambiguity arises from the presence of a specific KIR gene since the exact copy number (one or two) of that gene is unknown. Therefore, haplotype inference for these genes is becoming more challenging due to such large portion of missing information. Meantime, many haplotypes and partial haplotype patterns have been previously identified due to tight linkage disequilibrium (LD) among these clustered genes thus can be incorporated to facilitate haplotype inference. In this paper, we developed a hidden Markov model (HMM) based method that can incorporate identified haplotypes or partial haplotype patterns for haplotype inference from present-absent data of clustered genes (e.g., KIR genes). We compared its performance with an expectation maximization (EM) based method previously developed in terms of haplotype assignments and haplotype frequency estimation through extensive simulations for KIR genes. The simulation results showed that the new HMM based method outperformed the previous method when some incorrect haplotypes were included as identified haplotypes and/or the standard deviation of haplotype frequencies were small. We also compared the performance of our method with two methods that do not use previously identified haplotypes and haplotype patterns, including an EM based method, HPALORE, and a HMM based method, MaCH. Our simulation results showed that the incorporation of identified haplotypes and partial haplotype patterns can improve accuracy for haplotype inference. The new software package HaploHMM is available and can be downloaded at http://www.soph.uab.edu/ssg/files/People/KZhang/HaploHMM/haplohmm-index.html.

KIR gene content in amerindians indicates influence of demographic factors

Although the KIR gene content polymorphism has been studied worldwide, only a few isolated or Amerindian populations have been analyzed. This extremely diverse gene family codifies receptors that are expressed mainly in NK cells and bind HLA class I molecules. KIR-HLA combinations have been associated to several diseases and population studies are important to comprehend their evolution and their role in immunity. Here we analyzed, by PCR-SSP (specific sequencing priming), 327 individuals from four isolated groups of two of the most important Brazilian Amerindian populations: Kaingang and Guarani. The pattern of KIR diversity among these and other ten Amerindian populations disclosed a wide range of variation for both KIR haplotypes and gene frequencies, indicating that demographic factors, such as bottleneck and founder effects, were the most important evolutionary factors in shaping the KIR polymorphism in these populations.

Allele-level haplotype frequencies and pairwise linkage disequilibrium for 14 KIR loci in 506 European-American individuals

The immune responses of natural killer cells are regulated, in part, by killer cell immunoglobulin-like receptors (KIR). The 16 closely-related genes in the KIR gene system have been diversified by gene duplication and unequal crossing over, thereby generating haplotypes with variation in gene copy number. Allelic variation also contributes to diversity within the complex. In this study, we estimated allele-level haplotype frequencies and pairwise linkage disequilibrium statistics for 14 KIR loci. The typing utilized multiple methodologies by four laboratories to provide at least 2x coverage for each allele. The computational methods generated maximum-likelihood estimates of allele-level haplotypes. Our results indicate the most extensive allele diversity was observed for the KIR framework genes and for the genes localized to the telomeric region of the KIR A haplotype. Particular alleles of the stimulatory loci appear to be nearly fixed on specific, common haplotypes while many of the less frequent alleles of the inhibitory loci appeared on multiple haplotypes, some with common haplotype structures. Haplotype structures cA01 and/or tA01 predominate in this cohort, as has been observed in most populations worldwide. Linkage disequilibrium is high within the centromeric and telomeric haplotype regions but not between them and is particularly strong between centromeric gene pairs KIR2DL5∼KIR2DS3S5 and KIR2DS3S5∼KIR2DL1, and telomeric KIR3DL1∼KIR2DS4. Although 93% of the individuals have unique pairs of full-length allelic haplotypes, large genomic blocks sharing specific sets of alleles are seen in the most frequent haplotypes. These high-resolution, high-quality haplotypes extend our basic knowledge of the KIR gene system and may be used to support clinical studies beyond single gene analysis.

Killer cell immunoglobulin-like receptor (KIR) gene content variation in the HGDP-CEPH populations

In the present study, we investigate patterns of variation in the KIR cluster in a large and well-characterized sample of worldwide human populations in the Human Genome Diversity Project—Centre d'Etude du Polymorphisme Humain (HGDP-CEPH) panel in order to better understand the patterns of diversity in the region. Comparison of KIR data with that from other genomic regions allows control for strictly demographic factors; over 500,000 additional genomic markers have been typed in this panel by other investigators and the data made publicly available. Presence/absence frequencies and haplotypic associations for the KIR region are analyzed in the 52 populations comprising the panel and in accordance with major world regions (Africa, Middle East, Central Asia, East Asia, Europe, Americas, and Oceania). These data represent the first overview of KIR population genetics in the well-documented HGDP-CEPH panel and suggest different evolutionary histories and recent selection in the KIR gene cluster.

Differential association of gene content polymorphisms of killer cell immunoglobulin-like receptors with placental malaria in HIV- and HIV+ mothers

Pregnant women have abundant natural killer (NK) cells in their placenta, and NK cell function is regulated by polymorphisms of killer cell immunoglobulin-like receptors (KIRs). Previous studies report different roles of NK cells in the immune responses to placental malaria (PM) and human immunodeficiency virus (HIV-1) infections. Given these references, the aim of this study was to determine the association between KIR gene content polymorphism and PM infection in pregnant women of known HIV-1 status. Sixteen genes in the KIR family were analyzed in 688 pregnant Kenyan women. Gene content polymorphisms were assessed in relation to PM in HIV-1 negative and HIV-1 positive women, respectively. Results showed that in HIV-1 negative women, the presence of the individual genes KIR2DL1 and KIR2DL3 increased the odds of having PM, and the KIR2DL2/KIR2DL2 homozygotes were associated with protection from PM. However, the reverse relationship was observed in HIV-1 positive women, where the presence of individual KIR2DL3 was associated with protection from PM, and KIR2DL2/KIR2DL2 homozygotes increased the odds for susceptibility to PM. Further analysis of the HIV-1 positive women stratified by CD4 counts showed that this reverse association between KIR genes and PM remained only in the individuals with high CD4 cell counts but not in those with low CD4 cell counts. Collectively, these results suggest that inhibitory KIR2DL2 and KIR2DL3, which are alleles of the same locus, play a role in the inverse effects on PM and PM/HIV co-infection and the effect of KIR genes on PM in HIV positive women is dependent on high CD4 cell counts. In addition, analysis of linkage disequilibrium (LD) of the PM relevant KIR genes showed strong LD in women without PM regardless of their HIV status while LD was broken in those with PM, indicating possible selection pressure by malaria infection on the KIR genes.

Analytical methods for immunogenetic population data

In this chapter, we describe analyses commonly applied to immunogenetic population data, along with software tools that are currently available to perform those analyses. Where possible, we focus on tools that have been developed specifically for the analysis of highly polymorphic immunogenetic data. These analytical methods serve both as a means to examine the appropriateness of a dataset for testing a specific hypothesis, as well as a means of testing hypotheses. Rather than treat this chapter as a protocol for analyzing any population dataset, each researcher and analyst should first consider their data, the possible analyses, and any available tools in light of the hypothesis being tested. The extent to which the data and analyses are appropriate to each other should be determined before any analyses are performed.

Inference on the strength of balancing selection for epistatically interacting loci

Existing inference methods for estimating the strength of balancing selection in multi-locus genotypes rely on the assumption that there are no epistatic interactions between loci. Complex systems in which balancing selection is prevalent, such as sets of human immune system genes, are known to contain components that interact epistatically. Therefore, current methods may not produce reliable inference on the strength of selection at these loci. In this paper, we address this problem by presenting statistical methods that can account for epistatic interactions in making inference about balancing selection. A theoretical result due to Fearnhead (2006) is used to build a multi-locus Wright-Fisher model of balancing selection, allowing for epistatic interactions among loci. Antagonistic and synergistic types of interactions are examined. The joint posterior distribution of the selection and mutation parameters is sampled by Markov chain Monte Carlo methods, and the plausibility of models is assessed via Bayes factors. As a component of the inference process, an algorithm to generate multi-locus allele frequencies under balancing selection models with epistasis is also presented. Recent evidence on interactions among a set of human immune system genes is introduced as a motivating biological system for the epistatic model, and data on these genes are used to demonstrate the methods.

Impact of a functional KIR2DS4 allele on heterosexual HIV-1 transmission among discordant Zambian couples

Killer cell immunoglobulin-like receptors (KIRs) and their HLA ligands interact to regulate natural killer (NK) cell function. KIR gene content and allelic variations are reported to influence human immunodeficiency virus (HIV)-1 infection and pathogenesis. We investigated the impact of KIR genes on heterosexual HIV-1 transmission among 566 discordant couples from Lusaka, Zambia. KIR2DS4*001, the only allele of KIR2DS4 known to encode a functional activating receptor, was associated with relatively high viral load for HIV-1 in index (HIV-1 seroprevalent) partners (β [standard error (SE)], .17 [.8] log₁₀; P = .04) and with accelerated transmission of HIV-1 to cohabiting seronegative partners (relative hazard [RH], 2.00; P = .004). The latter association was independent of the direction of transmission (male-to-female or female-to-male), genital ulcers, and carriage of the putative ligand (HLA-Cw*04). No KIR-gene variant in the initially seronegative partners was associated with HIV-1 acquisition or early viral load following seroconversion. Further analysis of NK cell function should clarify the role of KIR2DS4*001 in HIV-1 transmission.

Human-specific evolution and adaptation led to major qualitative differences in the variable receptors of human and chimpanzee natural killer cells

Natural killer (NK) cells serve essential functions in immunity and reproduction. Diversifying these functions within individuals and populations are rapidly-evolving interactions between highly polymorphic major histocompatibility complex (MHC) class I ligands and variable NK cell receptors. Specific to simian primates is the family of Killer cell Immunoglobulin-like Receptors (KIR), which recognize MHC class I and associate with a range of human diseases. Because KIR have considerable species-specificity and are lacking from common animal models, we performed extensive comparison of the systems of KIR and MHC class I interaction in humans and chimpanzees. Although of similar complexity, they differ in genomic organization, gene content, and diversification mechanisms, mainly because of human-specific specialization in the KIR that recognizes the C1 and C2 epitopes of MHC-B and -C. Humans uniquely focused KIR recognition on MHC-C, while losing C1-bearing MHC-B. Reversing this trend, C1-bearing HLA-B46 was recently driven to unprecedented high frequency in Southeast Asia. Chimpanzees have a variety of ancient, avid, and predominantly inhibitory receptors, whereas human receptors are fewer, recently evolved, and combine avid inhibitory receptors with attenuated activating receptors. These differences accompany human-specific evolution of the A and B haplotypes that are under balancing selection and differentially function in defense and reproduction. Our study shows how the qualitative differences that distinguish the human and chimpanzee systems of KIR and MHC class I predominantly derive from adaptations on the human line in response to selective pressures placed on human NK cells by the competing needs of defense and reproduction.

Linkage disequilibrium organization of the human KIR superlocus: implications for KIR data analyses

An extensive family-based study of linkage disequilibrium (LD) in the killer cell immunoglobulin-like receptors (KIR) cluster was performed. We aimed to describe the LD structure in the KIR gene cluster using a sample of 418 founder haplotypes identified by segregation in a group of 106 families from Northern Ireland. The LD was studied at two levels of polymorphism: the structural level (presence or absence of KIR genes) and the allelic level (between alleles of KIR genes). LD was further assessed using the predictive value of one KIR polymorphism for another one in order to provide an interpretative framework for the LD effect in association studies. In line with previous research, distinct patterns of KIR genetic diversity within the genomic region centromeric to KIR2DL4 (excluding KIR2DL4) and within the telomeric region including KIR2DL4 were found. In a comprehensive PPV/NPV-based LD analysis within the KIR cluster, robust tag markers were found that can be used to identify which genes are concomitantly present or absent and to further identify groups of associated KIR alleles. Several extended KIR haplotypes in the study population were identified (KIR2DS2*POS–KIR2DL2*001–KIR2DL5B*002–KIR2DS3*00103–KIR2DL1*00401; KIR2DL4*011–KIR3DL1/S1*005–KIR2DS4*003–KIR3DL2*003; KIR2DL4*00802–KIR3DL1/S1*004–KIR2DS4*006–KIR3DL2*005; KIR2DL4*00801–KIR3DL1/S1*00101–KIR2DS4*003–KIR3DL2*001; KIR2DL4*00103–KIR3DL1/S1*008–KIR2DS4*003–KIR3DL2*009; KIR2DL4*00102–KIR3DL1/S1*01502/*002–KIR2DS4*00101–KIR3DL2*002; KIR2DL4*00501–KIR3DL1/S1*013–KIR2DL5A*001–KIR2DS5*002–KIR2DS1*002–KIR3DL2*007). The present study provides a rationale for analyzing associations of KIR polymorphisms by taking into account the complex LD structure of the KIR region.

Tag SNP selection using particle swarm optimization

Single nucleotide polymorphisms (SNPs) are the most abundant form of genetic variations amongst species. With the genome-wide SNP discovery, many genome-wide association studies are likely to identify multiple genetic variants that are associated with complex diseases. However, genotyping all existing SNPs for a large number of samples is still challenging even though SNP arrays have been developed to facilitate the task. Therefore, it is essential to select only informative SNPs representing the original SNP distributions in the genome (tag SNP selection) for genome-wide association studies. These SNPs are usually chosen from haplotypes and called haplotype tag SNPs (htSNPs). Accordingly, the scale and cost of genotyping are expected to be largely reduced. We introduce binary particle swarm optimization (BPSO) with local search capability to improve the prediction accuracy of STAMPA. The proposed method does not rely on block partitioning of the genomic region, and consistently identified tag SNPs with higher prediction accuracy than either STAMPA or SVM/STSA. We compared the prediction accuracy and time complexity of BPSO to STAMPA and an SVM-based (SVM/STSA) method using publicly available data sets. For STAMPA and SVM/STSA, BPSO effective improved prediction accuracy for smaller and larger scale data sets. These results demonstrate that the BPSO method selects tag SNP with higher accuracy no matter the scale of data sets is used.

Report from the killer immunoglobulin-like receptor (KIR) anthropology component of the 15th International Histocompatibility Workshop: worldwide variation in the KIR loci and further evidence for the co-evolution of KIR and HLA

The killer immunoglobulin-like receptor (KIR) anthropology component of the 15th International Histocompatibility Workshop (IHIWS) sought to explore worldwide population variation in the KIR loci, and to examine the relationship between KIR genes and their human leukocyte antigen (HLA) ligands. Fifteen laboratories submitted KIR genotype and HLA ligand data in 27 populations from six broad ethnic groups. Data were analyzed for correlations between the frequencies of KIR and their known HLA ligands. In addition, allelic typing was performed for KIR2DL2 and 3DL1 in a subset of populations. Strong and significant correlations were observed between KIR2DL2, 2DL3 genotype frequencies and the frequency of their ligand, HLA-C1. In contrast, only weak associations were seen for 3DL1, 3DS1 and the HLA-Bw4 ligand. Although some aspects of the correlations observed here differ from those reported in other populations, these data provide additional evidence of linked evolutionary histories for some KIR and HLA loci. Investigation of allele-level variation for the B haplotype locus KIR 2DL2 showed that two alleles, *001 and *003, predominate in all populations in this study. Much more allelic variation was observed for the A haplotype locus 3DL1, with several alleles observed at moderate frequencies and extensive variation observed between populations.

HLA haplotype and supertype associations with cellular immune responses and cytokine production in healthy children after rubella vaccine

Secreted rubella virus-specific cytokines reflect the immunologic mechanisms underlying adoptive immune responses and are significant markers of immunity to rubella. We studied the association between measures of cellular (cytokine and frequency of cytokine-secreted cells) immune responses and HLA haplotypes (with frequencies of > or =1%) and supertypes among 738 healthy children following two doses of rubella vaccine. Haplotype effects were estimated while accounting for linkage phase ambiguity via an expectation maximization algorithm. Importantly, the majority of HLA class I and class II haplotype associations with different cytokines were consistent between Th1, Th2 and/or innate/proinflammatory cytokine groups. We found few class I supertypes (A1, A2, A3, and B7) with potential associations with IL-10 ELISPOT counts and rubella-specific IL-2, IL-10, TNF-alpha, and IL-6 cytokine secretion levels. Our data indicate that the presence or absence of certain HLA haplotypes and/or supertypes may influence the cytokine immune response to rubella vaccine, and represents a more advanced analysis compared to individual candidate gene association studies.

KIR haplotypes defined by segregation analysis in 59 Centre d'Etude Polymorphisme Humain (CEPH) families

The killer cell immunoglobulin-like receptor (KIR) gene cluster exhibits extensive allelic and haplotypic diversity. Variation at the locus is associated with an increasing number of human diseases, reminiscent of the HLA loci. Characterization of diversity at the KIR locus has progressed over the past several years, particularly since the sequence of entire KIR haplotypes have become available. To determine the extent of KIR haplotypic variability among individuals of northern European descent, we genotyped 59 CEPH families for presence/absence of all KIR genes and performed limited allelic subtyping at several KIR loci. A total of 20 unique haplotypes differing in gene content were identified, the most common of which was the previously defined A haplotype (f = 0.52). Several unusual haplotypes that probably arose as a consequence of unequal crossing over events were also identified. Linkage disequilibrium (LD) analysis indicated strong negative and positive LD between several pairs of genes, values that may be useful in determining haplotypic structure when family data are not available. These data provide a resource to aid in the interpretation of disease association data involving individuals of European descent.

Methods for assessing gene content diversity of KIR with examples from a global set of populations

A number of statistical methods are widely used to describe allelic variation at specific genetic loci and its implication on the evolutionary history of these loci. Although the methods were developed primarily to study allelic variation at loci that are virtually always present in the genome, they are often applied to data of gene content variation (i.e., presence/absence of multiple homologous genes) at the killer cell immunoglobulin-like receptor (KIR) gene cluster. In this paper, we discuss methodological issues involved in the analysis of gene content variation data in the KIR region and also its covariation with polymorphism at the human leukocyte antigen class I loci, which encode ligands for KIR. A comparison of several statistical methods and measures (gene frequency, haplotype frequency, and linkage disequilibrium estimation) using the Centre d'Etude du Polymorphisme Humain data will be provided using KIR haplotypes that have been determined by segregation analysis, noting the strengths and weaknesses of the methods when only the presence/absence data is considered. Finally, application of these methods to a set of globally distributed populations is described (see Single et al., Nat Genet 39:1114-1119, 2007) in order to illustrate the challenges faced when inferring the joint effects of natural selection and demographic history on these immune-related genes.

NullHap--a versatile application to estimate haplotype frequencies from unphased genotypes in the presence of null alleles

BACKGROUND

Laboratory techniques used to determine haplotypes are often too expensive for large-scale studies and lack of phase information is commonly overcome using likelihood-based calculations. Whereas a number of programs are available for that purpose, none of them can handle loci with both multiple and null alleles.

RESULTS

Here we present a description of a modified Expectation - Maximization algorithm as well as its implementation (NullHap) which allow to effectively overcome these limitations. As an example of application we used Nullhap to reanalyze published data on distribution of KIR genotypes in Polish psoriasis patients and controls showing that the KIR2DS4/1D locus may be a marker of KIR2DS1 haplotypes with different effects on disease susceptibility.

CONCLUSION

The developed application can estimate haplotype frequencies for every type of polymorphism and can effectively be used in genetic research as illustrated by a novel finding regarding the genetic susceptibility to psoriasis.