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CUTTING EDGE |



* Basic Research Program, Science Applications International Corporation-Frederick, and
Laboratory of Genomic Diversity, National Cancer Institute, Frederick, MD 21702; and
Immunology Division, Department of Pathology, University of Cambridge, Cambridge, United Kingdom
| Abstract |
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| Introduction |
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The KIR genes map to 19q13.4 where they are arranged in a head to tail fashion spanning a region of roughly 150 Kb (12, 13). KIR genes are generally 8090% identical, whereas allelic variants of a single KIR gene tend to differ by 2% or less (14, 15). Except for a unique 14-kb sequence in the center of the KIR gene cluster just upstream of the KIR2DL4 gene, intervening segments between adjacent KIR genes are consistently 2 kb in length and are highly conserved (16). Three prototypic KIR haplotypes have been sequenced in their entirety (5, 6), providing fundamental information regarding KIR gene order across the cluster. Additional information regarding gene order has been garnered from a sequence-specific priming (SSP) protocol in which forward and reverse PCR primers were designed from gene-specific segments near the 3' end and 5' end of each KIR gene, respectively (3), which we will refer to as "intergenic SSP-PCR." By this approach, PCR products are produced only when the forward primer recognizing the 3' end of one gene and the reverse primer recognizing the 5' end of an immediately adjacent gene are used, thereby defining the pairwise order of KIR genes on that haplotype.
In this study, we describe an extended KIR haplotype in a family that contains two copies of both KIR2DL4 and KIR3DL1/S1, as well as a novel hybrid gene composed of half KIR2DL5A and half KIR3DP1. All individuals with the extended haplotype have three copies of both KIR2DL4 and KIR3DL1/S1, two of each on the extended haplotype and one of each on the homologous haplotype. We propose that the extended haplotype was generated by unequal crossing over, which represents a likely mechanism for the expansion and contraction of KIR haplotypes in general. Unequal crossing over may potentially maintain flux in the physical order of KIR genes within the set of KIR haplotypes present in a population.
| Materials and Methods |
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Genomic DNA from a three-generation Center dEtude du Polymorphisme Humaine family was genotyped for presence or absence of the following KIR genes: 2DL1, 2DL2, 2DL3, 2DL4, 2DL5, 2DS1, 2DS2, 2DS3, 2DS4, 2DS5, 3DL1, 3DL2, 3DL3, 3DS1, 2DP1, and 3DP1. Genotyping was performed using PCR amplification with two pairs of locus-specific primers (PCR-SSP) as previously described (10). Internal control primers that amplify a 796-bp fragment of the third intron of DRB1 were also included in each PCR to validate proper amplifications. Additional primers that recognize 3DP1 and 2DS4, respectively, are as follows: 3DP1F, 5'-GCAGCACCATGTCGCTCATG-3'; 3DP1R, 5'-AACGGTGTTTCGGAATAC-3'; 3DP1
exon 2F, 5'-CAGGGGGCCTGGCCACATGA-3'; 2DS4vF, 5'-GTTCAGGCAGGAGAGAAT-3'; 2DS4vR, 5'-GTTTGACCACTCGTAGGGAGC. Amplification was performed in a volume of 10 µl containing 200 µM dNTP, 500 nM primer, 1.5 mM MgCl2, 20 mM Tris-HCl (pH 8.4), 50 mM KCl, and 0.5U Platinum TaqDNA polymerase (Invitrogen, Carlsbad, CA), and 20 ng of DNA. Cycling was performed as follows: 2 min at 94°C; 5 cycles of 94°C for 15 s, 65°C for 15 s, 72°C for 30 s; 21 cycles of 94°C for 15 s, 60°C for 15 s, 72°C for 30 s; 4 cycles of 94°C for 15 s, 55°C for 1 min, 72°C for 2 min, and a final extension step of 10 min at 72°C. PCR products were electrophoresed in 1.5% agarose gels containing ethidium bromide, and predicted size products were visualized under UV light.
KIR haplotype determination
KIR haplotypes were determined by segregation analysis in the family (see Fig. 2). Because it was not always possible to define precisely the gene content of the haplotypes using segregation analysis, several assumptions were made in determining the haplotypes based on published gene frequencies and patterns of linkage disequilibrium between pairs of KIR genes: 1) 3DL3, 3DP1, 2DL4, and 3DL2 are present on all haplotypes, 2) if 2DL1 is present, 2DP1 is always present, 3) 3DS1 segregates as an allele of 3DL1, 4) 2DL2 and 2DL3 segregate as alleles of a single locus.
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Order of the genes on the c haplotype (see Fig. 1B) was determined by sequencing products derived from PCR in which forward primers recognized the 3' end and reverse primers recognized the 5' end of the various KIR genes (primers and annealing temperatures are provided in supplemental Tables I and II). In some cases it was necessary to reamplify the initial PCR product because yield of the amplicon was inadequate for sequencing. In these cases, an internal primer was used in reamplification. KIR gene sequences are based on the alignment provided in http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db = Books.
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KIR genes that were suspected to be duplicated on haplotype c (see Fig. 2) were sequenced for allelic determination. The PCR product derived from exons 35 of 2DL4 was also cloned for sequencing because it was not possible to assign alleles after direct sequencing. The amplified product was cloned into the expression vector pcDNA2.1-TOPO (Invitrogen) and eight clones were sequenced. Primers used for sequencing of 2DL4 and 3DL1 are provided in supplemental Table II. Primers were designed to amplify all known alleles of the genes. The primers used for amplification and sequencing of the hybrid 2DL5A/3DP1 gene are provided in supplemental Tables I and II. The amplified products were purified using the Qiaquick PCR purification kit (Qiagen, Valencia, CA). Cycle sequencing was performed using the ABI BigDye Terminator Cycle Sequencing Ready Reaction kit (Applied Biosystems, Foster City, CA), followed by isopropanol precipitation. The samples were then run on an ABI 377 sequencer. Allele nomenclature is derived from Ref.17 .
Determination of 2DL4 copy number
Simultaneous detection of the target KIR2DL4 sequence and an internal single-copy gene control in the same sample material was achieved by dual-color detection using the Lightcycler (Roche Diagnostic Systems, Indianapolis, IN). The KIR2DL4 forward and reverse PCR primers, sited within exon 3, were 5'-TCAGGA CAAGCCCTTCTG-3' and 5'-ACC CCATCT TTCTTG TACAGTG-3', respectively. The penultimate nucleotide of the reverse primer (underlined) is a mismatch to all KIR gene sequences to prevent nonspecific priming. The KIR2DL4 hybridization probes were 5'-CTGTGGTGCCTCAAGGAGG-fluorescein-3' and 5'-Red640-ACGTGACTCTTCGGTGTCAC-phosphate-3'. A proprietary internal control (
-globin gene; Roche Diagnostic Systems) was used. Final concentrations in 20-µl reaction volumes were 1x FastStart Reaction Mix (Roche Diagnostic Systems), 5 mM MgCl2, 0.5 ng/µl DNA template, 500 nM of each primer, 0.1 µM of each fluorescein probe, and 0.2 µM of each Red fluorophore probe. Cycling was performed as follows: 10 min at 95°C followed by 45 cycles of 95°C for 3 s, 62°C for 5 s, and 72°C for 8 s. The results of duplicate experiments are expressed as the mean relative ratio of KIR2DL4 to the reference gene (Relative Quantification Software (Roche Diagnostic Systems) using a precreated coefficient file) with SDs (Table I). All samples were tested blindly.
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| Results and Discussion |
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KIR2DL5A/3DP1 was identified in a three generation Center dEtude du Polymorphisme Humaine family. Extensive cloning, sequencing, and segregation analysis of KIR genes in the family indicated that two known alleles of both KIR2DL4 (X97229, AF034773) and KIR3DL1/S1 (AF262969, AF022044) segregated on the c haplotype, whereas a single distinct allele of each of these loci segregated on each of the a, b, and d haplotypes (Fig. 2). Using a quantitative PCR technique to measure gene dosage, we confirmed that individuals with the c haplotype had three copies of KIR2DL4, and those without the c haplotype had two copies of this gene (Table I). The order of the genes on the c haplotype was then determined by sequencing products derived from PCR in which forward primers recognized the 3' end and the reverse primers recognized the 5' end of the various KIR genes. Every sequence obtained supported the order of genes shown on the extended haplotype in Fig. 1B. Primer sequences used in this study and sequence of informative variant sites that allowed determination of gene order are provided in supplemental Tables I and II.
The gene duplication, gene order, and novel hybrid KIR2DL5A/3DP1 gene that characterize haplotype c strongly indicate that the mechanism by which this haplotype was derived involved unequal crossing over between two well-defined KIR haplotypes. We propose that this mechanism represents a common means by which expansion and contraction of KIR haplotypes occur, facilitating rapid evolution of the KIR gene complex. The truncated KIR haplotype that also would have been produced by the recombination event depicted in Fig. 1B has not been observed in any family studies published to date (3, 4, 5, 6, 7). However, the sequence of the hybrid gene in this putative haplotype, KIR3DP1/2DL5A (Fig. 3), is virtually identical to the gene KIR2DL5B (7) (AF217486). It follows that the truncated haplotype (or one similar to it) containing a hybrid KIR3DP1/2DL5A (i.e., KIR2DL5B) gene has been generated previously and has circulated in the population. KIR2DL5A and KIR2DL5B are highly homologous but distinct genes that are sometimes located on the same haplotype (6, 7), indicating that an unequal recombination event occurred subsequent to that which gave rise to KIR2DL5B, placing KIR2DL5A and KIR2DL5B on a single haplotype. Interestingly, of the three defined KIR2DL5B alleles, two alleles, including the most common one, are not expressed due to a mutation in their promoter region (18), partially reverting this gene to the pseudogene status of its ancestor KIR3DP1 and hybrid counterpart KIR2DL5A/3DP1.
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Evolution of tandem arrays of homologous genes might often occur through a mechanism involving unequal crossing over (20), which may underlie the "birth and death" of clustered genes (21). The mouse ly-49 region, the functional equivalent of KIR, behaves in a similar manner (22). Functional consequences of expanded (or truncated) KIR haplotypes in viral infections and cancer are quite plausible, and their characterization may illuminate our understanding of gene dosage effects in human disease.
| Footnotes |
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2 Address correspondence and reprint requests to Dr. Mary Carrington, National Cancer Institute, P.O. Box B, Frederick, MD 21702. E-mail address: carringt{at}ncifcrf.gov ![]()
3 Abbreviations used in this paper: KIR, killer Ig-like receptor; SSP, sequence specific priming. ![]()
Received for publication May 28, 2003. Accepted for publication July 11, 2003.
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