HLA-DRB intron 1 sequences: implications for the evolution of HLA-DRB genes and haplotypes

Personalized genome assembly for accurate cancer somatic mutation discovery using tumor-normal paired reference samples

BACKGROUND

The use of a personalized haplotype-specific genome assembly, rather than an unrelated, mosaic genome like GRCh38, as a reference for detecting the full spectrum of somatic events from cancers has long been advocated but has never been explored in tumor-normal paired samples. Here, we provide the first demonstrated use of de novo assembled personalized genome as a reference for cancer mutation detection and quantifying the effects of the reference genomes on the accuracy of somatic mutation detection.

RESULTS

We generate de novo assemblies of the first tumor-normal paired genomes, both nuclear and mitochondrial, derived from the same individual with triple negative breast cancer. The personalized genome was chromosomal scale, haplotype phased, and annotated. We demonstrate that it provides individual specific haplotypes for complex regions and medically relevant genes. We illustrate that the personalized genome reference not only improves read alignments for both short-read and long-read sequencing data but also ameliorates the detection accuracy of somatic SNVs and SVs. We identify the equivalent somatic mutation calls between two genome references and uncover novel somatic mutations only when personalized genome assembly is used as a reference.

CONCLUSIONS

Our findings demonstrate that use of a personalized genome with individual-specific haplotypes is essential for accurate detection of the full spectrum of somatic mutations in the paired tumor-normal samples. The unique resource and methodology established in this study will be beneficial to the development of precision oncology medicine not only for breast cancer, but also for other cancers.

Imputing Variants in HLA-DR Beta Genes Reveals That HLA-DRB1 Is Solely Associated with Rheumatoid Arthritis and Systemic Lupus Erythematosus

The genetic association of HLA-DRB1 with rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) is well documented, but association with other HLA-DR beta genes (HLA-DRB3, HLA-DRB4 and HLA-DRB5) has not been thoroughly studied, despite their similar functions and chromosomal positions. We examined variants in all functional HLA-DR beta genes in RA and SLE patients and controls, down to the amino-acid level, to better understand disease association with the HLA-DR locus. To this end, we improved an existing HLA reference panel to impute variants in all protein-coding HLA-DR beta genes. Using the reference panel, HLA variants were inferred from high-density SNP data of 9,271 RA-control subjects and 5,342 SLE-control subjects. Disease association tests were performed by logistic regression and log-likelihood ratio tests. After imputation using the newly constructed HLA reference panel and statistical analysis, we observed that HLA-DRB1 variants better accounted for the association between MHC and susceptibility to RA and SLE than did the other three HLA-DRB variants. Moreover, there were no secondary effects in HLA-DRB3, HLA-DRB4, or HLA-DRB5 in RA or SLE. Of all the HLA-DR beta chain paralogs, those encoded by HLA-DRB1 solely or dominantly influence susceptibility to RA and SLE.

Evolution of dimorphisms of the proteasome subunit beta type 8 gene (PSMB8) in basal ray-finned fish

The proteasome subunit beta type 8 (PSMB8) gene encodes a catalytic subunit of immunoproteasome that plays a central role in the processing of antigenic peptides presented by major histocompatibility complex class I molecules. The A- and F-type alleles defined by the 31st amino acid residue determining cleaving specificity have been identified from ray-finned fish, amphibia, and reptiles. These two types show extremely long-term trans-species polymorphism in Polypteriformes, Cypriniformes, and Salmoniformes, suggesting the presence of very ancient lineages termed A and F. To elucidate the evolution of the PSMB8 dimorphism in basal ray-finned fish, we analyzed Pantodon buchholzi (Osteoglossiformes), seven species of Anguilliformes, and Hypomesus nipponensis (Osmeriformes). Both A and F lineage sequences were identified from P. buchholzi and H. nipponensis, confirming that these two lineages have been conserved by basal ray-finned fish. However, both the A- and F-type alleles found in Anguilliformes species belonged to the F lineage irrespective of their types. This apparently suggests that the A lineage was lost in the common ancestor of Anguilliformes, and recovery of the A type within the F lineage occurred in Anguilliformes. The apparent loss of the F lineage and recovery of the F type within the A lineage have already been reported from tetrapods and higher teleosts. However, this is the first report on the reverse situation and reveals the dynamic evolution of the PSMB8 dimorphism.

The research of W.E. Mayer (1953-2012): a spectrum of immune systems

Over a period of some 20 years, Werner Eugen Mayer played a significant role in establishing a framework for molecular studies of Mhc genes in multiple vertebrates. His work largely concerned gene isolation, sequencing, and related bioinformatic analyses both for the Mhc and for immune system genes of about 200 species, ranging from apes, monkeys, rodents, and marsupials, through to birds, bony fishes, and lampreys. In addition to his exploration of diverse Mhc genes, Werner is remembered for playing a critical role in the development of two important insights into the evolution of immune systems. His was among the first published DNA sequence-based descriptions of trans-species evolution of Mhc alleles, including the first description of the long-lived polymorphisms shared by humans and chimpanzees. This research opened the way for using Mhc polymorphisms in demographic analyses. The second important insight in which he played a prominent role involved the characterization of immune cells and their expressed genes in the lamprey, a jawless vertebrate. His findings helped to indicate the considerable degree to which extant immune mechanisms were co-opted in the creation of the adaptive immune system of jawed vertebrates.

Long-lived dichotomous lineages of the proteasome subunit beta type 8 (PSMB8) gene surviving more than 500 million years as alleles or paralogs

On an evolutionary time scale, polymorphic alleles are believed to have a short life, persisting at most tens of millions of years even under long-term balancing selection. Here, we report highly diverged trans-species dimorphism of the proteasome subunit beta type 8 (PSMB8) gene, which encodes a catalytic subunit of the immunoproteasome responsible for the generation of peptides presented by major histocompatibility complex (MHC) class I molecules, in lower teleosts including Cypriniformes (zebrafish and loach) and Salmoniformes (trout and salmon), whose last common ancestor dates to 300 Ma. Moreover, phylogenetic analyses indicated that these dimorphic alleles share lineages with two shark paralogous genes, suggesting that these two lineages have been maintained for more than 500 My either as alleles or as paralogs, and that conversion between alleles and paralogs has occurred at least once during vertebrate evolution. Two lineages termed PSMB8A and PSMB8F show an A(31)F substitution that would probably affect their cleaving specificity, and whereas the PSMB8A lineage has been retained by all analyzed jawed vertebrates, the PSMB8F lineage has been lost by most jawed vertebrates except for cartilaginous fish and basal teleosts. However, a possible functional equivalent of the PSMB8F lineage has been revived as alleles within the PSMB8A lineage at least twice during vertebrate evolution in the amphibian Xenopus and teleostean Oryzias species. Dynamic evolution of the PSMB8 polymorphism through long-term persistence, loss, and regaining of dimorphism and conversion between alleles and paralogs implies the presence of strong selective pressure for functional polymorphism of this gene.

Analysis of the complete cDNA sequences of HLA-DRB1 alleles with group-specific amplification primers in the Chinese Han population

Currently for the majority of HLA-DRB1 alleles the focus has been mainly on exon 2 and complete cDNA sequences of HLA-DRB1 alleles are rare. In this study, we analyzed the complete coding sequences of partial alleles of HLA-DRB1 locus. The cDNA was amplified by polymerase chain reaction using the group-specific primers located in the 5'- and 3'-untranslated regions to obtain the complete coding sequences. The amplification products were sequenced using an ABI BigDye® Terminator Cycle Sequencing kit. The HLA-DRB1 allele phylogenetic tree was analyzed by dnaman software. Full-length cDNA sequences of 22 HLA-DRB1 alleles were obtained in this study. HLA-DRB1*08:09, DRB1 *12:02:01, and DRB1*13:12 alleles were first reported for complete coding sequences. The sequences of exon 1 of HLA-DRB1*04:06:01, DRB1*08:03:02, and DRB1 *14:07:01 were newly presented. The complete coding sequences of HLA-DRB1 *01:01:01, DRB1*03:01:01:01, DRB1*04:01:01, DRB1*04:05:01, DRB1*07:01:01: 01, DRB1*09:01:02, DRB1*10:01:01, DRB1*11:01:01, DRB1*12:01:01, DRB1*13: 01:01, DRB1*13:02:01, DRB1*14:04, DRB1*14:54, DRB1*15:01:01:01, DRB1*15: 02:01, and DRB1*16:02:01 were identical to those previously reported. Forty polymorphic positions in complete coding sequences outside exon 2 of these HLA-DRB1 alleles were confirmed. According to the phylogenetic tree of full-length coding sequence, the HLA-DRB1 allele was classified into seven major allelic lineages. In conclusion, a protocol for HLA-DRB1 cDNA amplification and sequencing was improved and the data may help to determine the polymorphism of coding sequences outside exon 2.

Transspecies dimorphic allelic lineages of the proteasome subunit beta-type 8 gene (PSMB8) in the teleost genus Oryzias

The proteasome subunit β-type 8 (PSMB8) gene in the jawed vertebrate MHC genomic region encodes a catalytic subunit of the immunoproteasome involved in the generation of peptides to be presented by the MHC class I molecules. A teleost, the medaka (Oryzias latipes), has highly diverged dimorphic allelic lineages of the PSMB8 gene with only about 80% amino acid identity, termed "PSMB8d" and "PSMB8N," which have been retained by most wild populations analyzed. To elucidate the evolutionary origin of these two allelic lineages, seven species of the genus Oryzias were analyzed for their PSMB8 allelic sequences using a large number of individuals from wild populations. All the PSMB8 alleles of these species were classified into one of these two allelic lineages based on their nucleotide sequences of exons and introns, indicating that the Oryzias PSMB8 gene has a truly dichotomous allelic lineage. Retention of both allelic lineages was confirmed except for one species. The PSMB8d lineage showed a higher frequency than the PSMB8N lineage in all seven species. The two allelic lineages showed curious substitutions at the 31st and 53rd residues of the mature peptide, probably involved in formation of the S1 pocket, suggesting that these allelic lineages show a functional difference in cleavage specificity. These results indicate that the PSMB8 dimorphism was established before speciation within the genus Oryzias and has been maintained for more than 30-60 million years under a strict and asymmetric balancing selection through several speciation events.

Impact of endogenous intronic retroviruses on major histocompatibility complex class II diversity and stability

The major histocompatibility complex (MHC) represents a multigene family that is known to display allelic and gene copy number variations. Primate species such as humans, chimpanzees (Pan troglodytes), and rhesus macaques (Macaca mulatta) show DRB region configuration polymorphism at the population level, meaning that the number and content of DRB loci may vary per haplotype. Introns of primate DRB alleles differ significantly in length due to insertions of transposable elements as long endogenous retrovirus (ERV) and human ERV (HERV) sequences in the DRB2, DRB6, and DRB7 pseudogenes. Although the integration of intronic HERVs resulted sooner or later in the inactivation of the targeted genes, the fixation of these endogenous retroviral segments over long time spans seems to have provided evolutionary advantage. Intronic HERVs may have integrated in a sense or an antisense manner. On the one hand, antisense-oriented retroelements such as HERV-K14I, observed in intron 2 of the DRB7 genes in humans and chimpanzees, seem to promote stability, as configurations/alleles containing these hits have experienced strong conservative selection during primate evolution. On the other hand, the HERVK3I present in intron 1 of all DRB2 and/or DRB6 alleles tested so far integrated in a sense orientation. The data suggest that multigenic regions in particular may benefit from sense introgressions by HERVs, as these elements seem to promote and maintain the generation of diversity, whereas these types of integrations may be lethal in monogenic systems, since they are known to influence transcript regulation negatively.

Full-length sequence analysis of the HLA-DRB1 locus suggests a recent origin of alleles

The HLA region harbors some of the most polymorphic loci in the human genome. Among them is the class II locus HLA-DRB1, with more than 400 known alleles. The age of the polymorphism and the rate at which new alleles are generated at HLA loci has caused much controversy over the years. Previous studies have mostly been restricted to the 270 base pairs that constitute the second exon and represent the most variable part of the gene. Here, we investigate the evolutionary history of the HLA-DRB1 locus on the basis of an analysis of 15 genomic full-length alleles (10-15 kb). In addition, the variation in 49 complete coding sequences and 322 exon 2 sequences were analyzed. When excluding exon 2 from the analysis, the diversity at the synonymous sites was found to be similar to the intron diversity. The overall diversity in noncoding region was also similar to the genome average. The DRB1*03 lineage has been found in human, chimpanzee, bonobo, gorilla, and orangutan. An ancestral "proto HLA-DRB1*03 lineage" appeared to have diverged in the last 5 million years into the human-specific lineages *08, *11, *13, and *14. With exception to exon 2, both the coding- and the noncoding diversity suggests a recent origin (<1 million years ago) for most of the alleles at the HLA-DRB1 locus. Sites encoding for amino acids involved in antigen binding [antigen recognizing sites (ARS)] appear to have a more ancient origin. Taken together, the recent origin of most alleles, the high diversity between allelic lineages, and the ancient origin of sequence motifs in exon 2, is consistent with a relatively rapid generation of novel alleles by gene conversion like events.

Conserved extended haplotypes of the major histocompatibility complex: further characterization

Since the complete sequencing of a human major histocompatibility complex (MHC) haplotype, interest in non-human leucocyte antigen (HLA) genes encoded in the MHC has been growing. Non-HLA genes, which outnumber the HLA genes, may contribute to or account for HLA and disease associations. Most information on non-HLA genes has been obtained in separate studies of individual loci. To comprehensively address polymorphisms of relevant non-HLA genes in 'conserved extended haplotypes' (CEH), we investigated 101 International Histocompatibility Workshop reference cell lines and nine additional anonymous samples representing all 37 unambiguously characterized CEHs at MICA, NFKBIL1, LTA, NCR3, AIF1, HSPA1A, HSPA1B, BF, NOTCH4 and a single nucleotide polymorphism (SNP) at HLA-DQA1 as well as MICA, NOTCH4, HSPA1B and all five tumour necrosis factor short tandem repeat (STR) polymorphisms. This work (1) provides an extensive catalogue of MHC polymorphisms in all CEHs, (2) unravels interrelationships between HLA and non-HLA haplotypical lineages, (3) resolves reported typing ambiguities and (4) describes haplospecific markers for a number of CEHs. Analysis also identified a DQA1 SNP and segments containing MHC class III polymorphisms that corresponded with class II (DRB3 and DRB4) lineages. These results portray the MHC where lineages containing non-HLA and HLA variants in linkage disequilibrium may operate in concert and can guide more thorough design and interpretation of HLA-disease relationships.

HLA-DRB1 and -DRB3 allele frequencies and haplotypic associations in Koreans

We have investigated the frequencies of human leukocyte antigen-DRB1 (HLA-DRB1) and -DRB3 alleles and DRB1-DRB3 haplotypic associations in 800 Koreans. DRB1 genotyping was done using polymerase chain reaction-sequence-specific oligonucleotide (PCR-SSO) and PCR-single strand conformation polymorphism (SSCP) methods. DRB3 genotyping was done on 447 samples carrying DRB3-associated DRB1 alleles (DRB1*03, *11, *12, *13, and *14) using PCR-SSCP method. The allele frequencies of DRB3*0101, DRB3*0202, and DRB3*0301 were 0.073, 0.136, and 0.120, respectively, and we found one case of a probable new allele (DRB3*01new, 0.001). DRB1-DRB3 haplotypes with frequency (HF) > 0.005 exhibited strong associations between DRB3*0101 and DRB1*1201, *1301, and *1403; between DRB3*0301 and DRB1*1202 and *1302; between DRB3*0202 and DRB1*0301, *1101, *1401, *1405, and *1406 alleles. Most of the DRB1 alleles with frequency > 0.005 were exclusively associated with particular DRB3 alleles with relative linkage disequilibrium values of 1.0, except for DRB1*1201, *1202 and *1301; the rare presence (HF < 0.005) of DRB3*0202 associations were observed for these DRB1 alleles. We also investigated and presented rare DRB1-DRB3 associations in additional 6000 Koreans. Comparison with other ethnic groups revealed that DRB1*0301 and *1301 related DRB1-DRB3 haplotypes vary among different populations, in that Koreans and other Asian populations show less diversity compared with Caucasoids or African Americans.

Increased heterozygosity for MHC class II lineages in newborn males

In plants, fungi and marine invertebrates, there are genetic compatibility systems to ensure diversity in the offspring. The importance of genetic compatibility in gametic union and selective abortion in vertebrate animals has also been appreciated recently. There have been suggestions that the major histocompatibility complex (HLA in humans) may be a compatibility system in vertebrates. HLA class II haplotypes often contain a second expressed DRB locus which can be either DRB3, DRB4 or DRB5. These encode the supertypical specificities and mark the ancestral lineages. The members of each lineage have related DNA sequences at the main class II locus HLA-DRB1. We analysed 415 newborns at all expressed DRB loci by PCR analysis to seek evidence for sex-specific prenatal selection events. While there was no significant change in heterozygosity rates between males and females at DRB1, the proportion of males carrying two DRB1 specificities from different ancestral lineages was significantly increased (53.7% in males vs 39.3% in females, P = 0.003). The genotypes consisting of phylogenetically most distinct ones, namely the DRB3 and DRB4 haplotypes, showed the most striking difference between sexes (P = 0.007). These results suggested a more favourable outcome for male concepti heterozygous for supertypical haplotypes. Heterozygosity for most divergent haplotypical families ensures the highest degree of functional heterozygosity at the main HLA class II locus DRB1 while increasing the likelihood of heterozygosity also at other MHC loci. Our observations agree with the previously reported heterozygote excess in male newborn rats and mice. Correlations between MHC class II heterozygosity and advertised male quality in deer and pheasant as well as increased reproductive success in MHC class II heterozygous male macaques are examples of postnatal benefits of heterozygosity in males that may be behind the development of prenatal selection mechanisms. The MHC-mediated prenatal selection of males may also be one of the selective events suggested by the very high primary (male-to-female) sex ratio at fertilization reaching close to unity at birth in humans. These results provide an appealing working hypothesis for further studies in humans and other vertebrates.

Independent origin of functional MHC class II genes in humans and New World monkeys

In previous studies, major histocompatibility complex (MHC) class II DP, DQ, and DR families of genes were characterized in different primate species mostly on the basis of their second exon sequences. Resemblances were found between Old World monkey (OWM) and New World monkey (NWM) genes and were interpreted as being the result of transspecies evolution. Subsequent analysis of intron sequences of catarrhine and platyrrhine DRB genes, however, revealed that the amplifiable genes were not, in fact, orthologous. To test other DRB genes and other families of the class II region Southern blot hybridizations were carried out with tamarin genomic DNA using probes specific for the third exons of the tamarin DQA, DQB, DPB, and DRB genes. The hybridizing bands were extracted from the gel and the third exons of the genes were amplified by PCR, cloned, and sequenced. With two exceptions, all NWM class II genes were found to group separately from the human sequences. Only the sequences of one nonfunctional DQB locus appeared to be more closely related to human genes than to other platyrrhine DQB genes. In the DRB family one gene was found that grouped with sheep and strepsirhine DRB sequences and might represent an old gene lineage. To extend the sequences to the second exon, long PCRs were performed on tamarin genomic DNA. This approach was successful for five of the ten third exon sequences. From these data, we conclude that at least the functional MHC class II genes have expanded independently in catarrhines and platyrrhines.

The noncoding regions of HLA-DRB uncover interlineage recombinations as a mechanism of HLA diversification

The mechanisms generating new alleles at the MHC loci are still unknown in detail, and several proposals have been made to explain the extent of polymorphism. The patchwork pattern of polymorphism in the 2nd exon of HLA-DRB1 recommends this locus as a model for the study of the potential of interallelic gene conversion. In general, the inference of gene conversion-like events based exclusively on exon sequence comparisons may be misleading because the identity of the putative donor allele remains unknown. In this study, we describe five alleles of the HLA-DRB1 gene, which intron regions give evidence for interlineage recombination events either strictly located at the 2nd exon or involving the adjacent introns. Furthermore, we show that the noncoding regions provide important clues to the mechanisms of the generation of new alleles, and our results indicate that interlineage recombinations may be hidden and are perhaps more frequent than currently expected.

A 39-kb sequence around a blackbird Mhc class II gene: ghost of selection past and songbird genome architecture

To gain an understanding of the evolution and genomic context of avian major histocompatibility complex (Mhc) genes, we sequenced a 38.8-kb Mhc-bearing cosmid insert from a red-winged blackbird (Agelaius phoeniceus). The DNA sequence, the longest yet retrieved from a bird other than a chicken, provides a detailed view of the process of gene duplication, divergence, and degeneration ("birth and death") in the avian Mhc, as well as a glimpse into major noncoding features of a songbird genome. The peptide-binding region (PBR) of the single Mhc class II B gene in this region, Agph-DAB2, is almost devoid of polymorphism, and a still-segregating single-base-pair deletion and other features suggest that it is nonfunctional. Agph-DAB2 is estimated to have diverged about 40 MYA from a previously characterized and highly polymorphic blackbird Mhc gene, Aph-DAB1, and is therefore younger than most mammalian Mhc paralogs and arose relatively late in avian evolution. Despite its nonfunctionality, Agph-DAB2 shows very high levels of nonsynonymous divergence from Agph-DAB1 and from reconstructed ancestral sequences in antigen-binding PBR codons-a strong indication of a period of adaptive divergence preceding loss of function. We also found that the region sequenced contains very few other unambiguous genes, a partial Mhc- class II gene fragment, and a paucity of simple-sequence and other repeats. Thus, this sequence exhibits some of the genomic streamlining expected for avian as compared with mammalian genomes, but is not as densely packed with functional genes as is the chicken Mhc.

Alu elements support independent origin of prosimian, platyrrhine, and catarrhine Mhc-DRB genes

The primate major histocompatibility complex (Mhc) genes fall into two classes and each of the classes into several families. Of the class II families, the DRB family has a long and complex evolutionary history marked by gene turnover, rearrangement, and molecular convergence. Because the history is not easily decipherable from sequences alone, Alu element insertions were used as cladistic markers to support the surmised phylogenetic relationships among the DRB genes. Intron 1 segments of 24 DRB genes from five platyrrhine species and five DRB genes from three prosimian species were amplified by PCR and cloned, and the amplification products were sequenced or PCR-typed for Alu repeats. Three Alu elements were identified in the platyrrhine and four in the prosimian DRB genes. One of the platyrrhine elements (Alu50J) is also found in the Catarrhini, whereas the other two (Alu62Sc, Alu63Sc) are restricted to the New World monkeys. Similarly, the four prosimian elements are found only in this taxon. This distribution of Alu elements is consistent with the phylogeny of the DRB genes as determined from their intron 1 sequences in an earlier and the present study. It contradicts the exon 2-based phylogeny and thus corroborates the conclusion that the evolution of DRB exon 2 sequences is, to some extent, shaped by molecular convergence. Taken together, the data indicate that each of the assemblages of DRB genes in prosimians, platyrrhines, and catarrhines is derived from a separate ancestral gene.

Rh gene evolution in primates: study of intron sequences

By amplification and sequencing of RH gene intron 4 of various primates we demonstrate that an Alu-Sx-like element has been inserted in the RH gene of the common ancestor of humans, apes, Old World monkeys, and New World monkeys. The study of mouse and lemur intron 4 sequences allowed us to precisely define the insertion point of the Alu-Sx element in intron 4 of the RH gene ancestor common to Anthropoidea. Like humans, chimpanzees and gorillas possess two types of RH intron 4, characterized by the presence (human RHCE and ape RHCE-like genes) or absence (human RHD and ape RHD-like genes) of the Alu-Sx element. This led us to conclude that in the RH common ancestor of humans, chimpanzees, and gorillas, a duplication of the common ancestor gene gave rise to two genes, one differing from the other by a 654-bp deletion encompassing an Alu-Sx element. Moreover, most of chimpanzees and some gorillas posses two types of RHD-like intron 4. The introns 4 of type 1 have a length similar to that of human RHD intron 4, whereas introns 4 of type 2 display an insertion of 12 bp. The latest insertion was not found in the human genome (72 individuals tested). The study of RH intron 3 length polymorphism confirmed that, like humans, chimpanzees and gorillas possess two types of intron 3, with the RHD-type intron 3 being 289 bases shorter than the RHCE intron 3. By amplification and sequencing of regions encompassing introns 3 and 4, we demonstrated that chimpanzee and gorilla RH-like genes displayed associations of introns 3 and 4 distinct to those found in man. Altogether, the results demonstrate that, as in humans, chimpanzee and gorilla RH genes experienced intergenic exchanges.

Mhc-DQ-DRB haplotype analysis in the rhesus macaque: evidence for a number of different haplotypes displaying a low allelic polymorphism

In the HLA-DRB subregion of man, five major groups of haplotypes, often displaying a remarkable polymorphism, are distinguishable. The polymorphism is thought to be generated by point mutation, microgene conversion and gene rearrangement by recombination. In order to gain insight into the organization of the rhesus macaque major histocompatibility complex (MHC) class II region, DRB genes from monkeys of different origins previously typed for their DQ genes were analyzed. At first DRB haplotypes were deduced from DQ-homozygous monkeys. The stability of these haplotypes was then examined in DQ-heterozygous monkeys by sequence-based typing for the presence of members of the DRB1*03 and DRB1*04 lineage, and for seven single alleles detected on the haplotypes. Six DRB haplotypes linked to the five most frequent and three haplotypes linked to less frequent DQ haplotypes were identified. Six novel DRB alleles were detected. The number of DRB genes per haplotype varied between two and four. The results altogether suggest that in rhesus macaques, in comparison to man, the DQ haplotypes are linked to only a small number of DRB haplotypes, the number and diversity of DRB haplotypes is larger, and the allelic polymorphism of a given haplotype is smaller. The diversity of the DRB haplotypes was partly due to the varying number and identity of genes linked to DRB1*03 and DRB1*04. Furthermore, the number of DRB1 genes themselves varied from zero to two.

Relative HLA-DRB1*13 allele frequencies and DRB3 associations of unrelated individuals from five US populations

The frequencies of 30 HLA-DRB1*13 alleles and 15 DRB3 alleles were determined for the 5 major U.S. ethnic populations: Caucasians, African Americans, Asian/Pacific Islanders, Hispanics, and Native Americans. A random sampling (163) of DRB1*13-positive individuals from each self-described ethnic group was selected out of a pool of 82,979 unrelated individuals, providing at least an 80% probability of detecting a rare allele that occurred at 1%. These 815 samples were subjected to allele-level SSOP typing and/or DNA sequencing which identified 11 different DRB1*13 alleles. DRB1*1301 and DRB1*1302 were the most common alleles seen in the five major ethnic groups while DRB1*1304 was not detected among Caucasians and DRB1*1305 was not detected among African Americans. DRB1*13 allele diversity was surprisingly more limited among African Americans compared to both Caucasians and Asian/Pacific Islanders. To determine the extent of DRB1*13-DRB3 associations, 504 of these samples expressing only one DRB3-associated DRB1 allele were subjected to PCR-SSOP typing and 14 DRB1*13-DRB3 haplotypes were detected. The distribution revealed that African Americans were significantly different from Caucasians, Asian/Pacific Islanders, and Hispanics. Allele frequency studies such as this further support previous findings that the distribution of HLA types can differ significantly among different ethnic populations.

Sequencing of HLA class II genes based on the conserved diversity of the non-coding regions: sequencing based typing of HLA-DRB genes

In this paper, we present a novel sequencing based typing strategy for the HLA-DRB1, 3, 4 and 5 loci. The new approach is based on a group-specific amplification from intron 1 to intron 2 according to the serologically-defined antigens. For this purpose, we have determined the 3' 500 bp-fragment of intron 1 and the 5' 340 bp-fragment of intron 2 of all serological antigens and their most frequent subtypes. We discovered a remarkably conserved diversity characterized by lineage-specific sequence motifs. This lineage-specificity of non-coding motifs in the 1st and 2nd intron offered the possibility to establish a clear serology-related amplification strategy. The method allows the complete analysis of the 2nd exon and the definition of the cis/trans linkage of sequence motifs by intron-mediated polymerase chain reaction (PCR)-based separation of the haplotypes in nearly all serologically heterozygous samples. In particular, the non-coding variabilities between the DR52-associated DRB1 groups made their independent amplification possible. Thus, compared to the standard procedures using exon-based amplification primers, the groups DR3, DR12, some DR13 alleles (1301, 1302) and the DR14 group could be amplified by specific primer mixes. The DR8 could be amplified with an individual primer mix not co-amplifying the DR12. The DR11 and DR13 did not show any individual motif in intron 1 or intron 2. In order to achieve a separate amplification, they had to be amplified by multispecific primer mixes (DR3/11/13/14; DR3/11/13 or DR11/13/14) excluding the other haplotype. Thus, exclusively the alleles in rare DR11,13 heterozygosities without a DRB1*1301 or 1302 could not be amplified separately. Fourteen primer mixes are used to amplify the specificities DR1-14, and 6 primer mixes for the specificities DR51-53. The sequence homology of the 3' end of intron 1 facilitated the application of only three different sequencing primers for all DRB alleles.

Major histocompatibility complex class II polymorphisms in primates

In the past decade, the major histocompatibility complex (MHC) class II region of several primate species has been investigated extensively. Here we will discuss the similarities and differences found in the MHC class II repertoires of primate species including humans, chimpanzees, rhesus macaques, cotton-top tamarins and common marmosets. Such types of comparisons shed light on the evolutionary stability of MHC class II alleles, lineages and loci as well as on the evolutionary origin and biological significance of haplotype configurations.

Phylogenetic history of hominoid DRB loci and alleles inferred from intron sequences

The evolutionary relationships among the MHC class II DRB4, DRB5 and DRB6 loci as well as the allelic lineages and alleles of the DRB1 locus were studied based on intron 1 and intron 2 sequences from humans, chimpanzee (Pan troglodytes), bonobo (Pan paniscus) and gorilla (Gorilla gorilla). The phylogenetic trees for these sequences indicate that most of the DRB1 allelic lineages predate the separation of the hominoid species studied, consistent with previous analysis of the coding sequences of these lineages. However, the intron sequence variation among alleles within DRB1 allelic lineages is very limited, consistent with the notion that the majority of the contemporary alleles have been generated within the last 250,000 years. The clustering of the DRB1 allelic lineages *08 and *12 with *03 supports a common ancestry for the DR8 and DR52 haplotypes. Similarly, the clustering of DRB1 allelic lineages *15 and *01 with the DRB3 locus is consistent with a common ancestry for the DR1 and DR51 haplotypes. Two cases of recombination around the second exon were observed: 1) the HLA-DRB6 locus appears to have been generated through a recombination between a DRB5 allele and an ancestral DRB6 allele, and 2) the gorilla sequence Gogo-DRB1 *03 appears to have been generated through a recombination between the DRB3 locus and an allele from the DRB1 *03 allelic lineage. The nucleotide substitution rate of DRB introns was estimated to 0.85-1.63 x 10(-9) per site per year, based on comparisons between the most closely related sequences from different hominoid species. This estimate is similar to the substitution rate for other intronic regions of the primate genome.

Antibodies of sharks: revolution and evolution

The combinatorial immune response is restricted to jawed vertebrates with cartilaginous fishes being the lowest extant species to have the mechanism for diversification and an extensive panoply of immunoglobulins, T-cell receptors and MHC products. Here, we review the molecular events of the "big bang" or rapid evolutionary appearance of the functionally complete combinatorial immune system coincident with the appearance of ancestral jawed vertebrates, suggesting that this event was catalyzed by horizontal transfer of DNA processing systems. We analyze the nature and extent of variable and constant domain diversity among the distinct immunoglobulin sets of carcharhine sharks focusing upon the lambda-like light chains and the mu and omega heavy chains. The detection and isolation of natural antibodies from the blood of unimmunized sharks illustrates a surprising range of recognition specificities and the existence of polyspecificity suggests that the antibody-forming system of sharks offers unique opportunities for studies of immunological regulation. Although the homologies between shark and mammalian immunoglobulins are unequivocal, major differences in segmental gene organization present challenges to our understanding of basic immunological phenomena such as clonal restriction.

The common marmoset: a new world primate species with limited Mhc class II variability

The common marmoset (Callithrix jacchus) is a New World primate species that is highly susceptible to fatal infections caused by various strains of bacteria. We present here a first step in the molecular characterization of the common marmoset's Mhc class II genes by nucleotide sequence analysis of the polymorphic exon 2 segments. For this study, genetic material was obtained from animals bred in captivity as well as in the wild. The results demonstrate that the common marmoset has, like other primates, apparently functional Mhc-DR and -DQ regions, but the Mhc-DP region has been inactivated. At the -DR and -DQ loci, only a limited number of lineages were detected. On the basis of the number of alleles found, the -DQA and -B loci appear to be oligomorphic, whereas only a moderate degree of polymorphism was observed for two of three Mhc-DRB loci. The contact residues in the peptide-binding site of the Caja-DRB1*03 lineage members are highly conserved, whereas the -DRB*W16 lineage members show more divergence in that respect. The latter locus encodes five oligomorphic lineages whose members are not observed in any other primate species studied, suggesting rapid evolution, as illustrated by frequent exchange of polymorphic motifs. All common marmosets tested were found to share one monomorphic type of Caja-DRB*W12 allele probably encoded by a separate locus. Common marmosets apparently lack haplotype polymorphism because the number of Caja-DRB loci present per haplotype appears to be constant. Despite this, however, an unexpectedly high number of allelic combinations are observed at the haplotypic level, suggesting that Caja-DRB alleles are exchanged frequently between chromosomes by recombination, promoting an optimal distribution of limited Mhc polymorphisms among individuals of a given population. This peculiar genetic make up, in combination with the limited variability of the major histocompatability complex class II repertoire, may contribute to the common marmoset's susceptibility to particular bacterial infections.

The nature of polymorphism of the HLA-DRB intron sequences is lineage specific

The sequence database of HLA-DRB genes is mainly derived from mRNA analysis or has focused exclusively on the polymorphism of the 2nd exon. Little is known about the non-coding sequences of the different DRB alleles which represent about 94% of the genes. In this study we have determined the sequence of the 3' 500 bp intron 1 fragment adjacent to exon 2 in all serologically defined HLA-DRB genes and their most frequent allelic subtypes. The intron sequences turned out to be highly polymorphic. Similar to the class I introns, this variability was not characterized by random point mutations but by a highly systematic diversity reflecting the lineage-specific relationship of the HLA-DR alleles. With a few exceptions in DRBI*15, 13 and 08 as well as DRB4 and 5, the variability mirrors the serological diversity. As well as delivering insight into the genetic relationship between the different DRB alleles, these sequences will provide an extremely valuable basis for developing advanced DRB sequencing strategies for clinical purposes.

Microsatellite single nucleotide polymorphisms in the HLA-DQ region

Sequencing studies were performed in three previously described microsatellite and minisatellite markers located within the HLA-DQ region, DQCAR, DQCARII and G51152. Multiple nucleotide substitutions that did not change size polymorphisms were observed in all three markers. In all loci, the number of core repeats did not correlate with neighboring DQ allele sequence motifs while single nucleotide changes within or flanking the microsatellite sequence did. This result indicates higher mutation rates for microsatellite expansions/contractions than for nucleotide substitutions in these loci. Further analysis indicated an almost complete phylogenetic correspondence between DQCAR single nucleotide polymorphisms (SNPs) and DQB1 sequences on one side (1.0-1.5 kb apart) and a complete relationship between DQCARII and DQA1 sequences on the other (4.5 kb apart). In contrast, G51152 sequences did not correspond perfectly with DQB1 allelic sequences, thus suggesting the existence of several ancestral crossovers between this marker and DQB1 (20-25 kb). Sequencing microsatellites might be useful in disease mapping studies by increasing marker informativeness and by helping in the interpretation of association study results. It is also proposed that SNPs within the flanking region of CA repeats could be used to develop biallelic markers from already available mapped microsatellite markers.

Presence of retroelements reveal the evolutionary history of the human DR haplotypes

Comparison of intron sequences has been a successful tool for drawing major conclusions about the evolutionary relationship of DRB genes. This complex family of genes is discussed in this review as well as a proposed model for the evolution of HLA-DR haplotypes. The model is based both on phylogenetic analysis of intron sequences as well as presence of ERV9 LTR elements located at identical position in intron 5 of a number of DRB genes. According to this model, two main evolutionary branches of DR haplotypes exist. The DR53 haplotype represents one branch, and the second branch contains the DR51, DR52, DR1, and DR8 haplotypes. After the divergence of the DR53 haplotype, an ERV9 LTR element was inserted in a primordial gene. Consequently, all DRB1 genes as well as the DRB3 gene within haplotypes of the second branch, contain this LTR element. In addition, conserved regulatory sequence motifs are found present within these LTR elements that might regulate DRB gene expression. Novel haplotypes are generated by recombinations and the maintenance of the DR haplotype variation as well as the frequent genetic rearrangements observed might be evolutionary advantageous.

Neutral behavior of shared polymorphism

Several cases have been described in the literature where genetic polymorphism appears to be shared between a pair of species. Here we examine the distribution of times to random loss of shared polymorphism in the context of the neutral Wright-Fisher model. Order statistics are used to obtain the distribution of times to loss of a shared polymorphism based on Kimura's solution to the diffusion approximation of the Wright-Fisher model. In a single species, the expected absorption time for a neutral allele having an initial allele frequency of 1/2 is 2.77 N generations. If two species initially share a polymorphism, that shared polymorphism is lost as soon as either of two species undergoes fixation. The loss of a shared polymorphism thus occurs sooner than loss of polymorphism in a single species and has an expected time of 1.7 N generations. Molecular sequences of genes with shared polymorphism may be characterized by the count of the number of sites that segregate in both species for the same nucleotides (or amino acids). The distribution of the expected numbers of these shared polymorphic sites also is obtained. Shared polymorphism appears to be more likely at genetic loci that have an unusually large number of segregating alleles, and the neutral coalescent proves to be very useful in determining the probability of shared allelic lineages expected by chance. These results are related to examples of shared polymorphism in the literature.

The HLA-DRB9 gene and the origin of HLA-DR haplotypes

HLA-DRB9 is a gene fragment consisting of exon 2 and flanking intron sequences. It is located at the extreme end of the DRB subregion, whose other end is demarcated by the DRB1 locus. We sequenced approximately 1400 base pairs of the segment encompassing the DRB9 locus from eight human haplotypes (DR1, DR10, DR2, DR3, DR5, DR6, DR8, and DR9, the DR4 and DR7 having been sequenced by others earlier), as well as two chimpanzee, five gorillas, one orangutan and one macaque haplotype. The analysis of these sequences indicates that the DRB9 locus, which we estimate to be more than 58 million years (my) old, has been coevolving with the DRB1 locus for the last 4.2 my. As a consequence of this coevolution, the human DRB9 alleles fall into groups that correlate with the DRB1 allelic groups and with the gene organization of the human haplotypes. This observation implies that the present-day HLA-DR haplotype groups (DR1, DR51, DR52, DR8, and DR53) were founded more than 4 my ago and have remained intact (barring minor internal rearrangements that did not recombine the DRB1 and DRB9 genes) for this period of time. The haplotypes have been transmitted during speciations from ancestral to emerging species just like allelic lineages at the DRB1 locus. Thus not only allelic but also haplotype polymorphism evolves trans-specifically.