Analysis of human leukocyte antigen-G polymorphism including intron 4 in Japanese couples with habitual abortion

Clinical variables and genetic variants associated with perioperative anaphylaxis in Chinese Han population: A pilot study

Background

Perioperative anaphylaxis (POA) can lead to severe consequences. Identifying clinical risk factors and genetic loci associated with POA through pre-prescription screening may help reduce its incidence.

Methods

Using univariate regression and covariate-adjusted multivariate regression, we retrospectively analyzed the association between clinical characteristics and POA in 72 POA patients and 72 non-POA individuals. The discovery study of whole-exome association relied on whole-exome sequencing of 73 POA cases and 1339 healthy individuals. A replication study involving an independent set of 16 POA cases and 1339 healthy individuals confirmed this association. The accurate typing of human leucocyte antigen through exome sequencing (ATHLATES) algorithm and the whole-exome sequencing data were used for genotyping the human leucocyte antigen G (HLA-G) of 73 POA patients. The HLA-G of 16 POA cases and 122 non-POA patients were genotyped through Sanger sequencing. We used Fisher's exact probability method to compare the allele and carrier frequencies between POA patients and healthy individuals or non-POA patients. A Pc (P/Bonferroni correction coefficient) < 0.05 represents statistical significance.

Results

Regression analysis identified female sex, an unconfirmed food allergy label, and a history of prior surgery as clinical variables associated with POA. The whole-exome association discovery study identified a strong signal in the major histocompatibility complex region on chromosome 6, with the rs1130356 being the most significant locus (P = 1.5E-10, OR = 3.4, 95% CI = 2.4-4.9). The replication study verified the association between the rs1130356-T allele and POA cases (P = 1.0E-6, OR = 6.3, 95% CI = 3.1-12.7). Compared with non-POA patients, HLA-G∗01:01 (Pc = 2.4E-4, OR = 2.4, 95% CI = 1.6-3.6) was significantly enriched, while HLA-G∗01:04 (Pc = 1.2E-6, OR = 0.3, 95% CI = 0.2-0.5) was lessened in POA patients.

Conclusion

Our study suggested an association between POA and the risk factors of female sex, an unconfirmed food allergy label, and prior surgery. HLA-G, located in the human leucocyte antigen (HLA) region, may act as a surrogate genetic marker for POA. This suggests a causal relationship between this specific genomic region and POA. Our findings shed light on the contribution of human exome genetic variants to the susceptibility to POA.

HLA class Ib in pregnancy and pregnancy-related disorders

The HLA class Ib genes, HLA-E, HLA-F, and HLA-G, were discovered long after the classical HLA class Ia genes. The elucidation of their functions had a modest beginning. However, their basic functions and involvement in pathophysiology and a range of diseases are now emerging. Although results from a range of studies support the functional roles for the HLA class Ib molecules in adult life, especially HLA-G and HLA-F have most intensively been, and were also primarily, studied in relation to reproduction and pregnancy. The expression of HLA class Ib proteins at the feto-maternal interface in the placenta seems to be important for the maternal acceptance of the semi-allogenic fetus. In contrast to the functions of HLA class Ia, HLA-G possesses immune-modulatory and tolerogenic functions. Here, we review an accumulating amount of data describing the functions of HLA class Ib molecules in relation to fertility, reproduction, and pregnancy, and a possible role for these molecules in certain pregnancy complications, such as implantation failure, recurrent spontaneous abortions, and pre-eclampsia. The results from different kinds of studies point toward a role for HLA class Ib, especially HLA-G, throughout the reproductive cycle from conception to the birth weight of the child.

Role of HLA-G and other immune mechanisms in pregnancy

Pregnancy loss (abortion) and pre-eclampsia represent the most common disorders in pregnant women. Besides infection, there are anatomical, endocrinological, genetic and immunological factors that can induce pregnancy disorders. Because the exact mechanisms of physiological pregnancy maintenance are still not clearly understood, the search for genes and proteins fulfilling this role is still in progress. One of the immune molecules that plays a beneficial role in pregnancy is the nonclassical HLA-G molecule. The molecule is mainly expressed on trophoblast cells in the foetal placenta and induces the immune tolerance of the foetus via its interaction with inhibitory receptors on maternal NK cells and CD8+ T lymphocytes. In relation to pregnancy disorders, associations between HLA-G polymorphism, HLA-G level and HLA-G function were described. Thus, the HLA-G molecule can be used as a new diagnostic marker and, potentially, for the future therapy of pregnancy disorders.

The importance of HLA-G expression in embryos, trophoblast cells, and embryonic stem cells

The nonclassical HLA-G molecule is a trophoblast-specific molecule present in almost every pregnancy. It differs from classical HLA class I molecules by the low degree of allelic variants and the high diversity of protein structures. HLA-G is reported to be a tolerogenic molecule that acts on cells of both innate and adaptive immunity. At the maternal-fetal interface HLA-G seems to be responsible largely for the reprogramming of local maternal immune response. This review will focus on the HLA-G gene expression profile in pregnancy, in preimplantation embryos, and in human embryonic stem cells with emphasis on the structural diversity of the HLA-G protein and its potential functional and diagnostic implications.

HLA-G polymorphism patterns show lack of detectable association with recurrent spontaneous abortion

Human leukocyte antigen-G (HLA-G) is a class I non-classical molecule that is predominantly expressed on the extravillous cytotrophoblasts at foetal-maternal interface during pregnancy. We recruited 143 recurrent spontaneous abortion (RSA) and 150 control couples for the study. DNA-based typing of the HLA-G was carried out to explore if we can validate the patterns of association reported elsewhere or find association of novel HLA-G alleles with RSA in the Indian population. We also evaluated the role of allele sharing in couples with RSA. We did not find association of any of the HLA-G alleles with RSA in our study. There is a general trend of increase in sharing among the RSA couples, but the increase is not significant. The results suggest that the HLA-G alleles or the allele sharing by couples may not play a significant role in the manifestation of RSA in the Indian context albeit more studies are required before making any definitive statement.

Human leukocyte antigen-G polymorphism in relation to expression, function, and disease

Human leukocyte antigen-G (HLA-G) is a nonclassical class Ib molecule belonging to the major histocompatibility complex. HLA-G appears to play a role in the suppression of immune responses and contribute to long-term immune escape or tolerance. The focus of this review is polymorphism in the HLA-G gene and protein and its possible importance in expression, function, and disease associations.

HLA-G polymorphism and in vitro fertilization failure in a Polish population

To investigate whether human leukocyte antigen-G (HLA-G) gene polymorphism is associated with in vitro fertilization (IVF) failure, we sequenced exons 2-4 of the HLA-G gene in 50 couples with three or more IVFs (including 10 couples with five or more IVFs) and 58 control fertile couples from a Polish population. Of the 10 different HLA-G alleles identified in our study subjects, neither allele was found to be associated with IVF. We also genotyped 50 couples with IVF and 71 control couples for the -725C>G variant in the promoter region and the 14 bp insertion or deletion polymorphism in the 3' untranslated region of the HLA-G gene. The frequency of -725GG or GC genotype in women with IVF and in control fertile women was similar [26% vs 25.3%; odds ratio (OR) = 1.0; P = 1.0]. The 14 bp ins/ins or ins/del genotype was more common in women with IVF than in control women (76.9% vs 59.1%; OR 2.4; P = 0.03), but the difference was not significant after Bonferroni correction for multiple comparisons. The frequency of the ins/ins or ins/del genotype was particularly high (90%) in women who experienced five or more IVFs (OR = 6.2; P = 0.08), but again, the excess was not statistically significant, possibly because of small sample sizes. These results are in line with functional studies that show lower levels of HLA-G mRNA and protein related to the HLA-G allele including the 14 bp sequence and suggest that the insertion allele may be associated with an increased risk of IVF.

HLA-G polymorphism in a Polish population and reproductive failure

To investigate whether human leukocyte antigen (HLA)-G gene polymorphism is associated with reproductive failure in a Polish population, we sequenced exons 2-4 of the HLA-G gene in 58 couples with three recurrent spontaneous abortions (RSAs) in the first trimester of pregnancy and 58 fertile control couples. We identified 12 different HLA-G alleles. Neither allele was found to be associated with an increased risk of RSA in the population. HLA-G allele sharing was similar in couples with RSA and in control fertile couples. All cases and controls were also genotyped for the -725C>G polymorphisms in the promoter region and the 14-bp insertion deletion in the 3' untranslated region of the HLA-G gene. The frequencies of both variants in RSA women and control fertile women were similar. These results suggest that HLA-G gene polymorphism does not influence the risk of RSA in the Polish population, but further studies are needed in this regard.

Recurrent spontaneous abortions patients have more −14 bp/+14 bp heterozygotes in the 3′UT region of the HLA-G gene in a Chinese Han population

Although human leukocyte antigen (HLA)-G gene polymorphism is low, it may influence levels of protein expression and, in some cases, has been associated with pregnancy diseases. Apart from the 23 HLA-G alleles, including a null allele known as HLA-G*0105N, a 14-bp deletion or insertion in the 3'UT region also contributes to the HLA-G gene polymorphism. In order to discover any correlation between the 14-bp deletion polymorphism and pregnancy diseases, we isolated the genomic DNA from 24 recurrent spontaneous abortions (RSA) patients and 88 normal fertile control individuals using the salted out method. Exon-8 of HLA-G gene of the two groups was amplified by polymerase chain reaction, respectively, and analyzed by electrophoresis on 9% nondenaturing polyacrylamide gel and stained with ethidium bromide. Differences between the two groups were compared using the chi(2) test. The results were there was no significant difference in allelic frequencies of 14-bp deletion polymorphism between normal control and RSA patients; frequency of -14 bp/+14 bp heterozygotes was significantly increased in RSA patients as compared with normal fertile controls. Our results are similar to a study of Indian women but contradictory to a study of Danish women. This discrepancy may be due to ethnic differences.

Association of the maternal 14-bp insertion polymorphism in the HLA-G gene in women with recurrent spontaneous abortions

Human leukocyte antigen (HLA)-G has been postulated as an important immunotolerant molecule in maintaining fetal-maternal relationship. Recent reports indicated that the 14-bp deletion/insertion polymorphism in exon 8 of HLA-G gene influences HLA-G mRNA stability and isoform splicing patterns, thus modulating the levels of HLA-G expression. This might play an immunomodulatory role of HLA-G during implantation and pregnancy. In the present study, 109 unrelated fertile control women and 79 women who had experienced recurrent spontaneous abortion (RSA) were genotyped for the 14-bp insertion/deletion polymorphism. No significant difference was observed in the distribution of 14-bp insertion/deletion genotype between controls and the RSA group. However, a greater number of 14-bp insertion alleles exist in the RSA group than in the controls.

The -56T HLA-G promoter polymorphism is not associated with pre-eclampsia/eclampsia in Australian and New Zealand women

OBJECTIVE

Decreased HLA-G expression has been linked to a number of pregnancy disorders, including preeclampsia, and a genetic basis for HLA-G regulation has yet to be found. The aim of this study was to determine whether a C-to-T base substitution 56 base pairs (bp) upstream from the HLA-G transcription start site is associated with preeclampsia.

METHODS

277 nulliparous women consisting of 113 normotensive, 118 preeclamptic, and 46 eclamptic patients were typed for the -56T polymorphism using restriction fragment length polymorphism and allelic discrimination analysis.

RESULTS

-56T allele frequencies for eclamptic, preeclamptic, and normotensive women were 0.053, 0.030, and 0.035, respectively. A chi2 test indicated that there was no significant association with the polymorphism in preeclamptic or eclamptic women with p > 0.05.

CONCLUSION

The -56T HLA-G polymorphism is not associated with preeclampsia or eclampsia in our population.

HLA-G polymorphism in a Chinese Han population with recurrent spontaneous abortion

Reproduction is an important biological phenomenon posing an immunological paradox because the semiallogeneic fetus survives by evading maternal immune recognition. The detailed mechanisms behind this maternal-fetal immunotolerance remain elusive. Human leucocyte antigen (HLA)-G, a non-classical HLA class I antigen, initially identified as a molecule selectively expressed on extravillous cytotrophoblasts and first studied in the context of pregnancy, has long been supposed to play a critical role in fetal-maternal immunotolerance. To investigate the role of HLA-G polymorphism in this process and whether the HLA-G genotype is associated with an increased risk for a subsequent miscarriage, 69 women with three or more recurrent spontaneous abortions (RSA) and 146 fertile control women were genotyped for the HLA-G locus in this study. To our knowledge, this is the first report on HLA-G polymorphism in RSA and in normal fertile women from a Chinese Han population. Nine HLA-G alleles were detected in the fertile control group; however, the allele HLA-G*0103 was absent in the RSA group. No statistical significance was observed in the distribution of HLA-G alleles between the two groups. The frequency of the null allele HLA-G*0105 N in the RSA group and in normal fertile women is 0.7% and 1.4%, respectively. Our data suggested that there was no association of HLA-G polymorphism with RSA.

HLA-G in human reproduction: aspects of genetics, function and pregnancy complications

The non-classical human leukocyte antigen (HLA) class Ib genes, HLA-E, -G and -F, are located on chromosome 6 in the human major histocompatibility complex (MHC). HLA class Ib antigens resemble the HLA class Ia antigens in many ways, but several major differences have been described. This review will, in particular, discuss HLA-G and its role in human reproduction and in the human MHC. HLA-G seems to be important in the modulation of the maternal immune system during pregnancy and thereby the maternal acceptance of the semiallogenic fetus. Recent findings regarding aspects of HLA-G polymorphism, the possible significance of this polymorphism in respect to HLA-G function and certain complications of pregnancy (such as pre-eclampsia and recurrent spontaneous abortions (RSA)) are discussed together with possible importance to IVF. Finally, aspects of a possible role of HLA-G in organ transplantation and in inflammatory or autoimmune disease, and of HLA-G in an evolutionary context, are also briefly examined.

Immunogenetic Determinants of Preeclampsia and Related Pregnancy Disorders

OBJECTIVE

We sought to systematically review the impact of immunogenetic factors, specifically human leukocyte antigen (HLA) allele frequencies, maternal homozygosity, couple sharing, and maternal-fetal sharing, on the risk of preeclampsia and intrauterine growth restriction.

DATA SOURCES

A computerized search of PubMed databases from 1975 to 2003 was performed with the terms "preeclampsia," "eclampsia," "intrauterine growth restriction," and "human leukocyte antigens" and limited to studies of human subjects in English. No restrictions were placed on study design. All bibliographies were cross-referenced to identify additional pertinent studies.

METHODS OF STUDY SELECTION

Titles and abstracts were reviewed carefully. Observational and basic science research studies were selected if their main objective was to assess the relationship of any aspect of HLA genotypes with preeclampsia and related disorders of pregnancy.

TABULATION, INTEGRATION, AND RESULTS

Data were abstracted and tabulated from 22 original research studies. Meta-analytic techniques were not performed owing to variations in disease and exposure definitions as well as research methodologies. Studies that examined maternal, paternal, and fetal HLA allele frequencies, maternal homozygosity, and couple sharing yielded inconsistent results. Although the cumulative evidence points to the HLA-DR locus (particularly DR4) as a correlate of preeclampsia, it remains unclear whether any specific HLA allele, haplotype, or susceptibility gene in linkage disequilibrium with the HLA region is responsible. Although genetic evidence is suggestive of gene-gene interaction between mother and fetus, few studies have evaluated the influence of maternal-fetal HLA sharing.

CONCLUSION

In the early 1990s, HLA genotypes were dismissed as possible etiologic factors for preeclampsia, based on studies that are heterogeneous with respect to study design, outcome, and exposure assessment. Many of these studies did not take into account the interactions between maternal, paternal, and infant genotypes. Thus, adequately powered studies designed specifically to assess the effect of maternal-fetal HLA sharing on risk of preeclampsia are needed.

Association of human leucocyte antigen sharing with recurrent spontaneous abortions

An estimated 15% of clinically recognized pregnancies abort spontaneously. Recurrent spontaneous abortion (RSA) is defined as three or more consecutive miscarriages conceived with the same partner in the absence of uterine, genetic or autoimmune abnormalities. Evidence points to human leucocyte antigens (HLA) as playing a role in the successful development of the foetus. In particular, HLA compatibility is more prevalent in couples experiencing reproductive failure, especially RSA couples, compared to fertile couples. According to the immunological hypothesis, an adequate immune response is necessary for proper implantation of the embryo; conversely, a depressed response of maternal lymphocytes to the stimulation by paternal antigens because of HLA sharing can result in disorders, such as RSA. The genetic hypothesis implicates homozygosity for recessive lethal alleles in linkage disequilibrium with specific HLA haplotypes. The specificity of HLA alleles or haplotypes responsible for or linked to other RSA susceptibility genes remains unclear. In this study, we identified 40 observational studies (32 case-control, five cohort, one cross-sectional, one case series and one basic science) that examined the associations between HLA and RSA, focusing on HLA allele couple and maternal-foetal sharing, and the special role of HLA-G. We sought to identify consistent findings among studies examining similar questions. Evidence remains divided concerning the role of HLA allele couple sharing. Of major concern is the focus of many studies on couple sharing as a proxy measure of maternal-foetal sharing. Therefore, adequately powered studies are needed, which employ standard case definitions and reproducible methodologies to directly assess the role of maternal-foetal HLA sharing on the risk of RSA.

Analysis of human leukocyte antigen (HLA)-G polymorphism in normal women and in women with recurrent spontaneous abortions

Human leukocyte antigen (HLA)-G is a non-classical class I antigen. It has limited expression, but is high at the foetomaternal interface. This unique expression pattern of HLA-G suggests that it might be important for survival of the foetus during pregnancy. In the present study, 120 women with recurrent spontaneous abortions (RSA) and 120 fertile control women were genotyped for the HLA-G locus. This is the first report describing HLA-G polymorphism in normal fertile and RSA women from India. The allele HLA-G*010103 was higher in women with recurrent foetal losses. Interestingly, the HLA-G*010105 and G*010108 alleles were totally absent in normal fertile women but present in RSA women with frequencies of 1.7% and 0.4%, respectively. Allele G*010107 was absent in both the groups. The frequency of the null allele G*0105N was high (13.8%) in our population as compared to other world populations. Our data support the hypothesis that HLA-G polymorphism may contribute to recurrent foetal loss.

Role of 14-bp deletion in theHLA-Ggene in the maintenance of pregnancy

Differential expression of human leukocyte antigens (HLAs) on trophoblast has been the focus of many studies, specially on extravillous cytotrophoblast cells, which migrates into the maternal uterine tissues. These invading cells do not express classical major histocompatibility complex class I (-A and -B) and class II molecules, along with low expression of HLA-C. HLA-G is the predominantly expressed antigen along with HLA-E. Hence, it is believed that expressed antigens may be involved in materno-fetal tolerance. In the present study, we have studied 14-bp deletion polymorphism in the exon-8 of the non-classical HLA-G antigen. There was no difference in the frequency of deletion/insertion polymorphism in fertile normal women and recurrent spontaneous abortion (RSA) women. However, the number of heterozygotes (-14b/+14b) were increased in RSA women. The probable mechanism for the increase of heterozygotes in recurrent fetal loss is discussed in light of soluble HLA-G.

Role of a KIR/HLA-C allorecognition system in pregnancy

Decidual natural killer (NK) cells are thought to play a significant role in the allorecognition mechanisms during pregnancy. Through their activating and inhibitory receptors they may recognize selectively class I HLA alleles expressed on invading trophoblast and provide self-signals to control NK responses, thus regulating the maternal immune response at the fetomatenal interface. Killer immunoglobulin-like receptors (KIR) constitute one of the families of class I MHC receptors which are expressed on NK cells. Their repertoire includes both activating and inhibitory receptors, most of which recognize specific epitopes on HLA-C molecules and can either activate NK cell responses or abort activating signals and inhibit NK cell functions. Since KIRs are expressed on decidual NK cells and the HLA-C molecules that they recognize are also expressed on invading trophoblast, KIR receptors may play a regulatory role in pregnancy by interacting with their trophoblastic HLA-C counterparts and providing trophoblast damage evading signals (KIR/HLA-C allorecognition system). Our hypothesis that the KIR/HLA-C system might be ineffective in some unsuccessful pregnancies, has been investigated in women with unexplained spontaneous abortions. Our results suggest that a limited maternal repertoire of inhibiting KIRs (inhKIRs) and/or lack of maternal inhKIR-fetal HLA-C epitope matching may predispose to miscarriage.

HLA-G allelic variants are associated with differences in the HLA-G mRNA isoform profile and HLA-G mRNA levels

During pregnancy, the human extra-villous trophoblast in the contact zone between maternal and fetal tissue in the placenta does not express the classical MHC class I and II molecules. Instead, HLA-G and -C, and possibly HLA-E, are expressed. HLA-G may modulate the immunological relationship between mother and fetus in several ways. Finally, the expression of membrane-bound HLA-G and soluble HLA-G has been proposed to influence the outcome of pregnancy, and an aberrant HLA-G expression in pre-eclamptic placentas and spontaneous abortions has been reported. Here, an association between certain HLA-G polymorphisms and the mRNA levels of the different alternatively spliced HLA-G isoforms in first trimester trophoblast cell populations is reported. Several alternatively spliced HLA-G mRNA isoforms, including a 14-bp polymorphism in the 3'UTR end (exon 8) of the HLA-G gene, are expressed at a significantly lower level than the corresponding HLA-G mRNA isoforms with the 14-bp sequence deleted. Furthermore, characteristic HLA-G mRNA isoform expression patterns were associated with specific HLA-G genotypes and alleles. In the HLA-G*01012 and - G*01013 alleles that include the 14-bp sequence, an additional alternative splicing was observed, with the first 92-bp of exon 8 spliced out. This was most pronounced in HLA-G genotypes with G*01013. These findings may have functional implications for the recent reports of aberrant HLA-G expression and reproductive success.

Natural killer (NK) cell receptors' repertoire in couples with recurrent spontaneous abortions

PROBLEM

Natural killer (NK) cell receptors (NKRs) have been suggested to protect trophoblast, but their function at the fetomaternal interface remains unknown. To investigate if the outcome of pregnancy depends on women's NKRs, we studied the NKR repertoire in couples with recurrent spontaneous abortions (RSA).

METHODS

Twenty-six childless couples with > or = 2 abortions, characterized by alloimmune abnormalities, and 26 control couples were genotyped for five killer immunoglobulin-like receptors (KIR) and two CD94/NKG receptors, known to have as ligands human leukocyte antigen (HLA) class I molecules with trophoblastic expression: inhibitory 2DL1,2,3 and activating 2DS1,4 KIRs, inhibitory NKG2A and activating NKG2C. Detected repertoires of women and partners were compared between the two groups.

RESULTS

Less aborters than controls were found to have all three inhibitory KIRs (30.77% versus 69.23%, P = 0.01), some of them had only one inhibitory KIR (19.23% versus 3.85%, P = 0.08) and most of them were lacking inhibitory KIRs possessed by their husbands (57.69% versus 15.38%, P = 0.001).

CONCLUSIONS

Women with alloimmune abortions have a limited inhibiting KIR repertoire and such miscarriages may occur because trophoblastic HLA class I molecules are recognized by decidual NK cells lacking the appropriate inhibitory KIRs.

HLA-G Molecules: from Maternal–Fetal Tolerance to Tissue Acceptance

Over the past few years, HLA-G, the non-classical HLA class I molecule, has been the center of investigations that have led to the description of its specific structural and functional properties. Although located in the HLA class I region of chromosome six, the HLA-G gene may be distinguished from other HLA class I genes by its low polymorphism and alternative splicing that generates seven HLA-G proteins, whose tissue-distribution is restricted to normal fetal and adult tissues that display a tolerogeneic function toward both innate and acquired immune cells. We review these points, with special emphasis on the role of HLA-G in human pathologies, such as cancer, viral infection, and inflammatory diseases, as well as in organ transplantation.

HLA-G polymorphisms in couples with recurrent spontaneous abortions

The etiology of a fraction of recurrent spontaneous abortions (RSA) may involve immunological mechanisms. Aberrant profiles of Th1 and Th2 cytokines have been observed which are not present in uncomplicated pregnancies. Studies of classical HLA class I and II antigens in relation to RSA have not been conclusive. Furthermore, these antigens are not expressed in the placenta with the exception of HLA-C. However, HLA-G is expressed on especially invasive cytotrophoblasts and exists in both membrane and soluble forms. HLA-G may be involved in materno-fetal tolerance. Therefore, 61 RSA couples (with three or more spontaneous abortions) and 47 fertile control couples were HLA-G genotyped by direct DNA sequencing and analyzed for specific polymorphisms. No statistically significant differences were observed in the distribution of HLA-G alleles between controls and RSA couples, however, 15% of the RSA women carried the HLA-G*0106 allele compared to 2% of the control women. The 14 bp deletion polymorphism in exon 8 was investigated separately. There were a greater number of heterozygotes for the 14 bp polymorphism in the group of fertile control women than expected, according to Hardy-Weinberg equilibrium. Furthermore, the HLA-G alleles without the 14 bp sequence were prominent in the RSA males in contrast to the RSA women in whom alleles including the 14 bp sequence were frequently observed, especially as homozygotes. These results are discussed in relation to two hypotheses concerning HLA-G and RSA. A hypothesis of HLA-G histo-incompatibility between fetus/placenta and the mother was not supported by the data. Another hypothesis concerned certain HLA-G alleles associated with an altered expression profile of HLA-G isoforms or reduced expression of certain HLA-G isoforms.

HLA-G exhibits low level of polymorphism in indigenous East Africans

Human leukocyte antigen G (HLA-G) is a nonclassical HLA class I antigen that is predominantly expressed on invasive cytotrophoblastic cells, and is postulated to be a mediator of maternal-fetal tolerance. Almost all studies in Caucasian and Asian populations have consistently reported that HLA-G exhibits low levels of allelic polymorphism unlike the classical class I genes. However, the concept that HLA-G is nonpolymorphic has recently been challenged in a single study of African-American subjects. We have examined the DNA sequences of the first seven HLA-G exons by single-strand conformational polymorphism (SSCP) and DNA direct sequencing procedures in 45 healthy individuals from an indigenous African population. Overall, we detected 14 sequence variations: 3 in the signal peptide (exon 1); 2 in the alpha-1 domain (exon 2); 5 in the alpha-2 domain (exon 3); 2 in alpha-3 domain (exon 4); 2 in transmembrane domain (exon 5); and none in the cytoplasmic tail (exons 6 and 7). Of these variants, only three result in amino acid substitutions at the protein level. Of particular interest, we identified a novel nucleotide substitution (C727T), 56 bp before the HLA-G gene transcription start site, located in the putative binding site for polyomavirus enhancer-binding protein 2 (PEBP2) transcriptor factor. These data confirm previous reports describing HLA-G exhibiting limited allelic polymorphism. Further studies are needed to determine the impact of the C727T polymorphism on the level or developmental regulation of HLA-G expression.