Natural Selection Impacts On The Polymorphism Of HLA-G3’UTR In5Ethnic Groups

Human Leucocyte antigen-G (HLA-G), a non-classic HLA I molecule, plays a key role in the regulation of the immune system, and is initially identified on the fetal extravillous cytotrophoblast cells, placental macrophages and mesenchymal chronic villi, which plays a crucial role in maternal-fetal tolerance. Recently,3’UTR polymorphism in the exon8of the HLA-G gene has been studied extensively, and found that it was associated with HLA-G mRNA stability and could influence HLA-G expression. At least three3’UTR polymorphic sites have been associated with HLA mRNA regulation. As genetic polymorphism observed in the HLA-G3’UTR region is associated with the posttranscriptional control of HLA-G expression, this gene has been studied in autoimmune, chronic inflammatory and chronic infectious disease, in allografting and in several types of cancer.Several reports have proposed a relationship between the HLA-G3’UTR polymorphism and diseases, however some different conclusions were drawn from the studies involved different populations. Considering HLA-G3’UTR polymorphism varied in populations, it is necessary to identify a reliable profile of HLA-G3’UTR polymorphism in populations. We explore the association between HLA-G3’UTR polymorphism and genetic background in the research. To detect the polymorphism of HLA-G3’UTR, PCR and DNA sequencing were used. The genetic background was investigated by the genotype of10STRs based on capillary electrophoresis. The DNA samples were obtained from432individuals from5minorities which were97Dai,110Gelao,135Shui,47Kirgiz and43Mongolian. Genetic backgrounds were investigated using10mircosatellite DNA makers on the Chromosome3with about40samples per minority group from5populations (Gelao and Shui population were explored by this research). The HLA-G3’UTR polymorphism frequencies were counted directly and the paired differences between populations were tested by using SPSS17.0. Arlequin3.11was used to calculate the parameters such as, expected heterozygosity (He), fixation index (FST) and Hardy-Weinberg Equilbrium (HWE) test. Ewens-Watterson test was also performed to measure the balancing selections based on the haplotype of HLA-G3’UTR polymorphisms. In order to avoid deviations caused by the difference of the distance calculation methods and catch the common features of population’s relationships, both Nei’s distance Da and FST were calculated. Phylogenetic tree was drawn based on these two genetic distances by Poptree2software. The genetic components of these five populations were clustered by STRUCTURE2.3. The outputs of STRUCTURE were graphically modified by DISTRUCT. We found8polymorphic sites respectively in the Gelao, Mangolian and Kirgiz population, and+3196G/C has no polymorphism and only7SNP were found in Shui and Dai people. All the polymorphic sites encompass the14bp Ins-Del,3003T/C,3010G/C,3027C/A,3035C/T,3142G/C,3187G/A and3196G/C. The heterozygote advantages seem to be a major force in the southern populations, consistent with a higher observed heterozygote than the expected under neutrality, and detectable results derived from Ewens-Watterson test. These two results suggest that the balancing selection may act on the profile of the HLA-G3’UTR region.Both natural selections and demographic events can impact the distribution of the heterozygote frequencies, so we need to compare the genetic background of these populations to rule out the influence of demographic events. The results of the10neutralized STR were not the same with the HLA-G3’UTR region. In the phylogenetic tree of10STRs we can found a distinguishable profile between the north and south. In the south of China, Gelao and Shui people shared a similar genetic background. However, in the gene tree of HLA-G3’UTR region, Gelao and Shui people located in the different branches. These results suggest that the distribution of HLA-G3’UTR region in southern populations is not always in accordance with their genetic backgrounds. Considering the balancing selection during HLA evolution, especially for the southern populations under a strong selection pressure in China, we propose that the balancing selection maybe break the connections between distributions HLA-G3’UTR region with their genetic backgrounds in southern populations.We speculate that pathogen-driven balancing selection may influence the HLA-G3’UTR region in southern populations which resulted in a higher heterozygosity in the southern populations. It is also demonstrated in the Ewens-Watterson test that people in the southern China may be influenced by balancing selection. Gelao and Shui people have the similar genetic background, origin, geographic location and experiences of infectious diseases, but they have distinct different HLA-G3’UTR polymorphic profiles. It probably suggests that the selective abortion be attributed to the difference of HLA-G in Dai and Shui people. So the mechanism of the balancing selection is complex, and more evidences are needed to distinguish in people of South China.