Gc (vitamin D binding protein) subtype polymorphism and variants distribution among Saharan Middle East and African populations.код для вставкиСкачать
AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 52435-441 (1980' Gc (Vitamin D Binding Protein) Subtype Polymorphism and Variants Distribution Among Saharan, Middle East, and Af r i can Popu I at io ns J . CONSTANS, Ph. LEFEVRE-WITIER, P. RICHARD, A N D G. JAEGER Centre dHemotypologie du C.N.R.S., CHU Purpan, 31052 TOULOUSE CEDEX (J.C., Ph.L.-W., P . R . ) Centres Europeens Assoads de Biologie Humaine, HGpttul Cochin, 75014 PARIS (G.J.) KEY WORDS Gc, VDBP, Polymorphism, Isoelectrofocusing, Sahara, Middle East, Africa ABSTRACT This article presents the results obtained by electrophoretic analysis of the group specific component polymorphism in more than 1,250 serum samples from populations living in the Sahara, the Middle East, and equatorial Africa. In addition to the alleles GcIt and Gels, five variants, including one previously unknown, were found. The distribution of the alleles herein described permits speculation on exchanges and relations among the groups considered. The lowest frequencies of the gene Gc' correspond to regions where sunlight is stronger. There is also a north-south gradient in the Gclb gene frequency. This seems to parallel the gradient seen in skin pigmentation. The study of serum protein polymorphisms by electrophoretic techniques has become a valuable tool for the modern physical anthropologist. It contributes to the description of genetic stocks, helps to trace migrations and exchanges, and may confirm the genetic isolation of some groups. As more refined techniques have developed, a large number of new alleles have been demonstrated in many serum protein systems. This permits a more precise definition of a population's genetic composition. When many populations have been studied, maps can be drawn showing the distribution of the various alleles. If these maps show that clines of gene frequencies cross linguistic, cultural, and racial barriers but are correlated with geographical factors, one can suppose an environmental (selective) effect which may be a s important as migration and genetic exchanges (Bodmer and Cavalli-Sforza, '76). The determination of the polymorphism of the serum group specific component is a good example of such analysis. Recently, Daiger et al. ('75; '77) showed that this protein is capable of binding vitamin D, and many other experiments have confirmed this (Van Baelen et al., '78; Haddad and Walgate, '76). At the same time, we have been able to discover the existence of the extensive polymorphism of this protein by developing a sensitive technique of 0002-9483/80/5203-0435$01 70 11 1980 ALAN R. LISS, INC isoelectrofocusing and immunofixation (Constans et al., '78a). Differences in the 25-OH vitamin D:, affinity of the protein produced by the GclF,GcIS,and Gc' alleles have been demonstrated by means of analytical isoelectrofocusing (Constans et al., '79b). The metabolic role of the protein and its polymorphism contribute to its importance in clinical and biochemical research. In addition, the fact that we observed different frequencies of the Gc genes in European, African, and South Amerindian groups (more than 3,000 samples studies in the laboratory) makes a knowledge of the genetic composition of a population imperative in studies of many disease states. This paper presents original data obtained in the study of the Gc polymorphism in Saharan, Middle-East, and African populations. MATERIAL AND METHODS In Near and Middle East, we examined 58 sera from a group of Kurds living in Iraq and 135 sera from agricultural communities of Bedouins who have settled in North Yemen. In Sahara and Sahel, it is necessary to distinguish three samples (Fig. 1):northern Algerian Sahara (Berber-speaking Harratines of the Received May 10, 1979. accepted Septemher 18. 1979 435 436 J. CONSTANS, Ph. LEFEVRE-WITIER, P. RICHARD, AND G. JAEGER Fig. 1. Location of the eight groups that a r e the subject of this work. Saoura valley), central Algerian S a h a r a (Twareg Isseqamarens of the Hoggar), and southern Sahara (Twareg Kel Kummer of Mali). In eastern Africa, we studied several families from the Afar (95persons) and Issa (92 persons) ethnic groups. In equatorial Africa, we examined samples from the Sara (291persons) who are farmers living in a savannah region of the Central African Empire. Each serum was simultaneously studied by two techniques according to the recommendations of the recent Gc workshop (Constans and Cleve, '79a): isoelectrofocusing on polyacrylamide gel 1 mm thick prepared with an Ampholine solution (LKB) a t pH 4.6 and electrophoresis on polyacrylamide gel. After electrophoresis, proteins a r e stained using a coomassie blue solution. After IEF, an immunofixation on cellulose acetate is necessary. The sera, conserved a t -2O"C, showed no bacterial degradation. All but one of the variants found during this study have already been described (Constanset al., '78a, b). The transmission of the new variant was confirmed by family studies. The results of this research are expressed, not according to the traditional nomenclature based on the diallelism of Gcl and Gc', but ac- cording to the recommendations of the International Workshop on Gc Protein. In Figure 2, we show the variants known a t this time with the correspondences between the old and the new nomenclatures. The genotype distributions were calculated by t h e maximum likelihood programm (Laudet, '68) according to codominant and autosomal transmission of the different alleles considered. The x 2 values were obtained by comparison between the numbers observed and expected in each genotype class. The degree of freedom for each sample was determined by subtracting the number of genes in the population (variant excluded) from the number of phenotypes expected, disregarding any phenotype classes with less t h a n five members (Schwartz, '77). RESULTS AND DISCUSSION The distribution of the observed phenotypes and of the genetic frequencies obtained are presented in Tables 1 and 2. These data show t h a t , based on the diallelism Gcl-GcY,the genotypic and phenotypic frequencies of these groups are similar to those given in the literature for the neighboring geographic zones (Cleve, '73). Gc SUBTYPES AND VARIANTS-SAHARA, MIDDLE EAST, AFRICA A: 2-1F 1C 1V2 1F 1Ab 1s B: 2-IF 1A6 1A5 IF 1Al 1s 1C3 2A5 2A3 2 Fig. 2. Isoelectrofocusing gel electrophoretic patterns of Gc subtypes and variants. Lane A corresponds to the usual denomination for Gc variants. Lane B corresponds to the new nomenclature adopted after the recent Gc workshop ( 7 ) which makes reference to the anodal or cathodal mobilities of the different bands. A set of two bands is a Gc' variant, while a single band is a Gc' variant. However, neither electrophoretic techniques have shown that the diallelism system is incomplete. In fact, there are three principal alleles in the Gc protein: Gcl", Gc'?,and Gc2.The electrophoretic mobilities of their protein products were described earlier (Constans and Cleve, '79a) and are compared to the variants detected in this study (Fig. 2 ) . A total of twenty nine variants are known to date. According to the usual definition, a variant is a n allele present at less than 0.01 frequency in a population (Neel, '78). The deviation between the number of individuals observed and expected in each phenotype (Table 2), assuming Hardy-Weinberg equilibrium, is small. Only the result obtained in the Afar and North Yemen groups show elevated xs values corresponding to a difference between the observed and expected heterozygotes Gc 2-1sand Gc-2-IF. The difference is a t the limit of significance (0.02 <p < 0.05). We have observed a similar distribution of the Gc'" and Gc"' genes in the Peulh population of Senegal (Constans et al., '78b). Only genetic studies done on families and on a larger population sample will allow us to distinguish between the effects of selection and chance. The high value of the x' expresses an anomaly in the population studied and cannot be explained by inbreeding alone. The Kel Kummer Twareg sample is an example of a population in which we have obtained a low x' value showing a good repartition, although this population is known 437 to be very inbred (Jacquard, '72; LefevreWitier, '74). In this group, descended from a very small number of ancestors, we notice the absence of the Gc' gene, which can be explained by the founder effect or by genetic drift. In other respects, the Kel Kummer can be regarded as belonging to the same genetic stock as the surrounding populations of the North Sahara. Since we have described that Gcl is actually made up of two alleles, we can see that this group has maintained a Gc polymorphism. Without modern electrophoretic techniques, this would not have been evident (Constans et al., '78a). The Gc", G P , and Gc' gene frequencies in the samples studied permit a division into three groups: First, there is the group made up of the populations of the northern Sahara, the Sahel, and East Africa (Djibouti samples), where the frequency of the gene Gc" is almost equal to that of the gene Gc'"; Second is the Twareg Kel Kummer genetic isolate, among whom the Gel" gene frequency is the highest yet known, and the Twareg Isseqamaren tribe whose Gcl" gene frequency is also superior to that of Gclt; Third, the Sara group (CAE) shows a Gc" gene frequency substantially higher than that of Gels. We have also observed such values in the Peulhs of Senegal (Constans et al., '78b) and among the Bi-Aka Pygmies (Constans et al., '78a). In the Gc system, European populations are characterized by a Gc' gene frequency greater than 0.25 and a frequency of Gc'" greater than Gc" (Constans et al., '78c; Cleve et al., '78; Kiihnl and Spielman, '78; Thymann and Henningsen, '78). The comparison between the European pattern and the data presented here contrast (Fig. 3).The populations of Europe and of black Africa represent the extremes on this graph and between these two extremes, the position occupied by other samples is especially interesting: The Afar and Issas groups are located not far from the populations of the Middle East, with a frequency of the gene Gc' greater than that of other African populations. However, the frequencies of their Gc'" and Gc'" genes are closer to those of the Twareg and Berber Arab groups of the Sahara. The Kurds and the Bedouins of North Yemen have Gcl" gene frequencies approaching those of European populations, while their Gc' gene frequencies distinguish them clearly from those groups. Clusters of populations can be seen in Figure 3. They correspond well to the geographical clustering of the same populations within the 438 J. CONSTANS, Ph. LEFEVRE-WITIER, P. RICHARD, AND G. JAEGER 0.481 0.425 161 160 Harratins and MRabtines (Saoura) Tuareg Isseqamaren (Hoggar) 260 357 291 751 Tuareg Kel Kummer (Menaka) Peuhls (Fula) Sara Pygmies Biaka Mali Senegal Empire Of Central Africa Algeria 0.619 0.835 0.182 0.091 0.115 0.780 0.039 0.005 0.027 - 0.074 - - 0.053 0.069 2.89 4.81 0.082 0.010 0.015 0.058 0 - - 0.008 - 0.677 2.52 0.015 2.55 2.87 7.24 2.74 7.17 XZ 0.003 0.054 0.125 0.179 0.172 0.137 Gc2 - 0.016 0.541 - - - - 0.010 0.005 - - 0.446 - - &‘A5 - \I 0.008 0.004 &I 0.005 0.008 0.430 0.315 0.430 92 Issa of Djibouti 0.358 0.463 95 Afar Republic 0.595 0.589 0.224 0.270 58 135 Kurds Miscellaneous Iraq North Yemen &” &IF N Population Country Gc Gene Frequencies TABLE 2. Frequency of Gc“, Gc”, GcZ, and Gc vanants in Middle East, S a h m n , and African groups (0 w b P “e M M F 5U 7 i -5 21 > ?? %U 4 k? 2 d n 440 J. CONSTANS, Ph. LEFEVRE-WITIER, P. RICHARD, AND G. JAEGER Gc Is gene frequencies t 'Itsa 'Afar q21 Fig, 3. Distribution of the eight groups which are the subject of this work according to their Gc" and Gc'gene frequencies and in comparison to other data puhltshed. clusters, the groups of the Sahara differ more by their Gc" gene frequencies, while in Europe the groups differ more by their GcLgene frequencies. Based on the existence of the three gene Gc", Gc" and Gc', it is interesting to notice such variability in the manifestation of the polymorphism. These data reveal certain phenomena which perhaps can be best explained by the action of geographical factors in relation to the metabolic role of this protein in the organism (Daiger and Cavalli-Sforza, '77; Constans et al., '79b). Kirk et a l . ('63), Walter and Steegmuller ('691, Mourant et al. ('76), and later Daiger ('79)have demonstrated that the frequency of the gene Gc2 follows a cline, diminishing a s the mean intensity of solar radiation increases. Our results confirm this assumption. The data recovered in this investigation show the presence of a gradient in the frequency of the gene Gel' which increases from Europe t o equatorial Africa. This gradient may be superposed on that of skin pigmentation (Loomis, '67) which has always been considered to be linked to genetic and adaptative conditions (Hiernaux, '77). Distribution of vuriunts The study of these samples has permitted us to discover one new variant, Gc' \Ii, in the Issa group. The other variants Gel.\' (Gc""), GclC':', Gc2.':<,Gc''.'~have already been described (Constans and Cleve, '79a). We observe that the variant Gc'." is present in the Kurds of Iraq, the Yemenites, the Sara samples, the Peulhs, and in the Harratins ofthe Saoura. Earlier studies suggested that the presence of the Gcl.'l variant was limited to the populations of sub-Saharan Africa, especially Bantus (Kitchin and Bearn, '66) or Pygmies (Constans et al., '78a). Its presence a t a low frequency among our different samples of Kurds, Yemenites, Sara, Fulani, and Harratins of the Saoura valley suggests a n introduction through contacts or the slave trade. This could also explain the presence of the negroid Gc"':' in two Harratin individuals (Algerian Saoura). Another variant, Gc"':', could be a good marker for populations of the Sahara and the Near East (Druzes studied by Cleve et al., '78). We found it in two Twareg samples in the Issas of Djibouti and the Fulani of Senegal (Constans Gc SUBTYPES AND VARIANTS-SAHARA, MIDDLE EAST, AFRICA et al., '78b). Up to this date, it has not been found in European and sub-Saharan samples. The G C " ~variant is also found in the Saharan populations, but has only been found in the small isolated tribe of Isseqamaren Twareg and the Fulani of Senegal. CONCLUSIONS Group specific component polymorphism, as studied by modern electrophoretic techniques, represents an extremely instructive marker system for anthropological research. The frequencies of the Gc alleles and their presence or absence in the populations presented in this paper help us t o evaluate the population dynamics of North Africa and the Arabian peninsula. Every human population studied to date is polymorphic for the Gc protein. Even in the Kel Kummer, who were completely lacking the Gc' allele, a polymorphism is present, because the Gc system is multiallelic. The distribution of the Gc alleles seems to correlate with geographical clines, notably the incidence of solar radiation, as might be expected for the protein which transports vitamin D. LITERATURE CITED Bodmer, W.F., and L.L. 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