Ethnic/Geographic Variation Of The Val34leu Polymorphism Of Coagulation Factor Xiii And Its Distribution In American Admixed Populations
M de Lugo, Dra., A Rodríguez-Larralde, B Guerrero, D de Guerra
american admixed populations, ethnic groups, factor xiii-val34leu, interpopulational variation coefficient gst`
M de Lugo, Dra., A Rodríguez-Larralde, B Guerrero, D de Guerra. Ethnic/Geographic Variation Of The Val34leu Polymorphism Of Coagulation Factor Xiii And Its Distribution In American Admixed Populations. The Internet Journal of Biological Anthropology. 2007 Volume 2 Number 1.
The Val34Leu polymorphism of coagulation Factor XIII gene shows a controversial relationship with homeostatic pathologies. Some studies have evidenced ethnic heterogeinity in its distribution but no patterns of genetic variability have been described so far; likewise, reports concerning admixed American populations remain scarce. We analyzed the diversity of inter and intra-group frequencies of the Leu34 using the estimators Dm and Gst', on data reported for populations classified into five genetic/geographic groups and also publish its frequencies in three Venezuelan populations, correlating these values with their levels of admixture.
We confirm ethnic heterogeneity in the prevalence of Leu34 (Gst`=10.41%), yet important intra continental group variations must also be considered. Their frequency in American admixed populations varies as a function of their parental contributions. Clarifying the degree of variability of this polymorphism, together with other aspects of homeostasis, should lead to a better understanding of different geographical patterns of coagulation disorders manifestations.
Several studies that have been carried out on diverse human populations to analyze the molecular bases of deficiencies of the blood clotting Factor XIII have led to the identification of five frequent polymorphisms located on the exonic regions of the gene which encodes for the A subunit of the plasmatic Factor XIIIA. One of those polymorphisms is the SNP (ie,
Preceding studies have pointed out the importance of adequate sample selection to evaluate frequency distributions of various polymorphisms in apparently healthy populations, prior to analyses of relationships with any kind of pathology. Particularly, with high proportions of racial admixture, mistakes of interpretation may be inherent to the process of genetic stratification of such populations 13,14,15,16 . For this reason and in order to minimize such errors, some researchers have preferred to restrict their samples to individuals from the same ethnic group 8 . The geographic distribution of the Leu34 variant may explain at least in part, the expressions of blood-clotting disorders in different population groups 9,10 ; although despite the high relevance of Val34Leu in epidemiological studies, little is known regarding its variations in populations from similar origins 17 . The aim of this study was to elucidate the genetic differentiation patterns of the Leu34 allele, analyzing its frequency distribution as reported for various worldwide populations. We also publish the frequencies of this polymorphism in apparently healthy urban people from three States of Eastern Venezuela, trying to correlate these values with their levels of racial admixture.
Populations And Methods
Frequency analyses of interpopulation diversity of this polymorphism were performed on data reported for apparently healthy control populations from several different geographic areas. Those populations were classified into five genetic/geographic groups: Europeans (N=3448), Africans (N=99), Asians (N=144), Native Americans (N=164) and American admixed (N=938) (Table 1). To study the three eastern Venezuelan populations, after approval by the IVIC (Instituto Venezolano de Investigaciones Científicas) Ethics Committee, we selected blood donors and other healthy volunteers. In previous works we have reported genetic differences between Venezuelan regions 18,19 ; therefore, in order to attain the highest genetic homogeneity in our samples we chose 186 biologically-unrelated individuals with parents and grandparents from the States of Monagas (N=41), Nueva Esparta (N=45) and Sucre (N=100), in northeastern Venezuela. All individuals gave informed consent to participate in this study.
For the Venezuelan samples DNA was isolated from 5-ml samples of peripheral blood, following standard procedures 20 . The Val34Leu system was amplified and digested by PCR-RFLP using Mse I as restriction enzyme, as prescribed by Franco et al. 5 . Admixture estimates were based on frequencies of blood groups ABO and RH (C, D, E) which were typed using standard procedures.
To obtain a measure of Leu34 gene differentiation for groups of populations, two estimators were used, Dm (interpopulation gene diversity) and Gst' (proportion of total genetic diversity explained by the diversity between subpopulations, corrected by the number of groups compared) 21,22 ; the DISPAN program 23 was used for this purpose. Genetic frequencies of Val34Leu for the Venezuelan populations were estimated by direct counting. For ABO and Rh the MAXLIK program was used, based on a maximum likelihood approach 24 . Hardy-Weinberg equilibrium was analyzed based on a χ 2 –test using the same program. The degree of admixture, was calculated using the gene identity method 25 with the ADMIX3 program; frequencies of the parental populations needed for this calculation were described elsewhere 26 .
Results And Discussion
Most studies dealing with Val34Leu frequencies have been done on European populations but little attention has been paid to regional and ethnic features of the individuals comprising the studied samples, thus narrowing the scope for broad conclusions. Review of the frequency distributions of the five ethnic/geographic groups chosen for the present analysis (Table 1) confirms that the highest values for the Leu34 allele correspond to native American people, with an average of 29.3%, followed by Europeans (25%) and Africans (18.1%) and the almost inexistence of this allele in Asians (0.6%). These values reveal an important pattern of genetic variation among main genetic/geographic groups, where the only allele frequencies not significantly different between each other were those between Europeans and Amerindians (p=0.1), in accordance with a previous study by Attié-Castro et al. 9 .
Livshits and Nei 21 propose estimates Dm and Gst` to compare the extent of genetic differentiation for various groups of populations. In the present analysis, the values of Dm and Gst` between four continental groups compared are, 0.0319 and 10.41%, respectively (Table 2), showing that genetic diversity and differentiation between these groups is higher than that observed for intra-continental/geographic comparisons. This result resembles those reported for some STR loci 27 and confirms an ethnic heterogeneity in the prevalence of the Leu34 allele.
A noticeable genetic diversity can also be detected within each continental group; native Americans, in addition to high frequency values, show marked differences between populations and these are reflected by the high values of Dm (0.0141) and Gst` (3.35%), as has been previously pointed out for other polymorphisms, attributed to pronounced evolutionary and populational events such as genetic drift 28,29 .
From Tables 1 and 2 it emerges that Europeans show an interesting pattern of Val34Leu frequency distributions, with an intermediate diversity value (Gst`=1.74%) which appears to be caused by minor regional differences. On that continent the highest values of Leu34 occur in the central-eastern (28.0%) and northern (25.0%) portions and the lowest in the southern ones (23.3%). At first glance, the most homogeneous region is the south (Gst`=0.8%), but the greater heterogeneity of the central-east diminishes considerably from 3.16 to 0.3% when Germans are excluded from the group, since they show an extremely low frequency of Leu34 (11%) in a rather small population sample (N=14). Excluding this country, the greatest European heterogeneity appears in the northern region, due to the data reported for Finland 2,30 where genetic drift events have been widely documented 31 . It may be stated that Europeans do not always show high Leu34 frequencies as has been claimed, as those from the southern or Mediterranean regions have lower values, analogous to those reported for African groups. The scarcity of data pertaining Africans and Asians does not allow any conclusive arguments; however, it is clear that reported frequencies are low in the case of Africans, even though with a high heterogeneity (Gst`=2.62%), and the very low frequencies of the Leu34 allele in Asian groups studied so far is reflected by a low value of internal diversity (Gst`= 0.6%).
It is not clear what biological advantage could be conferred to carriers of allele Leu34 that allows it to remain instead of becoming deleted by the process of selection. The high frequency of this allele in American natives is noteworthy, in contrast with the extremely low Asian values. It may be suggested that its frequencies in other Asian populations might be greater or alternatively, that the observed values support the influence of extreme events of genetic drift on Amerindian people, as proposed earlier by 28,29 . Such considerations have led to suggest that it is an old mutation which appeared in humans prior to the African/non-African racial divergence 9 , with subsequent variations throughout continents due to complex population and evolutionary processes. The groups belonging to the American admixed populations deserve special attention, as they have a shorter genetic history as the product of genetic exchanges between the other four groups thus far considered. The few available reports concern populations from the US and from Brazil, plus the data published here for Venezuela, with an overall wide range of allele Leu34 frequencies together with an intermediate coefficient of diversity Gst` of 1,97%. The kind of racial admixture experienced by those populations explains in part their frequency distribution. Thus, the highest Leu34 values correspond to whites from Washington (26%), analogous to their northern-European ancestors 7 . The data published by Attié-Castro et al. 9 for Brazilians of European extraction, most of them living in Riberão Preto, in the State of São Paulo, reveal higher frequencies (28.3%) than in their Portuguese ancestors; it might be inferred that an appreciable Amerindian contribution could have increased the Leu34 frequency in this population sample. In black North Americans and Brazilians the observed allele frequencies resemble the few available data known for their African ancestors, 19.5 and 14%, respectively 9 .
Our results for the three Venezuelan populations do not show significant differences between them, although the Leu34 frequency is higher in Monagas (23%) than in the States of Sucre (18%) or Nueva Esparta (17%). These last two values are among the lowest reported for American admixed people, even though these populations have a predominant European racial contribution, that is, 84.6% in Sucre and 79.8% in Nueva Esparta (Table 3); however, that European contribution came from southern Spain, in which the Leu34 prevalence is rather low. The higher frequency found in Monagas may be attributed to the greater Amerindian component (23%) noted in this State in the admixture analysis, as compared to those obtained in Sucre and Nueva Esparta, 6% and 5%, respectively.
This report represents the first approach for a populational study of the Val34Leu polymorphism. Previous data on frequencies for allele Leu34 came from healthy control groups in epidemiologic genetic studies, and this condition might produce some bias on allele frequency estimation. In our analysis, we used a wider number of European populations than those used in previous reports, and we evaluated the population diversity of allele frequencies with interpopulation gene diversity Dm and population genetic differentiation index (Gst`), which gives a clearer picture of the Leu34 distribution among Caucasoids.
In recent years various researchers have agreed to use the terminology “population genetic stratification” to refer to genetic features resulting in populations which are continuously subjected to racial mixing, or in those which have become mixed more recently, that is, those whose genetic structure has been modified only a few generations back 16,32,33 . In this sense, the current populations of the American continent show a wide heterogeneity, both between and within them, which translates into a broad genetic stratification; in some circumstances this could lead to wrongly associate some loci with diseases or other disorders 34 . Hence, in such association studies care should be taken to avoid false interpretations 35,36 . The Leu34 allele cannot elude this heterogeneity effect in its geographic and ethnic distribution in the Americas, as reflected by its interpopulation diversity coefficient, Gst`=1.97. The ethnic heterogeneity of the distributions of some related polymorphisms, besides that of Val34Leu, has already been documented by various studies of cardiovascular diseases 37,38 . Hence, it becomes important to elucidate the degree of variability of the genes involved to understand the diverse patterns of pathologies of the haemostatic system.
To conclude, the overall results indicate that whereas Val34Leu does not yield conclusive information regarding genetic affinities, however reveals differences between human groups with differing genetic origins and its frequency in mixed American populations varies in relation to their parental contributions. This polymorphism is associated with both thrombotic and bleeding pathologies and its functions ought to be assessed together with other polymorphisms associated to homeostasis, as well as their geographic and population heterogeneities. This should help to diminish any biases brought about by genetic stratification effects on populations with heavy proportions of racial admixture.
This work is part of the Doctoral Dissertation of M. Vívenes de Lugo. We thank all individuals who volunteered to participate in this study. To Marina Florez, Luis José Díaz, Mary Helen Izaguirre, Neida Gonzalez and Zoila Carvajal for assistance in laboratory work and data manipulation. This work was supported by IVIC.
Dinorah Castro de Guerra, M.Sc, Dr, Laboratorio de Genética Humana, Centro de Medicina Experimental, Instituto Venezolano de Investigaciones Científicas (IVIC), Apdo. 20632, Caracas 1020A, Venezuela. Tel. +582125041087; Fax +582125041086; E-mail: email@example.com