Association of the 5HTTLPR Polymorphism with Obesity in Mexican Women with High Native American Ancestry

Abstract

Aims:

The 5HTT gene has been associated with obesity; this study aimed to determine the association between L- and S-alleles at the 5HTTLPR polymorphism with obesity in indigenous Mexican populations.

Materials and Methods:

A total of 362 individuals, 289 belonging to eight Native American (NA) groups; 40 Mexican mestizos; and 33 Caucasian Mennonites were enrolled in a cross-sectional study. High (≥90%) and low (<90%) NA ancestry was molecularly determined. A body mass index >30 kg/m² was considered as obese. The L- and S-alleles of the 5HTTLPR locus were identified by PCR; the association between alleles and obesity was performed by logistic regression analysis.

Results:

A significantly lower prevalence of obesity (35%) was observed in participants from communities with high NA ancestry (p < 0.005). Under a dominant heritance model the L-allele was associated with obesity in women with high NA ancestry (odds ratio [OR] 7.27; 95% confidence interval [CI] 1.6-32.5; p = 0.009) but not in women with low NA ancestry (OR 0.83; 95% CI 0.3-2.2; p = 0.71); no association was observed in men.

Conclusion:

Our results suggest that the 5HTTLPR L-allele is a risk factor for developing obesity in Mexican women with high NA ancestry (≥90%).

Introduction

Obesity has become a global public health issue. In 2018, the prevalence of overweight/obesity in Mexico was 74.4% and 69.3% for women and men, respectively (Shamah-Levy et al., 2019). The Mexican Native American (NA) populations used to be lean because of their traditional lifestyles (Guerrero-Romero et al., 1997); however, a progressive acculturation process (Rodríguez-Morán et al., 2008) led to a rise in the total intake of calories in these populations (Rodríguez-Morán et al., 2009).

Metabolism depends not only on customary diet and physical activity but is also influenced by genetic susceptibility (Yu and Zinman, 2007); to date, 127 obesity susceptibility genes have been described (Castillo et al., 2017). Some genes, such as SLC6A4 (5HTT) encoding a serotonin transporter, are involved in maintaining energy balance (Lan et al., 2009). Brain serotonin transporters have a negative correlation with body mass index (BMI) in obese subjects (Erritzoe et al., 2010).

The promoter of the gene 5HTT contains the polymorphism 5HTTLPR (serotonin transporter-linked polymorphic region), which has two variants, a 43 bp insertion (long allele, L) or a deletion (short allele, S), with the former having three times more basal activity than the latter (Lesch et al., 1996). Paradoxically, both the S- (Sookoian et al., 2007) and L- (Peralta-Leal et al., 2012) alleles have been associated with obesity in Caucasian and Mexican mestizo populations, respectively. Therefore, this study aimed to evaluate whether this polymorphism is associated with obesity in 10 ethnic groups from Mexico with different degrees of NA ancestry.

Materials and Methods

Target population

The study was approved by the Ethics Committee of the Durango's General Hospital, at northern Mexico (031/007). Signed informed consent was obtained from all participants before the beginning of the study.

This study included 362 individuals, of them 289 belonging to eight NA groups, 40 Mexican mestizos, and 33 Mexican Mennonites.

Data related to demographic characteristics, medical condition, and customary diet were also collected.

Participants were recruited from their respective communities between 2010 and 2013. All seemingly healthy adult men and nonpregnant women were eligible for inclusion; persons with prior diagnoses of diabetes or hypertension were excluded.

Sampling strategy

Inhabitants from the eight NA groups were invited to join the study by community leaders. Initially, the participants themselves identified its NA ancestry by ratifying that their parents and grandparents belonged to the same ethnic group. Posteriorly, based on molecular characterization, the target population was divided into subjects with high (≥90%) or low NA ancestry (<90%) (Table 1).

Table 1.

General Characteristics of the Target Population According to Ethnicity, N = 362

	Tepehuano	Huichol	Mexicanero	Cora	Tarahumara	Seri	Guarijío	Mayo	Mestizo	Mennonite
N	92	20	13	55	64	8	9	28	40	33
Native American ancestry, %	96.4	96.3	94.5	93.9	92.1	88	81.6	65.6	47.2	0
Women, n (%)	62 (67.4)	10 (50)	8 (61.5)	38 (69.1)	43 (67.2)	7 (87.5)	7 (77.7)	22 (78.6)	29 72.5)	16 (48.5)
Age, years	36.6 ± 13.3	37.8 ± 19.3	43.5 ± 12.0	46.4 ± 20.1	42.8 ± 12.2	58.7 ± 15.5	60.0 ± 15.9	43.7 ± 17.6	44.8 ± 13.3	50.2 ± 12.3
BMI, kg/m²	22.2 ± 3.4	22.7 ± 3.6	23.0 ± 1.9	25.2 ± 5.0	23.9 ± 4.9	27.0 ± 4.3	28.4 ± 7.5	27.9 ± 4.5	28.5 ± 4.6	29.8 ± 5.0
SBP, mmHg	104.8 ± 16.4	120.6 ± 22.6	116.9 ± 16.5	119.2 ± 13.6	120.4 ± 15.8	130.0 ± 15.1	141.0 ± 12.8	127.3 ± 19.8	128.5 ± 15.6	122.4 ± 14.6
DBP, mmHg	68.6 ± 11.5	77.2 ± 11.2	73.8 ± 8.7	77.2 ± 9.8	78.6 ± 9.3	87.5 ± 8.7	90.7 ± 8.0	83.2 ± 11.9	82.0 ± 10.2	85.3 ± 12.5
Glucose, mg/dL	79.6 ± 14.6	73.4 ± 13.2	79.4 ± 13.7	81.6 ± 16.8	82.2 ± 13.5	94.6 ± 6.9	96.6 ± 10.2	69.7 ± 11.7	90.7 ± 7.4	67.4 ± 13.5
Triglycerides, mg/dL	111.3 ± 62.0	121.9 ± 59.1	160.1 ± 67.8.0	181.1 ± 111.1	166.6 ± 93.7	125.3 ± 44	264.1 ± 151.1	131.0 ± 79.0	197.1 ± 115	103.4 ± 67.0
HDL-c, mg/dL	50.4 ± 12.7	43.7 ± 8.6	34.3 ± 9.4	38.0 ± 10.8	41.4 ± 11.1	46.7 ± 10.3	51.1 ± 8.0	50.9 ± 11.4	50.0 ± 10.6	59.8 ± 12.6

BMI, body mass index; DBP, diastolic blood pressure; HDL-c, high-density lipoprotein-cholesterol; SBP, systolic blood pressure.

Measurements

Weight and height were measured using a standard stadimeter. Obesity and nonobesity were defined by BMI ≥30 kg/m² and BMI <30 kg/m² (WHO, 2018). Blood pressure was measured using a mercurial sphygmomanometer according to standard recommendations (Jones and Hall, 2004).

Clinical assays

After 10-12 h overnight fasting, glucose, triglycerides, and high-density lipoprotein-cholesterol (HDL-c) plasma levels were determined enzymatically, according to the manufacturer's procedures (BioSystems, Reagents & Instruments; Analyzer A15 © Biosystems, Barcelona).

Genotyping

DNA was extracted from peripheral blood using the QIAamp^® DNA blood kit (Qiagen, Hilden, Germany) and evaluated for integrity and concentration. SLC6A4 allelic variants were detected as previously described (Peralta-Leal et al., 2012). Genotyping was performed for 46 ancestry-informative markers in Mexican mestizos (Pereira et al., 2012) and 15 short tandem repeats loci in the NA groups (Rangel-Villalobos et al., 2013).

Statistical analyses

Data are presented as mean ± standard deviation or proportions. Differences between numerical variables were established with the Student's t-test for independent samples (Mann-Whitney U test for nonparametric data) and chi-squared test for categorical variables. Analysis of variance with a Games-Howell post hoc test was used to compare more than two groups. Genotype frequencies were obtained by direct counts and Hardy-Weinberg equilibrium (HWE) was determined by chi square. Admixture proportions were estimated with STRUCTURE v.2.2 (Falush et al., 2003), and included 327 individuals from three parental populations (Africa, Europe, and America). The analysis was performed using the correlated allele frequencies and admixture model and the number of clusters was determined (Evanno et al., 2005). A logistic regression analysis (LRA) adjusted by gender, age, and biochemical parameters was used to evaluate the association between 5HTTLPR polymorphisms (independent variables) with obesity (dependent variable). Statistical significance was defined as a p-value <0.05. Data were analyzed using the statistical package SPSS for Windows 25.0 (SPSS Inc., Chicago, IL).

Results

A total of 362 volunteers were enrolled in this study; of whom 244 (67.4%) had high NA ancestry. Most participants were women (n = 242; 66.8%), among them 161 (66.5%) had high NA ancestry (p = 0.38). Table 1 shows the anthropometric and biochemical variables of all participants. In the entire cohort, 60 individuals (16.6%) were identified as obese, 20 (33.3%) of whom had high NA ancestry (p < 0.005).

Participants with low NA ancestry were older and exhibited higher BMI, systolic blood pressure, diastolic blood pressure, and HDL-c levels than those with high NA ancestry (Table 2). The allele distribution in the whole population was in HWE (p > 0.05).

Table 2.

Anthropometric and Biochemical Characteristics of the Target Population, According to Gender and Native American Ancestry

	Women			Men
Native American ancestry, %	≥90	<90		≥90	<90
N	161	81	p^*	83	37	p^*
Age, years	39.0 ± 15.0	47.1 ± 14.9	<0.0005	44.9 ± 16.5	49.6 ± 15.8	0.14
BMI, kg/m²	24.1 ± 4.5	28.7 ± 5.2	<0.0005	22.5 ± 4.0	28.1 ± 4.3	<0.0005
SBP, mmHg	113.0 ± 16.9	127.8 ± 16.7	<0.0005	116.5 ± 19.4	126.8 ± 16.7	0.004
DBP, mmHg	73.6 ± 11.0	83.5 ± 10.6	<0.0005	75.4 ± 11.8	85.8 ± 12.6	<0.0005
Glucose, mg/dL	79.6 ± 13.5	80.0 ± 16.6	0.84	82.5 ± 16.9	79.1 ± 14.6	0.27
Triglycerides, mg/dL	147.1 ± 94.0	155.8 ± 107.5	0.54	138.9 ± 75.6	149.1 ± 105.5	0.60
HDL-c	43.2 ± 12.8	53.2 ± 12.7	<0.0005	45.0 ± 12.4	50.6 ± 9.6	0.008

Student's t-test for independent samples.

In the whole population, the adjusted LRA model showed that L-allele was associated with obesity (odds ratio [OR] 2.21; 95% confidence interval [CI] 1.1-4.4; p = 0.02). After stratification by NA, the adjusted LRA model showed that L-allele was significantly associated with obesity in the high NA ancestry group (OR 5.88; 95% CI 1.3-27.1; p = 0.02), but not in the low NA ancestry group (OR 1.25; 95% CI 0.5-3.0; p = 0.61).

Stratified according to gender, men in the group of low NA showed no association between L-allele and obesity, whereas in the group with high NA no obese men were identified. Under a dominant inheritance model, women with high NA, but not women with low NA ancestry, showed a strong association of L-allele with obesity (Table 3). Furthermore, the adjusted LRA showed that L-allele was associated with BMI (OR 1.57; 95% CI 1.1-2.9; p = 0.02) in women with high NA ancestry.

Table 3.

Association between 5HTTLPR Polymorphisms with Obesity in Mexican Women

Native American ancestry ≥90
		Obesity, n (%)
Model	Genotype	No (n = 141)	Yes (n = 20)	OR (95% CI)^*	p	AIC
Codominant	S/S	63 (44.7)	2 (10.0)	1.0
	L/S	53 (37.6)	13 (65.0)	7.73 (1.7-35.8)	0.009	116.4
	L/L	25 (17.7)	5 (25.0)	6.30 (1.1-34.6)
Dominant	S/S	63 (44.7)	2 (10.0)	1.0
Dominant	L/S-L/L	78 (55.3)	18 (90.0)	7.27 (1.6-32.5)	0.009	114.5
Recessive	S/S-L/S	116 (82.3)	15 (75.0)	1.0
Recessive	L/L	25 (17.7)	5 (25.0)	1.55 (0.5-4.6)	0.43	124.3
	Alleles				p
	S	179 (58)	17 (42)		0.011
	L	103 (42)	23 (58)		0.011
Native American ancestry <90
		Obesity, n (%)
Model	Genotype	No (n = 51)	Yes (n = 30)	OR (95% CI)^*	p	AIC
Codominant	S/S	15 (29.4)	10 (33.3)	1.0
	L/S	23 (45.1)	14 (46.7)	0.91 (0.3-2.6)	0.86	112.4
	L/L	13 (25.5)	6 (20.0)	0.69 (0.2-2.4)
Dominant	S/S	15 (29.4)	10 (33.3)	1.0
Dominant	L/S-L/L	36 (70.6)	20 (66.7)	0.83 (0.3-2.2)	0.71	110.6
Recessive	S/S-L/S	38 (74.5)	24 (80.0)	1.0
Recessive	L/L	13 (25.5)	6 (20.0)	0.73 (0.2-2.2)	0.57	110.5
	Alleles				p
	S	53 (52)	34 (56)		0.484
	L	49 (48)	26 (44)

Nonobesity, BMI <30 kg/m². Obesity, BMI ≥30 kg/m².

Model adjusted by age and biochemical parameters.

AIC, Akaike Information Criterion; CI, confidence interval; OR, odds ratio.

Finally, individuals with high NA ancestry had a significantly higher intake of alcoholic beverages and lower intake of canned foods and meat than individuals with low NA ancestry.

Discussion

Our results suggest that the L-allele of 5HTTLPR is a risk factor for developing obesity in Mexican women with high NA ancestry (≥90%). To our knowledge, this is the first study to evaluate the effects of indigenous and mestizo Mexican ethnicity on the association of this polymorphism with obesity.

We observed a high frequency of obesity in participants with low NA ancestry, which agrees with an earlier report from Stoddard et al. (2011) who found significantly higher rates of obesity in nonindigenous populations. This finding could be explained by geographical isolation and low exposure to industrialized diets in communities with high NA ancestry (Rodríguez-Morán et al., 2008).

The S-allele was linked with overweight/obesity in French (Fumeron et al., 2000) and Caucasian Argentinian (Sookoian et al., 2007, 2008) populations; in Hispanic men, the S-allele and SS genotype have been associated with obesity (Fuemmeler et al., 2008). Conversely, the L-allele was associated with obesity in the Turkish population (Mergen et al., 2007). In 2002, Camarena et al. (2002) found in Mexican obese women with impulsivity a preferential transmission of L-allele from the heterozygous parents, providing significant evidence for allelic association. These findings suggest a gender difference in the function of serotonin system, which could be a risk factor for obesity in women.

In male and female 5HTT knockout mice, both Cyp19a1 messenger RNA expression and 17β-estradiol serum levels are reduced, which is associated with an increase of fat mass (Zha et al., 2017). The effects of 5HTT ^−/− would be functionally equivalent to those of the 5HTT S/S genotype. Supporting this hypothesis, our results did not find any association between obesity and the S-allele.

Administration of estrogens to castrated male Wistar rats induces 5HTT expression (McQueen et al., 1999), and a similar induction of expression is also exerted by the L-allele. Therefore, the synergistic effect of estrogens and the L-allele could induce overexpression of 5HTT, explaining the association between the L-allele and obesity that we observed in women with high NA ancestry.

Mexican NAs have developed adaptive mechanisms to preserve energy during famine periods, with the thrifty genes playing an important role (Neel et al., 1998). The observation of the association between L-allele and obesity, exclusively in women with high NA ancestry, suggests inherited differences in gene action and its environmental adaptations.

Finally, we did not find significant difference in the customary diet between groups with high and low NA ancestry. These findings suggest that the former have adopted westernized diets, which, in the presence of the L-allele, may elicit a “saving phenotype” leading to obesity in women.

Some limitations of this study deserve to be mentioned: (1) an accurate recording of physical activity was not possible; (2) there were not obese men in the group with high NA; so, we cannot discard with certainty the gender-dependent differential functionality for the L- and S-alleles of 5HTTLPR; (3) we evaluated the influence of the L- and S-alleles in a sample of individuals from northern Mexico with different NA ancestry. So, these findings must be considered as tentative until its replication in larger samples and other ethnic groups.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This research was funded by grants from the Secretaría de Investigación y Posgrado (SIP) of Instituto Politécnico Nacional (SIP 20201479 and SIP 20201482).

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