D and MTHFR C677T polymorphisms are significantly associated with type 2 diabetes in Arab ethnicity: A meta-analysis

Gene 520 (2013) 166–177

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Gene journal homepage: www.elsevier.com/locate/gene

ACE I/D and MTHFR C677T polymorphisms are significantly associated with type 2 diabetes in Arab ethnicity: A meta-analysis Khalid Al-Rubeaan ⁎, Khalid Siddiqui, Amr T.M. Saeb, Nyla Nazir, Dhekra Al-Naqeb, Sara Al-Qasim Strategic Center for Diabetes Research, King Saud University, Saudi Arabia

a r t i c l e

i n f o

Article history: Accepted 6 February 2013 Available online 28 February 2013 Keywords: Type 2 diabetes Arabs Caucasians Polymorphism

a b s t r a c t In this meta-analysis study, SNPs were investigated for their association with type 2 diabetes (T2D) in both Arab and Caucasian ethnicities. A total of 55 SNPs were analyzed, of which 11 fulfilled the selection criteria, and were used for analysis. It was found that TCF7L2 rs7903146 was significantly associated with a pooled OR of 1.155 (95%C.I. = 1.059–1.259), p b 0.0001 and I2 = 78.30% among the Arab population, whereas among Caucasians, the pooled OR was 1.45 (95%C.I. = 1.386–1.516), p b 0.0001 and I2 = 77.20%. KCNJ11 rs5219 was significantly associated in both the populations with a pooled OR of 1.176(1.092–1.268), p b 0.0001 and I2 = 32.40% in Caucasians and a pooled OR of 1.28(1.111–1.475), p = 0.001 among Arabs. The ACE I/D polymorphism was found to be significantly associated with a pooled OR of 1.992 (95%C.I. = 1.774–2.236), p b 0.0001 and I2 = 83.20% among the Arab population, whereas among Caucasians, the pooled OR was 1.078 (95%C.I. = 0.993–1.17), p = 0.073 and I2 = 0%. Similarly, MTHFR C677T polymorphism was also found to be significantly associated among Arabs with a pooled OR of 1.924 (95%C.I. = 1.606–2.304), p b 0.0001 and I2 = 27.20%, whereas among Caucasians, the pooled OR was 0.986 (95%C.I. = 0.868–1.122), p = 0.835 and I2 = 0%. Meanwhile PPARG-2 Pro12Ala, CDKN2A/2B rs10811661, IGF2BP2 rs4402960, HHEX rs7923837, CDKAL1 rs7754840, EXT2 rs1113132 and SLC30A8 rs13266634 were found to have no significant association with T2D among Arabs. In conclusion, it seems from this study that both Arabs and Caucasians have different SNPs associated with T2D. Moreover, this study sheds light on the profound necessity for further investigations addressing the question of the genetic components of T2D in Arabs. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Type 2 diabetes (T2D) is a multi-factorial metabolic syndrome, characterized by high blood glucose level, mainly caused by insulin resistance

Abbreviations: ACE, angiotensin-I converting enzyme; ADAMTS9, ADAM metallopeptidase with thrombospondin type 1 motif, 9; CAMK1D, calcium/calmodulindependent protein kinase ID; CAPN10, calpain 10; CDC123, cell division cycle 123 homolog; CDKAL1, CDK5 regulatory subunit associated protein 1-like 1; CDK5, cyclin-dependent kinase 5; CDKN2A/B, cyclin-dependent kinase inhibitor 2A/B; EXT2, exostosin-2; FTO, fat mass and obesity associated; HHEX, hematopoietically expressed homeobox; IDE, insulin-degrading enzyme; IGF2BP2, insulin-like growth factor 2mRNA binding protein 2; JAZF1, juxtaposed with another zinc finger gene 1; KCNJ11, potassium inwardly rectifying channel, subfamily J, member 11; KCNQ1, potassium voltage-gated channel, KQT-like subfamily, member 1); LGR5, leucine-rich repeatcontaining G protein-coupled receptor 5; MC4R, melanocortin 4 receptor; MTHFR, methylenetetrahydrofolatereductase; NIDDM, noninsulin-dependent diabetes mellitus; NOTCH2, Notch homolog 2 Drosophila; PPARG, peroxisome proliferator-activated receptor gamma; SLC30A8, solute carrier family 30 member 8; SNP, single nucleotide polymorphism; T2D, type 2 diabetes; TCF7L2, transcription factor 7-like 2. ⁎ Corresponding author at: Strategic Center for Diabetes Research, King Saud University, P.O. Box 18397, Riyadh 11415, Saudi Arabia. Tel.: +966 1 478 6100x5403; fax: +966 1 477 5696. E-mail addresses: [email protected] (K. Al-Rubeaan), [email protected] (K. Siddiqui), [email protected] (A.T.M. Saeb), [email protected] (N. Nazir), [email protected] (D. Al-Naqeb), [email protected] (S. Al-Qasim). 0378-1119/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gene.2013.02.017

and relative insulin deficiency (Bonnefond et al., 2010). Type 2 diabetes has been considered by the World Health Organization (WHO) as a major healthcare problem all over the world. About 285 million people were estimated to live with diabetes in 2010, and the number is expected to rise to 438 million by 2030 (http://www.worlddiabetesfoundation. org/composite-35.htm). The Arabs, in general, are genetically very diverse and united by a history that dates back to 3500 BC. They originated from Semitic tribes in the Arabian Peninsula, initially spreading in the valley of the Tigris and Euphrates River, and later, in 2500 BC, across the East Mediterranean region, which has significantly contributed to their diversity (Teebi and Teebi, 2005) This metabolic syndrome is considered to be an endemic disease with a higher prevalence rate in the Eastern Mediterranean Region, South-Eastern Asia and the Arabian Peninsula. It has been observed that the Gulf States have a higher prevalence rate than other Middle Eastern countries. This provides a strong clue that Arab ethnicity is at a higher risk of developing type 2 diabetes mellitus when compared with other global ethnicities (Al-Rubeaan, 2010). The prevalence rates of T2D in Arab countries are increasing, and it has reached up to 35% in some Gulf countries. Saudi Arabia, for example, had 23.7% of T2D patients in the year 2004, with an additional 14.1% of pre-diabetic cases, thereby totaling to a prevalence rate of 37.8% (Al-Nozha et al., 2004). Moreover, the United Arab Emirates

K. Al-Rubeaan et al. / Gene 520 (2013) 166–177

showed an estimated diabetes prevalence of 19.5% in 2007, expected to rise to 21.9% in 2025 (http://www.worlddiabetesfoundation.org/ composite-35.htm). This increasing pattern may be attributed to the change in the environment towards a more sedentary lifestyle as a result of the oil boom within the Arab region, which plays a key role in the epidemic increase in the prevalence of T2D in the last 50 years. Arabs in the Middle East are still easily identifiable by their nationalities to Arab Countries because of their tribal and familial structure. They have minimal admixtures with other ethnicities like Africans, Berbers, Caucasians, Turkish and Persians. These minimal admixtures are limited and identifiable. Due to the high consanguinity rate and common tribal intermarriage the genetic structure is expected to be unique for this ethnic group. To gain an insight into the genes associated with T2D within the Arab population, a systematic literature review was undertaken to acquire a more focused and rigorous view of the bona-fide state of each gene studied within this population. In addition, we wanted to point out the relevant genetic basis of this disease within the different ethnic Arab communities (Tadmouri et al., 2009). This article aims to identify the unique risk of SNPs for T2D that are exclusive to Arab ethnicities, by performing a comparative meta-analysis of previously identified SNPs for the disease among Arabs and Caucasians. At the same time, this meta-analysis will identify SNPs for T2D that are shared by the two ethnicities or unique for either one, or SNPs that are not significant for both ethnicities. 2. Methodology A literature search was performed to collect articles relevant to this study. These articles were drawn from PubMed up to March 2011, by using an extensive search strategy for selecting terms and possible combinations to design queries, in order to retrieve a maximum number of relevant articles. The query was designed with a focus on three components: type 2 diabetes, genetic studies and the Arab population. A few accessible articles relating to SNPs in T2D published in Caucasian population were also retrieved in a similar manner. In this meta-analysis study, the Arab population was defined based on the nationality and/or geographical location of the tested population, i.e. within Middle Eastern countries. The query was created by identifying equivalent terms for each of these search components, for example T2D, NIDDM and “type 2 diabetes” was used for the component T2D. Different combinations of these terms were designed and tested, so as to retrieve a maximum number of articles for each component. Then, the query terms for the three components were combined, to retrieve articles that focus only on the different genes that have been studied in association with T2D within different Arab ethnic groups. A total of 625 articles were obtained and then filtered by screening through the titles and abstracts. These selected published articles were considered for our case collection, by fulfilling the following criteria. The articles published should be in English language only, and studied subjects have to be Arabs in ethnicities. Case controlled studies were only considered for this study where subjects clearly defined with normal glucose level as a controlled group irrespective of any diabetes risk factors such as obesity and hypertension. Type 2 diabetic cases and/or pre-diabetes cases were merged together when compared with their pair compared group. Studies were excluded if the control group consisted of individuals with pre-diabetes or the number of subjects involved in the study was less than 50 individuals. Data that pertained to non-Arab individuals or groups (i.e. Berbers, Turkish, and Persians) raising within the same geographical area were excluded. Lack of information related to the distribution of genotypes or alleles within the case and control groups is one of our exclusion criteria. A total of 34 articles fulfilled our inclusion and exclusion criteria and were considered for our analysis. All the studied populations

167

(i.e. Caucasian vs. Arabs), considered for this review were found to be stratified in their original context. The authors of the reviewed studies were not contacted for additional information, since this was not needed. 3. Data extraction and organization Thirty four (34) articles were selected and data of 55 SNPs was extracted and analyzed (Table 1). SNPs from different genes were organized on the basis of their association with type 2 diabetes in Arabs. In Table 1 there were three SNPs reported to be significantly associated with T2D among Arabs that were selected in order to compare their association status with the disease among Caucasians. An additional eight SNPs were selected randomly, that were found to have no significant association with T2D among Arabs, and were then compared to the Caucasian population. In total 11 SNPs were used in genetic analysis of type 2 diabetes. 4. Statistical analysis Deviation from Hardy–Weinberg equilibrium (HWE) was tested using the De Finetti program (http://ihg.gsf.de/cgi-bin/hw/hwa1.pl) for all studies analyzing the eleven SNPs. Studies not satisfying the HWE were excluded, and 48 studies were used for further analysis. The percentage of variation across studies due to heterogeneity rather than chance was measured via Higgin's I2 statistic using Stats Direct Statistical Software Version 2.7.8 (Higgins et al., 2003). OR was pooled based on heterogeneity; in six SNP studies, the heterogeneity was high, and the pooled OR was calculated based on the random-effect model, whereas, eight SNP studies had a low heterogeneity and the pooled OR was calculated based on the fixed-effect model. If I2 was greater than 50%, the heterogeneity was considered high, and vice-versa. The odds ratio (OR) and statistically significant P-values were validated for all the articles and recalculated for some of them, in order to remove any adjustments made within each study in cases where two different groups merged together, such as in the case of pre-diabetic and T2D groups. This was undertaken using statistical software SPSS 17.0 (SPSS, Chicago, IL, USA). A p-value b0.05 was considered to be statistically significant. The association of the analyzed 11 SNPs was represented using Forest plots (Comprehensive Meta Analysis Version 2.0, Biostat, Englewood NJ, 2005). Sensitivity analysis was performed by measuring the odds ratio (OR) if we have more than three studies for any identified SNP, by excluding the outlier study and recalculating the OR for any change in its value. Six SNPs were tested using this methodology where it has shown no false-positive effect, and therefore, none of the studies were excluded for the meta-analysis. 5. Scatter plot A comparative Scatter plot (Fig. 2) was created to show how the SNPs behave in both populations, based on a local scoring system that takes four main factors into consideration: OR values, sample size, number of studies, and the significance of association. This system was established in order to strengthen representative power and increase its precision. SNPs were given more weight if they were found to be significantly associated, to have a fairly large sample size, and if the number of studies was found to be greater than four. The score was calculated using the following formula: Score ¼ ðnscore þ Sscore þ Ascore Þ x Odds ratio where, nscore

is the score for the number of studies grouped together for each SNP group. The scores range from − 3 to 4 and are assigned based on the number of studies. The group with

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Table 1 Risk genes likely to be associated with type 2 diabetes mellitus in the Arab population. Risk genes

TCF7L2

Gene function

Chromosome SNPs/polymorphs no.

Implicated in blood glucose homeostasis, 10 regulation of proglucagon via the Wnt signaling pathway

rs7903146

rs12255372 HHEX

KCNJ11 GCK ENPP1

CDKAL1

LOC646279 CDKN2A/2B IGFBP2 EXT2

SLC30A8 MMP26 KCTD12 LDLR LOC387761 NGN3 CXCR4 PPAR-gamma2

10 Involved in developmental processes; may play a role in hematopoietic differentiation Allow potassium to flow into a cell rather 11 than out of a cell Glucokinase activity 7 6 Endonuclease, hydrolase and phosphodiesterase activity; cleaves a variety of substrates Catalytic activity; iron-sulfur cluster 6 binding 14 Cyclin-dependent protein kinase 9 inhibitor activity; tumor suppressor gene Binding to the 5′ UTR of the IGF2 mRNA 2 and regulating IGF2 translation Glycosyltransferase activity 11

Permit cellular efflux of zinc 8 Tissue remodeling events associated 11 with tumor progression Voltage-gated potassium channel activity 13 Cholesterol homeostasis 19 11 Central and peripheral nervous system 10 development 2 Regulation of transcription; regulation of 3 adipocyte differentiation

rs7923837 rs1111875 rs5219

Regulation of vasoconstriction; kidney development; regulation of smooth muscle cell migration

17

Remethylation of homocysteine; blood circulation; amino acid metabolic process IRS-2 Glucose metabolic process; insulin receptor signaling pathway IL-10 promoter Immunoregulation and inflammation

1

MTHFR

Calpain-10

Proteolysis

eNOS

GNB-3 CETP HLA class II GLUT-1

TNFα HSP70-2

13 1

2

7

Blood pressure regulation Cholesterol metabolic process; lipid transport Plays a central role in the immune system by presenting peptides Glucose transporter

12 16

Regulation of inflammatory response

6 14

6 1

Strongly-associated

Tunisian (R. Bouhaha, e-pub:2009) Saudi (Osama Alsmadi, 2008) Emirati (Hussein Saadi, 2008) Emirati (Hussein Saadi, 2008)

Tunisian (Intissar Ezzidi, 2009) Moroccan (Stepane Cauchi, 2007)

Moroccan (Stepane Cauchi, 2008) Tunisian (Intissar Ezzidi, 2009) Moroccan (Stepane Cauchi, 2008) Tunisian (Intissar Ezzidi, 2009)

rs1799884 rs1044498

Tunisian (Intissar Ezzidi, 2009) Tunisian (Intissar Ezzidi, 2009) Moroccan (Y. El Achhab, 2009)

rs7754840 rs10946398 rs7756992 rs1256517 rs10811661 rs564398 rs4402960 rs1470579 rs1113132 rs3740878 rs11037909 rs729287 rs13266634 rs2499953

Moroccan Moroccan Moroccan Moroccan Moroccan Moroccan Moroccan Moroccan Moroccan Moroccan Moroccan Moroccan Moroccan Moroccan

(Stepane (Stepane (Stepane (Stepane (Stepane (Stepane (Stepane (Stepane (Stepane (Stepane (Stepane (Stepane (Stepane (Stepane

Cauchi, Cauchi, Cauchi, Cauchi, Cauchi, Cauchi, Cauchi, Cauchi, Cauchi, Cauchi, Cauchi, Cauchi, Cauchi, Cauchi,

2008) 2008) 2008) 2008) 2008) 2008) 2008) 2008) 2008) 2008) 2008) 2008) 2008) 2008)

rs2876711 rs6413504 rs7480010 rs10823406

Moroccan Moroccan Moroccan Moroccan

(Stepane (Stepane (Stepane (Stepane

Cauchi, Cauchi, Cauchi, Cauchi,

2008) 2008) 2008) 2008)

rs932206 rs1801282

Moroccan (Stepane Cauchi, 2008) Tunisian (R. Bouhaha, 2008) Tunisian (Zouari Bouassida, 2005) Tunisian (Samir Ben Ali, 2009) Saudi (Salma M Wakil, 2006) Qatari (Ramin Badii, 2008) Tunisian (Ben Hadj Mohamed, 2007) Tunisian (Ben Hadj Mohamed, 2007)

rs1805192

ACE

Population-based association No-association

rs3856806 rs1800764 rs12449782 Insertion/deletion

Tunisian (Imen Arfa, 2008) Lebanese (Hania N Chmaisse, 2009)

C677T

Tunisian (Sounira Mehri, 2010)

A1298C Gly1057Asp

Tunisian (Thouraya Baroudi, 2009)

−1082G/A −819C/T −592C/A UCSNP-43 UCSNP-19 UCSNP-110 UCSNP-63 Glu298Asp/894G>T 4a4b −786T>C rs5443 TaqIB (B1/B2)

Palestenian (Suheir Ereqat, 2009) Emirati (Hussein Saadi, 2008)

Saudi (Osama Alsmadi, 2008)

Tunisian (R. Bouhaha, 2008)

Tunisian (Intissar Ezzidi, 2009) Tunisian (Intissar Ezzidi, 2009) Tunisian (Intissar Ezzidi, 2009) Tunisian (Sounira Mehri, 2010) Tunisian (Thouraya Baroudi, 2009) Tunisian (N. Koubaa, 2007) Tunisian (Nabil Mtiraoui, 2007) Tunisian (Nabil Mtiraoui, 2007)

Tunisian (Nabil Mtiraoui, 2009) Tunisian (Nabil Mtiraoui, 2009) Tunisian (Nabil Mtiraoui, 2009) Tunisian (C. Kifagi, 2008) Tunisian (C. Kifagi, 2008) Tunisian (C. Kifagi, 2008) Tunisian (C. Kifagi, 2008) Tunisian (Intissar Ezzidi, 2008) Tunisian (Intissar Ezzidi, 2008) Tunisian (Intissar Ezzidi, 2008) Emirati (Jawad G. Kiani, 2005) Tunisian (R. Chaaba, 2005)

DRB1 DQB1 Bahraini (Ayesha A Motala, 2005) HpyCH4V (rs710218) Enh2SNP1 (rs841847) Xbal (rs841853) TNF1/TNF2 P1/P2

Bahraini (Ayesha A Motala, 2005) Tunisian Tunisian Tunisian Tunisian Tunisian

(K. Makni, 2008) (K. Makni, 2008) (K. Makni, 2008) (Zouari Bouassida, 2004) (Zouari Bouassida, 2004)

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Sscore

Ascore

only one study scored −3, while the group with eight studies scored 4. is the score for the total number of T2Ds and Controls in each SNP group. The scores range from 1 to 20, and are assigned based on the total number of individuals in each SNP group. The group with a sample size b1000 scored 1, while groups with sample size ranging 19,000–20,000 scored 20. is the score given to each SNP group based on their association status. An additional score of 10 is given if the SNP group is found to be significantly associated with T2D, while a score of 10 is deducted if the SNP group was found to have no significant association with T2D.

The value obtained by adding the above scores are then multiplied by OR (odds ratio) in order to amplify the power of association, so that more representative data could be charted for each SNP group. In fact, SNPs do not lose any points; instead, they gain points based on different scales, some of which use negative values. According to these criteria, we were able to identify four major groups, as shown in Fig. 2. Group ‘A’ included positively associated SNPs in both Arab and Caucasian ethnicities, while Group ‘B’ included SNPs that are positively associated in Arab ethnic groups, but negatively associated in Caucasians. The third group, Group ‘C’ represented SNPs that were positively associated in Caucasians but were significantly negative in Arab ethnic groups. The last group, Group ‘D’ included SNPs that were negatively associated in both ethnicities.

6. Results In Arabs, the 31 studies that looked into the 30 genes and their corresponding 55 SNPs were considered, and the following are some of the genes that were studied (Table 1). In order to gain an insight into the genetic differences between the Arab population and the Caucasian population, we selected the following SNPs, namely TCF7L2 rs7903146, KCNJ11 rs5219, ACE I/D polymorphism, MTHFR C677T, PPARG-2 Pro12Ala, IGF2BP2 rs4402960, SLC30A8 rs13266634, CDKAL1 rs7754840, CDKN2A/2B rs10811661, HHEX rs7923837 and EXT2 rs1113132 based on their association status with T2D among the Arabs, and then compared them with their status within the Caucasian population. The SNPs have been grouped into four different groups: Group A, Group B, Group C and Group D and the corresponding pooled ORs and P-values of all the studied SNPs are presented in Table 2. Group ‘A’ included SNPs that were significantly associated within both Arab and Caucasian populations. TCF7L2 rs7903146 belonged to this group. TCF7L2 was one of the most studied genes, with a total of six studies. All of these studies focused on the association of SNP rs7903146 with T2D and two of these studies also investigated SNP rs12255372. For rs7903146, three studies conducted in Tunisian, Moroccan and Palestinian population showed a significant association with T2D with an OR of 1.25, 1.56 and 2.381 respectively (Cauchi et al., 2007; Ereqat et al., 2009; Ezzidi et al., 2009a). The remaining three studies conducted in Saudi, Emirati and an additional Tunisian population showed a weak or no association with p-values of 0.675, 0.397 and 0.671 respectively (Alsmadi et al., 2008a; Bouhaha et al., 2009; Saadi et al., 2008). For TCF7L2 rs1225372, neither of the studies conducted among Saudis and Emiratis showed a significant association with T2D with a p-value of 0.601 and 0.08 respectively.TCF7L2 rs7903146 was found to be studied in six Arab studies, namely Tunisians (Alsmadi et al., 2008a; Bouhaha et al., 2009; Cauchi et al., 2007; Ereqat et al., 2009; Ezzidi et al., 2009a; Saadi et al., 2008), Moroccans(Cauchi et al., 2007), Palestinians (Ereqat et al., 2009), Saudis (Alsmadi et al., 2008a), and Emiratis (Saadi et al., 2008). Similarly, six studies that looked into the association of TCF7L2 rs7903146 with T2D were considered for the

169

Caucasian population, namely Swedish (Mayans et al., 2007), Dutch (van Vliet-Ostaptchouk et al., 2007), Scottish (Kimber et al., 2007), British (De Silva et al., 2007), Norwegian (Thorsby et al., 2009), and French (Cauchi et al., 2006). The total number of T2D patients in those Arab studies was 2435, while it was 8146 within the above-mentioned Caucasian studies. The total number of controls within the Arabian studies was 1798, while the total number of controls within the Caucasian population was 9716. The pooled OR among Arab population was 1.336 (95%C.I. = 1.092–1.635), p = 0.005 and I2 = 78.30%, whereas among Caucasians, the pooled OR was 1.407 (95%C.I. = 1.265–1.564), p b 0.0001 and I2 = 77.20%. The other gene that showed positive association was KCNJ11 rs5219. It was found to be studied in two Arab studies, namely Tunisians (Ezzidi et al., 2009a) and Saudis (Alsmadi et al., 2008b). Meanwhile, there were seven studies that looked into the association of KCNJ11 rs5219 with T2D for the Caucasian population, namely Danish (Hansen et al., 2005), U.K (Gloyn et al., 2001; Inoue et al., 1997), Russian (Chistiakov et al., 2009), Czech (Cejkova et al., 2007), French (Hani et al., 1998), and Israeli Ashkenazi (Neuman et al., 2010). The total number of T2D patients in those Arab studies was 1355, while the total number of T2D patients was 3185 within the above-mentioned Caucasian studies. The total number of controls within the Arabian studies was 838, while the total number of controls within the Caucasian population was 3097. The pooled OR among Arab population was 1.28 (95%C.I. = 1.111– 1.475) and p = 0.001, whereas among Caucasians, the pooled OR was 1.176 (95%C.I. = 1.092–1.268), p b 0.0001 and I2 = 32.40%. Group ‘B’ included SNPs that were found to be significantly associated among Arabs, but not among Caucasians, namely, ACE I/D and MTHFR C677T polymorphisms. ACE I/D polymorphism was found to be studied in five Arab studies, namely Tunisians (Arfa et al., 2008; Baroudi et al., 2009; Ezzidi et al., 2009b; Mehri et al., 2010) and Lebanese (Chmaisse et al., 2009). Similarly, six studies that looked into the association of ACE rs1799752 with T2D were considered for the Caucasian population, namely Turks (Arzu et al., 2004; Degirmenci et al., 2005), Germans (Grammer et al., 2006), Finns (Huang et al., 1998) and the British (Nagi et al., 1995). The total number of T2D patients in these Arab studies was 1326, while the total number of T2D patients was 1958 within the above-mentioned Caucasian studies. The total number of controls within the Arabian studies was 1077, while the total number of controls within the Caucasian population was 2772. The pooled OR among Arab population was 1.960 (95%C.I. = 1.327– 2.895), p = 0.0007 and I 2 = 83.20%, whereas among Caucasians, the pooled OR was 1.062 (95%C.I. = 0.973–1.159), p = 0.183 and I2 = 0%. MTHFR C677T polymorphism was found to be studied in three Tunisian studies (Koubaa et al., 2007; Mehri et al., 2010; Mtiraoui et al., 2007). Meanwhile, there were six studies that looked into the association of MTHFR C677T with T2D for the Caucasian population, namely, Turkish (Eroglu et al., 2007; Tutuncu et al., 2005; Yilmaz et al., 2004), Czech (Benes et al., 2001), Polish (Ksiazek et al., 2004), and Italians (Mazza et al., 2005). The total number of T2D patients in those Arab studies was 561, while the total number of T2D patients was 1226 within the above-mentioned Caucasian studies. The total number of controls within the Arabian studies was 636, while the total number of controls within the Caucasian population was 932. The pooled OR among Arab population was 1.915 (95%C.I. = 1.598–2.294), p b 0.0001 and I 2 = 27.20%, whereas among Caucasians, the pooled OR was 0.996 (95%C.I. = 0.874–1.134), p = 0.977 and I 2 = 0%. Group ‘C’ included SNPs found to be significantly associated among Caucasians but not among Arabs, namely, PPARG-2 Pro12Ala, CDKN2A/2B rs10811661, CDKAL1 rs7754840 and IGF2BP2 rs4402960. PPARG-2 Pro12Ala was found to be studied in six Arab studies, namely, Saudi (Wakil et al., 2006), Qataris (Badii et al., 2008) and Tunisians (Ben et al., 2009; Bouassida et al., 2005; Bouhaha et al., 2008; Mohamed et al., 2007). While among Caucasians, seven studies that looked into the association of PPARG-2 Pro12Ala with T2D were considered, namely, Czech (Pinterova et al., 2004; Sramkova et al., 2002), Swedish (Ek et al., 2001), Danish (Ek et al., 2001; Hansen et al., 2005), Polish (Malecki

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The total number of controls within the Arabian studies was 1645, while the total number of controls within the Caucasian population was 3703. The pooled OR among Arab population was 0.865 (95%C.I. = 0.679–

et al., 2003) and French (Ghoussaini et al., 2005). The total number of T2D patients in these Arab studies was 1311, while the total number of T2D patients was 3650 within the above-mentioned Caucasian studies.

Table 2 A: Significant association of different SNPs in Arabs and Caucasians. B: Significant in Arabs and non-significant in Caucasians, ACE and MTHFR (Tunisian population). C: Non-significant association in Arabs and significant in Caucasians, CDKN2A/B, CDKAL1, IGFBP2 and PPARG-2 have shown no association in Arabs with single study but have significant association with Caucasians. D: Non-significant association of gene in Arab and Caucasian populations, (HHEX, EXT2, SLC30A8 show non-significant association but in Caucasians they show positive association in French studies).

Group ‘A’ ARABS (significant) Gene Name

CAUCASIANS (significant)

TCF7L2

KCNJ11*

TCF&L2

KCNJ11

rs7903146

rs5219

SNP

rs7903146

rs5219

No. of Studies Compiled

6

2

6

7

Population studied

Saudi, Tunisian, Emirati, Moroccan, Palestinian

Saudi, Tunisian

Scotts, Swedish, Dutch, French, Norwegian, British

Danish, U.K., Czech, Russian, French, Jewish

T2Ds (Total)

2435

1355

8146

3185

Controls (Total)

1798

838

9716

3097

Alleles

T

C

T

C

T

C

A

G

T2DMs

2278

2592

747

1963

5792

10500

2245

3323

Controls

1554

2042

384

1292

5356

14076

2221

3867

OR (C.I.)

1.155 (1.059-1.259)

1.28 (1.111-1.475)

1.45 (1.386-1.516)

1.176 (1.092-1.268)

P-value

0.001

0.001

<0.0001

<0.0001

I2

78.30%

-

77.20%

32.40

Group 'B' ARABS (significant) Gene Name

ACE

CAUCASIANS (non-significant)

MTHFR

ACE

MTHFR

SNP

rs1799752

(C677T)

rs1799752

(C677T)

No. of Studies Compiled

5

3

6

6

Population studied

Tunisian, Lebanon

Tunisian

U.K., Turkish, German, Finnish

T2Ds (Total)

1326

561

1958

1077

Controls (Total)

636

Polish, Turkish, Czech, Italian

1226

2772

932

Alleles

Del

Ins

T

C

Del

Ins

T

C

T2DMs

1470

1182

396

726

1470

1182

396

726

Controls

828

1326

281

991

828

1326

281

991

OR (C.I.)

1.992 (1.774-2.236)

1.924 (1.606-2.304)

1.078 (0.993-1.17)

0.986 (0.868-1.122)

P-value

0

0

0.073

0.835

I2

83.20%

27.20%

0%

0%

K. Al-Rubeaan et al. / Gene 520 (2013) 166–177

171

Table 2 (continued)

Group ‘C’ ARABS (non-significant) Gene Name

CDKN2A/B

CDKAL1

IGFBP2

PPARG-2

CDKN2A/B

CDKAL1

IGFBP2

PPARG-2

rs10811661

rs7754840

rs4402960

Pro12Ala

rs10811661

rs7754840

rs4402960

Pro12Ala

SNP

No. of Studies Compiled

1

1

Moroccan

Population studied

CAUCASIANS (significant)

1

Moroccan

6

Moroccan

5

3

4

French, Austrian, Israeli, Ashkenazi, Danish

French, Austrian, Israeli, Ashkenazi

French, Austrian, Saudi, Tunisian, Israeli, Ashkenazi, Qatari Danish

7

Polish, Danish, Swedish, Czech, French

T2Ds (Total)

518

518

519

1430

7227

1820

5651

3741

Controls (Total)

415

418

411

1864

8692

2057

6921

3746

Alleles

T

C

C

G

T

G

C

C

G

T

G

C

T2DMs

173

863

350

683

447

591

2674

C

184

G

9622

T

4674

1322

2318

3768

7534

6599

923

G

Controls

138

692

252

584

328

494

3535

193

11417

5781

1353

2716

4267

9575

6422

1070

OR (C.I.)

1.005 (0.787-1.284)

1.188 (0.976-1.445)

1.139 (0.946-1.372)

0.793 (0.644-0.977)

1.042 (0.994– 1.093)

1.164 (1.061.278)

1.122 (1.0641.184)

P-value

0.967

0.085

0.17

0.029

0.043

0.002

<0.0001

<0.0001

2

-

-

-

1.3%

33.60%

-

37.40%

17.5%

I

1.184 (1.0771.301)

Group ‘D’ ARABS (non-significant) Gene Name

HHEX

EXT2

rs7923837

SNP

CAUCASIANS (non-significant) SLC30A8

rs1113132

No. of Studies Compiled

2

Population studied

Tunisian, Moroccan

Moroccan

T2Ds (Total)

1313

Controls (Total)

926

HHEX

rs13266634

1

EXT2

rs7923837

1

SLC30A8

rs1113132

rs13266634

3

3

4

Moroccan

French, Austrian, Israeli, Ashkenazi

French, Austrian, Israeli Ashkenazi

French, Austrian, Israeli Ashkenazi, Russian

519

517

1839

1862

2371

419

421

2139

2170

2717

Alleles

A

G

C

G

C

T

A

G

C

G

C

T

T2DMs

655

1971

134

904

169

865

1118

2560

866

2858

1979

2763

Controls

498

1354

107

731

140

702

1353

2925

1018

3322

2245

3189

OR (C.I.)

0.904 (0.789-1.035)

1.013 (0.771-1.329)

P-value

0.142

0.928

0.87

0.237

0.831

0.668

I2

-

-

-

0%

73.60%

82.10%

0.98 (0.767-1.252) 0.944 (0.858-1.039) 0.989 (0.892-1.097)

1.017 (0.94-1.101)

*KCNJ11 is significant (only 2 studies, 1 has shown positive association and the other has no association).

1.051), p = 0.188 and I2 = 1.3%, whereas among Caucasians, the pooled OR was 1.163 (95%C.I = 1.056–1.281), p = 0.002 and I2 = 17.5%. SNPs CDKN2A/2B rs10811661, and IGF2BP2 rs4402960 were found to be studied in a Moroccan population studied by (Cauchi et al., 2008), while, the same study also looked into the association of these SNPs with T2D in three other populations, such as French, Austrian and Israeli Ashkenazi. SNPs CDKN2A/2B rs10811661 was also studied in French population (Marre et al., 2008) and along with IGF2BP2 rs4402960 in Danish (Grarup et al., 2007). The total number of T2D patients for CDKN2A/2B

rs10811661, and IGF2BP2 rs4402960 in the Arab study was 518, 518 and 519 respectively, while the total number of T2D patients was 1892, 1820 and 1819, respectively within the Caucasian population. The total number of controls for CDKN2A/2B rs10811661 and IGF2BP2 rs4402960 within the Arabian study was 415, 418 and 411, respectively, while the total number of controls within the Caucasian population was 2144, 2057 and 2059 respectively. The pooled OR for CDKN2A/2B rs10811661 was 1.005 (95%C.I. = 0.787–1.284, p = 0.967), and IGF2BP2 rs4402960 was 1.139 (95%C.I. = 0.946–1.372, p = 0.17) among Arab population,

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whereas among Caucasians, the pooled OR was 0.843 (95% C.I. 0.748– 0.949, p = 0.005 and I2 = 33.60%) for CDKN2A/2B rs10811661, and 1.121 (95%C.I. = 1.019–1.232, p = 0.019 and I2 = 37.40%) for IGF2BP2 rs4402960. The total number of T2D patients for CDKAL1 rs7754840 in the Arab study was 518 with a pooled OR of 1.188 (95%C.I. = 0.976– 1.445, p = 0.085), while the total number of T2D patients was 1820 with a pooled OR of 1.164 (95%C.I. = 1.06–1.278, p = 0.002 and I2 = 62.20%), within the Caucasian population. Group ‘D’ included SNPs that are neither significantly associated with Arabs, nor among Caucasians, namely HHEX rs7923837, EXT2 rs1113132, and SLC30A8 rs13266634. HHEX rs7923837 was found to be studied in two Arab studies, namely Tunisians (Ezzidi et al., 2009a) and Moroccans (Cauchi et al., 2008), while studies done in French, Austrian and Israeli Ashkenazi (Cauchi et al., 2008) populations were grouped together as the Caucasian population. The total number of T2D patients in these Arab studies was 1313, while the total number of T2D patients was 1839 within the above-mentioned Caucasian studies. The total number of controls within the Arabian studies was 926, while the total number of controls within the Caucasian population was 2139. The pooled OR among Arab population was 0.903 (95%C.I. = 0.789–1.035) and p = 0.143, whereas among Caucasians, the pooled OR was 0.960 (95%C.I. = 0.872–1.057), p = 0.421 and I 2 = 0%. In the case of Arabs, EXT2 rs1113132 was found to be studied in Moroccans (Cauchi et al., 2008), while for Caucasians, studies done in French, Austrian and Israeli Ashkenazi (Cauchi et al., 2008) populations were considered. The total number of T2D patients in the Arab study was 519, while the total number of T2D patients was 1862 within the above-mentioned Caucasian studies. The total number of controls within the Arabian study was 419, while the total number of controls within the Caucasian population was 2170. The pooled OR among Arab population was 1.013 (95%C.I. = 0.771– 1.329, p = 0.928), whereas among Caucasians, the pooled OR was 0.996 (95%C.I. = 0.808–1.227, p = 0.971 and I 2 = 73.60%). In the case of Arabs, SLC30A8 rs13266634 was found to be studied in Moroccans (Cauchi et al., 2008), while for Caucasians, studies done in French, Austrian Israeli Ashkenazi (Cauchi et al., 2008) and Russian (Grant et al., 2006) populations were considered. The total number of T2D patients in the Arab study was 517, while the total number of T2D patients was 2371 within the above-mentioned Caucasian studies. The total number of controls within the Arabian study was 421, while the total number of controls within the Caucasian population was 2717. The pooled OR among Arab population was 0.98 (95%C.I. = 0.767–1.252, p = 0.87), whereas among Caucasians, the pooled OR was 0.978 (95%C.I = 0.792–1.207, p = 0.837 and I 2 = 82.10%). Fig. 1 is a Forest plot representation of the four groups described above, in order to compare the association of the selected 11 SNPs among Arabs and Caucasians. In group A, TCF7L2 rs7903146 was found to be significantly associated with both the populations, but was more strongly associated in Caucasians than in Arabs with ORs of 1.450 and 1.155, respectively. Similarly, PPARG2 Pro12Ala was also found to be more strongly associated in Caucasians than in Arabs with ORs of 1.184 and 0.793, respectively. Moreover, KCNJ11 rs5219 was found to be similar in both Arab and Caucasian populations, with ORs of 1.28 and 1.206, respectively. In group B, ACE I/D and MTHFR C677T polymorphism was found to be significantly associated among Arabs, but not among Caucasians. In the case of MTHFR C677T, the OR was found to be two-folds higher among Arabs (OR = 1.924) when compared to Caucasians (OR = 0.986). Similar results were seen in the case of ACE I/D polymorphism, with ORs of 1.992 among Arabs and 1.078 among Caucasians. In group C, the SNPs CDKAL1 rs7754840, CDKN2A/2B rs10811661 and IGF2BP2 rs4402960, were found to be significantly associated with T2D among Caucasians with OR = 0.84, 1.164 and 1.157, respectively, but this was not the case among Arabs, with OR = 1.005, 1.188 and 1.139, respectively. In group D, HHEX rs7923837, EXT2 rs1113132 and SLC30A8 rs13266634 were not

significantly associated with T2D in both the Arab (OR = 0.904, 1.013 and 0.98, respectively) and Caucasian (OR = 0.944, 0.989 and 1.017, respectively) populations. Fig. 2 is a comparative scatter plot, demonstrating how these SNPs behave in these ethnicities, according to their groups. The first group, Group ‘A’ where SNPs were significantly positive in both Arabs and Caucasians, which included TCF7L2 rs7903146, showed a score of 22.71 for Arabs and 42.18 for Caucasians with a highly significant score. KCNJ11 rs5219 was also in Group A with 15.14 for Arabs and 23.52 for Caucasians. In Group ‘B’ SNPs, namely, ACE I/D and MTHFR C677T polymorphism, there was a highly significant positive score for the Arab population scoring, as 27.44 and 21.05 respectively, while Caucasian studies revealed a negative score of −3.19 and −4.98 respectively. The third group (Group ‘C’) of SNPs namely, PPARG-2 Pro12Ala, CDKN2A/B rs10811661, CDKAL1 rs7754840 and IGFBP2 rs4402960 were opposite, showing a significantly positive score for Caucasians, reading at 25.56, 28.13, 13 and 25.80 respectively, while the Arab ethnicities had a negative score measuring at −3.45, −12.06, −14.26 and −13.67 respectively. The last group (Group ‘D’) of SNPs namely, HHEX rs7923837, EXT2 rs1113132 and SLC30A8 rs13266634 scored negative for both Arabian and Caucasian populations with scores of −8.13, −12.16 and −11.76 respectively for Arabs and −6.72, −5.98, and −3.91 respectively for Caucasians.

7. Discussion To understand the high prevalence of T2D among Arab ethnicities, gene analysis could help to linking this disease with such ethnicities. Although medical literature does not have enough genetic analysis of Arab ethnicities, comparative analyses of different genes conducted by different researchers have demonstrated an interesting genetic pattern seen when these genes were analyzed. Among the 31 gene studies conducted in the Middle East for Arab ethnicities, 22 were from Arabs living in North African region (Tunisia and Morocco), seven genetic studies

Fig. 1. Forest plot representation comparing the association of the selected 11 SNPs. Group A — It shows that TCF7L2 and KCNJ11 are with significant association in both Arabs and Caucasians. Group B — Genes ACE and MTHRF are significantly associated in Arabs and non-significant in Caucasians. Group C — Consists of 4 genes namely, CDKAL1, CDKN2 A/B, PPARG2 and IGFBP2 that have no significant association in Arabs but significant in Caucasians. Group D — Includes HHEX, EXT2 & SLC30A8 that showed non-significant association in both Arab and Caucasian populations.

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173

Fig. 2. A comparative scatter plot of the 11 selected genes according to their score.

from the Arabian Peninsula countries (Saudi Arabia, Qatar, Bahrain, and U.A.E.) and two from the Mediterranean region (Lebanon and Palestine). Among the different SNPs that have been found to be associated with T2D in different ethnic groups around the world, TCF7L2 rs7903146. ACE I/D and MTHFR C677T polymorphisms were found to be significantly associated with the Arab population.

TCF7L2, a widely studied gene, has been found to have a marked, reproducible effect on T2D risk among different global ethnic groups. For example, 6 variants of TCF7L2 have been reported to have a strong association with the disease in an Icelandic study (Grant et al., 2006). Substantial association has been confirmed between variants in TCF7L2 and T2D among broad ethnic backgrounds, for example, populations of

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British (Groves et al., 2006), Dutch (van Vliet-Ostaptchouk et al., 2007), Finnish (Scott et al., 2006), Swedish (Mayans et al., 2007), French (Sladek et al., 2007), American (Zhang et al., 2006), Indian (Chandak et al., 2007) and Japanese (Hayashi et al., 2007) origins. However, a surprising lack of association between TCF7L2 variants and T2D was independently reported in two non-European populations, such as Chinese (Chang et al., 2007) and Pima Indians (Guo et al., 2007). Among Arabs, strong associations have been reported in Tunisians (Ezzidi et al., 2009a) Moroccans (Cauchi et al., 2007) and Palestinians (Ereqat et al., 2009), but studies done among Saudis (Alsmadi et al., 2008a) and Emiratis (Saadi et al., 2008), reported a relatively weak, or no association of TCF7L2 variants with T2D. The association reported among Tunisians and Moroccans were anticipated, as these populations have a significant amount of European input to their genetics, while among Arabs of the Arabian Peninsula, such as Saudis and Emiratis, the prevalence of consanguineous marriages is more likely to concentrate the gene pool, and therefore a different association. This might be due to gene–gene interactions, where variants in TCF7L2 interact with other unidentified genetic or environmental risk-factors that are highly prevalent in this population, thereby protecting the effect of the TCF7L2 gene. More in-depth investigations of an increased sample size need to be done among Arabs in order to support this hypothesis. When Arab studies were pooled together, a highly significant association of TCF7L2 rs7903146 with T2D was found with an OR of 1.336 and a p-value of 0.005. The angiotensin-I converting enzyme (ACE), a key component of the renin-angiotensin system (RAS), is an exo-peptidase, and catalyzes the conversion of angiotensin-I (Ang I) to a vasoconstrictor angiotensin-II (Ang-II), by cleaving the C-terminal dipeptide (His-Leu) (Davis and Roberts, 1997). A common polymorphism, characterized by the insertion (I allele) or deletion (D allele) of a 287-bp Alu repeat sequence in intron 16, has been suggested as being associated with T2D in several populations, such as in China (Feng et al., 2002), Japan (Daimon et al., 2003), Caucasians (Stephens et al., 2005). However, there were other reports that contradicted these studies, such as those in Caucasians (Grammer et al., 2006) and the Turkish population (Mazza et al., 2005). Similar contradictory results were seen among the Arab ethnic groups (Ben et al., 2009; Bouhaha et al., 2008, 2009; Mohamed et al., 2007; Saadi et al., 2008), but when the data of these studies were pooled together, the I/D polymorphism of ACE gene was found to be strongly associated with T2D, with an OR of 1.960 and a p-value of 0.0007. Similar results were seen in the case of MTHFR C677T polymorphism. MTHFR is a gene that encodes for methylenetetrahydrofolatereductase enzyme, which is involved in the transmethylation of homocysteine to methionine. C677T polymorphism results in a Ala-to-Val substitution at amino acid 226, which renders the enzyme thermolabile with a reduced activity, and causes elevated levels of amino acid homocysteine, thereby leading to hyperhomocysteinemia (Frosst et al., 1995). Reports revealed that C677T polymorphism was linked with moderate hyperhomocysteinemia when the folate levels were low, and this was seen more prominently in homozygotes (677T/T) compared with heterozygotes or non-carriers (Jacques et al., 1996). There was not much difference in the distribution of alleles between the T2D patients and healthy subjects among different Caucasian populations considered for this study, such as Polish (Ksiazek et al., 2004), Turkish (Eroglu et al., 2007; Tutuncu et al., 2005; Yilmaz et al., 2004), Italian (Mazza et al., 2005) and Czech (Benes et al., 2001). But among the Arab ethnic groups (Mehri et al., 2010; Mtiraoui et al., 2009), MTHFR C677T was found to be strongly associated with T2D; therefore, their pooled data also showed a highly significant association with T2D with an OR of 1.915 and a p-value of 0.0001. Similarly, the Peroxisome proliferator-activated receptor gamma 2 (PPARG-2), pt?>involved in the adipocyte differentiation and gene expression also plays an important role in type 2 diabetes and diabetes-related traits, including insulin sensitivity and energy metabolism. A frequent variation of PPARG-2 results in a Proline to Alanine

substitution at codon 12 (Yen et al., 1997). The Pro12Ala polymorphism seems to be associated with T2D in several populations, such as French (Ghoussaini et al., 2005), Czech (Pinterova et al., 2004), British (Zeggini et al., 2005) and Italian (Scacchi et al., 2007), but not in Czech (Sramkova et al., 2002), and two different Caucasian studies (Beamer et al., 1998). In several reports, the Ala variant has been found to have a protective effect against T2D, such as that reported in three independent Japanese studies (Nemoto et al., 2002), a Czech study (Pinterova et al., 2004) and a Scandinavian parent–offspring trios study (Altshuler et al., 2000). It has also been suggested that the Ala variant demonstrated improved insulin sensitivity, which was assessed by using hyperinsulinemiceuglycemic clamp techniques. (Chistiakov et al., 2010; Ek et al., 2001; Evans et al., 2001; Koch et al., 1999) In contrast, other studies done in German (Evans et al., 2001), Czech(Sramkova et al., 2002) and Polish (Malecki et al., 2003) populations suggest the association of Ala allele to increased susceptibility to T2D. While the five Arab studies relating to PPARG-2 showed no significant association with T2D and had a pooled OR of 0.865 and p-value of 0.188 (Badii et al., 2008; Ben et al., 2009; Bouassida et al., 2005; Bouhaha et al., 2008; Mohamed et al., 2007; Wakil et al., 2006). This indicates the need for large-scale studies to further investigate its role in the development of the disease in Arab ethnicity. KCNJ11 encodes the Kir6.2 subunits of the ATP-sensitive K + (KATP) channels. Signals derived from the metabolism of glucose in pancreatic β-cells ultimately result in insulin secretion via the closure of the KATP channels (Gloyn et al., 2003). E23K, the most common polymorphism of KCNJ11, where Glu-to-Lys substitution occurs at amino acid 23, has been analyzed in several studies, most of which have reported a positive association between the minor K allele and T2D (Chistiakov et al., 2009; Gloyn et al., 2001, 2003; Love-Gregory et al., 2003). A similar association was observed among Arab studies (Alsmadi et al., 2008b; Mohamed et al., 2007). rs10811661, a polymorphism on chromosome 9p is located 125 kb from the nearest annotated genes, CDKN2B (cyclin-dependent kinase inhibitor 2B) and CDKN2A (cyclin-dependent kinase inhibitor 2A) (Saxena et al., 2007; Scott et al., 2007; Zeggini et al., 2007). The genes CDKN2A and CDKN2B are tumor suppressor genes, and their products, p16INK4a and p15INK4b respectively, are inhibitors of cyclin-dependent kinase 6 (CDK6) and cyclin-dependent kinase 4 (CDK4), which are regulators of pancreatic beta cell replication (Saxena et al., 2007; Scott et al., 2007; Tsutsui et al., 1999; Wu et al., 2008; Zeggini et al., 2007). The association of CDKN2A/B rs10811661 with T2D was found to be significant in GWAS studies conducted among different Caucasian populations, such as Finns, Swedish, British and French, (Cauchi et al., 2008; Saxena et al., 2007; Scott et al., 2007; Zeggini et al., 2007) which was also replicated in several independent studies conducted among Asian populations such as Han Chinese (Han et al., 2010; Wu et al., 2008), Korean (Lee et al., 2008), while it was not reproducible in another Han Chinese study (Lin et al., 2010) Conversely, among Arabs, it was insignificant with an OR of 1.188 and a p-value of 0.085. This might be attributed to the insufficient number of studies conducted for this gene. Insulin-like growth factor 2 (IGF2) mRNA-binding protein 2 (IGF2BP2) gene at 3q27, is also referred to as IMP-2, and belongs to a family of three mRNA-binding proteins (IMP1, IMP2, and IMP3), involved in RNA localization, stability, and translation. IGF2 is mainly expressed during placental and fetal development and stimulates insulin action. However, the exact functional role of IGF2BP2 is not known, and even less is known about how variations in IGF2BP2 may affect the quantitative traits that contribute to diabetes risk (Duesing et al., 2008; Li et al., 2009; Van Hoek et al., 2009). SNP rs4402960, situated within the 125 kb intron 2 of IGF2BP2 at 3q27.2, was found to be significantly associated in a Han Chinese population (Han et al., 2010), Japanese population (Horikoshi et al., 2007; Hu et al., 2009; Sanghera et al., 2008; Takeuchi et al., 2009), and Indian Sikhs (Sanghera et al., 2008). On the contrary, studies conducted among French, (Cauchi et

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al., 2008; Duesing et al., 2008) Korean (Lee et al., 2008), Chinese (Hu et al., 2009) and Japanese (Horikoshi et al., 2007) populations showed no significant association of rs4402960 with T2D, which was in consistent with the Arab population with an OR of 1.139 and a p-value of 0.17. CDKAL1 is a gene with an unknown function; it encodes protein that is very similar to CDK5 regulatory subunit-associated-protein-1. This protein belongs to the methylthiotransferase family (Haupt et al., 2008). It was hard to study precisely the association of this gene with T2D, because there is an insufficient number of a study available. However, in this review, the CDKAL1 rs7754840 was analyzed, and found to be non-significantly associated with T2D in both Arabs and Caucasians. It was also found to be not significant with T2D in Asian Indian Sikhs (Sanghera et al., 2008), but not in Chinese (Hu et al., 2009), Japanese (Takeuchi et al., 2009) and Korean (Lee et al., 2008). HHEX is a hematopoietically expressed homeobox, one of its SNPs (rs7923837) was explored in this review and showed no significant association with T2D among both Arabs (OR of 0.903, p-value of 0.143), while, it showed a significant association with T2D in a Chinese (Hu et al., 2009), and two Japanese (Horikoshi et al., 2007) population studies. EXT2 gene, which encodes one of the two glycosyltransferases involved in the chain elongation step of heparin. EXT2 rs1113132, was studied in this review and was found to be non-significantly associated with T2D among Arabs (OR = 1.013 and p = 0.928) and Caucasians. It was found not to be significant among the Han Chinese (Wu et al., 2008) population. On the contrary, it was found to be significantly associated in the Japanese (Omori et al., 2008) population. SLC30A8 is a gene encoding a 369-amino acid zinc transporter protein ZnT-8. One of its SNPs (rs13266634) was analyzed in this review, and found to be insignificantly associated with T2D in both Arabs (OR = 0.98 and p = 0.87) and Caucasians. On the other hand, it was found to be significantly associated in the Han Chinese (Hu et al., 2009) and Japanese (Omori et al., 2008) populations. A comparative Scatter plot (Fig. 2) was plotted to support the findings represented by the forest plot. SNPs were grouped within the graph, based upon their association among Arabs and Caucasians. They are represented in order that the first group (A) will be significant in Arabs and Caucasians, and the last one (D) will be insignificant in Arabs and Caucasians. Group B includes SNPs which are significant in Arabs but non-significant in Caucasians whereas SNPs non-significant in Arabs and significant in Caucasians are placed in Group C. SNPs above the x-axis are significantly associated with T2D, and those below the x-axis have a weak or no association with T2D. As expected, SNPs that show no evidence for publication bias among Caucasian are greater than those with publication bias among Caucasians. This is likely to be the result of a good amount of published studies. However, there are four SNPs that show publication bias. This can be attributed to the sample size, as small sample-size studies may be more prone to publication bias. Meanwhile, among Arabs, publication, bias could not be evaluated for many SNPs, due to the insufficient amount of studies. Among those with a sufficient number of studies, two were found with bias, and this may also be attributed to the same reason, as above. Due to the lack of a sufficient number of studies, and the data not being stratified by other factors (subgroups), heterogeneity was decided to be high when below I2 = 50%, and low when above I2 = 50%. Nonetheless, around seven SNPs in Caucasians have shown low heterogeneity, whereas only two have done so among Arabs. 8. Conclusion It is clear from this study that we can easily group genes that are associated with Arab ethnic groups as TCF7L2 rs7903146, ACE I/D and MTHFR C677T polymorphisms. Although TCF7L2 rs7903146 is the only SNP found up to this moment to be significantly associated with Arabs and Caucasians, although KCNJ11 rs5219 shows a significant association in Arabs and Caucasians but number of studies is

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only two in Arabs one shows positive association and other no association. ACE I/D and MTHFR C677T are the most highly significant SNPs among the Arab population. In other SNPs, which were found to be associated with Caucasians namely, PPARG-2 Pro12Ala, CDKN2A/2B rs10811661, KCNJ11 rs5219, IGF2BP2 rs4402960, HHEX rs7923837, EXT2 rs1113132 and SLC30A8 rs13266634 were not found to be associated with Arabs. This meta-analysis demonstrates the SNPs associated with T2D among Arab ethnicities, according to what is available at the time of this study. Fig. 2 is more likely to be subject to a lot of changes in the future, when more genetic studies are conducted in these ethnicities. Funding This study was carried out and funded by Strategic Center for Diabetes Research, King Saud University, Saudi Arabia. Conflict of Interest None. References Al-Nozha, M.M., et al., 2004. Diabetes mellitus in Saudi Arabia. Saudi Med. J. 25, 1603–1610. Al-Rubeaan, K., 2010. Type 2 diabetes mellitus red zone. Int. J. Diabetes Mellit. 2, 1–2. Alsmadi, O., et al., 2008a. Weak or no association of TCF7L2 variants with type 2 diabetes risk in an Arab population. BMC Med. Genet. 9, 72–78. Alsmadi, O., et al., 2008b. Genetic study of Saudi diabetes (GSSD): significant association of the KCNJ11 E23K polymorphism with type 2 diabetes. Diabetes Metab. Res. Rev. 24, 137–140. Altshuler, D., et al., 2000. The common PPARγ Pro12Ala polymorphism is associated with decreased risk of type 2 diabetes. Nat. Genet. 26, 76–80. Arfa, I., et al., 2008. Lack of association between the angiotensin-converting enzyme gene (I/D) polymorphism and diabetic nephropathy in Tunisian type 2 diabetic patients. J. Renin Angiotensin Aldosterone Syst. 9, 32–36. Arzu, E.H., et al., 2004. Angiotensin-1 converting enzyme gene polymorphism in Turkish type diabetic patients. Exp. Mol. Med. 36, 345–350. Badii, R., et al., 2008. Lack of association between the Pro12Ala polymorphism of the PPAR-gamma 2 gene and type 2 diabetes mellitus in the Qatari consanguineous population. Acta Diabetol. 45, 15–21. Baroudi, T., et al., 2009. Association of the insertion/deletion polymorphism of the angiotensin-converting enzyme gene with type 2 diabetes in two ethnic groups of Jerba island in Tunisia. J. Renin Angiotensin Aldosterone Syst. 10, 35–40. Beamer, B.A., et al., 1998. Association of the Pro12Ala variant in the peroxisome proliferator-activated receptor-gamma2 gene with obesity in two Caucasian populations. Diabetes 47, 1806–1808. Ben, A.S., et al., 2009. Gender-specific effect of Pro12Ala polymorphism in peroxisome proliferator-activated receptor gamma-2 gene on obesity risk and leptin levels in a Tunisian population. Clin. Biochem. 42, 1642–1647. Benes, P., et al., 2001. Methylenetetrahydrofolatereductase polymorphism, type II diabetes mellitus, coronary artery disease, and essential hypertension in the Czech population. Mol. Genet. Metab. 73, 188–195. Bonnefond, A., Froguel, P., Vaxillaire, M., 2010. The emerging genetics of type 2 diabetes. Trends Mol. Med. 16, 407–416. Bouassida, K.Z., et al., 2005. The peroxisome proliterator activated receptorγ2 (PPARγ2) Pro12Ala variant: lack of association with type 2 diabetes in obese and non obese Tunisian patients. Diabetes Metab. 31, 119–123. Bouhaha, R., et al., 2008. Effect of ENPP1/PC-1-K121Q and PPARgamma-Pro12Ala polymorphisms on the genetic susceptibility to T2D in the Tunisian population. Diabetes Res. Clin. Pract. 8, 278–283. Bouhaha, R., et al., 2009. TCF7L2 is associated with type 2 diabetes in nonobese individuals from Tunisia. Pathol. Biol. (Paris) 58, 426–429. Cauchi, S., et al., 2006. Transcription factor TCF7L2 genetic study in the French population: expression in human beta-cells and adipose tissue and strong association with type 2 diabetes. Diabetes 55, 2903–2908. Cauchi, S., et al., 2007. TCF7L2 is reproducibly associated with type 2 diabetes in various ethnic groups: a global meta-analysis. J. Mol. Med. 85, 777–782. Cauchi, S., et al., 2008. Post genome-wide association studies of novel genes associated with type 2 diabetes show gene–gene interaction and high predictive value. PLoS One 3, e2031. Cejkova, P., et al., 2007. KCNJ11 E23K polymorphism and diabetes mellitus with adult onset in Czech patients. Folia Biol. (Praha) 53, 173–175. Chandak, G.R., et al., 2007. Common variants in the TCF7L2 gene are strongly associated with type 2 diabetes mellitus in the Indian population. Diabetologia 50, 63–67. Chang, Y.C., et al., 2007. Association study of the genetic polymorphisms of the transcription factor 7-like 2 (TCF7L2) gene and type 2 diabetes in the Chinese population. Diabetes 56, 2631–2637.

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