The Contributions of Thrombophilic Mutations to Genetic Susceptibility to Deep Venous Thrombosis in Iraqi Patients

Abstract

Background and Aims: There is a paucity of data on the contribution of various thrombophilic mutations to the development of venous thrombosis in Iraqi patients. Therefore we designed a study to assess the frequencies of known thrombophilic mutations in this population. Methods: 100 consecutive Iraqi patients with color Doppler confirmed deep venous thrombosis of the lower extremities and 100 age- and sex-matched healthy controls were enrolled in the study. Their DNAs were tested by multiplex polymerase chain reaction (PCR) followed by reverse hybridization for factor V Leiden (FVL), the prothrombin (PT) G20210A SNP, and the MTHFR C677T SNP. The factor V A4070G mutation was assessed by restriction fragment length polymorphism-PCR. Results: The prevalence of FVL was 13% in patients versus 2% in controls (odd ratios [OR] 7.3; p=0.007). Patients with recurrent thrombosis also had a significantly higher frequency of Factor V Leiden (OR 8.4, p=0.0007). The prothrombin G20210A, SNP, the factor V A4070G SNP, and the MTHFR 677 TT genotypes were present among patients at 5%, 9%, and 11%, respectively, and among controls at 2%, 6%, and 6%; none of these single mutation prevalence differences were significant. Interestingly, however, when these polymorphisms were studied in aggregate we found that 24% of patients had two or more thrombophilic alleles, compared to only 8% of the controls (OR 3.6; p=0.002). This subgroup included significantly more patients with proximal (p=0.007) and recurrent thrombosis (p=0.012), as well as younger patients (≤40 years) (p=0.026). Conclusion: Two or more thrombophilic alleles, as well as FVL on its own, were both significantly associated with an increased risk of venous thrombosis and recurrence in Iraqi patients. Single thrombophilic mutations on their own were not associated with an increased risk.

Introduction

Venous thromboembolism (VTE) is a common cause of morbidity and mortality worldwide, and deep venous thrombosis (DVT) of the lower extremities is its most frequent manifestation. The preponderance of latter is attributed to the high hydrostatic pressure and the low flow rate that affect the veins in that region (Dahlbäck, 2008).

DVT is a multifactorial disorder due to the interplay between a variety of genetic and acquired risk factors. Factor V Leiden (FVL) G1691A and prothrombin (PT) G20210A are among the most frequently investigated variants as putative inherited risk factors (Simone et al., 2013). FVL G1691A is a missense mutation at exon 10 of factor V gene affecting one of its activated protein C (APC) cleavage sites leading to the phenomenon of APC resistance (Kalafatis et al., 1995), while the PT G20210A is a mutation at 3′-untranslated region of the PT gene, altering the stability of mRNA and resulting in higher prothrombin (factor II) levels (Poort et al., 1996). Both the latter mutations have been implicated in significant increases in the risk of venous thrombosis in many populations (Simone et al., 2013). Other frequently investigated mutations in relevance to association with thrombosis include methylene tetrahydrofolate reductase (MTHFR C677T) and factor V A4070G polymorphism (HR2 haplotype). MTHFR C677T is responsible for Ala223Val substitution in a highly conserved residue of the molecule, rendering the resultant enzyme thermolabile, and leading to hyperhomocysteinemia (Gemmati et al., 1999). While, factor V A4070G, involves exon 13 of factor V gene and has been linked to a decrease in the factor V cofactor activity in APC degradation of FVIIIa and an increased procoagulant isoform FV1 compared to FV2 (Castaman et al., 2003). Unlike FVL and PT mutations, the associations of MTHFR 677 and HR2 haplotype with venous thrombosis are more or less controversial (Castaman et al., 2003; Simone et al., 2013).

Epidemiological studies have revealed differences in the incidence and genetic backgrounds of DVT among different populations (Roberts et al., 2009). In Iraq, which is a large country in the Eastern Mediterranean region, and except for a single study on the role of FVL in a small group of unselected patients with DVT (Al-Allawi et al., 2011), no study of significance focused on the role of these thrombophilic mutations in that population, and thus, their contributions to venous thrombosis remain largely unknown, which led to the initiation of the current study aiming to determine, for the first time, the contributions of four thrombophilic mutations to DVT among consecutive Iraqi patients.

Patients and Methods

A total of 100 consecutive patients visiting Azadi Teaching Hospital in Duhok, Kurdistan-Iraq, with a color Doppler confirming DVT of the lower limbs, were enrolled. Patients with pulmonary embolism and thrombosis in upper limbs or other sites were excluded. Their ages ranged from 12 to 80 years (Mean [SD]=45.6 [16.7]), and included 55 males and 45 females (M:F ratio of 1.2:1).

All patients had a short clinical history particularly regarding possible acquired risk factors. They were specifically asked for history of surgical intervention in the past month, immobilization for longer than 3 days, long-term travel (>6 h), pregnancy or postpartum, oral contraceptives, hormonal replacement therapy (HRT), or active malignant disease. Furthermore, venous thrombosis was classified according to the site as proximal (if it included popliteal vein or above) or distal (if it was restricted to the calf veins) (Scarvelis and Wells, 2006). DNA was extracted by a phenol-chloroform-based method. The DNA was used to detect FVL G1691A, PT G20210A, and MTHFR C677T mutations by multiplex polymerase chain reaction (PCR) and reverse hybridization of biotinylated PCR products to membrane-bound allele-specific oligonucleotides. The amplification, hybridization, and detection procedures were performed according to the manufacturer's instruction (ViennaLab Diagnostics). The HR2 haplotype (factor V A4070G) was then studied using restriction fragment length polymorphism-PCR as detailed elsewhere (Jadaon and Dashti, 2005).

A total of 100 age- and sex-matched healthy controls were concomitantly recruited. The control group had ages ranging from 14 to 78 years (Mean [SD]=44.1 [14.6]) and included 55 males and 45 females (1.2:1). The patients and the controls were age matched (p=0.535). The controls were either hospital staff or outpatients attending the laboratory for checkup. The inclusion criteria for the selection of controls were the absence of significant medical illness as well as the absence of history of venous and/or arterial thrombosis. Controls were similarly evaluated for the four thrombophilic mutations.

The study was approved by the appropriate ethics committees of the faculties of Science and Medical Sciences at the University of Duhok-Iraq, and informed consent was obtained from all participants. Statistical analysis utilized the chi-square test (with or without Yates correction) and Student's t-test where appropriate. Statistical evaluation was performed using SPSS (version 16.0) software package (SPSS, Inc.). A p<0.05 was considered significant.

Results

Among the 100 enrolled patients, 36 patients had acquired risk factors, including immobility in 16 cases, pregnancy related in 11/45 females (24.4%), and postoperative in 6 patients, while another 3 had a history of malignancy. No patient gave a history of recent long-term travel, contraception, or HRT. Among the remaining 64 patients, no acquired risk factors were identified at the time of thrombosis. Table 1 shows the distribution of four thrombophilic mutations among patients and controls. FVL 1691 (G>A) (combined heterozygous and homozygous states) was seen in 13% of patients and 2% of the controls (A allele frequencies of 7% and 1%, respectively) and was associated with a significant 7.3-fold increased risk of venous thrombosis compared to the controls. PT 20210 (G>A), on the other hand, was seen in the heterozygous state in 5% of patients and 2% of controls (A allele frequencies of 2.5% and 1% respectively). None of the patients or controls was homozygous for the latter mutation. Factor V HR2 mutation (A4070G) (combined heterozygous and homozygous) was seen in 9% of patients and 6% of controls (G allele frequencies of 5% and 3%, respectively), while the MTHFR C677 T was detected in 54% of patients and 44% of the controls (T allele frequencies of 32.5% and 25%, respectively). Other than FVL, none of the other three polymorphisms, on its own, was associated with a significantly increased risk of DVT (Table 1). When the homozygous state for MTHFR TT genotype was compared between patients and controls, this was also insignificant (p=0.205).

Table 1.

The Distribution of Four Thrombophilic Mutations Among Patients and Controls and Odds Ratios Associated with the Presence of Each

		Number
Mutations	Genotype	Patients (n=100)	Controls (n=100)	Odds ratio (95% CI)	p-Value	Allele frequencies % (patients/controls)
Factor V Leiden G1691A	GG	87	98	7.32 (1.6-33.4)	0.0073	G (93/99)
	GA	12	2			A (7/1)
	AA	1	0
Prothrombin G20210A	GG	95	98	2.58 (0.5-13.6)	0.447	G (97.5/99)
	GA	5	2			A (2.5/1)
	AA	0	0
MTHFR C677T	CC	46	56	1.49 (0.86-2.61)	0.157	C (67.5/75)
	CT	43	38			T (32.5/25)
	TT	11	6
Factor V HR2 A4070G	AA	91	94	1.55 (0.5-4.5)	0.421	A (95/97)
	AG	8	6			G (5/3)
	GG	1	0

CI, confidence interval.

Eleven patients (11%) had a history of recurrent thrombosis. When those with such history were compared to those with a single episode (Table 2), there was no significant difference between age, sex, site, secondary risk factors, or the frequency of any of the four thrombophilic mutations, except for FVL (odd ratios [OR] 8.4, 95% confidence interval [CI] 2.1-33.9; p=0.0007).

Table 2.

A Comparison Between Some Clinical Parameters and Thrombophilic Mutations of Patients with Recurrent and First-Episode Thromboses Enrolled in the Current Study

	Number (%)
Parameter	Recurrent	First episode	p-Value
Number of patients	11	89
Age ≤40 year	7 (63.6)	40 (44.9)	0.394
Sex: female	5 (45.5)	40 (44.9)	0.97
Site: proximal	8 (72.7)	59 (66.3)	0.93
Secondary risk factors	4 (36.4)	32 (36.0)	0.76
Factor V Leiden	5 (45.5)	8 (9.0)	0.0007
PT G20210A	1 (9.1)	4 (4.5)	0.94
Factor V A4070G	1 (9.1)	8 (9.0)	0.58
MTHFR C677T homozygous	1 (9.1)	10 (11.2)	0.77

When the number of thrombophilic alleles in each patient was taken into consideration, it was found that two or more thrombophilic alleles were 3.6 times (95% CI 1.5-8.6) higher in patients than controls (24 patients vs. 8 controls) (p=0.002). Furthermore, those with two or more thrombophilic alleles included significantly more young patients (≤40 years old) [p=0.026], were more likely to have proximal (p=0.007) and recurrent thromboses (p=0.012; OR 4.73, 95% CI 1.3-17.3), but there were no significant associations with gender or secondary risk factors (Table 3).

Table 3.

A Comparison Between Some Clinical Parameters in Those with ≥2 Versus <2 Thrombophilic Alleles

	Number of patients (%)
Parameter	<2 thrombophilic alleles	≥2 thrombophilic alleles	p-Value
Number of patients	76	24
Age ≤40 year	31 (40.8)	16 (66.7)	0.026
Sex: females	32 (42.1)	13 (54.2)	0.30
Site: proximal	46 (60.5)	22 (91.7)	0.007
Recurrent	5 (6.6)	6 (25)	0.012
Secondary risk factors	24 (31.6)	12 (50)	0.25

Alternatively, when patients were classed into those older than 40 and those younger, it was found that all four thrombophilic mutations were more frequent among the younger group of patients (FVL 17.0% vs. 9.4%; HR2 14.9% vs. 3.8%; PT 6.4 vs. 3.8%; MTHFR [TT] 14.9% vs. 7.5%), but none were significant (p=0.26, 0.112, 0.89, and 0.242, respectively). However, it was found that the younger age group was more likely to have two or more thrombophilic alleles and secondary associated factors than the older group (p=0.026 and 0.011, respectively).

A family history of venous thrombosis was documented in five patients, including two of the 13 FVL carriers, however, the latter finding was not significant (p=0.247). None of the remaining three patients with such a family history was a carrier of HR2, PT 20210A, or TT genotype of MTHFR 677.

Discussion

Few reports from Iraq have studied the background frequencies of thrombophilic mutations in the population and reported rates of 1.3-3% for FVL, 2-3% for PT 20210, and 6-8% for TT genotype of MTHFR 677 (Al-Allawi et al., 2004, 2009; Nerweyi, 2012; Nerweyi et al., 2013). This is to a great extent consistent with the frequencies we encountered in our control group. It is important here to note that while our frequencies of the MTHFR and PT mutations were to a great extent similar to those reported in the neighboring populations (Al-Allawi et al., 2009) that of FVL was much lower than those reported in countries to the west and north of Iraq like Lebanon, Syria, Jordan, and Turkey, where some of the highest frequencies worldwide were reported (up to 27.2%), but is nearer to figures from countries to the east and south of Iraq like Western Iran, Saudi Arabia, and Kuwait (rates of 0-4.5%) (Rahimi et al., 2010; Jadaon, 2011a). Of the three mutations, however, the current study has revealed that it was only FVL that was associated with a significantly approximate seven-fold increased risk of venous thrombosis. Most studies worldwide reported a significant increased risk of VTE with FVL, and in a recent meta-analysis, Simone et al. (2013) reported a pooled OR of 4.38 (crude OR varied between 1.27 and 15.9 in individual studies) among Caucasians. Furthermore, similar associations of FVL with VTE were reported by studies from other Eastern Mediterranean countries, which revealed that FVL is associated with 3.4- to 6.3-fold increased risk among Lebanese, Turks, and Western Iranians (Almawi et al., 2005; Rahimi et al., 2010; Eroglu et al., 2012). Such similarities, particularly with the latter West Iranian study, are not surprising, since the populations of Western Iran and Northern Iraq are both of a common Kurdish ethnic origin.

The PT G20210A mutation on the other hand, while having a comparative frequency to other populations in the Eastern Mediterranean region (Jadaon, 2011b), did not show any significant association with DVT despite the fact that it was seen in 5% of patients versus 2% of the controls. This is in contrast to studies from Lebanon showing around 6-fold increased risk and a meta-analysis on Caucasian populations reporting a significant pooled 3.17-fold increased risk (Almawi et al., 2005; Gohil et al., 2009). However, it is consistent with the observation of Rahimi et al. (2010) who found no significant increased risk among Western Iranians.

The role of MTHFR C677T as a risk factor for venous thrombosis has always been questionable (Simone et al., 2013). Our study did not show any significant increased risk of venous thrombosis in association with the homozygous state of this mutation. Our findings are consistent with a prospective and case-control studies from Europe, case-control studies from Iran and Lebanon, and those of two meta-analysis on Caucasians that revealed no association of the TT genotype with VTE (Frederiksen et al., 2004; Almawi et al., 2005; Bezemer et al., 2007; Gohil et al., 2009; Rahimi et al., 2010; Simone et al., 2013). However, such an association has been documented by several studies on Asian populations (Gohil et al., 2009; Jang et al., 2013).

The HR2 mutation received less attention than the other three polymorphisms, and the current study is the first from Iraq to determine its frequency among healthy individuals and those with venous thrombosis. The frequency among healthy controls varied in Western Europe and the United States from 8.1% to 12.1% (Castaman et al., 2003) and 7% to 10.1% in some neighboring countries in the Eastern Mediterranean region like Kuwait, Lebanon, and Turkey (Akar et al., 2000; Jadaon and Dashti, 2005; Zaatari et al., 2006). Our corresponding figure of 6% is slightly less than those reported in either group. On the other hand, the frequency of HR2 among VTE patients in Western Europe and USA varied between 7.8% and 18.5%, while figures of 16.5%, 16.4%, and 10.1% were reported from the Middle Eastern countries of Kuwait, Lebanon, and Turkey, respectively (Akar et al., 2000; Jadaon and Dashti, 2005; Otrock et al., 2008), again higher than the frequency of 9% among our own patients. Similar to many western studies, we could not find any significant association between the HR2 haplotype and venous thrombosis (Castaman et al., 2003), although a recent meta-analysis focusing mainly on Caucasians managed to document a significant but modest association with an OR of 1.24 (Gohil et al., 2009). Moreover, studies from Kuwait and Lebanon both found a significant 2.63- and 2.7-fold increased risk, respectively, in those with the HR2 mutation, a finding not shared by our study (Jadaon and Dashti, 2005; Otrock et al., 2008). Folsom et al. (2002) reported that while homozygosity to HR2 and combined heterozygosity with FVL are associated with 5.5 and 16.3 OR of VTE, heterozygosity is not. Two of our patients were combined heterozygous for FVL and HR2 (none in controls), one was homozygous for this mutation, and another three had the mutation in association with another thrombophilic allele.

The current study has also documented a significant 3.6-fold increased risk of venous thrombosis in those with two or more thrombophilic alleles, this has also been shown by several previous studies in neighboring countries, where there was also an increase in the risk of thrombosis in those with two or more thrombophilic alleles with a 5.2- to 22.2-fold increase (Salomon et al., 1999; Almawi et al., 2005; Jadaon and Dashti, 2005). Furthermore, the association with proximal DVT in those with two or more alleles has been documented by at least one earlier study (Pirtskhelani et al., 2014).

On the issue of recurrence and similar to several case control, prospective, as well as systemic reviews of literature, our study showed that the risk of recurrence was significantly increased in association with FVL (Simioni et al., 2000; Folsom et al., 2002; Marchiori et al., 2007; Segal et al., 2009; Sveinsdottir et al., 2012). Some of these studies, however, also found an increased risk associated with PT mutation (Folsom et al., 2002; Sveinsdottir et al., 2012), a finding not shared by the current study.

On the issue of age, several studies and similar to the current one, have revealed that the thrombophilic factors tend to occur more frequently in younger individuals (Gohil et al., 2009; Monreal et al., 2012; Simone et al., 2013; Weingarz et al., 2013), and although the current study did not show this to be significant for individual factors, however, it showed that two or more factors were significantly more frequent among those younger than 40 years.

In conclusion, this study is the first to document the frequencies of four thrombophilic mutations in Iraqis with venous thrombosis. Furthermore, it revealed that FVL as well as the combination of two or more thrombophilic alleles were associated with a significantly increased risk of DVT and its recurrence. Other thrombophilic factors on their own were insignificant in the studied population.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

References

Akar

, Akar

, Yilmaz

(2000) Factor V (His 1299 Arg) in Turkish patients with venous thromboembolism. Am J Hematol, 63:102-103.

Al-Allawi

, Jubrael

MSJ

, Hilmi

(2011) Factor V Leiden mutation in Iraqi patients with deep venous thrombosis. J Fac Med Baghdad, 53:293-295.

Al-Allawi

NAS

, Jubrael

JMS

, Baban

, Gedeon

(2009) Thrombophilic mutations in blood donors in Duhok-Iraq. Duhok Med J, 3:25-32.

Al-Allawi

NAS

, Jubrael

JMS

, Hilmi

(2004) Factor V Leiden in blood donors in Baghdad (Iraq). Clin Chem, 50:677-678.

Almawi

, Tamim

, Kreidy

, et al. (2005) A case control study on the contribution of factor V-Leiden, prothrombin G2010A, MTHFR C677T mutations to the genetic susceptibility of deep venous thrombosis. J Thromb Thrombolysis, 19:189-196.

Bezemer

, Doggen

, Vos

, Rosendaal

(2007) No association between the common MTHFR 677C- >T polymorphism and venous thrombosis: results from the MEGA study. Arch Intern Med, 167:497-501.

Castaman

, Faioni

, Tosetto

, Bernardi

(2003) The factor V HR2 haplotype and the risk of venous thrombosis: a meta-analysis. Haematologica, 88:1182-1189.

Dahlbäck

(2008) Advances in understanding pathogenic mechanisms of thrombophilic disorders. Blood, 112:19-27.

Eroglu

, Sertkaya

, Akar

(2012) The role of factor V Leiden in adult patients with venous thromboembolism: a meta-analysis of published studies from turkey. Clin Appl Thromb Haemost, 18:40-44.

10.

Folsom

, Cushman

, Tsai

, et al. (2002) A prospective study of venous thromboembolism in relation to factor V Leiden and related factors. Blood, 99:2720-2725.

11.

Frederiksen

, Juul

, Grande

, et al. (2004) Methylenetetrahydrofolate reductase polymorphism (C677T), hyperhomocysteinemia, and risk of ischemic cardiovascular disease and venous thromboembolism: prospective and case-control studies from the Copenhagen City Heart Study. Blood, 104:3046-3051.

12.

Gemmati

, Serino

, Trivellato

, et al. (1999) C677T substitution in the methylene tetrahydrofolate reductase gene as a risk factor for venous thrombosis and arterial disease in selected patients. Haematologica, 84:824-828.

13.

Gohil

, Peck

, Sharma

(2009) The genetics of venous thromboembolism. A meta-analysis involving ∼120,000 cases and 180,000 controls. Thromb Haemost, 102:360-370.

14.

Jadaon

(2011a) Epidemiology of activated protein C resistance and factor V leiden mutation in the mediterranean region. Mediterr J Hematol Infect Dis, 3:e2011037.

15.

Jadaon

(2011b) Epidemiology of prothrombin G20210A mutation in the mediterranean region. Mediterr J Hematol Infect Dis, 3:e2011054.

16.

Jadaon

, Dashti

(2005) HR2 haplotype in Arab population and patients with venous thrombosis in Kuwait. J Thromb Haemost, 3:1467-1471.

17.

Jang

, Jeon

, Choi

, et al. (2013) The 677C>T mutation of the MTHFR gene increases the risk of venous thromboembolism in Koreans and a meta-analysis from Asian population. Clin Appl Thromb Hemost, 19:309-314.

18.

Kalafatis

, Bertina

, Rand

, Mann

(1995) Characterization of the molecular defect in factor V R506Q. J Biol Chem, 270:4053-4057.

19.

Marchiori

, Mosena

, Prins

, Prandoni

(2007) The risk of recurrent venous thromboembolism among heterozygous carriers of factor V Leiden or prothrombin G20210A mutation. A systematic review of prospective studies. Haematologica, 92:1107-1114.

20.

Monreal

, Campo

, Papadakis

(2012) Thrombophilia and venous thrombo-embolism: RIETE experience. Best Pract Res Clin Haematol, 25:285-294.

21.

Nerweyi

FFA

(2012) Methylene tetrahydrofolate reductase (C677T) in healthy individuals in Erbil-Iraq. Duhok Med J, 6:34-42.

22.

Nerweyi

FFA

, Hussain

, Jubrael

JMS

(2013) Factor II prothrombin (G20210A) mutation among healthy individuals in Erbil/Iraq. Duhok Med J, 7:9-15.

23.

Otrock

, Taher

, Shamseddeen

, et al. (2008) Factor V HR2 haplotype: a risk factor for venous thromboembolism in individuals with absence of factor V Leiden. Ann Hematol, 87:1013-1016.

24.

Pirtskhelani

, Kochiashvili

, Makhaldiani

, et al. (2014) The role of point mutations in the genes, predisposing inherited thrombophilia in the pathogeneses of proximal and distal deep vein thrombosis in Georgian population. Georgian Med News, 227:98-102.

25.

Poort

, Rosendaal

, Reitsma

, Bertina

(1996) A common genetic variation in the 3′-untranslated region of the prothromboin gene is associated with elevated plasma prothrombin levels and an increase in venous thrombosis. Blood, 88:3698-3703.

26.

Rahimi

, Mozafari

, Shahriari-Ahmadi

, et al. (2010) Deep venous thrombosis and thrombophilic mutations in Western Iran: association with factor V Leiden. Blood Coagul Fibrinolysis, 21:385-388.

27.

Roberts

, Patel

, Arya

(2009) Venous thrombosis and ethnicity. Br J Haematol, 146:369-383.

28.

Salomon

, Steinberg

, Zivelin

, et al. (1999) Single and combined prothrombotic factors in patients with idiopathic venous thromboembolism: prevalence and risk assessment. Arterioscler Thromb Vasc Biol, 19:511-518.

29.

Scarvelis

, Wells

(2006) Diagnosis and treatment of deep-vein thrombosis. CMAJ, 175:1087-1092.

30.

Segal

, Brotman

, Necochea

, et al. (2009) Predictive value of factor V Leiden and prothrombin G20210A in adults with venous thromboembolism and in family members of those with a mutation: a systematic review. JAMA, 301:2472-2485.

31.

Simioni

, Prandoni

, Lensing

, et al. (2000) Risk for subsequent venous thromboembolic complications in carriers of the prothrombin or the factor V gene mutation with a first episode of deep-vein thrombosis. Blood, 96:3329-3333.

32.

Simone

, De Stefano

, Leoncini

, et al. (2013) Risk of venous thromboembolism associated with single and combined effects of factor V Leiden, prothrombin 20210A and methylenetethraydrofolate reductase C677T: a meta-analysis involving over 11,000 cases and 21,000 controls. Eur J Epidemiol, 28:621-647.

33.

Sveinsdottir

, Saemundsson

, Isma

, et al. (2012) Evaluation of recurrent venous thromboembolism in patients with factor V Leiden mutation in heterozygous form. Thromb Res, 13:467-471.

34.

Weingarz

, Schwonberg

, Schindewolf

, et al. (2013) Prevalence of thrombophilia according to age at the first manifestation of venous thromboembolism: results from the MAISTHRO registry. Br J Haematol, 163:655-665.

35.

Zaatari

, Otrock

, Sabbagh

, Mahfouz

(2006) Prevalence of factor V R2 (H1299R) polymorphism in the Lebanese population. Pathology, 38:442-444.