Rare F5 variants have previously been detected and studied in human populations from different parts of the world. Furthermore, many F5 variants have their common names reflecting cities in which they were first detected [10, 11, 20-22]. It is, however, necessary to conduct systematic and more elaborate studies in order to extend our understanding of the prevalence of F5 variants, and especially their functional and clinical significance.
In the general Bosnian-Herzegovinian population, two rare F5 variants have been detected, namely Met2120Thr and Asp2194Gly. The importance of Met2120Thr SNP has been demonstrated using recombinant FV protein. It has been shown that this variant is significantly associated with decreased FV levels in vivo and in vitro. MAF for Met2120Thr is 3% in Caucasians and it seems to be restricted to this population only since African and East Asian populations do not exhibit this variant in previously published studies [7]. The present study identified higher MAF of 7.3% with two individuals being homozygous carriers of 2120Thr allele.
On the other hand, Asp2194Gly variant constitutes HR2 haplotype that was first described in 1998 in terms of four linked missense variants with His1299Arg being functionally the most significant. It was immediately associated with lower FV levels in plasma [7, 23]. This haplotype was later characterized in more detail and is now believed to be consisted of 13 linked polymorphisms in exons 8 through 25 of F5 gene with seven of them being amino acid changes related to functional modifications of the protein [1, 23, 24]. The function of this haplotype is now mainly associated with an increased factor V1 to V2 ratio, thus increasing prothrombotic activity, as well as with decrease in APC-mediated degradation of FV, including individuals with wild-type Leiden genotype [23, 24]. Asp2194Gly is described as one of the major functional determinants of FVHR2 haplotype [25], interfering with the transport of mutant protein from ER to Golgi complex [26]. When compared to other rare variants, HR2 is relatively common with MAF of 5 to 17% in Europe, Asia and Africa, reaching its peak value of 50% in Indian tribes from Costa Rica [23].
There has been an ongoing debate of HR2 haplotype role in predisposition to thrombotic events with previous studies reporting opposing results. In their meta-analysis, Castaman and colleagues [23] found a statistically significant differences in VTE frequency in FV Leiden carriers alone when compared to FV Leiden/HR2 compound carriers. The authors have also concluded that HR2 haplotype can only be observed as a mild prothrombotic factor since this variant alone did not significantly increase the risk of VTE compared to wild-type individuals [23]. On the other hand, in one of the most extensive and detailed meta-analyses on this topic, Gohil and colleagues [27] identified, among others, F5 variants representing risk factors for VTE. FVHR2 haplotype was analyzed by comparing the results of 14 studies in Caucasian populations. Population attributable risk (PAR) for VTE was 2.6% in the presence of this variant, while MAF was 6% in both patients and healthy controls. This meta-analysis identified a significant risk of VTE in the presence of HR2 haplotype mutant allele [27].
Asp2194Gly SNP alone corroborates lower FV levels in vivo, which has been shown to contribute to increased APC resistance and, therefore, increased probability of thrombotic event. However, it is expected that this variant has a prothrombotic character if accompanied by FV Leiden mutation. MAF for this SNP is 0%, 7% and 5% in Africans, Caucasians and Asians, respectively [7]. This finding is in agreement with the present study, in which 2194Gly allele had frequency of 7.7%.
In the most detailed study of Met2120Thr and Asp2194Gly variants in Caucasian populations so far, 1013 Italians were genotyped and variant alleles were detected for both polymorphisms. The results report that 4.4% and 13.2% participants were Met2120Thr and Asp2194Gly carriers, respectively [25]. The same study offers extensive information on functional changes associated with these two variants. FV Leiden/Met2120Thr compound heterozygotes had significantly decreased APC ratios, while this difference was not significant for FV Leiden/Asp2194Gly compound heterozygotes. The prevalence of FV Leiden/Asp2194Gly genotype was significantly higher in symptomatic VTE patients than in asymptomatic controls. Higher frequency of FV Leiden/Met2120Thr genotype was also observed in symptomatic patients than in controls, but the difference was not statistically significant. Additional analysis confirmed that both variants lead to decrease of recombinant FV activity and antigen levels in vitro and protein levels in plasma in vivo. In heterozygous state, Met2120Thr decreased Factor V levels by 25% in vivo and 34% in vitro in expression studies. Comparable to this, heterozygous Asp2194Gly decreased FV levels by 20% and 45% in vivo and in vitro, respectively [25].
Human coagulation cascade is a delicate and fine-tuned pathway, while thrombophilia is a heterogenous condition being associated with many acquired and hereditary variants. Aside from FV Leiden, several other variants in F5 gene have been detected in clinical practice. We have here shown that Met2120Thr and Asp2194Gly are present in healthy population from Bosnia and Herzegovina and that the variant distribution is not significantly correlated with participants’ family history of cardiovascular and cerebrovascular diseases. In order to fully assess clinical significance of these and other F5 variants, it is necessary to design and implement studies that will aim at comparing general and patient population and assessing potential prothrombotic nature of rare F5 variants. Furthermore, functional consequences of rare F5 variants, including varying FV levels in blood and impaired protein secretion are to be investigated in order to get better understanding of their roles.