**4. DNA polymorhisms for a pharmacogenonic approach to COVID-19 treatment**

The risk of thromboembolism in COVID-19 is documented in an article published in The Lancet [6]. This finding brings up an important issue to be screened on COVID-19 patients: the impact of inherited predisposition to thrombotic events in patients affected by COVID-19.

Given this, thrombophilic genetic abnormalities in variants were widely reported in the medical sciences such as Factor V Leiden (F5), Prothrombin (F2), and the polymorphism in methylenetetrahydrofolate reductase (MTHFR), among others [7]. These polymorphisms could put a patient's carriers of mutant alleles in the Risk Group, beyond the well-known factors, such as elderly patients, hypertension, cardiac and respiratory diseases, cancer, and diabetes [8].

In this chapter, we present a brief review of the three main DNA polymorphisms associated with thrombophilic events and suggest the inclusion of these, as well as the coagulation profiles of their carriers, as aggravating comorbidities of COVID-19.

Firstly, a brief review of the main molecular characteristics of these polymorphisms is as follows:

#### **4.1 Factor V Leiden (FVL or F5)**

It represents one of the main causes of resistance to protein C, as mutation increases the risk of thrombotic disease three to ten times for heterozygous carriers and eighty times for homozygous carriers [4, 9]. About 90% of cases of protein C resistance result from point mutation in the coagulation factor V gene. This mutation occurs in exon 10 of the factor V gene, causing a substitution of the G/A base (Guanine/Adenine) in nucleotide 1691, resulting in the exchange of Arg (Arginine) by Gln (Glutamine) at position 506 of the protein, one of the main cleavage sites for protein C activation [10]. FVL is the most common inherited cause of venous thrombosis.

In patients with increased protein C resistance, venous thrombosis without known etiology and familiar history of unexplained thrombosis, the FVL mutations' screening should be considered beyond a strong clinical investigation. The diagnosis for FVL mutations is based on well-known molecular biology approaches. The clotting time-based functional assays and genetic biomarkers' screening become together the basis for clinical decisions. It is a very important step to guide the clinical approach, balancing the long-term anticoagulation with its side effects and benefits [11].

Briefly, the mechanism of action of factor V could be described as follows. Factor V is cleaved by thrombin on its B domain at cleavage sites R709, R1018, and R1545, producing an amino-terminal heavy chain and a carboxy-terminal light chain, which binding themselves create a dimer called Factor Va (FVa). In turn, FVa binds with Factor Xa creating a prothrombinase complex which on the platelet surface converts prothrombin (II) to thrombin (IIa). FV can also be split by the action of activated protein C (APC) at the cleavage site R506 before it is cleaved by thrombin. It results in the inactivation of factor V to factor Vi and the generation of an imperfect peptide, the Factor Vac, which apparently has anticoagulant characteristics by stimulating APC- and protein S-mediated inactivation of factor VIIIa. A second mechanism of thrombosis observed with FVL is its diminished cofactor activity with APC and phospholipid in the inactivation of factor VIIIa to factor

VIIIi. Taken together, FVL is a prothrombotic mutation due to a combination of a gain of function, with higher prothrombin activation, and loss of function due to low cofactor activity with APC in the inactivation of factor VIIIa. Normally, patients with defective FVL have a variable thrombophilia phenotype, have increased thrombin generation, have a longer factor Va half-life in plasma, and are resistant to factor Va inactivation [11]. A larger C-terminal peptide results if factor V is cleaved by APC before it is cleaved by thrombin [12].
