Hemophilia A is an X‐chromosome‐linked recessive bleeding disorder defined by a qualitative and/or quantitative factor VIII (FVIII, F8) deficiency (Salen P & Babiker HM 2019). Patients affected by the mild form of the disease (FVIII activity 0.05–0.4 IU/mL) suffer from bleedings occurring after trauma or surgery. In severe hemophilia A patients (FVIII activity<0.01 IU/mL) bleedings occur spontaneously, whereas moderate hemophilia A patients (FVIII activity 0.01–0.05 IU/mL present with an intermediate bleeding phenotype (White GC 2nd et al. 2001). In healthy individuals, FVIII is synthesized as an ∼ 300-kDa glycoprotein by hepatocytes, liver sinusoidal endothelial cells, and certain types of endothelial cells (Wion KL et al. 1995; Jacquemin M et al. 2006; Shahani T et al. 2009; Turner NA & Moake JL 2015). The FVIII protein contains a domain sequence A1-A2-B-ap-A3-C1-C2 and circulates as an A1-A2-B:ap-A3-C1-C2 heterodimer bound noncovalently to the von Willebrand factor (vWF) protein. vWF protects FVIII from rapid clearance (Lenting PJ et al. 2007). During the activation of FVIII by thrombin to FVIIIa, the B domain and an activation peptide (ap) are released, and cleavage between the A1 and A2 domains produces an A1:A2:A3-C1-C2 heterotrimer (Lollar P & Parker ET 1991; Nogami K et al. 2005; Newell JL & Fay PJ 2007; 2009). Once activated, FVIIIa dissociates from vWF and binds to the membrane of activated platelets to assemble with activated factor IX (FIXa) (Gilbert GE & Arena AA 1996; Ahmad SS et al. 2003; Panteleev MA et al. 2004; Ngo JC et al. 2008). At physiologic concentrations, the A2 subunit spontaneously dissociates, leading to loss of FVIIIa cofactor activity (Lollar P & Parker CG 1990).
Hemophilia A results from a broad spectrum of mutations that occur along the entire length of the F8 gene causing diverse molecular phenotypes that result in similar disease states (Peyvandi F et al. 2016). Together with missense mutations being the most common type of mutations in hemophilia A, a relatively frequent cause is ascribable to nonsense and splice site mutations, deletions/insertions and promoter mutations (Hakeos WH et al. 2002; Wei W et al. 2017; Jacquemin M et al. 2000; Amano K et al. 1998; Gilbert GE et al. 2012; Pahl S et al. 2014; Peyvandi F et al. 2016). In addition, the inversion of intron 1 or 22 in the F8 gene is responsible for approximately half of severely affected hemophilia A patients (Antonarakis SE et al. 1995). Although specific FVIII missense mutations correlate with defects including decreased secretion or stability and specific functional impairment of FVIII, the mechanisms of the majority of missense mutations are poorly understood (Hakeos WH et al. 2002; Wei W et al. 2017, 2018; Jacquemin M et al. 2000; Amano K et al. 1998; Gilbert GE et al. 2012; Pahl S et al. 2014). The Reactome module describes several molecular mechanisms underlying hemophilia A which include:(1) low-level secretion of defective FVIII molecule as a result of impaired FVIII folding and intracellular processing, (2) reduced ability of FVIII variants to bind to von Willebrand factor (VWF) that leads to instability of FVIII variants in the plasma, (3) abnormal interaction of defective FVIII with FIXa. Defects in FVIII activity may also result in potentially slowing down FVIII activation by thrombin or altering stability of activated FVIIIa.