Combined Factor V and Factor VIII Deficiency
Combined deficiency of Factor V (FV) and Factor VIII (FVIII) (F5F8D, Online Mendelian Inheritance in Man® [OMIM] # 227300) is an autosomal recessive bleeding disorder characterized by concomitantly low levels of the two coagulation factors. This condition was first described by Oeri et al. in 1954.1 The molecular mechanism of the association of the two factor deficiencies had not been understood until Nichols et al2, 3 discovered that the cause of the deficiency was associated with null mutations in the ERGIC-53 gene, now called LMAN1 gene (lectin mannose binding protein). LMAN1 encodes an endoplasmic reticulum (ER)–Golgi intermediate compartment (ERGIC) marker protein. Approximately 70% of affected patients have mutations in LMAN1.
In 2003, Zhang et al4 identified a second locus associated with the deficiency in about 15% of affected families with no mutation in LMAN1: the MCFD2 (multiple coagulation factor deficiency 2) gene encoding for a cofactor for LMAN1.5 Until now previous biochemical studies have failed to identify additional components of the LMAN1–MCFD2 receptor complex,5 supporting the idea that F5F8D might be limited to the LMAN1 and MCFD2 genes. 6
The LMAN1 and MCFD2 complex functions as a cargo receptor to transport FV and FVIII from the endoplasmic reticulum to the Golgi to be secreted. FV and FVIII are essential coagulation factors that circulate in plasma as precursors. These proteins exhibit cofactor activities in the activation of prothrombin by FXa, and in the activation of FX by FIXa, respectively, ultimately leading to clot formation.
At the end of the coagulation process the intravascular clots have to be dissolved and removed to avoid thrombosis. Inactivation of procoagulant factors is therefore needed. Inactivation of FVa and FVIIIa is accomplished by activated protein C in the presence of protein S and phospholipids through several proteolytic cleavages at distinct sites.7
LMAN1 is a 53-kDa type 1 transmembrane, nonglycosylated protein with homology to leguminous lectin proteins.8 It displays different oligomerization states—monomer, dimer, and hexamer—which have been implicated in its exit/retention within the ER. Efficient transport of coagulation FV and FVIII along the secretory pathway requires the integrity of their heavily glycosylated B domains and a functional LMAN1 protein.5, 9 LMAN1 is believed to bind correctly folded glycosylated cargo proteins, including FV and FVIII in the ER. It recruits the cargo for package into coat protein complex II (COPII)-coated vesicles and transports them first to the ERGIC and then to the Golgi.10
MCFD2 is a small (146 residues) soluble protein of 16 kDa with a signal sequence mediating translocation into the ER and two EF-hand motifs that may bind Ca2+ ions in the C-terminal region. MCFD2 forms a Ca2+-dependent 1:1 stoichiometric complex with LMAN1, which works as a cargo receptor for efficient ER–Golgi transfer of coagulation FV and FVIII during their secretion.4
In 2009, the three-dimensional structure of the complex between MCFD2 and LMAN1 was determined suggesting that MCFD2 is converted into the active form upon complex formation with LMAN1, thereby being able to capture polypeptide segments of FV and FVIII. The coagulation factors bind the LMAN1 oligomer in the ER, but are released upon arrival in acidic post-ER compartments because the sugar-binding of ERGIC-53 is pH-dependent.11
To date, more than 50 mutations in LMAN1 and MCFD2 genes have been described (http://www.isth.org/?MutationsRareBleedin ). Almost 70% of these mutations were located on the LMAN1. Mutations in LMAN1 and MCFD2 are associated with indistinguishable phenotypes.4 Zhang et al12 performed a genotype–phenotype analysis to evaluate whether mutations in the two genes are associated with differences in the FV and FVIII plasma levels. They found that the mean FV and FVIII levels in patients with MCFD2 mutations were significantly lower than the corresponding levels in patients with LMAN1 mutations.
Congenital F5F8D is estimated to be extremely rare (1:1,000,000) in the general population.13 However, this disorder was reported to be particularly prevalent among Middle Eastern Jewish and non-Jewish Iranians, where the incidence was estimated to reach approximately 1:1,00,000,14 probably due, at least in part, to the high incidence of consanguinity. Other cases were also reported from different countries of the world: Europe, Asia, Africa, and America.15
The last WFH 2010 global survey and the European Network of Rare Bleeding Disorders (EN-RBD) project seem to indicate that the worldwide prevalence of F5F8D patients is about 3% of the total number of patients affected by rare coagulation disorders (www.rbdd.eu ). This figure suggests that F5F8D is one of the rarest coagulation disorders. Nonetheless, F5F8D may be significantly underdiagnosed because of the often mild bleeding symptoms associated with this condition.