Page 43 - 2021_02-Haematologica-web

Coagulation factor VII, hemostasis and thrombosis
Table 3. Overview of molecular genetics findings in factor VII deficiency. Molecular genetics of FVII deficiency
Genetic diagnosis
Mutation zygosity and disease severity
Mutation type frequency
Recombinant expression
Sanger sequencing of exons and splicing junctions in >1,000 individuals with FVII deficiency Next-generation sequencing in a few individuals
Phenotypic assays do not define specific genetic defects*
No highly frequent gene lesions
Large heterogeneity of genetic causes (n>300 different mutations) Geographical/ethnic clustering of identical-by-descent mutations (n>15) Several recurrent mutations in different populations
Severe deficiencies caused by compound heterozygous/homozygous mutations
Increased prevalence of homozygotes in genetic isolates/consanguineous marriages
Otherwise asymptomatic heterozygous variants associated with frequent FVII-lowering single nucleotide polymorphism in mild deficiencies
Missense>>Small deletions>Splicing>Nonsense>Large deletions (not reported in homozygosity)
Numerous variants expressed after site directed mutagenesis A few purified protein variants
Translational read through over premature termination codons Gain-of-function premature termination codon
*The asymptomatic Arg364Gln is detectable by using different thromboplastins59. Small deletions include also insertions and indels. FVII: factor VII.
Table 4. Management of factor VII deficiency using recombinant factor VII (synopsis of the STER study data).
Clinical Context
Spontaneous bleeding
Major surgery^
Minor surgery and invasive procedures
Treatments (n)
79
24
29
Treatment days Median, range
1 (1-14)
3.5 (1-16)
1 (1-4)
3/week or 2/week
Total dose of rFVIIa
Median, range (μg/kg of body weight)
60 (10-3,600)
31.9 (12-120)
20 (7.2-510)
30 (3 per week) 24 (3 per week)
Prophylaxis§ 31
18 (3/week)
13 (2/week)
Outcome
3 rebleeding 1 inhibitor
3 bleeding
(orthopaedic surgery) 1 inhibitor
Partly effective in one case in each of the two arms§
The data express data analysis, not recommendations; ^ Mariani et al. 2011; §Napolitano et al. 2013 (no statistical difference between arms). FVII: factor VII; rFVII: recombinant FVII; STER: Seven Treatment Evaluation Registry.
shortly after birth due to severe hemorrhage.5 Complete FVII deficiency causes perinatal mortality in “knockout” mice, in which the presence of a trace amount of FVII (0.7%) seems sufficient for survival with symptoms.61 Several cases with heterozygous combined FVII and FX deficiency, due to large deletions within the terminal end of chromosome 13, have been reported.62
Recombinant expression after site-directed mutagenesis has been performed for a number of mutations detected in patients, which permits one to investigate the molecular bases of the deficiency. Recombinant studies of the rare and potentially null homozygous nonsense mutations support the notion that gain-of-function and translational readthrough over premature stop codons may prevent truly null conditions and may contribute to the unexpect- edly variable bleeding phenotype in individuals homozy- gous for F7 nonsense changes.63 Whereas life-threatening symptoms have been associated with the p.Ser52(112)X, moderate bleeding was observed in p.Cys82 (132)X homozygotes. The Arg402(462)X change permits the secretion of a small amount of protein with gain-of-func- tion features, causing a mild phenotype paradoxically associated with a “null” mutation.64
Chimeric fluorescent FVII molecules have also been constructed to investigate intracellular processing of vari- ants, and chemical chaperones have been used to stimu-
late their secretion.54 The FVII protein has been purified and biochemically characterized for very few variants, among these the frequent Arg304(364)Gln50 and Ala294(354)Val55 changes, as well as the severe Gln160Arg33 and the activation sequence Val154(214)Gly65 changes. These laborious experiments provided strong support for the understanding of the mild or severe coag- ulation phenotypes in vivo and for FVII altered expression and properties.
The homozygous conditions for polymorphisms pre- dicting lower FVII levels might not be pathogenic but mimic Mendelian heterozygous FVII deficiency35 (Table 1), which frequently leads to the request for clotting defect diagnosis in pre-surgical screenings, and generates confu- sion in estimating the incidence of inherited FVII deficien- cy (see dedicated section). In light of their effects on low- ering FVII plasma levels, these variants might contribute to pathogenicity of co-inherited variants.52 As a matter of fact the frequencies of the FVII-lowering alleles were signifi- cantly higher51 in the FVII-deficient individuals tested than in controls, suggesting that the presence of these alleles may have increased, through increased clinical signifi- cance (Table 2), the likelihood of identifying “causative” F7 gene lesions. Genotyping of these frequent variants would be especially useful for the interpretation of mild and moderate FVII deficiency.
haematologica | 2021; 106(2)
357