A focus on dominant negative variants in a series of 170 heterozygous FXI-deficient patients

INTRODUCTION

Dominant-negative effects have been described for 10 F11 variants in the literature.

AIM

The current study aimed at identifying putative dominant-negative F11 variants.

MATERIAL AND METHODS

This research consisted in a retrospective analysis of routine laboratory data.

RESULTS

In a series of 170 patients with moderate/mild factor XI (FXI) deficiencies, we identified heterozygous carriers of previously reported dominant-negative variants (p.Ser243Phe, p.Cys416Tyr, and p.Gly418Val) with FXI activities inconsistent with a dominant-negative effect. Our findings also do not support a dominant-negative effect of p.Gly418Ala. We also identified a set of patients carrying heterozygous variants, among which five out of 11 are novel, with FXI activities suggesting a dominant-negative effect (p.His53Tyr, p.Cys110Gly, p.Cys140Tyr, p.Glu245Lys, p.Trp246Cys, p.Glu315Lys, p.Ile421Thr, p.Trp425Cys, p.Glu565Lys, p.Thr593Met, and p.Trp617Ter). However, for all but two of these variants, individuals with close to half normal FXI coagulant activity (FXI:C) were identified, indicating an inconstant dominant effect.

CONCLUSION

Our data show that for some F11 variants recognized has having dominant-negative effects, such effects actually do not occur in many individuals. The present data suggest that for these patients, the intracellular quality control mechanisms eliminate the variant monomeric polypeptide before homodimer assembly, thereby allowing only the wild-type homodimer to assemble and resulting in half normal activities. In contrast, in patients with markedly decreased activities, some mutant polypeptides might escape this first quality control. In turn, assembly of heterodimeric molecules as well as mutant homodimers would result in activities closer to 1:4 of FXI:C normal range.

The natural occurrence of human fibrinogen variants disrupting inter-chain disulfide bonds (A{alpha}Cys36Gly, A{alpha}Cys36Arg and A{alpha}Cys45Tyr) confirms the role of N-terminal A{alpha} disulfide bonds in protein assembly and secretion

Analyses of site-directed fibrinogen mutants expressed in several recombinant models have previously shown that both inter- and intra-chain disulfide bonds are critical for fibrinogen assembly and secretion. Four naturally occurring mutations on AαCys36 and AαCys45 residues are reported here to be associated with decreased fibrinogen levels. This confirms the main role of the AαCys36-BβCys65 and AαCys45-γCys23 disulfide bonds in reaching a normal fibrinogen plasma level. Decreased coagulant/antigen ratios indicate abnormal species secretion in heterozygous subjects which varies between individuals. However, in contrast to overexpression in experimental models, disruption of the AαCys36-BβCys65 disulfide bond did not result in the appearance of Aα-Bβ-γ moieties in vivo. A 188 kDa molecule reacting only with anti Aα and anti Bβ chains was found in the plasma of the AαCys45Tyr variant. Heterozygous carriers of Aα chain mutations usually have normal fibrinogen levels, in contrast to the AαCys36Gly, AαCys36Arg and AαCys45Tyr variants that are shown here to cause hypofibrinogenemia.

Identification of five novel mutations in the factor XI gene (F11) of patients with factor XI deficiency

Factor XI (FXI) deficiency is an inherited autosomal recessive disorder associated with bleeding of variable severity. However, many cases of dominant disease transmission have been recently described. This disorder is rare in the general population, whereas it is commonly found in individuals of Ashkenazi Jewish ancestry. This study reports the molecular genetic analysis of FXI deficiencies in 11 unrelated families of different origin. Five novel mutations have been identified. Severe FXI deficiency of two unrelated patients resulted from two novel mutations: one deletion (960-961delGT) in exon 9 predicting a frameshift, and a Ser-4Leu mutation located in the signal peptide. In addition, three novel missense mutations associated with partial FXI deficiency have been identified: Cys122Tyr, Glu297Lys and Glu579Lys.

Factor XI deficiency: identification of six novel missense mutations (P23L, P69T, C92G, E243D, W497C and E547K)

Factor XI (FXI) deficiency is a rare coagulation disorder associated with bleeding of variable severity but without a clear relationship between bleeding and FXI levels. This study reports the molecular genetic analysis of FXI deficiencies in thirteen patients. Six novel missense mutations were identified: P23L, P69T, C92G, E243D, W497C and E547K.

Molecular basis of severe factor XI deficiency in seven families from the west of France. Seven novel mutations, including an ancient Q88X mutation

Inherited factor (F)XI deficiency is a rare disorder in the general population, though it is commonly found in individuals of Ashkenazi Jewish ancestry. In particular, two mutations--a stop mutation (type II) and a missense mutation (type III)--which are responsible for FXI deficiency, predominate. The bleeding tendency associated with plasma FXI deficiency in patients is variable, with approximately 50% of patients exhibiting excessive post-traumatic or postsurgical bleeding. In this study, we identified the molecular basis of FXI deficiency in 10 patients belonging to six unrelated families of the Nantes area in France and one family of Lebanese origin. As in Ashkenazi Jewish or in French Basque patients, we have identified a new ancient mutation in exon 4 resulting in Q88X, specific to patients from Nantes, that can result in a severely truncated polypeptide. Homozygous Q88X was found in a severely affected patient with an inhibitor to FXI and in three other unrelated families, either as homozygous, heterozygous or compound heterozygous states. Other identified mutations are two nonsense mutations in the FXI gene, in exon 7 and 15, resulting in R210X and C581X, respectively, which were identified in three families. A novel insertion in exon 3 (nucleotide 137 + G), which causes a stop codon, was characterized. Finally, sequence analysis of all 15 exons of the FXI gene revealed three missense mutations resulting in G336R and G350A (exon 10) and T575M (exon 15). Two mutations (T575M and G350A) with discrepant antigen and functional values are particularly interesting because most of the described mutations are associated with the absence of secreted protein.