Amelioration of the severity of heparin-binding antithrombin mutations by posttranslational mosaicism: a post-translational mosaicism for heparin binding mutations

The balance between procoagulant and anticoagulant factors protects organisms against bleeding and thrombotic disorders. Thus, antithrombin deficiency increases the risk of thrombosis and complete quantitative deficiency causes intra-uterine lethality. Only homozygous patients for the L99F and R47C mutations are viable. These mutations do not modify the folding or secretion of the protein, but abolish the glycosaminoglycan-induced activation of antithrombin by affecting the heparin binding domain. We speculated that natural β-glycoform of antithrombin might compensate the effect of heparin binding mutations. We purified α and βantithrombin glycoforms from plasma of two homozygous L99F patients. Heparin affinity chromatography and intrinsic fluorescence kinetic analyses demonstrated that the reduced heparin affinity of the α-L99F (KD:107.9±3 nM) was restored in the β-L99F glycoform (KD:53.9±5 nM) to values close to the α-wild type (KD: 43.9±0.4 nM). Accordingly, β-L99F glycoform was fully activated by heparin. Similar results were observed for recombinant R47C and P41L, other heparin binding mutations. In conclusion, we identified a new type of mosaicism associated with mutations causing heparin binding defect on antithrombin. The presence of a fully functional β-glycoform together with the progressive activity retained by these variants contributes to explain the viability of homozygous and the milder thrombotic risk of heterozygous patients.