MOLECULAR-BASES OF PSEUDO-HOMOZYGOUS APC RESISTANCE - THE COMPOUND HETEROZYGOSITY FOR FV R506Q AND A FV NULL MUTATION RESULTS IN THE EXCLUSIVE PRESENCE OF FV LEIDEN MOLECULES IN PLASMA

Pseudo-homozygous APC resistance, the condition resulting from compoun d heterozygosity for FV R506Q (FV Leiden) and quantitative FV deficien cy, provides a natural model to study the interaction between procoagu lant and anticoagulant defects. This paper reports a complete FV chara cterization of a pseudo-homozygous APC resistant thrombotic patient. T he expression of the patient's non-leiden gene was found to be severel y impaired both at the mRNA and protein levels. In particular, only FV Leiden molecules were detected in the patient's plasma by immunoblott ing, which accounts for the observed marked APC resistance. Analysis o f the FV cDNA obtained by reverse transcription of platelet RNA reveal ed that the mRNA of the non-Leiden gene was extremely reduced in amoun t. A PAC clone containing the whole FV gene was used to design primers for a complete FV exon scanning. A 2-bp insertion at nucleotide 3706 in the large exon 13 of the non-leiden gene, predicting a frame-shift and premature termination of protein synthesis, was identified as resp onsible for the FV defect. Failure to find any case of pseudo-homozygo us APC resistance in a large sample (6,804) of blood donors suggests t hat this condition is extremely rare among normal controls and that it s detection is favoured by the thrombotic risk that it may confer.