A Tyr346-->Cys substitution in the interdomain acidic region a1 of factor VIII in an individual with factor VIII:C assay discrepancy.
The interdomain acidic region a1 is a unique structural feature of coagulation factor VIII (FVIII) and may mediate the proteolytic activation of FVIII and the inactivation of FVIIIa. We report an individual with a Tyr346-->Cys substitution within region a1, who presented with a one-stage FVIII activity (FVIII:C) of 0.34 iu/ml (normal range 0.5-2.0) but normal two-stage FVIII:C and FVIII antigen values. In a factor Xa (FXa)-generation assay for FVIII in which the activation time with thrombin was varied, the variant plasma showed normal FVIII:C at both short and long activation times. However, at intermediate activation times the FXa generation of the variant plasma was less than that of normal pooled plasma. In a modified one-stage FVIII:C assay in which partially purified FVIII was activated with thrombin at low concentrations, the variant FVIII showed less activation than wild-type FVIII, although this defect corrected with increasing concentrations of thrombin. When partially purified variant FVIII was activated with a large molar excess of thrombin, the subsequent rate of decay of FVIII:C was greater for variant FVIII. The complex defects in activation and inactivation displayed by FVIII Tyr346-->Cys support the hypothesis that the a1 sequence is a key regulator of FVIII activity.
Haemostasis Research, MRC Clinical Sciences Centre, Faculty of Medicine at Imperial College, Hammersmith Hospital, Du Cane Road, London W12 0NN, UK.
Andrew D
AD
Mumford
Michael
M
Laffan
James
J
O'Donnell
John H
JH
McVey
Daniel J D
DJ
Johnson
Richard A
RA
Manning
Geoffrey
G
Kemball-Cook
D000368:Aged; D019943:Amino Acid Substitution / Q000235:genetics*; D015153:Blotting, Western; D005169:Factor VIII / Q000737:chemistry / Q000235:genetics*; D015944:Factor VIIIa / Q000235:genetics; D006467:Hemophilia A / Q000235:genetics; D006801:Humans; D008297:Male; D009154:Mutation / Q000235:genetics; D010314:Partial Thromboplastin Time; D017434:Protein Structure, Tertiary / Q000235:genetics*
D015944:Factor VIIIa; D005169:Factor VIII