ACTIVATED PROTEIN C RESISTANCE
Cortese Hassett, Ph.D.
A. Bontempo, M.D.
phenomenon of activated protein C resistance (APCr) was first reported by
Dahlback (1) et al. in 1993 and refers to the ability to mount an effective
anticoagulant response. Clinically this results in an increased risk of
thrombosis. Most cases of APC resistance are associated with a single point
mutation in the factor V gene (Leiden mutation), which results in the
substitution of arginine at position 506 by glutamine.
Cleavage of this site by APC is necessary for exposure of the two
additional cleavage sites needed for inactivation.
The rate of inactivation of factor V Leiden (FVL) is therefore slower
than that of normal factor V.
In vivo this manifests as an 8 fold increased risk of thrombosis in
heterozygotes and a 50 to 100 fold increased risk of thrombosis in
of the FVL allele is the major cause of APCr and a well-documented risk
factor for venous thrombosis. APC
resistance testing is performed on citrated plasma (blue tops) preferably
away from the time of the acute event, when the patient is not on treatment
with anticoagulants. The first generation, original assay consists of a
standard APTT test performed in the absence and presence of commercially
available activated protein C. In
a normal patient, the APTT is prolonged in the presence of APC due to the
anticoagulant action of this protein. Samples
from patients with FVL will usually prolong the clotting time resulting in
“resistance to APC”. The
results are reported as a ratio of the APC-APTT/APTT.
Affected patients will have an abnormally low ratio compared to
controls, usually less than 2.0; levels elevated above normal currently have
no clinical significance.
generation, modification of the original assay is currently the method of
choice. Patient plasma is
diluted with factor V-deficient plasma. This limits variations in plasma
handling, minimizes the effects of oral anticoagulant therapy, and improves
the sensitivity and specificity for the Leiden allele to 100 percent, while
making it cost efficient. If a
patient has a normal result with this assay, one does not need to proceed
with costly DNA testing for the Leiden mutation.
penetrence of clinical symptoms, i.e., thrombosis, preeclampsia, placental
abruption, intrauterine growth retardation and stillbirth, is highly
variable in APC resistant individuals. Some never get thrombosis,
whereas others suffer from recurrent severe thrombotic events. Homozygosity
for the Leiden mutation gives a much higher risk of thrombosis than
heterozygosity. The risk of thrombosis is increased by the coexistence
of other genetic or acquired risk factors (4). Other genetic risk
factors for thrombosis include deficiencies of protein C, protein S, or
antithrombin III. Mutations in the genes for factor II (prothrombin)
and the homocysteine-related, methylene tetrahydrafolate reductase (MTHFR)
polymorphism may also increase thrombotic risk. The most common
acquired risk factors are the use of oral contraceptives, pregnancy, trauma,
surgery and the presence of antiphospholipid antibodies. It is therefore
important to consider all of these factors when determining the true risk of
thrombosis in a given patient with a positive APCr test.
with first time venous thrombosis and FVL mutation who resolve with standard
courses of anticoagulant therapy and have no other thrombotic risk factors
can have anticoagulant therapy stopped.
Should a second venous thrombosis occur, indefinite anticoagulant
therapy is recommended.
Patients with venous thrombosis, FVL and a second congenital or
acquired risk factor usually require indefinite anticoagulation.
Asymptomatic carriers of FVL must be prophylactically anticoagulated
for surgical procedures.
Anticoagulation for an asymptomatic pregnant carrier is currently not
As always treatment regiments may need to be individualized.
The factor V
Leiden mutation accounts for approximately 90% of patients with APCr.
Recent investigations have centered on the elucidation of other
causes of APCr. At least two
other genetic mutations, which contribute to an increased risk of
thrombosis, have been identified, both of which may lead to an abnormal test
for APCr (R2 allele and FV Cambridge; (5-6)).
In addition, studies are currently underway to determine whether a
reduced sensitivity for APC, not due to a genetic mutation leads to an
increased risk for venous thrombosis (7).
Ongoing investigation into these other causes of APCr will lead to an
increased understanding of yet more mechanisms underlying thromboembolic
Dahlback B., et. al. Familial thrombophilia due to a previously
unrecognized mechanism characterized by poor anticoagulant response to
activated protein C: Prediction
of a cofactor to activated protein C. Proc. Natl. Acad Sci. 1993: 90:
Rosendaal FR., et. al. High
risk of thrombosis in patients homozygous for factor V Leiden (activated
protein C resistance). Blood 1995; 85: 1504-1508.
Ridker PM., et. al. Mutation in the gene coding for coagulation
factor V and the risk of myocardial infarction, stroke, and venous
thrombosis in apparently healthy men. N Engl. J. Med. 1995; 332: 912-917.
Zoller B., et. al. Identification of the same factor V gene mutation
in 47 out of 50 thrombosis-prone families with inherited resistance to
activated protein C. J. Clin. Invest. 1994; 94: 2521-2524.
Bernardi, EM., et. al. A factor V genetic component differing from
factor V R506Q contributes to the activated protein C resistance phenotype.
Blood 1997; 90: 1552-1557.
Williamson, D., et. al. Factor V Cambridge: A new mutation (Arg306àThr)
associated with resistance to activated protein C. Blood 1998; 91:
deBisser, MCH., et. al., A reduced sensitivity for activated protein
C in the absence of factor V Leiden increases the risk of venous thrombosis.
Blood 1999; 93: 1271-1276.