Factor V Leiden: the venous thrombotic risk
in thrombophilic families
RUUD LENSEN, FRITS ROSENDAAL, JAN VANDENBROUCKE AND ROGIER BERTINA Department ofClinical Epidemiology and the Haemostasis and Thrombosis Research Centre, Leiden Universily Medical Centre, Leiden, The Netherlands
Received 30 December 1999; accepted for publication 4 May 2000
Summary. Factor V Leiden (FVL) leads to a sevenfold increased risk of venous thrombosis and is present in 50% of individuals from families referred because of unexplained familial thrombophilia. We assessed the association of FVL with venous thromboembolism (VTE) in 12 thrombophilic families of symptomatic probands with FVL in a retro-spective follow-up study. We screened 182 first- and second-degree relatives of the 12 unrelated propositi for the FVL mutation and the occurrence of VTE. The incidence rate of VTE in carriers of FVL (0-56%/year) was about six times the incidence for the Dutch population (0-1%/year). The incidence rate in non-carriers also appeared to be higher (0-15% per year). At the age of 50 years, the probability of not being affected by VTE was reduced to 75% for carriers and to 93% for non-carriers (P = 0-009). Identification of
carriers of FV Leiden may be worthwhile in young symptomatic individuals and their relatives with a strong positive family history of venous thromboembolism or a history of recurrent venous thrombosis who may be at risk (e.g. pregnancy, use of oral contraceptives). After adjust-ment for prothrombin G20210A (present in two families), even higher thrombotic incidence rates were found in carriers and non-carriers of FVL. This makes the presence of other unknown prothrombotic risk factors more probable in these families.
Keywords: venous thromboembolism, factor V Leiden, hypercoagulability, thrombophilia, activated protein C resistance.
Venous thromboembolism is a major cause of morbidity with an incidence of about one per 1000 per year (Nordström et al, 1992; National Medical Registration of the Foundation Information Centre for Health, 1996). Predisposing factors can be either genetic or environmental. Environmental risk factors include recent surgery, immobi-lization, oral contraceptives, pregnancy and puerperium (Nachman & Silverstein, 1993). Until 1993, a specific genetic defect was identifled in a maximum of 10-15% of affected subjects (Allaart & Briet, 1994). These defects included deficiencies of the main Inhibitors of the clotting System: protein C, protein S and anti-thrombin (Hirsh et al, 1986). Familial resistance to activated protein C (APC-resistance), first described in 1990 (Dahlbäck et al, 1990), is the most frequent genetic risk factor for thrombosis (Griffin et al, 1993; Koster et al, 1993; Svensson & Dahlbäck, 1994; Dahlbäck, 1995; Lane et al, 1996). In the large majorily of cases, APC resistance is associated with a single point mutation (1691 G to A Substitution) in the factor V gene Correspondence: Dr Ruud Lensen, Department of Clinical Epide-miology, Leiden University Medical Centre, Building l CO-P45, PO Box 9600, 2300 RC Leiden, The Netherlands. E-mail: ruudle nsen@hotmail.com
that predicts the synthesis of a factor V molecule (Factor V Leiden) that is not properly inactivated by activated protein C (APC) (Bertina et al, 1994; Greengard et al, 1994; Voorberg et al, 1994). In Caucasians, this mutation is present in about 5% of healthy individuals (Rees et al, 1995; Zivelin et al, 1997). Factor V Leiden leads to a sevenfold increased risk of venous thrombosis (Rosendaal et al, 1995). It is present in 20% of unselected, consecutive patients with deep vein thrombosis and in 50% of individuals from families referred because of unexplained familial thrombo-philia (Bertina ei al, 1994; Zöller et al, 1994; Lane et al, 1996).
We demonstrated an earlier age of onset in a series of selected patients from thrombophilic families with factor V Leiden than in a panel of unselected patients with a first venous thrombosis who turned out to be carriers of the factor V Leiden mutation (Lensen et al, 1996). This suggests a higher thrombotic tendency in members from selected families than in consecutively diagnosed patients, even if both carry the same or a similar molecular defect. Important to clinicians is the question of what prophylactic measures are advisable for patients and their relatives in these selected thrombophilic families with factor V Leiden. Before this question can be answered, the risk of thrombosis
940 R. Lensen et al
in these individuals needs lo be assessed. Therefore, we studied 12 thrombophilic families with the factor V Leiden mutation in which, next to the proband, at least two persons had experienced thrombosis.
PATIENTS AND METHODS
Paüents and families. The 12 probands originate from a larger panel of 28 patients who were referred to our centre for diagnostic work-up for venous thrombophilia, i.e. patients with a positive family history of venous thrombosis (at least two symptomatic relatives in addition to the proband) and who did not have deficiencies of protein C, protein S or anti-thrombin. These 28 patients were screened for the presence of the factor V Leiden mutation which was detected in 12 patients. We invited the siblings, parents and children of these 12 probands, äs well äs uncles and aunts of the affected parental side, and, if these were carriers, their children (first cousins of the proband) to participate. Family members under 15 years of age were excluded for practical purposes. Of the 12 probands, 182 first- and second-degree family members (93%) participated in the study and 12 did not, three members because they lived abroad and nine for reasons unknown. In all cases of couples with factor V Leiden-positive children, we verified from which parent the mutant allele was transmitted by testing the other parent. In 30 of these cases, none of the other parents who had married into these pedigrees, carried the factor V Leiden allele.
The 12 probands and their 182 family members were seen by the same physician for venepuncture and interview; risk factor Status was assessed by the Interviewer before carrier Status was determined (except for the proband). A standardized history was taken of the occurrence of deep venous thrombosis (excluding superficial thrombophlebitis) and pulmonary embolism, including the age at each episode, the method of diagnosis, treatment and predispos-ing circumstances. The history included occurrence of risk factors in the past: surgery, hospital admission, immobiliza-tion (period exceeding two weeks), plaster casts, malignan-cies, pregnanmalignan-cies, postpartum periods and use of oral contraceptives, hormonal replacement therapy or oral anticoagulants. When a patient reported to have experi-enced a thrombotic event, clinical information was gathered for confirmation from the physician responsible for the treatment of the patient. We counted äs venous thrombotic events only those that had been diagnosed by physicians (80% of deep venous thrombotic events were objectively diagnosed, i.e. by ultrasound, venography, pulmonary angiography or lung Ventilation perfusion scan; 20% were not, mainly because of events occurring before objective diagnostic techniques were available).
Laboratory methods. Blood samples were collected from the antecubital vein into 0-106 mmol/1 trisodium citrate. Plasma was prepared by centrifugation for 10 min at 2000g at room temperature and stored at — 70°C in 1-5 ml volumes. The mutation of the factor V gene was detected with the use of amplification and restriction-enzyme digestion. Normalized APC-sensitivity ratios were
measured äs described previously (de Ronde & Bertina, 1994). APC resistance was defined äs normalized APC-sensitivity ratio of <0·84; carriers of factor V Leiden usually have a normalized APC-sensitivity ratio of <0·71 (Bertina et al, 1994). We measured normalized APC-sensitivity ratios in all relatives who were not treated with oral anticoagulants (n = 166) and found a clear association of factor V Leiden and a normalized APC-sensitivity ratio of <0·71. We screened all individuals for deficiencies of protein C, protein S, anti-thrombin and the 20210 G to A prothrombin variant. These laboratory measurements were carried out by one laboratory technician who had no knowledge of the medical history.
Statistics. We analysed the lifetime risk of thrombosis by Standard life-table techniques (Kaplan-Meier method). To compare the two curves we used the logrank test, resulting in a chi-square distribution with one degree of freedom. This analysis of lifetime risk of thrombosis based on retrospective data is valid under the assumption that there is no excess mortality in these families. Absence of excess mortality for factor V Leiden has been reported by Mari et al (1996) and Hille et al (1997), and is analogous to other reports on the absence of excess mortality in thrombophilic families with deficiencies of anti-thrombin or protein C (Rosendaal et al, 1991; van Boven ei al, 1994; Allaart et al, 1995). We have assessed occurrences of superficial thrombophlebitis (diag-nosed by a physician), but we performed further analyses without this type of venous thrombosis (because of difficulties in the subjective diagnosis and for reasons of comparability with other published studies). When neces-sary, we also excluded second-degree relatives for compar-ability with other studies.
We compared the incidence rates of first venous thrombotic events in relatives with factor V Leiden (carriers) and without factor V Leiden (non-carriers) of the 12 probands. We calculated these incidence rates by counting patient-years of observation (follow-up Urne) and dividing the number of events in each group by the total number of patient-years of all the individuals in the group. Follow-up for symptomatic individuals started at birth and ended at the date of the first venous thrombosis. Follow-up for asymptomatic individuals started at birth and ended at the date of the interview. For all participants, follow-up was complete. Incidence rates of recurrent venous thrombotic events were computed äs the number of recurrences divided by the total of follow-up time in symptomatic relatives between the first event and the date of the recurrence or the date of the interview, whichever came first.
We used Poisson regression analysis to study the role of the putative risk factors: age (seven 10-year age groups ranging from age 10-19 years to 70-79 years), sex, heterozygosity for factor V Leiden, surgery (exposition window: l year per Operation), immobilizalion longer than 2 weeks (including hospital admissions and plaster casts; exposition window: l year), obesity (body mass Index > 25 kg/m2), smoking, pregnancy/post partum period
families, we adjusted for this coagulation defect in order to assess more precisely the thrombogenicity of factor V Leiden.
RESULTS Clinical data
We studied 182 relatives of 12 probands with factor V Leiden. Of these 182 family members, 91 were heterozygous for the factor V Leiden mutation (44 men and 47 women; median age 39 years). One relative was homozygous (62-year-old man) and 90 relatives were non-carriers (46 men and 44 women, median age 36 years).
Twenty-five (14%) of the 182 relatives were symptomatic. Of the 92 factor V Leiden carriers (probands excluded), 20 (22%) had experienced one or more venous thrombotic events in their lifetime (first event was a pulmonary embolism in four carriers and a venous thrombosis in a lower extremity in 16 carriers). Five of the 90 non-carriers (6%) experienced a venous thrombotic event (first event was a pulmonary embolism in three non-carriers and a deep venous thrombosis in a lower extremity in two non-carriers). Among carriers, the incidence of venous throm-bosis was 0-56%/year (20 events on 3585 person-years), which was 3-7-fold (95% CI 1-4-10-0) higher than the incidence of 0-15%/year among non-carriers (five events on 3354 person-years). First event was a superficial throm-bophlebitis in nine carriers and in one non-carrier.
We calculated the thrombosis-free survival by Kaplan-Meier analysis (Fig 1). At the age of 50 years, the probability of being free of deep venous thromboembolism was reduced to 75% for heterozygotes; for non-carriers, 93% were still free of thrombosis at this age (P = 0-009). These significant differences could not be explained by different frequencies of other risk factors in the two groups. Overall relative risk of carriers vs. non-carriers, äs calculated by the Cox proportional hazards model, was 3-4 (95% CI 1-3-9-2). Interestingly, the symptomatic fraction of first-degree family members with factor V Leiden was significantly higher than the symptomatic fraction of second-degree family members with factor V Leiden. Among 36 first-degree relatives carrying factor V Leiden, 17 (47%) had experienced venous thrombosis, äs compared with three (5%) among 56 second-degree relatives with factor V Leiden. This is visualized in Fig 2, which shows the thrombosis-free curves for the first-degree relatives only.
The incidence of first venous thrombosis in the 92 carriers of factor V Leiden (probands excluded) was 0-2%/ year up to age 24 years (vs. 0-05%/year in non-carriers). Between 25 years and 45 years of age the incidence was 1-1%/year (vs. 0-3%/year in non-carriers) and over 45 years of age the incidence was 1-2%/year (vs. 0-3%/ year in non-carriers) (Table I).
There had been 18 recurrent events in ten of the 20 symptomatic carriers; none of them was taking oral anticoagulants at the time of the second event. One symptomatic non-carrier experienced four recurrent events despite treatment with oral anticoagulants. The incidence of first recurrence was 3-7%/year in carriers, which was 3-7
] non camers 0-5
l
H 0-0 10 20 30 40 50 60 70 Age (years)Fig 1. Venous Ihrombosis-free survival curves in 92 carriers (lower line, probands excluded) and 90 non-carners (upper line). The difference in curves was significant (logrank test: P = 0-009). times higher (95% CI: 0-5-29-9) than the incidence of recurrence in non-carriers (1-0%/year).
The median age at first thrombosis for the 20 sympto-matic relatives with the factor V Leiden mutation was 30 years (ränge, 16-75 years). This median age of onset was equal in both sexes. The median age of onset for the five non-carriers was 28 years (ränge, 23-54 years).
Effect of other risk factors
In none of the families was deficiency of protein C, protein S, or anti-thrombin found. However, two of the 12 propositi (16-7%) were heterozygous carriers of the recently described 20210 G to A prothrombin variant, i.e. they had a second defect in combination with factor V Leiden (Poort et al, 1996). After exclusion of these two families with the prothrombin 20210A variant in addition to factor V Leiden, the thrombotic incidence rate in factor V Leiden carriers of
non carriers
Age (years)
Fig 2. Venous thrombosis-free survival curves in 36 carriers (lower line, probands excluded) and 31 non-carriers (upper line) after exclusion of second-degree relatives. The difference in curves was significant (logrank test: P = 0-002).
942 R. Lensen et al
Table I. Incidence rates of deep venous thromboembohsm per year in carriers and non carners per age categoiy in thrombophilic lamihes with factor V Leiden
Age < 24 24-45 > 45 Total Factor V Leiden + Person years (n) 2305 959 321 3585 Events (n) 5 11 4 20 Incidence (95% Clf 0 2 (0-0 4) 1 1 (0 5-1 8) 1 2 (0-2 5) 0 56 (0 3-0 8) Factor V Leiden -Person years (n) 2159 874 321 3354 Events (n) 1 3 1 5 Incidence (95% CI) 0 05 (0-0 1) 0 3 (0-0 7) 0 3 (0-0 9) 0 15 (0-0 3)
1 Incidence rates are in percentage per yeais
the remaming ten famdies was 0 58%/year and was 0 17%/ year m non-carners (relative risk 3 4, 95% CI l 2-10 1) Cox regression analysis revealed for the 12 famihes to have a risk ratio of 3 5 (95% CI l 3-9 3) for factor V Leiden and a risk ratio of 2 2 (95% CI 0 5-9 2) for the prothrombin 20210A vanant (multivanate analysis)
The distnbuüon of environmental risk factors in carners and non-carners was very similar with the exception of pregnancies the 47 female carners expenenced 100 pregnancies, while the 44 female non-carners expenenced 50 pregnancies Precipitatmg risk factors dunng or shortly before the first event were present m 15 (75%) of the 20 carners and m three (60%) of the five non-carners with thrombosis (Table II) Four of the 20 carners with thrombosis expenenced their first episode post partum Other risk iactors in carners were surgery, use of oral contraceptives, pregnancy and immobihzation, m only five carners (25%) was the first episode apparently spontaneous Risk factors in the five non-carners with thrombosis were surgery and pregnancy, two non-carriers expenenced a spontaneous thrombotic event All carners expenenced, in total, 244 episodes of surgery (n = 84), immobilization (n = 60) or pregnancy (mcludmg puerpenum, n = 100) and 12 of these episodes were comphcated by a venous thrombotic event (4 9%), which was the case m l 6% of the non-carriers (185 episodes were comphcated by three thrombotic events) Eleven of the 18 recurrent events (61%) m carners were apparently spontaneous
In Table III, rate ratlos are presented when the mam risk
factors were entered in a Poisson regression model In this multivanate-adjusted model the strengest environmental risk factor was pregnancy and puerpenum (rate ratio 37 7), followed by surgery (rate ratio 14 1), immobilization (rate ratio 8 9) and use of oral contraceptives (rate ratio 4 1) We found a highly mcreased risk (mteraction under a multi-phcaüve model) when factor V Leiden was combmed with pregnancy and puerpenum, and only a moderate mcreased risk (mteraction under an additive model) when factor V Leiden was combmed with surgery As the thrombotic risk of surgery was also high in non-carners, the jomt risk estimates are high for all combmaüons of risk iactors with factor V Leiden, with annual rates of 2-7%
DISCUSSION
We periormed a study on 182 members of 12 referred iamihes with thrombophiha At the age of 50 years, 25% of carners had expenenced at least one venous thrombotic event (vs 7% m non-carners) The mcidence rate of deep vein thrombosis (superficial thrombophlebitis excluded) in carners is six times higher than the reported mcidence rate for the general Dutch population (about 0 11%/year) (Nordstrom et al, 1992, Briet et al, 1994, National Medical Registration of the Foundation Information Centre for Health Care, 1996) This mcidence rate oi 0 11%/year for the general population is probably an overesümation äs recurrences are mcluded Interesüngly m the famihes we studied the mcidence among non-carners (0 17%/year,
Table II. Potential risk factors associated with the first venous thrombosis
Potential risk factor Symptomaüc cameis (n = 20) Symptomaüc non cameis (n = 5) Puerpenum Pregnancy Surgery Oral contraceptives Immobilization Total 4 3 3 3 2 15 (75%) 1 0 2 0 0 3 (60%)
Probands excluded There were five spontaneous venous thrombotic events among carners and two among non carriers
Table III Adjusted rate idüos (95 /o CI) for thc mam potential nsk lactors on venous thrombotic events for the 1S2 relatives
Potential nsk factor Rate latio (95/ CI) Tactor V Leiden
Age
Picgnancy & puerpenum Surgeiy
Contraceplive pill use Immobili/ation 30(1 l 8 2 ) l 6 (l 3-2 1) 37 7 (13 9-101 7) 14 l (4 S 41 8) 4 l (l 1-15 1) 8 9 (l 9-41 1)
The rate ratios aie the risks of thiombosis m the piesence of the nsk factor ai> compaied to its absence adjusted foi the othei iactois m the model All vanables aie coded äs ycb/no age u, coded m 10 year mlervals
adjusted for the prothrombm G20210A allele) dlso exceeded this population figure This mcreased mcidence of venous thrombosis m non camers is probably caused by the selection of famihes that die very prone to thrombosis m which the presence of more than one defect may be suspected (Lane et al 1996 Lensen et al 1996 Rosendaal 1997) We also thmk that the significant higher thrombotic mcidence found m first-degree relatives äs compared with second degree relatives (which is charactenstic m polygenic mhentance) gives suppoit to the hypothesized presence ot more co existing genetic defects and justifies furthei research work mto this matter
Previously in a population-based case-control study on venous thrombosis (Leiden Thrombophilia Study LETS Rosendaal ei al 1995) among 471 unselected consecutive patients with a flrst objectively confirmed deep vem thrombosis (all younger than 70 years) and 471 unrelated age- and sex matched controls we found an odds tatio for heterozygosity of 7 3 The lower nsk latio of 3 7 for heterozygosity we found in thrombophilic famihes with factor V Leiden may be the result of the high mcidence of venous thrombosis m non carriers and the difference in study design
Considenng Tables I and II we thmk that the mimmally higher thrombotic nsk found in carriers older than 45 years äs compared with the 25-45 year gioup is caused by the occurrence of many thrombotic events dunng pregnancy and pueipenum or the use of oial contraceptives in the 25-45 years group demonstrating their important role äs a thiombogenic nsk factors in this age group
Recenlly Middeldorp U al (1998) presented their results of a lamily study on factor V Leiden They investigated first-degree family members of consecutive patients with venous thromboembolism äs opposed to i eierred high-iisk famihes m oui study They and Zoller U al (1994) found that 97 /o öl cariiers were still iree of thrombosis at age 30 years We found lhat 28% of carneis m thrombophilic tarmhes had expenenced thrombosis at this age (Fig 2) The mcidence rate for first-degree heterozygous family members younger than 61 yeais of age was 0 9%/yeai moie lhan two times the mcidence rate found by Middeldorp a al (1998) (0 4 X,/ year m both calculaüons follow up started at the age of
15 years) These compansons emphasize that the estimated nsk of thrombosis for carneis of factor V Leiden depends on the mclusion critena and is much higher m selected and refeired famihes than in lelatives of consecutive unselected patients This is also illustrated by comparmg our data with data from a study lecently performed by Simiom et al (1999) They included relatives of unselected patients and found an annual thrombotic mcidence of 0 28% m carriers and 0 09% m non-carriers (rate ratio 2 8 95% CI l l 8 6) (m first degiee relatives we found l 7% m carriers and 0 3% in non camers resultmg in a doubled late ratio of 5 7 95% CI 2 0 26 8) We recently found thrombotic incidences in first-degree lelatives of 47 unselected consecutive patients with a first venous thrombosis that were five times Iowei m camers and three times lower m non-carriers than the thrombotic incidences we found m the thrombophilic lamilies Only one of these 47 lamilies (2%) met the mclusion critena used in the current study (more than one symptomatic lelative besides the proband) and 8% of the first degiee relatives had expenenced a deep vem thiombosis (äs Middeldorp et al 1998 found in their study population Lensen et al 2000) vs 47% m the current study This illustrates the role of a strong family history äs a piobable nsk factor for venous thrombosis
The mclusion of thrombotic events that were not objectively diagnosed (20%) could have led to an over-estimation of the thrombotic mcidence As the possibility of misclassification conceins only this fraction of all events and äs the medical histoiy m these patients strongly mdicated a deep venous thrombosis this cannot have affected our figures matenally (after exclusion of the 20% who were not objectively diagnosed we found similar nsk estimates with wider confldence intervals)
Two years and 6 yeais after the fnst event 10% (two out of 20) and 25% (five out of 20) of the symptomatic carriers had expenenced a second event (superficial thiombophlebi tis excluded) These high recuuence iisks m factor V Leiden cairiers are similar to the results of previous studies from the United States and Italy (Ridker U al 1995 Simiom et al
1997 Prandom et al 1998) The protective role of oral anticoagulants is well illustrated m these 20 symptomatic camers None of the eight camers who received oral anticoagulant tieatment expenenced a recurrence while ten of the 12 carriers who received no oral anticoagulant treatment expenenced a recurrence
Remaikably two (16 7%) of the propositi had a combmed defect l e factor V Leiden and the prothiombin G20210A vanant This mutation has a population prevalence of about 2°/i and therefore it seems probable that the combination of defects led to these patients becommg index patients (Poort et al 1996) The elevated mcidence m non-carriers could not be explamed by the presence of the prothrombin G20210A vanant none of the relatives carrying only the prothiombin G20210A vanant had expenenced a venous thiombosis and the thrombotic mcidence rate in non camers was even higher after exclusion of the two famihes with the piothiombm G20210A vanant
944 R. Lensen et al
famibes with heterozygous protein C deficiency (who were also known to our centre) for thrombophiha work-up. Usmg these data, we iound that the median of thrombotic mcidence and recurrence rates were very similar for both defects, which does not lend support to different thrombotic nsks for protein C deficiency and factor V Leiden.
In conclusion, m climcal practice special attention should be paid to young symptomatic mdividuals and their relatives with a strong positive family history of venous thromboem-bolism or a history of recurrent venous thrombosis who are at risk, especially women who would like to use oral contraceptives or who mtend to become pregnant. Identi-fication of carners of factor V Leiden may be worthwhile m these persons m order to discourage contraceptive pill use among carners and to protect carners dunng pregnancies agamst venous thrombosis However, data that such a policy would be beneficial are lacking In addition, our data are based on a study among selected families with thrombophiha and should not be applied to screenmg of other asymptomatic mdividuals (i.e. prior to prescnbmg oral contraceptives). Finally, our data provide no grounds to treat patients with factor V Leiden differently from patients with heterozygous protein C deficiency However, consider-ing the fact that the prevalence of factor V Leiden is at least tenfold higher than the prevalence of all other known genetic deficiencies, further prospective studies will be needed to evaluate clinical policy.
ACKNOWLEDGMENTS
This study was supported by a grant from the Netherlands Heart Association (grant no 95 026).
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