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B. Paulsen et al.
fibrinogen gamma gene (FGG) located on chromosome 4, has two isoforms, gA and g’. In the Leiden Thrombophilia Study, the FGG rs2066865 single nucleotide polymorphism (SNP) was first proposed as a risk factor for VTE by reducing fibrinogen g’ levels.14 Several later genotyping15,16 and genome-wide association studies (GWAS)17,18 confirmed an association between rs2066865 and VTE risk, whereas two cohort studies found no significant association.19,20 In a recent meta- analysis including seven studies, rs2066865 was associated with an increased risk of VTE (OR 1.61, 95% CI: 1.34- 1.93).21
The majority of the genetic studies have excluded indi- viduals with cancer-related thrombosis. However, as pro- thrombotic genotypes are fixed, and not influenced by dis- ease, interventions and complications, they may be attrac- tive candidates as biomarkers of VTE risk in cancer patients. Recent studies have suggested that interactions between cancer and other prothrombotic genotypes (fac- tor V variants rs6025 and rs4524 and prothrombin G20210A) have synergistic effects on the risk of VTE.22-25 To the best of our knowledge, no study has investigated the impact of rs2066865 on the risk of VTE in cancer patients. Therefore, we aimed to investigate the joint effect of rs2066865 and active cancer on the absolute and relative risks of VTE in a population-based case-cohort.
Methods
Study population
The Tromsø Study is a single-center population-based cohort, following residents of the municipality of Tromsø, Norway, with repeated health surveys. The case-cohort was derived from the fourth survey (Tromsø 4), which included 27,158 participants aged 25-97 years. A detailed cohort profile of the Tromsø study has been published previously.26 The study was approved by the Regional Committee for Medical and Health Research Ethics in Northern Norway, and all participants provided informed written consent to participation. From enrolment in Tromsø 4 (1994/95), subjects were followed until December 31, 2012. Detailed infor- mation regarding identification and validation of VTE-events are described in the Online Supplementary Material and Methods.
In total, 710 participants developed VTE during follow-up. Of these, 26 did not have blood samples available or of sufficient quality for DNA analyses. The remaining 684 subjects were included as the cases in our study. A subcohort (n=3,931) was composed by randomly sampling individuals from Tromsø 4 weighted for the age distribution of the cases in 5-year age-groups. Due to the nature of the case-cohort design, where each partici- pant has the same probability of sampling, 72 of the cases were also in the subcohort. Subjects with a history of cancer prior to inclusion (n=232) and subjects with missing information on rs2066865 (n=9) were excluded from the analysis. The final case- cohort consisted of 4,374 subjects, with 640 cases and 3,734 in the subcohort. A flow chart of the case-cohort is displayed in Figure 1.
Baseline measurements and genotyping
Baseline measurements and genotyping methods are described in the Online Supplementary Materials and Methods.
Cancer exposure
Cancer assessment is described in the Online Supplementary Materials and Methods. Previous studies have shown a strong tem- poral relation between cancer diagnosis and incident VTE, and up
to 50 % of cancer-related VTE events presents within a 2.5-year interval (from six months preceding the cancer diagnosis until 2 years following the cancer diagnosis).27,28 Therefore, a VTE was defined as related to active cancer if it occurred within this time period.
Subjects who survived the active cancer period without a VTE were censored at the end of the active cancer period (i.e. 2 years after cancer was diagnosed). The censoring was performed because information regarding remission and relapse of cancer was unavailable, and extension of the observation period of cancer could result in the dilution of the estimates due to inclusion of VTE cases not necessarily caused by cancer. This approach result- ed in censoring of 14 VTE cases that occurred after the active can- cer period. Thus, 626 VTE cases were included in the final analy- ses.
Statistical analysis
Statistical analyses were performed using STATA version 15.0 (Stata Corporation LP, College Station, TX, USA). Cox proportion- al hazards regression models were used to obtain age- and sex- adjusted HR with 95% CI for VTE across categories of cancer sta- tus (no cancer/active cancer) and FGG risk alleles. Cancer was assessed as a time-dependent covariate in the model. Subjects who developed cancer contributed person-time as unexposed from the inclusion date until six months prior a cancer diagnosis, and thereafter contributed person-time in the active cancer group as exposed. Absolute incidence rates (IR) were calculated based on person-time from the original cohort (n=27,128). To calculate joint effects conferred by active cancer and FGG risk alleles, subjects with no cancer and no risk alleles were used as the reference group in the Cox model. Based on the total active cancer person-time at risk derived from the source cohort, 1-Kaplan-Meier curves were used to estimate the cumulative incidence of VTE in subjects with active cancer according to the presence of FGG risk alleles. Methods for assessing synergism between FGG and active cancer on the risk of VTE are described in detail in the Online Supplementary Materials and Methods.
Results
The mean follow-up of the case-cohort was 12.6 years. In total, 854 subjects had active cancer, of which 167 expe- rienced an incident VTE. The baseline characteristics of
Table 1. Baseline characteristics in the entire case-cohort and in the active cancer group.
Subjects (n)
Age (years)
Sex (males)
BMI (kg/m2)
Daily smoking
WBC count (109/L) Platelet count (109/L) rs2066865*
1 risk allele
2 risk alleles
Entire case-cohort
4374
58 ± 13
47.0 (2,048)
26.0 ± 4
34.5 (1,464)
7.1 ± 1.8
251 ± 60
0.26
1,723
289
Active cancer
854
62 ± 10
53.0 (456)
26.0 ± 4
43.5 (364)
7.2 ± 1.8
250 ± 58
0.26
334
51
Values are numbers or percentages with numbers in parenthesis or means ± standard deviation (SD). Active cancer: period from six months before a cancer diagnosis until two years after; BMI: body mass index; Daily smoking indicates smoking at the time of enrollment; WBC: white blood cell; *: allele frequency.
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haematologica | 2020; 105(7)