Journal of Thrombosis and Haemostasis, 1 239–242 ORIGINAL ARTICLE

ORIGINAL ARTICLE

The association between birth weight and plasma ?brinogen is abolished after the elimination of genetic in?uences

R.G.I J Z E R M A N ,C.D.A.S T E H O U W E R ,E.J.D E G E U S ,y C.K L U F T *and D.I.B O O M S M A y

Department of Internal Medicine,Institute for Cardiovascular Research-Vrije Universiteit,VU University Medical Center,Amsterdam;*Gaubius Laboratory,TNO-PG,Leiden;and y Department of Biological Psychology Vrije Universiteit,Amsterdam,the Netherlands

Summary.Low birth weight is associated with an increased risk of atherothrombosis,which may be related in part to the association between low birth weight and high plasma ?brino-gen.The association between birth weight and ?brinogen may be explained by intrauterine,socio-economic or genetic factors.We examined birth weight and ?brinogen in 52dizygotic and 56adolescent monozygotic (genetically identical)twin pairs.The dizygotic but not the monozygotic twins with the lowest birth weight from each pair had a ?brinogen level that was higher compared with their co-twins with the highest birth weight [dizygotic twins: 2.62?0.46g L à1vs. 2.50?0.41g L à1(P ?0.04);monozygotic twins:2.42?0.45g L à1vs.2.49?0.39g L à1(P ?0.2)].These ?ndings suggest that the associa-tion between birth weight and plasma ?brinogen is abolished after the elimination of genetic in?uences and therefore that this association has genetic causes.Improvement of intrauterine nutrition may not lower ?brinogen levels in later life.Keywords :birth weight,?brinogen,genetics,twins.

Introduction

Indices of fetal growth,such as birth weight,are inversely associated with the prevalence and mortality of coronary heart disease and stroke [1,2].It has been suggested that increased plasma concentrations of ?brinogen in later life may play a role in these associations [3–5].The inverse association between birth weight and ?brinogen has been attributed to a programmed response to intrauterine malnutrition that induces permanent changes in the structure and function of organs,which cause increased levels of ?brinogen in later life [1].This theory may be supported by a study demonstrating that people who were exposed to the Dutch famine in early gestation had slightly elevated levels of ?brinogen in later life [5].If the association between birth weight and ?brinogen is due to intrauterine

nutrition,improvements in intrauterine nutrition may lower ?brinogen levels in later life.However,the alternative view is that some factors other than intrauterine nutrition may in?uence both growth in utero and levels of ?brinogen.Envir-onmental causes,particularly those associated with socio-eco-nomic status,as well as genetic factors have been proposed as alternative explanations [6].If the association between birth weight and ?brinogen is due to socio-economic or genetic causes,improvement of intrauterine nutrition may not lower ?brinogen levels in later life.Studies in dizygotic and mono-zygotic twin pairs still living with their parents offer a unique opportunity to distinguish between intrauterine,socio-eco-nomic and genetic in?uences [7–9].Studying differences in twin pairs avoids socio-economic factors that could confound the association between size at birth and cardiovascular disease in later life.Furthermore,investigating differences in mono-zygotic (genetically identical)twin pairs allows elimination of the in?uence of genotype on this association.

We investigate herein the association between birth weight and ?brinogen in a group of adolescent dizygotic and mono-zygotic twin pairs still living with their parents.The underlying hypotheses for the within-pair analyses were that if intrauterine nutrition is responsible for the association between birth weight and ?brinogen,the association would be present within both dizygotic and monozygotic twin pairs.If socio-economic fac-tors are responsible,the association would be absent within both dizygotic and monozygotic twin pairs.If a genetic predisposi-tion is responsible,birth weight would be associated with ?brinogen within dizygotic twin pairs,but not within mono-zygotic twin pairs.Methods

Subjects

This study is part of a larger project in which cardiovascular risk factors were studied in 160adolescent twin pairs and their parents [8–10].Addresses of twins living in Amsterdam and neighboring cities were obtained from City Council population registries.Twins still living with their biological parents were contacted by letter.A questionnaire was used to gather infor-mation on various factors including the use of medication and smoking behavior.The maternal questionnaire included ques-tions regarding birth weight and gestational age of the children.

Journal of Thrombosis and Haemostasis ,1:239–242

#2003International Society on Thrombosis and Haemostasis

Correspondence:Professor Dr Coen Stehouwer,VU University Medical Center,Department of Medicine,De Boelelaan 1117,1007MB Amsterdam,the Netherlands.

Tel.:t31204440629;fax:t31204440502;e-mail:cda.stehouwer@vumc.nl Received 13March 2002,revised 24June 2002,accepted 24June 2002

This questionnaire was sent to the mothers a few weeks ahead of their visit to our department,allowing them to obtain birth data from birth certi?cates[8,9].Opposite-sex dizygotic twin pairs (n?28)were excluded from the analyses because of the effects of sex differences within a pair on both birth weight and ?brinogen.Subjects using oral contraceptives were also excluded(six dizygotic twin pairs and seven monozygotic twin pairs).None of the subjects used any other medication that may affect plasma concentrations of?brinogen.Thus,52dizygotic and56monozygotic twin pairs were available for analysis. Measurements

Height and weight were measured in a standardized way.After acclimatization blood was obtained between08.30and 10.30am by venipuncture after overnight fasting.Total?brino-gen antigen in EDTA plasma was determined with an enzyme immunoassay that uses a pool of rabbit anti-human?brinogen immunoglobulins(IgGs)as catching antibodies[11]and per-oxidase-conjugated monoclonal antibodies against fragment DD(DD13)[12]as tagging antibodies.Pooled plasma of healthy volunteers is used as a standard(100%)from which the?brinogen content was determined additionally with a gravimetric method to express?brinogen in grams per liter[13]. Statistical analysis

The paired Student’s t-test was used to compare twins with the lowest birth weight from each pair with their co-twins with the highest birth weight.For this analysis,two dizygotic twin pairs and one monozygotic twin pair had to be excluded because the birth weight of the twins within a pair was equal.Differences in dizygotic twin pairs and in monozygotic twin pairs were compared using the independent samples t-test.In addition, linear regression analysis was used to analyze whether intrapair differences in birth weight in?uenced intrapair differences in ?brinogen before and after adjustment for differences in current weight or body mass index(BMI;including the three twin pairs in which the birth weight of the twins within a pair was equal). Interaction analysis was performed to investigate whether zygosity or differences in current weight in?uenced the asso-ciations between intrapair differences in birth weight and ?brinogen.Linear regression analysis was used to analyze the association between birth weight and?brinogen in the overall sample.Interaction analysis was performed to investi-gate whether zygosity in?uenced this association.A two-tailed P-value<0.05was considered signi?cant.All analyses were performed on a personal computer using the statistical software package SPSS version9.0(SPSS Inc).

Results

As a?rst intrapair analysis,we compared co-twins with the lowest birth weight from each pair with their co-twins with the highest birth weight.The dizygotic but not the monozygotic twins with the lowest birth weight from each pair had?brinogen levels that were higher compared with their co-twins with the highest birth weight(Table1).The differences in?brinogen between the co-twins with the lowest and the co-twins with the highest birth weight were signi?cantly different in dizygotic compared with monozygotic twin pairs(P?0.02).In both the dizygotic and the monozygotic twins,current BMI and smoking habits were similar in co-twins with the lowest and the co-twins with the highest birth weight.In an additional analysis,in-trapair differences in birth weight were associated with differ-ences in?brinogen in dizygotic twins{regression coef?cient à0.25g Là1kgà1[95%con?dence interval(CI)à0.49to à0.01],P<0.05}suggesting that the larger the difference in birth weight,the higher the?brinogen in the twin with the lowest birth weight compared with the co-twin with the highest birth weight.In monozygotic twins,however,intrapair differ-ences in birth weight were not associated with differences in ?brinogen[t0.16g Là1kgà1(95%CIà0.12–0.45),P?0.3]. Interaction analyses con?rmed that these associations were signi?cantly different between dizygotic and monozygotic twins(P?0.03).In the overall sample of twins,birth weight was not associated with?brinogen[regression coef?cient 0.01g Là1kgà1(95%CIà0.10–0.13),P?0.8,adjusted for age and sex].The results were similar after adjustment for (differences in)current weight,BMI or smoking.In addition, the results were similar after the exclusion of smokers.Inter-action analyses demonstrated that the(intrapair)associations between birth weight and?brinogen were not different between men and women(P for interaction was always>0.4).

Table1Clinical characteristics of the co-twins with the lowest and the highest birth weight in dizygotic and monozygotic twin pairs Dizygotic twin pairs Monozygotic twin pairs

Co-twins with the lowest birth weight Co-twins with the

highest birth weight P

Co-twins with the

lowest birth weight

Co-twins with the

highest birth weight P

Birth weight(g)2226?4772604?540<0.0012339?5242637?475<0.001 Gestational age(weeks)37?2.837?2.8–36?8.436?8.4–

n(male/female)50(29/21)50(29/21)–55(29/26)55(29/26)–Age(years)17.0?1.717.0?1.7–16.0?1.816.0?1.8–Current BMI(kg mà2)20.0?1.920.3?2.20.519.5?2.319.8?2.30.2 Smoking(n)79–44–Fibrinogen(g Là1) 2.62?0.46 2.50?0.410.04 2.42?0.45 2.49?0.390.2 Mean?SD.BMI,body mass index.

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Discussion

We found that low birth weight was associated with high ?brinogen within dizygotic twin pairs,but not within mono-zygotic twin pairs.These data provide the?rst evidence that the association between birth weight and?brinogen is abolished after the elimination of genetic in?uences.Importantly,these ?ndings contradict the hypothesis that improvement of intrau-terine nutrition may lower?brinogen levels in later life.

It could be argued that besides genetic factors,intrauterine factors may also differ between dizygotic and monozygotic twins and may be the cause of the difference in the intrapair association between birth weight and?brinogen.However, previous studies have demonstrated that the associations be-tween low birth weight and increased cardiovascular risk factors in overall samples of twins were similar in dizygotic and monozygotic twins[8,9,14].In addition,intrapair differences in birth weight were related to differences in diabetes[15],high density lipoprotein(HDL)cholesterol[9],and height[16,17]in both dizygotic and monozygotic twins.These studies suggest that intrauterine differences between dizygotic and monozygo-tic twins do not explain the differences in the intrapair associa-tion between birth weight and?brinogen in dizygotic and monozygotic twins.

It could be suggested that twin pairs cannot be used as a model to study the association between birth weight and cardiovascular risk factors in singletons.However,birth weight in twins has been associated with many variables that have been related to birth weight in singletons[8,9,14–17].In addition,?brinogen levels in our adolescent twins were not different from levels found in studies in singletons[18,19].

The absence of an association between birth weight and ?brinogen in the overall sample is consistent with several studies in singletons[20–22],but not all[3,4].Our results in dizygotic twin pairs demonstrate that the association between birth weight and?brinogen may be strengthened after the elimination of socio-economic factors.In contrast,this associa-tion is abolished after the elimination of genetic factors.Genetic and socio-economic in?uences may be different across popula-tions and may explain the contradictory?ndings of studies in singletons.Interestingly,it has been demonstrated that although size at birth was not associated with plasma levels of?brinogen in people born around the Dutch famine,people who were exposed to famine in early gestation had slightly elevated levels of?brinogen in later life[5].

The results from the Dutch famine birth cohort could be interpreted as a speci?c effect of the intrauterine environment on?brinogen levels[5].However,an alternative explanation is that this?nding re?ects a selective survival advantage of fetuses genetically susceptible to an increased cardiovascular risk[23]. During the famine,the number of conceptions was about50% lower than the prefamine level and perinatal mortality as well as mortality in the?rst year after birth were highest in those who were born during the famine[24].

Our?ndings suggest that genetic factors account for the association between birth weight and?brinogen.However,the genetic factors that may be responsible are not known.Inter-estingly,it has recently been demonstrated that several inherited risk factors for thrombophilia were related to low birth weight in Caucasian children[25].Therefore,further research into the genetic factors responsible for the association between birth weight and?brinogen is warranted.

In summary,we found that low birth weight was associated with high levels of?brinogen within dizygotic twin pairs,but not within monozygotic twin pairs.These data suggest that genetic factors account for the association between birth weight and?brinogen.Therefore,improvements in intrauterine nutri-tion may not lower?brinogen levels in later life. References

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