Respiratory Medicine (2014) 108, 1321e1326

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Heredity of chronic bronchitis: Aregistry-based twin study

Howraman Meteran a,*, Vibeke Backer a, Kirsten Ohm Kyvik b,c,Axel Skytthe c, Simon Francis Thomsen a

a Respiratory and Allergy Research Unit, Bispebjerg Hospital, Copenhagen, Denmarkb Odense Patient Data Exploratory Network, Institute of Regional Health Services Research,University of Southern Denmark, Odense, Denmarkc The Danish Twin Registry, University of Southern Denmark, Odense, Denmark

Received 10 April 2014; accepted 28 June 2014Available online 4 July 2014

KEYWORDSChronic bronchitis;COPD;Heritability;Twin studies

* Corresponding author. RespiratoryDenmark. Tel.: þ45 6067 7296; fax: þ

E-mail address: hmeteran@gmail.c

http://dx.doi.org/10.1016/j.rmed.2010954-6111/ª 2014 Elsevier Ltd. All rig

Summary

Background: Smoking is a major risk factor for lung diseases and lower respiratory symptoms,but since not all smokers develop chronic bronchitis and since chronic bronchitis is also diag-nosed in never-smokers, it has been suggested that some individuals are more susceptible todevelop chronic bronchitis due to genetics.Objective: To study the relative influence of genetic and environmental factors on the varia-tion in the susceptibility to chronic bronchitis.Methods: In a population-based questionnaire study of 13,649 twins, 50e71 years of age, fromthe Danish Twin Registry, we calculated sex-specific concordance rates and heritability ofchronic bronchitis. The response rate was 75%.Results: The prevalence of chronic bronchitis was 9.3% among men and 8.5% among women.The concordance rate for chronic bronchitis was higher in monozygotic twins than in dizygotictwins among women; 0.30 vs. 0.17, but not among men; 0.15 vs. 0.18. The heritability ofchronic bronchitis adjusted for smoking and age was 55% (36e71%) in women, whereas the sus-ceptibility to chronic bronchitis in men for 25% (8e41%) was ascribable to familial environmentbut not to genetic factors.Conclusions: Chronic bronchitis shows a moderate familial aggregation, particularly in women.Increased susceptibility to respiratory disease among female smokers relative to male smokersmay have a genetic origin.ª 2014 Elsevier Ltd. All rights reserved.

and Allergy Research Unit, Bisp45 35312179.om (H. Meteran).

4.06.010hts reserved.

ebjerg Hospital, Bispebjerg Bakke 23, DK-2400 Copenhagen NV,

1322 H. Meteran et al.

Introduction

Chronic bronchitis is a disease characterized by chroniccough and sputum from the airways for at least threemonths in each of two successive years, as long as there areno other causes of chronic cough [1]. The prevalence andmortality associated with chronic obstructive pulmonarydisease (COPD) is increasing throughout the world andCOPD is estimated to be one of the most common causes ofdeath in 2020 [2,3]. COPD poses an important challenge forsociety as consultation rates in primary care are high andCOPD exacerbations account for a substantial part of hos-pital admissions [4].

Smoking is a well-known risk factor for chronic bron-chitis and COPD [5] and the risk of chronic bronchitis in-creases with the amount of tobacco consumed and withage [5e8]. Pelkonen and colleagues found that by the ageof 75 years the cumulative incidence of chronic bronchitiswas 42% in continuous smokers, 26% in ex-smokers and22% in never-smokers in the finnish population [9]. Otherrisk factors such as farming, industrial work and asthmahave also been suggested [10e12]. Furthermore, somestudies have shown an association between passivesmoking during childhood and accelerated decline in lungfunction and lower respiratory symptoms in adulthood[13,14].

Smoking remains the major risk factor for chronicbronchitis and COPD, but since not all smokers developobstructive respiratory disease, and more interestingly,since chronic bronchitis and COPD have been diagnosed innever-smokers, it has been suggested that the susceptibil-ity could be due to genetics. The most important geneticrisk factor for COPD is a1-antitrypsin deficiency, which hasbeen known for several decades. However, a1-antitrypsindeficiency only accounts for 1e2% of the susceptibility toCOPD in the population [15,16].

Previous studies have shown a familial aggregation ofobstructive pulmonary disease [17e21]. Furthermore, it hasbeen shown that women have a greater degree of airwayobstruction for a given amount of tobacco consumptioncompared with men [22,23]. This low degree of airflowobstruction was also found among non-smokers and in-dividuals with less than five pack-years’ smoking history[24] and it has been suggested that women are more sus-ceptible to smoking induced diseases compared with men[25,26].

Hallberg and colleagues have assessed to what extentgenetic factors contribute to the development of chronicbronchitis [27]. The study, which examined Swedish twins,found a moderate heritability of chronic bronchitis of 40%[27]. Their criterion used to define chronic bronchitis waswider than the classical criterion, which we have used inour study. Furthermore, the Swedish authors dichotomizedsmoking, using 10 pack e years as cutoff and thereby lightsmokers were classified as nonsmokers.

Twin studies offer a powerful approach to examine theimpact of genes and environment on human diseases. Theaim of this study was, in a large Danish twin sample, tostudy the relative influence of genetic and environmentalfactors on the variation in susceptibility to chronicbronchitis.

Methods

Design

Data was collected from the Danish Twin Registry and thepresent study sample comprised twins born between 1931and 1952 [28]. These corresponded to 69% of all twin pairsborn in Denmark during these years. In 2002, these twins(and twins born between 1953 and 1982) were sent amultidisciplinary questionnaire concerning health and life-style in which a history of smoking and chronic bronchitiswas recorded. The response rate to the questionnaire was75%.

The study was approved by The Danish National Com-mittee on Biomedical Research Ethics.

Study population

In total, 13,649 twin individuals (4030 intact pairs withcomplete data on chronic bronchitis and smoking) werefrom the cohort born 1931e1952. Chronic bronchitis wasdefined as an affirmative response to the question ‘Haveyou experienced at least three months per year of coughingwith production of phlegm during the past two years? Twinzygosity was determined using four questions of similarityand mistaken identity, which assign zygosity correctly inmore than 95% of the cases [29].

Statistical analysis

The prevalence and concordance rate for chronic bronchitiswere calculated for the different sex-by-zygosity groups.The concordance rate denotes the probability that one twinis affected given that the co-twin is affected. Since wehave complete ascertainment of twins the concordancerate was estimated as two times the number of concordantpairs (both twins are affected) divided by two times thenumber of concordant affected pairs plus the number ofdiscordant pairs (one twin is affected) [30].

Latent factor models of genetic and environmentalfactors were fitted to the raw data following the methodsdescribed by Neale & Cardon [31]. These are customarytwin analyses assuming that the variation in the suscepti-bility to chronic bronchitis can be partitioned into geneticand environmental sources of variance. Genetic variancecan be further partitioned into additive genetic (locicontributing additively to disease risk, A) and non-additivegenetic variance (interacting alleles, either from the samelocus e genetic dominance e or from separate loci eepistasis, D). Environmental variance can be further parti-tioned into shared environmental (environmental factorsthat increase the resemblance between members of thesame family, C) and non-shared e random e environmentalvariance (influences unique to individuals that result indifferences between family members, E) [32]. For mosthuman traits it is reasonable to assume that all four sourcesof variance (A, D, C, and E) act simultaneously. However,components C and D are not identified under the samemodel in studies that include only twins reared together[33]. Therefore, the likelihood of the data was determined

Heredity of chronic bronchitis 1323

under a saturated model that included components A, C,and E since there was evidence that shared environmentcould influence the disease liability (correlation in mono-zygotic (MZ) twins below twice the correlation in dizygotic(DZ) twins). The resulting expected phenotypic variance isdefined as the sum of these sources of variance, i.e.A þ C þ E. Since MZ twins are genetically identical, theexpected covariance for MZ twins equals A þ C, while thecovariance between DZ twins, who share on average 50% oftheir genes, equals 0.5*A þ C. The significance of thecontribution of the individual parameters to the variance indisease liability was determined by a likelihood ratio testfor the difference between the full ACE model and subse-quently fitted nested models (AE, CE and E models). Thestatistical packages SPSS 20.0 (SPSS Inc., Chicago, IL) andMx was used for the analyses [34].

Results

The mean age of the study participants was 58.8 years and52.7% were females. The overall prevalence of currentsmokers in the population was 33%, whereas 30% wereformer smokers. The prevalence of chronic bronchitis was9.3% among men and 8.3% among women and there was nosignificant difference in prevalence between men andwomen, p Z 0.286. The population was divided into twoage groups, 50e59 and 60e71 years, in which the preva-lence of chronic bronchitis was 8.4% and 9.6%, respectively,p Z 0.01. The prevalence of chronic bronchitis in neverand former smoker was 4% and 6.1%, respectively,p < 0.001. The total population of current smokers wasdecomposed into three groups based on number of ciga-rettes smoked per day; 1e10, 11e20 and 21þ, and theprevalence in the three groups was 9.9%, 16.9% and 31.6respectively. Compared with never-smokers currentsmokers had an increased risk of chronic bronchitis in adose-dependent manner with an up to tenfold increasedrisk in the heaviest smokers, OR Z 10.21 (7.93e13.15),p < 0.001.

We found a higher concordance rate in MZ comparedwith DZ twins among women, 0.30 vs. 0.17, p Z 0.166, butnot among men, 0.15 vs. 0.18, p Z 1.00 (Table 1).

Results from variance components analysis adjusted forage and smoking are shown in Table 2. In women, a modelthat included additive genetic effects and non-sharedenvironment best described the familial aggregation ofchronic bronchitis with a heritability of 55% (36e71%). Inmen, the heritability of chronic bronchitis was small and

Table 1 Resemblance between Danish twins for chronic bronch

Zygosity group Pairs (n) Affected (%) Discorpairs

MZMales 430 66 (7.7) 56Females 535 74 (6.9) 52

DZMales 683 123 (9.0) 101Females 862 130 (7.5) 108

MZ, monozygotic; DZ, dizygotic same sex.

statistically insignificant: 2% (0e52%), and susceptibility tochronic bronchitis was best described by a model thatincluded only effects of shared and non-shared environ-ment, with an estimate of shared environmental effects of25% (8e41%).

Discussion

Previous studies have reported contradicting resultsregarding the prevalence of chronic bronchitis in men andwomen [7,27]. We did not find any significant difference inthe prevalence of chronic bronchitis between men andwomen, and our data thereby support the results of anotherlarge population-based study of Danish adults: The Copen-hagen City Heart Study [7]. In contrast, we found a higherconcordance rate for chronic bronchitis among MZ womencompared with DZ women, but this was not the case formen, indicating a genetic influence on the risk of devel-oping chronic bronchitis in women, but not in men. Thispattern was further examined in a variance componentsanalysis adjusted for age and smoking. For women, wefound that the best fitting model describing susceptibilityto chronic bronchitis included additive genetic effects andnon-shared environmental affects with a heritability of55%, suggesting a genetic predisposition to chronic bron-chitis in women. However, the results for men did notsuggest a genetic predisposition to chronic bronchitis butrather that familial aggregation was due to shared envi-ronmental factors with a preferred model including onlyshared and non-shared environmental effects.

When changing the outcome from the classical chronicbronchitis definition to only include chronic cough and notthe time restriction, we found similar heritabilityestimates.

A study of 44 probands with severe, early-onset COPDperformed by Silverman and colleagues found a signifi-cantly increased risk of forced expiratory volume in the firstsecond (FEV1) <80% of predicted (OR Z 4.5) and anincreased risk of chronic bronchitis among current or ex-smoking first-degree relatives, whereas no differenceswere found in non-smoking subjects [17]. This suggests thatthe increased risk of reduced FEV1 and chronic bronchitismay relate to genotype-by-environment interaction be-tween cigarette smoking and one or more genetic variants.In a later report, Silverman and colleagues performedanother study of 40 early-onset COPD families and assessedthe sex-related differences in spirometric values, bron-chodilator responsiveness and chronic bronchitis. They

itis.

dant(n)

Concordantpairs (n)

Concordancerate

P-value

5 0.1511 0.30

11 0.18 1.011 0.17 0.166

Table 2 Variance components analysis of chronic bronchitis in Danish twins.

Variance componentsa Fit statistics

A C E De2 logQ Ddf P-value

MalesACE 0.02 (0.00e0.52) 0.23 (0.00e0.41) 0.75 (0.48e0.92)AE 0.33 (0.09e0.54) - 0.67 (0.46e0.91) 0.851 1 0.356CE e 0.25(0.08e0.41) 0.75 (0.59e0.92) 0.002 1 0.963

E e e 1.00 7.98 2 0.019FemalesACE 0.49 (0.00e0.71) 0.05 (0.00e0.47) 0.46 (0.29e0.68)AE 0.55 (0.36e0.71) - 0.45 (0.29e0.64) 0.05d 1 0.817

CE e 0.40 (0.25e0.53) 0.60 (0.47e0.75) 2.80 1 0.094E e e 1.00 28.06 2 0.000

For each model (ACE, AE, CE and E) variance components sum to 1.00 (100%); the preferred model based on parsimony and goodness offit is highlighted in bold print. The estimate of each component (A, C and E) is the proportion of the total phenotypic variance (sus-ceptibility to disease) that is accounted for by the specific components.De2 logQ, difference in model fit (log-likelihood) between saturated model and nested model.Ddf, difference in degrees of freedom between saturated model and nested model.Most parsimonious models are highlighted in bold.a Variance components (95% confidence intervals) of chronic bronchitis susceptibility due to additive genetic factors (A), shared

environmental factors (C), and non-shared environmental factors (E) adjusted for age and smoking.

1324 H. Meteran et al.

found increased risk of airflow obstruction and increasedbronchodilator responsiveness, but no increased risk ofchronic bronchitis in female first-degree relatives of early-onset COPD probands [21]. Similar results were reported byMontnemery and colleagues, who found that familialoccurrence of chronic bronchitis/emphysema in first-degree relatives is a stronger risk factor for developmentfor these traits than is smoking [18]. Another study by Pateland colleagues showed familial aggregation of airway wallthickening and emphysema, with siblings of an effectedproband having a more than twofold increased risk ofemphysema compared with controls [20]. Furthermore, theauthors reported that the association between airway wallthickening and chronic cough and sputum production actsas a marker of chronic bronchitis, characterized by mucushypersecretion. The authors concluded that, since smokingis the only environmental exposure than confers this levelof risk, familial aggregation of the two major COPD phe-notypes is due to genetic susceptibility [20].

Several studies have shown that women are more sus-ceptible to smoking-induced respiratory diseases [22e26].However it has not previously been shown whether thissusceptibility is due to genetic factors. It has been sug-gested that more women smoke and that women are moreaware of symptoms than men. Moreover women seek healthcare earlier or more often than men, and also tend to un-derreport, whereas men tend to overreport their tobaccoconsumption [8,35].

If women underreport phlegm, prevalence estimates ofCB may be too low. We cannot exclude that the observeddifferences in resemblance and heritability of chronicbronchitis in women and men may represent differentphenotypes measured in men and women. COPD withmainly coughing symptoms in women may show decreasedheritability in COPD with phlegm in men. Thus, under-reporting of phlegm in women will result in differences inthe variance components when comparing men and

women. That women are more predisposed to developsmoking-related respiratory disease could also beexplained by the differences in airway size as women maybe exposed to a greater amount of tobacco smoke thanmen [35].

A criticism of twin studies it that they may overestimateheritability. The argument being that monozygotic twinsare generally treated more similarly to each other thandizygotic twins. E.g. parents often dress their identicaltwins alike, treat them identically, and, in general, accenttheir similarities. In this case, the MZ concordance wouldincrease relative to the DZ concordance [36].

It is uncertained whether this potential bias plays a rolein chronic bronchitis. However, studies on psychiatric dis-orders have shown that the degree of environmentalsharing does not predict concordance rates [37].

Since data were self-reported we acknowledge limita-tions such a recall bias and lack of clinical verification ofthe airway symptoms. However, we used the definition setby American Thoracic Society [1] and a good correlationbetween self-reported chronic bronchitis and actualsputum volume has been shown [38]. We did not takeoccupation and outdoor pollution into consideration andprevious studies have shown associations between theserisk factors and respiratory symptoms [39,40]. Since ourstudy is nation-wide, it includes subjects from both ruraland urban areas in Denmark and therefore we do notbelieve it would have changed our results considerably.Conclusively, our study suggests that familial aggregation ofchronic bronchitis in women is mainly due to genetic fac-tors, whereas the familial aggregation in men can beascribed to shared environmental factors. Therefore phy-sicians should question all current and former smokers,especially female patient, about their family history ofchronic bronchitis. Furthermore, we encourage physician tobe aware of undetected COPD in relatives to patients withCOPD.

Heredity of chronic bronchitis 1325

Conflict of interest statements

The authors have nothing to disclose.

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