Genetic influence on the age at onset of asthma: A twinstudy

Simon Francis Thomsen, MD, PhD,a David Lorenzo Duffy, MD, PhD,b Kirsten Ohm Kyvik, MD, PhD, MPM,c

and Vibeke Backer, MD, DMSca Copenhagen and Odense, Denmark, and Brisbane, Australia

Background: Although the genetics of asthma susceptibilityhave been frequently explored, little is known about geneticfactors that influence the age at onset of asthma.Objective: To study the variation in the age at onset of asthmaattributable to genetic and environmental factors.Methods: Data on the age at onset and predictors of asthmawere collected in 2002 via a multidisciplinary questionnairestudy of 34,782 Danish twins 20 to 71 years of age. Survivalanalytic methods were applied to partition variation in the ageat onset of asthma into genetic and environmental components.Results: Sex, hay fever, atopic dermatitis, smoking, andexposure to passive smoking in childhood were significant riskfactors, whereas BCG vaccination was protective for asthma.The risk of asthma in the co-twin of an affected twin was higherin monozygotic than in dizygotic twins (hazard ratio, 2.59; 95%CI, 1.83-3.68; P < .001). The risk of asthma in the co-twindecreased with increasing age at onset of asthma in the indextwin (hazard ratio per ten years, 0.86; 95% CI, 0.76-0.98;P 5 .019). The effect was attenuated in dizygotic twins relativeto monozygotic twins (P 5 .005). Genetic factors explained 34%of the variation in the age at onset of asthma, and environmentalfactors accounted for 66%.Conclusion: Host-related differences in genetic makeup causedifferent individuals to develop asthma at different ages. Betterunderstanding of the causes for variation in the age at onsetof disease may ultimately lead the way to targeted treatments.(J Allergy Clin Immunol 2010;126:626-30.)

Key words: Asthma, genetics, twin study, age at onset, heritability,epidemiology

Although asthma can occur at any age, most cases develop inchildhood with a gradual decrease in incidence after adolescence.Boys have a greater risk of asthma in the first years of life,whereas girls are more frequently affected during adolescenceand in adulthood.1 Allergy plays a pivotal role in childhood-onsetasthma, whereas adult-onset asthma seems less related to allergicmechanisms.2 Accordingly, individuals with early-onset asthmamore often have atopic dermatitis and hay fever than subjects

From athe Department of Respiratory Medicine, Bispebjerg Hospital, Copenhagen; bthe

Queensland Institute of Medical Research, Brisbane; and cthe Danish Twin Registry

and Institute of Regional Health Services Research, University of Southern Denmark,

Odense.

S.F.T. is sponsored by the Danish Medical Research Council grant.

Disclosure of potential conflict of interest: The authors have declared that they have no

conflict of interest.

Received for publication January 14, 2010; revised April 30, 2010; accepted for publica-

tion June 4, 2010.

Available online August 5, 2010.

Reprint requests: Simon Francis Thomsen, MD, PhD, Department of Respiratory Med-

icine, Bispebjerg Hospital, DK-2400 Copenhagen NV, Denmark. E-mail: sft@city.dk.

0091-6749/$36.00

� 2010 American Academy of Allergy, Asthma & Immunology

doi:10.1016/j.jaci.2010.06.017

626

whose asthma develops in adulthood. Moreover, subjects withearly-onset asthma more frequently have a family history ofatopic disease than subjects with later-onset asthma.3 Individualswith adult-onset asthma also tend to have lower lung function, tobe smokers, and to have a poorer prognosis than individuals withchildhood-onset asthma.4 On the contrary, parental smoking dur-ing pregnancy or in early life of the child seems to be a strongerpredictor of early-onset persistent asthma than later-onset asthma,particularly in children with atopic heredity.3 Subjects with child-hood asthma who relapse in adulthood appear to have a moresevere form of disease than subjects whose asthma occurs inadulthood,5 consistent with a lower lung function according to alonger duration of disease.6

Genetic studies of asthma have so far mainly identified asthmasusceptibility loci,7 whereas studies of genetic variation underly-ing the age at onset of disease are sparse. Bouzigon et al8 foundevidence of 2 regions (5q13 and 1p31) with suggestive linkageto time to onset of asthma in French families. Interestingly, the5q13 region was also linked to asthma severity. Furthermore, a re-gion on 7q showed suggestive linkage to asthma in the samepopulation but with different genotype relative risks accordingto the age at onset of disease.9 Hizawa et al10 found that the-28G allele of the RANTES promoter region at chromosome17q increased the risk of late-onset asthma (>40 years of age)compared with early-onset and middle age–onset asthma in Jap-anese. In German children, age at onset of wheezing was stronglylinked to chromosome 6q24-q25.11 Although these markers havebeen identified, the overall contribution of genetic factors to theage at onset of asthma has not been estimated. We consequentlyanalyzed questionnaire data on a large sample of adult Danishtwins with the aim to study the overall contribution of the geneticmakeup to the variation in the age at onset of asthma.

METHODS

SampleThe study population was composed of the twin cohorts born between 1931

and 1982 who were enrolled in the Danish Twin Registry.12 In 2002, when the

study subjects were between 20 and 71 years of age, they were mailed a mul-

tidisciplinary questionnaire on health and lifestyle. Asthma was identified on

the basis of an affirmative response to the question, ‘‘Do you have or have your

ever had asthma?’’ whereas the age at onset of asthma was determined by the

question, ‘‘How old were you (in years) when you got asthma?’’ Additional

questions used herein were on history of hay fever, atopic dermatitis, smoking,

vaccination with BCG, exposure to passive smoking in childhood, and symp-

toms of chronic bronchitis (‘‘having experienced at least 3 months per year of

coughing with production of phlegm during the past 2 years’’). Four questions

on physical similarity and mistaken identity determined twin zygosity. This

method has a misclassification rate of less than 5%.13 In total, 34,782 subjects

participated in the study (response rate, 75%). Of these, 11,671 were intact

pairs, 3408 monozygotic pairs, 4273 dizygotic pairs of same sex, and 3691 di-

zygotic pairs of opposite sex, leaving 299 pairs of unknown zygosity. Asthma

was present in 1714 of the intact twin pairs. Of these, 1614 pairs had complete

information on age at onset.

TABLE I. Characteristics of 34,782 twin subjects, 20 to 71 years of age, who participated in a nationwide questionnaire study

Age group 20-30 (n 5 6,690) 31-40 (n 5 7,933) 41-50 (n 5 7,281) 51-60 (n 5 7,855) 61-71 (n 5 5,023) Total (n 5 34,782)

Men 2,907 (43.5) 3,402 (42.9) 3,463 (47.6) 3,677 (46.8) 2,397 (47.7) 15,846 (45.6)

Women 3,783 (56.5) 4,531 (57.1) 3,818 (52.4) 4,178 (53.2) 2,626 (52.3) 18,936 (54.4)

Ever asthma 676 (10.2) 722 (9.2) 600 (8.3) 549 (7.1) 457 (9.3) 3004 (8.7)

Men 260 (9.0) 293 (8.7) 240 (7.0) 195 (5.4) 204 (8.7) 1192 (7.6)

Women 416 (11.1) 429 (9.6) 360 (9.5) 354 (8.6) 253 (9.9) 1812 (9.7)

Current asthma 356 (5.4) 421 (5.4) 379 (5.3) 367 (4.7) 343 (7.0) 1866 (5.4)

Men 131 (4.5) 167 (4.9) 141 (4.1) 125 (3.4) 147 (6.2) 711 (4.5)

Women 225 (6.0) 254 (5.7) 238 (6.3) 242 (5.9) 196 (7.7) 1155 (6.2)

Ever smoking 2,762 (42.1) 4,056 (52.0) 4,361 (61.3) 4,861 (64.2) 3,184 (67.9) 19224 (57.0)

Men 1,173 (41.1) 1,656 (49.5) 2,058 (60.7) 2,513 (70.5) 1,738 (76.2) 9,138 (59.2)

Women 1,589 (42.8) 2,400 (54.0) 2,303 (61.9) 2,348 (58.7) 1,446 (60.1) 10,086 (55.2)

Current smoking 2,077 (31.6) 2,627 (33.7) 2,676 (37.6) 2,748 (36.3) 1,526 (32.6) 11654 (34.6)

Men 932 (32.6) 1,150 (34.4) 1,263 (37.2) 1,374 (38.5) 790 (34.6) 5509 (35.7)

Women 1,145 (30.9) 1,477 (33.2) 1,413 (38.0) 1,374 (34.3) 736 (30.6) 6,145 (33.6)

Chronic bronchitis

With asthma

Men 22 (9.5) 40 (14.8) 51 (23.0) 54 (30.0) 65 (33.5) 232 (21.1)

Women 55 (15.9) 79 (21.5) 91 (28.6) 91 (28.5) 98 (41.7) 414 (26.1)

Without asthma

Men 69 (2.7) 125 (4.2) 188 (6.1) 250 (7.4) 171 (8.2) 803 (5.7)

Women 80 (2.5) 166 (4.3) 230 (7.0) 225 (6.2) 118 (5.3) 819 (5.1)

Data presented as n (%).

J ALLERGY CLIN IMMUNOL

VOLUME 126, NUMBER 3

THOMSEN ET AL 627

Statistical analysisWe performed 2 different regression analyses to identify predictors for the

age at onset of asthma. Initially, predictors of the age at onset of asthma were

determined with a Cox proportional hazards regression model with age as the

underlying time ignoring the familial relationships between the twins.

Covariates were age group (birth cohort) in 10-year bands, sex, atopic

dermatitis, hay fever, BCG vaccination, smoking (pack-years), and exposure

to passive smoking in childhood. We subsequently fitted a Cox proportional

hazards regression model to asthma-free survival of co-twins of a proband

(index twin) who was the first member of that sibship to develop asthma.14 The

survival time for the co-twin started at the age the proband was affected and

continued until the co-twin developed asthma or was censored at the age of

last follow-up. In this analysis, an increased hazard ratio for monozygotic

co-twins relative to dizygotic co-twins signals genetic influences on the age

at onset of asthma. As covariates, we included the age at onset of asthma in

the proband and age group (birth cohort) in 10-year bands, sex, BCG vaccina-

tion, and smoking (pack-years) in the co-twin.

We then explored the correlation between the age at onset of asthma within

twin pairs and partitioned the variation in the age at onset of asthma into

genetic and environmental components. Because of the correlation between

members of a twin pair, nonparametric estimation of the bivariate survival

function for the twins was estimated by the Dabrowska approach. To carry out

variance components analysis of age at onset, we tested 2 different

approaches. One was the Cox proportional hazards frailty model implemented

in the R kinship package (www.r-project.org),15 and the other involved fitting

the Gaussian mixed model to martingale residuals calculated from the margi-

nal cumulative hazard curve for the twins.16

RESULTSThe mean age of the participants was 44.5 years, and 54.4%

were women. A total of 9.7% of the women and 7.6% of the menhad a history of asthma (P < .001). The prevalence of asthma de-creased with increasing age until age 60 years, after which a smallincrease was observed. In total, 50% of the men with asthma re-ported an age at onset below 15 years, whereas 50% of the womenreported an age at onset below 24 years. Symptoms of chronicbronchitis were significantly associated with asthma both in

men and in women. However, among subjects with asthma, thefrequency of chronic bronchitis was higher in women than men(26.1% vs 21.1%; P 5 .003), whereas among subjects withoutasthma the frequency of chronic bronchitis was higher amongmen than women (5.7% vs 5.1%; P 5 .012; Table I).

According to the Cox proportional hazards model applied tothe entire population of twins, sex, hay fever, atopic dermatitis,smoking, and exposure to passive smoking in childhood weresignificant risk factors, whereas BCG vaccination was protectivefor asthma (Table II). The effect of sex disappeared when restrictingthe analysis to subjects without symptoms of chronic bronchitis.

Fig 1 shows the risk of asthma in a co-twin of an affected twin.According to the Cox proportional hazards model restricted to af-fected sibships, the risk of asthma was increased about 3 times in amonozygotic co-twin relative to a dizygotic co-twin, indicatingthat variation in the age at onset of asthma was influenced by ge-netic factors (Table III). Conversely, the risk of asthma in dizy-gotic co-twins of the opposite sex was about half that ofdizygotic co-twins of the same sex, suggesting that the geneticfactors, which regulate the age at onset of asthma, may act differ-entially in men and women. The risk of asthma in the co-twin de-creased with increasing age at onset of asthma in the index twin,indicating that familial aggregation was more substantial aroundearly-onset asthma than late-onset asthma. The effect was atten-uated in dizygotic twins relative to monozygotic twins (P 5

.005), consistent with early-onset asthma being under tighter ge-netic control compared with late-onset asthma.

Fig 2 shows the correlation between the age at onset of asthmawithin monozygotic and dizygotic twin pairs. The correlation washigher in monozygotic than in dizygotic twins, particularlyamong males (Table IV). According to the frailty model ap-proach, the proportion of variation in the age at onset of asthmaexplained by (dominance) genetic variance was 34%, whereasthe proportion explained by environmental variance was 66%.Variance components analysis with the Gaussian mixed modelapproach revealed a heritability of 12%.

TABLE II. Predictors of asthma in twin subjects, 20 to 71 years of

age

All subjects

Subjects without

chronic bronchitis

Predictors HR (95% CI)

P

value HR (95% CI)

P

value

Age group (birth years) .000 .000

1972-1982 1.00 1.00

1962-1971 0.66 (0.57-0.77) 0.71 (0.60-0.85)

1952-1961 0.46 (0.39-0.54) 0.50 (0.41-0.61)

1942-1951 0.34 (0.29-0.41) 0.41 (0.33-0.50)

1931-1941 0.33 (0.27-0.41) 0.36 (0.28-0.45)

Female sex 1.21 (1.11-1.32) .000 1.08 (0.98-1.20) .138

Atopic dermatitis 2.52 (2.26-2.82) .000 2.52 (2.20-2.89) .000

Hay fever 7.29 (6.69-7.95) .000 8.20 (7.38-9.12) .000

BCG vaccination 0.87 (0.76-0.99) .039 0.84 (0.72-0.99) .032

Smoking 1.16 (1.11-1.21) .000 1.15 (1.09-1.22) .000

Passive smoking 1.17 (1.05-1.21) .004 1.18 (1.03-1.34) .014

HR, Hazard ratio.

Smoking is per 10 pack-years. Passive smoking is exposure to environmental tobacco

smoke in the childhood rearing environment. HRs are multivariately adjusted.

FIG 1. Risk of asthma in the co-twin of an affected twin. DZ-os, Opposite-

sex dizygotic twins; DZ-ss, same-sex dizygotic twins; MZ, monozygotic

twins.

TABLE III. Predictors of asthma in a co-twin of an affected twin

brother or sister, in twin pairs, 20 to 71 years of age

All subjects

(n 5 1614 pairs)

Subjects without

chronic bronchitis

(n 5 1013 pairs)

Predictors HR (95% CI) P value HR (95% CI) P value

Age at onset 0.86 (0.76-0.98) .019 0.70 (0.57-0.86) .001

Zygosity .000 .000

DZ-ss 1.00 1.00

MZ 2.59 (1.83-3.68) 3.07 (1.87-5.06)

DZ-os 0.55 (0.35-0.87) 0.46 (0.23-0.94)

DZ-os, Opposite-sex dizygotic twins; DZ-ss, same-sex dizygotic twins; HR, hazard

ratio; MZ, monozygotic twins.

Age at onset is the age at onset of asthma in the index twin (per 10 years). Adjusted for

age group, sex, smoking (pack-years), and BCG vaccination. The total number of

events is 229 in all subjects and 113 in subjects without chronic bronchitis.

J ALLERGY CLIN IMMUNOL

SEPTEMBER 2010

628 THOMSEN ET AL

DISCUSSIONThis study showed that genetic factors contribute to the

variation in the age at onset of asthma. In particular, around onethird of the interindividual variation in the age at onset of asthmawas ascribable to genetic factors. However, environmental factorswere also of importance and accounted for two thirds of thevariation. Furthermore, we found that female sex, hay fever,atopic dermatitis, smoking, and exposure to passive smoking inchildhood were risk factors for asthma, whereas BCG vaccinationwas protective for asthma. The risk of asthma in the co-twindecreased with increasing age at onset of asthma in the affected

twin, and more so in monozygotic twins compared with dizygotictwins, consistent with the hypothesis that early-onset asthma has agreater genetic component than late-onset asthma.3

We used questionnaire responses from a large nationwide twinregistry in which asthma was among many diseases and healthattributes studied. Because the purpose of the study was not onlyto report on asthma, this method made prevalence estimates ofself-reported asthma less prone to selection bias. However, thefrequency of adult-onset asthma was high and may to some extentreflect childhood-onset asthma relapsing in adulthood. Moreover,it is likely that older subjects provide less accurate estimates ofage at onset and that some older subjects without asthma mayhave had asthma in childhood without remembering. Further-more, some cases of late-onset asthma may represent chronicobstructive pulmonary disease, consistent with our finding thatasthma in females tended to be confounded with chronic bron-chitis. There was a tendency for affected subjects to prefer certainround numbers—for example, 10 or 20 years of age at onset ofasthma—indicative of some recall bias. However, the asthmaquestion has previously been shown to have a high specificity forthe diagnosis of asthma.17 Notably, we validated the asthma diag-nosis in a subset of 575 clinically examined twins from the cohortborn 1953 to 1982 who themselves reported a history of asthma onthe questionnaire, had their co-twins reported this history, orboth.18 Of these, a total of 89% with questionnaire asthma weresubsequently diagnosed with asthma on the basis of the findingsat the clinical examination. Likewise, only 12% of those who de-nied a history of asthma on the questionnaire survey were diag-nosed with asthma at the clinical examination.19

We found that exposure to passive smoking in childhoodsignificantly predicted asthma. Previous studies have recognizeda role of environmental tobacco smoke in the onset of asthma andwheezing in childhood.20 Furthermore, some studies have foundthat exposure to smoking may be a risk factor for adult asthmaand low lung function in adulthood even after adjusting for indi-vidual smoking habits.21,22 Also, individual smoking has been as-sociated with onset of asthma in adults although evidence iscircumstantial.23 In line with this, we found that smoking was asignificant risk factor for asthma in subjects both with and withoutsymptoms of chronic bronchitis.

BCG vaccination significantly protected against asthma.24 Therole of BCG vaccination in the etiology of asthma has not beencompletely established, although a recent review of studies foundno evidence of an association, either protective or provocative.25

FIG 2. Correlation between age at onset of asthma within twin pairs. The black dots are pairs in which both

have asthma; the gray circles are pairs in which 1 twin has asthma. The contours are the Dabrowska

estimates of the bivariate survival function.

TABLE IV. Correlation between asthma onset residuals in twin

pairs, 20 to 71 years of age

Zygosity Pairs (n 5 9773) Correlation (95% CI)

MZ twins 2976 0.372 (0.341-0.403)

DZ twins 6797 0.092 (0.068-0.115)

MZ male 1227 0.421 (0.298-0.381)

MZ female 1749 0.340 (0.298-0.381)

DZ male 1548 0.080 (0.031-0.130)

DZ female 2099 0.180 (0.139-0.222)

DZ opposite sex 3150 0.038 (0.003-0.072)

DZ, Dizygotic; MZ, monozygotic.

Twin correlations are for disease-free survival martingale residuals arising from

Weibull regression of individual age at onset versus age group, sex, smoking

(pack-years), BCG vaccination, and exposure to passive smoking in childhood.

J ALLERGY CLIN IMMUNOL

VOLUME 126, NUMBER 3

THOMSEN ET AL 629

However, the timing of BCG vaccination may be of importance.In particular, vaccination at an early age was inversely relatedto atopic outcomes in a rodent model, whereas vaccination at alater age was unrelated to these outcomes.26 On the contrary, Ba-ger et al27 showed that the age at BCG vaccination in human be-ings was not of importance for the development of asthma usingretrospective data on self-reported asthma.

Irrespective of the identification of these diverse risk profilesfor different ages at onset, it is not completely understood whetherasthma represents a single disease or should be regarded as acomplex of multiple, separate syndromes that overlap.28 Ouranalysis assumed a uniform risk of asthma throughout the life-span. We adjusted for age group and found a decrease in therisk of asthma with age, consistent with the idea that younger sub-jects have a higher risk of asthma than older subjects. However,the observation could reflect both a cohort and a period effect—-for example, because of changes in environmental exposures overthe past decades. Furthermore, other factors that we did not con-trol for, such as respiratory infections, dietary factors, and socio-economic status, were likely also of importance for variation inthe age at onset of asthma.

We infer from our study that the interindividual variation in theage at onset of asthma is ascribable partly to genetic factors. Thisfinding indicates that host-related differences in genetic makeup

cause different individuals to develop asthma at different ages,which is consistent with previous reports, suggesting that severalgenetic loci are linked to variation in the age at onset of disease.The idea of a genetic component interacting with differentenvironmental triggers at different ages opens the possibility ofmodifying the course of the disease through identification of thesedeterminants. Better understanding of the causes for variation inthe age at onset of disease may ultimately lead the way to targetedtreatments.

Key messages

d Little is known about genetic factors that influence theage at onset of asthma. This study explores the variationin the age at onset of asthma attributable to genetic andenvironmental factors using a large population of twins.

d Genetic factors explain 34% of the variation in the age at on-set of asthma, and environmental factors account for 66%.

d Host-related differences in genetic makeup cause differ-ent individuals to develop asthma at different ages.

REFERENCES

1. Reed CE. The natural history of asthma. J Allergy Clin Immunol 2006;118:543-8.

2. Romanet-Manent S, Charpin D, Magnan A, Lanteaume A, Vervloet D. EGEA

Cooperative Group. Allergic vs. nonallergic asthma: what makes the difference?

Allergy 2002;57:607-13.

3. London SJ, James Gauderman W, Avol E, Rappaport EB, Peters JM. Family his-

tory and the risk of early-onset persistent, early-onset transient, and late-onset

asthma. Epidemiology 2001;12:577-83.

4. ten Brinke A. Risk factors associated with irreversible airflow limitation in asthma.

Curr Opin Allergy Clin Immunol 2008;8:63-9.

5. Segala C, Priol G, Soussan D, Liard R, Neukirch F, Touron D, et al. Asthma in

adults: comparison of adult-onset asthma with childhood-onset asthma relapsing

in adulthood. Allergy 2000;55:634-40.

6. Hsu JY, King SL, Kuo BI, Chiang CD. Age of onset and the characteristics of

asthma. Respirology 2004;9:369-72.

7. Ober C, Hoffjan S. Asthma genetics 2006: the long and winding road to gene

discovery. Genes Immun 2006;7:95-100.

8. Bouzigon E, Ulgen A, Dizier MH, Siroux V, Lathrop M, Kauffmann F, et al. Evidence

for a pleiotropic QTL on chromosome 5q13 influencing both time to asthma onset

and asthma score in French EGEA families. Hum Genet 2007;121:711-9.

J ALLERGY CLIN IMMUNOL

SEPTEMBER 2010

630 THOMSEN ET AL

9. Dizier MH, Besse-Schmittler C, Guilloud-Bataille M, Selinger-Leneman H, Kauff-

mann F, Clerget-Darpoux F, et al. Indication of linkage and genetic heterogeneity

of asthma according to age at onset on chromosome 7q in 107 French EGEA fam-

ilies. Eur J Hum Genet 2001;9:867-72.

10. Hizawa N, Yamaguchi E, Konno S, Tanino Y, Jinushi E, Nishimura M. A

functional polymorphism in the RANTES gene promoter is associated with

the development of late-onset asthma. Am J Respir Crit Care Med 2002;

166:686-90.

11. Alcaıs A, Plancoulaine S, Abel L. An autosome-wide search for loci underlying

wheezing age of onset in German asthmatic children identifies a new region of in-

terest on 6q24-q25. Genet Epidemiol 2001;21(suppl 1):S168-73.

12. Skytthe A, Kyvik K, Holm NV, Vaupel JW, Christensen K. The Danish Twin Reg-

istry: 127 birth cohorts of twins. Twin Res 2002;5:352-7.

13. Christiansen L, Frederiksen H, Schousboe K, Skytthe A, von Wurmb-Schwark N,

Christensen K. Age- and sex-differences in the validity of questionnaire-based zy-

gosity in twins. Twin Res 2003;6:275-8.

14. Thomas DC, Langholz B, Mack W, Floderus B. Bivariate survival models for anal-

ysis of genetic and environmental effects in twins. Genet Epidemiol 1990;7:

121-35.

15. Pankratz VS, de Andrade M, Therneau TM. Random-effects Cox proportional

hazards model: general variance components methods for time-to-event data.

Genet Epidemiol 2005;28:97-109.

16. Yoo B, Pankratz VS, de Andrade M. Practical application of residuals from sur-

vival models in quantitative trait linkage analysis. Genet Epidemiol 2001;

21(suppl 1):S811-6.

17. Toren K, Brisman J, Jarvholm B. Asthma and asthma-like symptoms in adults

assessed by questionnaires. Chest 1993;104:600-8.

18. Thomsen SF, Ulrik CS, Kyvik KO, Ferreira MAR, Backer V. Multivariate genetic

analysis of atopy phenotypes in a selected sample of twins. Clin Exp Allergy 2006;

36:1382-90.

19. Thomsen SF. Risk factors for asthma in adult twins. PhD thesis. Copenhagen:

Faculty of Health Sciences, University of Copenhagen; 2006.

20. Gergen PJ, Fowler JA, Maurer KR, Davis WW, Overpeck MD. The burden of en-

vironmental tobacco smoke exposure on the respiratory health of children 2 months

through 5 years of age in the United States: Third National Health and Nutrition

Examination Survey, 1988 to 1994. Pediatrics 1998;101:E8.

21. Hu FB, Persky V, Flay BR, Richardson J. An epidemiological study of asthma

prevalence and related factors among young adults. J Asthma 1997;34:67-76.

22. Eisner MD. Environmental tobacco smoke exposure and pulmonary function

among adults in NHANES III: impact on the general population and adults with

current asthma. Environ Health Perspect 2002;110:765-70.

23. Jamrozik E, Knuiman MW, James A, Divitini M, Musk AW. Risk factors for adult-

onset asthma: a 14-year longitudinal study. Respirology 2009;14:814-21.

24. Thomsen SF, Kyvik KO, Bryld CE, Backer V. BCG vaccination and risk of atopic

disease in a twin cohort. Clin Respir J 2008;2:63-4.

25. Balicer RD, Grotto I, Mimouni M, Mimouni D. Is childhood vaccination associated

with asthma? a meta-analysis of observational studies. Pediatrics 2007;120:e1269-77.

26. Shen H, Huang H, Wang J, Ye S, Li W, Wang K, et al. Neonatal vaccination with

Bacillus Calmette-Guerin elicits long-term protection in mouse-allergic responses.

Allergy 2008;63:555-63.

27. Bager P, Rostgaard K, Nielsen NM, Melbye M, Westergaard T. Age at bacille Calmette-

Guerin vaccination and risk of allergy and asthma. Clin Exp Allergy 2003;33:1512-7.

28. Wenzel SE. Asthma: defining of the persistent adult phenotypes. Lancet 2006;368:

804-13.