Introduction

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Rheumatoid arthritis is a systemic inflammatory autoimmune disease of unknown cause. Environmental and genetic risk factors have been identified, but no single risk factor has emerged as necessary or sufficient to cause the disease.

Twin studies represent one of the simplest ways to unravel the relative importance of genetic and environmental effects. In studies of specific diseases or traits in twins who volunteer to take part, monozygotic, concordant, and female twins tend to be over- represented. ^{1 2} Hence, much of the available literature on rheumatoid arthritis in twins overestimates the contribution of genetic factors.^3-5 Only two previous studies were population based, but confirmation of the diagnosis according to validated classification criteria was not performed. ^{1 6}

We undertook a nationwide study among twins in Denmark to estimate the importance of genetic effects in the development of rheumatoid arthritis.

	Methods

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Ascertainment of twinsThe study comprised two nationwide twin populations. The older birth cohort comprised 1631 same sex pairs of twins born 1921-40 in which both twins were alive in 1994.⁷ The younger birth cohort comprised 34 076 surviving twins from same and opposite sex pairs of twins born 1953-82.⁷ To estimate a possible selection bias introduced by the requirement that both twins had to be alive we also sent questionnaires to 990 surviving individuals from same sex twin pairs born 1921-30.

Ascertainment and verification of rheumatoid arthritis In 1994 the twins were asked by questionnaire if they had ever suffered from rheumatoid arthritis. Twins who reported that they had rheumatoid arthritis subsequently received a clinical profile questionnaire followed by a telephone interview. If rheumatoid arthritis could not be ruled out they were asked for permission to approach their non-affected cotwin. Both twins were invited to have a clinical examination. They underwent a structured interview and clinical examination, and blood samples were drawn for measurement of rheumatoid factor, HLA typing, and determination of zygosity by blood group analysis. We used the modified 1987 revised criteria of the American Rheumatism Association to confirm the diagnosis.⁸ Time of onset was defined as the time when the diagnosis was established for the first time. Time of discordance was defined as the time from onset in one twin until onset in the second twin or the end of observation.

Validation of completenessWe used record linkage with the Danish discharge registry to find any twins with rheumatoid arthritis that we had not already identified. We estimated the completeness of the study with capture-recapture.⁹

We linked records of the twin registry and the Danish national registry of deaths. We sent a questionnaire to individual twins who would have been eligible for participation except for the death of their cotwin. The surviving twin was asked whether he or she or the deceased cotwin had ever had rheumatoid arthritis.

EthicsThe study was approved by all the regional scientific ethics committees in Denmark and the Danish data protection board.

AnalysisWe used the probandwise concordance rate. ^{10 11} In our study a proband was a twin who independently of his or her cotwin reported rheumatoid arthritis and who fulfilled our classification criteria for rheumatoid arthritis. A secondary case was a twin ascertained through a cotwin and who fulfilled our classification criteria for rheumatoid arthritis. We excluded pairs of twins in which who both twins had rheumatoid arthritis and neither twin fulfilled the proband criteria.

The calculation of concordance rates is given in more detail in the full version on the BMJ's website bmj.com.

	Results

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The overall response rate after one reminder was 75% (2445/3262) in older twins and 86% (29 433/34 076) in younger twins. Forty nine twins satisfied the modified criteria of the American Rheumatism Association.

There were no significant differences between monozygotic and dizygotic twins regarding sex, age, age at onset, mean discordance time, presence of rheumatoid factor, bony erosions, or shared epitope (table). Nodules were present more often in monozygotic twins than in dizygotic twins.

There was no difference between monozygotic and dizygotic twin pairs in concordance rates. The probandwise concordance rate was 0 (95% confidence interval 0 to 24.7) in monozygotic twins and 8.8 (1.9 to 23.7) in dizygotic twins, and the pairwise concordance rate was 0 (0 to 24.7) and 5.9 (0.7 to 19.7), respectively.

The record linkage study identified 46 twins with possible rheumatoid arthritis, but in 23 cases the diagnosis could not be verified. Among the remaining 23 twins, we identified five solely through the Danish discharge registry. On the basis of the capture-recapture model our questionnaire had an estimated probability of ascertainment of 78%, and hence nine cases (95% confidence interval 3 to 16) may have been missed. We found no concordant monozygotic pairs through the Danish discharge registry.

The response rate from twins whose cotwin had died was 74%. There was no difference in response rate between monozygotic and dizygotic twins nor between men and women. There was no difference in concordance for rheumatoid arthritis between monozygotic and dizygotic twin pairs.

From the record linkage study with the registry of deaths we identified six twins from six different twin pairs. In three cases the cotwins of the dead twins who had had rheumatoid arthritis were still alive and responded to the questionnaire. In one case a dizygotic twin pair was concordant for rheumatoid arthritis according to self report. We found no monozygotic concordant pairs.

	Discussion

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In this large twin study monozygotic twins were no more likely to be concordant for rheumatoid arthritis than dizygotic twins. Our results do not support a genetic contribution to the development of rheumatoid arthritis. This is the first study to combine the recruitment of twins with rheumatoid arthritis through a questionnaire sent to a population based random sample with a subsequent clinically based validation of the diagnosis.

Cross sectional studies cannot show the lifetime risk for cotwins, but this does not imply any bias as long as the mean and distribution of discordance time does not differ between monozygotic and dizygotic twins, as in our study. None of the previously published twin studies on rheumatoid arthritis have dealt with this potential bias. The association between rheumatoid arthritis and HLA markers is related to age at onset and sex. ^{12 13} We did not find any difference in age at onset and sex between monozygotic and dizygotic probands, though rheumatic nodules, an indicator of more severe disease, were more common in monozygotic twins. The UK rheumatoid arthritis twin study is the only other such study that can reject this potential bias.⁵ Even though our study was population based, twins with rheumatoid arthritis did not differ from twins recruited from clinical settings with regard to the traditional measures of severity, including HLA associated antigens.

The lack of concordant pairs could be due to observer bias, but this is unlikely as the cotwins in pairs discordant for disease did not show any signs of arthritis, and in most cases disease classification was based on medical records and blood samples, both of which are resistant to observer bias. We identified only a small number of twins with rheumatoid arthritis because participants were mostly young, with 90% of the twins born in 1952-83.

Our study had an estimated probability of ascertainment of 78%. As there is always bias in ascertainment towards monozygotic twin pairs and twin pairs concordant for specific diseases and traits, complete ascertainment would not change our estimate of concordance for rheumatoid arthritis.

We acknowledge that our sample was relatively small, but we consider our results to be the most unbiased estimate of the genetic contribution to rheumatoid arthritis and support the observation of a weak familial aggregation. ^{14 15} Genetic makeup seems of minor importance in the development of rheumatoid arthritis.

	Acknowledgments

We thank Dorte Viborg for help with the establishment of a blood and DNA repository. We also thank the staff at the tissue typing laboratory in Skejby and Copenhagen for use of laboratory facilities and the staff at Statens Serum Institut for the determination of rheumatoid factor and the use of laboratory facilities.

Contributors: see bmj.com

	Footnotes

Funding: Danish Rheumatism Association; Clinical Institute, Odense University; Karen Hansens Foundation; Danish Medical Association's Research Foundation; Købmand Hans Christensens Foundation; Grosserer Valdemar Foersom og hustru Thyra Foersom, født Otto's Foundation; Henny og Helge Holgersens Foundation; Ingemann O Buck's Foundation; Ingeniør af Frederikssund Søren Alfred Andersen's Foundation.

Competing interests: None declared.

The full version of this article appears on bmj.com

	References

Top Abstract Introduction Methods Results Discussion References

1.	Harvald B, Hauge M. Hereditary factors elucidated by twin studies. In: Neel JV, Shaw MV, Schull WJ, eds. Genetics and the epidemiology of chronic diseases. Washington, DC: Department of Health, Education and Welfare, 1965:61-76 (Public Health Service Publication No 1163).
2.	Lykken DT, Tellegen A, DeRubeis R. Volunteer bias in twin research: the rule of two-thirds. Soc Biol 1978; 25: 1-9[Medline].
3.	Lawrence JS. Heberden oration, 1969. Rheumatoid arthritisnature or nurture? Ann Rheum Dis 1970; 29: 357-379[Medline].
4.	Bellamy N, Duffy D, Martin N, Mathews J. Rheumatoid arthritis in twins: a study of aetiopathogenesis based on the Australian twin registry. Ann Rheum Dis 1992; 51: 588-593[Abstract].
5.	Silman AJ, MacGregor AJ, Thomson W, Holligan S, Carthy D, Farhan A, et al. Twin concordance rates for rheumatoid arthritis: results from a nationwide study. Br J Rheumatol 1993; 32: 903-907[Medline].
6.	Aho K, Koskenvuo M, Tuominen J, Kaprio J. Occurrence of rheumatoid arthritis in a nationwide series of twins. J Rheumatol 1986; 13: 899-902[Medline].
7.	Kyvik KO, Christensen K, Skytthe A, Harvald B, Holm NV. The Danish twin register. Dan Med Bull 1996; 43: 467-470[Medline].
8.	MacGregor AJ, Bamber S, Silman AJ. A comparison of the performance of different methods of disease classification for rheumatoid arthritis. Results of an analysis from a nationwide twin study. J Rheumatol 1994; 21: 1420-1426[Medline].
9.	Hook EB, Regal RR. Capture-recapture methods in epidemiology: methods and limitations. Epidemiol Rev 1995; 17: 243-264[Medline].
10.	Holm NV. A note on ascertainment probability in the Allen/Hrubec twin model. Acta Genet Med Gemellol (Roma) 1983; 32: 37-47[Medline].
11.	Smith C. Concordance in twins: methods and interpretation. Am J Hum Genet 1974; 26: 454-466[Medline].
12.	Jaraquemada D, Ollier W, Awad J, Young A, Silman A, Roitt IM, et al. HLA and rheumatoid arthritis: a combined analysis of 440 British patients. Ann Rheum Dis 1986; 45: 627-636[Abstract].
13.	MacGregor A, Ollier W, Thomson W, Jawaheer D, Silman A. HLA-DRB1*0401/0404 genotype and rheumatoid arthritis: increased association in men, young age at onset, and disease severity. J Rheumatol 1995; 22: 1032-1036[Medline].
14.	Del Junco D, Luthra HS, Annegers JF, Worthington JW, Kurland LT. The familial aggregation of rheumatoid arthritis and its relationship to the HLA-DR4 association. Am J Epidemiol 1984; 119: 813-829[Abstract].
15.	Barrera P, Radstake TR, Albers JM, van Riel PL, van De Putte LB. Familial aggregation of rheumatoid arthritis in the Netherlands: a cross-sectional hospital-based survey. European consortium on rheumatoid arthritis families (ECRAF). Rheumatology (Oxford) 1999; 38: 415-422[Abstract].

(Accepted 1 October 2001)

Commentary: Do genes or environment influence development of rheumatoid arthritis?

Alan J Silman, professor. 

ARC Epidemiology Unit, Manchester University Medical School, Manchester M13 9PT

a.silman{at}man.ac.uk

The aetiology of rheumatoid arthritis, like several other chronic disorders, is widely accepted to be "multifactorial." This term suggests that the disease results from one or more environmental influences acting on a genetically susceptible background. Although the relative contributions of genetic and environmental influences are the source of several investigations, in the strictest sense, as with most other diseases, neither is enough on its own to explain onset of the disease.

The size of a genetic effect is typically estimated from the familal risk of recurrence, defined as the increased risk in first degree relatives of affected individuals compared with the background occurrence in the population. With this approach the genetic contribution to rheumatoid arthritis is indeed small. The risk of recurrence in a sibling is possibly not much greater than 2,¹ substantially lower than that seen, for example, with other autoimmune disorders such as multiple sclerosis and thyroid disease.² An extension to that approach is to consider the comparative risk between identical and non-identical cotwins of affected individuals. The underlying hypothesis is that identical (monozygotic) and (same sex) non-identical (dizygotic) twins are similar in their sharing of the environment and hence the magnitude of any excess risk of disease in the monozygotic twins quantifies the genetic effect.

Two nationwide twin studies, one from Finland,³ the other from the United Kingdom,⁴ yielded similar concordance rates in monozygotic twins of around 15%, four times greater than that seen in dizygotic twins. Indeed, from these two studies it was estimated that shared genetic factors in the monozygotic twins explained about 60% of the incidence of rheumatoid arthritis.⁵ The assumption about similarity in environment between the two types of twins might not be true,⁶ and hence this proportion represents the upper limit.

Against this background the results of Svendsen et al seem somewhat surprising. In this carefully conducted investigation with a national twin register they found no concordant monozygotic pairs and two concordant dizygotic pairs. The authors conclude that their data argue against a major genetic influence. There are some limitations in their interpretation. Small numbers and consequently wide confidence intervals mean that their results are not inconsistent with those from previously published studies. Also, the authors relied on recalled diagnosis that might have underestimated the true occurrence of the disease.

There is an undoubted genetic contribution to rheumatoid arthritis which, at least in part, is explained by a susceptibility allele at the HLA-DRB1 locus.⁷ Indeed, possession of susceptibility alleles at this locus explains why some monozygotic twins are, and others are not, concordant for rheumatoid arthritis.⁸ HLA, however, may explain only about half of the genetic contribution to rheumatoid arthritis,⁹ although it has been a difficult task to show genetic susceptibility factors other than HLA consistently among studies. ^{10 11}

In summary the study by Svendsen et al cannot disprove a genetic component in susceptibility to rheumatoid arthritis. However, their results emphasise that the genetic effects are weak compared with environmental ones in explaining differences in occurrence of the disease. This makes the task more difficult for those attempting large scale linkage studies aimed at revealing the genetic basis for rheumatoid arthritis.

	Footnotes

   Competing interests: None declared.

	References

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2.	Cooper GS, Miller FW, Pandey JP. The role of genetic factors in autoimmune disease: implications for environmental research. Environ Health Perspect 1999; 107(suppl 5): 693-700.
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4.	Silman AJ, MacGregor AJ, Thomson W, Holligan S, Carthy D, Farhan A, et al. Twin concordance rates for rheumatoid arthritis: results from a nationwide study. Br J Rheumatol 1993; 32: 903-907[Medline].
5.	MacGregor AJ, Snieder H, Rigby AS, Koskenvuo M, Kaprio J, Aho K, et al. Characterizing the quantitative genetic contribution to rheumatoid arthritis using data from twins. Arthritis Rheum 2000; 43: 30-37[Medline].
6.	Phillips DIW. Twin studies in medical research: can they tell us whether diseases are genetically determined? Lancet 1993; 341: 1008-1009[Medline].
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8.	Jawaheer D, Thomson W, MacGregor AJ, Carthy D, Davidson J, Dyer PA, et al. "Homozygosity" for the HLA-DR shared epitope contributes the highest risk for rheumatoid arthritis concordance in identical twins. Arthritis Rheum 1994; 37: 681-686[Medline].
9.	Rigby AS, Silman AJ, Voelm L, Gregory JC, Ollier WER, Khan MA, et al. Investigating the HLA component in rheumatoid arthritis: an additive (dominant) mode of inheritance is rejected, a recessive mode is preferred. Genet Epidemiol 1991; 8: 153-175[Medline].
10.	Cornelis F, Faure S, Martinez M, Prud'homme JF, Fritz P, Dib C, et al. New susceptibility locus for rheumatoid arthritis suggested by a genome-wide linkage study. Proc Natl Acad Sci U S A 1998; 95: 10746-10750[Abstract/Full Text].
11.	Jawaheer D, Seldin MF, Amos CI, Chen WV, Shigeta R, Monteiro J, et al. A genomewide screen in multiplex rheumatoid arthritis families suggests genetic overlap with other autoimmune diseases. Am J Hum Genet 2001; 68: 927-936[Medline].