Environment, Not Genetics, More Important In Rheumatoid Arthritis

[Guest guest]

Environment, Not Genetics, More Important In Rheumatoid Arthritis

01/31/2002

By Harvey McConnell

Investigation of 37,338 monozygotic and dizygotic twins indicates that

environment rather than genetics is more important in the development of

rheumatoid arthritis.

" Genetic makeup seems of minor importance, " declares Dr Anders Svendsen

and colleagues at the Department of Rheumatology, Odense University

Hospital, Odense, Denmark.

Although rheumatoid arthritis is a systemic inflammatory autoimmune

disease of unknown cause, the investigators point out, " 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 overrepresented. Hence, much of

the available literature on rheumatoid arthritis in twins overestimates

the contribution of genetic factors. "

This has prompted the researchers to undertake a nationwide study among

twins in Denmark to estimate the importance of genetic effects in the

development of rheumatoid arthritis. A cohort study with record linkage

between a twin registry and the Danish discharge registry as well as the

Danish national registry of deaths were used to estimate completeness.

The 37,338 twins in the study were questioned about rheumatic diseases.

Any case of rheumatoid arthritis was verified by clinical examination

and from medical records.

Dr Svendsen and colleagues found rheumatoid arthritis was verified in 13

monozygotic and 36 dizygotic twins. There were no concordant monozygotic

twin pairs and two concordant dizygotic twin pairs.

" 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, " the clinicians declared.

In addition, the clinicians said, " 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. "

Dr Svendsen concludes: " 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. "

BMJ 2002; 324: 264-67