Evidence against HLA and immunological dependence of disease outbreak in SLE. Immunological characterisation of identical twins clinically discordant for SLE

Lessons learned from twins in autoimmune and chronic inflammatory diseases

Autoimmunity and chronic inflammation recognize numerous shared factors and, as a result, the resulting diseases frequently coexist in the same patients or respond to the same treatments. Among the convenient truths of autoimmune and chronic inflammatory diseases, there is now agreement that these are complex conditions in which the individual genetic predisposition provides a rate of heritability. The concordance rates in monozygotic and dizygotic twins allows to estimate the weight of the environment in determining disease susceptibility, despite recent data supporting that only a minority of immune markers depend on hereditary factors. Concordance rates in monozygotic and dizygotic twins should be evaluated over an observation period to minimize the risk of false negatives and this is well represented by type I diabetes mellitus. Further, concordance rates in monozygotic twins should be compared to those in dizygotic twins, which share 50% of their genes, as in regular siblings, but also young-age environmental factors. Twin studies have been extensively performed in several autoimmune conditions and cumulatively suggest that some diseases, i.e. celiac disease and psoriasis, are highly genetically determined, while rheumatoid arthritis or systemic sclerosis have a limited role for genetics. These observations are necessary to interpret data gathered by genome-wide association studies of polymorphisms and DNA methylation in MZ twins. New high-throughput technological platforms are awaited to provide new insights into the mechanisms of disease discordance in twins beyond strong associations such as those with HLA alleles.

Twin studies in rheumatic diseases

Twin studies attract both clinicians and geneticists because of the value of the twin method in helping unravel the genetic predisposition to diseases and the role of environment in their causation. In the field of rheumatology, there are many case reports on twins concordant or discordant for diseases. Interesting as such reports may be, very few generalizations can be made from them. The concordance rate among monozygotic (MZ) twins indicates the maximum level of genetic contribution. Based on studies of systematically compiled twin series, the concordance rate is about 15% for rheumatoid arthritis; the rate is probably of the same order of magnitude for systemic lupus erythematosus (SLE). The fine specificity of antinuclear antibodies of MZ twins at least one of whom is affected by SLE is very similar, despite somatic generation of immune diversity. Up to now, twin studies have provided little information on the role of environmental factors in rheumatic diseases. A notable exception is a case-control study of MZ twins discordant for smoking that reinforced the role of smoking as a cause of lumbar intervertebral disc degeneration.

A revised estimate of twin concordance in systemic lupus erythematosus

OBJECTIVE

Based on a small clinical series and previously published case reports, concordance for systemic lupus erythematosus (SLE) among monozygous (MZ) twins has been reported to be as high as 69%. Using a larger and less biased sample, we provide another estimate of this percentage.

METHODS

We established a registry of twins with SLE, based upon self-reports and information provided by the patients' physicians. We used DNA fingerprinting to validate the reported zygosity in a sample of these twins.

RESULTS

Of 107 twin pairs meeting the American College of Rheumatology 1982 revised criteria for the diagnosis of SLE, 24% of 45 MZ pairs and 2% of 62 dizygous (DZ) pairs were concordant. The frequency distributions of diagnostic criteria and disease symptoms in the SLE patients were similar to those in other published reports of SLE patients. Zygosity was confirmed by DNA fingerprinting in a subsample of 15 self-described MZ twins and 7 self-described DZ twins. All individuals had correctly predicted their zygosity.

CONCLUSION

MZ concordance for SLE is similar to that for other autoimmune diseases and is much lower than previously believed.

A pair of monozygotic twins who are concordant for myasthenia gravis but became discordant for systemic lupus erythematosus post-thymectomy

We describe a pair of monozygotic twins who are concordant for myasthenia gravis but discordant for systemic lupus erythematosus (SLE). SLE developed in twin 1 18 years post-thymectomy and has been characterized by recurrent transverse myelitis and optic neuritis. Twin 2 remains well post-thymectomy, except for a skin rash and persistent leukopenia. Both twins have developed autoimmune thyroid disease. We review genetic and environmental factors of importance in the pathogenesis of SLE and discuss the possible role of thymectomy in the etiology of the disease.

Nonlinkage of the T cell receptor alpha, beta, and gamma genes to systemic lupus erythematosus in multiplex families

To test the possibility that T cell antigen receptor (TcR) genes are linked to the genes involved in the pathogenesis of systemic lupus erythematosus (SLE), genomic DNA restriction fragment length polymorphisms were studied, using the Southern blot technique, in 5 families with multiple members with SLE, 14 unrelated SLE patients, and 14 normal controls. Polymorphic patterns were detected with probes for all 3 TcR chains, but there was no significant difference in the distribution of the restriction fragment length polymorphism pattern among the patients, the relatives, and the controls. Furthermore, in the families with at least 2 individuals with the disease, each of the 3 TcR chain genes did not cosegregate with the disease. We conclude that TcR alpha, beta, and gamma chain genes are not likely to be linked to genes related to SLE.