Segregation of autoantibodies with disease in monozygotic twin pairs discordant for systemic sclerosis. Three further cases

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 autoimmune disease: genetics, gender and environment

Twin studies are powerful tools to discriminate whether a complex disease is due to genetic or environmental factors. High concordance rates among monozygotic (MZ) twins support genetic factors being predominantly involved, whilst low rates are suggestive of environmental factors. Twin studies have often been utilised in the study of systemic and organ specific autoimmune diseases. As an example, type I diabetes mellitus has been investigated to establish that that disease is largely affected by genetic factors, compared to rheumatoid arthritis or scleroderma, which have a weaker genetic association. However, large twin studies are scarce or virtually non-existent in other autoimmune diseases which have been limited to few sets of twins and individual case reports. In addition to the study of the genetic and environmental contributions to disease, it is likely that twin studies will also provide data in regards to the clinical course of disease, as well as risk for development in related individuals. More importantly, genome-wide association studies have thus far reported genomic variants that only account for a minority of autoimmunity cases, and cannot explain disease discordance in MZ twins. Future research is therefore encouraged not only in the analysis of twins with autoimmune disease, but also in regards to epigenetic factors or rare variants that may be discovered with next-generation sequencing. This review will examine the literature surrounding twin studies in autoimmune disease including discussions of genetics and gender.

Induction of apoptosis and fibrillin 1 expression in human dermal endothelial cells by scleroderma sera containing anti–endothelial cell antibodies

OBJECTIVE

Sera from patients with scleroderma (systemic sclerosis [SSc]) contain anti-endothelial cell antibodies (AECAs) capable of inducing endothelial cell apoptosis. We sought to determine whether SSc sera containing anticentromere antibodies (ACAs) or anti-topoisomerase I antibodies (or, anti-Scl-70 antibodies) contain subsets of AECAs that trigger distinct pathways of apoptosis and gene expression in normal adult human dermal endothelial cells (HDECs).

METHODS

Adult HDECs were grown to subconfluence and treated with control or SSc patient sera. Apoptosis was investigated by differential interference contrast (DIC) microscopy, microarrays of proapoptotic gene expression, caspase 3 protease activity, and flow cytometry for phosphatidyl serine translocation.

RESULTS

Flow cytometry and DIC microscopy demonstrated that HDECs exposed to SSc sera containing either SSc autoantibody underwent apoptosis at much higher levels than those treated with control sera. While unique gene expression profiles were induced in HDECs by stimulation with SSc sera containing the respective autoantibody, similar patterns of increased gene expression of transcripts for the proapoptotic protease caspase 3 as well as the SSc autoantigen fibrillin 1 were demonstrated. Caspase 3 gene expression correlated with increased protease activity, and targeted inhibition of this protease partly blocked SSc serum-induced apoptosis. Immunohistochemistry studies of serum-stimulated HDECs demonstrated the aberrant expression of fibrillin 1 protein only in apoptotic endothelial cells treated with SSc sera containing AECAs.

CONCLUSION

There are distinct AECA subsets in the sera of patients with limited SSc (with ACAs) and diffuse SSc (with anti-Scl-70) that induce unique patterns of HDEC gene expression in the setting of apoptosis associated with increased caspase 3 activity and the reexpression of endothelial cell fibrillin 1.

Analysis of systemic sclerosis in twins reveals low concordance for disease and high concordance for the presence of antinuclear antibodies

OBJECTIVE

To examine concordance for systemic sclerosis (SSc) in monozygotic (MZ) and dizygotic (DZ) twins.

METHODS

MZ and DZ twins were recruited nationwide. Zygosity was confirmed by DNA fingerprint analysis. The presence of antinuclear antibodies (ANAs) was determined using indirect immunofluorescence with HEp-2 cells as substrate. Identification of SSc-associated serum autoantibodies was performed by immunoprecipitation and double immunodiffusion. Major histocompatibility complex class II alleles were identified by polymerase chain reaction-restriction fragment length polymorphism analysis.

RESULTS

Concordance for SSc was found to be similar in MZ and DZ twins. Overall concordance for SSc was low in the twins (4.7%). Concordance for the presence of ANAs was significantly higher in MZ twins compared with DZ twins. SSc-associated serum autoantibodies occurred exclusively in patients with SSc. The distribution of SSc-associated serum autoantibodies was similar to that observed in our large database of SSc patients. Increased HLA allele sharing was detected in DZ twins, irrespective of disease concordance.

CONCLUSION

These results indicate that inherited genetic factors are not sufficient to explain the development of SSc. Rather, these data indicate that inheritance may play a role in the development of serum autoantibodies in the "healthy" twin sibling of an SSc patient.

Association of 5'-untranslated region of the Fibrillin-1 gene with Japanese scleroderma

Excessive production of extracellular matrix (ECM) constituents is a hallmark scleroderma or systemic sclerosis (SSc). Fibrillin-1, a major component of microfibrils in the ECM, may play a role in the pathogenesis of SSc. The TSK1 mouse model of SSc bears an in-frame duplication of the Fibrillin-1 gene (FBN1) which results in a larger than normal protein that is more susceptible to proteolysis. Metabolic labeling studies of Fibrillin-1 in human SSc dermal fibroblasts demonstrated that while normal amounts of Fibrillin-1 are synthesized, the protein itself appears to be unstable. Moreover, autoantibodies specific for Fibrillin-1 have been demonstrated in serum from SSc patients and TSK1 mice. In particular, a high frequency of anti-Fibrillin-1 was observed in Japanese patients with diffuse and limited scleroderma or CREST (calcinosis, Raynaud phenomenon, esophageal dysmotility, sclerodactyly, telangiectasia) syndrome. Genetic studies in a Native American population with high prevalence of using microsatellite marker showed strong association between FBN1 haplotypes and SSc. Subsequently, studies of FBN1 single nucleotide polymorphisms (SNPs) demonstrated that certain FBN1 haplotypes were associated with SSc in both Native American and Japanese patients with limited scleroderma. Thus, FBN1 was sequenced in 22 Japanese SSc patients to ascertain the presence of any relevant mutations or SNPs. Sequence analysis revealed eight coding and 14 non-coding SNPs and other polymorphisms. Among them, a CT insertion in the 5'-untranslated region of exon A had a significant negative association with disease.

The genetics of systemic sclerosis

The etiopathogenesis of systemic sclerosis (SSc) is unclear. With no definitive evidence supporting an environmental cause, recent attention has focused on genetic factors. Familial clustering and ethnic influences have been demonstrated. Human leukocyte antigen (HLA) associations exist but are more related to the presence of particular autoantibodies rather than to the disease. In addition, no single major histocompatibility complex (MHC) allele predisposes to SSc in all ethnic groups. The role of microchimerism in SSc is a novel yet unproven hypothesis that may be related to intergenerational HLA compatibility. Recent studies investigating polymorphisms in genes coding for extracellular matrix proteins and cell-signaling molecules implicate non-MHC areas in SSc pathogenesis. The data reviewed suggest that SSc is a multigenic complex disorder.

Familial occurrence frequencies and relative risks for systemic sclerosis (scleroderma) in three United States cohorts

OBJECTIVE

To determine the frequency with which scleroderma (systemic sclerosis; SSc) recurs in families and the familial relative risk (lambda) in the US.

METHODS

Family histories of SSc were prospectively surveyed in 3 large US cohorts of SSc patients, 2 in Texas and 1 in Michigan. Diagnoses of familial SSc were verified by rheumatologist evaluation and/or review of medical records. Familial relative risks for first-degree relatives (lambda1) and siblings (lambdas) were calculated using actual reported counts of first-degree relatives in 2 cohorts and recent estimates of SSc prevalence in the US.

RESULTS

Compared with the estimated prevalence of SSc in the US (2.6 cases/10,000 population [0.026%]), the disease occurred in 1 or more first-degree relatives in 1.5-1.7% of SSc families in the 3 cohorts (or 11 of 703 families [1.6%]), a significant increase. Familial relative risks in first-degree relatives in the 3 cohorts ranged from 10 to 16 (13 combined), and in siblings they ranged from 10 to 27 (15 combined).

CONCLUSION

SSc occurs significantly more frequently in families with scleroderma (1.6%) than in the general population (0.026%). A positive family history of SSc is the strongest risk factor yet identified for SSc; however, the absolute risk for each family member remains quite low (<1%).

Genetic factors in the etiology of systemic sclerosis and Raynaud phenomenon

There is increasing evidence that genetic factors play important roles in susceptibility to and expression of systemic sclerosis (SSc), as well as primary Raynaud phenomenon. Familial aggregation for SSc, although infrequent (1.2%-1.5% of SSc families), has now been established, and when compared with population prevalence represents a significant risk factor for the disease and lays a firmer foundation for genetics in etiopathogenesis. Major histocompatibility complex class II alleles increase disease risk in some populations but are more strongly correlated with specific autoantibody profiles. Microchimerism influenced by human leukocyte antigen also remains an intriguing hypothesis. A variety of extracellular matrix genes, including fibrillin-1, have become additional candidates for contributing to what is likely a complex genetic disease. Reviewed here is evidence relating to these concepts, especially new data reported over the last year.

Association of microsatellite markers near the fibrillin 1 gene on human chromosome 15q with scleroderma in a Native American population

OBJECTIVE

To localize disease genes for scleroderma, or systemic sclerosis (SSc), in a population of Choctaw Native Americans with a high prevalence of SSc, in which there is evidence of a possible founder effect.

METHODS

A candidate gene approach was used in which microsatellite alleles on human chromosomes 15q and 2q, homologous to the murine tight skin 1 (tsk1) and tsk2 loci, respectively, were analyzed in Choctaw SSc cases and race-matched normal controls for possible disease association. Genotyping first-degree relatives of the cases identified potential disease haplotypes, and haplotype frequencies were obtained by expectation-maximization and maximum-likelihood estimation methods. Simultaneously, the ancestral origins of contemporary Choctaw SSc cases were ascertained using census and historical records.

RESULTS

A multilocus 2-cM haplotype was identified on human chromosome 15q homologous to the murine tsk1 region, which showed a significantly increased frequency in SSc cases compared with controls. This haplotype contains 2 intragenic markers for the fibrillin 1 (FBN1) gene. Genealogical studies demonstrated that the SSc cases were distantly related, and their ancestry could be traced back to 5 founding families in the mid-eighteenth century. The probability that the SSc cases share this haplotype due to familial aggregation effects alone was calculated and found to be very low. There was no evidence of any microsatellite allele disturbances on chromosome 2q in the region homologous to the tsk2 locus or the region containing the interleukin-1 family.

CONCLUSION

A 2-cM haplotype on chromosome 15q that contains FBN1 is associated with scleroderma in Choctaw Native Americans from Oklahoma. This haplotype may have been inherited from common founders about 10 generations ago and may contribute to the high prevalence of SSc that is now seen.