Download Genetic background of systemic sclerosis: autoimmune genes take

RHEUMATOLOGY
Rheumatology 2010;49:203–210
doi:10.1093/rheumatology/kep368
Advance Access publication 18 November 2009
Review
Genetic background of systemic sclerosis:
autoimmune genes take centre stage
Yannick Allanore1,2, Philippe Dieude3 and Catherine Boileau2,4
SSc is a complex multiorgan disease. The key steps in its pathogenesis include early endothelial damage,
dysregulation of the immune system with abnormal autoantibody production and fibroblast activation
resulting in hyperproduction of extracellular matrix. The disease is caused by an interaction between
susceptibility genes and environmental triggers since epidemiological data, including family and twin
studies, reveal a genetic component in the pathogenesis of SSc. The candidate gene approach has
mainly been employed to identify SSc susceptibility genes. We will focus on data obtained through
large samples of well-phenotyped patients and replicated in independent cohorts. These case–control
association studies have enabled the identification of several genes that are shared with other connective
tissue disorders, and for some of these, putative autoimmune susceptibility genes have been identified.
Indeed, we will mainly focus on IRF5 (rs2004640), STAT4 (rs7574865), PTPN22 (rs2476601) and
BANK1 (rs3733197 and rs10516487) data. Some of these genes/loci are common to several autoimmune diseases, indicating a shared genetic background also contributing to SSc. Among
connective tissue disorders, similarities for genetic markers with SLE are noteworthy. Most likely, these
immune-modifying genes could interact and influence both disease phenotype and severity. Less
evidence is available yet with regard to genetic markers relating to the vascular and fibrotic aspects
of the disease.
Key words: SSc, Genes, Polymorphisms, Autoimmunity.
Introduction
SSc is a complex multiorgan disease affecting the
immune system, the microvascular network and the
connective tissue. Only few aspects of the disease pathogenesis are known, starting from the early inflammatory
phase to the fibrosis of cutaneous and internal organs.
Progressive organ failure renders SSc a severe and
lethal disease: it may display an acute evolution that
may become chronic leading to a major disability and
loss of quality of life and even death. Many points have
been clarified by recent epidemiological studies [1–5].
It has been confirmed that SSc is the most severe form
1
Université Paris Descartes, Service de Rhumatologie A, Hôpital
Cochin, 2Université Paris Descartes, Unité INSERM U781, Hôpital
Necker, 3Université Diderot Paris 7, Service de Rhumatologie, Hôpital
Bichat Claude Bernard, APHP, Paris and 4Service de Biochimie
Génétique et Hormonologie, Université Versailles Saint Quentin en
Yvelines, Hôpital Ambroise Paré, Boulogne, France.
Submitted 30 June 2009; Revised version accepted 7 October 2009.
Correspondence to: Yannick Allanore, Service de Rhumatologie A,
Hôpital Cochin, 27 rue du faubourg St Jacques, 75014 Paris, France.
E-mail: [email protected]
of CTD: the standardized mortality ratio was estimated at
1.5–7.2 in an international study [2] and at 2.7 in Canada
[3]. Several organ involvements remain untreatable and no
specific drug counteracting the fibrotic process has been
found. This has been highlighted by the poor results
recently reported in SSc-related pulmonary arterial hypertension [6] and interstitial lung disease [7].
Prevalence of the disease varies worldwide and
population-based studies yield higher prevalence than
hospital records-based studies. In North America, the
prevalence of SSc has been reported as 74.4 cases per
100 000 women [95% CI (confidence interval) 69.3, 79.7]
and 13.3 cases per 100 000 men (95% CrI 11.1, 16.1) in a
Canadian study [8], whereas in the USA figures of 26 per
100 000 [9] vs 75 per 100 000 [10] were reported by medical records- vs population-based studies, respectively.
In Europe, the prevalence was reported as 16 per
100 000 in Denmark [11] and 15.8 per 100 000 adults
(95% CI 129, 187) in France [12].
The scope of this review is to present and discuss the
more relevant data on genetic susceptibility factors
obtained through large samples and replicated in independent cohorts. We will focus on genes regulating the
! The Author 2009. Published by Oxford University Press on behalf of the British Society for Rheumatology. All rights reserved. For Permissions, please email: [email protected]
R EV I E W
Abstract
Yannick Allanore et al.
immune system (i.e. innate immunity, cell signalling and
T-cell biology) for which the data are more consistent. We
will discuss shared genetics and mechanisms with other
connective tissue disorders and autoimmune diseases,
and also the interaction of environment with host susceptibility genes.
Genetic background
SSc is not inherited in a Mendelian fashion, and although
SSc pathogenesis is unclear, it is believed that both
genetic and environmental factors contribute to disease
susceptibility and clinical expression or progression [13].
Complex genetic diseases are influenced by the interplay
of multiple genes and/or the environment; susceptibility
genes act in concert to increase an individual’s risk of
disease. Thus, in contrast to the situation in monogenic
traits, most susceptibility genes exert only a minor individual effect on the disease itself.
Two complementary analytical methods (linkage with
positional cloning and association studies) have been
employed to detect the specific genetic regions involved
in the disease process. However, in complex diseases,
due to the large number of loci that may be involved,
and the genetic heterogeneity underlying the phenotypic
heterogeneity, it has often been difficult to replicate linkage evidence. The availability of genome-wide markers,
combined with multipoint analyses using closely linked
markers, has increased the power of studies. The
genome-wide association (GWA) approach has yielded
a wealth of new genetic susceptibility genes/loci in complex genetic disorders. The completion of the Human
Genome Project [14] and high-throughput genotyping
technology have enabled researchers to carry out case–
control association studies on thousands of samples
with several hundreds of thousands of markers throughout the human genome. This case–control approach is
designed to find the loci that fit with the common
disease–common variant hypothesis of human disease.
It has provided some important results in immunemediated diseases, such as RA, SLE [15] and Crohn’s
disease [16]. However, despite extensive efforts, only a
few susceptibility genes have been identified to date.
The risk of SSc is increased among first-degree relatives of patients, compared with the general population.
In a study of 703 families in the USA, including 11 multiplex SSc families, the familial relative risk in first-degree
relatives was 13, with a 1.6% recurrence rate, compared
with 0.026% in the general population [17]. The sibling risk
ratio was 15 (range 10–27 across cohorts). Although a
family history of SSc is the highest risk factor identified to
date, the likelihood of developing the disease is <1%
among offsprings of patients and others. Furthermore,
familial associations might relate to some environmental
exposure. The only twin study reported to date included
42 twin pairs (24 monozygotic) [18]. The data show a
similar concordance rate in both the monozygotic (4.2%)
and dizygotic twins (5.6%; NS), and an overall crosssectional concordance rate of 4.7% (Table 1). However,
concordance for the presence of ANAs was significantly
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TABLE 1 Genetic background of SSc: evidence from
epidemiological data and prevalence analyses
Population
Prevalence, %
References
General population:
unrelated individuals
Family approach:
first-degree relatives
Monozygotic twins
0.026
[16]
1.6
[16]
4.7
[17]
higher in the monozygotic twins (90%) than in the dizygotic twins (40%). It is noteworthy that for one concordant
pair, the onset of disease in the first twin was 15 years
after the onset of disease in her sister. In addition, the
mean age at evaluation of the monozygotic twins was
48 years and that of the dizygotic twins was 48.5 years
(range for both 28–69 years), which is commonly recognized as the age at risk for occurrence of SSc and thus
one may argue that a longer follow-up is required before
drawing a definitive conclusion for clinical concordance.
A further study showed a high level of concordance
among monozygotic twins regarding the gene expression
profile of cultured dermal fibroblasts [19]. These results
might support the role of the genetic background, at
least for some component of the disease, but also clearly
confirm that genetic susceptibility alone is not sufficient
for the development of SSc.
Familial and individual aggregation of
SSc with other autoimmune diseases
One recent study attempted to ascertain whether a higher
risk of developing rheumatic diseases in first-generation
immigrant parents in Sweden is associated with a higher
risk of developing rheumatic diseases in the next generation [20]. Polish-born immigrants and second-generation
Yugoslavs and Russians showed a significantly increased
risk of SSc, consistent with the genetic component of the
disease. However, this cannot rule out that environmental
exposure is relevant for these individuals who may have
been exposed to similar environmental factors.
History of the co-occurrence of other autoimmune
diseases (AIDs) in SSc patients as well as a family history
of autoimmunity has been the subject of several other
recent studies. Polyautoimmunity (i.e. AID co-occurring
within patients) and familial autoimmunity (i.e. diverse
AID co-occurring within families) have been investigated
[21]. Among 719 SSc patients, 273 (38%) had at least
one other AID according to a cross-sectional study. The
most frequent autoimmune diseases were thyroid disease
(38%), RA (21%), SS (18%) and primary biliary cirrhosis
(4%). There were 260 (36%) SSc patients with their firstdegree relatives affected by at least one AID, of which the
most frequent were RA (18%) and thyroid disease (9%)
[21]. We have confirmed these findings in European
Caucasian patients. A study on these patients reported
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Autoimmune genes in SSc
that about a quarter of a large series including 1132 SSc
patients developed at least one associated AID [22]. In
addition, we observed that the co-existence of at least
one AID was associated with the limited cutaneous
subtype and a lower prevalence of digital ulcers.
Therefore, this latter study identified a subset of patients
with milder disease, potentially associated with a specific
B-cell-mediated autoimmunity.
Another study has been undertaken to estimate associations of RA with any of 33 AIDs and related conditions
among parents and offspring, singleton siblings, twins and
spouses [23]. Familial standardized incidence ratio (SIR)
for RA in relation to other AIDs and related conditions
according to the presence of SSc in the proband were
1.65 (1.13, 1.33) for parents only and 1.99 (0.87, 4.32)
for siblings only. For discordant diseases, offspring had
an increased risk of RA when parents were hospitalized
for SSc (SIR 1.65). These associations support the concept of a shared genetic background.
Susceptibility genes in SSc:
autoimmunity
The immune system plays a crucial part in the host
defence against harmful antigens and in the balance
between tolerance and immunity to other antigens.
A very large number of associations between the HLA
system and autoimmune disorders has long been established [24]. In addition, through the candidate gene
approach and more recently by genome-wide scans, a
large number of non-HLA candidate genes have been
identified including those that have consistently been
found to be associated with multiple autoimmune disorders [24]. These results inspired several investigations in
SSc and our review aims to discuss the more relevant
results obtained, focusing on those that could be
replicated in independent populations.
In AIDs, the only unambiguous susceptibility loci identified is the HLA complex, located on the short arm of
chromosome 6. Indeed, the MHC region has been
the most prominently associated region of the human
genome, with different allelic variants associated with
each disease. Previous HLA genetic association studies
of SSc patients have suggested that these genes exert
their influence primarily on autoantibody expression
[25, 26]. ACA-positive and anti-topo I antibody-positive
SSc subsets have been associated with different
HLA class II alleles and haplotypes. HLA-DRB1*11–
DQB1*0301 haplotypes have been associated with
anti-topo I positivity in SSc, whereas HLA-DRB1*01–
DQB1*0501 haplotypes are more common in ACA-positive SSc patients [27]. A recent case–control association
study was conducted to determine the HLA class II
(DRB1, DQB1, DQA1 and DPB1) alleles, haplotypes and
shared epitopes in 1300 SSc cases (961 whites) and 1000
controls in the US population [27]. The strongest positive
class II associations with SSc in whites and Hispanics
were the DRB1*1104, DQA1*0501, DQB1*0301 haplotypes and DQB1 alleles encoding a non-leucine residue
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at position 26 (DQB126 epi), whereas the DRB1*0701,
DQA1*0201, DQB1*0202 haplotypes and DRB1*1501
haplotype were negatively correlated and possibly protective in dominant and recessive models, respectively.
However, several other alleles/haplotypes which appear
to be unique to several autoantibody-positive groups
were observed, highlighting the influence of ethnic diversity and SSc clinical heterogeneity. This will need to be
replicated and clinical translation investigated.
The highly complex linkage disequilibrium (LD) structure
of this locus has limited the dissection of the risk variants
responsible for the considerable involvement of the HLA
locus in AID susceptibility. One particular point for a
subset of MHC haplotypes is that there is a higher
amount of LD observed between segments of strong LD
[28, 29]. Therefore, such tight segment-to-segment LD
limits MHC research: if one identifies a disease association with a variant in the region, it may not be possible to
determine whether the variant is causal or whether its
association simply reflects LD with the true causal variation. A further complication is the great variability exhibited by some of the HLA genes [29]. Studies performed in
much larger cohorts that evaluate the entire HLA (classes
I, II and III) locus or a dense map of variation rather than
specific regions, and that are inclusive of non-Europeans,
could be helpful in understanding the contribution of HLA
in SSc pathogenesis.
Regarding non-HLA genes, we present below the more
consistent results and in particular those that were
obtained through large samples and were replicated.
According to a recent review on shared autoimmunity
[24], we have classified SSc susceptibility genes according to their involvement in specific pathways: (i) innate
immunity, (ii) immune-cell signalling regulation and (iii)
T-cell differentiation.
Genes involved in innate immunity
A growing body of evidence has provided a new paradigm
for understanding autoimmunity in CTDs and particularly
SLE [30, 31]. Type I IFN is a central mediator of innate
immunity and IFN production is normally a feature of the
immune response to microbial infections and has multiple
effects on the immune system. IFN stimulates monocyte
maturation into dendritic cells, plasma cell maturation and
immunoglobulin class switching, cytotoxic T- and natural
killer cell activity, and chemokine secretion. Microarray
studies have underlined a pivotal role of the type I IFN in
SLE and primary SS and these data have been strengthened by the identification of the IFN regulatory factor 5
(IRF5) gene, and particularly IRF5 rs2004640, as a susceptibility genetic factor in these diseases [30, 31]. The
IRF5 rs2004640 T allele, which creates a donor splice
site in intron 1 of the gene, results in the transcription of
the alternative exon 1B of no clear biological effect. Gene
expression profiling is another complementary genetic
analytical method as changes in expression are at least
in part related to genetic regions involved in the disease.
Several studies have demonstrated its importance in
SSc, its potential in prognostication, as well as in the
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Yannick Allanore et al.
elucidation of the underlying pathogenesis [32]. Using
peripheral blood cells, a type 1 IFN signature has been
reported in SSc [33] and raised the hypothesis of a role
of IFN genes in this disorder.
An association of IRF5 was shown from a case–control
study of our group (1641 subjects of French European
Caucasians split into discovery and replication cohorts)
[34]. In both sets, the TT genotype was significantly
more common in patients with SSc than in control subjects, with an OR of 1.58 (95% CI 1.18, 2.11) in the combined analyses. Among the whole SSc population, we
found a significant association between homozygosity
for the T allele and the presence of fibrosing alveolitis
(OR 2.07; 95% CI 1.38, 3.11). In addition, in a multivariate
analysis model including the diffuse cutaneous subtype
of SSc and positivity for anti-topo I antibodies, the IRF5
rs2004640 TT genotype remained associated with fibrosing alveolitis (P = 0.029; OR 1.92; 95% CI 1.07, 3.44).
Preliminary data from the US cohort, not yet published,
have confirmed these findings [35]. Logistic regression
analysis showed in this population that the TT genotype
of single nucleotide polymorphism (SNP) rs2004640 was
an independent risk factor for SSc (OR 1.56; 95% CI 1.3,
2.0) and also for anti-topo I antibody positive and SScrelated fibrosing alveolitis [35]. A replication of significance
has been reported in an independent population from
Japan [36]. A case–control association study (281 SSc
patients and 477 healthy controls) was performed for
rs2004640 as well as for rs10954213 and rs2280714. All
three SNPs were significantly associated with SSc, with
the rs2280714 A allele having the strongest association
(OR 1.42; 95% CI 1.15, 1.75). Association was preferentially observed in subsets of patients with dcSSc and antitopo I antibody positivity. Conditional analysis revealed
that rs2280714 could account for most of the association
of these SNPs. The genotype of rs2280714 was strongly
associated with IRF5 mRNA expression. This replication is
noteworthy as it is obtained in a different ethnic group.
Altogether, these data provide new insight into the pathogenesis of SSc, including clues to the mechanisms
leading to specific disease subtypes. Nevertheless, considering discrepancies between SLE and SSc regarding a
fibrotic component, no unifying hypotheses can be put
forward. Further research into the role of IRF5 and IFN
in promoting specific fibrotic phenotype expression in
SSc is warranted.
Genes involved in T-cell differentiation
Recent GWA studies for SLE have also identified new
associations with genes in T- and B-cell pathways
[14, 37]. Indeed, various association studies have convincingly identified the STAT4 gene as a susceptibility factor
for different connective tissue disorders and inflammatory
bowel diseases [14, 38, 39]. STAT4 encodes the signal
transducer and activator of transcription 4, which transduces IL-12, IL-23 and type 1 IFN cytokine signals in
T cells and monocytes. The identified STAT4 susceptibility
gene variant is rs7574865, located in the third intron of the
gene. Although it is a non-functional variant, it tags an
206
LD block which is correlated with high expression of
STAT4 mRNA [39]. Two studies have concomitantly identified the association between STAT4 rs7574865 and
SSc. One study included a total of 1317 SSc patients
and 3113 healthy controls, from an initial case–control
set of Spanish Caucasian ancestry and five independent
cohorts of European ancestry [40]. The rs7574865 T allele
variant was significantly associated with susceptibility
to lcSSc in the Spanish population (OR 1.61; 95% CI
1.29, 1.99), but not with dcSSc. The combined analysis
showed a strong risk effect of the T allele for lcSSc susceptibility (pooled OR 1.54; 95% CI 1.36, 1.74). The other
study included 1855 individuals, all of French Caucasian
origin [41]. STAT4 rs7574865 was found to be associated
with SSc (OR 1.29; 95% CI 1.11, 1.51). However, this
association was not restricted to a particular phenotype
and an association with fibrosing alveolitis was observed.
A case–control association study has been performed in
the Japanese population and included 282 patients with
SSc and 590 controls. In spite of the difference in the risk
allele frequency between the populations, the data confirmed an association between rs7574865 and SSc,
preferentially with lcSSc subset [42]. Therefore, although
these three studies established STAT4 as a genetic risk
factor, genotype–phenotype correlations remain unclear
and will need further research. In our study, we also investigated the joint effect on SSc susceptibility of the risk
allele IRF5 rs2004640 T and STAT4 rs7574865 T [41].
No evidence for dominance or interactions was observed
between the two risk alleles. We observed an additive
effect on SSc susceptibility. The ORs for SSc were 1.72
(95% CI 1.23, 2.14) for carriers of one risk allele; 2.17
(95% CI 1.55, 3.04) for carriers of two risk alleles; and
2.72 (95% CI 1.86, 3.99) for carriers of three or four risk
alleles. We also observed a strong association between
the three risk alleles and SSc interstitial lung disease (OR
1.79; 95% CI 1.235, 2.582), which remained significant in
multivariate analyses including other known risk factors
for lung involvement. This point is of major interest as an
additive effect has been reported in an SLE Caucasian
population, for contribution to a general loss of tolerance
[43]. Hence, STAT4 and IRF5 could both contribute to a
disease-specific phenotype.
Genes involved in immune-cell signalling
PTPN22 is a selective phosphatase that modulates signal
transduction in T cells, and represents a case in which
a causal variant that contributes to disease susceptibility
has been identified. The known p.R620W (1858C to T;
rs2476601) risk allele is a gain of function variant with
increased catalytic activity, and is thought to be a more
potent suppressor of TCR signalling [24, 43]. The PTPN22
gene has been convincingly associated with Crohn’s
disease, RA, SLE, type 1 diabetes and Graves’ disease
[44, 45]. This clustering of genetic risk factors for many
AIDs shows that these diseases may share, at least partly,
similar underlying causal mechanisms. In SSc, the first
evidence of association of PTPN22 was reported by
a case–control study of >1000 SSc patients in
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Autoimmune genes in SSc
the USA [46]. The PTPN22 CT/TT genotype showed
significant association with anti-topo I antibody-positive
SSc in white patients (OR 2.21; 95% CI 1.3, 3.7) and
with ACA-positive white patients with SSc (OR 1.70;
95% CI 1.1, 2.7). Frequency of the T allele also showed
significant association with anti-topo I antibody-positive
SSc in white patients (OR 2.03; 95% CI 1.3, 3.2).
Replication came from performing a case–control study
(659 SSc patients and 504 healthy controls) enlarged to
additional SPNs (n = 7) and by the performance of a metaanalysis [47]. In a first step, we assessed our SSc patients
for concomitant AID and excluded those with known
PTPN22 risk allele-associated diseases. No association
was detected for any of the SNPs tested. However,
PTPN22 haplotype analysis identified a strong association
between SSc and the presence of a risk haplotype
carrying the 1858T allele (P = 1.52 10 7) and a protective
haplotype carrying the 1858C allele (P = 2.20 10 16). The
meta-analysis provided further evidence that the PTPN22
1858T allele is involved in genetic susceptibility to SSc in
Caucasian (OR 1.08; 95% CI 1.02, 1.15) and mixed (OR
1.09; 95% CI 1.04, 1.16) populations; particularly in the
anti-topo I-positive subset (OR 1.48; 95% CI 1.09, 2.0 for
Caucasians and OR 1.09; 95% CI 1.04, 1.16 for mixed).
Altogether, these results driven by large studies indicate
that PTPN22, a shared genetic factor of multiple autoimmune diseases, also contributes to the genetic background of SSc. Regarding the overall data obtained with
PTPN22, a role for genetic factors in autoantibody seropositivity has emerged. Hence, autoantibody positivity
might represent sub-phenotypes within autoimmune
diseases, and differential genetic associations in autoantibody-positive and -negative groups could help us to
understand differences in pathogenesis and disease
development.
Scaffold protein with ankyrin repeat gene, BANK1, is a
B-cell-specific scaffold protein and LYN tyrosine kinase
substrate that promotes tyrosine phosphorylation of
inositol 1,4,5-trisphosphate receptors. BANK1 overexpression leads to the enhancement of BCR-induced
calcium mobilization by connecting protein tyrosine
kinases to IP3R, facilitating phosphorylation and activation
of IP3R by LYN and subsequent release of Ca2+ from
endoplasmic reticulum stores. Nevertheless, to date, the
exact role of BANK1 remains unclear. Functional variants
of BANK1 were recently found to be associated with SLE
[48]. Therefore, we tested this newly identified risk factor
for SLE for association with SSc, and for its putative
gene–gene interaction with the pre-identified genetic
risk factors IRF5 and STAT4 [49]. BANK1 nonsynonymous functional substitution rs10516487 and
rs3733197 SNPs were genotyped in 2407 individuals
comprising a French set (874 SSc patients and 955 controls, previously genotyped for both IRF5 rs2004640 and
STAT4 rs7574865) and a German set (421 SSc patients
and 182 controls). The BANK1 variants were found to be
associated with dcSSc in both samples, providing an
OR of 0.77 (95% CI 0.64, 0.93) for the rs10516487
T rare allele in the combined populations compared with
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controls and an OR of 0.73 (95% CI 0.61, 0.87) for the
rs3733197 A rare allele. BANK1 haplotype analysis
found the A-T haplotype to be protective in dcSSc
(OR 0.70; 95% CI 0.57, 0.86); significant differences
were also observed when the lcSSc subset was compared with dcSSc, both for rare alleles and haplotypes.
Moreover, the BANK1, IRF5 and STAT4 risk alleles
displayed a 1.43-fold increased risk of dcSSc. These
results suggest a role of B cells in the dcSSc subset of
patients, in accordance with the findings for the B-cell
stimulator, BAFF, which showed higher serum levels in
dcSSc patients that were correlated with the skin score
[50]. It is noteworthy that we could identify no link with
autoantibody status. Similarly, a recent study in RA that
reported a modest effect at the BANK1 locus could
identify no link with specific autoantibodies [51].
Although autoantibodies are supposed to be of greater
importance in SLE, a relationship with BANK1 variants
has not yet been queried in this setting. We are reaching
a new area in complex genetic diseases where identification of the genes involved in disease susceptibility or
severity is becoming a reality. Therefore, it is of interest
to understand the relationship between the various
genetic effects. We examined the possibility of a genetic
interaction between a B-cell genetic marker, BANK1, and
type I IFN genes, IRF5 and STAT4. Our results suggested
that the IRF5 and STAT4 SNPs act additively with BANK1
to increase the risk for dcSSc and SSc [49]. Therefore, at
least two major pathways seem to contribute to dcSSc
pathogenesis, implicating both innate and adaptative
immunity (associated autoimmune genes are summarized
in Table 2).
Susceptibility genes in SSc: outside
autoimmunity
Several studies have been performed during the
same period to identify variants from vascular or fibrotic
candidate genes that may be associated with SSc.
Despite their major relevance from a pathogenetic point
of view and the use of large studies in some cases, no
gene could be consistently identified and replicated
[13, 52]. Probably the most important data came from
the CTGF gene. A functional SNP, at 945 bp from the
start codon in the promoter region (rs6918698), was found
to be associated in a UK population including 1000 subjects [53]. However, other studies of larger sample size
(2964 individuals of European ancestry and 2315 from
the USA) failed to replicate these findings [54, 55]. It is
noteworthy that overall these studies reported similar
allele or genotype frequencies in the SSc group but
discrepancies regarding the control group that supported
the association we found in the initial report. However,
a study from Japan including 659 individuals has recently
suggested an association with the same SNP [56].
Therefore, the definitive involvement of CTGF variants in
the genetic background of SSc remains to be established
and new studies and/or meta-analyses remain to clarify
this issue.
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Yannick Allanore et al.
TABLE 2 Autoimmunity susceptibility genes associated with SSc
Marker
Gene/locus
Chromosomal
region
rs 2004640 (exon 1B, alternative
splicing)
IRF5
7q32
rs7574865 (intron)
STAT4
2q32.3
rs2476601 (p.R620W, causal variant)
PTPN22
1p13.2
rs3,733,197and rs10,516,487
(exon, missense variant)
BANK1
4q24
Future directions
These genetic studies indicate that certain genes/loci
seem to confer predisposition to multiple immune-related
disorders, thereby supporting the hypothesis that a
shared group of genes contributes to the spectrum of
immune diseases. They confirm the clinical and epidemiological observations regarding the co-occurrence of
immune-related diseases. They also provide clues indicating why these immune diseases form clusters. Both the
innate and adaptative immunities appear to play a role in
these diseases. Innate immunity is particularly interesting
from a clinical perspective, as it provides links to
environmental triggers of the disease. It is noteworthy
that despite their pleiotropic effects, these genes seem
to be associated with some severe sub-phenotypes
of the immune disorders. This situation may lead in the
future to reconsideration of the classification of some
diseases according to a better understanding of their
pathogenesis.
An important issue is the research agenda from the
current situation. It is important to correlate genotypic
and phenotypic data that may provide insight into disease
pathogenesis and, more importantly, new therapeutic
approaches. It will be important to determine whether
the genetic variants identified to date fall into specific
mechanistic pathways or involve a multitude of different
pathways. There is a great need for post-genomic studies
paired with comprehensive genotypic data. Indeed, the
identification of the different molecular pathways involved
may provide innovative targets for therapeutic intervention. New therapies are particularly expected in SSc
which still has no specific efficient therapy. Longitudinal
studies will be instrumental in assessing the contributions
of both genetic and environmental factors. They may also
allow prediction of disease progression. The genetic predisposition to impairment of a given pathway could help
define clinical subgroups of the disease and prioritize
208
Relevance to pathogenesis
Member of the IRF family; transcription factors
with diverse roles, including modulation of cell
growth, differentiation, apoptosis and immune
system activity
Transcription factor; transduces IL-12, IL-23 and
type 1 IFN cytokine signals in T cells and
monocytes
Intracellular protein tyrosine phosphatase;
expressed primarily in lymphoid tissues;
involved in TCR signalling
BANK1 is a B-cell-specific scaffold protein and
LYN (MIM #165120) tyrosine kinase substrate
promotes tyrosine phosphorylation of inositol
1,4,5-trisphosphate
patient groups towards a specific therapy. To that end,
unified efforts from clinicians and scientists of complementary expertise will be mandatory.
In SSc, no GWA study has been published yet and one
may expect important information to be driven through
this approach. It is likely that many different susceptibility
alleles contribute to SSc, each of which has only a modest
effect. Therefore, one may speculate that many genes
remain to be identified. Genes or more probably pathways
that could act at the interplay between the respective
disturbances will be particularly scrutinized. An advantage
of these approaches, as compared with more typical
association studies that test for a connection between
disease and candidate-gene markers, is that they screen
most of the genes in the human genome—thus allowing
the investigation of new mechanisms, sometimes unexpected, of disease susceptibility. Nevertheless, the heterogeneity of SSc may limit somehow the establishment
of true associations and this will need careful multistep
statistical analyses. One may hope that GWAS will identify
not only a number of genome-wide significant associations but also additional sets of ‘suggestive’ findings
that will have to be followed up and replicated in other
independent populations. Thereafter, meta-analyses will
be required to identify associations. In addition, examining
rare variants (rarer variants of larger effects) and copy
number variants may provide further insight into genetic
mechanisms of complex diseases to which SSc belongs.
Finally, and in contrast to genetic alterations, an epigenetic change is defined as a heritable change in gene
expression that does not involve a change in the
DNA sequence. Epigenetic mechanisms play an essential
role in eukaryotic gene regulation by modifying the
chromatin structure, which in turn modulates gene
expression. Convincing evidence indicates that epigenetic
mechanisms, and in particular impaired T-cell DNA
methylation, provide additional factors contributing to
connective tissue disorders [57] and this will also have
www.rheumatology.oxfordjournals.org
Autoimmune genes in SSc
to be addressed in the investigation of the pathogenesis
of SSc.
Rheumatology key messages
SSc belongs to the group of complex genetic
disorders.
. The more consistent susceptibility genes identified
until now belong to autoimmunity pathways.
. Several genetic factors are shared with other
autoimmune diseases.
.
Acknowledgements
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Funding: This work was funded by Association des
Sclérodermiques de France, Institut National de la Santé
et de la Recherche Médicale (INSERM), Agence Nationale
pour la Recherche (ANR) [Grant no. R07094KS]. It was
supported by Groupe Français de Recherche sur la
Sclérodermie.
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Disclosure statement: The authors have declared no
conflicts of interest.
17 Arnett FC, Cho M, Chatterjee S et al. Familial occurrence
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