Genetics and genomic studies in scleroderma (systemic sclerosis).

Abstract

It currently is believed that scleroderma is a complex polygenic disease that occurs in genetically predisposed individuals who have encountered specific environment exposures and/or other stochastic factors. The nature of these genetic determinants and how they interact with environmental factors are areas of active investigation. This article discusses the evidence that supports a strong genetic link to scleroderma. These studies implicate potential pathogenetic mechanisms involved in scleroderma, which, it is hoped, may translate into clinical utility, including determination of disease risk, diagnosis, prognosis, and novel therapeutics.

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Genetics and Genomic Studies
in Scleroderma (Systemic Sclerosis)
Sandeep K. Agarwal, MD, PhD*,
Filemon K. Tan, MD, PhD,
Frank C. Arnett, MD
Division of Rheumatology, Department of Internal Medicine, The University of Texas Health
Science Center at Houston, 6431 Fannin, MSB 5.270, Houston, TX 77030, USA
Scleroderma or systemic sclerosis is an autoimmune connective tissue dis-
ease clinically characterized by fibrosis of the skin and internal organs and
obliterative vasculopathy. The complexities of scleroderma are evident from
the variability in its clinical manifestations, which probably reflects the
diverse mechanisms that underlie the development of disease subtypes.
Despite recent advances in the understanding of some of the molecular path-
ways involved in scleroderma, the etiopathogenesis remains unknown.
Although fibrosis and endothelial dysfunction are hallmarks of the disease,
autoimmunity is probably the root cause. Autoimmunity and inflammation
currently are best exemplified by the multiple but not overlapping patterns
of specific autoantibodies in patients who have scleroderma. In fact, each of
these autoantibodies tends to mark a distinct clinical subset of disease [1].
The presence of inflammatory infiltrates in the dermis early in the disease
and increased circulating levels of cytokines and chemokines in patients
who have scleroderma further implicate inflammation in the pathogenesis
of scleroderma. It remains unknown how these autoimmune responses
lead to certain patterns of organ damage that vary among different clinical
subsets of scleroderma.
It currently is believed that scleroderma is a complex polygenic disease
that occurs in genetically predisposed individuals who have encountered
specific environment exposures and/or other stochastic factors. The nature
of these genetic determinants and how they interact with environmental fac-
tors are areas of active investigation. This article discusses the evidence that
* Corresponding author.
E-mail address: sandeep.k.agarwal@uth.tmc.edu (S.K. Agarwal).
0889-857X/08/$ - see front matter ? 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.rdc.2007.10.001rheumatic.theclinics.com
Rheum Dis Clin N Am
34 (2008) 17–40

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supports a strong genetic link to scleroderma. Also reviewed are the family
and twin studies that suggest a genetic component in scleroderma, and re-
cent genetic-association studies implicating specific genes in the pathoge-
netic triad of autoimmunity, endothelial dysfunction, and fibroblast
activation. Last, this article highlights recent studies in scleroderma that
use gene-expression microarray profiles seeking to identify pathogenetic
pathways in scleroderma. Together these studies implicate potential patho-
genetic mechanisms involved in scleroderma, which, it is hoped, may trans-
late into clinical utility, including determination of disease risk, diagnosis,
prognosis, and novel therapeutics.
Familial aggregation and twin studies
Determining that a disease occurs more commonly in families than in the
general population is a first step in implicating potential genetic contribu-
tions. Robust estimates of prevalence and incidence rates of scleroderma,
which are essential in determining familial aggregation, have varied widely
in the general population, ranging from 3.1 to 20.8 per 100,000 and from
0.4 to 1.2 per 100,000, respectively [2–5]. A recent, well-conducted study
estimated the prevalence of scleroderma to be 24.2 per 100,000 adults
with an annual incidence of 1.93 per 100,000 adults [6]. Initial reports sug-
gested that familial clustering was uncommon, but only recently has it been
quantitated [7]. Ten cases of scleroderma in first-degree relatives among 710
proband cases were reported in the Sydney, Australia population, confer-
ring a familial risk conservatively estimated at 11 (95% confidence interval
[CI], 2.7–19.3) [8]. It was demonstrated subsequently that scleroderma re-
curred in 1.6% of families of scleroderma cases in three separate cohorts
that had an estimated population risk of only 0.026% [9]. Although this ab-
solute risk of familial scleroderma was relatively low, the familial relative
risk was approximately 15-fold higher for siblings and 13-fold higher for
first-degree relatives. A recent study suggested that the affected members
within multicase scleroderma families tend to have concordant sclero-
derma-specific autoantibodies, further supporting the concept of a genetic
predisposition [10]. Based on these studies, a positive family history of
scleroderma confers the strongest known relative risk for disease.
The investigation of monozygotic twins is an important approach for
assessing and quantifying the role of genetic versus environmental factors
in specific diseases. Case reports have described concordance for sclero-
derma in twins [11,12]. Examination of 42 monozygotic and dizygotic twins
collected from across the United States demonstrated similar scleroderma
concordance rates (w5%), thus implying no genetic susceptibility [13].
The concordance rate of antinuclear antibodies was significantly higher in
monozygotic twins (90%) than in dizygotic twins (40%), however [13]. A
subsequent study compared gene-expression microarray profiles of cultured
fibroblasts from 15 discordant mono- and dizygotic twin pairs [14].
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Unsupervised hierarchical clustering segregated cultured fibroblasts into
two distinct groups. Fibroblast lines from unaffected monozygotic, but
not dizygotic, twins tended to group with cultured fibroblast cell lines
from affected scleroderma patients rather than with normal controls. To-
gether, as summarized in Table 1, these data suggest that genetic factors
may play a significant role in susceptibility to scleroderma with regards to
the production of autoantibodies and in vitro fibroblast activation. Al-
though these data may partially explain the clustering of autoimmune dis-
eases observed in some families, it is necessary to define the genetic
factors that underlie scleroderma susceptibility.
Ethnic factors
It is apparent that ethnicity influences the susceptibility to autoimmune
diseases, including scleroderma. African Americans have been reported to
have a higher incidence of scleroderma (22.5 cases per million per year)
than white women (12.8 cases per million per year) [15]. Furthermore, Afri-
can American women have been reported likely to have more severe disease,
earlier age of onset, and worse survival rates [15]. Reveille and colleagues
[16] demonstrated in a prospective cohort that Hispanics and African Amer-
icans were more likely than whites to have diffuse skin involvement, digital
ulcerations, and pulmonary hypertension.
Among the more intriguing observations with regards to ethnicity and
scleroderma were those made in the Oklahoma Choctaw Indians [17]. The
prevalence in full-blooded Choctaws was estimated at 469 cases per
100,000 over a 4-year period. This prevalence was significantly higher
than that seen in non–full-blooded Choctaws, other Native Americans in
Oklahoma, and whites. Furthermore, a majority of the Choctaw sclero-
derma cases had diffuse scleroderma, pulmonary fibrosis, and circulating
anti-topoisomerase I antibodies and could be traced genealogically to a com-
mon founding family in the late 1700s. No environmental factors were iden-
tified in the Oklahoma environment, and the strongest risk factor was
a nearly unique Amerindian HLA class II haplotype (DRB1*1602,
DQA1*0501, DQB1*0301). Investigators using microsatellite markers in
Table 1
Estimates of scleroderma occurrence and risk in families
Population Frequency (%)
General population (prevalence)
First-degree relatives of persons who have scleroderma (prevalence)
Siblings of persons who have scleroderma (prevalence)
Monozygotic twins of persons who have scleroderma (concordance)
Monozygotic twins with antinuclear antibody–positive concordance
Monozygotic twins with fibroblast gene profile concordance
0.026
1.60
0.40
5.00
90
50
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three candidate regions found a shared haplotype on chromosome 15q con-
taining the fibrillin-1 (FBN1) gene, an important structural protein and reg-
ulator of transforming growth factor-beta (TGF-b) within the extracellular
matrix, to be significantly overrepresented in Choctaw scleroderma cases
[18]. Similar studies of single-nucleotide polymorphisms (SNPs) supported
a role for FBN1 in Choctaw and Japanese scleroderma cases but did not
show associations in whites [19]. Interestingly, a mouse model of sclero-
derma, the tight skin mouse (tsk-1), is caused by a genomic duplication of
fibrillin-1 [20].
These studies demonstrate the influence of ethnicity on scleroderma sus-
ceptibility.Althoughmultiplefactorsincludingsocioeconomicfactors,access
to health care, and even environmental exposures may help explain these dif-
ferenceinsclerodermasusceptibilityorevenclinicalexpression,geneticdiffer-
ences among ethnic groups probably are key determinants as well.
Genetic factors
HLA associations
The major histocompatibility complex (MHC) or HLA region is the most
polymorphic region of the genome. Polymorphisms in HLA have been
linked to a number of autoimmune diseases including rheumatoid arthritis,
ankylosing spondylitis, systemic lupus erythematosus, and many others
[1,21–23]. Scleroderma also has been associated with HLA polymorphisms,
and these associations have been reviewed previously [24,25]. The associa-
tions of HLA polymorphisms with scleroderma susceptibility itself are mod-
est but, more importantly, are consistent and are reproducible across
different populations; these include associations with the HLA-DR5/11
and DR3 haplotypes in white patients and with HLA-DR2 haplotypes in
Japanese and Choctaw Indian patients. Of particular interest is a finding
of a significantly higher frequency of HLA-DQA1*0501 in male patients
who have scleroderma [26].
Stronger associations with HLA haplotypes exist for specific autoanti-
body subsets of scleroderma, including HLA DRB1*1104 and, indepen-
dently, DPB1*1301 in whites, DQB1*0301 and DPB1*1301 in African
Americans, and DR2 haplotypes in Japanese (DRB1*1502, DQB1*0601,
DPB1*090) and Choctaws (DRB1*1602, DQB1*0301, DPB1*1301) with
anti-topoisomerase antibody [27–32]. Furthermore, HLA DQB1*0501 and
other DQB1 alleles encoding nonpolar amino acids in position 26 are asso-
ciated with anti-centromere antibody [27,33]. An association of HLA
DRB1*1302, DQB1*0604/0605 haplotypes has been found with anti-fibril-
larin (anti-U3-RNP)–positive patients, who are more often male African
Americans, and the HLA DRB1*0301 haplotype has been shown to be as-
sociated with anti-PM-Scl antibody positivity in patients who tend to be
nearly exclusively white [34,35]. Finally, an association was observed with
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