Skewed X-Chromosome Inactivation in Scleroderma
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- Uz, E., Loubiere, L.S., Gadi, V.K. et al. Clinic Rev Allerg Immunol (2008) 34: 352. doi:10.1007/s12016-007-8044-z
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Scleroderma is a female-prevalent autoimmune disease of unclear etiology. Two fundamental gender differences, skewed X-chromosome inactivation (XCI) and pregnancy-related microchimerism, have been implicated in scleroderma. We investigated the XCI patterns of female scleroderma patients and the parental origin of the inactive X chromosome in those patients having skewed XCI patterns (>80%). In addition, we investigated whether a correlation exists between XCI patterns and microchimerism in a well-characterized cohort. About 195 female scleroderma patients and 160 female controls were analyzed for the androgen receptor locus to assess XCI patterns in the DNA extracted from peripheral blood cells. Skewed XCI was observed in 67 (44.9%) of 149 informative patients and in 10 of 124 healthy controls (8.0%) [odds ratio (OR) = 9.3 (95% confidence interval (CI) 4.3–20.6, P < 0.0001)]. Extremely skewed XCI (>90%) was present in 44 of 149 patients (29.5%) but only in 3 of 124 controls (2.4%; OR = 16.9; 95% CI 4.8–70.4, P < 0.0001). Parental origin of the inactive X chromosome was investigated for ten patients for whom maternal DNA was informative, and the inactive X chromosome was of maternal origin in eight patients and of paternal origin in two patients. Skewed XCI mosaicism could be considered as an important risk factor in scleroderma.
Scleroderma (systemic sclerosis, SSc) is an autoimmune disease characterized by fibrosis and alterations in the microvasculature . Scleroderma is three to ten times more prevalent in women than in men . Most autoimmune diseases are more prevalent in females than in males , and fundamental differences between male and female biology, such as hormone status , pregnancy , and X-chromosome inactivation [6, 7] have been proposed as the underlying pathophysiological mechanisms leading to the female prevalence of autoimmune diseases. Indeed, both pregnancy-related microchimerism [8–11], and skewed (i.e., not the expected 50:50 balance) X-inactivation mosaicism  have been shown to be associated with scleroderma.
This study was performed to investigate the X-inactivation patterns of female scleroderma patients along with the parental origin of the inactive X chromosome in those patients with extremely skewed X-inactivation. In addition, the correlation between skewed X-inactivation and both maternal and fetal microchimerism was examined in a well-characterized cohort of scleroderma patients.
Materials and Method
DNA samples were obtained from 195 scleroderma patients and 160 control women. Clinical characteristics of the patients and controls have been published elsewhere [12, 13]. The ethics review boards at the participating institutions approved the study protocol. Informed consent was obtained from all subjects. The X-chromosome inactivation status of the patients and controls was determined by genotyping a highly polymorphic CAG repeat in the first intron of the androgen receptor as previously described [12, 14]. Depending on the definition and quantitative accuracy of the measurement method, a few percent to nearly one fifth of apparently healthy women display skewed (i.e., non-random) patterns of X-inactivation. Whereas ratios in the range of 50–79% are usually regarded as normal variation, deviation from this range in 80–89% of cells is defined as skewed, and above 90% of cells as extremely skewed X-inactivation. [15–17] Fisher’s exact test was used for statistical analyses.
Proportions of scleroderma patients and controls with skewed X-inactivation
Degree of skewing (%)
No. (%) observed with skewing
Present study (n = 94)
Previous study  (n = 55)
Combined (n = 149)
Control (n = 124)
Parental Origin of the Inactive X Chromosome
Parental origin of the inactive X chromosome in scleroderma patients with skewed X-inactivation
Degree of skewing
Parental origin of the inactive X chromosome
Evaluation of Skewed X-Inactivation for Correlation with Microchimerism
Proportion of maternal (MMc) and fetal (FMc) microchimerism in scleroderma patients
58.3% positive for MMc
57.1% positive for FMc
66.7% positive for MMc
56.0% positive for FMc
Autoimmune disorders affect more than 5% of the population, and a high female prevalence is characteristic of most autoimmune diseases including scleroderma, autoimmune thyroid diseases, systemic lupus erythematosus, and Sjögren’s syndrome . Pregnancy-related microchimerism  and skewed X-inactivation  have been proposed as potential contributors to the pathophysiology of scleroderma. In this study, we observed skewed X-inactivation patterns in a significant proportion (34%) of women with scleroderma. Of these women, approximately 18% displayed extreme skewing (>90:10). This result is consistent with our previous study  and indicates that skewed X-inactivation could be a common finding in different population groups.
We do not know the cause of skewed X inactivation in scleroderma, and probable mechanisms have been discussed in the accompanying manuscript . Among them, X-linked lethal mutations, which would be compatible with life in females because of X-inactivation mosaicism, are appealing causative mechanism. If the cause of skewing is indeed X-linked mutations, these should be inherited from the maternal lineage unless they occur de novo during gametogenesis. We therefore analyzed the parental origin of the inactive X chromosome in 19 patient–mother pairs, and of the 10 pairs with informative genotyping results, inactive X was found to be of maternal origin in 8 pairs. This result is on the border of statistical significance (P = 0.055). Assuming this result holds, we will be left with an interesting puzzle. Why are maternally inherited X chromosomes more likely to be inactivated in highly skewed patients? One possibility is that some X chromosomes confer a selective disadvantage to both the organism itself and the peripheral cells within the organism. This precise situation has been documented in X-linked immunodeficiency, where heterozygous (female) carriers have high X-inactivation skew (because of natural selection within the organism), whereas affected males have a survival disadvantage . When an X-encoded genotype is selectively disadvantageous to cells, skew is an outcome. When an X-encoded genotype is selectively disadvantageous to an organism, preferential maternal inheritance is an outcome. Together, this leads us to the hypothesis that some highly skewed patients have an X chromosome that would confer a selective disadvantage on homozygotic women and on males.
Because scleroderma is the first disease in which pregnancy-related microchimerism has been documented, we investigated a subset of patients for whom microchimerism data were also available for correlation of skewed X-inactivation with microchimerism. Neither maternal nor fetal microchimerism was found to be correlated with skewed X-inactivation. In conclusion, the two types of female mosaicism—skewed X-inactivation and maternal/fetal microchimerism—appear to be independent risk factors in scleroderma. One possible explanation for these data is that mosaicism itself is an underlying cause of scleroderma and, by extension, female-prevalent autoimmune disease in general.
We would like to thank Iclal Ozcelik for critical reading of the manuscript. Supported by grants from the Scientific and Technical Research Council of Turkey–TUBITAK-SBAG 3334, International Centre for Genetic Engineering and Biotechnology–ICGEB-CRP/TUR04-01, and Bilkent University Research Fund (to Dr. Ozcelik).