Q J Med 2003; 96: 575-578
© 2003 Association of Physicians
The interleukin-6 promoter polymorphism in Gaucher disease: a new modifier gene?
From the 1Genetics Institute, 2Gaucher Clinic, and 3Department of Medicine, Shaare Zedek Medical Center, and 4Osteoporosis Center, Hadassah University Hospital, Jerusalem, Israel
Received 3 February 2003 and in revised form 24 April 2003
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Summary |
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Background: Individuals with Gaucher disease vary significantly with regard to degree of bone disease, but there are no predictive markers for severity of skeletal involvement.
Aim: To determine the frequency of polymorphisms of interleukin-6 (IL-6) among patients with Gaucher disease, and the relationship to bone mineral density (BMD) and other markers of disease severity.
Design: Case-control study.
Methods: Genotyping for the 174G 
C promoter polymorphism of IL-6 was performed in adult patients with Gaucher disease for whom there was concurrent bone mineral density (BMD) data and in healthy Ashkenazi Jewish controls.
Results: The prevalence of allelic variants (58% G/G, 36% G/C, and 6% C/C) was similar in Ashkenazi Jewish adults with Gaucher disease as in Ashkenazi Jewish controls, but significantly different (p < 0.05) from that reported among Caucasians. No statistically significant correlation was found between IL-6 genotypes and BMD or markers of severity of Gaucher disease. Patients with the C/C genotype had relatively mild Gaucher disease.
Discussion: The IL-6 polymorphisms appear to be distributed differently in Ashkenazi Jews than among other Caucasians. In Gaucher disease, the C/C genotype may be associated with a milder Gaucher phenotype, and may serve as a mitigating genetic modifier.
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Introduction |
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Gaucher disease is an autosomal recessive lysosomal storage disorder resulting from mutations in the gene for ß-glucocerebrosidase. The prevalence of the disease varies among ethnic groups, but there is a predilection among Ashkenazi Jews for the non-neuronopathic (type I) form.1 Accumulation of lipid-engorged cells of the monocyte-macrophage lineage (Gaucher cells) has traditionally been implicated as the cause of organ involvement in Gaucher disease. These cells are capable of secreting many cytokines, such as sCD14, and hence it has been hypothesized that they are a source of the pathological cytokine levels in the serum of patients with Gaucher disease.2
One of the clinical hallmarks of Gaucher disease is bone involvement, which may cause significant morbidity. Many adult patients have some skeletal involvement that is perceptible only in radiological studies, e.g. the Erlenmeyer flask deformity and osteopenia. There is, however, clinically significant skeletal involvement with considerable morbidity, which varies greatly among individual patients, including localized osteolysis, avascular necrosis, and pathological fractures that are slow to heal; the pathogenesis of these features has not been explained.3
Abnormal cytokine release from activated macrophages has been postulated as an important cause of organ disease. Activated macrophages secrete various cytokines including the interleukins (IL) IL-6, IL-1, TNF-
, and IL-10.4 Among these, IL-6 has been considered to have a pathogenic role in bone disease,5 particularly post-menopausal bone loss,6 and is also elevated in multiple myeloma.7 Interestingly, the prevalence of multiple myeloma is high in Gaucher disease,8 a fact that may be related to the abnormal cytokine milieu in the bone marrow. Indeed, IL-6 and IL-10 are both significantly elevated in patients with Gaucher disease, compared to controls.9
A C/G polymorphism in the promoter region of the IL-6 gene has been identified at position –174 of the 5' flanking region of the gene. Compared with the G/C and G/G genotypes, the C/C genotype was associated with lower bone resorption and lesser decrease in bone mass in older postmenopausal women, suggesting that these alleles may be significant determinants of the risk for osteoporosis.10
Within the Gaucher disease patient population, even individuals who carry the same genotype vary significantly with regard to the degree of bone disease. It would therefore be of interest to ascertain whether IL-6 alleles may also be a determinant, i.e. a genetic modifier, of the severity of bone involvement in Gaucher disease. The purpose of this study was to identify the frequency of the IL-6 promoter polymorphism in adult patients with Gaucher disease and correlate these findings with the BMD and other parameters of severity.
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Methods |
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All adult patients arriving for follow-up at a referral clinic for Gaucher disease who had undergone BMD assessment within one month of the clinic visit were enrolled in this study. In addition, 103 Ashkenazi Jewish healthy controls were recruited by canvassing persons with both parents of Ashkenazi Jewish ethnicity. All patients and controls signed informed consent forms, and the study was approved by the local Institutional Review Board.
Skeletal involvement was evaluated by dual energy X-ray absorptiometry (DEXA; QDR-4500, Hologic Inc) of the lumbar spine (LS) and femoral neck (FN) in adults with Gaucher disease only. Information about Gaucher genotype, avascular necrosis (AVN) of the femoral head, and splenectomy were included. The severity score index (SSI),11 with a range of 0–30 points based on increasing disease parameter involvement, was calculated at initial presentation. Use of enzyme replacement therapy (as per criteria of the Israeli Ministry of Health requirements) was documented from later visits.
DNA from patients and controls was extracted and amplified by polymerase chain reaction (PCR) using forward and reverse primers, respectively: 5'-TGACTTCAGCTTTACTCTTGT-3' and 5'-CTGATTGGAAACCTTATTAAG-3' as previously described.10 The 190-bp PCR product was digested overnight at 37°C with NlaIII (New England Biolabs). The C allele in position –174 creates a cut site for the restriction endonuclease NlaIII.
Statistical analysis
Student's t-test was used to compare independent variables between groups, with a p value of 0.05 or less considered statistically significant.
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Results |
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A total of 79 patients were enrolled, with a mean age of 45 ± 6 (range 18–64) years. Mean SSI was 7.7 ± 4.9 (range 1–26) points. Demographic characteristics are shown in Table 1. The prevalence of the polymorphisms of IL-6 is presented in Table 2. There was no significant difference between patients and controls regarding the prevalence of wild type (G/G), heterozygotes (G/C) or homozygotes (C/C).
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Mean LS z-score was –1.55 ± 1.2 (range –4.5 to 1.51); mean FN z-score –1.01 ± 1.36 (range –3.91 to 3.01), i.e. the mean was approximately 1–1.5 SD below the BMD values of age- and sex-matched healthy controls. The mean values of SSI and BMD z-scores of adult patients, segregated by IL-6 polymorphism, are presented in Table 3. No statistically significant correlation was found between z-scores of BMD at the LS or FN sites and polymorphisms of IL-6, nor between SSI and polymorphisms of IL-6.
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No significant correlation was found between polymorphisms of IL-6 and AVN of the femur, SSI, splenectomy, or Gaucher disease genotype (data not shown). None of the patients with the C/C genotype had been splenectomized, nor had any AVN of the femur; only one patient required enzyme treatment (but developed thrombocytosis and was withdrawn from therapy).12
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Discussion |
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The purpose of this study was to ascertain the prevalence of IL-6 polymorphisms among a cohort of patients with Gaucher disease, compared with a control group of Ashkenazi Jews. The genotype distribution among Caucasians has previously been ascertained as 37% G/G, 44% G/C, and 18% C/C;13 in the current cohort, a different distribution consistent with fewer C/C was found, both among the patients and controls, all of whom were Ashkenazi Jews. The difference between the study cohorts and Caucasians was statistically significant (p < 0.05), whereas there was no difference in IL-6 genotype distribution between the patients and the controls. This represents a new distribution of IL-6 polymorphisms in this ethnic group that is universally regarded as Caucasian, and has no parallel among other ethnic populations.
Another interesting finding in this study was that patients with Gaucher disease and the C/C genotype generally had a milder Gaucher phenotype. These results suggest a protective effect of the IL-6 genotype C/C on expression of Gaucher disease; however, the C/C sample size was very small, reflecting an apparently equivalent low prevalence of the C/C allele among Ashkenazi Jews. Nonetheless, we analysed a further sample of adults and children with Gaucher disease specifically for the C/C genotype in order to corroborate this speculation. Five other patients were identified as having the C/C genotype, and all had mild disease, apart from one non-Jewish child whose genotype was N370S/W381STOP. This latter patient had been splenectomized as a toddler and had suffered avascular necrosis of three joints as well as pathological fractures and the sequelae of poor healing after orthopedic surgeries (Jmoudiak et al., in press). In that her genotype is predictive of severe Gaucher disease by virtue of the ‘stop' codon, it may be that she benefited from the protective effect of the IL-6 C/C genotype, since her organ involvement could have been more extensive, including life-threatening lung disease, for example.
Normal healthy young men with the C/C genotype had significantly higher BMD scores both at the LS and FN than teenagers with either the G/G or G/C genotypes.14 This implies that lower plasma IL-6 levels (of the C/C genotype) are correlated with either more rapid bone building or less bone resorption. In patients with Gaucher disease, plasma IL-6 levels are high,9 but no expression studies have yet been done.
The IL-6 C/C polymorphism is protective against post-menopausal osteoporosis15 and juvenile chronic arthritis.13 Hence, it may be that genetic control by the IL-6 genotype C/C induces less bone resorption than the wild-type G/G or the heterozygote G/C, both in normal and pathological states. Thus, the current findings are in concordance with this mitigating effect of the C/C genotype. What is surprising is that the former disorders involve the skeletal system, and the C/C genotype reduces pathological involvement, whereas in Gaucher disease we did not find any association between the C/C genotype and measures of skeletal involvement. However, the IL-6 C/C genotype is correlated with less severe manifestations of Gaucher disease and hence may be a genetic modifier of Gaucher disease by affecting other IL-6 effectors.
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Footnotes |
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Address correspondence to Dr D. Elstein, Gaucher Clinic, Shaare Zedek Medical Center, P.O. Box 3235, Jerusalem 91031, Israel. e-mail: gaucher{at}szmc.org.il
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References |
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