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Max Planck Institute for Molecular Genetics and Institute for Medical Genetics (U.K.), Charité University Hospital, D-13353 Berlin, Germany; Institut National de la Santé et de la Recherche Médicale U 606 (A.O., S.B., O.B., M.-C.d.V.) and Fédérationde Rhumatologie (M.-C.d.V.), Hôpital Lariboisière, 75010 Paris, France
Address all correspondence and requests for reprints to: Marie-Christine de Vernejoul, Institut National de la Santé et de la Recherche Médicale U 606, Hôpital Lariboisière, 2, rue Ambroise Paré, 75010 Paris, France. E-mail: uwe.kornak{at}charite.de (U.K.); nezumi{at}netcourrier.fr (S.B.); agnes.ostertag{at}larib.inserm.fr (A.O.); benizeph{at}free.fr (O.B.); christine.devernejoul{at}inserm.lrb.ap-hop-paris.fr (M.-C.d.V.).
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Abstract |
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Objective: The objective of the study was to investigate the association between polymorphisms in the CLCN7 gene and BMD in postmenopausal women and with clinical variability in ADOII.
Design: This was a genetic association study using five single-nucleotide polymorphisms and a variable number tandem repeat (VNTR) polymorphism in the CLCN7 gene.
Participants: A total of 425 postmenopausal women aged 64 ± 7 yr participated in the study. We also investigated an ADOII family with low penetrance comprising 18 mutation carriers.
Main Outcome Measure(s): In our postmenopausal cohort, individual single-nucleotide polymorphism genotypes and haplotypes were analyzed for association with BMD at the lumbar spine and the femoral neck and with the bone resorption marker deoxypyridinoline (D-Pyr/Crea). The same polymorphisms on the nonmutated CLCN7 allele were investigated for association with the variability of the ADOII phenotype.
Results: Analysis by multiple linear regression revealed a significant association between the ss genotype of the VNTR and higher Z-score values (P = 0.029). The haplotype 4, which comprises the long allele of the VNTR, was found to be significantly associated with lower femoral neck Z-score values (P = 0.011). Furthermore, we found an association of the ss genotype of the VNTR with lower levels of the bone resorption marker D-Pyr/Crea (P = 0.015), whereas haplotype 4 was associated with higher D-Pyr/Crea levels (P = 0.039). In the ADOII family, we could demonstrate that haplotype 3, which contains the s-allele of the VNTR, is associated with a slightly higher probability that mutation carriers develop osteopetrosis (P = 0.029). In both cases the association seems largely to be driven by the VNTR genotype but is further strengthened if surrounding polymorphisms are added to the analysis.
Conclusion: We observed a significant association of CLCN7 polymorphisms with the variance of BMD and bone resorption marker levels in postmenopausal women and with the variability of the ADOII phenotype.
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Introduction |
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TopAbstractIntroductionPatients and MethodsResultsDiscussionReferences |
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A large number of endogenous (PTH, vitamin D3, kidney function, etc.) and exogenous factors (physical activity, calcium intake, smoking, etc.) are known to influence the coordinated action of osteoblasts and osteoclasts in humans (reviewed in Ref.1). Investigation of the variation of bone mineral density (BMD) in families and twin studies suggest that the heritability of the peak bone mass is 60–80%. Many association studies investigating polymorphisms in candidate genes have revealed several genetic factors underlying variability of BMD and/or fracture rate (2).
Osteoclasts produce a low pH microenvironment at the contact surface between the cell and the bone matrix in the so-called resorption lacuna. It has become clear recently that this process relies on the cooperation of a v-ATPase complex and the chloride channel ClC-7 (3). ClC-7 is a broadly expressed member of the CLC chloride channel family and resides mainly in late endosomes, lysosomes, and the ruffled membrane of osteoclasts (3). Different groups have identified CLCN7 mutations in autosomal recessive osteopetrosis and autosomal dominant osteopetrosis type II (ADOII) (4, 5, 6). The different phenotypes can be explained by a reduction of ClC-7 chloride channel function of a varying degree. Hence, in ADOII osteoclast function is only mildly impaired, leading to a rather benign increase in bone mass, whereas in ARO bone resorption seems to be completely abolished. The fact that the penetrance of the ADOII phenotype is incomplete (7, 8) indicates that there are modifying factors that determine whether mutation carriers become symptomatic.
We hypothesized that polymorphisms in CLCN7 can lead to a mild alteration of chloride channel function, which could either produce a beneficial higher peak bone mass or aggravate the phenotype in osteopetrosis. To test this hypothesis, we investigated a panel of six different polymorphisms in CLCN7 in a large family with ADOII and postmenopausal women. Our results demonstrate a weak but significant association with BMD in both groups.
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Patients and Methods |
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TopAbstractIntroductionPatients and MethodsResultsDiscussionReferences |
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For the BMD association study, 425 Caucasian women were recruited and investigated at a single clinical center (Hôpital Lariboisière, Paris, France). Two hundred eight-two women were selected from a population of 500 postmenopausal women already being monitored to evaluate determinant factors for bone loss (Viggos study). We included women between 50 and 80 yr of age who agreed with DNA sampling. Individuals who had been treated with corticosteroids for more than a month, bisphosphonates, or fluoride were excluded. Thirty percent of the included individuals had a T score less than –2.5 at the femoral neck (FN) or lumbar spine (LS). One hundred forty-three postmenopausal women between 50 to 80 yr of age were referred to our outpatient clinic for osteoporosis. These patients underwent clinical, biochemical, and radiological assessment to exclude secondary causes of osteoporosis. All of these included individuals had a T score less than –2.5 at the FN or LS.
All 425 women were interviewed to evaluate risk factors for osteoporosis (personal and familial history of fractures, physical exercise, nutritional habits) and had an x-ray of the spine. Vertebral crush fractures were defined by a decrease in height of 20% or greater. This population is described in Table 1
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Informed consent for genotyping of osteoporosis candidate genes was obtained from all participants, and the study was approved by the Ethics Committee of the Hôpital Lariboisière in Paris.
Samples from ADOII family
The mutation 797fsX925 in family K affected with ADOII and the phenotypical findings had been reported before (6, 9). In brief, compared with other families affected with ADOII, the phenotype was exceptional because of low radiological penetrance: 47% of the mutation carriers presented with segmentary osteosclerosis and 11% with a high bone density measured by dual-energy x-ray absorptiometry without segmentary osteosclerosis on x-rays. Nine affected carriers, nine nonaffected carriers, and 14 noncarriers from this family were available for genotyping.
Genotyping
rs3751884 and rs12926089 were detected by restriction digestion of PCR products by SmaI and MslI, respectively. rs2235580, rs2235579, and rs2294542 were analyzed using the Snapshot method (Applied Biosystems, Foster City, CA) according to the manufacturer’s instructions. Primers used for amplification by a standard PCR protocol and probes for Snapshot primer extensions are given in Table 2. Length of the DNA fragments was determined by electrophoresis on 2.5% agarose gels and fragment length analysis using an ABI 3100 capillary sequencer (Applied Biosystems).
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Statistical methods
Results were expressed as mean ± SD. The means were compared by ANOVA for the quantitative data. All tests were two sided, and the significance level was adjusted for multiple testing according to the recommendations given by Sankoh et al. (11). Taking into account the average correlation of the six polymorphisms (R2 = 0.54), we determined the level for significance as P < 0.032 by the Armitage and Parmar method. A modified Bonferroni correction as proposed by Holm was used to determine the significance level for the association of haplotypes with Z-score or BMD.
Using a multiple regression model, we evaluated the Z-score [adjusted for hormone replacement therapy (HRT) use and duration, as a continuous response data] as a function of the genotype (included as binary variable) and three environment variables (weight, height, and duration of menopause as quantitative variables). The model included the four factors and their combined effects (first-order interaction terms). To select the best-fitting and most parsimonious subsets, we conducted a stepwise model selection. Diagnostic tests were performed to detect nonnormality and outliers.
Statistical analysis was performed using S-PLUS 6.1 software program for Windows (S-PLUS 6 for Windows, Insightful Corp., Seattle, WA.).
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Results |
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TopAbstractIntroductionPatients and MethodsResultsDiscussionReferences |
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). Only SNP rs12926089 leads to an amino acid change (V418M). Whereas five polymorphisms are SNPs, one is a 50-bp variable number tandem repeat (VNTR) residing in intron 8. In the general population, the number of the repeat units ranges between two and nine; the most frequent alleles contain three or five repeat units. We defined alleles harboring two or three repeat units as short (s) and those with four to nine repeats as long (l) to simplify the statistical analysis. A similar approach was taken by Becherini et al. (12) in the analysis of a TA polymorphism in the ESR1 gene. All polymorphisms did not deviate significantly from the Hardy-Weinberg equilibrium (P > 0.05).
We were not able to detect any significant association of the genotype of any individual polymorphism with Z-score corrected only for the use and duration of HRT (data not shown). The lowest P value was observed for the association of the intron 8 VNTR length with FN Z-score (P = 0.15) (Table 3). Multiple linear regression analysis of the Z-score was performed using weight, height, duration of menopause, and CLCN7 polymorphisms as covariables. Using this method, the intron 8 genotype could be shown to have an impact on FN Z-score (P = 0.028), whereas no association of the intron 8 VNTR with LS Z-score could be detected. The linear regression model was highly significant (P < 0.0001) and predicted that 1% of the observed Z-score variance can be explained by the intron 8 genotype (see Table 5). Furthermore, individuals with the ss genotype of the intron 8 VNTR had significantly lower urine levels of the bone resorption marker deoxypyridinoline (D-Pyr/Crea) than the remaining individuals (P = 0.015) (Fig. 1).
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2 = 50.2). Four haplotypes with a frequency of more than 5% were deduced that explained 84% of the genotypes found in our postmenopausal patient sample. None of these haplotypes showed a significant association with Z-score without any further correction (Table 4). In the multiple regression analysis, however, haplotype 4 (T-l-A-C) could be shown to have a significant association with FN Z-score (P = 0.011) and accounted for 1.3% of its variance (Table 5).
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As previously reported, the heterozygous CLCN7 mutation 797fsX925 in family K gives rise to a rather mild ADOII phenotype, presumably because the C-terminally elongated channel protein is still functional (6). Nine mutation carriers from this family clearly showed radiographic signs of osteopetrosis (either sandwich vertebrae or diffuse osteosclerosis), whereas the other nine carries were unaffected (9). Eighteen mutation carriers and 14 noncarrier DNAs were available for genotyping. Because of the existing linkage disequilibrium among markers rs2235580, intron 8 VNTR, rs2235579, and rs12926089, haplotype analysis was performed. Of the 13 individuals carrying haplotype 3 (T-s-A-C), nine were affected, whereas all of the haplotype 2 (C-l-T-C) carriers were unaffected (P = 0.029) (Fig. 2). One nonaffected person showed haplotype 5 (C-l-T-T), which comprises the V418M polymorphism. No difference existed between the frequency of the haplotypes of nonmutated alleles from unaffected carriers and noncarriers (haplotype 3: four of nine vs. 11 of 28; P = 0.7). This suggests that the polymorphisms on the nonmutated allele might partially contribute to the susceptibility for the disease. As seen in the osteoporosis cohort, the haplotype comprising the intron 8 s-allele seems to be associated with an increase in BMD, although the contribution of the individual tested markers cannot be disentangled due to their mutual correlation.
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Discussion |
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TopAbstractIntroductionPatients and MethodsResultsDiscussionReferences |
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The putative chloride channel ClC-7 has been shown to be of crucial importance for the function of osteoclasts (3). A 50% reduction of ClC-7 expression, however, is not sufficient to considerably alter bone density in heterozygous human and murine carriers of ClC-7 loss-of-function mutations. In ADOII, however, heterozygous dominant mutations further lower the dosage of functional ClC-7 channel protein dimers, thereby reaching the threshold at which ClC-7 becomes rate limiting for the resorption process (13). At this threshold any further disturbance of ClC-7 expression, e.g. by polymorphic sequence changes, could result in changes in BMD.
Individuals showing the intron 8 ss genotype have a slightly higher FN Z-score, whereas haplotype 4, which contains the intron 8 l-allele, is associated with lower bone density. These findings suggest that the association of CLCN7 with BMD is largely driven by the intron 8 genotype. The analysis of haplotype 4, however, is justified because it decreases the P value so that the association remains significant even after a very conservative Bonferroni correction for multiple testing. Additionally, the fraction of the FN Z-score variance explained by CLCN7 sequence changes is higher for haplotype 4 than for intron 8 alone. Furthermore, we were able to assess only a dominant effect of haplotype 4 due to the limited number of homozygous individuals in our sample. It can be speculated that the observed effect on BMD might be even stronger if a sufficiently high number of homozygous haplotype 4 carriers is investigated. Surprisingly, no significant association of CLCN7 polymorphisms or haplotypes with LS Z-score could be detected. This is in contrast with the increased vertebral bone density observed in ADOII (14).
In postmenopausal osteoporosis bone loss is related to an increase in bone resorption, which is reflected by elevated levels of bone resorption markers (15). In line with our finding that intron 8 and haplotype 4 genotypes are associated with Z-score values, we could show that there was an association between the intron 8 VNTR length and haplotype 4 with a marker of bone resorption (D-Pyr/Crea). It seems therefore likely that the higher BMD seen in individuals with the ss genotype or without haplotype 4, respectively, stems from a reduced bone resorption activity, which is compatible with the physiological role of ClC-7.
Due to the globally reduced osteoclast activity, ADOII normally has a segmental radiological appearance (7, 8). In contrast, a diffuse bone sclerosis is seen in autosomal dominant osteopetrosis type I, which results from mutations in the LRP5 gene that regulates osteoblast differentiation (16). Similar to the findings in LRP5, the ADOII family described here demonstrates that the phenotype of patients with heterozygous mutations in CLCN7 can consist of high bone density only (Fig. 2) (9). Furthermore, polymorphisms in LRP5 turned out to be associated with higher BMD in a population sample (17). It has been reported by other groups that the penetrance of ADOII can be incomplete (4). In our ADOII family only 50% of the mutation carriers are affected. To understand the variability of the phenotype in this family, we asked whether the nonmutation-carrying CLCN7 allele could be associated with the expression of the disorder. The frequency of the SNPs rs2235580, rs2235579, and rs12926089 and the s-allele of the intron 8 VNTR were significantly different in affected and nonaffected carriers. There are, however, three unaffected individuals with a mutated allele and a haplotype 3 allele showing that the nonmutated CLCN7 allele might be only one factor among others influencing penetrance. Chu et al. (7) found suggestive linkage to a modifier locus on chromosome 9, but due to the low number of tested individuals, we could not test for further modifying loci. In an earlier work, Frattini et al. (5) did not find any association of expression of the ADOII phenotype and intron 8 repeat length alone by comparing carrier parents with more severely affected siblings. We cannot exclude that in our ADOII family the haplotype 2 allele might contain another so-far-unknown sequence variation that limits ClC-7 function. Because this family is of the same ethnic background as the postmenopausal women investigated in our association study, this could be true for these individuals as well.
V418M (rs12926089) is the nonsynonymous polymorphism with the highest frequency (10%) in the CLCN7 gene. Although V418M was described elsewhere to be associated with BMD in the general population, we could not detect any significant effect, which could be due to the size of our sample (18). Our study had a power of 30% to detect a significant difference between subgroups. Furthermore, Campos-Xavier et al. (8) as well as Chu et al. (7) found an association of V418M with the severity of the ADOII phenotype in several families. We could not reproduce this finding because only one of the 18 mutation carriers of our ADOII family harbored this polymorphism (Fig. 2). This person was not affected. On the other hand, our study and the data of Pettersson et al. (18) demonstrate that the intron 8 VNTR and V418M are in linkage disequilibrium so that the contributions of both polymorphisms can hardly be separated from each other. The mechanism by which the intronic VNTR might influence the function of the CLCN7 gene product (ClC-7) is elusive. It is remarkable that each repeat unit contains the complete splice acceptor consensus sequence of the intron 8/exon 9 border. We could not detect any alternative splice products by RT-PCR (data not shown). Furthermore, this VNTR is a predicted CpG island that could have regulatory function. No repetitive elements can be found in intron 8 of the murine ortholog, thus precluding an analysis in the mouse. Further work will be needed to specify whether the VNTR exerts any direct influence or whether another polymorphism within the haplotype block might be causative.
Several studies have investigated the association between polymorphisms within candidate genes implicated in osteoclast function and BMD. The gene TNFRSF1B encodes the receptor activator of nuclear factor-
B receptor that is expressed by osteoclast precursors and governs their differentiation (19). In a population study using a multiple linear regression model, polymorphisms in TNFRSF1B accounted for a similar level of variance of BMD as the CLCN7 polymorphism we report here (20). The gene TCIRG1 encodes the 
3-subunit of the proton pump protein complex that is functionally linked to ClC-7 in the osteoclast acid secretion mechanism. Two studies (21, 22) investigated whether TCIRG1 is associated with osteoporosis and came to conflicting conclusions.
Thus, a growing amount of data indicates that genes essential for osteoclast function are worthwhile candidates for studying the polygenic regulation of bone mass. Association studies on such genes allow for an evaluation of candidate genes that might represent new drug targets. In line with that, a recent report (23) indicates that a putative ClC-7 inhibitor has a beneficial effect on BMD in a rat osteoporosis model.
Although the contribution of CLCN7 polymorphisms to BMD variance in postmenopausal women identified in our study is in the same range as that of other tested candidate genes, it is well known that association results can vary greatly, depending on the investigated sample (24). Further replication of our findings in other populations is therefore necessary to finally demonstrate the degree of relevance of CLCN7 for the complex pathogenesis of osteoporosis.
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Acknowledgments |
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Footnotes |
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Abbreviations: ADOII, Autosomal dominant osteopetrosis type II; BMD, bone mineral density; D-Pyr/Crea, deoxypyridinoline; FN, femoral neck; HRT, hormone replacement therapy; l, long; LS, lumbar spine; s, short; SNP, single-nucleotide polymorphism; VNTR, variable number tandem repeat.
U.K. had a Poste Vert position from the Institut National de la Santé et de la Recherche Médicale to realize part of this work. M.-C.d.V. received a grant Direction de la Recherche Clinique at AP/HP (AOR 01049).
Received September 9, 2005.
Accepted December 8, 2005.
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References |
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TopAbstractIntroductionPatients and MethodsResultsDiscussionReferences |
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gene and their relationship to bone mass variation in postmenopausal Italian women. Hum Mol Genet 9:2043–2050This article has been cited by other articles:
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  S. Ferrari Single Gene Mutations and Variations Affecting Bone Turnover and Strength: a Selective 2006 Update IBMS BoneKEy, December 1, 2006; 3(12): 11 - 29. [Abstract] [Full Text] [PDF]
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) and nonaffected (
) individuals was significantly different (P = 0.029) as assessed by Fisher’s exact test. B, Representative x-rays of three members of family K. II-1 displays typical sandwich vertebrae, whereas a diffuse sclerosis is seen in II-2. The LS of mutation carrier III-6 appears completely normal.