Abstract
Objective. Unconfirmed reports describe association of ankylosing spondylitis (AS) with several candidate genes including ANKH. Cellular export of inorganic pyrophosphate is regulated by the ANK protein, and mutant mice (ank/ank), which have a premature stop codon in the 3’ end of the ank gene, develop severe ankylosis. We tested the association between single-nucleotide polymorphisms (SNP) in these genes and susceptibility to AS in a population of patients with AS. We investigated the role of these genes in terms of functional (BASFI) and metrological (BASMI) measures, and the association with radiological severity (mSASSS).
Methods. Our study was conducted on 355 patients with AS and 95 ethnically matched healthy controls. AS was defined according to the modified New York criteria. Four SNP in ANKH (rs27356, rs26307, rs25957, and rs28006) were genotyped. Association analysis was performed using Cochrane-Armitage and linear regression tests for dichotomous and quantitative variables. Analyses of Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), BASFI, and mSASSS were controlled for sex and disease duration.
Results. None of the 4 markers showed significant single-locus disease associations (p > 0.05), suggesting that ANKH was not a major determinant of AS susceptibility in our population. No association was observed between these SNP and age at symptom onset, BASDAI, BASFI, BASMI, or mSASSS.
Conclusion. These results confirm data in white Europeans that ANKH is probably not a major determinant of susceptibility to AS. ANKH polymorphisms do not markedly influence AS disease severity, as measured by BASMI and mSASSS.
Ankylosing spondylitis (AS) is a chronic inflammatory arthropathy, with an estimated prevalence of 0.1%–0.9% in white populations1. Genetic factors play a major role in the risk of developing AS2,3, and influence several measures of disease severity, including the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), Functional Index (BASFI)4,5, and Bath AS radiographic index (BASRI)6. Studies have shown that sex influences susceptibility and disease severity. The prevalence of AS is 2.5 times higher in men than in women7, and women have a later onset of the disease and less thoracic and lumbar spinal radiographic severity8. The pathophysiological mechanisms underlying these differences remain unclear. It is unlikely that the major genetic factors involved are X-linked because there is no linkage of AS susceptibility with X-chromosome markers9.
The ANKH gene is of particular interest in AS, as mice with a loss-of-function mutation in the homologous gene, ank, develop severe ectopic mineralization and skeletal ankylosis resembling AS10. Humans with gain-of-expression mutations and polymorphisms in this gene develop calcium pyrophosphate chondrocalcinosis11,12, whereas loss-of-function mutations cause excess hydroxyapatite deposition in Jackson’s craniometaphyseal dysplasia disease13,14. A family has recently been described with a spondyloarthropathy with some similarities to AS due to homozygosity for a loss-of-function ANKH mutation15. An initial study of ANKH showed no association with susceptibility to AS16, and no association has been identified with this gene in genomewide association studies in AS to date17,18. However, weak positive findings have been reported by some investigators19, and it has been suggested that the association may be more strongly observed in women20. Further, no study has investigated the association of ANKH variants with radiographic or joint metrology indices. We sought to test the association between single-nucleotide polymorphisms (SNP) in ANKH and susceptibility to AS in a Portuguese population. Additionally we investigated the association of ANKH with functional (BASFI) and metrological (Bath Ankylosing Spondylitis Metrology Index; BASMI) measures, and for association with radiological severity (modified Stoke Ankylosing Spondylitis Spine Score; mSASSS).
MATERIALS AND METHODS
Subjects
The study was conducted on 355 unrelated patients with AS and 95 ethnically matched healthy controls, all of Portuguese ancestry. AS was defined according to the modified New York criteria21. Cases were recruited from hospital outpatient departments; controls were healthy Portuguese bone marrow donors. Our study was approved by the ethics board of the study centers involved, and written informed consent was obtained from the participating individuals.
Patients completed a questionnaire self-assessment of clinical features, including the BASDAI and the BASFI. Age at disease onset was defined as the age at symptom onset, and disease duration was defined as the period of time (years) after symptom onset. Metrology investigation was performed by 1 investigator (FPS) to obtain the BASMI score. Radiological evaluation was performed using the mSASSS; all radiographs were scored independently by 2 authors (FPS, AFM). Where there was discordance between the scores, they were reevaluated by both reviewers for a consensus score. Data on current therapy were collected.
Genotyping
Genomic DNA from cases and controls was prepared from peripheral blood lymphocytes using standard techniques. Samples were genotyped for ANKH allelic variants (rs27356, rs26307, rs25957, and rs28006) that had previously been associated with AS in either men or women20. Taqman® SNP assays (Applied Biosystems, Foster City, CA, USA) were used for genotyping, performed according to the manufacturer’s protocols. Genotyping reactions were performed with an ABI 7900HT instrument, and the allele call by analysis of allelic discrimination plots with ABI SDS 2.3 software. Replicate known and negative genotype control samples were typed in each 96-well plate.
Statistical analysis
SNP genotype data were assessed for missing data and for Hardy-Weinberg equilibrium in controls. Individuals with > 10% missingness were excluded. Association analysis was performed using the Cochrane-Armitage test as implemented in the PLINK program (Harvard University, Cambridge, MA, USA; Website: http://pngu.mgh.harvard.edu/~purcell/plink/gplink.shtml). Association between SNP and the quantitative variables age of symptom onset, BASDAI, BASFI, BASMI, and mSASSS were tested by linear regression assuming an additive model using PLINK, taking into account sex and disease duration as covariates. Statistical power was tested using the Genetic Power Calculator22.
RESULTS
The AS cohort population (n = 355) included 224 (63.1%) men and 131 (36.9%) women with a mean age of 45.4 (SD ± 13.2) years (range 20–79 yrs) and a mean disease duration of 19.1 (SD ± 12.6) years (range 0–60 yrs), of whom 82% were HLA-B27-positive. The therapies used were similar in all patients, with the exception of nonsteroidal antiinflammatory drugs, which were used in a greater percentage of women (92.6%) than men (80.4%). Thus differences in therapy between sexes are unlikely to be an explanation of observed differences in AS activity or severity. Epidemiological data of the cases are summarized in Table 1.
All genetic markers studied were in Hardy-Weinberg equilibrium in the control group, with missing-ness rates < 10%, and there were no observations of differential missing-ness in cases and controls (p < 0.01). The minor allele frequencies (MAF) of the 4 SNP are presented in Table 2.
None of the 4 studied markers showed significant single-locus disease associations (p > 0.05), in the whole group and in subanalysis by sex, suggesting that ANKH gene is not a major determinant of AS susceptibility in a Portuguese population (Table 2). In addition, no association was observed between these SNP and age of symptom onset, BASDAI, BASFI, BASMI, or mSASSS when considering the whole population (Table 3). The sample size in individual sex groups was too small for a worthwhile analysis.
The study had 80% power to detect association with AS (p < 0.05, assuming a population prevalence of 0.5%, with D’ = 1) with an additive OR of 1.9, and to detect association with quantitative measures (BASDAI, BASFI, BASMI, mSASSS) contributing > 3% of the trait variance.
DISCUSSION
We analyzed 4 intronic markers previously described as associated with AS in men (rs26307, rs27356) or women (rs28006, rs25957), in a study of 201 multiplex families20. In our study, involving unrelated patients with AS, we demonstrate that ANKH is not significantly associated with either susceptibility to AS or measures of its activity or severity, either in the whole group or in men or women separately. There are several possible explanations for the discrepancy of the results observed in the 2 studies: intrinsic differences between the 2 populations (North Americans vs Portuguese subjects) or differences in the patient populations (multiplex families vs unrelated individuals). Finally, both studies were underpowered to detect genes with small effects consistently, potentially leading to discrepancies between results. Despite the methodological differences (ethnicity, case ascertainment approaches, and ANKH marker variants analyzed), this investigation reinforces the results of another study in white Europeans16, where no associations with disease susceptibility or phenotypic characteristics were seen. Our current study extends these previous observations, in that it is the first study to test ANKH associations with metrological (BASMI) and radiological (mSASSS) indices. Given the previous findings in mice and humans with loss-of-function ANKH mutants, we hypothesized that ANKH polymorphisms may contribute to spinal ossification in AS. This effect would be more easily detected by the potential influence on variables of the BASMI and mSASSS. Even considering these aspects, ANKH variants appeared to have no significant role in our population.
Several major ossification pathways have been identified that may play a central role in diseases characterized by bone formation, such as AS. They involve transforming growth factor-ß23,24, bone morphogenetic proteins25,26, and the wingless (Wnt) proteins27,28. Further studies investigating these pathways in larger datasets are indicated to identify genes influencing the severity and rate of ankylosis in AS.
Our results confirm previous data in white Britons that ANKH is not a major determinant of susceptibility to AS16, and also demonstrate that ANKH variants do not have a major influence on severity of AS (measured by BASDAI, BASFI, BASMI, or mSASSS) or age at disease onset.
Acknowledgment
We thank the individuals who shared their clinical data with us to complete this study, the Ankylosing Spondylitis Portuguese Patients Association, ANEA, and all members of the CORPOREA Study Group.
APPENDIX
List of study collaborators. The CORPOREA Study Group: Centro Hospitalar de Lisboa Ocidental, Hospital de Egas Moniz EPE, Lisbon: A.F. Mourão, A.A. de Matos, C. Ribeiro, F.M. Pimentel-Santos, J. Bravo Pimentão, J.C. Branco, M. Mateus, P. Nero, P. Araújo, S. Falcão, T.L. Pinto, W. Castelão; Unidade de Investigação em Reumatologia, Instituto de Medicina Molecular, Faculdade de Medicina da Universidade de Lisboa: E. Vieira de Sousa, J. Caetano-Lopes, J.E. Fonseca; Instituto Português de Reumatologia, Lisbon: C. Silva, E. Simões, H. Madeira, H. Santos, J. Vaz Patto, J. Ferreira, M. Micaelo, M.J. Mediavilla, M. Sousa; Hospital Curry Cabral EPE, Lisbon: P. Soares Branco; Hospital Garcia de Orta EPE, Almada: F. Godinho, J. Canas da Silva, S. Garcês, V. Tavares; Centro Hospitalar do Alto Minho, Hospital Conde de Bertiandos EPE, Ponte de Lima: A. Ribeiro, D. Araújo, J.A. Costa, L. Costa, M.C. Afonso, M. Bogas, S. Alcino; Centro Hospitalar de Vila Nova de Gaia/Espinho EPE, Vila Nova de Gaia: P. Pinto; Hospital Infante D Pedro EPE, Aveiro: A. Barcelos, I. Silva, C. Ambrósio; Centro Hospitalar de Oeste Norte, Centro Hospitalar das Caldas da Rainha, Caldas da Rainha: M. Cruz; Hospital de Faro EPE, Faro: G. Sequeira; Hospital Espírito Santo EPE, Évora: A.R. Cravo; Hospital Militar Principal, Lisbon: R.A. Santos.
Footnotes
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Supported by Bolsa de Investigação da Sociedade Portuguesa de Reumatologia/Schering-Plough 2007, the FCML 2007 Grant, and the Wyeth Lederle Portugal Grant. Dr. Brown is supported by a National Health and Medical Research Council (Australia) Principal Research Fellowship.
- Accepted for publication August 25, 2011.