Thorvaldur Ingvarsson

Multiple Genetic Loci for Bone Mineral Density and Fractures

BACKGROUND Bone mineral density influences the risk of osteoporosis later in life and is useful in the evaluation of the risk of fracture. We aimed to identify sequence variants associated with bone mineral density and fracture. METHODS We performed a quantitative trait analysis of data from 5861 Icelandic subjects (the discovery set), testing for an association between 301,019 single-nucleotide polymorphisms (SNPs) and bone mineral density of the hip and lumbar spine. We then tested for an association between 74 SNPs (most of which were implicated in the discovery set) at 32 loci in replication sets of Icelandic, Danish, and Australian subjects (4165, 2269, and 1491 subjects, respectively). RESULTS Sequence variants in five genomic regions were significantly associated with bone mineral density in the discovery set and were confirmed in the replication sets (combined P values, 1.2x10(-7) to 2.0x10(-21)). Three regions are close to or within genes previously shown to be important to the biologic characteristics of bone: the receptor activator of nuclear factor-kappaB ligand gene (RANKL) (chromosomal location, 13q14), the osteoprotegerin gene (OPG) (8q24), and the estrogen receptor 1 gene (ESR1) (6q25). The two other regions are close to the zinc finger and BTB domain containing 40 gene (ZBTB40) (1p36) and the major histocompatibility complex region (6p21). The 1p36, 8q24, and 6p21 loci were also associated with osteoporotic fractures, as were loci at 18q21, close to the receptor activator of the nuclear factor-kappaB gene (RANK), and loci at 2p16 and 11p11. CONCLUSIONS We have discovered common sequence variants that are consistently associated with bone mineral density and with low-trauma fractures in three populations of European descent. Although these variants alone are not clinically useful in the prediction of risk to the individual person, they provide insight into the biochemical pathways underlying osteoporosis.

New sequence variants associated with bone mineral density.

In an extended genome-wide association study of bone mineral density among 6,865 Icelanders and a follow-up in 8,510 subjects of European descent, we identified four new genome-wide significant loci. These are near the SOST gene at 17q21, the MARK3 gene at 14q32, the SP7 gene at 12q13 and the TNFRSF11A (RANK) gene at 18q21. Furthermore, nonsynonymous SNPs in the C17orf53, LRP4, ADAM19 and IBSP genes were suggestively associated with bone density.

Identification of new susceptibility loci for osteoarthritis (arcOGEN): A genome-wide association study

Summary Background Osteoarthritis is the most common form of arthritis worldwide and is a major cause of pain and disability in elderly people. The health economic burden of osteoarthritis is increasing commensurate with obesity prevalence and longevity. Osteoarthritis has a strong genetic component but the success of previous genetic studies has been restricted due to insufficient sample sizes and phenotype heterogeneity. Methods We undertook a large genome-wide association study (GWAS) in 7410 unrelated and retrospectively and prospectively selected patients with severe osteoarthritis in the arcOGEN study, 80% of whom had undergone total joint replacement, and 11 009 unrelated controls from the UK. We replicated the most promising signals in an independent set of up to 7473 cases and 42 938 controls, from studies in Iceland, Estonia, the Netherlands, and the UK. All patients and controls were of European descent. Findings We identified five genome-wide significant loci (binomial test p≤5·0×10−8) for association with osteoarthritis and three loci just below this threshold. The strongest association was on chromosome 3 with rs6976 (odds ratio 1·12 [95% CI 1·08–1·16]; p=7·24×10−11), which is in perfect linkage disequilibrium with rs11177. This SNP encodes a missense polymorphism within the nucleostemin-encoding gene GNL3. Levels of nucleostemin were raised in chondrocytes from patients with osteoarthritis in functional studies. Other significant loci were on chromosome 9 close to ASTN2, chromosome 6 between FILIP1 and SENP6, chromosome 12 close to KLHDC5 and PTHLH, and in another region of chromosome 12 close to CHST11. One of the signals close to genome-wide significance was within the FTO gene, which is involved in regulation of bodyweight—a strong risk factor for osteoarthritis. All risk variants were common in frequency and exerted small effects. Interpretation Our findings provide insight into the genetics of arthritis and identify new pathways that might be amenable to future therapeutic intervention. Funding arcOGEN was funded by a special purpose grant from Arthritis Research UK.

Genomewide Scan for Hand Osteoarthritis: A Novel Mutation in Matrilin-3

Osteoarthritis (OA) is the most common human joint disease, characterized by loss and/or remodeling of joint synovium, cartilage, and bone. Here, we describe a genomewide linkage analysis of patients with idiopathic hand OA who were carefully phenotyped for involvement of either or both the distal interphalangeal (DIP) joints and the first carpometacarpal (CMC1) joints. The best linkage peaks were on chromosomes 4q and 3p and on the short arm of chromosome 2. Genomewide significance was reached for a locus on chromosome 2 for patients with affected CMC1 joints (LOD = 4.97); this locus was also significant for patients with OA in both CMC1 and DIP joints (LOD = 4.44). The peak LOD score at this locus coincides with a gene, MATN3, encoding the noncollagenous cartilage extracellular matrix protein, matrilin-3. Subsequent screening of the genomic sequence revealed a missense mutation, of a conserved amino acid codon, changing threonine to methionine in the epidermal growth factor-like domain in matrilin-3. The missense mutation cosegregates with hand OA in several families. The mutation frequency is slightly more than 2% in patients with hand OA in the Icelandic population and has a relative risk of 2.1.

A Genome-Wide Association Study Identifies an Osteoarthritis Susceptibility Locus on Chromosome 7q22

OBJECTIVE To identify novel genes involved in osteoarthritis (OA), by means of a genome-wide association study. METHODS We tested 500,510 single-nucleotide polymorphisms (SNPs) in 1,341 Dutch Caucasian OA cases and 3,496 Dutch Caucasian controls. SNPs associated with at least 2 OA phenotypes were analyzed in 14,938 OA cases and approximately 39,000 controls. Meta-analyses were performed using the program Comprehensive Meta-analysis, with P values <1 x 10(-7) considered genome-wide significant. RESULTS The C allele of rs3815148 on chromosome 7q22 (minor allele frequency 23%; intron 12 of the COG5 gene) was associated with a 1.14-fold increased risk (95% confidence interval 1.09-1.19) of knee and/or hand OA (P = 8 x 10(-8)) and also with a 30% increased risk of knee OA progression (95% confidence interval 1.03-1.64) (P = 0.03). This SNP is in almost complete linkage disequilibrium with rs3757713 (68 kb upstream of GPR22), which is associated with GPR22 expression levels in lymphoblast cell lines (P = 4 x 10(-12)). Immunohistochemistry experiments revealed that G protein-coupled receptor protein 22 (GPR22) was absent in normal mouse articular cartilage or synovium. However, GPR22-positive chondrocytes were found in the upper layers of the articular cartilage of mouse knee joints that were challenged with in vivo papain treatment or methylated bovine serum albumin treatment. GPR22-positive chondrocyte-like cells were also found in osteophytes in instability-induced OA. CONCLUSION Our findings identify a novel common variant on chromosome 7q22 that influences susceptibility to prevalence and progression of OA. Since the GPR22 gene encodes a G protein-coupled receptor, this is potentially an interesting therapeutic target.

Large‐scale analysis of association between GDF5 and FRZB variants and osteoarthritis of the hip, knee, and hand

OBJECTIVE GDF5 and FRZB have been proposed as genetic loci conferring susceptibility to osteoarthritis (OA); however, the results of several studies investigating the association of OA with the rs143383 polymorphism of the GDF5 gene or the rs7775 and rs288326 polymorphisms of the FRZB gene have been conflicting or inconclusive. To examine these associations, we performed a large-scale meta-analysis of individual-level data. METHODS Fourteen teams contributed data on polymorphisms and knee, hip, and hand OA. For rs143383, the total number of cases and controls, respectively, was 5,789 and 7,850 for hip OA, 5,085 and 8,135 for knee OA, and 4,040 and 4,792 for hand OA. For rs7775, the respective sample sizes were 4,352 and 10,843 for hip OA, 3,545 and 6,085 for knee OA, and 4,010 and 5,151 for hand OA, and for rs288326, they were 4,346 and 8,034 for hip OA, 3,595 and 6,106 for knee OA, and 3,982 and 5,152 for hand OA. For each individual study, sex-specific odds ratios (ORs) were calculated for each OA phenotype that had been investigated. The ORs for each phenotype were synthesized using both fixed-effects and random-effects models for allele-based effects, and also for haplotype effects for FRZB. RESULTS A significant random-effects summary OR for knee OA was demonstrated for rs143383 (1.15 [95% confidence interval 1.09-1.22]) (P=9.4x10(-7)), with no significant between-study heterogeneity. Estimates of effect sizes for hip and hand OA were similar, but a large between-study heterogeneity was observed, and statistical significance was borderline (for OA of the hip [P=0.016]) or absent (for OA of the hand [P=0.19]). Analyses for FRZB polymorphisms and haplotypes did not reveal any statistically significant signals, except for a borderline association of rs288326 with hip OA (P=0.019). CONCLUSION Evidence of an association between the GDF5 rs143383 polymorphism and OA is substantially strong, but the genetic effects are consistent across different populations only for knee OA. Findings of this collaborative analysis do not support the notion that FRZB rs7775 or rs288326 has any sizable genetic effect on OA phenotypes.

Nonsense mutation in the LGR4 gene is associated with several human diseases and other traits

Low bone mineral density (BMD) is used as a parameter of osteoporosis. Genome-wide association studies of BMD have hitherto focused on BMD as a quantitative trait, yielding common variants of small effects that contribute to the population diversity in BMD. Here we use BMD as a dichotomous trait, searching for variants that may have a direct effect on the risk of pathologically low BMD rather than on the regulation of BMD in the healthy population. Through whole-genome sequencing of Icelandic individuals, we found a rare nonsense mutation within the leucine-rich-repeat-containing G-protein-coupled receptor 4 (LGR4) gene (c.376C>T) that is strongly associated with low BMD, and with osteoporotic fractures. This mutation leads to termination of LGR4 at position 126 and fully disrupts its function. The c.376C>T mutation is also associated with electrolyte imbalance, late onset of menarche and reduced testosterone levels, as well as an increased risk of squamous cell carcinoma of the skin and biliary tract cancer. Interestingly, the phenotype of carriers of the c.376C>T mutation overlaps that of Lgr4 mutant mice.

Meta-analysis of genome-wide association studies confirms a susceptibility locus for knee osteoarthritis on chromosome 7q22

Objectives Osteoarthritis (OA) is the most prevalent form of arthritis and accounts for substantial morbidity and disability, particularly in older people. It is characterised by changes in joint structure, including degeneration of the articular cartilage, and its aetiology is multifactorial with a strong postulated genetic component. Methods A meta-analysis was performed of four genome-wide association (GWA) studies of 2371 cases of knee OA and 35 909 controls in Caucasian populations. Replication of the top hits was attempted with data from 10 additional replication datasets. Results With a cumulative sample size of 6709 cases and 44 439 controls, one genome-wide significant locus was identified on chromosome 7q22 for knee OA (rs4730250, p=9.2×10−9), thereby confirming its role as a susceptibility locus for OA. Conclusion The associated signal is located within a large (500 kb) linkage disequilibrium block that contains six genes: PRKAR2B (protein kinase, cAMP-dependent, regulatory, type II, β), HPB1 (HMG-box transcription factor 1), COG5 (component of oligomeric golgi complex 5), GPR22 (G protein-coupled receptor 22), DUS4L (dihydrouridine synthase 4-like) and BCAP29 (B cell receptor-associated protein 29). Gene expression analyses of the (six) genes in primary cells derived from different joint tissues confirmed expression of all the genes in the joint environment.

Insights into the genetic architecture of osteoarthritis from stage 1 of the arcOGEN study

Objectives The genetic aetiology of osteoarthritis has not yet been elucidated. To enable a well-powered genome-wide association study (GWAS) for osteoarthritis, the authors have formed the arcOGEN Consortium, a UK-wide collaborative effort aiming to scan genome-wide over 7500 osteoarthritis cases in a two-stage genome-wide association scan. Here the authors report the findings of the stage 1 interim analysis. Methods The authors have performed a genome-wide association scan for knee and hip osteoarthritis in 3177 cases and 4894 population-based controls from the UK. Replication of promising signals was carried out in silico in five further scans (44 449 individuals), and de novo in 14 534 independent samples, all of European descent. Results None of the association signals the authors identified reach genome-wide levels of statistical significance, therefore stressing the need for corroboration in sample sets of a larger size. Application of analytical approaches to examine the allelic architecture of disease to the stage 1 genome-wide association scan data suggests that osteoarthritis is a highly polygenic disease with multiple risk variants conferring small effects. Conclusions Identifying loci conferring susceptibility to osteoarthritis will require large-scale sample sizes and well-defined phenotypes to minimise heterogeneity.

Hand osteoarthritis in older women is associated with carotid and coronary atherosclerosis: the AGES Reykjavik study

Objective: There is evidence that atherosclerosis may contribute to the initiation or progression of osteoarthritis. To test this hypothesis, the presence and severity of hand osteoarthritis (HOA) was compared with markers of atherosclerotic vascular disease in an elderly population. Patients and Methods: The AGES Reykjavik Study is a population-based multidisciplinary study of ageing in the elderly population of Reykjavik. In a study of 2264 men (mean age 76 years; SD 6) and 3078 women (mean age 76 years; SD 6) the severity of HOA, scored from photographs, was compared with measures of atherosclerosis. These included carotid intimal thickness and plaque severity, coronary calcifications (CAC) and aortic calcifications and reported cardiac and cerebrovascular events. Results: After adjustment for confounders, both carotid plaque severity and CAC were significantly associated with HOA in women, with an odds ratio of 1.42 (95% CI 1.14 to 1.76, p = 0.002) for having CAC and 1.25 (95% CI 1.04 to 1.49, p = 0.016) for having moderate or severe carotid plaques. Both carotid plaques and CAC also exhibited significant linear trends in relation to HOA severity in women in the whole AGES Reykjavik cohort (p<0.001 and p = 0.027, respectively, for trend). No significant associations were seen in men. Despite this evidence of increased atherosclerosis, women with HOA did not report proportionally more previous cardiovascular or cerebrovascular events. Conclusions: The results indicate a linear association between the severity of HOA and atherosclerosis in older women. The pathological process of HOA seems to have some components in common with atherosclerosis. Prospective studies may help elucidate the possible mechanisms of this relationship.