The DOT1L rs12982744 polymorphism is associated with osteoarthritis of the hip with genome-wide statistical significance in males

Osteoarthritis as an Enhanceropathy: Gene Regulation in Complex Musculoskeletal Disease

PURPOSE OF REVIEW

Osteoarthritis is a complex and highly polygenic disease. Over 100 reported osteoarthritis risk variants fall in non-coding regions of the genome, ostensibly conferring functional effects through the disruption of regulatory elements impacting target gene expression. In this review, we summarise the progress that has advanced our knowledge of gene enhancers both within the field of osteoarthritis and more broadly in complex diseases.

RECENT FINDINGS

Advances in technologies such as ATAC-seq have facilitated our understanding of chromatin states in specific cell types, bolstering the interpretation of GWAS and the identification of effector genes. Their application to osteoarthritis research has revealed enhancers as the principal regulatory element driving disease-associated changes in gene expression. However, tissue-specific effects in gene regulatory mechanisms can contribute added complexity to biological interpretation. Understanding gene enhancers and their altered activity in specific cell and tissue types is the key to unlocking the genetic complexity of osteoarthritis. The use of single-cell technologies in osteoarthritis research is still in its infancy. However, such tools offer great promise in improving our functional interpretation of osteoarthritis GWAS and the identification of druggable targets. Large-scale collaborative efforts will be imperative to understand tissue and cell-type specific molecular mechanisms underlying enhancer function in disease.

The Genetic Markers of Knee Osteoarthritis in Women from Russia

Osteoarthritis is a chronic progressive joint disease that clinically debuts at the stage of pronounced morphologic changes, which makes treatment difficult. In this regard, an important task is the study of genetic markers of the disease, which have not been definitively established, due to the clinical and ethnic heterogeneity of the studied populations. To find the genetic markers for the development of knee osteoarthritis (OA) in women from the Volga-Ural region of Russia, we conducted research in two stages using different genotyping methods, such as the restriction fragment length polymorphism (RFLP) measurement, TaqMan technology and competitive allele-specific PCR-KASPTM. In the first stage, we studied polymorphic variants of candidate genes (ACAN, ADAMTS5, CHST11, SOX9, COL1A1) for OA development. The association of the *27 allele of the VNTR locus of the ACAN gene was identified (OR = 1.6). In the second stage, we replicated the GWAS results (ASTN2, ALDH1A2, DVWA, CHST11, GNL3, NCOA3, FILIP/SENP1, MCF2L, GLT8D, DOT1L) for knee OA studies. The association of the *T allele of the rs7639618 locus of the DVWA gene was detected (OR = 1.54). Thus, the VNTR locus of ACAN and the rs7639618 locus of DVWA are risk factors for knee OA in women from the Volga-Ural region of Russia.

Genetics of osteoarthritis

Osteoarthritis genetics has been transformed in the past decade through the application of large-scale genome-wide association scans. So far, over 100 polymorphic DNA variants have been associated with this common and complex disease. These genetic risk variants account for over 20% of osteoarthritis heritability and the vast majority map to non-protein coding regions of the genome where they are presumed to act by regulating the expression of target genes. Statistical fine mapping, in silico analyses of genomics data, and laboratory-based functional studies have enabled the identification of some of these targets, which encode proteins with diverse roles, including extracellular signaling molecules, intracellular enzymes, transcription factors, and cytoskeletal proteins. A large number of the risk variants correlate with epigenetic factors, in particular cartilage DNA methylation changes in cis, implying that epigenetics may be a conduit through which genetic effects on gene expression are mediated. Some of the variants also appear to have been selected as humans adapted to bipedalism, suggesting that a proportion of osteoarthritis genetic susceptibility results from antagonistic pleiotropy, with risk variants having a positive role in joint formation but a negative role in the long-term health of the joint. Although data from an osteoarthritis genetic study has not yet directly led to a novel treatment, some of the osteoarthritis associated genes code for proteins that have available therapeutics. Genetic investigations are therefore revealing fascinating fundamental insights into osteoarthritis and can expose options for translational intervention.

Associations between adipokines gene polymorphisms and knee osteoarthritis: a meta-analysis

Background

Adipokines gene polymorphisms are speculated to be associated with the risk of knee osteoarthritis (OA), but evidence remains conflicting. This study therefore aimed to examine whether associations exist between adipokines gene polymorphisms and knee OA by considering the evidence collected from eligible studies through a meta-analysis.

Methods

A systematic search was performed on PubMed, Embase, Web of Science, China National Knowledge Infrastructure (CNKI), and Wanfang up to March 31, 2020. Meta-analysis was carried out by focusing on the associations between adipokines gene polymorphisms and knee OA with the allele model, dominant model, and recessive model.

Results

The present meta-analysis included 5 eligible studies for ADIPOQ rs1501299 with 1,021 cases and 1,097 controls, 3 eligible studies for ADIPOQ rs2241766 with 549 cases and 544 controls, 3 eligible studies for LEPR rs1137101 with 808 cases and 856 controls, 2 eligible studies for VISFATIN rs4730153 with 339 cases and 680 controls and 2 eligible studies for VISFATIN rs16872158 with 339 cases and 680 controls. Significant association was observed between LEPR rs1137101 and knee OA in the overall population (recessive: OR = 0.40, 95% CI 0.21–0.79). Limited data revealed that associations may exist between ADIPOQ rs2241766 and knee OA in Asians (dominant: OR = 1.35, 95% CI 1.03–1.78), between VISFATIN rs4730153 and knee OA in Asians (allele: OR = 0.58, 95% CI 0.41–0.83; dominant: OR = 0.57, 95% CI 0.39–0.83), and between VISFATIN rs16872158 and knee OA in Asians (allele: OR = 1.84, 95% CI 1.26–2.68; dominant: OR = 1.94, 95% CI 1.31–2.89).

Conclusions

Adipokines gene polymorphisms may be associated with knee OA. The association was observed in LEPR rs1137101 in the present study. In addition, limited data revealed that associations may also exist in ADIPOQ rs2241766, VISFATIN rs4730153 and VISFATIN rs16872158.

Prospero registration

CRD42020187664.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-022-05111-4.

The Genetic Epidemiology of Joint Shape and the Development of Osteoarthritis

Congruent, low-friction relative movement between the articulating elements of a synovial joint is an essential pre-requisite for sustained, efficient, function. Where disorders of joint formation or maintenance exist, mechanical overloading and osteoarthritis (OA) follow. The heritable component of OA accounts for ~ 50% of susceptible risk. Although almost 100 genetic risk loci for OA have now been identified, and the epidemiological relationship between joint development, joint shape and osteoarthritis is well established, we still have only a limited understanding of the contribution that genetic variation makes to joint shape and how this modulates OA risk. In this article, a brief overview of synovial joint development and its genetic regulation is followed by a review of current knowledge on the genetic epidemiology of established joint shape disorders and common shape variation. A summary of current genetic epidemiology of OA is also given, together with current evidence on the genetic overlap between shape variation and OA. Finally, the established genetic risk loci for both joint shape and osteoarthritis are discussed.

Osteoarthritis: Insights Offered by the Study of Bone Mass Genetics

PURPOSE OF REVIEW

This paper reviews how bone genetics has contributed to our understanding of the pathogenesis of osteoarthritis. As well as identifying specific genetic mechanisms involved in osteoporosis which also contribute to osteoarthritis, we review whether bone mineral density (BMD) plays a causal role in OA development.

RECENT FINDINGS

We examined whether those genetically predisposed to elevated BMD are at increased risk of developing OA, using our high bone mass (HBM) cohort. HBM individuals were found to have a greater prevalence of OA compared with family controls and greater development of radiographic features of OA over 8 years, with predominantly osteophytic OA. Initial Mendelian randomisation analysis provided additional support for a causal effect of increased BMD on increased OA risk. In contrast, more recent investigation estimates this relationship to be bi-directional. However, both these findings could be explained instead by shared biological pathways. Pathways which contribute to BMD appear to play an important role in OA development, likely reflecting shared common mechanisms as opposed to a causal effect of raised BMD on OA. Studies in HBM individuals suggest this reflects an important role of mechanisms involved in bone formation in OA development; however further work is required to establish whether the same applies to more common forms of OA within the general population.

Genome-wide association study on Northern Chinese identifies KLF2, DOT1L and STAB2 associated with systemic lupus erythematosus

OBJECTIVES

To identify novel genetic loci associated with systemic lupus erythematosus (SLE) and to evaluate potential genetic differences between ethnic Chinese and European populations in SLE susceptibility.

METHODS

A new genome-wide association study (GWAS) was conducted from Jining, North China, on 1506 individuals (512 SLE cases and 994 matched healthy controls). The association results were meta-analysed with existing data on Chinese populations from Hong Kong, Guangzhou and Central China, as well as GWAS results from four cohorts of European ancestry. A total of 26 774 individuals (9310 SLE cases and 17 464 controls) were included in this study.

RESULTS

Meta-analysis on four Chinese cohorts identifies KLF2 as a novel locus associated with SLE [rs2362475; odds ratio (OR) = 0.85, P=2.00E-09]. KLF2 is likely an Asian-specific locus as no evidence of association was detected in the four European cohorts (OR = 0.98, P =0.58), with evidence of heterogeneity (P=0.0019) between the two ancestral groups. Meta-analyses of results from both Chinese and Europeans identify STAB2 (rs10082873; OR= 0.89, P=4.08E-08) and DOT1L (rs4807205; OR= 1.12, P=8.17E-09) as trans-ancestral association loci, surpassing the genome-wide significance.

CONCLUSIONS

We identified three loci associated with SLE, with KLF2 a likely Chinese-specific locus, highlighting the importance of studying diverse populations in SLE genetics. We hypothesize that DOT1L and KLF2 are plausible SLE treatment targets, with inhibitors of DOT1L and inducers of KLF2 already available clinically.

Mesenchyme-specific loss of Dot1L histone methyltransferase leads to skeletal dysplasia phenotype in mice

Chromatin modifying enzymes play essential roles in skeletal development and bone maintenance, and deregulation of epigenetic mechanisms can lead to skeletal growth and malformation disorders. Here, we report a novel skeletal dysplasia phenotype in mice with conditional loss of Disruptor of telomeric silencing 1-like (Dot1L) histone methyltransferase in limb mesenchymal progenitors and downstream descendants. Phenotypic characterizations of mice with Dot1L inactivation by Prrx1-Cre (Dot1L-cKO^Prrx1) revealed limb shortening, abnormal bone morphologies, and forelimb dislocations. Our in vivo and in vitro data support a crucial role for Dot1L in regulating growth plate chondrocyte proliferation and differentiation, extracellular matrix production, and secondary ossification center formation. Micro-computed tomography analysis of femurs revealed that partial loss of Dot1L expression is sufficient to impair trabecular bone formation and microarchitecture in young mice. Moreover, RNAseq analysis of Dot1L deficient chondrocytes implicated Dot1L in the regulation of key genes and pathways necessary to promote cell cycle regulation and skeletal growth. Collectively, our data show that early expression of Dot1L in limb mesenchyme provides essential regulatory control of endochondral bone morphology, growth, and stability.

Osteoarthritis year in review 2019: genetics, genomics and epigenetics

Although osteoarthritis (OA) aetiology is complex, genetic, genomic and epigenetic studies published within the last decade have advanced our understanding of the molecular processes underlying this common musculoskeletal disease. The purpose of this narrative review is to highlight the key research articles within the OA genetics, genomics and epigenetics fields that were published between April 2018 and April 2019. The review focuses on the identification of new OA genetic risk loci, genomics techniques that have been used for the first time in human cartilage and new publicly available databases, and datasets that will aid OA functional studies. Fifty-six new OA susceptibility loci were identified by two large scale genome wide association study meta-analyses, increasing the number of genome-wide significant risk loci to 90. OA risk variants are enriched near genes involved in skeletal development and morphology, and show genetic overlap with height, hip shape, bone area and developmental dysplasia of the hip. Several functional studies of OA loci were published, including a genome-wide analysis of genetic variation on cartilage gene expression. A specialised data portal for exploring cross-species skeletal transcriptomic datasets has been developed, and the first use of cartilage single cell RNAseq analysis reported. This year also saw the systematic identification of all microRNAs, long non-coding RNAs and circular RNAs expressed in human OA cartilage. Putative transcriptional regulatory regions have been mapped in human chondrocytes genome-wide, providing a dataset that will facilitate the prioritisation and characterisation of OA genetic and epigenetic loci.

The Role of Dot1l in Prenatal and Postnatal Murine Chondrocytes and Trabecular Bone

Osteoarthritis and osteoporosis are widely prevalent and have far-reaching public health implications. There is increasing evidence that epigenetics, in particular, histone 3 lysine 79 methyltransferase DOT1L, plays an important role in the cartilage and bone biology. In this study, we evaluated the role of Dot1l in the articular cartilage, growth plate, and trabecular bone utilizing conditional KO mouse models. We generated chondrocyte-specific constitutive and inducible conditional Dot1l KO mouse lines using Col2a1-Cre and Acan-CreER systems. Prenatal deletion of Dot1l in mouse chondrocytes led to perinatal mortality, accelerated ossification, and dysregulation of Col10a1 expression. Postnatal deletion of Dot1l in mouse chondrocytes resulted in trabecular bone loss decreased extracellular matrix production, and disruption of the growth plate. In addition, pharmacological inhibition of DOT1L in a progeria mouse model partially rescued the abnormal osseous phenotype. In conclusion, Dot1l is important in maintaining the growth plate, extracellular matrix production, and trabecular bone. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Epigenetics as a New Frontier in Orthopedic Regenerative Medicine and Oncology

Skeletal regenerative medicine aims to repair or regenerate skeletal tissues using pharmacotherapies, cell-based treatments, and/or surgical interventions. The field is guided by biological principles active during development, wound healing, aging, and carcinogenesis. Skeletal development and tissue maintenance in adults represent highly intricate biological processes that require continuous adjustments in the expression of cell type-specific genes that generate, remodel, and repair the skeletal extracellular matrix. Errors in these processes can facilitate musculoskeletal disease including cancers or injury. The fundamental molecular mechanisms by which cell type-specific patterns in gene expression are established and retained during successive mitotic divisions require epigenetic control, which we review here. We focus on epigenetic regulatory proteins that control the mammalian epigenome at the level of chromatin with emphasis on proteins that are amenable to drug intervention to mitigate skeletal tissue degeneration (e.g., osteoarthritis and osteoporosis). We highlight recent findings on a number of druggable epigenetic regulators, including DNA methyltransferases (e.g., DNMT1, DNMT3A, and DNMT3B) and hydroxylases (e.g., TET1, TET2, and TET3), histone methyltransferases (e.g., EZH1, EZH2, and DOT1L) as well as histone deacetylases (e.g., HDAC3, HDAC4, and HDAC7) and histone acetyl readers (e.g., BRD4) in relation to the development of bone or cartilage regenerative drug therapies. We also review how histone mutations lead to epigenomic catastrophe and cause musculoskeletal tumors. The combined body of molecular and genetic studies focusing on epigenetic regulators indicates that these proteins are critical for normal skeletogenesis and viable candidate drug targets for short-term local pharmacological strategies to mitigate musculoskeletal tissue degeneration. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1465-1474, 2019.

Functional testing of thousands of osteoarthritis-associated variants for regulatory activity

To date, genome-wide association studies have implicated at least 35 loci in osteoarthritis but, due to linkage disequilibrium, the specific variants underlying these associations and the mechanisms by which they contribute to disease risk have yet to be pinpointed. Here, we functionally test 1,605 single nucleotide variants associated with osteoarthritis for regulatory activity using a massively parallel reporter assay. We identify six single nucleotide polymorphisms (SNPs) with differential regulatory activity between the major and minor alleles. We show that the most significant SNP, rs4730222, exhibits differential nuclear protein binding in electrophoretic mobility shift assays and drives increased expression of an alternative isoform of HBP1 in a heterozygote chondrosarcoma cell line, in a CRISPR-edited osteosarcoma cell line, and in chondrocytes derived from osteoarthritis patients. This study provides a framework for prioritization of GWAS variants and highlights a role of HBP1 and Wnt signaling in osteoarthritis pathogenesis.

Osteoarthritis year in review 2018: biology

This Year in Review highlights a selection of articles published between the 2017 and 2018 Osteoarthritis Research Society International (OARSI) World Congress meetings within the field of osteoarthritis biology, presented at OARSI 2018. Selected articles were obtained from a PubMed search covering cartilage, subchondral bone, inflammation, ageing, pain and animal models. Studies focused on biomechanics, biomarkers, genetics and epigenetics, imaging and clinical studies were excluded due to their coverage in other articles within the OARSI Year in Review series. Significant themes including the role of progenitor cells in cartilage homeostasis and repair, novel signalling mechanisms controlling chondrocyte phenotypic stability and the influence of disrupted or senescent chondrocytes were identified and are discussed in this review. Overarching conclusions derived from these study areas indicate that promising avenues of intervention are on the horizon, however further understanding is required in order to target therapeutic treatments to suitable patient subgroups and disease stages.

Role of Epigenomics in Bone and Cartilage Disease

Phenotypic variation in skeletal traits and diseases is the product of genetic and environmental factors. Epigenetic mechanisms include information-containing factors, other than DNA sequence, that cause stable changes in gene expression and are maintained during cell divisions. They represent a link between environmental influences, genome features, and the resulting phenotype. The main epigenetic factors are DNA methylation, posttranslational changes of histones, and higher-order chromatin structure. Sometimes non-coding RNAs, such as microRNAs (miRNAs) and long non-coding RNAs (lncRNAs), are also included in the broad term of epigenetic factors. There is rapidly expanding experimental evidence for a role of epigenetic factors in the differentiation of bone cells and the pathogenesis of skeletal disorders, such as osteoporosis and osteoarthritis. However, different from genetic factors, epigenetic signatures are cell- and tissue-specific and can change with time. Thus, elucidating their role has particular difficulties, especially in human studies. Nevertheless, epigenomewide association studies are beginning to disclose some disease-specific patterns that help to understand skeletal cell biology and may lead to development of new epigenetic-based biomarkers, as well as new drug targets useful for treating diffuse and localized disorders. Here we provide an overview and update of recent advances on the role of epigenomics in bone and cartilage diseases. © 2019 American Society for Bone and Mineral Research.

Involvement of epigenetics in osteoarthritis

Osteoarthritis (OA) is the most prevalent chronic age-related arthritic disease that mainly affects the diarthrodial joints. Nevertheless, there is no treatment currently available that can effectively reduce symptoms or slow down or stop disease progression. The lack of disease-modifying therapies could be explained by the complex pathogenesis of OA, which is still not completely understood. Intertwined epigenetic mechanisms such as DNA methylation, histone modifications, and noncoding RNAs (ncRNAs) have been indicated as important cellular tools to maintain tissue homeostasis upon environmental challenges. The current review illustrates that dysfunctional epigenetic control mechanisms in the articular cartilage likely play an important role in driving OA pathophysiology.

Pathways to understanding the genomic aetiology of osteoarthritis

Osteoarthritis is a common, complex disease with no curative therapy. In this review, we summarize current knowledge on disease aetiopathogenesis and outline genetics and genomics approaches that are helping catalyse a much-needed improved understanding of the biological underpinning of disease development and progression.

Increased DOT1L in synovial biopsies of patients with OA and RA

The studies aimed to determine the changes of histone methylation in synovial tissues of patients with osteoarthritis (OA) and rheumatoid arthritis (RA). Synovial tissues were obtained from 30 patients including 12 OA, 16 RA, and 2 trauma that were used as control. A histone methyltransferase DOT1L of the tissues was examined for transcript level with quantitative RT-PCR and protein expression with western blot. Methylation status of DOT1L substrate, H3K79, was examined with immunohistochemistry and western blot. Two-tailed non-pair T test and chi-square test were applied for age/disease duration and gender distribution, respectively. Kruskal-Wallis test and Post hoc Dunn's test were used for examine the difference between control, OA and RA. Both transcript and protein levels of DOT1L appeared the highest in synovial tissues of RA patients and increased in that of OA patients compared to the controls with ratios of 13.8/4.7/1 and 15.5/11.2/1.0 for RA/OA/control, respectively. The changes between RA and control, and RA and OA patients were statistically significant. Both immunohistochemistry study and western blot showed an increased methylation of H3K79 in synovial tissues of OA and RA patients. Gene and protein expression of DOT1L was increased in synovial tissues of both OA and RA patients. A high level of di-methylated H3K79 was also observed in the patients. Considering the important functions of DOT1L and H3K79 contributing to the initiation and maintenance of active transcription in the genome, these unprecedented findings, although still unclear how to impact diseases, may provide novel insights to further explore pathological mechanism of OA and RA.

Genetic Determinants of Radiographic Knee Osteoarthritis in African Americans

OBJECTIVE

The etiology of knee osteoarthritis (OA), the most common form of arthritis, is complex and may differ by race or ethnicity. In recent years, genetic studies have identified many genetic variants associated with OA, but nearly all the studies were conducted in European whites and Asian Americans. Few studies have focused on the genetics of knee OA in African Americans.

METHODS

We performed a genome-wide association study of radiographic knee OA in 1217 African Americans from 2 North American cohort studies: 590 subjects from the Johnston County Osteoarthritis Project and 627 subjects from the Osteoarthritis Initiative. Analyses were conducted in each cohort separately and combined in an inverse variance fixed effects metaanalysis, which were then included in pathway analyses. We additionally tested 12 single-nucleotide polymorphisms robustly associated with OA in European white populations for association in African Americans.

RESULTS

We identified a genome-wide significant variant in LINC01006 (minor allele frequency 12%; p = 4.11 × 10-9) that is less common in European white populations (minor allele frequency < 3%). Five other independent loci reached suggestive significance (p < 1 × 10-6). In pathway analyses, dorsal/ventral neural tube patterning and iron ion transport pathways were significantly associated with knee OA in African Americans (false discovery rate < 0.05). We found no evidence that previously reported OA susceptibility variants in European whites were associated with knee OA in African Americans.

CONCLUSION

These results highlight differences in the genetic architecture of knee OA between African American and European whites. This finding underscores the need to include more diverse populations in OA genetics studies.

Genome-Wide Association Study of Radiographic Knee Osteoarthritis in North American Caucasians

OBJECTIVE

A major barrier to genetic studies of osteoarthritis (OA) is the need to obtain large numbers of individuals with standardized radiographic evaluations for OA. To address this gap, we performed a genome-wide association study (GWAS) of radiographically defined tibiofemoral knee OA in 3,898 cases and 3,168 controls from 4 well-characterized North American cohorts, and we performed replication analysis of previously reported OA loci.

METHODS

We performed meta-analysis using a 2-stage design. Stage 1 (discovery) consisted of a GWAS meta-analysis of radiographic knee OA carried out in the Osteoarthritis Initiative and the Johnston County Osteoarthritis Project. Knee OA was defined as definite osteophytes and possible joint space narrowing or total joint replacement in one or both knees. Stage 2 (validation) was performed in the Multicenter Osteoarthritis Study and the Genetics of Osteoarthritis study. We genotyped lead meta-analysis variants (P ≤ 1 × 10^-4 ) from stage 1 and tested the association between these variants and knee OA. We then combined results from all cohorts in a meta-analysis.

RESULTS

Lead variants from stage 1, representing 49 unique loci, were analyzed in stage 2; none met genome-wide significance in the combined analysis of stage 1 and stage 2. We validated 1 locus (rs4867568 near LSP1P3) with nominal significance (P < 0.05), which was also our top finding in the combined meta-analysis (odds ratio [OR] 0.84 [95% confidence interval (95% CI) 0.79-0.91], P = 3.02 × 10^-6 ). We observed nominally significant associations (P < 0.05) with 3 previously reported OA loci: rs143383 in GDF5 (OR 1.12 [95% CI 1.04-1.21], P = 2.13 × 10^-3 ), rs835487 in CHST11 (OR 0.93 [95% CI 0.85-0.99], P = 0.03), and rs8044769 in FTO (OR 1.10 [95% CI 1.03-1.19], P = 6.13 × 10^-3 ).

CONCLUSION

These findings provide suggestive evidence of a novel knee OA locus and confirm previously reported associations in GDF5, CHST11, and FTO.

Radiographic endophenotyping in hip osteoarthritis improves the precision of genetic association analysis

OBJECTIVE

Osteoarthritis (OA) has a strong genetic component but the success of previous genome-wide association studies (GWAS) has been restricted due to insufficient sample sizes and phenotype heterogeneity. Our aim was to examine the effect of clinically relevant endophenotyping according to site of maximal joint space narrowing (maxJSN) and bone remodelling response on GWAS signal detection in hip OA.

METHODS

A stratified GWAS meta-analysis was conducted in 2118 radiographically defined hip OA cases and 6500 population-based controls. Signals were followed up by analysing differential expression of proximal genes for bone remodelling endophenotypes in 33 pairs of macroscopically intact and OA-affected cartilage.

RESULTS

We report suggestive evidence (p<5×10-6) of association at 6 variants with OA endophenotypes that would have been missed by using presence of hip OA as the disease end point. For example, in the analysis of hip OA cases with superior maxJSN versus cases with non-superior maxJSN we detected association with a variant in the LRCH1 gene (rs754106, p=1.49×10-7, OR (95% CIs) 0.70 (0.61 to 0.80)). In the comparison of hypertrophic with non-hypertrophic OA the most significant variant was located between STT3B and GADL1 (rs6766414, p=3.13×10-6, OR (95% CIs) 1.45 (1.24 to 1.69)). Both of these associations were fully attenuated in non-stratified analyses of all hip OA cases versus population controls (p>0.05). STT3B was significantly upregulated in OA-affected versus intact cartilage, particularly in the analysis of hypertrophic and normotrophic compared with atrophic bone remodelling pattern (p=4.2×10-4).

CONCLUSIONS

Our findings demonstrate that stratification of OA cases into more homogeneous endophenotypes can identify genes of potential functional importance otherwise obscured by disease heterogeneity.

DOT1L safeguards cartilage homeostasis and protects against osteoarthritis

Osteoarthritis is the most prevalent and crippling joint disease, and lacks curative treatment, as the underlying molecular basis is unclear. Here, we show that DOT1L, an enzyme involved in histone methylation, is a master protector of cartilage health. Loss of DOT1L disrupts the molecular signature of healthy chondrocytes in vitro and causes osteoarthritis in mice. Mechanistically, the protective function of DOT1L is attributable to inhibition of Wnt signalling, a pathway that when hyper-activated can lead to joint disease. Unexpectedly, DOT1L suppresses Wnt signalling by inhibiting the activity of sirtuin-1 (SIRT1), an important regulator of gene transcription. Inhibition of SIRT1 protects against osteoarthritis triggered by loss of DOT1L activity. Modulating the DOT1L network might therefore be a therapeutic approach to protect the cartilage against osteoarthritis.

Novel Genetic Variants for Cartilage Thickness and Hip Osteoarthritis

Osteoarthritis is one of the most frequent and disabling diseases of the elderly. Only few genetic variants have been identified for osteoarthritis, which is partly due to large phenotype heterogeneity. To reduce heterogeneity, we here examined cartilage thickness, one of the structural components of joint health. We conducted a genome-wide association study of minimal joint space width (mJSW), a proxy for cartilage thickness, in a discovery set of 13,013 participants from five different cohorts and replication in 8,227 individuals from seven independent cohorts. We identified five genome-wide significant (GWS, P≤5·0×10-8) SNPs annotated to four distinct loci. In addition, we found two additional loci that were significantly replicated, but results of combined meta-analysis fell just below the genome wide significance threshold. The four novel associated genetic loci were located in/near TGFA (rs2862851), PIK3R1 (rs10471753), SLBP/FGFR3 (rs2236995), and TREH/DDX6 (rs496547), while the other two (DOT1L and SUPT3H/RUNX2) were previously identified. A systematic prioritization for underlying causal genes was performed using diverse lines of evidence. Exome sequencing data (n = 2,050 individuals) indicated that there were no rare exonic variants that could explain the identified associations. In addition, TGFA, FGFR3 and PIK3R1 were differentially expressed in OA cartilage lesions versus non-lesioned cartilage in the same individuals. In conclusion, we identified four novel loci (TGFA, PIK3R1, FGFR3 and TREH) and confirmed two loci known to be associated with cartilage thickness.The identified associations were not caused by rare exonic variants. This is the first report linking TGFA to human OA, which may serve as a new target for future therapies.

Analysis of genetics and DNA methylation in osteoarthritis: What have we learnt about the disease?

Osteoarthritis (OA) is a chronic musculoskeletal disease characterised by the destruction of articular cartilage, synovial inflammation and bone remodelling. Disease aetiology is complex and highly heritable, with genetic variation estimated to contribute to 50% of OA occurrence. Epigenetic alterations, including DNA methylation changes, have also been implicated in OA pathophysiology. This review examines what genetic and DNA methylation studies have taught us about the genes and pathways involved in OA pathology. The influence of DNA methylation on the molecular mechanisms underlying OA genetic risk and the consequence of this interaction on disease susceptibility and penetrance are also discussed.

Translating genomics into mechanisms of disease: Osteoarthritis

Osteoarthritis (OA) is the most common age-related arthritic disorder and is characterized by aberrant extracellular matrix (ECM) content and surface disruptions that range from fibrillation, clefting and delamination, leading to articular surface erosion. Worldwide, over 20% of the population is affected with OA and 80% of these patients have limitations in movement, whereas 25% experience inhibition in major daily activities of life. OA is the most common disabling arthritic disease; nevertheless, no disease-modifying treatment is available except for the expensive total joint replacement surgery at end-stage disease. Lack of insight into the underlying pathophysiological mechanisms of OA has considerably contributed to the inability of the scientific community to develop disease-modifying drugs. To overcome this critical barrier, focus should be on translation of identified robust gene deviations towards the underlying biological mechanisms.

The effect of forced exercise on knee joints in Dio2(-/-) mice: type II iodothyronine deiodinase-deficient mice are less prone to develop OA-like cartilage damage upon excessive mechanical stress

OBJECTIVE

To further explore deiodinase iodothyronine type 2 (DIO2) as a therapeutic target in osteoarthritis (OA) by studying the effects of forced mechanical loading on in vivo joint cartilage tissue homeostasis and the modulating effect herein of Dio2 deficiency.

METHODS

Wild-type and C57BL/6-Dio2(-/-) -mice were subjected to a forced running regime for 1 h per day for 3 weeks. Severity of OA was assessed by histological scoring for cartilage damage and synovitis. Genome-wide gene expression was determined in knee cartilage by microarray analysis (Illumina MouseWG-6 v2). STRING-db analyses were applied to determine enrichment for specific pathways and to visualise protein-protein interactions.

RESULTS

In total, 158 probes representing 147 unique genes showed significantly differential expression with a fold-change ≥1.5 upon forced exercise. Among these are genes known for their association with OA (eg, Mef2c, Egfr, Ctgf, Prg4 and Ctnnb1), supporting the use of forced running as an OA model in mice. Dio2-deficient mice showed significantly less cartilage damage and signs of synovitis. Gene expression response upon exercise between wild-type and knockout mice was significantly different for 29 genes.

CONCLUSIONS

Mice subjected to a running regime have significant increased cartilage damage and synovitis scores. Lack of Dio2 protected against cartilage damage in this model and was reflected in a specific gene expression profile, and either mark a favourable effect in the Dio2 knockout (eg, Gnas) or an unfavourable effect in wild-type cartilage homeostasis (eg, Hmbg2 and Calr). These data further support DIO2 activity as a therapeutic target in OA.

The Genetic Epidemiological Landscape of Hip and Knee Osteoarthritis: Where Are We Now and Where Are We Going?

Osteoarthritis (OA) is a complex disease that affects the whole joint, with multiple biological and environmental factors contributing to its development. The heritable component for primary OA accounts for ∼50% of susceptibility. So far, candidate gene studies and genome-wide association scans have established 18 OA-associated loci. These findings account for 11% of the heritability, explaining a rather small fraction of the genetic component. To further unravel the genetic architecture of OA, the field needs to facilitate more precise phenotypic definitions, high genome coverage, and large sample metaanalyses, expecting the identification of rare and low frequency variants with potentially higher penetrance, and more accurate methods for calculating phenotype-genotype correlation. Expression analysis, epigenetics, and investigation of interactions can also help clarify the implicated transcriptional regulatory pathways and provide insights into further novel pathogenic OA mechanisms leading to diagnostic biomarker identification and new, more focused therapeutic disease approaches.

Association study of MMP8 gene in osteoarthritis

OBJECTIVES

Osteoarthritis (OA) is a joint disease common in the elderly. There is a prior functional evidence for different matrix metalloproteinases (MMPs), such as MMP8 and MMP9, having a role in the breakdown of cartilage extracellular matrix in OA. Thus, we analyzed whether the common genetic variants of MMP8 and MMP9 contribute to the risk of OA.

MATERIALS AND METHODS

In total, 13 common tagging single-nucleotide polymorphisms (SNPs) were studied in a discovery knee OA cohort of 185 cases and 895 controls. For validation, two knee OA replication cohorts and two hand OA replication cohorts were studied (altogether 1369 OA cases, 4445 controls in the five cohorts). The χ(2) test for individual study cohorts and Cochran-Mantel-Haenszel test for combined meta-analysis were calculated using Plink.

RESULTS

The rs1940475 SNP in MMP8 showed suggestive association in the discovery cohort (OR = 0.721, 95% CI 0.575-0.906; p = 0.005). Other knee and hand OA replication study cohorts showed similar trend for the predisposing allele without reaching statistical significance in independent replication cohorts nor in their meta-analysis (p > 0.05). Meta-analysis of all five hand and knee OA study cohorts yielded a p-value of 0.027 (OR = 0.904, 95% CI 0.826-0.989).

CONCLUSIONS

Initial analysis of the MMP8 gene showed suggestive association between rs1940475 and knee OA, but the finding did not replicate in other study cohorts, even though the trend for predisposing allele was similar in all five cohorts. MMP-8 is a good biological candidate for OA, but our study did not find common variants with significant association in the gene.

The role of inflammation-related genes in osteoarthritis

In this review article we examine the role of inflammation-related genes in osteoarthritis (OA) from the perspective of genetics, epigenetics and gene expression. There have been great strides in such genomic analyses of OA in recent years thanks to the study of adequately powered patient cohorts, the detailed analysis of candidate genes, and the application of genome-wide approaches. These have led to some unexpected and therefore exciting discoveries, implicating pathways that would not necessarily have been predicted to have a role in this common arthritis. Inflammatory-related genes sit firmly in the candidate camp based on prior observations that the OA disease process can have an inflammatory component. What is clear from the genetic studies published to date is that there is no compelling evidence that DNA variation in inflammatory genes is an OA risk factor. This conclusion may of course change as ever more powerful association studies are conducted. There is, however, compelling evidence that epigenetic effects involving inflammatory genes are a component of OA and that alteration in the expression of these genes is also highly relevant to the disease process. We may in fact be close to demonstrating, at the genomic level, a clear separation of OA patients into those in whom inflammation is a key driver of the disease and those in whom it is not. This has obvious implications for the design of trials of novel OA interventions and may also guide the intelligent re-purposing of anti-inflammatory therapies.

Methylation quantitative trait locus analysis of osteoarthritis links epigenetics with genetic risk

Osteoarthritis (OA) is a common, painful and debilitating disease of articulating joints resulting from the age-associated loss of cartilage. Well-powered genetic studies have identified a number of DNA polymorphisms that are associated with OA susceptibility. Like most complex trait loci, these OA loci are thought to influence disease susceptibility through the regulation of gene expression, so-called expression quantitative loci, or eQTLs. One mechanism through which eQTLs act is epigenetic, by modulating DNA methylation. In such cases, there are quantitative differences in DNA methylation between the two alleles of the causal polymorphism, with the association signal referred to as a methylation quantitative trait locus, or meQTL. In this study, we aimed to investigate whether the OA susceptibility loci identified to date are functioning as meQTLs by integrating genotype data with whole genome methylation data of cartilage DNA. We investigated potential genotype-methylation correlations within a 1.0-1.5 Mb region surrounding each of 16 OA-associated single-nucleotide polymorphisms (SNPs) in 99 cartilage samples and identified four that function as meQTLs. Three of these replicated in an additional cohort of up to 62 OA patients. These observations suggest that OA susceptibility loci regulate the level of DNA methylation in cis and provide a mechanistic explanation as to how these loci impact upon OA susceptibility, further increasing our understanding of the role of genetics and epigenetics in this common disease.

Chondrocyte hypertrophy in skeletal development, growth, and disease

Most of our bones form through the process of endochondral ossification, which is tightly regulated by the activity of the cartilage growth plate. Chondrocyte maturation through the various stages of growth plate physiology ultimately results in hypertrophy. Chondrocyte hypertrophy is an essential contributor to longitudinal bone growth, but recent data suggest that these cells also play fundamental roles in signaling to other skeletal cells, thus coordinating endochondral ossification. On the other hand, ectopic hypertrophy of articular chondrocytes has been implicated in the pathogenesis of osteoarthritis. Thus, a better understanding of the processes that control chondrocyte hypertrophy in the growth plate as well as in articular cartilage is required for improved management of both skeletal growth disorders and osteoarthritis. This review summarizes recent findings on the regulation of hypertrophic chondrocyte differentiation, the cellular mechanisms involved in hypertrophy, and the role of chondrocyte hypertrophy in skeletal physiology and pathophysiology.

Individuals with high bone mass have an increased prevalence of radiographic knee osteoarthritis

We previously reported an association between high bone mass (HBM) and a bone-forming phenotype of radiographic hip osteoarthritis (OA). As knee and hip OA have distinct risk factors, in this study we aimed to determine (i) whether HBM is also associated with knee OA, and (ii) whether the HBM knee OA phenotype demonstrates a similar pattern of radiographic features to that observed at the hip. HBM cases (defined by DXA BMD Z-scores) from the UK-based HBM study were compared with unaffected family controls and general population controls from the Chingford and Hertfordshire cohort studies. A single blinded observer graded AP weight-bearing knee radiographs for features of OA (Kellgren-Lawrence score, osteophytes, joint space narrowing (JSN), sclerosis) using an atlas. Analyses used logistic regression, adjusting a priori for age and gender, and additionally for BMI as a potential mediator of the HBM-OA association, using Stata v12. 609 HBM knees in 311 cases (mean age 60.8years, 74% female) and 1937 control knees in 991 controls (63.4years, 81% female) were analysed. The prevalence of radiographic knee OA, defined as Kellgren-Lawrence grade≥2, was increased in cases (31.5% vs. 20.9%), with age and gender adjusted OR [95% CI] 2.38 [1.81, 3.14], p<0.001. The association between HBM and osteophytosis was stronger than that for JSN, both before and after adjustment for BMI which attenuated the ORs for knee OA and osteophytes in cases vs. controls by approximately 50%. Our findings support a positive association between HBM and knee OA. This association was strongest for osteophytes, suggesting HBM confers a general predisposition to a subtype of OA characterised by increased bone formation.

A meta-analysis of genome-wide association studies identifies novel variants associated with osteoarthritis of the hip

OBJECTIVES

Osteoarthritis (OA) is the most common form of arthritis with a clear genetic component. To identify novel loci associated with hip OA we performed a meta-analysis of genome-wide association studies (GWAS) on European subjects.

METHODS

We performed a two-stage meta-analysis on more than 78,000 participants. In stage 1, we synthesised data from eight GWAS whereas data from 10 centres were used for 'in silico' or 'de novo' replication. Besides the main analysis, a stratified by sex analysis was performed to detect possible sex-specific signals. Meta-analysis was performed using inverse-variance fixed effects models. A random effects approach was also used.

RESULTS

We accumulated 11,277 cases of radiographic and symptomatic hip OA. We prioritised eight single nucleotide polymorphism (SNPs) for follow-up in the discovery stage (4349 OA cases); five from the combined analysis, two male specific and one female specific. One locus, at 20q13, represented by rs6094710 (minor allele frequency (MAF) 4%) near the NCOA3 (nuclear receptor coactivator 3) gene, reached genome-wide significance level with p=7.9×10(-9) and OR=1.28 (95% CI 1.18 to 1.39) in the combined analysis of discovery (p=5.6×10(-8)) and follow-up studies (p=7.3×10(-4)). We showed that this gene is expressed in articular cartilage and its expression was significantly reduced in OA-affected cartilage. Moreover, two loci remained suggestive associated; rs5009270 at 7q31 (MAF 30%, p=9.9×10(-7), OR=1.10) and rs3757837 at 7p13 (MAF 6%, p=2.2×10(-6), OR=1.27 in male specific analysis).

CONCLUSIONS

Novel genetic loci for hip OA were found in this meta-analysis of GWAS.

Osteoarthritis year in review 2014: genetics and genomics

Recent developments in genetics/genomics of osteoarthritis (OA) are discussed to improve our understanding of OA pathophysiology. The discovery of a novel variant near the NCOA3 (nuclear receptor coactivator 3) gene associated with hip OA and the regulation of GDF5 gene by four transcription factors via the OA susceptibility locus rs143383 are among important findings in OA genetics. Several microarray-based gene expression studies were published for different tissues of the joint. In OA synovium elevation of collagens and cross-linking enzymes (COL1A1, COL5A1, PLOD2, LOX and TIMP1) responsive to TGF-β was found as well as differential expression pattern between different areas of the osteoarthritic synovial membrane. In OA peripheral blood the role of apoptotic genes was highlighted, while whole genome expression profiling in OA subchondral bone and cartilage revealed common genes in cartilage and bone to be involved in OA development. In epigenetics, several microRNAs (miRNAs) were found to regulate genes' expression in chondrocytes, among which miR-125, miR-127b miR-21, miR-148a and their use as potential drug targets was highlighted. Future studies must focus on the integration of genetics, genomics and epigenetics for the identification of signaling pathways and regulatory networks responsible for OA development.

Genome-wide association and functional studies identify a role for IGFBP3 in hip osteoarthritis

OBJECTIVES

To identify genetic associations with hip osteoarthritis (HOA), we performed a meta-analysis of genome-wide association studies (GWAS) of HOA.

METHODS

The GWAS meta-analysis included approximately 2.5 million imputed HapMap single nucleotide polymorphisms (SNPs). HOA cases and controls defined radiographically and by total hip replacement were selected from the Osteoporotic Fractures in Men (MrOS) Study and the Study of Osteoporotic Fractures (SOF) (654 cases and 4697 controls, combined). Replication of genome-wide significant SNP associations (p ≤5×10(-8)) was examined in five studies (3243 cases and 6891 controls, combined). Functional studies were performed using in vitro models of chondrogenesis and osteogenesis.

RESULTS

The A allele of rs788748, located 65 kb upstream of the IGFBP3 gene, was associated with lower HOA odds at the genome-wide significance level in the discovery stage (OR 0.71, p=2×10(-8)). The association replicated in five studies (OR 0.92, p=0.020), but the joint analysis of discovery and replication results was not genome-wide significant (p=1×10(-6)). In separate study populations, the rs788748 A allele was also associated with lower circulating IGFBP3 protein levels (p=4×10(-13)), suggesting that this SNP or a variant in linkage disequilibrium could be an IGFBP3 regulatory variant. Results from functional studies were consistent with association results. Chondrocyte hypertrophy, a deleterious event in OA pathogenesis, was largely prevented upon IGFBP3 knockdown in chondrocytes. Furthermore, IGFBP3 overexpression induced cartilage catabolism and osteogenic differentiation.

CONCLUSIONS

Results from GWAS and functional studies provided suggestive links between IGFBP3 and HOA.

Assessment of osteoarthritis candidate genes in a meta-analysis of nine genome-wide association studies

Objective

To assess candidate genes for association with osteoarthritis (OA) and identify promising genetic factors and, secondarily, to assess the candidate gene approach in OA.

Methods

A total of 199 candidate genes for association with OA were identified using Human Genome Epidemiology (HuGE) Navigator. All of their single-nucleotide polymorphisms (SNPs) with an allele frequency of >5% were assessed by fixed-effects meta-analysis of 9 genome-wide association studies (GWAS) that included 5,636 patients with knee OA and 16,972 control subjects and 4,349 patients with hip OA and 17,836 control subjects of European ancestry. An additional 5,921 individuals were genotyped for significantly associated SNPs in the meta-analysis. After correction for the number of independent tests, P values less than 1.58 × 10⁻⁵ were considered significant.

Results

SNPs at only 2 of the 199 candidate genes (COL11A1 and VEGF) were associated with OA in the meta-analysis. Two SNPs in COL11A1 showed association with hip OA in the combined analysis: rs4907986 (P = 1.29 × 10⁻⁵, odds ratio [OR] 1.12, 95% confidence interval [95% CI] 1.06−1.17) and rs1241164 (P = 1.47 × 10⁻⁵, OR 0.82, 95% CI 0.74−0.89). The sex-stratified analysis also showed association of COL11A1 SNP rs4908291 in women (P = 1.29 × 10⁻⁵, OR 0.87, 95% CI 0.82−0.92); this SNP showed linkage disequilibrium with rs4907986. A single SNP of VEGF, rs833058, showed association with hip OA in men (P = 1.35 × 10⁻⁵, OR 0.85, 95% CI 0.79−0.91). After additional samples were genotyped, association at one of the COL11A1 signals was reinforced, whereas association at VEGF was slightly weakened.

Conclusion

Two candidate genes, COL11A1 and VEGF, were significantly associated with OA in this focused meta-analysis. The remaining candidate genes were not associated.

Osteoarthritis year 2013 in review: genetics and genomics

Progress in genetic research has delivered important highlights in the last year. One of the widest impact is the publication of the Encyclopedia of DNA Elements (ENCODE) project showing the impressive complexity of the human genome and providing information useful for all areas of genetics. More specific of osteoarthritis (OA) has been the incorporation of DOT1-like, histone H3 methyltransferase (DOT1L) to the list of 11 OA loci with genome-wide significant association, the demonstration of significant overlap between OA genetics and height or body mass index (BMI) genetics, and the tentative prioritization of HMG-box transcription factor 1 (HBP1) in the 7q22 locus based on functional analysis. In addition, the first large scale analysis of DNA methylation has found modest differences between OA and normal cartilage, but has identified a subgroup of OA patients with a very differentiated phenotype. The role of DNA methylation in regulation of NOS2, SOX9, MMP13 and IL1B has been further clarified. MicroRNA expression studies in turn have shown some replication of differences between OA and control cartilage from previous profiling studies and have identified potential regulators of TGFβ signaling and of IL1β effects. In addition, non-coding RNAs showed promising results as serum biomarkers of cartilage damage. Gene expression microarray studies have found important differences between studies of hip or knee OA that reinforce the idea of joint specificity in OA. Expression differences between articular cartilage and other types of cartilage highlighted the WNT pathway whose regulation is proposed as critical for maintaining the articular cartilage phenotype. Many of these results need confirmation but they signal the exciting progress that is taking place in all areas of OA genetics, indicate questions requiring more study and augur further interesting discoveries.

Advances in osteoarthritis genetics

Osteoarthritis (OA), the most common form of arthritis, is a highly debilitating disease of the joints and can lead to severe pain and disability. There is no cure for OA. Current treatments often fail to alleviate its symptoms leading to an increased demand for joint replacement surgery. Previous epidemiological and genetic research has established that OA is a multifactorial disease with both environmental and genetic components. Over the past 6 years, a candidate gene study and several genome-wide association scans (GWAS) in populations of Asian and European descent have collectively established 15 loci associated with knee or hip OA that have been replicated with genome-wide significance, shedding some light on the aetiogenesis of the disease. All OA associated variants to date are common in frequency and appear to confer moderate to small effect sizes. Some of the associated variants are found within or near genes with clear roles in OA pathogenesis, whereas others point to unsuspected, less characterised pathways. These studies have also provided further evidence in support of the existence of ethnic, sex, and joint specific effects in OA and have highlighted the importance of expanded and more homogeneous phenotype definitions in genetic studies of OA.