Genetic insights into ossification of the posterior longitudinal ligament of the spine

Background Ossification of the posterior longitudinal ligament of the spine (OPLL) is an intractable disease, leading to severe neurological deficits. Its etiology and pathogenesis are mostly unknown. The relationship between OPLL and comorbidities, especially type 2 diabetes (T2D) and body mass index (BMI), has been the focus of attention; however, no trait has been proven to have a causal relationship. Methods To clarify the etiology and pathogenesis of OPLL, we conducted a meta-analysis of genome-wide association studies (GAWSs) using 22,016 Japanese individuals. We classified OPLL into cervical, thoracic and the other types, and conducted GWAS sub-analyses. We conducted a gene- based association analysis and a transcriptome-wide Mendelian randomization approach to identify other potential causal genes. To investigate cell groups related to OPLL, we conducted cell type group enrichment analysis. To identify traits with a causal effect on OPLL, we evaluated the genetic correlation with 99 complex traits and then performed mendelian randomization (MR) analyses. Finally, we generated polygenic risk score (PRS) to investigate the genetic impact of the causal trait on OPLL subtypes. Results A GWAS meta-analysis identified 14 significant loci, including eight novel loci. GWAS sub-analyses identified subtype-specific signals. A Gene-based association analysis and a transcriptome-wide Mendelian randomization approach identified five and three potential causal genes, respectively. These loci/genes contained bone metabolism-related genes. Cell type group enrichment analysis observed significant enrichment of the polygenic signals in the active enhancers of the connective/bone cell group, especially H3K27ac in chondrogenic differentiation cells. Genetic correlations showed positive correlation with T2D and BMI and negative correlation with cerebral aneurysm and osteoporosis. MR analyses demonstrated a significant causal effect of increased BMI and high bone mineral density (BMD) on OPLL, but not of T2D, indicating that high BMI confounded the T2D correlation. A PRS for BMI demonstrated that the effect of BMI was particularly strong in thoracic OPLL. Conclusion We identified multiple causative genes involved in bone metabolism that are candidates for future therapeutic targets. By MR analyses, we showed for the first time a causal relationship between the common metabolic conditions (high BMI and BMD) and OPLL. We successfully linked intervenable traits to OPLL. ### Competing Interest Statement Yoshiharu Kawaguchi: Consulting fees from Medacta International, Masashi Yamazaki: A representative of Japanese Organization of the Study for Ossification of Spinal Ligament, Atsushi Okawa: A representative of Japanese Organization of the Study for Ossification of Spinal Ligament, The other authors declare that no competing interests exist. ### Funding Statement This work was supported by a JSPS KAKENHI Grant (no. 19K09566 to T.E.), a grant from Japan Orthopaedics and Traumatology Foundation (no. 452 to Y. Koike), a grant from AO Spine Japan Research (to M. Takahata), a grant from Suhara Memorial Foundation (to M. Takahata), grants from Japan Agency for Medical Research and Development (AMED) (no. JP21kk0305013, JP21tm0424220, and JP21ck0106642 to C.T.), and the JCR Grant for Promoting Basic Rheumatology (to C.T.). ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: This research project was approved by the ethical committees of RIKEN Centers for Integrative Medical Sciences, Hokkaido University, Keio University School of Medicine, Nagoya University, Osaka University, Kanazawa University, the University of Toyama, Hakodate Central General Hospital, Spinal Injuries Center, Tokyo Medical and Dental University, the University of Tokyo, National Okayama Medical Center, Kagoshima University, the University of Fukui, Shiga University of Medical Science, Jichi Medical University, Kyoto University, Yamaguchi University, Niigata University, Hirosaki University, Kochi Medical School, Hamamatsu University School of Medicine, Kyorin University School of Medicine, Kurume University School of Medicine, and National Center for Geriatrics and Gerontology. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable. Yes Full GWAS results will be available after acceptance via the website of the Japanese ENcyclopedia of GEnetic associations by Riken (JENGER, ).