Genetic and histopathologic features of chronic nonbacterial osteomyelitis in the Irish population

Chronic nonbacterial osteomyelitis (CNO) is a rare autoinflammatory disease predominantly affecting the paediatric population. Diagnosis is based on clinical presentation, imaging, laboratory investigations and, in cases of diagnostic uncertainty, histology. CNO is associated with other inflammatory diseases such as pustulosis, psoriasis, arthritis and inflammatory bowel disease. Diagnostic, classificantion and monitoring criteria are yet to be validated and there is phenotypic heterogeneity between and within cohorts. In the Irish cohort, diagnostic delay persists and there is a high rate of multifocal disease and extraosseous manifestations, in particular psoriasis. Remission rates in response to treatment are variable. Complications are common, particularly psychosocial complications. A genetic basis for sporadic CNO is suspected and the genetics of syndromic forms of the disease and murine models are well-established. Genetic associations in sporadic cases have not been consistently replicated outside the original reports, suggesting significant genetic heterogeneity. Whole exome sequencing data was used to perform gene burden testing against a geographically and ethnically matched population from the UK Biobank. This showed no association between CNO in the Irish population and previously described candidate genes for syndromic or sporadic forms of CNO. Gene burden testing showed a statistically significant increased burden of variants in HMCN1 in the Irish CNO cohort suggesting that this gene may play a role in CNO pathogenesis. It demonstrates the possible application of gene burden testing against a closely matched population in the identification of candidate genes for rare diseases. There are no confirmed associations between CNO and HLA alleles or haplotypes. A possible association between HLA-B*27 and CNO remains unclear with studies both supporting and opposing it. Whole exome sequencing data and bioinformatic HLA prediction tools were used to predict human leucocyte antigen (HLA) class and haplotypes in the Irish cohort. PCR based HLA typing was performed in two other cohorts from the UK and Germany. A meta-analysis of these results show HLA-B*27 is associated with CNO in three European populations. HLA-B*37-C*06 and HLA-B*27-C*02 haplotypes are associated with CNO in the Irish cohort. CNO shares HLA haplotype associations with psoriasis, strengthening the known clinical association between these diseases. Histologic evaluation of bone remains an important tool in excluding alternative diagnoses, particularly when access to specialist radiology is limited. However, semiquantitative evaluation of samples does not demonstrate any association between disease duration and the predominant subset of inflammatory cells. HTG Molecular Edge-Seq extraction-free gene expression analysis was used on archival formalin fixed paraffin embedded (FFPE) bone biopsy samples. Genes associated with neutrophil chemotaxis are up-regulated in chronic compared to subacute CNO biopsies and the IL10 signalling network is functionally enriched in CNO. Application of this technique can overcome some of the challenges associated with RNA extraction from archival FFPE bone biopsy samples and may provide additional insight into the pathogenesis of CNO.