Genetic predisposition for CNO/CRMO has been suspected by the occurrence of familial monogenic disorders including non-infectious osteomyelitis as one of the main clinical features. Patients with Majeed syndrome (caused by homozygous mutations in the
LPIN2 gene) [
27], the deficiency of interleukin-1 receptor antagonist (DIRA; caused by autosomal recessive loss-of-function mutations in
IL1RN) [
28‐
30], and pyogenic arthritis, pyoderma gangrenosum and acne syndrome (PAPA; caused by autosomal dominant loss-of-function mutations in
PSTPIP1) [
31] develop severe aseptic osteomyelitis. Because of the central involvement of increased IL-1 signaling and inflammasome activation in these monogenic disorders involving CNO as a key symptom, inflammasome activation and IL-1 release appeared likely involved in the pathophysiology of “sporadic” CNO. However, screening of
IL1RN did not deliver disease-causing mutations in a cohort of CNO patients [
32].
Associations of CNO/CRMO with other autoinflammatory disorders, such as psoriasis or inflammatory bowel disease, and the occurrence of familial clusters further suggest a common genetic predisposition for sporadic CNO [
1,
7,
33,
34]. Golla et al. performed an association study in CNO patients identifying a potential susceptibility locus on chromosome 18q21.3-22 [
35]. However, findings of this study were not confirmed in subsequent efforts in larger and independent cohorts. Recently, we analyzed the polymorphisms in the proximal promoter region of the immune-regulatory cytokine interleukin 10 (
IL10) in a cohort of CRMO patients. Three
IL10 promoter polymorphisms rs1800896 (c.-1082A>G), rs1800871 (c.-819T>C), and rs1800872 (c.-592A>C) form distinct haplotype blocks (GCC, ACC, and ATA) that influence transcription factor recruitment capacities to regulatory elements within the proximal promoter [
16,
17]. Based on promoter haplotypes, IL-10 expression can be stratified from “high” (GCC), over “medium” (ACC), to “low” (ATA). Previously, recruitment of Sp-1 was shown to depend on “permissive” GCC
IL10 promoter haplotypes, which may contribute to disease outcomes in infectious disorders [
15,
17,
36]. Unexpectedly, we determined an enrichment of
IL10 promoter haplotypes encoding for “high” IL-10 expression (GCC), while we did not find a single CNO patient with homozygous ATA haplotypes in our cohort. Thus,
IL10 polymorphisms can by themselves certainly not explain impaired IL-10 secretion by monocytes from CRMO patients [
15,
17]. Therefore, it appeared likely that additional molecular mechanisms contribute to altered IL-10 activation and secretion in CNO (see above). Indeed, the absence of
IL10 promoter haplotypes encoding for “low” IL-10 expression suggests that individuals with the aforementioned molecular defects contributing to reduced IL-10 expression in CNO/CRMO and ATA haplotypes may develop more severe symptoms and will therefore not be diagnosed with CNO but potentially some other inflammatory condition. While this hypothesis sounds promising, it currently remains to be investigated and proven in larger multi-national cohorts.
Recently Cox et al. identified mutations in
FBLIM1 in two CNO patients from South Asia using whole-exome sequencing [
37]. While the exact function of Filamin Binding LIM Protein 1 (FBLIM1) remains somewhat unclear, it acts as an anti-inflammatory molecule by regulating receptor activator of NF-κB ligand (RANKL) activation [
38]. Interactions between RANK receptors on the surface of osteoclast precursor cells and soluble RANKL result in osteoclast generation and activation, thereby contributing to bone remodeling and inflammatory bone loss [
13,
25,
26]. Expression of
FBLIM1 is regulated by STAT3 [
37]. Since IL-10 induces the activation of STAT3, reduced IL-10 expression may result in impaired STAT3 activation and subsequently altered
FBLIM1 expression. Indeed, both reported patients carry
IL10 promoter haplotypes encoding for “low” IL-10 expression. Therefore, the combination of
IL10 promoter haplotypes encoding for “low” gene expression together with
FBLIM1 variants may result in CNO [
16,
39].