Recessive coding and regulatory mutations in FBLIM1 underlie the pathogenesis of chronic recurrent multifocal osteomyelitis (CRMO)

Abstract

Chronic recurrent multifocal osteomyelitis (CRMO) is a rare, pediatric, autoinflammatory disease characterized by bone pain due to sterile osteomyelitis, and is often accompanied by psoriasis or inflammatory bowel disease. There are two syndromic forms of CRMO, Majeed syndrome and DIRA, for which the genetic cause is known. However, for the majority of cases of CRMO, the genetic basis is unknown. Via whole-exome sequencing, we detected a homozygous mutation in the filamin-binding domain of FBLIM1 in an affected child with consanguineous parents. Microarray analysis of bone marrow macrophages from the CRMO murine model (cmo) determined that the Fblim1 ortholog is the most differentially expressed gene, downregulated over 20-fold in the cmo mouse. We sequenced FBLIM1 in 96 CRMO subjects and found a second proband with a novel frameshift mutation in exon 6 and a rare regulatory variant. In SaOS2 cells, overexpressing the regulatory mutation showed the flanking region acts as an enhancer, and the mutation ablates enhancer activity. Our data implicate FBLIM1 in the pathogenesis of sterile bone inflammation and our findings suggest CRMO is a disorder of chronic inflammation and imbalanced bone remodeling.

Chronic Recurrent Multifocal Osteomyelitis and Related Diseases—Update on Pathogenesis

“…Mutations in LPIN2 , Pstpip2 , IL1RN and FBLIM1 have been found in human and murine models of CRMO(24–29). However, mutations in these genes explain a very small minority of patients with CRMO and for most, the genetic factors predisposing to disease remain unclear.…”

“…Recently, we reported on a South Asian child with consanguineous parents who presented with CRMO and psoriasis (29). Whole exome sequencing detected a rare, homozygous missense mutation in the gene FBLIM1 , which codes for FBLP1, a filamin-binding protein that anchors cytoskeletal adhesion proteins at cell-extracellular matrix (ECM) and cell-cell contacts (50, 51).…”

“…In macrophages, FBLIM1 expression is significantly increased in IL-10 treated cells compared to untreated cells, and this upregulation is a function of regulation by STAT3 as part of the anti-inflammatory response (53). We performed a microarray experiment measuring gene expression among bone marrow-derived macrophages from cmo mice, cmo IL1R −/− mice and cmo IL1R −/+ mice, and Fblim1 was the most significantly differentially expressed gene, downregulated 26-fold in the cmo mouse (29). Sequencing FBLIM1 in a large cohort of CRMO patients identified a second child harboring compound heterozygous functional mutations, a novel frameshift mutation and a rare variant in the third intron (29).…”

“…We performed a microarray experiment measuring gene expression among bone marrow-derived macrophages from cmo mice, cmo IL1R −/− mice and cmo IL1R −/+ mice, and Fblim1 was the most significantly differentially expressed gene, downregulated 26-fold in the cmo mouse (29). Sequencing FBLIM1 in a large cohort of CRMO patients identified a second child harboring compound heterozygous functional mutations, a novel frameshift mutation and a rare variant in the third intron (29). The intronic variant (rs41310367) is in an enhancer and a STAT3 binding region, as characterized by ENCODE (54), and disrupts an NR4A2 recognition site (29).…”

“…Sequencing FBLIM1 in a large cohort of CRMO patients identified a second child harboring compound heterozygous functional mutations, a novel frameshift mutation and a rare variant in the third intron (29). The intronic variant (rs41310367) is in an enhancer and a STAT3 binding region, as characterized by ENCODE (54), and disrupts an NR4A2 recognition site (29). Functional validation of rs41310367 in osteoblast-like cells showed that the region surrounding the variant exhibited significant enhancer activity, and that the mutation completely negated the activity (29).…”