Abstract 966 Suppressed DNA Repair Mechanisms in Rheumatoid Arthritis.
S-H Lee, DK Chang, CR Boland, Z han, GS Firestein. La Jolla, CA.

Reactive oxygen and nitrogen species are produced in inflamed joints and can induce mutations in key genes like p 53. Normally these mutations are prevented by a DNA mismatch repair (MMR) system that maintains sequence fidelity. Key members of the MMR system include MutSα (comprised of hMSH2 and hMSH6), which repairs single base mismatches, and MutsΒ (comprised of hMSH2 and hMSH3), which repairs longer insertion/deletion loops. In this study, the investigators hypothesized that oxidative stress would relax the DNA MMR system and permit microsatellite instability (MSI) and mutagenesis in RA.

Methods: Human synovial fibroblasts were isolated from RA synovial tissue and incubated with H202 or the nitroc oxide donor S-nitroso-N-acetylpenicillamine (SNAP). MMR protein expression and mRNA levels were monitored by Western blot and RT-PCR, respectively. MSI was examined by PCR on genomic DNA of matched synovial cells and peripheral blood cells.

Results: Constitutive expression of hMSH2, 3 and 6 was observed in RA and OA synovial fibroblasts. In fibroblasts exposed to SNAP, hMSH6 decreased by 46%+12% within 6 hrs (p<0.05). H202 also induced suppression of hMSH6. Clear evidence of MSI was observed in 3 of 5 microsatelitte loci examined.

Conclusion: Oxidative stress reduces hMSH6. Decreased hMSH6 interferes with repair of single base mutations, which are the type observed in RA. Longer deletions/insertions are regulated by hMSH2 and 3, which are not suppressed. The presence of MSI in RA synovial cells provides in vivo confirmation of relaxed MMR. Enhancing repair mechanisms in RA might prevent somatic mutations and MSI.

Editorial Comment: This effect on hMSH6 may explain, at least in part, the observed single base mutations observed in RA synovium, and may allow for a phenotypic change in synovium to a more aggressive, invasive state.

Abstract 1075 Expression of Nuclear Factor kB ligand in Rheumatoid Bone Erosion.
Y Kuga, K Taniguchi, s Uchida, H Oda, K Nakamura.

The mechanisms of bone erosion and cartilage destruction are significant issues in the design of effective therapies for rheumatoid arthritis and other types of inflammatory arthritides. Although clearly there is considerable overlap, the pathophysiologic pathways mediating the signs and symptoms of inflammation may differ from those mediating structural damage. There is considerable information that osteoclasts play an important role in focal bony erosion.

Tumor necrosis factor related activation induced cytokine also known as receptor activator of NF-kB ligand (TRANCE/RANKL) is an important factor in osteoclastic differentiation. In this study, metatarsal head from patients with RA and femoral heads of patients with OA were obtained from arthroplasty and immunohistochemistry for RANKL was performed. In RA, RANKL positive fibroblast-like synoviocytes were found adjacent to the bone within the synovial pannus. In addition, osteoblasts within the bone marrow cavity were also positive for RANKL. Similar cells were also seen within the synovial tissues along the surface of the femoral neck in OA.

Editorial Comment: Although not a surprising finding, localization of RANKL positive cells within the synovial pannus at the site of rheumatoid bone resorption is an important confirmation the role of osteoclasts in rheumatoid bony erosion.

Abstract 1638 TRANCE/RANKL Knockout Mice are Protected From Bone Erosion in the k/BxN Serum Transfer Model of Arthritis.
AR Pettit, H Ji, D von Strechow, R Mueller, Y Choi, SR Goldring, C Benoist, E Gravallese.

Osteoclasts (OC) are thought to be important mediators of the bony erosion seen in rheumatoid arthritis and TRANCE/RANKL is an essential factor for OC differentiation and augments T cell-dendritic cell interactions. The k/BxN trangenic mouse spontaneous develops a chronic synovitis with pannus formation similar to human rheumatoid arthritis. The arthritis is due to the induction of autoantibodies against glucose-6-isomerase and the arthritis can be induced in allogeneic and syngeneic mouse strains by passive transfer of serum.

To examine the role of OC in inflammatory arthritis, arthritis was generated in a TRANCE/RANKL knockout (KO) mouse by serum transfer from a k/BxN mouse strain. By comparison with control arthritic littermates, TRANCE/RANKL KO mice had similar levels of joint inflammation assessed clinically and histologically. The TRANCE/RANKL mice had complete absence of OC and dramatically reduced bone erosion. However cartilage damage occurred in both the KO mice and controls, demostrating the role of OC in bone erosion but not in cartilage degradation.

Editorial Comment: These mice are powerful models that confirm the importance of OC on bone erosion in inflammatory arthritis. This further delineates the pathophysiologic mechanisms of inflammation and joint destruction. Like the previous abstracts, this study strengthens support for the clinical utility of an inhibitor of OC activity in inflammatory arthritis.

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