Primary Sjögren's Syndrome: Studies of DNA damage responses and autoantibodies

Primary Sjögren’s syndrome (SS) is a chronic autoimmune disease of unknown etiology. The disease primarily involves lachrymal and salivary glands, leading to dryness of the eyes and mouth, but a wide spectrum of exocrine and non-exocrine disease manifestations may be seen. A characteristic property of primary SS is the production of autoantibodies directed against intracellular, often DNA/RNA-modifying, proteins. The hypothesis initiating the present studies was that these autoantibodies reflect some alteration of the antigen, causing not only increased immunogenicity but possibly also abnormal DNA/RNA processing, for instance during repair of DNA damage. Accordingly, this thesis was conducted to explore whether an abnormal DNA damage response is involved in the pathogenesis of primary SS.

Initially, the mutations formed in a DNA-damaged shuttle vector plasmid repaired and replicated by SS B cell lines were found to include a reduced frequency of multiple point mutations. In a following study of DNA damage-inducible proteins it was observed that protein extracted from peripheral blood lymphocytes exposed to a DNA-damaging agent showed an aberrant DNA-binding pattern as well as autoantigenic reactivity with SS patient serum IgG.

DNA-dependent protein kinase (DNA-PK) is a crucial component of both the DNA repair machinery and the V(D)J recombination process creating immune diversity. No significant difference in DNA-PK activity in T cell lines was found between primary SS patients and healthy individuals. In contrast, primary SS patients positive for SS-A/SS-B autoantibodies displayed both an enhanced capacity to phosphorylate a synthetic p53 peptide and an enhanced G1 cell cycle arrest in response to DNA damage. Furthermore, SS-A/SS-B positive primary SS patients also showed a reduced frequency of t(14;18) chromosomal translocations in blood lymphocytes, a mutation thought to be generated by V(D)J recombinase.

An additional aim was to investigate whether primary SS patients display antibodies against CD4, and if so, to determine whether a correlation exists between these antibodies and CD4+ T lymphocyte depletion. Anti-CD4 antibodies were detected in 12.6% of the patients and in 0.6% of the healthy individuals, however, no correlation was found between these antibodies and the CD4+ T lymphocytopenia seen in some SS patients.

In summary, the present thesis has documented DNA damage response abnormalities in primary SS cells which may be involved in the pathogenesis of this disease.