Increased binding of anti-dsDNA antibodies to short oligonucleotides modified with topoisomerase I reveals a potential new enzyme function independent from DNA relaxation.

Topoisomerase I (topo I) is a highly conserved enzyme which is known to reduce torsional stress at double-stranded (ds) DNA. Torsional stress induced by supercoiling of dsDNA requires either very long dsDNA existing in genomic DNA or circulation as presented in plasmid DNA. To enable DNA relaxation, topo I induce a transient single-strand break followed by stress-relieving rotation of the released DNA strand. Our group found by serendipity that the topo I inhibitor irinotecan is able to suppress murine systemic lupus erythematosus (SLE), an autoimmune disease which is characterized by the existence of pathogenic anti-dsDNA antibodies (abs). As a possible mechanism we demonstrated in the absence of immunosuppression an increased binding of anti-dsDNA abs to long genomic or circulated plasmid dsDNA modified with topo I. Here we show that this effect requires active site tyrosine of topo I which is known to facilitate DNA relaxation activity. Moreover, topo I enhanced anti-dsDNA abs binding to short linear oligonucleotides down to a size of 42 bp. Since oligonucleotides of such length are devoid of torsional stress and relaxation respectively, our results suggest a new and unknown function for the enzyme topo I.

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