Comparison of filler DNA at immune, nonimmune, and oncogenic rearrangements suggests multiple mechanisms of formation.

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Abstract
Extra nucleotides (termed filler DNA) are commonly found at the junctions of genetic rearrangements in mammalian cells. The filler DNA at immune system rearrangements, which are called N regions, are generated at VDJ joints primarily by terminal deoxynucleotidyl transferase. However, the origin of filler DNA at genetic rearrangements in nonlymphoid cells is uncertain. In an analysis of more than 200 junctions that arose by circularization of transfected linear DNA (D. B. Roth and J. H. Wilson, Mol. Cell. Biol. 6:4295-4304, 1986), we found 18 junctions with extra nucleotides exactly at the point of circularization. Analysis of these 18 junctions indicated that nonlymphoid cells could add extra nucleotides to the ends of duplex DNA. The characteristics of the extra nucleotides at these junctions and at 31 other rearrangement junctions from nonlymphoid cells were quite similar, suggesting that many genetic rearrangements may pass through a stage with free DNA ends. A comparison of the filler DNA at these 49 nonimmune system rearrangements with 97 N regions derived from immune system rearrangements suggested that lymphoid and nonlymphoid cells use different mechanisms for insertion of filler DNA, as expected from the absence of detectable terminal deoxynucleotidyl transferase in cells from nonlymphoid tissues. The filler DNAs at a smaller group of 22 translocations associated with cancer had features in common with both immune and nonimmune system rearrangements and therefore may represent a mixture of these two processes. Mechanisms that might account for the presence of filler DNA in nonlymphoid cells are discussed.