Chromosomal rearrangements resulting in an interstitial inverted duplication with concomitant terminal deletion were first described for the short arm of chromosome 8 in 1976. Since then, this type of alteration has been identified and characterized for most chromosome arms. Three mechanisms are commonly proposed to explain the origin of this type of rearrangement. All three mechanisms involve formation of a dicentric chromosome that then breaks in a subsequent meiotic division to produce a monocentric duplicated and deleted chromosome. However, the events leading to the formation of the dicentric chromosome differ between the mechanisms. In one mechanism, either parent carries a paracentric inversion. This results in formation of a loop during meiotic pairing with a recombination event occurring in the loop. In the second mechanism, inverted low-copy repeats in the same chromosome arm allow partial folding of one homolog onto itself with a recombination event between the inverted repeats. The third mechanism involves a pre-meiotic double-strand break with subsequent fusion, or U-type exchange, between the sister chromatids. The first two mechanisms require a single copy region to exist between the duplicated and deleted regions on the derivative chromosome, and therefore high-resolution analysis of the rearrangement can be used to distinguish between these mechanisms. Using G-banded chromosome analysis, FISH and array CGH, we describe 17 new cases of inverted duplication with terminal deletion of 2q, 4p, 5p, 6q, 8p, 9p, 10q, 13q, 15q, 18p, 18q, and 22q. These new cases combined with previously described cases demonstrate that U-type exchange is the most frequent mechanism for this rearrangement and can be observed on most, or perhaps all chromosome arms.
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