Tandem duplication - A novel type of triplet repeat instability

Triplet repeat sequence (TRS) inserts containing (CTG.CAG)(n) (17-175 units in length) were tandemly duplicated when propagated in plasmids in Escheri chia coli, The products of this novel type of TRS genetic instability are t racts of as many as 34 multiple units, which contain the entire TRS as well as 129 base pairs of nonrepetitive flanking sequence. The duplication proc ess required the presence of two or more TRS-containing units. Close proxim ity (170 base pairs) of the TRS to the R6K gamma origin of replication of t he pUTminiTn5Cm-derived constructs stimulated the tandem duplication proces s. These events are proposed to occur due to secondary structure formation, stalling of DNA synthesis, and slippage-mediated misalignment of the compl ementary strands relative to each other during DNA replication. This mechan ism may account for the TRS-associated duplications in protein kinase and m etalloprotease genes in neuroblastomas and melanomas, as well as the massiv e repeat expansions in type II triplet repeat neurological diseases.