The Molecular Basis of Multiple Vector Insertion by Gene Targeting in Mammalian Cells

Gene targeting using sequence insertion vectors generally results in integration of one copy of the targeting vector generating a tandem duplication of the cognate chromosomal region of homology. However, occasionally the target locus is found to contain >1 copy of the integrated vector. The mechanism by which the latter recombinants arise is not known. In the present study, we investigated the molecular basis by which multiple vectors become integrated at the chromosomal immunoglobulin μ locus in a murine hybridoma. To accomplish this, specially designed insertion vectors were constructed that included six diagnostic restriction enzyme markers in the Cμ region of homology to the target chromosomal μ locus. This enabled contributions by the vector-borne and chromosomal Cμ sequences at the recombinant locus to be ascertained. Targeted recombinants were isolated and analyzed to determine the number of vector copies integrated at the chromosomal immunoglobulin μ locus. Targeted recombinants identified as bearing >1 copy of the integrated vector resulted from a Cμ triplication formed by two vector copies in tandem. Examination of the fate of the Cμ region markers suggested that this class of recombinant was generated predominantly, if not exclusively, by two targeted vector integration events, each involving insertion of a single copy of the vector. Both vector insertion events into the chromosomal μ locus were consistent with the double-strand-break repair mechanism of homologous recombination. We interpret our results, taken together, to mean that a proportion of recipient cells is in a predetermined state that is amenable to targeted but not random vector integration.