Altered expression after expansion of a v-erbA transgene in transgenic mice

Repetitive DNA is known to undergo size variations based on an increase or decrease in the number of monomer units. We describe here the spontaneous expansion of an experimentally introduced tandem array of repeats in a transgenic mouse consisting of the human beta-actin promoter fused to the viral oncogene v-erbA and an SV40 polyadenylation signal (hAP/v-erbA). The expansion of the transgene was identified during routine screening of transgenic offspring for heterozygosity or homozygosity in one founder line. The control heterozygote genome consisted of the hAP/v-erbA transgene organized as a tandem array approximately five monomer units at a single chromosomal locus. The number of units increased to 20-21 copies, and germline transmission of the expanded units was stable for at least two generations. The majority of animals carrying the expanded monomer units retained their RNA expression patterns; however, some of these animals had drastically reduced expression levels. We discuss the possibility that expansion of repeated units may provide a mechanism by which the expression of a deleterious transgene is reduced.