DNA bridging of yeast chromosomes VIII leads to near-reciprocal translocation and loss of heterozygosity with minor cellular defects.

Loss of heterozygosity (LOH) of tumor suppressor genes in somatic cells is a major process leading to several types of cancer; however, its underlying molecular mechanism is still poorly understood. In the present work, we demonstrate that a linear DNA molecule bridging two homologous chromosomes in diploid yeast cells via homologous recombination produce LOH-generating regions of hemizygosity by deletion. The result is a near-reciprocal translocation mutant that is characterized by slight cell cycle defects and increased expression of the multidrug-resistant gene VMR1. When the distance between target regions is approximately 40 kb, the specificity of gene targeting becomes less stringent and an ensemble of gross chromosomal rearrangements arises. These heterogeneous genomic events, together with the low frequency of specific translocation, confirm that several pathways contribute to the healing of a broken chromosome and suggest that uncontrolled recombination between parental homologs is actively avoided by the cell. Moreover, this work demonstrates that the common laboratory practice of making targeted gene deletions may result in a low, but not negligible, frequency of LOH due to the recombination events triggered between homologous chromosomes in mitosis.