Mathew Thayer, PhD
Chromosome structure, DNA replication, chromosome translocations in cancer, genomic instabilityGenetic instability occurs in cancer cells at distinct levels. In most cancers, the instability occurs at the chromosome level, resulting in gains or losses of whole chromosomes or large portions of chromosomes. In contrast to standard molecular genetic approaches, somatic cell genetics offers the ability to ascertain the functional significance of complex genetic lesions, such as gene amplifications, duplications, inversions, or translocations, which occur frequently in tumor cells. By utilizing a combined somatic cell and molecular genetic approach, my laboratory has recently characterized a new type of chromosomal abnormality that occurs with a subset of chromosomal alterations (Smith et al., PNAS 98:13300-05, 2001). We found that four different translocation chromosomes display a delay in mitotic chromosome condensation (DMC) that is associated with a delay in the mitosis-specific phosphorylation of histone H3. Furthermore, this DMC phenotype is preceded by a delay in chromosome replication timing (DRT) that is characterized by a delay in the initiation as well as the completion of DNA synthesis. In addition, chromosomes with this phenotype participate in numerous secondary translocations and rearrangements, indicating that these chromosomes display chromosomal instability. Chromosomes with this phenotype are common in tumor cell lines and primary tumor samples. Furthermore, chromosomes with DMC/DRT can be generated by ionizing radiation. Our findings suggest that certain chromosomal alterations cause a significant delay in replication timing of the entire chromosome that subsequently results in delayed mitotic chromosome condensation and ultimately in chromosomal instability. The primary goal of this research is to determine the genetic basis for the DMC/DRT phenotype. We are considering two possibilities to explain why certain chromosome rearrangements display DMC/DRT. First, because all of the translocations that display DMC/DRT involve deletion and/or rearrangement of the affected chromosomes, it is possible that deletion or mutation of a cis element that normally establishes early replication timing has occurred. Deletion of this element would then result in delayed replication of the entire chromosome. Second, because the chromosomes with DMC/DRT involve translocations or rearrangements in or near their centromeres, it is possible that this type of rearrangement actively interferes with normal replication timing by an unknown mechanism. We are currently using chromosome engineering strategies, combined with somatic cell and molecular genetics, to generate and characterize chromosomes with DMC/DRT.