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One of the areas of research in the lab is the study of chromosome aberrations in cancer and determining the effect of these genomic alterations on gene expression. In keeping with this theme, I am particularly interested in the events that cause cells to develop translocations (chromosome rearrangements) and aneuploidy (loss or addition of chromosomes).
Chromosome Structural Aberrations and Cancer
Tumors are characterized by their inability to maintain the integrity of the genome, either through mutation or rearrangement of genomic sequences. Cells have multiple mechanisms whereby they can repair DNA damage or for those cells in which repair is not possible, induce cell death. The non-homologous end-joining DNA double strand break repair pathway is one such mechanism and involves the coordinated effort of many different proteins. Of particular importance is the DNA-PK complex consisting of the proteins DNA-PKcs, Ku70 and Ku80. Mice deficient for either of the genes encoding the Ku proteins are particularly sensitive to the DNA damaging effects of ionizing radiation. Our previous studies demonstrated that cells in the developing embryos of these mice have a propensity to develop DNA breaks, chromosome rearrangements and aneuploidy even in the absence of IR. Most of these cells, however, either undergo cell death or senescence, resulting in the runted growth of these mice relative to their normal siblings. Breeding of these mice with mice deficient for the protein p53, which is known to induce cell death, results in mice that develop tumors in the B cell lineage at a very early age. These tumors are characterized by a specific chromosomal rearrangement involving the immunoglobulin heavy chain gene (IgH) and the tumor promoting gene c-myc. Such an IgH - c-myc rearrangement is seen in Burkitt's Lymphoma in humans and is believed to result in the misregulated growth characteristic of cancer cells. Further characterization of these tumors revealed that the chromosome translocations were occurring through the process of break-induced replication (BIR). Subsequent amplification of the IgH / c-myc fusion was the result of repeated cycles of breakage-fusion-bridge (BFB) and eventual stabilization of the aberrant chromosome end via telomere capture from another chromosome. We are currently studying the role of DNA-PKcs in the process of tumorigenesis.
Chromosome Numerical Aberrations and Cancer
The partitioning of genomic material during cell division is critical for maintaining the genomic content of each daughter cell. Centrosomes are one of the primary cellular structures responsible for this process. As such, I am looking at defects in these structures and the proteins that regulate them in order to determine how they can lead to the development of aneuploidy. One model system we are using is colorectal tumors because they can be classified into two categories based on the type of genomic defects they contain. Diploid colorectal tumors have a normal number (1 - 2) of centrosomes (as determined by localization of centrosome proteins) while aneuploid colorectal tumors have localization of centrosome proteins to more than 2 discrete structures. We and others have postulated that these extra structures are directly responsible for the mis-segregation of chromosomes leading to aneuploidy and eventually tumor formation. This is seen very nicely in cells from mice lacking the p53, BRCA1 or ATM genes where aberrant partitioning of chromosomes is clearly visible. We are therefore using this system to understand the events leading up to aneuploidy and tumor formation.
The Effects of Genomic Alterations on Gene Expression
Following on the heels of earlier studies by Dr. Upender in our lab showing that introduction of an additional copy of a chromosome into cells results in an average increase in the expression levels of genes located on that chromosome, we are performing a similar analysis in tumors and tumor-derived cell lines. Because aneuploid tumors contain multiple chromosomes in a numerically unbalanced state, the effect is less pronounced, however, there remains a high concordance between chromosome copy number and gene expression, particularly for those chromosomes that are characteristically affected in a particular tumor type.