Original articleGenes that contribute to cancer fusion genes are large and evolutionarily conserved
Introduction
The study of cancer fusion genes, arising from chromosomal translocations during cancer cell development, has led to a much more sophisticated understanding of the basis of cancer and to designer drugs specifically targeted to certain cancers. For example, translocations that have fused the MYC gene with the IgH gene have led to the understanding that part of the development of Burkitt's lymphoma is caused by an abnormal and apparently perennial activation of the c-myc gene, which in turn stimulates cell proliferation [1], [2], [3]. In addition, the understanding of the structure of the bcr-abl protein, resulting from the fusion of the BCR and ABL genes [4], led to the discovery and use of Gleevec [5], [6], which efficiently retards the progress of chronic myelocytic leukaemia without the side of effects of less specific, anti-proliferative drugs. Finally, there is the hope of using fusion proteins to generate cancer-specific immune responses [7].
Most, if not all, fusion genes have been discovered as a result of pursuing the specific goal of isolating a fusion gene that is expected to be associated with a chromosomal translocation, with one recent exception representing a more general approach [8]. A case-by-case approach to identifying fusion genes is inefficient, and with more than 50,000 reported disease-associated chromosomal rearrangements in the Mitelman database, a more systematic approach toward identifying fusion genes is needed. Thus, we have selected 59 fusion genes from the published literature and the Atlas of Genetics and Cytogenetics in Oncology and Haematology (http://www.atlasgeneticsoncology.org/index.html) to determine whether these genes have features in common that would distinguish them from neighboring genes. This type of information offers the prospect of being able to predict which genes, within the large regions defined by karyotype analyses of chromosomal translocations, are most likely to be involved in a fusion event. Our results indicate that, with a high degree of probability and statistical significance, the majority of fusion genes are either very large, compared to their neighbors, or have an unusually high degree of evolutionary conservation. The possible connection between these characteristics and the occurrence of the translocation, as well as the potential value of identifying a very large number of DNA segments involved in translocations, are discussed below.
Section snippets
Selection of cancer fusion genes
We selected 36 genes involved in fusion events using the PubMed database for a preliminary study. We then selected an additional 23 genes involved in fusion events from the Atlas of Genetics and Cytogenetics in Oncology and Haematology (http://www.atlasgeneticsoncology.org/index.html). We began the selection of the latter 23 genes with chromosome 22 and worked toward larger chromosomes until 23 genes that did not involve either the immunoglobulin or T-cell receptor loci were selected.
Basic method of data acquisition
To compare
Results
We determined the rank order of size of 59 genes involved in fusion events (Table 1), among all genes present within one million base pairs on either side of each fusion gene. Genes involved in fusion events that were not among the top five in size were labeled as being below 5 (b5) and not ordered further. The size range was selected because karyotype analyses in general has the potential of identifying a region of DNA represented by a chromosomal translocation with a lower limit of about 2
Discussion
The above analyses represent previously identified genes involved in fusion events in cancer. Thus, the analyses do not take into consideration biases that could exist in the process of discovery of these genes. For example, it is possible that larger genes were discovered more frequently because their size made them more accessible to any or all of the technologies used in the process of identifying the fusion gene. Likewise, evolutionarily conserved genes may be more fundamental to cellular
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