Technique Identifies Initial Events in Tumor Development

Translocations occur when a broken strand of DNA from one chromosome is erroneously joined with that of another chromosome. Sometimes these irregularities can be beneficial in that they enable the immune system to respond to a vast number of microorganisms and viruses. However, translocations can also result in tumors.

The study’s findings were reported in the September 30, 2011, issue of the journal Cell. Translocations can take place during the course of normal cell division, when each chromosome--a single strand of DNA containing many genes--is copied verbatim to provide genetic information for the daughter cells. Occasionally, during this process, byproducts of normal metabolism or other factors can cause breaks in the DNA.

“The cell expresses specific enzymes whose primary purpose is to repair such lesions effectively, but when the enzymes mistakenly join pieces of two different chromosomes, the cell’s genetic information is changed,” said Rafael C. Casellas, PhD, senior investigator in the Genomics and Immunity Section at the NIAMS, who led the research team along with Michel C. Nussenzweig, MD, PhD, from Rockefeller University.

Scientists have known since the 1960s that recurrent translocations play a key role in cancer. What was not clear was how these genetic abnormalities are created, since very few of them were examined, and only within the context of tumors, according to Dr. Casellas. To better understand the nature of these tumor-inducing rearrangements, the authors had to create a system to visualize their appearance in normal, nontransformed cells.

The system the scientists designed involved introducing enzymes that recognize and cause damage at a particular sequence in the DNA into cells from mice, thereby constructing a genome where a unique site is broken continuously. The investigators then utilized a method called polymerase chain reaction (PCR)--which allows scientists to rapidly quickly amplify short sequences of DNA--to inspect all the sites in the genome that would get translocated to this particular break. Using this technique, they were able to explore more than 180,000 chromosomal rearrangements from 400 million white blood cells, called B cells.

Based on this large data set, the scientists were able to make several significant observations about the translocation process. They determined that most of the translocations involve gene domains, instead of the space on the DNA between the genes. They also discovered that most translocations target active genes, with a clear predisposition for the beginning of the gene, as opposed to its middle or end. The team also showed that a specific enzyme that normally creates DNA breaks in B cells dramatically increases the incidence of translocations during the immune response. This characteristic clarifies the long-standing observation that more than 95% of human lymphomas and leukemias are of B cell origin.

“This knowledge is allowing us to understand how tumors are initiated,” concluded Dr. Casellas. “It is the kind of information that in the near future, might help us prevent the development of cancer.”

Related Links:
National Institute of Arthritis and Musculoskeletal and Skin Diseases
National Cancer Institute
National Institutes of Health, and the Rockefeller University