Squaring the Circle

A new TAU technique is helping scientists understand a hitherto mysterious agent - circular DNA molecules that are not part of normal chromosomes. These renegade molecules signal, and perhaps even cause, the onset of cancer.

For most cancer victims, the earlier the cancerous growth is diagnosed, the greater the chances for successful treatment. A new method for identifying circular DNA molecules developed at TAU's Lamdan Laboratory for Cancer Research has shown a link between these molecules and the very earliest stages of oncogenesis - the process by which a normal cell turns into a cancerous cell. Clinical use of this method, the researchers believe, will allow very early detection of the cancer process, provide a tool for studying exposure to carcinogenic materials, and help identify new carcinogens.

Developed by Dr. Sarit Cohen and her PhD supervisor, Prof. Sara Lavi of the Department of Cell Research and Immunology, the patented method is the first in the world to identify and quantify circular DNA molecules, called spcDNA molecules.

Molecular biologists had previously reported the appearance of rounded DNA bits floating in the cell's nucleus, but had no tools to monitor their level or function. Regular DNA is arranged linearly, in strands, which form the 46 chromosomes in the human cell. SpcDNA, by contrast, are small circular molecules found only on the outside of chromosomes.

Locating spcDNA

The technique developed at TAU uses special gels that sift out DNA molecules according to their size and structure. Circular DNA can be separated from linear DNA, then quantified and studied.

Using this tool, Cohen and Lavi, together with master's student Aviv Regev, found spcDNA in cancerous cells and in cancerous tissues. They also found it in the non-cancerous cells of a patient with Fanconi's anemia - an hereditary disease characterized by genetic instability and vulnerability to the cancer-inducing effects of radiation. SpcDNA molecules were not found in healthy human skin cells.

The Fanconi's finding was particularly intriguing, because the presence of spcDNA molecules seemed to be linked to these cells' ultra-sensitivity to the carcinogen, radiation. The researchers thus decided to expose normal skin cells to carcinogenic materials, to see if circular DNA molecules would form. Indeed, two to three days after exposure, spcDNA molecules appeared. Cohen and Lavi believe that the spcDNA molecules are a very early sign of the genetic instability caused by exposure to carcinogens, damage which occurs long before the cancer process sets in irrevocably.

SpcDNA can be added, they say, to the list of genome anomalies that are known to be preludes to cancer: extra copies of chromosomes, lack of chromosomes, fragments of chromosomes not in the right place, and a lack or an excess of sections of a chromosome.

Bystander or perpetrator?

What the TAU team knows with certainty is that extra-chromosomal spcDNA molecules signal genetic instability. But Prof. Lavi believes that there may be a further, more explicitly causal, connection. She hypothesizes that these molecules contribute to the process of oncogenesis by reentering the chromosome and disrupting its orderliness, causing breakages in the DNA, rearrangement of DNA segments, or genetic mutations.

The spcDNA may also suppress the cell's repair mechanism, routinely activated in response to damage. And it seems that even when cell repair has apparently taken place, some of these circular cells may linger, "waiting" to reenter the chromosome and wreak their havoc.

Fig. 1: Healthy chromosomes, color-coded and arranged in pairs.	Fig. 2: Abnormal, genetically unstable chromosomes - made up of DNA bits and pieces.
(Photos courtesy of Dr. Irit Bar-Am of Applied Spectral Imaging, Migdal Ha'emek, Israel.)