The resulting mutation made the tumor cells highly unstable genetically, a characteristic long associated with more aggressive cancers. In addition, it rendered the cells resistant to taxanes.

The results of the study, published in the January 18 issue of Nature, have implications for drug development -- and may provide a new marker for diagnosing the potential aggressiveness of tumors.

In 1996, Drs. Robert Benezra and Yong Li of Memorial Sloan-Kettering identified MAD2, a member of a class of proteins referred to as mitotic checkpoint proteins. These proteins ensure the equal distribution of chromosomes to the two daughter cells during cell division.

The loss of this checkpoint results in a form of chromosome instability in which whole chromosomes can be lost or gained. Cancers that exhibit this type of chromosome instability are usually more aggressive -- and have a poor prognosis.

Correlations between chromosome instability and the loss of the mitotic checkpoint have been identified in human colon cancer cell lines. However, there was previously no evidence providing a direct relationship between these two phenomena. Now, researchers in the Benezra laboratory have found that the loss of MAD2 in a genetically stable cancer cell line created chromosome instability.

"When we took a particularly stable human colon carcinoma cell line and genetically engineered the loss of one copy of the MAD2 gene, we were able to visualize the cell's chromosomes falling apart prematurely during cell division by using a simple test," said Loren Michel, MD, the study's lead author.

"Although the loss of one copy of MAD2 caused only subtle decreases in the amount of MAD2 protein levels, it had a great impact on the cell's genetic behavior," Michel said. "The tumors became highly genomically unstable and continued to grow even in the presence of chemotherapy drugs in the taxane family.

"Our results suggest that developing a similar test to detect the changes in this genetic pathway in human cancers could be used to predict disease progression."

Taking their findings one step further, the researchers found that the identical genetic mutation that had such a dramatic effect on a pre-existing tumor cell could also contribute to the initiation of cancerous tumors in mice.

"Mice with complete absence of MAD2 protein die during embryonic development. We introduced a mutation that inactivated just one copy of the MAD2 gene in mice and this resulted in cancer," explained Vasco Liberal, a study author from Memorial Sloan-Kettering. "Uniquely, this mutation resulted in a high frequency of lung carcinomas, despite the fact that these genes are found in every cell of the body and the disease is extremely rare in most mice.

"Why the lung tissue is specifically affected is unknown, but it does show that disruption of this process participates in the development of cancer. Interestingly, in humans, low levels of MAD2 have been observed in breast tumor cell lines."

The researchers also found that small changes in the MAD2 protein level result in a partial loss of the mitotic checkpoint.

"When the cell was missing half a dose, it became resistant to taxane drugs. This was a surprise, since the yeast results suggested the exact opposite," said Robert Benezra, PhD., senior author of the study and head of the Molecular Mechanisms of Differentiation Laboratory at Memorial Sloan-Kettering. "This could have implications as to why a cancer cell suddenly develops drug resistance, and needs further investigation."

Also participating in this study were V.V.V.S. Murty, PhD., and Anupam Chatterjee, PhD., Columbia Presbyterian College of Physicians and Surgeons; Boris Pasche, PhD., Northwestern University Medical School; Max Dobles, PhD., and Peter K. Sorger, PhD., Massachusetts Institute of Technology and Regina Kirchwegger, PhD., of Memorial Sloan-Kettering.

The work was supported by a grant from the National Institutes of Health.

23-Jan-2001