Scientists know cancer develops over a period of years through a series of genetic changes or mutations. Scientists have also long accepted that these genetic mutations occur in a single cell type for any given cancer. But the sequence of events-which "hits" happen first, and which specific cells are involved-has not been clearly documented until now.
Writing in the September 1 issue of Human Molecular Genetics, a research team, led by Dr. Charis Eng, director of the Clinical Cancer Genetics Program at Ohio State, says it has uncovered evidence suggesting that genetic changes leading to breast cancer occur first in the epithelium of breast tissue, and then are followed by corresponding alterations in the surrounding structures, or stroma.
"This is almost paradigm-breaking," says Eng. "In breast cancer, the alterations that lead to malignancy are thought to occur only in the epithelial cells of the breast. We have shown for the first time that genetic mutations can occur with some frequency in the stromal cells, too."
Utilizing laser capture microdissection, (LCM) investigators examined the tissue of 41 invasive breast cancers. In LCM technology, a laser beam only 7.5 microns in width - roughly one-tenth the size of a human hair -- acts as a "knife"- enabling researchers to isolate cancerous cells from normal cells, and to separate epithelial cells from stromal cells.
"Before LCM, the study of cancer genetics entailed grinding up a lump of tumor comprising a mixture of cells and examining the 'mixed bag' of cells for gene alterations," says Eng. "We really couldn't attribute genetic changes to any one cell type. LCM allows us to do that." Next, they extracted the DNA from each set of cells, and measured the loss of heterozygosity (LOH), an indicator of gene alteration, in each sample.
"LOH is a technical indicator of the loss of a tumor suppressor gene, says Eng. "A tumor suppressor gene is like the brakes of a car, which keep the cell from reproducing over and over again. Normally, a cell has a pair of suppressors working for it. LOH simply means at least one - or maybe both sets of brakes -- are broken."
LOH was frequent in both types of cells, ranging from 25 to 69 per cent in the cancerous epithelial cells, and from 17 to 61 per cent in the stromal cells. Eng says the higher frequency of LOH in the epithelial tissue suggests that mutations there might precede changes in the surrounding stroma. "In the field of human cancer genetics, it is believed that markers with the highest frequency of LOH represent those with the earliest genetic alterations, the so-called first "hits" and the one with the lowest frequency of LOH, the latest "hit" in carcinogenesis."
"On top of that, we were able to associate certain changes in the epithelial cells with corresponding changes in the stroma," says Eng. "We used to think that the surrounding structure of the cancer cell was just a silent bystander in the process of carcinogenesis. Now we know better."
The research was funded in part by proceeds from the Jimmy V Golf Classic, an event sponsored by the V Foundation for Cancer Research, dedicated to the memory of former North Carolina State Basketball coach Jim Valvano.
Journal
Human Molecular Genetics