The study will be reported in the Proceedings of the National Academy of Sciences' Online Early Edition (www.pnas.org/papbyrecent.shtml) the week of June 27.
"Previous studies have identified a small set of mutated, or abnormal, genes that are associated with non-small cell lung cancer," says the study's lead author, Giovanni Tonon, MD, PhD, of Dana-Farber. "But we also know that the chromosomes of these cells contain a large number of irregular regions –?where genes have either been deleted or copied over and over again –?which suggests that a large number of cancer genes remain to be discovered. The purpose of this study was to locate the likeliest candidates."
The study is part of a renewed effort by scientists worldwide to uncover the basic biology of lung cancer, the number one cause of cancer-related deaths in the United States. Non-small cell lung cancer (NSCLC) accounts for about 75 percent of all lung cancers and is responsible for nearly 120,000 deaths in this country annually. It is one of the most difficult cancers to treat, with only 15 percent of patients surviving more than five years after diagnosis.
In recent years, technological advances have brought new precision to the search for gene abnormalities associated with cancer. In the current study, Dana-Farber researchers used two forms of microarray technology to bring such abnormalities into focus.
Using tumor samples from 44 NSCLC patients and 34 laboratory-grown lines of NSCLC cells, investigators scanned the cells with high-resolution cDNA (oligonucleotide) microarray equipment to find chromosome regions containing unusual numbers of gene copies. The technology, developed in conjunction with Agilent Technologies, was 50-100 times more powerful than had been used on NSCLC cells in the past, enabling researchers to identify irregular sites more precisely. They found a total of 93 regions, each containing about 11 genes, where gene deletions or over-copying had occurred.
Researchers re-analyzed the tumor and cell samples with the latest oligonucleotide expression microarray technology from Affymetrix, which indicates if individual genes are active. Using this data, they scanned the genes in these 93 regions to see if any were missing (and inactive) or present in unusually large amounts (and therefore highly active) in deleted or overcopied regions, respectively. This enabled them to narrow the search for genes that were the targets of the irregular regions. Intriguingly, all of the genes already known to be involved in NSCLC reside within the abnormal regions identified by the Dana-Farber team.
"This is compelling evidence that we're on the right track," says the study's other first author, Kwok-Kin Wong, MD, PhD, of Dana-Farber. "It's likely that the genetic mutations already linked to NSCLC constitute only a portion of all the genetic errors that drive the disease. Our work provides a good starting point for scientists looking for others."
As part of the study, investigators did microarray analyses on the two major subtypes of NSCLC, adenocarcinoma and squamous cell carcinoma, and found that their genomic profiles overlap in every area but one: squamous cell carcinomas contain an area of gene amplification, or over-copying, not found in adenocarcinomas. Among the few genes in that area is one called p63, which is known to play a role in the ability of skin cells to reproduce. The new finding raises the possi bility that adenocarcinoma and squamous cell carcinoma arise from an error in the same cell type and are driven to malignancy by similar genetic routes, the study authors say.
Finally, the researchers compared their data for NSCLC with similar data for pancreatic cancer, and found that both diseases have some chromosomal irregularities in common, suggesting that in both disorders, some of the same genes may be driving the tumors.