Damage occurs to our genes every day, some of it due to chemical or physical agents that have the potential to cause mutations leading to cancer. Luckily, cell proteins detect such damage and repair it before the cell reproduces, preventing a mutation. Scientists have long suspected that defects in this repair process could lead to an elevated risk of cancer. But until fairly recently, specific DNA repair mechanisms were not well understood.
Two biochemists--Paul L. Modrich, phd, Professor of Biochemistry and Investigator at the Howard Hughes Medical Institute at Duke University Medical Center and Richard D. Kolodner, phd, Professor of Biological Chemistry and Molecular Pharmacology at Harvard Medical School and Chief of Human Cancer Genetics at the Dana Farber Cancer Institute, unraveled the mystery of DNA "mismatch repair" and its relationship to hereditary nonpolyposis colon carcinoma (HNPCC). The most common cancer susceptibility syndrome, HNPCC may affect as many as 1 in every 200 Americans, according to some experts. DNA mismatch repair defects could now provide "markers" of risk for HNPCC and the potential for treatments. The pioneering work of Drs. Kolodner and Modrich over the past 14 years has been honored with the 1996 Charles S. Mott Prize for Outstanding Research in Cancer Causation or Prevention, awarded by the General Motors Cancer Research Foundation.
Elucidating the Genetic Repair System
It was in the 1980s that both researchers began their studies of protein enzymes that help repair genetic damage.
Genes can be damaged either by mutagens or errors in replication, resulting in damage, deletion, or mismatching of base pairs (twin segments on the two strands of DNA that encode the function of each gene). Changes in a base alter the function of a gene and can result in the uncontrolled cell growth that forms tumors. Normal genes contain proteins that detect discrepancies in genetic information and correct DNA mismatches. When these enzymes are defective or missing, the mismatched base pairs are copied each time the cell reproduces, resulting in a genetic mutation that can set the stage for disease. Although mutated genes that cause disease are rapidly being discovered, without understanding the defective mechanism within those genes, there's no way to develop preventive or therapeutic strategies.
Dr. Modrich first focused his attention on mismatch repair activities within Escherichia coli. In 1983 he discovered a set of enzymes that repairs mismatches in this bacteria. After developing biochemical assays, Dr. Modrich and colleagues were able to identify 10 proteins responsible for mismatch repair in E coli. Then Dr. Modrich looked at human cells, and in 1990 discovered a similar mismatch repair system, subsequently shown to depend on proteins similar to the bacterial MutS and MutL.
"Whereas normal cells have a mismatch repair mechanism, hypermutable cells, either bacterial or human, did not," remarks Dr. Modrich. "The kind of mutations we identified were associated with a variety of human tumors. We had also found a mismatch repair defect in hypermutable human cell lines resistant to chemical agents similar to those used in cancer."
"We have now purified proteins that can restore mismatch repair function in vitro, which are virtually identical to bacterial MutL or MutS. Three of those proteins encode for the genes which carry defects identified in the HNPCC families," adds Dr. Modrich. "A fourth repair protein is associated with a gene where defects have been frequently found in sporadic cases of colon cancer, not associated with HNPCC families."
The Role of Humble Yeast
While Dr. Modrich was in pursuit of the mismatch repair proteins, Dr. Kolodner was using model organisms like yeast to investigate the repair enzymes and find the genes involved and their relationship to human cancer susceptibility.
Based on the studies with E coli, Dr. Kolodner and colleagues at Dana Farber developed in vitro assays for mismatch repair enzymes in yeast cells. They also found equivalents to MutS, as well as homologs to other bacterial repair enzymes.
Using information based on similarities in sequencing of yeast and bacterial MutS and MutL proteins, his laboratory and collaborators were able to isolate human DNA encoding for corresponding genes. These genes were found to be located in the same region as genes causing inherited forms of colon cancer.
A defect in the mismatch repair mechanism was simultaneously uncovered in tumor cells removed from patients with HNPCC by Dr. Modrich's group. Several other research teams found that the genetic mutation in HNPCC involved defects in a protein similar to bacterial MutS.
In 1993, Dr. Kolodner and collaborators reported that mutations in one mismatch repair gene, MSH2, predisposed people to HNPCC. The following year, he and his collaborators reported that mutations in a second mismatch repair gene, MLH1, also predisposed carriers to colon cancer. Dr. Kolodner's laboratory has since cloned the genomic regions encoding both genes and demonstrated that inherited mutations in these genes cause HNPCC (also known as Lynch II syndrome).
So studies of the humble yeast cell, in part, have led to the promise of genetic tests to identify individuals at risk for hereditary colon cancer, who could be followed with heightened surveillance to catch the cancer in its earlier, curable stages.
Similar defects in repair systems are likely associated with a variety of sporadic cancers. "Lack of these repair enzymes could potentially be markers for risk in other cancers as well," remarked Joseph G. Fortner, md, President of the General Motors Cancer Research Foundation. And, he said, research into repair enzymes could also aid in cancer diagnosis and development of more effective treatments.
Lifetimes in the Lab
Dr. Kolodner received his undergraduate and graduate degrees in biology and biochemistry from the University of California at Irvine. After postdoctoral studies at Harvard Medical School (1975 to 1978), he came to Harvard Medical School and the Dana-Farber Cancer Institute as an Assistant Professor of Biochemical Chemistry in 1978. He became the Chair of the Charles A. Dana Division of Human Cancer Genetics at Dana Farber in 1995.
Dr. Modrich received his bs in biology from Massachusetts Institute of Technology and his phd in biochemistry from Stanford University and pursued postdoctoral studies at Harvard from 1973 to 1974. He joined the Duke University faculty in 1976 and was appointed Investigator at the Howard Hughes Medical Institute there in 1994.