Minnie Geller Professor
My lab is active in three somewhat related research areas: 1) the cellular mechanisms that regulate genome stability, 2) the genetic control of the replication and maintenance of chromosome ends (telomeres), and 3) the mechanisms that control the level of meiotic recombination. Almost all of our studies are done using the yeast Saccharomyces cerevisiae.
Genetic regulation of genome stability
In wild-type cells, the frequency of genomic alterations of any type (point mutations, deletions, insertions, and chromosome rearrangements) is very low. We are interested in the genes that regulate genome stability. One of our studies involved mechanisms that affect the stability of simple repetitive DNA sequences (microsatellites). Alterations in the length of microsatellites are linked to some human diseases, such as Huntington’s disease. Unstable microsatellites are also associated with certain forms of colorectal cancer. We found that microsatellite stability in yeast is not affected by mutations that alter the rate of recombination, but is greatly decreased by mutations that eliminate DNA mismatch repair. These results were one clue that implicated DNA mismatch repair as the causal defect in familial colorectal cancers.
Treatment of mammalian cells with drugs that inhibit DNA polymerase results in breakage of the chromosomes at specific sites, termed “fragile sites”. We have recently shown that yeast cells with low levels of DNA polymerase also have frequent chromosome breaks. We are interested in the mechanisms by which these breaks occur. Our analysis suggests that fragile sites in yeast reflect the processing of secondary DNA structures (“hairpins”) formed during DNA replication.
Genetic control of telomere length
In Saccharomyces cerevisae, the chromosomes terminate in a simple repetitive sequence that has the form poly G1-3T/ poly C1-3A. In wild-type strains, the telomeres are about 400 bp in length. We have identified mutants with substantially shorter telomeres. For example, a strain with the tel1 mutation has telomeres that are only 50 bp in length. We cloned and sequenced TEL1 and showed that is homologous to the human gene ATM, the gene that is defective in patients with the genetic disease ataxia telangiectasia. The TEL1 gene is also homologous to the yeast MEC1 gene which is known to be involved in a DNA damage checkpoint pathway. Although yeast strains with either the tel1 or the mec1 mutation have rates of chromosome loss and deletions that are similar to wild-type strains, the tel1 mec1 double mutant strains have greatly elevated rates of chromosome loss and deletions. We also showed that the double mutant strains have very high rates of telomere-telomere fusions (T-TFs). We are currently investigating the mechanisms involved in producing these fusions. Since these fusions would generate dicentric chromosomes, we hypothesize that the fusions are likely to be the cause of chromosome deletions and chromosome losses. Similar fusions are observed in human cells with the ATM mutation and in aging human cells.
Regulation of meiotic recombination activity
Our analysis has been concentrated on a hotspot for meiotic exchange located near the HIS4 gene. Hotspot activity at this site requires the binding of several transcription factors, but is not directly related to the level of transcription. This binding is required for efficient generation of a double-strand break at HIS4. In collaboration with Joe DeRisi and Pat Brown, we have recently used microarrays to map all of the recombination hotspots and coldspots in the yeast genome. We have shown that genomic regions near the centromere and telomeres of the chromosome have suppressed meiotic recombination. We are currently investigating whether this suppression is related to histone modifications or other features of chromatin structure.