Chromosome Instability Mechanisms and Disease
A primary aim of my lab is to elucidate the mechanisms involved in triplet repeat instability and fragility, and determine how these two unusual characteristics are interrelated. To this aim, we are analyzing tract stability and fragility in yeast mutants deficient in processes important for genome stability such as DNA replication, DNA repair (including chromatin structure and modification), and the DNA damage/genome integrity checkpoint.
Figure 1. The drawing illustrates lesions known to occur in several genetic diseases that arise as a result of repeat expansions.
We found that proteins involved in the S-phase DNA damage checkpoint are important for preventing fragility and instability of expanded CAG/CTG tracts, indicating that repair of lesions within the repetitive DNA is especially important during S-phase. We are now further studying the role of this pathway in maintaining repetitive sequences and preventing cell death. We are also using both genetic and physical assays to study the types of proteins that mediate repair of repeat-induced chromosomal damage, including proteins involved in chromatin modification and remodeling. To find other pathways involved in repeat instability, we are using a novel genetic assay to screen for proteins whose expression influences repeat expansion or fragility.
Figure 2. The left panel illustrates the S-phase DNA checkpoint and mechanisms by which chromosome breakage can occur. The right panel illustrates a human X chromosome with an expanded CGG/GCC tract . This illustration is from Jacky, Fragile X Syndrome, 1991
In addition to triplet repeats, several other types of minisatellite sequences act as "common" fragile sites on human chromosomes - sites present in all individuals that are prone to breakage. The causes of chromosome fragility are not well understood, but the results can be significant: a broken chromosome is vulnerable to loss or rearrangement, which can lead to cancer or other genetic diseases.
My lab is interested in the basis for chromosome fragility and its implications for cell growth and survival. We are studying both sequence elements and protein requirements for breakage at common fragile sites, and are testing whether the conditions and proteins that affect triplet repeats also affect these sequences. The results could shed light on the genomic instability that occurs during tumorigenesis.