Laboratory Research

DNA Mismatch Repair

DNA is vulnerable to many types of damage caused by external insults such as UV radiation or carcinogenic agents, as well as, by intrinsic cellular processes such as DNA replication or DNA recombination.  Failure to correct these lesions predisposes to cancer development and a number of severe genetic disorders.  To prevent this from happening, cells possess a variety of DNA repair systems that correct these errors or trigger cell death when damage cannot be repaired.  Our laboratory studies the DNA mismatch repair pathway, which corrects errors that have escaped proofreading during DNA replication.  Defects that inactivate mismatch repair genes are the primary cause of hereditary non-polyposis colorectal cancer and Turcot syndrome, a hereditary disorder characterized by primary tumors of the central nervous system.  The MutL protein is central to this system as it orchestrates the sequence of events that recognize, remove and correct mismatched DNA.  Our work is aimed at understanding how MutL coordinates the action of mismatch repair and replication proteins.

Regulation of DNA replication in Escherichia coli

DNA replication is one of the fundamental processes in all living cells. As with other pathways, regulation occurs predominantly at the initiation stage. In E. coli, initiation of DNA replication is a precisely regulated event that occurs only once and at a precise moment during the cell cycle.  Initiation is regulated at two levels. The first must guarantee that initiation starts at a proper time during the cell cycle, while the second ensures that the genome is replicated once and only once per cell cycle. The SeqA protein negatively regulates timing of initiation and prevents premature re-initiation events in a process known as origin sequestration.  We have made significant contributions to the field by unveiling how a key protein (SeqA) organizes newly replicated DNA into higher order nucleoprotein complexes, the ultimate cause of origin sequestration.  Our lab is currently studying the less understood role of SeqA in structural organization of the newly replicated chromosomes.

Regulation of DNA replication in eukaryotes

Initiation of DNA replication in eukaryotes is a far more complicated process and it requires the assembly and licensing of a pre-replicative complex formed by more than 30 proteins at the onset of the synthesis phase. The regulatory mechanisms that govern these processes are an active area of research because mistakes may have consequences as severe as the establishment of cells with uncontrollable proliferation, a hallmark in cancer cells. The Dbf4-dependent kinase Cdc7 (DDK) from S. cerevisiae phosphorylates targets found at licensed origins triggering the onset of DNA replication and plays an integral role during checkpoint responses mediated by Rad53.  Additionally, Dbf4 regulates the Cdc5 kinase that has critical roles in mitotic progression and cytokinesis.  Given their roles in the two main events of the cell cycle, Cdc7 and Cdc5 are two important cell cycle regulators.  Not surprisingly, the human homologues of these kinases are upregulated in cancer cells.  Our lab studies how Dbf4 inter-regulates initiation of DNA replication, checkpoint response upon fork arrest and mitotic progression.