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T. Prolla |
DNA mismatch repair (DMR) pathways play an important role in several
aspects of DNA metabolism, including mutation avoidance, genetic
recombination, and transcription coupled repair. The recent identification
of mutations in the DMR genes MLH1, PMS2 and MSH2 in hereditary colon cancer has given strong support to the notion
that genetic instability plays an important role in cancer development.
In my previous work using S. cerevisiae as a model system, I have identified and characterized the MLHZ DMR gene (13). In a collaboration with Dr. Tom Petes at UNC, we
first demonstrated that mutations in DNA mismatch repair genes
result in microsatellite instability (15), a finding that guided
several groups in the identification of genes mutated in hereditary
colon cancer (3,5,8,10). More recently I have studied physical
interactions between DMR proteins and proposed a model for the
initiation of eucaryotic DNA mismatch repair (13). As a postdoctoral
fellow in the laboratory of Dr. Allan Bradley at Baylor I have
used gene targeting in the mouse to generate mouse models of human
disease and to study basic aspects of DNA repair. My general research
interests m the area of DNA repair include:
The genetic characterization of yeast DMR genes, and the rapid
translation of this knowledge into the field of human genetics,
has been cited as strong evidence in favor of the use of multiple
model systems in the elucidation of complex eucaryotic systems.
I plan to use molecular biology techniques in both S. cerevisiae and the mouse in order to continue characterizing the eucaryotic
DMR system, and other poorly characterized mammalian repair pathways.
Very little is known regarding functional domains of DMR proteins.
Recently we have begun analysis of functional domains of the MLH1
and PMS2 proteins in S. cerevisiae through site specific mutagenesis and yeast two-hybrid assays
(11). This study is a first step in identifying specific motifs
in eucaryotic DMR proteins, and may lead to a better understanding
of how DMR is coordinated, and perhaps linked to DNA replication.
Future interests in this area also include the generation of altered
DMR proteins that may be able to form stable complexes suitable
to structural analysis.
In Dr. Allan Bradley's laboratory at Baylor, I obtained training
in the use embryonic stem (ES) cell technology in the generation
of mice carrying targeted mutations in several genes. My work
with MLH1-1- mice has confirmed the role of MLH1 as a tumor suppressor gene and has helped to characterize its
role in genetic recombination (1). We are currently performing
long-term tumor studies with these mice. Hopefully, MLH1 deficient mice will prove to be a good model for human hereditary
colon cancer. Interestingly, my preliminary work in the analysis
of mice deficient for the mouse PMS1 (10) DNA mismatch repair homolog suggests that this gene is the
component of a novel DMR pathway. Recently, I have also characterized
mice deficient for both DMR and p53. These animals develop tumors
at an extremely rapid rate, and may be valuable in cancer research.
If possible1 I would like to use DMR deficient mice, and mice
deficient for both DMR and p53, in chemoprevention studies. Additionally,
since lack of DMR function is common in sporadic tumors, DMR deficient
mice may be extremely helpful in the identification of therapeutic
agents that target DMR deficient tumors.
The usefulness of DMR deficient mice in cancer research is partially
compromised due to early lymphoma development in these animals.
As a result of our ongoing study on the functional characterization
of DMR proteins in yeast I have identified specific mutations
in the MLH1 gene that result in a dominant loss of DMR activity in wild-type
cells (9). I plan to model these mutations in the mouse in order
to generate transgenic animals with tissue specific deficiency
in DNA mismatch repair. Alternatively, Cre-loxp mediated recombination
may be used to delete DMR genes in a tissue specific fashion (6).
These animals would be valuable in the study of the long-term
effects of a very high mutational load in important processes
such as aging and tumorigenesis
Despite our growing understanding of structural proteins involved
in DNA repair, proteins that signal and regulate the DNA repair
response are poorly understood. One family of proteins that may
activate a phosphorylation cascade that signals the presence of
DNA damage and/or
cytoskeletal abnormalities is Casein kinase-I (CKI), an ubiquitous
serine/threonine-specific kinase activity that was first described
over 15 years ago. Many proteins are in vitro substrates of CKI, including cytoskeletal proteins, enzymes involved
in DNA metabolism and some metabolic enzymes. However, the in vivo function of CKI is unknown. Casein kinase I-a associates with
the mitotic spindle (2), and may be a mitotic checkpoint protein
similar in function to the recently characterized MAD proteins
(4) Our current understanding of the role of nuclear isoforms
of CKI in DNA repair comes from studies in S. cerevisiae and S. pombe. In yeast, deletion of the CKI homologue HRR25 (HO and radiation repair deficient) results in a severe deficiency in double-strand
break repair, sterility and chromosomal instability (7). We and
others have identified the mammalian HRR25 homologs CKI-alpha, delta and epsilon (14). The discovery of a CKI activity that phoshorylates p53
in cell extracts suggests that a CKI gene family member is a regulator
of p53 activity (9).
Since CKI isoforms may be at least partially redundant in their biological activities, I have decided to study the role of this gene family through gene targeting in mice in a collaboration with Dr. Merl Hoekstra (ICOS Corporation), who first characterized the role of CKI in DNA repair. I have generated mice deficient for CKI-£, and we are currently generating mice deficient for other members of the CKI family. We plan to characterize these mice at the cellular level regarding several aspects of DNA metabolism, including sensitivity to ionizing radiation, DNA damaging drugs, and sensitivity to mitotic inhibitors. Additionally, CKI deficient mice are being monitored for tumor development, and will also be crossed to p53 deficient mice in order to check functional overlap between CKI and p53 mediated functions. Mice deficient for the various CKI isoforms will also be crossed to each other in order to generate compound deficiencies. These studies are at an early stage, but they will likely help to elucidate the role of CKI in mammalian DNA repair, genetic recombination and other cellular processes. Since the CKI mediated repair pathway in yeast is poorly understood, but likely to be at least partially conserved in mammals, I am also interested in complementing the mammalian work with additional studies in S. cerevisiae. In yeast, studies would include determining how CKI mediated signaling is coupled to known checkpoint genes such as the RAD, MAD and BUB genes. Longer term interests in this general area include the identification of novel repair-signaling proteins through genetic and biochemical approaches in both mammals and yeast.
Site admin: - wrengels@facstaff.wisc.edu, 18 June 1997