|Michael J Hendzel, Ph.D.
|Darin McDonald, M.Sc.
|Ismail Ismail, Ph.D.
Hilmar Strickfaden, Ph.D.
|Mohammed Ali, Ph.D.
from Our Laboratory
Polycomb repressive complex 1 is involved in ubiquitin signaling from DNA double-strand breaks.
The polycomb repressive complex 1 (PRC1) is an E3 ubiqutin ligase complex that is responsible for the ubiquitylation of histone H2A and its variants on lysine 119. This plays a critical gene silencing function during development. We recently discovered that this E3 ubiquitin ligase complex also ubiquitylates histone H2A in response to DNA double-strand breaks. The upregulation of this complex in cancer stem cells may confer the radiation resistance commonly observed in cancer stem cells.
Proline Isomerization mediated by Pin1 regulates histone H1 binding.
Histone H1 is known to be regulated by phosphorylation of cdk1/cdk2 consensus target sequences, primarily located in the large carboxy-terminal domain. This domain has long been thought to be unstructured and has not been amenable to crystallization. Currently, it is thought that the C-terminus adopts a defined conformation upon binding to chromatin. In this study, we show that proline isomerization, which can dramatically alter the structure and function of a peptide sequence, takes place within the carboxy-terminal domain. This isomerization is mediated by PIN1 and, surprisingly, counteracts the destabilizing effects of phosphorylation to stabilize the binding of H1 to chromaitn.
Development of a quantitative method for sites of
H2AX phosphorylation and the objective measurement
of DNA double-strand break-associated foci.
Antibodies that allow the quantification of DNA
double-strand breaks are being developed in
pre-clinical studies to quantify the damage and
persistence of damage caused by radiation treatment
or chemotherapeutic agents. Jianxun Han, a
Ph.D. student co-supervised by Dr. Joan Turner,
developed a method for objectively measuring DNA
damage by quantifying sites of histone H2AX
ATM phosphorylates histone H2AX
during mitosis and in the absence of DNA damage.
The importance of developing an objective method for
measuring double-strand breaks (see above) is
highlighted by this study where Kirk McManus, a
recent Ph.D. graduate, determined that histone H2AX
is phosphorylated at hundreds of sites in the
absence of DNA damage. This phosphorylation is
cell cycle-regulated and ATM, a protein important in
maintaining genome stability, is required for this
phosphorylation. This may reflect the
existence of a second mechanism where ATM
contributes to genome stability independent of its
role in DNA double-strand break repair.
Approximately 40% of Canadians
are expected to have cancer at some point in their lives and
approximately 25% of Canadians are expected to die of cancer.
It is only recently that modern biology has begun to impact upon
the clinical treatment of cancer patients. Despite early
successes in so-called rationally designed therapy, much of the
fundamental biology that provides the foundation for rationally
designed therapy remains to be discovered and characterized.
My research laboratory investigates the basic biology of the
genome and the cell nucleus, which houses the genome. The
maintenance of genome stability (mechanisms that ensure the
faithful transmission of chromosome number of sequence content),
the regulation of DNA double strand break repair, and the
regulation of the genome through epigenetic mechanisms are being
studied at the level of single cells with the objective of
identifying mechanisms that have the potential to be translated
into novel rationale therapies. We are currently funded by
the Canadian Institutes of Health Research
Taking Place in Our Laboratory
A HeLa cell treated with a protein
methylation inhibitor for 2 hours. The treatment
results in a failure to properly align the chromosomes
Genome Stability and Epigenetics in
the coding for information in the genome that is
important in the control of cell phenotype but is not
directly encoded in the DNA. It is now appreciated
that 50% or more of the cumulative changes in a cell
necessary to convert it into a cancer cell is
epigenetic, rather than genetic, in origin. We are
currently studying how epigenetic modifications of
chromatin function in organizing and regulating the
genome during interphase and mitosis. In
collaboration with Drs.
J.B. Rattner, and
Alan Underhill, we are developing reagents and drugs
to exploit the requirement for histone methylations in
maintaining genome stability. Read more about this
A pair of mouse embryonic fibroblast
cells stained with an antibody recognizing
phosphorylated histone H2AX. The cells have been
exposed to ionizing radiation prior to staining
generating the charactgeristic "γH2AX"
foci at sites of DNA damage.
DNA Damage and DNA Damage Repair.
current cancer therapies, including radiation therapy,
kill cancer cells by introducing double-strand breaks in
the DNA. We have found a novel function for
nuclear actin in repairing DNA double-strand breaks and
are working to identify novel actin-related targets to
enhance the effectiveness of radiation therapy. In
Dr. Joan Turner, we are investigating
how changes in genome organization alter the ability of
cells to repair radiation-induced double-strand breaks
(and thereby fail to kill cells). We are also
collaborating with Drs.
Guy Poirier and
to determine how cells sense and signal the existence of
DNA double-strand breaks. Collectively, these
studies have the objective of identifying mechanisms to
improve cancer therapies that work through the
introduction of double-strand breaks in the DNA.
Read more about this research.
A mouse fibroblast stained for DNA
(blue), transfected with SC35-GFP (a fluorescently
tagged RNA splicing factor) and microinjected with 40 nm
beads. A contour map was prepared from the colour
Nuclear Dynamics and Genome Regulation.
One of the frontiers in cell and cancer cell biology is
to determine the dynamic properties of important
cellular proteins inside of living cells. These
studies complement genomics and proteomics studies by
providing the details of the spatial and temporal
regulation that takes place inside of living cells.
Our laboratory is particularly interested in the
dynamics of nuclear proteins that take place in the
orchestration of genome regulation, chromatin structure,
and genome repair. In collaboration with
Th'ng, we are studying the regulation of the histone H1
protein during cellular transformation and terminal
differentiation. We are also characterzing the
basic physical properties of the living nucleus in order
to understand how these properties influence the
regulation of the genome in normal and cancer cells.
This is transdisciplinary work involving collaborations
Gerda de Vries (Mathematics) and
Jack Tuszynski (Biophysics and Computational
Biology). Read more.