Rona Graham
RONA K GRAHAM- PhD DEFENSE
“In vivo Characterization of Caspase Resistant Huntingtin:
Insights into the Pathogenic Mechanism of Huntington Disease”
B.Sc. Concordia University, 1987
Wednesday, January 25, 2006, 9:00 am, Room 200, Graduate
Student Centre
SUPERVISORY COMMITTEE
Dr. Michael Hayden, Research Supervisor (Medical Genetics)
Dr Shoukat Dedhar (Biochemistry and Molecular Biology)
Dr Lynn Raymond (Psychiatry)
Dr Wendy Robinson (Medical Genetics)
Dr Cheryl Wellington (Pathology)
EXAMINING COMMITTEE
Chair: Dr. Stelvio Bandiera (Pharmaceutical Sciences)
Supervisory Committee: Dr. Michael Hayden, Research
Supervisor (Medical Genetics), Dr. Cheryl Wellington (Pathology), Dr. Shoukat
Dedhar (Biochemistry and Molecular Biology)
University Examiners: Dr. Ken Baimbridge (Physiology), Dr.
Lorne Clarke (Medical Genetics)
External Examiner: Dr. Patrik Brundin, Department of
Experimental Medical Science. Wallenberg Neuroscience Center. Lund, Sweden
ABSTRACT
Proteolytic cleavage of htt is regarded as a critical
event in the pathogenesis of HD. Expression of htt fragments containing an
expanded polyglutamine repeat are toxic in vitro and in vivo, and accumulation
of N-terminal truncated products of htt are observed in human and mouse HD
brain. Notably, the presence of htt fragments prior to clinical onset of HD
suggests that htt cleavage may be a crucial, causal event in the pathogenesis
of HD. However the relationship between specific huntingtin fragments and the
pathogenesis of HD is unknown. Mutagenesis of all caspase sites in mutant
huntingtin prevents toxicity in cultured cells and caspase inhibitors improve
survival of neurons transfected with mutant htt. Caspase resistant (CR) htt
mouse models therefore would be ideal systems in which to assess whether
creation of caspase generated fragments of htt underlie the pathogenesis of HD
in vivo. To examine whether a specific caspase cleavage fragment of mutant
huntingtin is responsible for the selective neurodegeneration observed in HD,
we generated YAC transgenic mice expressing selective mutations of the caspase
cleavage sites within mutant huntingtin. We show, using sequential mutagenesis,
that caspase-6 and not caspase-3, mediated cleavage of mutant htt is responsible
for the HD-related behavioral phenotype and selective striatal neurodegeneration
observed in the YAC128 model of HD. Activation of caspase-6 and nuclear
translocation of htt fragments coincide with onset of motor dysfunction in the
YAC128 model, supporting a role for a specific nuclear htt fragment in initiating
neuronal dysfunction. Furthermore, caspase-6 cleavage of mutant htt influences
susceptibility to excitotoxic stress highlighting caspase-6 mediatedproteolysis
of htt and excitotoxicity as a primary mechanism underlying motor dysfunction
and neuropathology in HD. The results presented in this thesis support and
further refine the toxic fragment hypothesis by identifying a specific
proteolytic cleavage site in htt that is required for initiating a sequence of
events which culminate in the death of selective neurons affected in HD. This
evidence demonstrates that generation of a specific fragment of mutant htt in
vivo represents an initiating event in the pathogenesis of HD and identifies
novel approaches for inhibiting cell death in neurodegenerative disorders such
as HD.
SELECTED PUBLICATIONS
Rona K.
Graham, Yu Deng, Elizabeth J. Slow, Nagat Bissada, Ge Lu,
Jacqueline
Rona K.
Graham, Elizabeth J. Slow, Yu Deng, Nagat Bissada, Ge Lu,
Jacqueline
Elizabeth J. Slow and Rona
K. Graham, Alexander P. Osmand, Rebecca S. Devon,
Maya Saleh, John P Vaillancourt, Rona K Graham, Mathew Huyck, Srinivasa M
SELECTED PRESENTATIONS
Rona K. Graham, Elizabeth J. Slow, Rebecca S. Devon, Kuljeet
Vaid and Michael R.
Rona K Graham, Yu Deng, Elizabeth Slow, Brendan Haigh,
Nagat Bissada, Ge Lu,
Rona K Graham, Anat Yanai, Alaa El Husseini and Michael R
Hayden. Links between proteolysis and neurotoxicity identify novel approaches
for modifying the pathogenesis of HD. HD Congress, 2005.
AWARDS
University Graduate Fellowship – Master’s 2000-2001
Michael Smith Foundation for Health Research – Master’s
2001-2003
Michael Smith Foundation for Health Research – Doctoral
2003-2005
Canadian Institute of Health Research – Doctoral 2002-2005
GRADUATE STUDIES
Field of Study: Neurodegeneration and Programmed Cell
Death
Courses
MEDG505 Genome Analysis Dr. P. Heiter
MEDG520 Advanced Human Genetics Dr. C. Brown
MEDG530 Advanced Human Genetics Dr. B. Simpson
MEDG540 Seminar Dr. F. Dill
MEDG548 Directed Studies Dr. C. Brown
BIOL530 Biology of the Cell Dr. D. Moerman
Rona Graham - PhD Defense program- pdf