Supervisory Committee: Dr. Carolyn Brown, Research
Supervisor (Medical Genetics), Dr. Louis Lefebvre (Medical Genetics)
University Examiner: Dr. Sally Otto (Zoology)
SUPERVISORY COMMITTEE
Dr. Carolyn Brown, Research Supervisor (Medical Genetics)
Dr. Louis Lefebvre (Medical Genetics)
Dr. Evica Rajcan-Separovic (Pathology/Cytogenetics)
Dr. Wyeth Wasserman (Medical Genetics)
ABSTRACT
X chromosome inactivation has not been well studied in
mammals other than humans and mice. In both species, the inactive X expresses
the XIST/Xist (Xinactivation specific transcript) non-coding RNA that is
crucial for dosage compensation in females. Although both species belong to
the same mammalian subclass, Eutheria, they show significant differences in
imprinting patterns, negative regulation of XIST/Xist, and extent of silencing on the
inactive X chromosome. Furthermore, the mechanism by which the Xist transcript
coats and silences the X in cis is unknown. This study focuses on X-inactivation in
other eutherians, first to unravel domains within XIST/Xist of biological
significance, and second to investigate whether incomplete silencing in humans
is unique within the mammalian subclass. Comparative analysis to predict
conserved secondary structures between seven eutherian orthologs revealed
common stems in the sequence before the Xist A repeat, the A repeat, F repeat,
and exon 4. Several complex secondary structures were also similar between
rodents but were not conserved in other species. These included the D repeat;
structures between the B and D, as well as A and F repeats; and the unique
rodent exon 5. The significance of these conserved domains in the context of
potential biological functions, and how the structural differences might
account for some species-specific differences, is discussed in this thesis. To
investigate the species variability in the extent of silencing, methylation analysis
was performed on Zfx, Jarid1C, Crsp2, Utx, Ube1, Ar, and Fmr1 in the cow and
coast mole, in addition to human and mouse. Results from this study suggest that
mouse is distinct in its more complete inactivation at several loci – Zfx,
Crsp2 – on the evolutionary newer part of the X, and Ube1 on an evolutionary
older part of the chromosome. In addition to evolutionary age, factors
such as the position of the centromere, distance from the X inactivation centre
(XIC), and presence of Y homologs failed to consistently explain or predict whether
the genes on the X chromosome would escape or be subject to inactivation. Further
epigenetic analysis is necessary to understand the distinct mechanisms leading
to escape versus
inactivation amongst different mammals.
PUBLICATIONS
Chow, J., Yen, Z., Ziesche, S., and C.J. Brown. Silencing
of the Mammalian X Chromosome. Annu. Rev. Genomics Hum. Genet. 2005. 6:69–92.
PRESENTATIONS
Poster, American Society of Human Genetics Annual Meeting,
2004
Poster, Medical Genetics Research Day, 2004
AWARDS
NSERC PGS-M, 2004-2005
Medical Genetics Research Day Poster Award, 2004
NSERC USRA, 2003
NSERC USRA, 2002
GRADUATE STUDIES
Field of
Study: Mammalian X Inactivation, Department of Medical Genetics
Courses:
MEDG 510 Advanced Immunogenetics Dr. Kelly McNagny
MEDG 520 Advanced Human Molecular Genetics Dr. Rob
McMaster
MEDG 530 Advanced Human Genetics Dr. Jan Friedman
MEDG 545 Current Topics Medical Genetics Dr. Wendy
Robinson
