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P&S Journal

P&S Journal: Winter 1997, Vol.17, No.1
P&S Graduate Affairs: Two Ph.D. Graduates Receive Dean's Award

Philip Feigelson, Ph.D., Assistant Vice President and Associate Dean for Graduate Affairs
Richard E. Abbott, Ph.D., Assistant Dean for Graduate Affairs

The graduating Ph.D. student judged by a faculty-student committee to be most outstanding in his or her Ph.D. thesis research receives the Dean's Award for Excellence in Research each year. Two graduating students received the 1996 Dean's Award--Dr. Judy P. Masucci, a student in the laboratory of Dr. Eric Schon (genetics and neurology), and Dr. Stephen Hing-Lam Tsang, a student in the lab of Dr. Stephen Goff (biochemistry & molecular biophysics and microbiology). The awards were presented at graduation exercises in May, and the recipients also were honored at the fourth annual graduation dinner celebrating all Ph.D. graduates and the 1996 dean's award recipients.

Abstracts of the thesis projects:

"Molecular Biology of Mitochondria: Analysis of Respiratory Chain Function"

By Judy P. Masucci

I worked on human neuromuscular disorders caused by mutations in the mitochondrial DNA. Specifically, I was studying one such disorder known as MERRF (myoclonus epilepsy with ragged-red fibers). It is characterized clinically by myoclonus (muscle jerks), seizures, ataxia (unsteady gait), ragged-red fibers in the muscle biopsy (indicating an accumulation of defective mitochondria), and less frequently by hearing loss and dementia. Genetically, MERRF has been associated with two different point mutations in the mitochondrial tRNA-Lys gene at nt positions 8344 and 8356.

My work focused on the pathogenetic mechanisms underlying these two point mutations, both of which result in clinical and biochemical defects. We were able to detect a severe decrease in both the levels of protein synthesis as well as the steady-state levels of the mitochondrial translation products in cell lines harboring 100 percent mutated mitochondrial DNAs. Our hypothesis for the pathogenetic mechanisms behind these defects was a phenomenon related to what is known as the "stringent response" in bacteria in which all proteins are downregulated in response to amino acid starvation, regardless of their requirement for the particular amino acid in question. This is exemplified by the fact that all the mitochondrial translation products are downregulated regardless of their lysine content, including ND4L which contains no lysine residues at all! Our second hypothesis involves ribosomal frameshifting as described by Gallant and colleagues in 1993. This is a pathway that is induced during starvation conditions in which the ribosome will shift either one nucleotide forward or one nucleotide backward when the cell is starved for a particular amino acid. Gallant's group just happened to be studying ribosomal frameshifting in bacteria that had been starved for lysine and found that ribosomal frameshifting occurs at a particular heptanucleotide consensus sequence (under starvation conditions). So we searched the human mitochondrial genome for the presence of this sequence and found that it occurred five times, which would generate four different sized aberrant translation products. These predicted translation products correspond precisely with those we observed in our mutant cell lines. If proved, this will be the first description of this type of frameshifting described in human cells.

"Retinal Degeneration in Gene Targeted Mutant Mice at the Phosphodiesteraseg (Pdeg) locus and its Deceleration by B-cell Lymphona Gene (BCL2)"

By Stephen Hing-Lam Tsang

More is known about the development, organization, physiology, and biochemistry of the retina than any other part of the central nervous system. Our research group is interested in understanding in great detail how the retina can detect a single photon and generate a nerve impulse.

An important element in the retina is an enzyme called the cGMP phosphodiesterase (PDE). This enzyme contains PDEa and PDEb subunits bound to two inhibitory PDEg subunits. To examine the function of the inhibitory PDEg subunits, I removed the gene for the PDEg subunit (Pdeg) in the mouse. This alteration led to a retinal degeneration that resembles incurable retinal diseases that affect up to 6 million Americans.

These mutant mice revealed several novel functions for the PDEg gene. First, the gene product is necessary for the survival and function of the retina. Second, it has an unanticipated role in promoting PDE activity. These mutant mice demonstrate how genetic studies can reveal unexpected roles for components of a signal transduction cascade. Future studies of PDEg mutant mice lines will reveal the basis for the degeneration and important steps in vision and sensory transduction.

I also studied why the cells die in these animals. Many cells are known to die in a programmed way, and a particular gene, termed B cell lymphoma 2 (BCL2), can prevent that. Histology suggested that the mutant retina displays features of premature program cell death. Elucidation of the cell death program in Pdegtm1/Pdegtm1 mutants may lead to identification of agents that interfere with cell deaths in human degenerative diseases. To modify the rate of this degeneration, the BCL2 gene was introduced into the mutant retina. This BCL2 was effective in slowing the rate of retinal degeneration. This thesis project, therefore, created a model of central nervous system degeneration and demonstrated that retarding programmed cell death is a new way to treat human retinal or neuronal degenerative diseases.

copyright ©, Columbia-Presbyterian Medical Center

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