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Molecular Genetic Approaches to Neurodegenerative Disease
Summary: Nancy Bonini is interested in mechanisms of human neurodegenerative diseases, including Huntington's, Parkinson's, and Alzheimer's diseases. She is using the powerful genetics of Drosophila to create models for human neurodegeneration in the fly, learn new insight into disease mechanisms, and uncover novel means of mitigating neuronal loss.
Our laboratory is applying the genetics of Drosophila melanogaster—the common fruit fly—to the problem of human neural degeneration. Many human neurodegenerative diseases are poorly understood and untreatable, including Huntington's, Parkinson's, and Alzheimer's diseases. Overall, genes in Drosophila are highly conserved to humans; thus, genes found to be critical to maintenance of the brain in flies are likely to have conserved homologs in humans—with conservation not only of protein sequence but also of protein function.
We introduce mutant human disease genes into Drosophila to re-create the respective human disease in flies. Then, using molecular and genetic manipulations, we define mechanisms of disease and find genes that modulate disease manifestation and progression. These studies provide the foundation to determine whether similar mechanisms function in vertebrates, with the goal to provide the foundation for new approaches and techniques to interfere with disease progression in humans.
The Fly as a Model for Human Neurodegenerative Disease
We initiated these studies by introducing into Drosophila the human disease gene for spinocerebellar ataxia type 3, also called Machado-Joseph disease (SCA3/MJD). SCA3 is one of a class of human neurodegenerative diseases called polyglutamine repeat diseases and is the most prevalent dominantly inherited ataxia worldwide. Polyglutamine diseases include several additional spinocerebellar ataxias, as well as Huntington's disease. In SCA3 and the other polyglutamine diseases, a CAG repeat becomes expanded within the open reading frame of the gene. This expansion leads to incorporation of an abnormally long polyglutamine run within the disease protein. A normal polyglutamine repeat of about 10–30 becomes expanded to 60–80 in SCA3. The expanded polyglutamine domain confers dominant toxicity to the protein, and this leads to neural dysfunction and cell loss.
Initially, we expressed in flies a normal human ataxin-3 protein (the protein encoded by the SCA3 gene) and a human disease form of ataxin-3 with an expanded polyglutamine repeat. Expression of the normal protein in flies has no effect; expression of the expanded polyglutamine protein causes an effect reminiscent of human disease: late-onset, progressive neural degeneration. Moreover, expanded polyglutamine proteins in flies form intracellular inclusions—pathological protein aggregates—characteristic of human polyglutamine diseases. These inclusions are similar to the amyloid fibrillar aggregates of Alzheimer's disease, suggesting they may be akin to the aggregates of Alzheimer's, and other neurodegenerative diseases. In the latter cases, the aggregated protein is cytoplasmic or extracellular, although mechanisms of protein aggregation, and of subsequent neural degeneration, may be similar.
We have extended our studies to mechanisms of Parkinson's disease, a movement disorder caused by loss of dopaminergic neurons. Rare familial forms of Parkinson's disease are associated with mutations in the alpha-synuclein protein, a small protein found in neurons that accumulates in the Lewy body aggregates that characterize Parkinson's disease. Directed expression of alpha-synuclein in Drosophila induces adult-onset compromise of dopaminergic neurons, as in human Parkinson's disease. These studies emphasize the relevance of using fly genetics to approach mechanisms of human disease and to define genes that may mitigate neural degeneration in humans.
Mechanisms of Degeneration and Preventing Disease
Our goal is to pioneer methods by which to prevent or delay human neurodegenerative disease. One approach involves testing candidate modifier proteins. These have included the molecular chaperones, which modulate protein solubility. Because polyglutamine disease may involve abnormal protein folding, we initially addressed whether altering molecular chaperone levels can affect polyglutamine-induced neurodegeneration in Drosophila. These studies showed remarkable rescue of the degenerative disease by molecular chaperones and revealed unexpected aspects of selective chaperone activity in disease pathogenesis. Moreover, they revealed that compromising molecular chaperone pathways may normally contribute to disease.
We have extended these findings to Parkinson's disease models to show that up-regulation of molecular chaperone activity—by genes or drugs—protects against deleterious effects of alpha-synuclein on dopaminergic neurons. Our studies also reveal that compromised chaperone activity normally contributes to pathology in Parkinson's disease and other human neurodegenerative diseases associated with abnormal alpha-synuclein accumulations. Most significantly, data from others has subsequently shown that compromised chaperone activity may contribute to Parkinson's disease in humans. These studies reveal the power of model organisms like the fly to reveal insight into human disease, and have provided the foundation for a new approach to the treatment of Parkinson's disease with drugs that prevent neuron loss, rather than with drugs that treat the consequences of neuron loss. We have extended this approach by collaborating with researchers who use other powerful model organisms to discover new mechanisms of disease pathogenesis. Thus, studies of the fly, together with approaches from other systems, can help pioneer new approaches to treatment.
An additional power of the fly rests in our ability to perform genetic screens to define new and unsuspected mechanisms that contribute to disease and to disease suppression. This approach, which does not require prior knowledge of the precise mechanism of action of the mutant disease gene or the nature of the suppressor, includes performing genetic screens, some based on loss-of-function interactions, while others use gain-of-function genetics. By applying multiple approaches to the problem of neurodegenerative disease, we are defining various mechanisms by which disease occurs, as well as different ways by which to interfere in the disease process. Already, our approaches have confirmed a striking role for protein-misfolding pathways. They have also revealed new pathways that have not previously been implicated in maintenance of neuronal integrity. These new pathways include revealing that the RNA that codes for the mutant disease protein may itself be toxic in polyglutamine disease. This finding indicates that there may be both RNA- and protein-level toxicities that contribute to the disease, revealing an additional layer of complication and new types of pathways and processes that contribute to neurodegeneration. By using the fly models in such a manner, we are using the uniquely powerful genetics of the fly to build a foundation for insights into understanding and treatment of human neurodegenerative diseases.
Aspects of this research have received support from the David and Lucile Packard Foundation, the National Institute on Aging, and the National Institute of Neurological Disorders and Stroke.
Last updated December 19, 2008
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