Huntingtin: new marker along the road to death?

Canadian Association of Neurosciences Review: Polyglutamine Expansion Neurodegenerative Diseases

Since the early 1990s, DNA triplet repeat expansions have been found to be the cause in an ever increasing number of genetic neurologic diseases. A subset of this large family of genetic diseases has the expansion of a CAG DNA triplet in the open reading frame of a coding exon. The result of this DNA expansion is the expression of expanded glutamine amino acid repeat tracts in the affected proteins, leading to the term, Polyglutamine Diseases, which is applied to this sub-family of diseases. To date, nine distinct genes are known to be linked to polyglutamine diseases, including Huntington's disease, Machado-Joseph Disease and spinobulbar muscular atrophy or Kennedy's disease. Most of the polyglutamine diseases are characterized clinically as spinocerebellar ataxias. Here we discuss recent successes and advancements in polyglutamine disease research, comparing these different diseases with a common genetic flaw at the level of molecular biology and early drug design for a family of diseases where many new research tools for these genetic disorders have been developed. Polyglutamine disease research has successfully used interdisciplinary collaborative efforts, informative multiple mouse genetic models and advanced tools of pharmaceutical industry research to potentially serve as the prototype model of therapeutic research and development for rare neurodegenerative diseases.

Huntington's disease and dentatorubral-pallidoluysian atrophy: proteins, pathogenesis and pathology

Each of the glutamine repeat neurodegenerative diseases has a particular pattern of pathology largely restricted to the CNS. However, there is considerable overlap among the regions affected, suggesting that the diseases share pathogenic mechanisms, presumably involving the glutamine repeats. We focus on Huntington's disease (HD) and Dentatorubral-pallidoluysian atrophy (DRPLA) as models for this family of diseases, since they have striking similarities and also notable differences in their clinical features and pathology. We review the pattern of pathology in adult and juvenile onset cases. Despite selective pathology, the disease genes and their protein products (huntingtin and atrophin-1) are widely expressed. This presents a central problem for all the glutamine repeat diseases-how do widely expressed gene products give rise to restricted pathology? The pathogenic effects are believed to occur via a "gain of function" mechanism at the protein level. Mechanisms of cell death may include excitotoxicity, metabolic toxicity, apoptosis, and free radical stress. Emerging data indicate that huntingtin and atrophin-1 may have distinct protein interactions. The specific interaction partners may help explain the selective pathology of these diseases.

Role of apoptosis in gastric epithelial turnover

Gastric epithelial turnover is a dynamic process. It is characterized by continuous cell proliferation, which is counterbalanced by cell loss. The biological principle that mediates the homeostasis of epithelium is programmed cell death, or apoptosis. Currently, several subtypes of apoptosis are distinguished, which are mediated by different mechanisms. Various subtypes of apoptosis also occur in the gastric epithelium under various conditions. In the normal stomach, apoptosis due to cell isolation (anoikis) mediates the physiological epithelial turnover. Albeit rarely seen in routine histology, approximately 2% of epithelial cells in the normal stomach are apoptotic. In Helicobacter pylori-induced gastritis, apoptosis and epithelial proliferation are moderately increased, with approximately 8% apoptotic epithelial cells. In gastritis, factors such as CD95 ligand or tumor necrosis factor (TNF) alpha act as death factors. They bind to specific receptors, CD95 and TNF-R, which are induced either by other cytokines, such as interferon gamma, or by Helicobacter pylori itself. In addition to CD95, H.pylorican also induce upregulation of CD95 ligand expression. Taken together, the upregulated expression of CD95, and the presence of CD95L in the close proximity to apoptotic gastric epithelial cells suggest a functional role of the CD95-CD95L system in the induction of apoptosis in H.pylori-gastritis. The role of other pathways to apoptosis is currently under study. Apart from being a biological phenomenon, apoptosis in the stomach may also have direct clinical consequences. An extreme example is given in gastric graft-vs.-host disease when epithelial denudement occurs.

Apoptosis, neurotrophic factors and neurodegeneration

Apoptosis is an active process of cell death characterized by distinct morphological features, and is often the end result of a genetic programme of events, i.e. programmed cell death (PCD). There is growing evidence supporting a role for apoptosis in some neurodegenerative diseases. This conclusion is based on DNA fragmentation studies and findings of increased levels of pro-apoptotic genes in human brain and in in vivo and in vitro model systems. Additionally, there is some evidence for a loss of neurotrophin support in neurodegenerative diseases. In Alzheimer's disease, in particular, there is strong evidence from human brain studies, transgenic models and in vitro models to suggest that the mode of nerve cell death is apoptotic. In this review we describe the evidence implicating apoptosis in neurodegenerative diseases with a particular emphasis on Alzheimer's disease.