Understanding Huntington's Disease

Researchers are inching ever closer to a treatment for the inherited neurodegenerative disorder Huntington's disease (HD). Following close on the heels of recent optimistic reports on fetal cell implants1,2 comes a report from Johns Hopkins University that sheds light on a possible mechanism of neuronal destruction in HD.3 The work reveals possible new drug targets. Legendary folksinger Woody Guthrie's fight with HD brought the disease into the public eye in the 1960s. Today, some 30,000 people in the United States are affected by the late-onset, autosomal dominant condition. Early symptoms--such as anger or depression, repetitive fidgety movements, and a clumsiness and tendency to fall--may go unnoticed for years. Neurological deterioration, reflecting cell death in the brain's striatum, typically continues for 15 to 20 years. HD has left its mark on the recent history of genetics, but has remained frustratingly untreatable. Discovery of a marker in 1983,4 and of the gene in 1993,5 gave affected families the option of predictive testing, but without any treatments that could halt or even slow the disease process. As one of the first "triplet repeat" disorders to be described, HD is also one of eight "polygln" disorders in which an expanded repeat of the RNA triplet CAG adds extra glutamines (gln) to the protein product huntingtin. The fact that individuals who lack the huntingtin gene do not develop the disease suggested early on that the expanded triplet repeat mutation adds a function, rather than destroying one. Max Ferdinand Perutz of the Laboratory of Molecular Biology in Cambridge, U.K., who won the Nobel Prize in chemistry in 1962 for determining the structures of hemoglobin and other globular proteins, hypothesized that the gain in function was rooted in the extra glutamines. Clinicians subsequently teamed with geneticists and after further study it became clear that with an increased number of glutamines, the earlier the onset and more serious the symptoms of HD. At the same time, work with transgenic mice revealed telltale inclusions in the cytoplasm and the nuclei of affected neurons.6 The search was on for the mechanism whereby extra glutamines gum up the works in the striatum. Was abnormal huntingtin glomming together with itself, or perhaps binding to and snatching away other proteins?7 Several transcription factors were known to harbor short stretches of glutamines, and some of them indeed bound to huntingtin in vitro. Finding Huntingtin's Mate Johns Hopkins University professor of psychiatry and neurology Christopher Ross and co-workers and those at Niigata University in Japan homed in on CREB binding protein (CBP), which includes a stretch of 18 glutamines.8 "CBP is a co-activator. It helps transcription factors turn on probably hundreds of different genes, controlling a large number of proteins vital for neuronal survival. Growth factors, for example, may act through CBP," explains Ross. "The real problem is that the abnormally shaped huntingtin protein attracts and becomes entangled with a smaller, critical protein in the cell nucleus," adds doctoral student Frederick Nucifora Jr. In a series of experiments using neuroblastoma cells in culture, transgenic mouse brains, and post-mortem brains from HD patients, the researchers used immunohistochemistry and confocal microscopy to show that mutant huntingtin indeed binds the polyglutamine portion of CBP. "We were aiming to see if huntingtin can remove CBP from its normal localization in the nucleus. It had been shown that the two proteins aggregate, so we did a co-transfection with cDNA for huntingtin and for CPB. Normal CBP is in the nucleus around the DNA as a transcription factor. When we cotransfect with the gene encoding normal huntingtin, CBP location doesn't change, but when we cotransfect with mutant huntingtin, it interacts with it and traps it," explains Ross. Further experiments showed that CRB missing its polygln stretch is not vulnerable to the gobbling mutant huntingtin. The researchers also demonstrated that mutant huntingtin squelched the transcriptional activity of CBP, and that overexpression of CBP can overcome the effects of mutant huntingtin. "These experiments showed that the interaction depends on the mutation and the presence of polyglutamines in CBP," sums up Ross. Questions Remain Implications of the experiments are both basic and applied. "Our work provides a molecular explanation for all polyglutamine diseases. But CBP is not the only other protein involved. Others have short polyglutamine repeats, and these may be the basis of other disorders," says Ross. Still, the tango between huntingtin and CBP reveals a potentially powerful drug target. Ross describes the interaction as a "noncovalent polar zipper." The new findings provide a piece to an ever-expanding pathogenic puzzle. Questions remain however, such as just how mutant huntingtin slithers into the nucleus. "Huntingtin is normally in the cytoplasm, but the pathological form gets into the nucleus. I think maybe it is truncated first, leaving a piece with the poly-glutamines. But this is a controversial idea, and other people think it all gets through," comments Ross. Mutant huntingtin also grabs CBP in the cytoplasm, where proteins are synthesized. Ongoing work in many labs seeks to sort out how the polygln disorders are alike and how they differ. "Even though most scientists in the field agree that there will be shared pathogenic processes among most, and perhaps all polygln disorders, there clearly are important disease-specific features," says Henry Paulson, assistant professor of neurology at the University of Iowa College of Medicine in Iowa City. "The obvious, and probably most important factor is the particular protein context in which each of the expanded polygln domains reside. Depending on the surrounding protein, the function of that protein, and the various proteins with which it normally interacts, pathogenesis will differ," he adds. Meanwhile, for those with HD, the long-awaited news that researchers are finally beginning to understand how the disease happens is most welcome. Ricki Lewis (rickilewis@nasw.org) is a contributing editor for The Scientist. References 1. T.B. Freeman et al., "Transplanted fetal striatum in Huntington's disease: phenotypic development and lack of pathology," Proceedings of the National Academy of Sciences, 97:13877-82, Dec. 1, 2000. 2. A.-C. Bachoud-Levi et al., "Motor and cognitive improvements in patients with Huntington's disease after neural transplantation," The Lancet, 356:1975-79, Dec. 9, 2000. 3. F.C. Nucifora Jr. et al., "Interference by Huntingtin and atrophin-1 with CBP-mediated transcription leading to cellular toxicity," Science, 291:2423-28, March 23, 2001. 4. J. Gusella et al., "A polymorphic DNA marker genetically linked to Huntington's disease," Nature, 306:234-8, 1983. 5. H.D. Collaborative Research Group, "A novel gene containing a trinucleotide repeat that is expanded and unstable in Huntington's disease chromosomes," Cell, 72:91-983, 1993. 6. G.P. Bates et al., "Transgenic models of Huntington's disease," Human Molecular Genetics, 6:1633-7, 1997. 7. H.L. Paulson, "Protein fate in neurodegenerative proteinopathies: polyglutamine diseases join the (mis)fold," American Journal of Human Genetics, 64:339-45, 1999. 8. A. McCampbell et al., "CREB-binding protein sequestration by expanded polyglutamine," Human Molecular Genetics, 9:2197-202, 2000.

Understanding Huntington's Disease

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