Marcy E. MacDonald

A novel gene containing a trinucleotide repeat that is expanded and unstable on Huntington's disease chromosomes

The Huntingtons disease (HD) gene has been mapped in 4p16.3 but has eluded identification. We have used haplotype analysis of linkage disequilibrium to spotlight a small segment of 4p16.3 as the likely location of the defect. A new gene, IT15, isolated using cloned trapped exons from the target area contains a polymorphic trinucleotide repeat that is expanded and unstable on HD chromosomes. A (CAG)n repeat longer than the normal range was observed on HD chromosomes from all 75 disease families examined, comprising a variety of ethnic backgrounds and 4p16.3 haplotypes. The (CAG)n repeat appears to be located within the coding sequence of a predicted approximately 348 kd protein that is widely expressed but unrelated to any known gene. Thus, the HD mutation involves an unstable DNA segment, similar to those described in fragile X syndrome, spino-bulbar muscular atrophy, and myotonic dystrophy, acting in the context of a novel 4p16.3 gene to produce a dominant phenotype.

Homozygous defect in HIV-1 coreceptor accounts for resistance of some multiply-exposed individuals to HIV-1 infection

Rare individuals have been multiply exposed to HIV-1 but remain uninfected. The CD4+ T-cells of two of these individuals, designated EU2 and EU3, are highly resistant in vitro to the entry of primary macrophagetropic virus but are readily infectable with transformed T-cell line adapted viruses. We report here on the genetic basis of this resistance. We found that EU2 and EU3 have a homozygous defect in CKR-5, the gene encoding the recently described coreceptor for primary HIV-1 isolates. These individuals appear to have inherited a defective CKR-5 allele that contains an internal 32 base pair deletion. The encoded protein is severely truncated and cannot be detected at the cell surface. Surprisingly, this defect has no obvious phenotype in the affected individuals. Thus, a CKR-5 allele present in the human population appears to protect homozygous individuals from sexual transmission of HIV-1. Heterozygous individuals are quite common (approximately 20%) in some populations. These findings indicate the importance of CKR-5 in HIV-1 transmission and suggest that targeting the HIV-1-CKR-5 interaction may provide a means of preventing or slowing disease progression.

A novel moesin-, ezrin-, radixin-like gene is a candidate for the neurofibromatosis 2 tumor suppressor

Neurofibromatosis 2 (NF2) is a dominantly inherited disorder characterized by the occurrence of bilateral vestibular schwannomas and other central nervous system tumors including multiple meningiomas. Genetic linkage studies and investigations of both sporadic and familial tumors suggest that NF2 is caused by inactivation of a tumor suppressor gene in chromosome 22q12. We have identified a candidate gene for the NF2 tumor suppressor that has suffered nonoverlapping deletions in DNA from two independent NF2 families and alterations in meningiomas from two unrelated NF2 patients. The candidate gene encodes a 587 amino acid protein with striking similarity to several members of a family of proteins proposed to link cytoskeletal components with proteins in the cell membrane. The NF2 gene may therefore constitute a novel class of tumor suppressor gene.

Huntingtin Controls Neurotrophic Support and Survival of Neurons by Enhancing BDNF Vesicular Transport along Microtubules

Polyglutamine expansion (polyQ) in the protein huntingtin is pathogenic and responsible for the neuronal toxicity associated with Huntingtons disease (HD). Although wild-type huntingtin possesses antiapoptotic properties, the relationship between the neuroprotective functions of huntingtin and pathogenesis of HD remains unclear. Here, we show that huntingtin specifically enhances vesicular transport of brain-derived neurotrophic factor (BDNF) along microtubules. Huntingtin-mediated transport involves huntingtin-associated protein-1 (HAP1) and the p150(Glued) subunit of dynactin, an essential component of molecular motors. BDNF transport is attenuated both in the disease context and by reducing the levels of wild-type huntingtin. The alteration of the huntingtin/HAP1/p150(Glued) complex correlates with reduced association of motor proteins with microtubules. Finally, we find that the polyQ-huntingtin-induced transport deficit results in the loss of neurotrophic support and neuronal toxicity. Our findings indicate that a key role of huntingtin is to promote BDNF transport and suggest that loss of this function might contribute to pathogenesis.

Integrated systems approach identifies genetic nodes and networks in late-onset Alzheimer's disease.

The genetics of complex disease produce alterations in the molecular interactions of cellular pathways whose collective effect may become clear through the organized structure of molecular networks. To characterize molecular systems associated with late-onset Alzheimers disease (LOAD), we constructed gene-regulatory networks in 1,647 postmortem brain tissues from LOAD patients and nondemented subjects, and we demonstrate that LOAD reconfigures specific portions of the molecular interaction structure. Through an integrative network-based approach, we rank-ordered these network structures for relevance to LOAD pathology, highlighting an immune- and microglia-specific module that is dominated by genes involved in pathogen phagocytosis, contains TYROBP as a key regulator, and is upregulated in LOAD. Mouse microglia cells overexpressing intact or truncated TYROBP revealed expression changes that significantly overlapped the human brain TYROBP network. Thus the causal network structure is a useful predictor of response to gene perturbations and presents a framework to test models of disease mechanisms underlying LOAD.

Detection of Huntington’s disease decades before diagnosis: the Predict-HD study

Objective: The objective of the Predict-HD study is to use genetic, neurobiological and refined clinical markers to understand the early progression of Huntington’s disease (HD), prior to the point of traditional diagnosis, in persons with a known gene mutation. Here we estimate the approximate onset and initial course of various measurable aspects of HD relative to the time of eventual diagnosis. Methods: We studied 438 participants who were positive for the HD gene mutation, but did not yet meet the diagnostic criteria for HD and had no functional decline. Predictability of baseline cognitive, motor, psychiatric and imaging measures was modelled non-linearly using estimated time until diagnosis (based on CAG repeat length and current age) as the predictor. Results: Estimated time to diagnosis was related to most clinical and neuroimaging markers. The patterns of association suggested the commencement of detectable changes one to two decades prior to the predicted time of clinical diagnosis. The patterns were highly robust and consistent, despite the varied types of markers and diverse measurement methodologies. Conclusions: These findings from the Predict-HD study suggest the approximate time scale of measurable disease development, and suggest candidate disease markers for use in preventive HD trials.

Polyglutamine-expanded human huntingtin transgenes induce degeneration of Drosophila photoreceptor neurons

Huntingtons disease (HD) is an autosomal dominant neurodegenerative disorder. Disease alleles contain a trinucleotide repeat expansion of variable length, which encodes polyglutamine tracts near the amino terminus of the HD protein, huntingtin. Polyglutamine-expanded huntingtin, but not normal huntingtin, forms nuclear inclusions. We describe a Drosophila model for HD. Amino-terminal fragments of human huntingtin containing tracts of 2, 75, and 120 glutamine residues were expressed in photoreceptor neurons in the compound eye. As in human neurons, polyglutamine-expanded huntingtin induced neuronal degeneration. The age of onset and severity of neuronal degeneration correlated with repeat length, and nuclear localization of huntingtin presaged neuronal degeneration. In contrast to other cell death paradigms in Drosophila, coexpression of the viral antiapoptotic protein, P35, did not rescue the cell death phenotype induced by polyglutamine-expanded huntingtin.

Huntingtin is required for neurogenesis and is not impaired by the Huntington's disease CAG expansion

Huntingtons disease (HD) is an autosomal-dominant neurodegenerative disorder caused by a CAG repeat expansion that lengthens a glutamine segment in the novel huntingtin protein. To elucidate the molecular basis of HD, we extended the polyglutamine tract of the mouse homologue, Hdh, by targetted introduction of an expanded human HD CAG repeat, creating mutant HdhneoQ50 and HdhQ50 alleles that express reduced and wild-type levels of altered huntingtin, respectively. Mice homozygous for reduced levels displayed characteristic aberrant brain development and perinatal lethality, indicating a critical function for Hdh in neurogenesis. However, mice with normal levels of mutant huntingtin did not display these abnormalities, indicating that the expanded CAG repeat does not eliminate or detectably impair huntingtins neurogenic function. Thus, the HD defect in man does not mimic complete or partial Hdh inactivation and appears to cause neurodegenerative disease by a gain-of-function mechanism.

Molecular genetics: Unmasking polyglutamine triggers in neurodegenerative disease

Two decades ago, molecular genetic analysis provided a new approach for defining the roots of inherited disorders. This strategy has proved particularly powerful because, with only a description of the inheritance pattern, it can uncover previously unsuspected mechanisms of pathogenesis that are not implicated by known biological pathways or by the disease manifestations. Nowhere has the impact of molecular genetics been more evident than in the dominantly inherited neurodegenerative disorders, where eight unrelated diseases have been revealed to possess the same type of mutation — an expanded polyglutamine encoding sequence — affecting different genes.

Regulation of intracellular accumulation of mutant Huntingtin by Beclin 1.

Intracellular accumulation of mutant Huntingtin with expanded polyglutamine provides a context-dependent cytotoxicity critical for the pathogenesis of Huntington disease (Everett, C. M., and Wood, N. W. (2004) Brain 127, 2385-2405). Here we demonstrate that the accumulation of mutant Huntingtin is highly sensitive to the expression of beclin 1, a gene essential for autophagy. Moreover, we show that the accumulated mutant Huntingtin recruits Beclin 1 and impairs the Beclin 1-mediated long lived protein turnover. Thus, sequestration of Beclin 1 in the vulnerable neuronal population of Huntington disease patients might further reduce Beclin 1 function and autophagic degradation of mutant Huntingtin. Finally, we demonstrate that the expression of beclin 1 decreases in an age-dependent fashion in human brains. Because beclin 1 gene is haploid insufficient in regulating autophagosome function (Qu, X., Yu, J., Bhagat, G., Furuya, N., Hibshoosh, H., Troxel, A., Rosen, J., Eskelinen, E. L., Mizushima, N., Ohsumi, Y., Cattoretti, G., and Levine, B. (2003) J. Clin. Invest. 112, 1809-1820; Yue, Z., Jin, S., Yang, C., Levine, A. J., and Heintz, N. (2003) Proc. Natl. Acad. Sci. U. S. A. 100, 15077-15082), we propose that the age-dependent decrease of beclin 1 expression may lead to a reduction of autophagic activity during aging, which in turn promotes the accumulation of mutant Htt and the progression of the disease.