Genetics

Journal of Neurochemistry, 2006, 97, 1690–1699

SPECIAL ISSUE

doi:10.1111/j.1471-4159.2006.03979.x

The genetics of neurodegenerative diseases

REVIEW

John Hardy*,  and Harry Orrà *Laboratory of Neurogenetics, National Institute on Aging, Bethesda, Maryland, USA  Reta Lila Weston Institute of Neurological Studies, The National Hospital for Neurology and Neurosurgery, London, UK àDepartment of Biochemistry, Molecular Biology, Biophysics, Department of Laboratory Medicine and Pathology and the Institute of Human Genetics, The University of Minnesota, Minneapolis, USA

Abstract In the last 50 years, an enormous amount of progress has been made in dissecting the etiology of hereditary neurodegenerative diseases, including the dementias, the parkinsonisms, the ataxias and the motor-neuron diseases. In addition, these genetic findings are beginning to provide insights into the pathogeneses of the sporadic forms of the diseases. Through animal and cellular modeling studies we are begin-

ning to gain insights into the pathogenic pathways to disease. This mechanistic understanding is now leading to therapeutic strategies based on this new understanding. As yet, however, no mechanistic therapies are in use in the clinic. Keywords: ataxia, dementia, parkinsonism, polyglutamine, a-synuclein, tau. J. Neurochem. (2006) 97, 1690–1699.

In our aging societies, neurodegenerative diseases are among those most feared. In the developed world, they afflict about 2% of the population at any time. In many cases genetics plays a role in their etiology. Studying the genetics of diseases is important for several reasons: first, each time a mutation is identified in a family, it potentially has a direct clinical impact on that family in terms of diagnosis and presymptomatic and prenatal testing; second, the identification of pathogenic loci leads to a greater understanding of the pathogenesis of the disease in general, not just the simple genetic form of the disease, and thus offers target pathways for therapy; third, it leads to the development of animal models of the disease that can be used to develop ideas about pathogenesis and to test therapies on; and fourth, it gives direct insight into gene and protein function. An unanticipated bonus from the study of the genetics of neurodegenerative diseases has been the realization that many of these diseases fall into one of three, or possibly four, classes of diseases that almost certainly share pathogenic mechanisms, including the deposition of misfolded proteins, and may therefore be treatable using similar approaches. In this review, we discuss these genetic findings in these four classes of neurodegenerative disease and the implications of these findings for the development of ideas concerning pathogenesis and disease-modifying treatments. The disease classes include, first, the tauopathies, second, the

synucleinopathies, a third possible class is the ubiquitin inclusion diseases and fourth are the polyglutamine diseases. In each of these families of diseases, the pathway to cell death is probably broadly similar and offers a therapeutic target suitable for all of them.

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The tauopathies

Either tangle or other tau pathology is found in a large variety of neurological diseases (Table 1, Lee et al. 2001). Received April 5, 2006; revised manuscript received April 25, 2006; accepted April 25, 2006. Address correspondence and reprint requests to John Hardy, Laboratory of Neurogenetics, National Institute on Aging, Porter Neuroscience Building, 35 Convent Drive, Bethesda, MD20892, USA. E-mail: [email protected] Abbreviations used: ALS, amyotropic lateral sclerosis; APP, amyloid-b precursor protein; AR, androgen receptor; BDNF, brain-derived neutrophic factors; CBD, corticobasal degeneration; FTD, frontal temporal dementia; FTDP-17T, FTD with parkinsonism linked to chromosome 17; HD, Huntington’s disease; MJD, Machado Joseph disease; PDC, Parkinson’s disease/ALS complex of Guam; PEP, von Ecomono’s disease (post-encephalitic parkinsonism); PS1/2, presenilin 1 and presenilin 2 genes; PSP, progressive supranuclear palsy; SBMA, spinobulbar muscular atrophy; SCA1, spinocerebellar ataxia type 1; SCA3, SCA type 3; SOD, superoxide dismutase; SSPE, subacute sclerosing panencephalitis; VCP, valosin-containing protein.


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Table 1 Diseases with Tau Pathology Alzheimer’s disease Argyrophilic grain dementia Corticobasal degeneration Dementia pugilistica Frontotemporal dementia with parkinsonism linked to chromosome 17 (Pick’s Disease) Hallervorden-Spatz disease

seen in the context of a neurodegenerative process the tau dysfunction contributes to the cell loss. Tangle formation and neurodegeneration has been modeled in transgenic mice carrying MAPT mutations (Hutton et al. 2001). These mice are thus useful in testing prospective therapies for tauopathies: for example, lithium treatment, which inhibits tau phosphorylation, reduces tangle formation and cell loss in these animals (Noble et al. 2005) and thus has been proposed as a therapy for tangle diseases.

Myotonic dystrophy Niemann-Pick disease, type C Parkinsonism–dementia complex of Guam Postencephalitic parkinsonism Prion diseases (some) Progressive subcortical gliosis Progressive supranuclear palsy Subacute sclerosing panencephalitis

Although the most common tauopathy is Alzheimer’s disease, the prototypic tau disease is frontal temporal dementia with parkinsonism linked to chromosome 17 (FTDP-17T). FTDP-17T Both the clinical and pathological phenotype of FTDP-17T is remarkably variable. The disease can appear either as a purely dementing syndrome or present parkinsonian or even amyotrophic features. Tau pathology always occurs, but varies from wispy tau filaments, through Pick bodies to neurofibrillary tangles. In this disease, mutations in either the open reading frame or in exon-10 splice elements of the tau (MAPT) gene lead to tau pathology and to neurodegeneration (Hutton 2001). The precise pathology appears to be largely determined by the particular MAPT mutation (Ingram and Spillantini 2002; http://www.molgen.ua.ac.be/ADMutations/default.cfm?MT ¼ 1&ML ¼ 1&Page ¼ MutByGene), with, in general, mutations in the exon-10 splice elements leading to the deposition of wispy tau filaments, mutations in exon 10 leading to dense deposits of four-repeat tau, and mutations elsewhere in the MAPT gene leading either to deposits of both three- and fourrepeat tau or to deposits of three-repeat tau in Pick bodies (Ingram and Spillantini 2002; Sergeant et al. 2005). The relationship of the mutation to the clinical phenotype is less clear. The mechanism by which the mutations lead to disease is also not clear. There are two main theories, which are not mutually exclusive: either the mutations lead to decreased microtubule binding, and thus to a greater opportunity for selfaggregation, or they lead to increased self-aggregation and thus to less microtubule binding (Hutton 2001; Lee et al. 2001; Ingram and Spillantini 2002). Whatever the precise mechanism of pathogenesis, these data show that tau mutations lead directly to neurodegeneration and that tau deposition, either as tangles or Pick bodies is associated with this neurodegenerative process. These findings have importance beyond FTDP17T families, in that they imply that whenever tau pathology is

Progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) PSP and CBD are relatively rare sporadic diseases characterized by neurofibrillary tangles consisting largely of fourrepeat tau (Sergeant et al. 2005). In Europeans the MAPT gene has an unusual genetic structure, with a very divergent and inverted H2 haplotype that has an allele frequency of
25% (Evans et al. 2004; Stefansson et al. 2005). This has made the genetic analysis of PSP and CBD easy in this population and has shown a robust association between the H1 haplotype of the MAPT locus and both PSP and CBD (Hutton 2001). It is likely that there will be a MAPT association with these diseases in other populations, but the relevant analyses are more difficult to perform in these other populations because of the absence of the H2 haplotype. A haplotypic association, in the absence of coding changes, implies that the biological effect is mediated by either differences in expression or differences in splicing between haplotypes. The fact that the splicing of the MAPT gene is one of the causes of FTDP-17T, and the fact that the tangle deposits are, almost exclusively, of four-repeat tau, suggests that either or both of the above are equally likely explanations. A more detailed analysis of the structure of the H1 haplotype revealed that it has considerable complexity, and that, in fact, the haplotypic association between H1 and PSP and CBD is driven by a variant of the haplotype named H1c, which runs from the promoter to intron 10 of the gene. Analysis of this haplotype has not yet allowed the determination of whether it is a particularly high-expressing haplotype, one that particularly expresses the exon-10 containing transcript, or a mixture of both. This haplotypic association is an example of the general principle that genetic variability at the loci causing autosomal dominant disease (in this case FTDP-17T) is part of the genetic contribution to the sporadic diseases (in this case PSP and CBD) (Singleton et al. 2004). Parkinson’s disease/ALS complex of Guam (PDC), von Ecomono’s disease (post-encephalitic parkinsonism) (PEP) and subacute sclerosing panencephalitis (SSPE) When US military medical personnel arrived in Guam shortly after the World War 2, they were astonished to find that, in the southern village of Umatac, amyotropic lateral sclerosis, ALS (for which the Chamorro word was lytico, from the Spanish for paralysis) was a major cause of death


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(Mulder et al. 1954). Later, they realized that a parkinsonian syndrome (for which the local word was bodig from the Chamorro word for shuffle) was also highly prevalent in the local community (Kurland et al. 1961). Both diseases were familial. Neuropathological examination revealed that both diseases were underpinned by extensive tangle pathology (Kurland et al. 1961). However, since that time, both diseases have gradually disappeared and now, although there are still a few elderly individuals with bodig, lytico has essentially disappeared and neither disease has convincingly occurred in an individual born since the turmoil when Guam was liberated in 1944 (Waring et al. 2004). The disease has occurred in Chamorros who moved overseas, but not convincingly in immigrants to Guam. Although it was familial, the fact that it has died out suggests that it was not a simple genetic disorder, and indeed a genetic linkage scan showed that the disease was not a single-gene disorder (Morris et al. 2004). However, it seems that, like PSP and CBD, bodig shows an association with the MAPT locus (Poorkaj et al. 2001) (lytico could not be tested because it has disappeared). Although there have been many theories concerning the etiology and pathogenesis of Guam disease, based on researchers’ views of the Chamorros’ diet (Cox and Sacks 2002), these have not received either epidemiological or experimental support. Perhaps lytico and bodig most resemble von Economo’s disease, which was a parkinsonian tangle disorder believed to have been caused by a delayed onset reaction to a viral infection, possibly the 1919 ‘Spanish flu’ epidemic memorably portrayed in the film ‘Awakenings’ (Dickman 2001). Similarly, SSPE is a rare but serious complication of measles infection that is also characterized by tangles (Bancher et al. 1996). These diseases, PDC, PEP and SSPE, show that although, apart from Alzheimer’s disease (discussed below), tangle diseases are rare, they have the potential to become epidemics: lytico was the major cause of death in Umatac, and clearly PEP and SSPE have the potential to reach epidemic proportions given a suitable infectious agent. Alzheimer’s disease Alzheimer’s disease is easily the most common neurodegenerative disease and afflicts
5% of those over 65 years. It is characterized clinically by a dementia, classically starting with problems forming recent memories, and is characterized pathologically by neuritic plaques of which the peptide Ab42 is the principle component. The autosomal dominant forms of the disease are caused by mutations in the amyloid-b precursor protein (APP) and the presenilin 1 and presenilin 2 genes (PS1/2) (Rogaeva 2002). The mutations in APP occur at its cleavage sites, thereby altering APP processing such that more Ab42 is produced (Hardy and Selkoe 2002). The presenilins are a central component of c-secretase, the enzyme responsible for liberating from the C-terminal fragment of APP, and mutations in the presenilins also alter

APP processing such that more Ab42 is produced (Wilquet and De Strooper 2004). These genetic data are the intellectual basis for the amyloid hypothesis of the disorder, and suggest that Ab42 is the initiating molecule in Alzheimer’s disease. Recent data have reinforced this view, with APP gene duplications also causing the disease (Rovelet-Lecrux et al. 2006). These genetic data have been largely recapitulated in transgenic mice. Mice with APP mutant transgenes develop amyloid plaque pathology, and this pathology is hastened by crossing them with mice with mutant presenilin transgenes. However, such mice neither develop extensive cell loss nor exhibit tangle pathology (LaFerla and Oddo 2005). Crossing the mice with plaque pathology with the mutant MAPT mice potentiates the tangle pathology but has no effect on the plaque pathology, suggesting that the Ab/amyloid pathology is upstream, in the pathological cascade, to the tau/tangle pathology (Hardy et al. 1998). Although there is a consensus that Ab is upstream of tau in the pathological cascade in Alzheimer’s disease, there is little hard evidence on either the nature of the interaction or how direct it is. The most promising line of enquiry in this regard is the observation that Ab was much less toxic to neurons in which the MAPT gene had been knocked out (Rapoport et al. 2002). The nature of the toxic species of Ab is also not entirely clear, but an evolving view has been that it is some form of Ab oligomer: this evolving consensus has centred around a species called A4 in the original APP cloning paper, or more recently Alzheimer’s Disease Diffusible Ligands, Ab oligomers or Ab* (Klein et al. 2004). Strictly, these species have not been shown to be toxic, but rather have been shown to be potent synaptic depressants (Lesne et al. 2006). Much of the effort for developing mechanistic therapeutics has been aimed at APP processing (Fig. 1): by inhibiting either BACE or c-secretase either directly or indirectly (Hardy and Selkoe 2002; Golde 2005). The greatest publicity has been given to trials of immunization against Ab. This immunization clearly and surprisingly results in the clearance of Ab from the brains of mice and humans, and has behavioral benefits in the mice, but the human trial resulted in serious and occasionally fatal meningoencephalitis in a proportion of Alzheimer cases without clear clinical benefit (Schenk et al. 2004). Whether this was an adverse effect of the particular protocol used or was centrally related to the immunological approach to Ab clearance is currently unclear. In the Alzheimer field there is the general sense that mechanistic approaches to therapy should emerge soon, but there is also a growing impatience amongst those who believe that Ab-centred therapy should have worked by now if indeed it was a valid approach to the disease (Lee et al. 2005). The synucleinopathies

Like the tauopathies, a-synuclein pathology is a feature of many neurodegenerative diseases (Table 2).


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Indeed there is considerable overlap between the diseases suggesting that there are mechanistic relationships between them. This overlap includes, for example, Alzheimer’s disease, where a-synuclein pathology occurs in cases with APP mutations, Down’s syndrome and in presenilin mutation cases (Lee et al. 2004: Fig. 2). An analogous interaction to that between Ab and tau in the APP/MAPT double transgenic mice alluded to above (Hutton et al. 2001) has been shown between Ab and a-synuclein in APP/synuclein double transgenic mice (Hashimoto et al. 2003). In both cases, the APP transgene potentiates the intracellular pathology.

Fig. 1 APP processing.

Table 2 Diseases with a-Synuclein Pathology Alzheimer’s disease Gaucher’s disease Hallervorden-Spatz disease Lewy Body Dementia Multisystem atrophy Parkinson’s disease Prion diseases (some)

Parkinson’s disease The prototypic synucleinopathy is Parkinson’s disease (Spillantini et al. 1997). However, the nosology of Parkinson’s disease is messy (Hardy and Lees 2005). About 90% of individuals with the clinical diagnosis of Parkinson’s disease have Lewy bodies (Hughes et al. 2001), but there is no agreement about whether the disease should be defined by clinicians or as a clinicopathologic entity. This becomes important because none of the genetic findings fit cleanly with the diagnoses. Five genes have been identified that cause mendelian diseases which have been claimed to be Parkinson’s disease (Cookson et al. 2005; Table 3) and in no case was there a one-for-one mapping of gene defect with the diagnosis of Lewy Body Parkinson’s disease. However, genetic variability at the a-synuclein locus contributes to the risk of the sporadic disease, indicating that a-synuclein expression contributes to the risk of the sporadic disease in an analogous fashion to tau expression and sporadic tangle disease (Singleton et al. 2004). Whether all these genetic loci map onto one pathway to disease analogous to the amyloid

Fig. 2 An outline of the relationship between Ab, tau and a-synuclein according to the amyloid hypothesis.


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1694 J. Hardy and H. Orr

Table 3 Loci claimed to cause Parkinson’s disease Gene

Mode of inheritance

Putative function

Problem

a-synuclein

Dominant

Vesicle trafficking?

parkin

Usually recessive

Protein turnover

Some cases have Lewy body dementia rather than Parkinson’s disease clinically Most cases don’t have Lewy bodies

DJ-1

Usually recessive

Signaling to mitochondrion

No pathological reports have been published

PINK1

Usually recessive

Mitochondrial kinase

No pathological reports have been published

LRRK2

Dominant

Cytoplasmic kinase

Pathology usually involves Lewy bodies but sometimes it is a tangle disease and sometimes there seems to be no histopathology

Fig. 3 An outline of the putative relationship between dardarin (encoded by LRRK2) and a-synuclein and tau.

hypothesis for Alzheimer’s disease remains unclear (AbouSleiman et al. 2006). The most puzzling gene for Parkinson’s disease is LRRK2, which encodes the protein, dardarin. Mutations in this gene are a very common cause of typical Lewy body Parkinson’s disease (Gilks et al. 2005), but can also cause tangle pathology (Zimprich et al. 2004). It is not clear how this relates to the pathogenesis of these lesions (Singleton 2005), although in this respect one could think of LRRK2/dardarin having a similar biochemical effect as an Ab load (Fig. 3 q.v. Fig. 2). Thus, from a genetic perspective, the mutations in LRRK2, which appear to be a gain of kinase function (West et al. 2005), have a similar action to Ab suggesting that dardarin may be on the pathway from Ab to tau and synuclein. In terms of mechanistic treatment, perhaps the two most promising approaches are synuclein immunization, which remarkably cleared intraneuronal synuclein pathology in the transgenic mice (Masliah et al. 2005), and developing an inhibitor for dardarin, as the pathogenic mutations seem to lead to overactive kinases.

The ubiquitin-inclusion diseases For the tauopathies, the synucleinopathies and the polyglutamine diseases, molecular histochemistry has shown that the lesions in the different diseases are essentially identical, suggesting that their pathogenesis is probably also very similar. This is not true for the ubiquitin-inclusion diseases for which the molecular substrate is not yet known, and so designation of this as a class of diseases is not as certain as for the other diseases. Two genetic loci have been reported for this disease, frontal temporal dementia (FTD) on chromosome 17, close to the MAPT locus and FTD/ALS on chromosome 9p. FTDP-17U Remarkably, a proportion of families that showed genetic linkage to chromosome-17 markers did not have MAPT mutations, although their clinical features were indistinguishable from those that did (Foster et al. 1997; van der Zee et al. 2006). At autopsy, these cases have no tangle pathology, but rather had ubiquitin-positive inclusions (Mackenzie et al. 2006). The majority of cases that show this linkage


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have FTD, although a minority have an ALS presentation. It will not be entirely clear how prevalent this condition is until the gene is identified, but a recent study from Flanders suggested it may be quite common (van der Zee et al. 2006). It seems a remarkable coincidence that this locus should map essentially to the same point on the genome to FTDP17T, and yet, apparently, be pathogenically distinct. Only when the gene is identified will we be able to determine whether its function relates to tau biology (Hardy et al. 2006). FTD/ALS chromosome-9p linked Recently, linkage to markers on chromosome 9p has been independently reported by two groups (Morita et al. 2006; Vance et al. 2006): this linkage apparently supersedes previous reports of linkages to chromosome 9q and 16q markers (ibid.). The phenotype of these families includes both FTD and ALS, and the pathology is again of ubiqitinated inclusions. Inclusion body myopathy associated with Paget disease This rare disease classically has an unusual mixture of features of Paget’s disease of bone and a progressive dementia. Genetic analysis linked it to chromosome 9p (although to a separate location from the FTD/ALS syndrome described above). The locus was identified as the valosin-containing protein (VCP), an AAA-ATPase that is believed to be involved in the ubiquitin proteasome pathway (Watts et al. 2004). In some individuals, the phenotype is of a pure FTD, and the pathology is also of ubiquitinated inclusions (Schroder et al. 2005). These ubiquitinated inclusions also stain for VCP. ALS Interestingly, most cases of ALS, but not those with superoxide dismutase 1 (SOD1) mutations, have ubiquitinated inclusions (Leigh et al. 1991; Mackenzie and Feldman

2005). This implies that typical ALS may have a pathogenesis related to this pathway, but that the SOD-encoded ALS represents a different pathogenesis. From a treatment perspective, because the SOD mutant mice are typically used to test therapies, it is important to determine whether they do indeed share the same pathogenesis or whether the grouping of diseases should rather be as outlined herein. The polyglutamine diseases

The group of inherited neurodegenerative diseases designated the polyglutamine diseases share many seminal features with the other families of neurodegenerative diseases (Zoghbi and Orr 2000). They typically manifest with a late age of onset and, at least during their initial stages, these disorders are characterized by a specific set of clinical signs with pathology limited to a distinct subset of neurons. Like many other neurodegenerative diseases, the polyglutamine diseases have, as a hallmark of pathology, the accumulation of insoluble material within neurons, adding further to the concept of a common pathogenic theme, the generation and accumulation of misfolded proteins. Whether the polyglutamine inclusions have a direct role in pathogenesis remains controversial. Nine neurodegenerative diseases have as their diseasecausing mutation the expansion of a polyglutamine tract (polyQ). This involves the unstable expansion of a CAG sequence within the coding region of each gene. Thus, these diseases fall within a broader class of disorders, i.e. diseases that involve the expansion of an unstable repetitive element, usually triplet sequences (Gatchel and Zoghbi 2005). Interestingly, expansions of an unstable nucleotide repeat is a mutational mechanism that appears to be unique to the human genome. Furthermore, although genes that are highly homologous to the polyglutamine genes are present in the genomes of other mammals, the polyglutamine tract is not conserved (Table 4), suggesting

Table 4 CAG repeat lengths in mammalian species

Disease

Gene Locus

SBMA

Xq11–12

Human wild-type alleles

Human mutant alleles

Chimpanzee

Orangutan

Macaque

Marmoset

Rodent

40–63

18–23

5

7–9

NA

2

Protein

Protein Location Nuclear & cytoplasmic Cytoplasmic

6–39 6–34

36–121

NA

NA

NA

NA

7

Nuclear (neurons)

8–44

39–83

20–26

20–25

9–15

9–15

2

HD

4p16.3

Androgen receptor Huntingtin

SCA1

6p22–23

Ataxin-1

SCA2

12q23–24

Ataxin-2

Cytoplasmic

13–33

32–77

22–27

16–17

NA

NA

QPQ

SCA3/MJD

14q24.3–31

Atxain-3

Cytoplasmic

12–40

54–89

14–20

24–25

13–14

NA

NA

SCA6

19p3

CACNA1A

Cell membrane

4–18

21–33

9–13

11–13

NA

NA

NA

SCA7

3p12–p21.1

Ataxin-7

Nuclear

4–35

37–306

NA

NA

NA

NA

NA

SCA17

22q13

TATA-BP

Nuclear

29–42

47–55

NA

NA

NA

NA

NA

DRPLA

12q

Atrophin-1

Cytoplasmic

6–36

49–84

11–17

15

NA

NA

NA


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that the polyglutamine stretches are not necessary for normal function. The polyglutamine disorders include spinobulbar muscular atrophy, Huntington’s disease (HD), the spinocerebellar ataxias (SCA1, SCA2, SCA3/MJD, SCA6, SCA7 and SCA17), and dentatorubral-pallidoluysian atrophy (DRPLA). All of these disorders are progressive, typically striking at midlife and having a course that consists of an extended period of neuronal dysfunction followed by neuronal loss and eventually death 10–20 years after onset. Other features that characterize this group of neurodegenerative disorders include a direct relationship between the length of the polyglutamine tract and the severity of the disease, and between the number of glutamines and the age of onset and the severity of the disease. Mutant CAG repeats show both germline and somatic instability. Germline instability along with the relationship between repeat length and disease course are the basis for the phenomena of genetic anticipation. As the repeat grows upon passage from generation to generation, affected members in successive generations have an earlier age of onset and more rapidly progressing form of the disease. In the following sections some of the polyglutamine disorders are discussed in more detail. Attention is directed to the disorders that are more prominent and to other instances where research suggests insights that may have broader implications for the neurodegenerative disorders. Spinobulbar muscular atrophy (SBMA) SBMA (also known as Kennedy’s disease) is characterized by the loss of anterior horn, bulbar motorneurons and dorsal root ganglia sensory neurons. Except for SBMA, which is caused by a mutation in the androgen receptor (AR) gene on the X chromosome, the polyglutamine disorders are inherited in an autosomal dominant fashion. Yet SBMA is in many ways the archetypical polyglutamine disease. SBMA was the first polyglutamine-based disorder for which the gene was cloned (La Spada et al. 1991). More importantly the genetics of AR provide clear evidence that the polyglutamine expansion is a gain-of-function mutation and not a loss-offunction mutation. SBMA (or Kennedy’s disease) is an adultonset motorneuron disease with only very limited signs of androgen insensitivity in adults. A loss of function mutation in AR does not result in either SBMA or motorneuron disease. Rather the deletion of the AR gene leads to testicular feminization, with abnormal fetal sexual development and no signs of motorneuron disease such as weakness. The distinct phenotype caused by a loss of AR function compared with SBMA provides a strong genetic argument that the polyglutamine expansion associated with SBMA is not a loss-offunction mutation. Moreover, the genetics of SBMA indicate that the pathogenic mechanism triggered by the polyglutamine expansion gain-of-function mutation in AR is linked to the normal function of this protein. SBMA is essentially a

disorder that affects only males. In fact women who are homozygosis for expanded CAG repeats in the AR were reported to have minimal disease, suggesting that a sexspecific factor impacts the expression of SBMA (Schmidt et al. 2002). It appears that the higher levels of circulating androgen hormone in males are required for mutant AR to cause SBMA, connecting the pathogenic pathway of mutant AR to its normal function. Recent studies that administered androgen antagonists to a mouse and Drosophila (fly) model of SBMA support the premise that ligand-induced nuclear translocation of mutant AR is a crucial aspect of SBMA pathogenesis (Katsuno et al. 2002, 2003; Takeyama et al. 2002). One agent, leuprorelin, was found to be very effective in rescuing all aspects of the disease phenotype. Leuprorelin is a luteinizing hormone releasing hormone agonist that reduces the release of testosterone from the testis and thereby reduces circulating levels of testosterone. This result supports the concept that ligand-induced nuclear translocation of mutant AR is a crucial aspect of SBMA pathogenesis. The second drug used to treat the SBMA mice, flutamide, had no effect on motor neuron disease. Flutamide has a very high affinity for AR and functions as an androgen antagonist by competitively competing with testosterone for binding to AR. Although flutamide suppresses the androgen-dependent transactivation activity of AR, it neither reduces plasma testosterone nor blocks nuclear translocation of mutant AR in either the mouse or the fly. Besides providing an important mechanistic insight into SBMA, this result suggests that leuprorelin is a candidate for the treatment of SBMA. Huntington’s disease (HD) HD is the most prevalent polyglutamine disease. In addition to motor symptoms (chorea), HD often presents with cognitive changes that include memory deficits, depression and changes in personality. Pathologically HD is characterized by the loss of medium-sized spiny GABAergic neurons from the striatum. There is also a loss of cortical neurons that project to the striatum. Transgenic mice expressing a small N-terminal fragment of huntingtin with an expanded polyglutamine tract develop abnormal neurological signs and neuropathology (Mangiarini et al. 1996). This result led to the hypothesis that in HD pathogenesis a crucial step is the proteolytic production of a toxic N-terminal fragment from intact mutant huntingtin. This concept is still widely held with considerable effort directed at understanding the proteolytic processes that act upon huntingtin (Tarlac and Storey 2003.). These mice were also instrumental in bringing to the forefront the inclusion/ aggregate model of HD pathogenesis, as well as the pathogenesis of polyglutamine diseases in general. After much study, the concept that large inclusions/aggregates of a mutant polyglutamine are the pathogenic species seems to have fallen into disfavor (Arrasate et al. 2004).


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In regards to specific cellular pathways that might be targets of the toxic gain-of-function property of mutant huntingtin (either intact or an N-terminal fragment), three have gained the most attention: deregulation of transcription (Sugars and Rubinsztein 2003), disruption of intracellular trafficking (Gunawardena and Goldstein 2005), and a change in mitochondrial/energy metabolism (Browne and Beal 2004). An interesting point where a role of huntingtin in cytoplasmic transport and transcription deregulation might intersect is centered on the function of brain-derived neutrophic factors (BDNF). BDNF is a prosurvival factor for striatal neurons and its levels have been found to be decreased in HD brains (Ferrer et al. 2000). Two nonexclusive mechanisms that have been suggested for this decrease are a mutant huntingtin-induced decrease in BDNF transcription (Zuccato et al. 2001) and an induced transport deficit of BDNF vesicles (Gauthier et al. 2004). More recently a correlation of [ATP/ADP] with the length of the polyglutamine tract in intact huntingtin was reported (Seong et al. 2005). An inverse association throughout the range of wild-type and mutant polyglutamine tract lengths was found. This observation suggests that variation in the length of the polyglutamine tract in huntingtin is by some mechanism a regulator of the energy status of striatal neurons. If confirmed, this relationship may underlie the vulnerability of medium-sized spiny striatal neurons to excitotoxicity, which would have important implications for HD pathogenesis. Spinocerebellar ataxia type 1 (SCA1) SCA1 is one of several inherited forms autosomal dominant ataxia. Typical of most ataxias, SCA1 consists clinically of gait ataxia dysarthria and bulbar dysfunction, with death usually ocurring between 10 and 15 years after the onset of symptoms. Despite the protein, ataxin-1, being widely expressed in the CNS, the most frequently seen and most severe pathological alterations are restricted to loss of Purkinje cells in the cerebellar cortex, as well as loss of neurons in the inferior olivary nuclei, the cerebellar dentate nuclei and the red nuclei. With the identification of an expanded polyglutamine tract as the mutational basis for several neurodegenerative disorders, a pathogenic mechanism largely dependent on the biochemical property of the polyglutamine tract itself gained quick favor. In contrast, two experiments utilizing SCA1 transgenic mice showed that amino acid residues outside of the polyglutamine had a crucial role in pathogenesis. In the first example an amino acid substitution was made in the nuclear localization signal of ataxin-1, such that the protein could no longer be translocated into the nucleus. When this substitution was placed into a mutant allele of ataxin-1 with an expanded polyglutamine tract that was then used to generate transgenic mice, the mice failed to develop disease (Klement et al. 1998). Thus by restricting the subcellular

distribution of mutant ataxin-1 to the cytoplasm of susceptible neurons the protein was no longer pathogenic. Perhaps a more dramatic illustration of the importance of ‘host’ protein sequence for pathogenesis was shown recently when a site of phosphorylation of ataxin-1 was identified, the serine at position 776 (Emamian et al. 2003). Replacing this serine with an alanine yielded a protein that still was transported to the nucleus, but when transported in a mutant ataxin-1 with 82 glutamines failed to cause disease. Spinocerebellar ataxia type 3 (SCA3)/Machado Joseph disease (MJD) SCA3, also known as MJD, is the most common of the autosomal dominantly inherited ataxias. SCA3 has several genetic features that distinguish it from many of the other polyglutamine disorders. In contrast to HD and SCA1, where the repeat threshold for mutant alleles is near 40, in SCA3 the repeat threshold for mutant alleles is longer than 50 repeats. Moreover, although other polyglutamine disorders behave as pure dominant diseases, SCA3/MJD homozygous patients have a more severe disease presentation than individuals with only a single mutant allele. Ataxin-3, the SCA3-encoded polyglutamine protein, is a polyubiquitin binding protein by virtue of its ubiquitin interaction motifs (UIMs) located close to the polyglutamine tract (Burnett et al. 2003; Chai et al. 2004). Ataxin-3 also has ubiquitin protease activity and interacts with components of the proteasome complex. These aspects of ataxin-3 provide strong evidence that this protein normally functions in the ubiquitin-proteasome pathway. Recently in a Drosophila model of SCA3/MJD it was shown that wild-type ataxin-3 is a suppressor of polyglutamine-induced neurodegeneration (Warrick et al. 2005). This work highlights further the importance of the link between protein-folding/ clearance pathways and neurodegeneration. Summary and conclusions

Each of the diseases briefly reviewed in this article share pathogenic mechanisms within their class, and therefore it is likely that developing an understanding of one member of the class will help more generally with the entire class of diseases. However, there are similarities between classes of disease: most obviously between the tauopathies and the synucleinopathies where there are both genetic parallels and overlap between the mechanisms by which they can be initiated. In all of these diseases, the process of protein deposition, if not the deposits themselves, appear to play a key role in initiating the disease, suggesting that at a deeper level all of these diseases may share similarities. A recent suggestion is that there are oligomeric intermediates with similar properties that mediate toxicity in all of the diseases (Glabe 2006). Despite the enormous amount of progress we have made in terms of understanding the etiologies of these diseases in


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the last 15 years, the most important questions remain unanswered. What are the mechanisms of cell death in these diseases? Why does each disease preferentially affect certain neuronal types? Why do diseases have onset ages from middle to late life? and of course, most importantly, can we realize the promise of knowledge-based treatments for these devastating disorders? Much has been done, but much remains… Acknowledgements JH is part of the intramural NIA/NIH program. HO is supported by the NINDS.

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