Huntington’s Disease
Running head: REVIEW AND PROPOSAL
Huntington’s Disease: Past, Present, and Potential Future Analyses
Nicholas E. Glass
Creighton University
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Huntington’s Disease
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Huntington’s Disease: Past, Present, and Potential Future Analyses
To date, research efforts have produced substantial information regarding the causes of
Huntington’s Disease (HD). But despite this proliferation of knowledge, the subcellular
mediators of HD remain its most poorly understood causes. Consequently, our understanding of
what leads to HD is still too rudimentary to proffer any kind of curative therapy. And given the
devastating nature of HD, this inadequacy in our understanding is, though itself understandable,
quite unacceptable.
Therefore, this paper will first address the known features of HD. Explanation of the
known causes of HD will follow. The paper will then define the HD problem and propose a
course of investigation which may help to solve it.
The State of the Science
Regarding the Disease
HD is perhaps most remarkable for the gradual but relentless debilitation and eventual
death which it entails. Movement disturbances are usually the first manifestations of HD that
carriers recognize (Blumenfeld, 2002). Among these is Huntington’s chorea, the motor hallmark
of the disease. Banich (1997) characterizes HD chorea as “a variety of jerky movements that
appear to be well coordinated but are performed involuntarily and ceaselessly in an irregular
manner” (p. 151). During the early stages of HD, the choreiform movements may affect only
parts of limbs, but eventually most of the body, including whole limbs, the head, face, and trunk
will become involved and all movement will be uncontrollable (Banich, 1997). HD sufferers also
have difficulty initiating and executing movements, especially voluntary eye movements (Banich,
1997; Blumenfeld, 2002). This motor difficulty is known as bradykinesia or akinesia depending
on whether the intended movements are slowed or stopped, respectively. At later stages, HD
patients become unable even to walk, speak, or swallow (Banich, 1997; Blumenfeld, 2002).
The effects of HD are by no means restricted to body movement. With respect to mental
processes, HD usually impairs memory first, then personality, mood, affect, and attention,
eventually resulting in severe dementia marked by the virtually complete absence of emotional
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and intellectual functioning (*, *; Cotran, Kumar, & Collins, 1999). The severe memory loss
coincident with late-stage HD can be categorized as temporally extensive flat retrograde amnesia,
since memories for both the recent and remote past are affected to a similarly profound extent
(Banich, 1997). Moreover, depression, anxiety, obsessive-compulsive disorder, impulsive or
destructive manic behavior, and even psychosis have been among the psychiatric symptoms
brought on by HD (Blumenfeld, 2002).
Along with the vicious quality of the impairments inflicted by HD, the disease is also
alarming from the standpoint of how many people it impairs. In America alone, HD afflicts
approximately 30,000 individuals, with almost twice as many more people carrying the gene
asymptomatically, i.e. without yet suffering from HD symptoms (Reiner, Dragatsis, Zeitlin, and
Goldowitz, 2003). Of course, many thousands more people around the world also have HD in its
latent and manifest forms. Unfortunately, accurate estimates as to the number of HD sufferers
and carriers worldwide have been unfeasible for several reasons, the foremost being that data for
Africa and Eastern Asia are lacking (Harper, 1992). A second major reason is that the numbers of
people affected by HD can fluctuate rapidly and significantly due to global immigration and
travel, and to the mode of transmission of HD (Harper, 1992). Nonetheless, the newest data
affirm that populations of European descent have an unusually high prevalence of HD (4-8 per
100,000) and that HD frequency is high in India and parts of central Asia but surprisingly low in
Finland and Japan (Harper, 1992). A standard estimate of worldwide HD incidence independent
of demography is 5 to 10 victims for every 100,000 people (*, *; Li et al., 2003). Other sources
propose much lower frequencies, such as 1.6 cases per million (Banich, 1997) or 4 to 5 cases per
million (Blumenfeld, 2002). By all accounts, though, HD is a relatively rare condition.
Granted the scarcity of HD, epidemiological statistics are negligible consolation to people
who have received the veritable death sentence that is a HD diagnosis. No matter how rare the
condition may be, it still occurs. And when it does, it amounts to an inexorable loss of control,
identity, and life. Not surprisingly, then, patients who grasp this reality are significantly more
likely to commit suicide, attempt suicide, or be hospitalized for psychiatric reasons. Factors
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associated with suicide, suicide attempts, and psychiatric hospitalization include unemployment
and a psychiatric history ≤5 years before being diagnosed. Out of all possible factors, though, the
strongest impetus for despair seems to be the actual onset of symptoms (Almqvist et al., 1999).
With regard to onset of symptoms, the precise age at which this occurs in HD is a matter
of moderate disagreement, since certain reports conflict as to when HD symptoms normally
appear. The general consensus is that onset is most common in the fourth and fifth decades of life
(Cotran et al., 1999). However, Klug and Cummings (2002) state that “symptoms usually appear
in the fifth decade of life” (p.624). It is unclear, though, whether they are referring to initial
symptoms or the more progressive symptoms. Much less ambiguously, Li et al. (2003) claim the
mean age of onset to be 38 years. Importantly, a number of sources point out that HD symptoms
can appear at any age (*, *; Glass, Van Dellen, Blakemore, Hannan, & Faull, 2004). Considering
all of the aforementioned accounts, the reported age of onset for HD seems to be dropping as
time passes. Possible causes for the apparent decrease in reported age of onset include increased
awareness and vigilance on the part of medical practitioners dealing with initial HD symptoms.
Advances in and greater utilization of, diagnostic technologies such as CT and MRI might have
contributed as well.
Inconsistency is even more appreciable among reports of life expectancy following
diagnosis. According to older literature, the majority of HD victims die within 10 to 15 years
after the onset of symptoms (Banich, 1997; Klug & Cummings, 2002). Blumenfeld (2002)
similarly puts the median survival after the onset of initial symptoms at 15 years. The most recent
literature, however, sets typical life expectancy closer to 20 years (Li et al., 2003; Reiner et al.,
2003). Treatment for symptoms (as there is no treatment for the condition per se) may play a
marginal role in the apparent increase in reported life expectancy. Additionally, similar to the
discrepancy in reported age of onset, the variation in reported life expectancy might be traceable
to the attainment of timelier diagnoses.
With the HD gene having been identified, the potential for timely and accurate diagnosis
is unprecedented. In fact, it is now possible to diagnose a predisposition to HD in utero. (This
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possibility raises immense concerns about whether people should undergo genetic testing for
presently incurable diseases. These concerns, however, are the topic of another paper.) This
possibility exists because HD is inherited in an autosomal dominant fashion and has complete
penetrance (Blumenfeld, 2002; Sugars & Rubinsztein, 2003). In other words, an individual with
the HD mutation will predictably develop HD regardless of (1) his or her sex, (2) the sex of the
parent from whom a mutant allele was inherited, and (3) whether one or both inherited alleles are
of the mutant form (Klug & Cummings, 2002). The specific autosome on which the gene
responsible for HD resides is chromosome 4. More precisely, it was located at 4p16.3 through
the use of restriction fragment length polymorphisms (The Huntington’s Disease Collaborative
Research Group, 1993).
Transmission of HD, while adhering to the classical model of complete autosomal
dominance, is not without some idiosyncrasies. Specifically, with respect to HD being
autosomally rather than sex chromosomally linked, it is notable that children of fathers bearing
the HD mutation are prone to developing symptoms earlier. This susceptibility is due to the
phenomenon of genetic anticipation, which will be discussed later (Cotran, Kumar, & Collins,
1999; Klug & Cummings, 2002).
Another interesting idiosyncrasy pertains to homozygous individuals, both of whose
alleles are of the mutant form and so predispose them to HD. (Such individuals are exceedingly
rare in any given population, since they can only be born to two parents who both have the gene
for HD.) The expected clinical course of these individuals is the subject of pointed disagreement.
Squitieri et al. (2003) demonstrated that once the characteristic HD degeneration began, it was
more severe in a sample of eight homozygotes as compared to a group of 75 unrelated
heterozygotes. On the other hand, Alonso et al. (2002) observed a single sibling pair consisting of
one hetero- and one homozygote. In their study, the age and type of symptoms at onset were the
same for both siblings, but the disease had a more severe progression in the heterozygote. If a
statistically significant difference between hetero- versus homozygous disease severity can be
firmly established, it will have considerable mechanistic implications.
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Regarding the Mechanisms of the Disease
Even when speaking only of what is already verifiably known about the processes that
give rise to HD, the means by which HD takes its effects are rather complicated. The nature and
number of interacting levels can be daunting. But they are also comprehensible, provided the
appropriate systematic inspection.
In attempting to guide such an inspection, I will present findings and theories in order of
ascending material magnitude. That is, the causes of HD will be presented first at the level of the
involved genes, then at the levels of involved proteins, organelles, and, lastly, cells and tissues.
Genes
The genetic mutation associated with HD is only responsible for the disease insofar as the
gene encodes an abnormal form of a protein. The protein in question is known as huntingtin (Htt)
and will be discussed at length in the next section. The gene of interest encodes abnormal Htt
because of its own abnormality. Specifically, the problematic HD gene contains an excessive
number of repeats of the trinucleotide sequence CAG (cytosine-adenine-guanine) (Bolivar,
Manley, & Messer, 2003; Squitieri et al. 2003). Normal Htt-encoding genes have fewer CAG
repeats along a particular stretch of exon 1—the first segment of DNA expressed in protein form
following translation of mRNA (Sugars & Rubinsztein, 2003). There is approximate agreement
as to the number of repeats that counts as excessive. The most seminal studies on the topic assert
that people with fewer than 35 repeats do not develop HD, while those with 35-39 repeats have a
heightened risk of developing the disease, and that people with 40 or more repeats will develop
HD within a normal lifetime (Bates, 2003; Huntington’s Disease Collaborative Research Group,
1993). Some later sources truncate this assessment, stipulating that possession of any more than
35 CAG repeats is sufficient for HD (Li et al., 2003; Reiner et al., 2003; Sugars & Rubinsztein,
2003).
However long a pathological repeat expansion may be, it is a result of genetic
anticipation, as noted previously. Anticipation, in the somewhat peculiar genetic sense, is “the
phenomenon of a progressively earlier age of onset and increasing severity of symptoms for a
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genetic disorder in a successive generation” (Klug & Cummings, 2002, p. A-18). In the particular
case of HD, the anticipation stems from erroneous DNA replication during spermatogenesis, the
process whereby male gametes are produced (Cotran et al., 1999). This conclusion can be made
because HD-positive sons sometimes have twice or three times as many CAG repeats as their
HD-positive fathers, while other HD-positive sons exhibit a CAG complement very similar to, if
not congruent with, that of their HD-positive mothers (*, *). On the topic of anticipation, it bears
mentioning that HD is one of a dozen genetic disorders that affect the nervous system by means
of mutant expansion of normal trinucleotide repeats (Campbell, Reece, & Mitchell, 1999). Other
disorders of this kind include Fragile X syndrome and Myotonic dystrophy, the most common
adult form of muscular dystrophy (Klug & Cummings, 2002).
As with these other triplet-expansion-based disorders, the severity of HD is related to the
extent of the expansion. Basically, age of onset and life expectancy, the cardinal measures of HD
severity, can be thought of as functions of the trinucleotide expansion. Both of these quantities
vary inversely with the degree of expansion. This relationship would imply that a greater number
of repeats induces a younger age of onset and a shorter life expectancy. And indeed, this
prediction has been borne out repeatedly in human HD patients (*, *; Blumenfeld, 2002;
Squitieri et al., 2003) as well as in mouse models (Bolivar et al., 2003; Klug & Cummings,
2002).
Proteins
To reiterate, the protein of primary concern in HD is huntingtin (Htt). The fact that its
presence (in either normal or mutant form) is necessary for extraembryonic membrane function
and hence embryonic development has been demonstrated by numerous studies with knockout
and transgenic mice (Cotran et al., 1999; Reiner, 2003). Furthermore, instrumental roles for Htt
have been discovered in several other crucial functions, namely both the morphogenesis (i.e.
arrangement) and survival of neurons in the adult forebrain (i.e. cerebrum, thalamus, and
hypothalamus) (Reiner et al., 2003). Studies are currently underway to more fully elucidate
patterns of Htt’s ordinary action in both neuronal and nonneuronal signal transduction
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(MacDonald, 2003). It seems we may only be realizing a fraction of the entire range of functions
performed by Htt in its normal capacity.
In its abnormal capacity, when at least one Htt gene is sufficiently expanded, there are
even more intricacies to consider. Chief among these are the CAG repeats discussed above. The
CAG repeats are translated into repeats of the amino acid glutamine (Q) (*, *; Glass et al., 2004).
The resultant polyQ segment is what wreaks cellular havoc in HD (Glass et al., 2004; Klug &
Cummings, 2002). And, like the trinucleotide repeats from which they arise, the lengths of these
polyQ tracts can reliably predict the severity of HD that will be experienced by the individuals
possessing them (*, *; Bolivar et al., 2003; Cotran et al., 1999).
Exactly how the polyQ region generates the phenotypic symptoms of HD is not entirely
known, although we do have evidence to support several ideas about what transpires. For
instance, there are many credible assertions that mutant Htt, normal Htt, and fragments of either
type of Htt tend to group together and form aggregations (Cotran et al., 1999; Li et al., 2003;
Zhou et al., 2003). The significance of these aggregations can best be seen in light of their actions
relative to the subcellular structures which make up their environment.
Organelles
The nucleus was the first organelle to be included in hypotheses about the pathogenesis of
HD. It attained this status because the Htt aggregations described above have shown a propensity
for being transported into the nucleus (Saudau, Finkbeiner, Devys, & Greenberg, 1998; Zhou et
al., 2003). Htt and/or Htt fragments containing the polyQ repeat are transported into the nucleus
as part of so-called intranuclear inclusions (Cotran et al., 1999). These inclusions, in turn, were
found to coincide with increased apoptosis, i.e. programmed cell suicide, the eventual precipitant
of HD (Saudou et al., 1998). This observation suggested that the inclusions might be critically
interfering with transcription within the nucleus (Trushina et al., 2003). However, by using
cultured neurons of the kind involved in HD, it was determined that preventing nuclear
importation of the inclusions did not prevent apoptosis (Saudou et al., 1998). On the contrary,
mutant-Htt-induced cell death increased with suppression of nuclear Htt inclusions (Saudou et
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al., 1998). Hence there must be other at least one other subcellular structure besides the nucleus
that interacts with Htt.
And in fact the mitochondria do interact heavily with Htt. As the headquarters of essential
apoptotic enzymes, the mitochondria have necessarily intimate biochemical exchanges with
mutant Htt aggregations (Li et al., 2003; Zhou et al., 2003). The precise natures of all of these
interactions remain undetermined, although a significant amount of insight has been compiled.
This insight is partially reflected in Figure 1, which tabulates a variety of molecules according to
how they are believed to interact with Htt (Sugars & Rubinsztein, 2003).
Another collection of subcellular structures with some at least hypothetical connection
with Htt are microtubules. Trushina et al. (2003) recently made a convincing case that
destabilization of microtubules (a component of the cellular frame, in effect) is a necessary
condition for mutant Htt to exert its toxic effects. This study was the first to invoke such a central
role for microtubules in the molecular pathology of HD. So naturally, many new questions arise
from it, not the least of which is whether the results will be replicated. The replicability has to be
questioned because the conclusions of the study rely pivotally on data obtained from cultured
cells, which are widely acknowledged as exhibiting erratic behavior in response to even minute
alterations to the culture medium.
Cells and Tissues
The types of cells that undergo apoptosis and are thus implicated in HD are of course
neurons. The principally involved neurons, however, are located in the caudate nucleus and
putamen, which jointly comprise the striatum of the basal ganglia (Banich, 1997). The initial
symptoms of HD correspond to a circumscribed kind of cerebral atrophy, wherein the caudate
nucleus is especially affected (Coltran et al., 1999). Within the striatum and, to a lesser extent,
the adjacent globus pallidus, the medium-sized spiny neurons that use GABA or acetylcholine
are especially vulnerable to Htt-induced apoptosis (Banich, 1997; Coltran, 1999). Due to the
apoptosis of these neurons, a particular neural pathway is interrupted. The indirect pathway
(which synapses on the external globus pallidus from the striatum as part of a circuit connecting
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the cortex, basal ganglia, and thalamus) becomes underactive (Banich, 1997). This underactivity
subsequently allows the direct pathway to overstimulate the internal globus pallidus (Banich,
1997). The overstimulated internal globus pallidus in turn allows the thalamus to overstimulate
physical activity, thus yielding the chorea and other disturbances characteristic of HD
(Blumenfeld, 2002). And as the cortex begins its own irreversible atrophic course, the disease
progresses toward its usual culmination in global dementia (Banich, 1997).
Research Proposal
Questions in Need of Address
Knowing the aforementioned mechanisms by which HD arises, we would do well to
formulate some questions, the answers to which could complement what is known and thereby
lead to more and better treatment options. The ultimate overarching question seems to be: Why,
at the age of about 38 years, would GABA-ergic and cholinergic neurons in the striatum begin
undergoing apoptosis as a result of polyQ aggregates that had been made there since before birth?
This question can be subdivided into the following: (1) Why would GABA-ergic and cholinergic
neurons ever undergo apoptosis? (2) Under what conditions can polyQ aggregates trigger
apoptosis? (3) Why do the GABA-ergic and cholinergic neurons die off before the other types of
neurons? (4) Might normal Htt have some properties that are more pronounced or enhanced due
to the polyQ extension? (5) What biochemical events typically occur at the age of about 38 years
that might make a cell more prone to apoptosis? (6) Is there a way to disassemble or deactivate
polyQ aggregates in vivo? (7) Could there be a way to safely prevent the initial aggregation of
polyQ segments? (8) What is/are the means of disposal of polyQ aggregates? (9) What are the
effects of polyQ aggregates on other cellular proteins? (10) Is there a way to locally inhibit
apoptosis in the striatum?
Approaches to Answering the Above Questions
A myriad of tools and strategies could prove useful in enabling response to the above
queries. In any case, I would approach them as follows. (Due to length restrictions not all
questions are addressed.)
Huntington’s Disease
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This question would be best addressed in a separate and thorough literature review, where
due consideration can be given to caspases and their regulation.
2.
This question could be answered by setting up a cell culture assay. An optimal cell line
would have a known number of CAG or polyQ repeats, and controllable transcription of the Htt
gene. One could then administer variable quantities and combinations of other chemical factors
endogenous to the intra- or extracellular environment of the striatum. If this method were less
than practical in terms of introducing foreign factors into the cells, other genetic vectors or
engineered endosomes could be utilized. Alternatively, DNA microarray analysis could probe
differences between the genes expressed by apoptotic and surviving cells.
3.
Given enough documented knowledge of the upstream precursors and downstream
metabolites of GABA and acetylcholine, one could proceed as described for (2) above, with the
modification that the chemical factors being introduced are a controlled set of precursors and/or
metabolites of GABA and/or acetylcholine.
5 & 8. An answer to these questions may already have been very fortuitously provided. Zhou et
al. (2003) report that toxic polyQ fragments accumulate and aggregate more readily when
proteasomes (protein-degrading bodies within cells) become inactive the way they normally do
when organisms and their constituent cells get older. In conjunction with this study, the findings
of Squitieri et al. (2003) strongly suggest that until a certain age, the proteasomes are responsible
for staving off HD. It would seem that prior to the critical age, proteasomal resources of cells are
capable of degrading the polyQ aggregations produced by either the hetero- or homozygous HD
genotype. But after the proteasomal decline that marks a particular age, the two-fold greater
production of mutant Htt by homozygous cells could induce the more rapid degeneration of
homozygous HD patients reported by Squitieri et al. This line of research should be especially
beneficial because it invites plausible theories of how to boost neural proteasome activity,
possibly with a pill or an injection. And in the event that such a prophylactic breakthrough is
made, HD could conceivably become a thing of the past. On top of that, there might be some
poetic justice in successfully countering genetic anticipation with clinical anticipation.
Huntington’s Disease
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As indicated before, Sugars and Rubinsztein (2003) offer a synopsis of the effects of
polyQ aggregates on a variety of other important proteins (See Figure 1). Likewise, Figure 2
shows a currently valid (though almost certainly incomplete) reckoning of certain events that are
likely to mediate HD presentation (Bates, 2003). So essentially, this question amounts to an
appeal for the continuation of polyQ interaction research.
But regardless of the exact methods used and avenues of inquiry followed, it is only in
answering the kinds of questions posed above that hope exists for finding the elusive knowledge
required for curing illness.
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References
*. (*). [The textbook that had the “Chapter 3: What are the units of brain function?” excerpt in
our first set of readings.]
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Huntington’s Disease
Figure 1. Taken from Sugars and Rubinsztein, 2003.
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Huntington’s Disease
Figure 2. Taken from Bates, 2003.
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