The Role Of Polyglutamine Repeats In Huntingtons Diseases Biology Essay

Huntington ‘s diseases ( HD ) is a annihilating autosomal-dominant, progressive neurodegenerative upset whose clinical features includes progressive cognitive damage, choreiform motion, dystonia, psychiatric perturbation and dementedness. Pathologically, HD is characterized by a prevailing loss of nerve cells in the striated and cortical countries of the encephalon with the formation of neural inclusions. ( Imarisio, S et al. , 2008 ; Walker, F.O, 2007 ) This upset had been antecedently reported, nevertheless the initial description of the disease was from that of George Huntington, a medical practician of Pomeroy, Ohio, in 1872. ( G.Huntington, 1872 ) HD is categorized to be in the same household of neurodegenerative disease that is genetically distinguishable with a gain-of-function that is caused by CAG repetitions mutant. This household of disease includes dentatorubralpallidoluysian wasting ( DRPLA ) , SBMA and spinocerebellar ataxy 1,2,3,6,7 and 17. ( SCA1/2/3/6/7/17 ) ( Zoghbi & A ; Orr. , 2000 ; Nakamura, K. et al. , 2001 ) HD mutant is caused by enlargement of CAG repetitions of more than 36 in the first coding DNA of huntingtin ( Htt ) cistron on chromosome 4p16.3 ( MIM ID *613004 ) which is translated into a polyglutamine ( PolyQ ) repetition near the amino end point of Huntingtin protein which is a ubiquitously expressed ctyoplasmic protein. ( The Huntington ‘s Disease Collaborative Research Group, 1993 ) ( Rubinsztein et al. , 1996 ) studied a immense group of persons whom carried 30 to 40 CAG repetitions in their Htt Gene. Using the PCR method which allowed the scrutiny of CAG repeats merely, consequences show that persons with 35 or fewer CAG repetitions had no clinical manifestation of HD while most single with 36 to 39 CAG repetitions were clinically affected. Persons with 35 CAG repetitions show no penetrance of the disease whereas persons with 36 to 39 shows uncomplete penetrance. When the repetitions reach above 40 or more the disease is to the full penetrance. The length of the CAG repetitions correlates reciprocally with the age of oncoming of HD. While juvenile HD ( JHD ) characterized with enlargement of more than 60 repetitions and an age of oncoming before 20 old ages old. JHD is normally inherited paternally. This reappraisal will discourse the function of Polyglutamine repetitions in Huntington ‘s disease which ranges from heritages form, age of oncoming associating to CAG repetitions and pathogenesis construct of HD.

Clinical Feature

Huntington ‘s Disease is one of the chronic neurodegenerative upsets that affect the cardinal nervous system. ( Gusella and Macdonald. 1995 ) Its classical marks are that of motor symptoms, cognitive symptoms, psychiatric symptoms ( Kirkwood et al. , 2001 ) and metabolic symptoms. ( Van der Burg et al. , 2009 ) It is found that motion troubles are linked with both nonvoluntary and voluntary motion, which conditions worsens increasingly. One of the most common clinical manifestation of motor symptom is Chorea ; arrhythmic nonvoluntary motion superimposed on a purposeful act. ( Cummings JL, 1995 ) Badness of chorea additions during the initial old ages of HD but is subsequently replaced by bradykinesia and rigidness ( Craufurd, 1996 ) . Patients develop rigidness as disease progresses, and its more common in Juvenile oncoming of HD. Behavior abnormalcies comes along with personality alterations, anxiousness and crossness ( James et al.,1994 ) . At the terminal phase of HD, patient ‘s mental capablenesss become slower and dementedness is being develop. This is normally characterized with hapless concentration, inefficient usage of memory and damage of executive maps such as forming, planning, look intoing and acquisition of new motor accomplishments. ( Craufurd, 1996 ; Rubinsztein et al. , 2003 )

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Molecular genetic sciences of Huntington ‘s disease

The characteristics of mutant in the HD cistron elucidated many of the trademarks of HD which includes the opposite relationship of age of oncoming correlating with the repetitions length and Juvenile HD linkage to meiosis in spermatogenesis doing enlargement in CAG repetitions. ( Myers et al. , 1998 ; Patrick et al. , 1999 ; Yvon et al. , 1994 ) The Homo sapiens HD cistron is located on the chromosome 4p16.3 by manner of findings from linkage analysis followed by positional cloning in 1993. The HD cistron is besides known as IT-15 cistron, consists of 67 coding DNAs. It encode for a protein of 350kda. With an addition enlargement of the nucleic acids CAG in the coding part of the first coding DNA coding part for the glutamine residues ( polyQ ) followed by two short stretches of prolines causes the disease. ( The Huntington ‘s Disease Collaborative Research Group, 1993 ) Normally the back-to-back repetitions of CAG that codes for glutamine in a healthy person is repeated about 10-29 ( average, 18 ) . While on the other manus, HD patients have an increased in repetitions figure of 36 to 121 ( average, 44 ) . There are indicants that length of the polyglutamine repetition sequence is reciprocally correlated with the age of disease oncoming. ( Kremer et al. , 1994 ) Another surveies shows four CAG repetitions size interval associated with changing disease hazard in HD ( Figure 1 ) . ; 1 ) Normal, 2 ) Meiotic instability scope, 3 ) Reduced penetrance scope and 4 ) Pathogenic scope

Normal scope is where allelomorphs with 26 or less CAG repetitions. This is non related to disease in any instance and farther adding on that these allelomorphs have ne’er been expanded and causes disease in the following coevals.

Meiotic instability scope is where allelomorphs with 27-35 CAG repetitions in HD cistron which do non take develop the disease. However theses allelomorphs may demo meiotic instability which may increase the figure of repetitions and causes disease in the following coevals. ( Telenius et al. , 1995 ) An illustration would be a 27 CAG repetitions in a healthy male parent has been reported to spread out to 38 repetitions in his morbid boy ( McGlennan et al. , 1995 )

Reduced Penetrance scope or intermediate scope is where lone portion of the persons with 36-39 CAG repetitions will develop HD. The repetition in this scope do no show full penetrance ( Rubinsztein et al. , 1996 ) . As the CAG repeats figure additions, the penetrance besides increases. This sort of decreased penetrance is an utmost instance, which implies that familial background like familial qualifier and environmental factors could be portion of the disease oncoming.

Pathological scope is persons with more than 40 CAG repetitions. These persons will develop HD if they live long plenty till the disease oncoming. There will be a higher opportunity of inheriting the disease to the following coevals.

With the addition in CAG repetitions enlargement can do earlier oncoming and patterned advance of the disease. In comparing with patients with less CAG repeats enlargement shows their first symptoms in the ulterior portion of their lives. The expanded CAG repetitions encodes for a polyglutamine piece of land in the huntingtin protein which is widely expressed within the cardinal nervous system and in extra-neural tissues. Huntingtin protein is expressed much more in nerve cells than in glial cells. With the accretion of proteolytic huntingtin protein fragments and their collection will trip a cascade that leads to increasing neural disfunction through oxidative hurt, transcriptional dysregulation, glutamate exitotoxicity ( Jarabek et al. , 2004 ; Li et al. , 2003 ) , apoptotic signals, mitochondrial disfunction and energy depletion ( Beal and Ferrante, 2004 ) . However, these above alterations are accompanied by neurochemical changes besides affect on other receptors, such as the Dopastat ( DA ) and adenosine receptors involved in motor maps and non entirely on glutamate receptors. ( Ferre et al. , 1993 ; Pavase et al. , 2003 ; Teunissen et al. , 2001 ) in the look of the same mutant. ( Wells and Warren, 1998 )

Figure 1. CAG Repeats size with association in Risk of Huntington ‘s Disease. Adapted from ( Wells et al. , 1998 )

Repeats Instability

HD cistrons are extremely unstable as CAG repeats shows enlargement and contraction in figure of repetitions over coevalss. There are several factors which may impact the size and way of the unstable repetitions. The sex of the affected parent plays a portion. As in maternal transmittal, repetitions on offspring shows about equal figure or enlargement and contraction are besides observed. However in paternal transmittal there are more frequent enlargements and there is an addition in Numberss of repetitions. Furthermore meiotic instability and grade of instability have been described within bodily tissues of HD patients ( Telenius etal. , 1994 ) Accounting for repetition instability there are three general mechanisms propose ; 1 ) Misalignment with subsequent deletion fix, 2 ) slippage during DNA reproduction and unequal crossing over and 3 ) recombination Slippage-mediated length alteration during DNA reproduction. These could be usage to better explicate the addition in figure of repetitions in HD ( Richards and Sutherland, 1994 )

Age of oncoming

A strong opposite correlativity between age of oncoming and CAG length have been review ( Brinkman etal. , 1997 ) . The length of CAG repetitions in the HD cistron is said to be the most of import factor impacting the age of oncoming which accounts for 70 % of the fluctuation in the age of oncoming ( Andrew et al. , 1993 ) . However such wide scope of oncoming can be link to figure of CAG repetitions, itself entirely can non be usage to foretell the age of disease oncoming ( Sneel et al. , 1993, Rubinsztein et al. , 1996 ) . A 2nd factor which affects the age of oncoming is the sex of the parent. ( Rubinsztein et al. , 1996 ) shows that JHD patients are more likely to inherit the disease from their male parent and patient with late age of onset normally from their affected female parent ( Hall et al. , 1983, farrer et al. , 1992 ) . Recent surveies besides shows that other cistrons besides modify the age of oncoming in HD. For exaple In ( Elahe et al. , 2009 ) shows linkage of PCG-1 alpha suppression by mutant htt protein taking to transcriptional deregulating and mitochondrial disfunction in HF. A polymorphism within noncoding DNA 2 shows a statistically important variableness in the age of oncoming. It farther suggests PGC-1 alpha has modifying effects on the infective procedure of HD. ( Rubinsztien et al. , 1996 ) reported some HD patients with 36 repetitions and some really old persons with 36-39 repetitions but have no recognizable HD symptoms which indicates that disease is non ever to the full penetrant. The definition of penetrance is the proportion of persons with a specified genotype who present the expected phenotype in their expected lifetime. Thus the penetrance of HD additions with the figure of repetitions with 36-39 as intermediate scope fig1 and above 40 repetitions as complete penetrance. Due to spread out repetitions sequences, repetitions instability in HD has a distinguishable characteristic of familial expectancy as symptoms appear at earlier ages with greater badness in following consecutive coevals. Particularly in paternal transmittal, as it passes through the germ line, the repetitions Numberss additions and consequence in a much earlier age of oncoming of HD which can take to JHD ( Jennings, 1995 )

Juvenile HD VS Classical HD

Juvenile HD ( JHD ) is represented normally by a big enlargement of beyond 60 CAG repetitions with disease onset before 20 old ages old. ( Harper,1991 ) surveies show 70 % to 80 % of JHD is due to paternal transmittal of expanded repetitions. Large mutant repetitions above 80 – 100 CAG repetitions are rare and histories for 5 % of JHD instances ( Cannella et al. , 2004 ) and may ensue childish HD with disease onset 10 old ages earlier which has a peculiarly annihilating patterned advance rate ( Squitieri et al. , 2002, 2003b ) . Beside before onset, they besides presented a peculiar phenotype which showcase larning jobs, rigidness, dystonia and ictuss ( Nance et al. , 1999 ; Cannella et al.,2004 ) . JHD big enlargement of CAG causes a addition of map and shows association in altered energy metamorphosis in peripheral tissues ( Sawa et al. , 1999 ; Lodi et al. , 2000: Panov et al. , 2002 ) . An addition in protein

Table 1

Neurological and psychiatric symptoms in Huntington ‘s diseaseSymptoms manifested from grownup and juvenile HDAdditional symptoms preponderantly manifested in childish HDBehavioral abnormalcies

Depression and psychosis


Cognitive change

Eyemovement abnormalcies

Canine chorea




Memory loss

Gait Disturbance






Severe behavioural alteration

Seizures and myoclonic epilepsy

Predominant cerebellar characteristics

Learning jobs

EEG abnormalcies

School failure

SpasticityTable 1: Symptoms which may be shared by grownup and juvenile HD patients are on the left side. On the other side are patients with unusual symptoms that are displayed in childish HD and non in grownup HD or Juvenile, HD. Table is adapted from ( F. Squitieri et al. , 2006 )

sums roll uping in nerve cell is besides present ( Scherzinger et al. , 1999 ) .

Classical HD exhibit untypical motor symptoms ( Squitieri et al. , 2000a ) yet normally without manifestation of chorea, bradykinesia, cerebellar characteristics or rigidness. However in JHD, rigidness occurs more often and is associated with a widespread encephalon wasting ( Squitieri et al. , 2000a ) . Despite the age of oncoming and differences in symptoms, an overlap set of common symptoms at oncoming between classical grownup HD and JHD does be in Table 1. In the instance of infantile HD where CAG repetitions are more than 80 – 90, contribute to a phenotype which clinical characteristics are really different from classical HD and early oncoming of HD. This suggests extra pathogenic mechanisms are involved in instances of infantile HD which causes it to be more terrible.

Familial factors play a portion in act uponing JHD. The negative correlativity of age of oncoming and figure of CAG repetition has been widely reported ( Guseella and McDonalds, 2002 ) . There are many surveies which shows a rigorous additive correlativity between repetitions of more than 60 and age of oncoming, proposing that JHD is much more dependent on lengths of repetitions than classical HD ( Telenius et al. , 1993 ) . However certain surveies take history of immature patients with more than 60 repetitions but really frequently less than high repetitions figure of 80-90 CAG repetitions. Such barrier occurs because of the rareness of such patients with childish oncoming which are normally publish as individual studies ( Nance, 1997 ; Sue et al. , 1998 ; Nance et al. , 1999 ; Rasmussen et al. , 2000 ; Squitieri et al. , 2000b, 2003b ; Gambardella et al. , 2001 ; Landau and Cannard, 2003 ; Cannella et al. , 2004 ; Duesterhus et al. , 2004 ; Seneca et al. , 2004 ; Schapiro et al. , 2004 ; Ullrich et al. , 2004 ) put the above surveies together, of patients with high repetitions figure, it shows a decreased correlativity between figure of repetitions and age of oncoming.

Pathogenesis Concept

The pathogenesis of HD have yet to be to the full elucidated, nevertheless there are emerging constructs on the pathogenesis of the disease ; 1 ) Proteolytic Cleavage, 2 ) Conformational alterations, 3 ) Transcription 4 ) Metamorphosis and mitochondrial Dysfunction, 5 ) Proteotoxic Stress, 6 ) Collection and inclusion formation, 7 ) Change of normal protein map

1 ) Proteolytic Cleavage

Several surveies have indicate clear links of proteolytic cleavage which frees toxic CAG polyglutamine- containing fragments ( Li et al. , 2007 ) . In ( Wellington et al. , 1998, 2002 ) surveies of full length HD mouse theoretical accounts, theoretical accounts are created with a 6 caspase cleavage site in the huntingtin ( htt ) protein, consequences show that the infective phenotype was attenuated in the theoretical accounts ( Graham et al. , 2006 ) . Possibilities of multiple cleavage events might ensue in a assortment of toxic fragments still remain ( Hoffner et al. , 2005 ) .

Question from this construct would it be a individual peptidase which is responsible to originate the pathogenesis of HD or a assortment of events?

2 ) Conformation alterations

A protein with an expanded CAG polyglutamine is prone to aggregation in-vitro ( Sherzinger et al. , 1997 ) , which may do the passage to a novel and toxic conformation Schaffer et al. , 2004 ; Nagai et al. , 2007 ) . Aggregation conformation is non an terminal effect of a protein with polyglutamine piece of land as the collection conformation may differ for cell to cell environment. In ( Diamond et al. , 2000 ) surveies a alteration in conformation in a expanded htt peptide was found it the following subsequent collection in vitro ( Schaffer et al. , 2004 ) In a separate survey ( Nagai et al. , 2007 ) which demonstrate 2 conformational alterations which result from

Figure 2. Pathogenesis of HD and different drug marks. Adapted from ( Puneet et al. , 2010 )

the merger of an expanded polyglutamine piece of land to thioredoxin ( Nagai et al. , 2007 ) . After protein purification it conforms to an alpha-helical province nevertheless after a few yearss it conforms to a beta-sheet rich conformation. This beta sheet rich conformation is much more prone to collection and causes toxicity to cell when microinject into civilized cells ( Nagai et al. , 2007 ) . In contrast a polyglutamine-binding peptide ( QBP1 ) is found to forestall the toxic conformation passage ( Nagai et al. , 2007 ) . With the above information from surveies it indicates that polyglutamine piece of land in protein may ease conformation alterations which in return can impact Numberss of factors and protein-protein interaction.

3 ) Transcription

It is suggested that protein with glutamine piece of lands may disrupt with specific transcriptional factors that may interrupt cistron look which may take to neurodegeneration. Study of ( Okazawa, 2003 ) shows that interaction of such protein and transcriptional and co-factors may take enlisting into sum. Transcription factors and co-factors like CREB-binding protein ( CBP ) and SP1. SP1 is link with soluble signifier of htt protein and interaction causes a repression of SP1 written text activities due to polyglutamine piece of land ( Li et al. , 2002 ; Dunah et al. , 2002 ) . Further more it is found that mutant htt protein reduced the cytoplasmatic interaction with repressor element-1 written text factor/neuron restrictive silencer factor ( REST/NRSF ) which leads to atomic enrichment of REST/NRSF, its enhanced binding to the nerve cell restrictive silencer component, and transcriptional repression of the cistron encoding brain-derived neurotrophic factor ( BDNF ) ( Zuccato et al. , 2003 ) . Recent work besides suggests that soluble mutant htt protein selectively represses the written text of PGC-1a, a regulator of indispensable mitochondrial cistrons, via interfering with the CREB/TAF4-dependent transcriptional tract ( Cui et al.,2006 ) . CBP is besides known to be found in atomic inclusion formed by mutant htt protien ( Nucifora et al. , 2001 )

4 ) Metamorphosis and mitochondrial disfunction

It is still yet to the full elucidated if mitochondrial shortages are specific to HD as patient with HD shows metabolic defects ( Jenkins et al. , 1993 ) which are described by weight loss despite holding Calories intake and foods ( Djousse et al. , 2002 ) . Mutant htt protein may hold several effects on chondriosome map. ( Lin and Beal, 2006 ; Browne and Beal, 2004,2006 ) ( Sawa et al. , 1999 ; Panov et al. , 2002 ; Yu et al. , 2003 ) shows increased mitochondrial depolarisation and early Ca defect in patients and mouse theoretical accounts. This may be due direct binding of mutant htt protein to mitochondria itself. However ( Cui et al. , 2006 ; Weydt et al. , 2006 ) propose that indirect consequence by manner of the transcriptional repression of PGC-1alpha, which is a transcriptional co-activator that regulates mitochondria biosynthesis and respiration.

5 ) Proteotoxic Stress

The infective polyglutamine piece of land in mutant htt protein causes human disease which is closely similar to that of polyglutamine proteins to aggregate in vitro ( Scherzinger et al. , 1997 ) . A cardinal function in HD pathogenesis is protein misfolding. ( Muchowski and Wacker, 2005 ; Rubinsztien et al. , 2007 ; Ross and Pickart et al. , 2004 ) . It is besides shown that the encephalon is more susceptible to protein misfolding as HD is associated with big intracellular inclusion and protein quality control worse as with age which is besides a factor of HD onset ( Zhou et al. , 2003 ; Soti and Csermely, 2002 ) . In ( Ravikumar et al. , 2002,2004 ; Berger et al. , 2006 ) had describe autophagy as a procedure where cells can degrade aggregative proteins, the procedure had been implicated to be opposition to polyglutamine pathology in cells in Drosophila and mice surveies. Further in ( Hara et al. , 2006 ; Komatsu et al. , 2006 ) cogent evidence that loss of autophagy map induces neurodegeneration in mouse theoretical accounts and leads to accretion of misfolded proteins. On the similar subject, ( Bence et al. , 2001 ) besides shows that proteasome disfunction has been link to polyglutamine pathogenesis. The survey demonstrated that cultured cells with big intracellular inclusion formed by mutant htt protein are associated with proteasome damage. These non merely bespeak proteasome obstruction underlies damage but besides inability of proteasome to to the full digest soluble expanded polyglutamine protein and coevals of such proteins ( Venkatraman et al. , 2004 ; Holmber et al. , 2004 ) .

6 ) Aggregation vs. inclusion formation

7 ) Change of normal protein map

HD is dominantly inherited with a addition of map due to toxic activity of polyglutamine expanded proteins responsible for its pathogenesis. ( Dragatsis et al. , 2000 ) propose that that HD dominant might come to visible radiation in the cognition of normal CAG repeats map. as it shows that normal htt protein inactivation leads to progressive neurodegeneration and htt protein map is indispensable for neurogenesis and postpartum development ( Duyao et al 1995 ; White et Al 1997 ; Zeitlin et Al 1995 ) . ( Wexler et al. , 1987 ) besides shown that homozygous HD patient has similar disease badness and age of oncoming comparison to heterozygote patients which in bend regulation out the loss of map as the chief mechanism behind HD.

Disease survey theoretical accounts.


Future surveies


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