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前言 Introduction
亨廷顿氏病,也被称为亨廷顿舞蹈病,或障碍(HD),是一种无法治愈的神经退行性遗传性疾病,其特征是纹状体神经元的损失,影响肌肉协调，还相应的造成知识能力的损害,情感上的障碍,以及认知功能的伤害。这是最常见的异常的遗传原因，不随意肌运动称为舞蹈病。退化的神经元影响基底神经节细胞,这是因为大脑深处结构有许多重要的功能,包括协调运动。在基底神经节中,血液透析将纹状体神经元作为目标。血液透析也会影响大脑的外表面,或皮层,控制思想,认知,和记忆。
这种疾病是由显性突变引起的，一个特定基因有两个副本,位于染色体4。任何受父母遗传影响的孩子得这种疾病的风险为50%。在极少数情况下,父母双方有一个受影响的基因,或父母任意一方有两个受影响的基因副本,这种风险就会大大增加。
Huntington's disease, also known as huntington chorea, or disorder (HD), is an incurable neurodegenerative genetic disorder characterized by loss of striatal neurons that affects muscle coordination, loss of intellectual faculties, and emotional disturbance, as well as impairment of some cognitive functions. It is the most common genetic cause of abnormal involuntary muscle movements called chorea. Degeneration of neurones specifically affects cells of the basal ganglia, which are structures deep within the brain that have many important functions, including coordinating movement. Within the basal ganglia, HD targets neurons of the striatum. HD also affects the brain's outer surface, or cortex, which controls thought, perception, and memory.
The disease is caused by a dominant mutation on either of the two copies of a specific gene, located on chromosome 4. Any child of an affected parent has a 50% risk of inheriting the disease. In rare situations where both parents have an affected gene, and either parent has two affected copies, this risk is greatly increased.
Physical symptoms of Huntington's disease can begin at any age from infancy to old age, but usually begin between 35 and 44 years of age. On rare occasions, when symptoms begin before about 20 years of age, they progress faster and vary slightly, and the disease is classified as juvenile, akinetic-rigid or Westphal variant HD. HD-HTT gene
The HD mutation was found in 1993 as an unstable expansion of the CAG (trinucleotide) repeat within the coding region of the gene "IT15". This gene on chromosome 4 (4p63), encodes the protein, Huntington (htt). HD is thought to have been caused by the tri-nucleotide repeat in the Huntington gene which translates as a polyglutamine (polyQ) repeat in the protein product.
Htt gene is a 348-kDa multidomain protein that contains a polymorphic glutamine/ proline-rich domain at its amino-terminus. The longer polyQ domain seems to induce conformational changes in the protein, which causes it to form intracellular aggregates that, in most cases, manifest as nuclear inclusions. However, aggregates can also form outside the nucleus. Despite its large size, the normal function of htt has been difficult to establish because it contains very little sequence homology to other known proteins, is ubiquitously expressed and is localized in many subcellular compartments (The hunt for huntingtin function - Harjes & Wanker, 2003).
Htt is present in the nucleus, cell body, dendrites and nerve terminals of neurons, and is also associated with a number of organelles including the Golgi apparatus, endoplasmic reticulum and mitochondria. Various approaches have been used to determine the function of htt and its pathological effects, and it is becoming Apparent that the role of htt is complex and that it operates at many different cellular levels (Harjes & Wanker, 2003). Htt forms part of the dynactin complex, co-localizing with Microtubules and interacting directly with β-tubulin, which suggests a role in vesicle transport and/or cytoskeletal anchoring. Interestingly, htt has also been shown to have a role in clathrin-mediated endocytosis, neuronal transport and postsynaptic signalling. Furthermore, htt can protect neuronal cells from apoptotic stress and therefore may have a pro-survival role. Pathogenesis of Huntington’s Disease
The cAMP-responsive element pathway and the SP1 pathway.
Of the transcription pathways that are affected in HD, the cAMP- responsive element (CRE) - and the SP1-mediated pathways are widely studied. (Fig 1A. Description of the CRE pathway).
The ablation of CREB results in an HD-like phenotype in mice, in which there is progressive neurodegeneration in the hippocampus and striatum (Disruption of CREB function in brain leads to neurodegeneration - Mantamadiotis et al, 2002), and the down-regulation of CRE-regulated genes has also been detected in HD patients. The CRE pathway initially focused on CBP, which is sequestered into aggregates, and which led to the proposal that the availability of CBP for CRE-mediated transcription is disrupted (Huntingtin-protein interactions and the pathogenesis of Huntington's disease - Li & Li, 2004).
However, CBP can be sequestered into aggregates formed by several other polyQ proteins, including atrophin 1(protein found in nervous tissue, associated with a form of Trinucleotide repeat disorder), the androgen receptor (DNA binding receptor which regulates gene expression) and ataxin3 (in Machado-Joseph disease also known as spinocerebellar ataxia-3).
Htt has been shown to interact with CBP through both its glutamine-rich and acetyltransferase domains; this may account for the observed decrease in CRE-mediated transcription and the loss of acetyltransferase activity seen in polyQ models of disease.
A study by Obrietan & Hoyt in 2004 on CRE-mediated transcription is in Huntington's disease reported that CRE-dependent transcription is upregulated in double-transgenic mice expressing mutant htt and a CRE-β- galactosidase reporter construct. Moreover, phosphorylated CREB levels were also elevated, as were the levels of the CREB-regulated gene product CCAAT-enhancer binding protein, which suggests that mutant htt facilitates CRE-dependent transcription. In addition to CBP, CRE-mediated transcription also appears to be affected by the coactivator TAFII130. This coactivator has also been found in aggregates and, on over-expression, inhibition of CREB dependent transcription can be overcome (Expanded polyglutamine stretches interact with TAFII130, interfering with CREB-dependent transcription - Shimohata et al, 2000). Fig 1. Description of the CRE pathway
Htt is a multi-domain protein with many functions, including transcriptional regulation, intracellular transport and involvement in the endosome–lysosome pathway. The protein also has pro-survival properties.
The chromosomal position of the Huntington gene is large, spanning 180 kb and consisting of 67 exons. The Huntington gene is widely expressed and is required for normal development. It is expressed as 2 alternatively polyadenylated forms displaying different relative abundance in various fetal and adult tissues. The larger transcript is approximately 13.7 kb and is expressed predominantly in adult and fetal brain whereas the smaller transcript is 10.3 kb and is more widely expressed. The genetic defect leading to Huntington disease may not necessarily eliminate transcription, but may alter the function of the protein. Location of HTT Gene:
Figure1. The HTT gene is located on the short arm (p) of chromosome 4 at position 4p16.2. More precisely, the HTT gene is located from base pair 3,046,205 to base pair 3,215,484 on c-some 4. Expansion of PolyQ
The mutation in Huntington produces an expanded stretch of glutamine residues attached to its amino terminal. Expansions beyond a threshold of 36 CAG's at 5’ end of the transcript encoding huntingtin cause Huntington’s disease. This mutation increases the size of the CAG segment in the htt gene. People with Huntington disease have 36 to more than 120 CAG repeats. People with 36 to 40 CAG repeats may or may not develop the signs and symptoms of Huntington disease, while people with more than 40 repeats almost always develop the disorder.
The expanded CAG segment leads to the production of an abnormally long version of the Huntingtin protein. The lengthened protein is cut into smaller, toxic fragments that bind together and accumulate in neurons, disrupting the normal functions of these cells. This process particularly affects regions of the brain that help coordinate movement and control thinking and emotions (the striatum and cerebral cortex). The dysfunction and eventual death of neurons in these areas of the brain underlie the signs and symptoms of Huntington disease.
A larger number of repeats are usually associated with an earlier onset of signs and symptoms. This phenomenon is called anticipation. People with the adult-onset form of Huntington disease (which appears in mid-adulthood) typically have 40 to 50 CAG repeats in the HTT gene, while people with the less common, early-onset form of the disorder (which appears in childhood or adolescence) tend to have more than 60 CAG repeats.
Individuals who have 27 to 35 CAG repeats in the HTT gene do not develop Huntington disease, but they are at risk of having children who will develop the disorder. As the gene is passed from parent to child, the size of the CAG trinucleotide repeat may lengthen into the range associated with Huntington disease (36 repeats or more).#p#分页标题#e# Classification of the trinucleotide repeat, and resulting disease status, depends on the number of CAG repeats Repeat count Classification Disease status <28 Normal Unaffected 28–35 Intermediate Unaffected 36–40 Reduced Penetrance +/- Affected >40 Full Penetrance
Affected Table 1. Trinucleotide repeats and classification of infection
This neuro-degenerative family includes eight other disorders: dentatorubralpallidoluysian atrophy (DRPLA), spinobulbar muscular atrophy (SBMA) and spinocerebellar ataxia (SCA) types 1–3, 6, 7 and 17, also Machado Joseph Disease. Although all of these disease proteins contain extended polyQ tracts (usually >36 glutamines) and are widely expressed in the body and brain, they cause distinct neuropathology in particular brain regions. Transgenic Mice Model
Human trials in HD are difficult, costly and time-consuming due to the slow disease course, dangerous onset and patient-to-patient variability. Identification of molecular biomarkers associated with disease progression will aid development of effective therapies by allowing further confirmation of animal models and by providing more sensitive measures to show progression of the disease.
In the 12 years since Huntington’s disease mutations were identified, considerable progress has been made with modeling pathogenesis in cell and animal models. Mutant huntingtin accumulates in intraneuronal inclusions. Huntingtin is cleaved to form N-terminal fragments consisting of the first 100–150 residues that contain the expanded polyQ tract and these are believed to be the toxic species found in the aggregates (Aggregation of huntingtin in neuronal intranuclear inclusions and dystrophic neurites in brain - DiFiglia et al., 1997)
The extent to which mouse models expressing N-terminal fragments of mutant huntingtin repeat the human disease phenotype is unclear, although studies on mice have shown pathological and behavioral similarities to HD.
Studies on transgenic mice reported in Neuron, Vol. 23, 181–192, May, 1999 have used mice that replicate the full-length HD gene expressed in the same developmental tissue and cell-specific manner as seen in patients with the disease and cloned using homologous recombination in yeast to introduce expanded CAG repeats (46 and 72) into YACs with the same CAG sizes similar to those seen in adult onset (YAC46 containing 46 CAG repeats) or in juvenile cases (YAC72 containing 72 CAG repeats) HD. The YAC46 and YAC72 mice developed progressive electrophysiological abnormalities that precede nuclear translocation and aggregation of htt. YAC72 mice have behavioural abnormalities, with onset influenced by the level of mutant protein.
A mouse expressing mutant htt with 72 glutamines at higher levels presented with an early onset behavioural phenotype and had intranuclear aggregates and neuro-degeneration specifically in the striatum.
Other YAC72 mice expressing lower levels of mutant protein had progressive electrophysiological abnormalities at 6 and 10 months, followed by selective degeneration of medium spiny neurons in the lateral striatum associated with translocation of N-terminal htt fragments to the nucleus. These mice represented the first animal model expressing full length mutant human htt under the control of its own promoter, providing insight into the sequential molecular and cellular events underlying HD.
Neuronal degeneration can be assessed by light microscopy in 1.5um thick toluidine blue-stained sections from the striatum, the cortex adjacent to the lateral striatum, the hippocampus, and the cerebellum. Degenerating neurones were present in the striatum of YAC 46 and YAC72 mouse at 12 month of age. They were hyperchromatic and shrunken. Unlike striatal neurones, which have normally round nuclei and round cellular outlines, degenerating neurons had oval fusiform perikarya and nuclei. The chromatin within degenerating neurones was also condensed and marginated.
Electron microscopy using the same tissues confirmed the degenerative and abnormal nature of these morphologic changes. The changes varied in severity and included reduced neuronal size, the presence of nuclear and plasma membrane irregularities, increased electron density of the cytoplasm and the nucleus, swelling of some mitochondria, nuclear shrinkage and margination of heterochromatin. These degenerative features were consistent with apoptosis.
Significant degeneration was present in the striatum of the YAC72 mouse at 12 months of age. No degeneration was found in the YAC46 or in the wild type controls used. In the striatum of mouse YAC72 there was extensive neuronal degeneration. Selective neurodegeneration affected specifically the medium spiny neurons of the striatum.
Neurodegeneration can occur in the absence of aggregates. The YAC transgenic mice expressing mutant htt with the 46 CAG repeats did not have any clinical phenotype by detailed behavioural analysis for up to 20months of age. However, the electrophysiological abnormalities were evident much earlier. The severity of abnormalities is more obvious in mice with 72 CAG compared with mice with 46 CAG repeats. Therapeutic silencing of mutant huntingtin with siRNA
Therapies aimed at delaying disease progression have been tested in these mouse models. Beneficial effects have been reported in animals treated with substances that increase transcription of neuroprotective genes (histone and deacetylase), prevent apoptosis (caspase inhibitors), enhance energy metabolism (co-enzyme Q_remacemide and creatine), and inhibit the formation of polyglutamine aggregates (trehalose, Congo red, and cystamine). These approaches target downstream and possibly indirect effects of HD gene expression.
When mutant htt is inducibly expressed, pathological and behavioral features of the disease develop, including a characteristic neuronal inclusions and abnormal motor behavior. Upon repression of transgene expression in affected mice, pathological and behavioral features resolved. Thus, reduction of htt expression by using RNA interference (RNAi) may allow protein clearance mechanisms within neurons to normalize mutant htt-induced changes. Direct inhibition of the mutant htt expression will slow or prevent HD associated symptom.
Small interfering RNA (siRNA), are a class of double-stranded RNA molecules, 20-25 nucleotides long, they play a variety of roles in biology. Most notably, siRNA are involved in the RNA interference (RNAi) pathway, where they interfere with the expression of a specific gene.
http://www.newscientist.com/data/images/ns/cms/dn3493/dn3493-1_550.jpg
Using gene therapy to switch off genes instead of adding new ones could slow down or prevent the fatal brain disorder. Improving motor and neuropathological abnormalities in Huntington’s disease.
This mechanism involves pieces of double-stranded RNA or siRNAs that trigger the degradation of any other RNA in the cell with a matching sequence. If a siRNA is chosen to match the RNA copied from a particular gene, it will stop production of the protein the gene codes for. In HD the mutations in the huntingtin gene result in defective large protein clumps that gradually kill off part of the brain. The above studies in transgenic mice showed that reducing production of the defective protein can slow down the disease. Discussion
Neurons in the brain of transgenic mice, expressing exon 1 of the human HD gene contained an expanded polyQ, it degenerated within those specific areas of the brain known to be affected in HD. Degenerating neurons of similar ultrastructural appearance can be found in postmortem brain from patients with HD, but have never been reported in other neurodegenerative diseases such as Alzheimer’s disease and Parkinson’s disease. Neuronal death found in the brain in HD is by a process that is morphologically and biochemically distinct from apoptosis, and is associated with the presence of intracellular aggregates of mutant protein. This defines a temporal progression of protein aggregation, inclusion formation, appearance of neurological symptoms, and finally neurodegeneration. Potential treatments for HD directed toward prevention of protein aggregation may thus prove more effective than antiapoptotic therapies. |
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