Machado Joseph disease (MJD) is a neurodegenerative disorder associated with deposits of an aberrant form of the protein ataxin-3 in the brain. The disease is also fatal and the most common hereditary motor neurodegenerative disease in many countries.
Despite this, not much is known about MJD including the neurological basis of some of its symptoms, which cannot be linked to the brain damage found in patients. But now, researchers in Portugal and France using a new animal model of the disease were able to show, for the first time, that MJD also affects the striatum, a brain area associated with movement and balance control. These new findings, just published as advance online publication in the journal Human Molecular Genetics, finally clarify the cause of previously unexplained symptoms, such as muscle twisting and abrupt dance-like movements of the limbs.
The research helps to understand better a still incurable pathology while also providing a new animal model to study the disease as well as potential treatments.
MJD, also called spinocerebellar ataxia type 3, belongs to a group of disorders linked to an abnormal repetition of three nucleotides (nucleotides are the DNA building blocks) within a gene. The altered gene produces an abnormal protein, which, incapable of working properly, accumulates instead in insoluble deposits in the patients’ brain and is linked to the neural death characteristic of these disorders.
In MJD the pathology results from a mutation in the MJD1 gene that encodes a protein called ataxin-3. The name Machado-Joseph comes from two families of Portuguese/Azorean descent who were the first patients identified and, today, disease prevalence is still highest among people with a link to Azores. In fact, disease incidence among immigrants of Portuguese ancestry in New England is 1 in 4,000 while in the Azores island, Flores is even higher - 1 in 140. Symptoms include increasing limb weakness (ataxia means lack of muscle control) and widespread clumsiness, difficulty with speech and a general loss of motor control that eventually confines the patient to a wheelchair and, in most severe cases, leads to premature death. Although the disease has been discovered in the 1970s much is still unknown including its cause and also the reason behind many of its symptoms as they do not seem to relate to the brain areas damaged by the disease.
In order to better understand this and because animal models that mimic closely the pathology are necessary Sandro Alves, Nicole Déglon, Luis Pereira de Almeida and colleagues working at the University of Coimbra, Coimbra, Portugal, and the Institute of Biomedical Imaging (I2BM) and Molecular Imaging Research Center (MIRCen), Fontenay-aux-Roses, France together with others in Switzerland and the USA created a virus that contained in its genome the gene for the mutant form of ataxin-3 and injected it into rats .
The idea was that the virus injected into the rat brain, when inserted into the DNA of the infected cells, would also add the mutant ataxin-3 gene. As result, infected cells would produce mutant ataxin-3 and (hopefully) reproduce a MJD-like disease in these animals that could then be studied.
When injecting the altered virus Alves, Almeida and colleagues chose three different brain areas: the substantia nigra, which was known to be affected by MJD in humans so it could serve as positive control for neurodegeneration - but also the cortex and the striatum areas of the brain, which recent imaging techniques have suggested to be linked to MJD. Control rats were injected with a virus that contained DNA for a normal (no-mutated) version of ataxin-3.
Injection of mutated ataxin-3 into the rats’ substantia nigra replicated some of the motor dysfunction of MJD as well as induced brain degeneration and deposits of mutant protein within the injected area. Mutant ataxin-3 deposits in MJD patients are characteristically insoluble fibrillar structures localised inside of the cell nucleus. In agreement with this the deposits found in the rats’ substantia nigra were insoluble, also localised in the nucleus, and of high molecular weight. Supporting the specificity of this new MJD model animals injected with a virus containing the normal form of ataxin-3 showed even distribution of the protein throughout the cell and no neural death..
When it was established that Alves, Almeida and colleagues’ protocol created a valid animal model for MJD the next step was to inject the virus with mutant or normal proteins into the cortex or the striatum of the rats. The idea was to look for MJD-like behavioural and neurologic symptoms in these animals suggesting an involvement of these areas in MJD. Interestingly, although animals injected in the cortex showed deposits of mutant ataxin3 there was almost no neurodegeration detected. Rats injected in the striatum, however, were very different as they had insoluble mutant ataxin-3 deposits and clear indication of neural changes around these deposits. Most interestingly, the amount of neural dysfunction observed was directly linked to the amount of mutant ataxin-3 gene injected (so dose-dependent) supporting the toxic role of this protein in the disease.
To further confirm that the striatum degeneration was characteristic of MJD the researchers decided to look into the striatum tissues of deceased MJD patient and of a mice model of MJD where animals are genetically modified to produce mutant ataxin-3 throughout the brain. In both cases it was found deposits of mutant ataxin-3 and signs of neuropathology within the striatal tissue.
Sandro Alves, Luis Pereira de Almeida and colleagues’ results finally explain why MJD patients suffer so many times from tightening and twisting of the limb and, although less frequently, of abrupt irregular movements since both motor problems can now be explained by a disruption of the neurological paths in the striatum, a brain area connected to balance and movement. The work also provides a new genetic model to study the disease and possible therapeutic approaches to combat it.
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