[Gene-selective treatment approaches for Huntington's disease]. / Genselektive Therapieansätze bei der Huntington-Krankheit.

In Germany at least 8000 and probably up to ca. 14,000 people currently suffer from clinically manifest Huntington's disease (HD). In addition, an estimated 24,000 Germans carry the HD mutation in the huntingtin (HTT) gene and will develop HD during their lifetime. Although HD is a rare neurodegenerative disease, it is currently in the focus of general medical interest: clinical trials have begun that provide a rational basis for hope to slow down the so far relentless progression of the disease, ultimately resulting in patients becoming entirely dependent on nursing care. If treatment is started early enough it may be possible to mitigate the clinical manifestation of HD. These innovative therapeutic approaches aim at inhibiting the de novo production of mutant HTT gene products. A first clinical drug trial to demonstrate the efficacy (phase III) of intrathecal antisense oligonucleotides (ASO, active substance RG6042) was started in 2019. Additional clinical studies on alternative treatment approaches with allele-selective ASOs as well as gene therapeutic approaches using RNA molecules and zinc finger repressor complexes are imminent. This article gives an overview of the current gene-selective therapeutic approaches in HD under discussion.

Impaired glutamate transport and glutamate-glutamine cycling: downstream effects of the Huntington mutation.

The pathogenesis of Huntington's disease is still not completely understood. Several lines of evidence from toxic/non-transgenic animal models of Huntington's disease suggest that excitotoxic mechanisms may contribute to the pathological phenotype. Evidence from transgenic animal models of Huntington's disease, however, is sparse. To explore potential alterations in brain glutamate handling we studied transgenic mice expressing an N-terminal fragment of mutant huntingtin (R6/2). Intracerebral microdialysis in freely moving mice showed similar extracellular glutamate levels in R6/2 and littermate controls. However, partial inhibition of glutamate transport by L-trans-pyrrolidine-2,4-dicarboxylate (4 mM) disclosed an age-dependent increase in extracellular glutamate levels in R6/2 mice compared with controls, consistent with a reduction of functional glutamate transport capacity. Biochemical studies demonstrated an age-dependent downregulation of the glial glutamate transporter GLT-1 mRNA and protein, resulting in a progressive reduction of transporter function. Glutamate transporters other than GLT-1 were unchanged. In addition, increased extracellular glutamine levels and alterations to glutamine synthetase immunoreactivity suggested a perturbation of the glutamate-glutamine cycle. These findings demonstrate that the Huntington's disease mutation results in a progressively deranged glutamate handling in the brain, beginning before the onset of symptoms in mice. They also provide evidence for a contribution of excitotoxicity to the pathophysiology of Huntington's disease, and thus Huntington's disease may be added to the growing list of neurodegenerative disorders associated with compromised glutamate transport capacity.

SCA7 mouse models show selective stabilization of mutant ataxin-7 and similar cellular responses in different neuronal cell types.

Accumulation of expanded polyglutamine proteins and selective pattern of neuronal loss are hallmarks of at least eight neurodegenerative disorders, including spinocerebellar ataxia type 7 (SCA7). We previously described SCA7 mice displaying neurodegeneration with progressive ataxin-7 accumulation in two cell types affected in the human pathology. We describe here a new transgenic model with a more widespread expression of mutant ataxin-7, including neuronal cell types unaffected in SCA7. In these mice a similar handling of mutant ataxin-7, including a cytoplasm to nucleus translocation and accumulation of N-terminal fragments, was observed in all neuronal populations studied. An extensive screen for chaperones, proteasomal subunits and transcription factors sequestered in nuclear inclusions (NIs) disclosed no pattern unique to neurons undergoing degeneration in SCA7. In particular, we found that the mouse TAF(II)30 subunit of the TFIID initiation complex is markedly accumulated in NIs, even though this protein does not contain a polyglutamine stretch. A striking discrepancy between mRNA and ataxin-7 levels in transgenic mice expressing the wild-type protein but not in those expressing the mutant one, indicates a selective stabilization of mutant ataxin-7, both in this model and the P7E/N model described previously. These mice therefore provide in vivo evidence that the polyglutamine expansion mutation can stabilize its target protein.

Expanded polyglutamines induce neurodegeneration and trans-neuronal alterations in cerebellum and retina of SCA7 transgenic mice.

Among the eight progressive neurodegenerative diseases caused by polyglutamine expansions, spinocerebellar ataxia type 7 (SCA7) is the only one to display degeneration in both brain and retina. We show here that mice overexpressing full-length mutant ataxin-7[Q90] either in Purkinje cells or in rod photoreceptors have deficiencies in motor coordination and vision, respectively. In both models, although with different time courses, an N-terminal fragment of mutant ataxin-7 accumulates into ubiquitinated nuclear inclusions that recruit a distinct set of chaperone/proteasome subunits. A severe degeneration is caused by overexpression of ataxin-7[Q90] in rods, whereas a similar overexpression of normal ataxin-7[Q10] has no obvious effect. The degenerative process is not limited to photoreceptors, showing secondary alterations of post-synaptic neurons. These findings suggest that proteolytic cleavage of mutant ataxin-7 and trans-neuronal responses are implicated in the pathogenesis of SCA7.

Expression analysis of ataxin-7 mRNA and protein in human brain: evidence for a widespread distribution and focal protein accumulation.

Spinocerebellar ataxia 7 (SCA7) is an autosomal dominant neurodegenerative disorder caused by the expansion of a CAG-trinucleotide repeat in the coding region of the SCA7 gene. The expansion is translated into an extended polyglutamine stretch in the protein ataxin-7, a protein of unknown function. By Northern blot analysis expression of ataxin-7 was detected in numerous regions of human brain and some peripheral tissues. It is unknown, however, if ataxin-7 is enriched at sites of the SCA7 pathology. We studied the regional and cellular expression pattern of ataxin-7 at the mRNA level by in situ hybridization histochemistry in normal human brain. Furthermore we used a monoclonal and two polyclonal antibodies raised against the normal ataxin-7 to establish the distribution of this protein in brain, retina and peripheral organs. At the mRNA level ataxin-7 was preferentially expressed in neurons; the regional distribution reflected neuronal packing density. Ataxin-7 immunoreactivity (IR) was similarly widely expressed. In most neurons, ataxin-7 IR was preferentially localized to the cytoplasmatic compartment although some nuclear ataxin-7 IR was detected in most neurons. A more intense and more prominently nuclear ataxin-7 IR was observed in neurons of the pons and the inferior olive, brain regions severly affected by the disease, suggesting that the subcellular localization and abundance of ataxin-7 is regulated in a regionally specific way. Since neurons displaying more intense and more prominently nuclear ataxin-7 IR belonged to the class of susceptible cells in SCA7, an enrichment of normal ataxin-7 in the nuclear compartment may contribute to neurodegeneration. However not all sites of SCA7 pathology displayed a strong cytoplasmatic and nuclear immunoreactivity.