Polyglutamine disorders

Principal investigator

Dr. Willeke M.C. van Roon

The polyglutamine research group studies the molecular pathogenesis of several neurodegenerative protein aggregation disorders with the main focus on Huntington disease, Spinocerebellar Ataxia type 1 and 3, and Hereditary Cerebral Hemorrhage With Amyloidosis – Dutch type (HCHWA-D). In the group there is a strong focus on translational research of molecular medicine, using both patient material as well as cell, animal and induced pluripotent stem cell (iPSC) models to unravel disease pathology, discover novel drug targets/therapies and the identification of biomarkers for disease stage and progression. We implement novel sequencing technologies and biosemantic analysis methods, making optimal use of novel and existing data sets and closely collaborate with clinical departments.   


Figure 1: Translational research from bench to bed side 

Huntington disease 

Huntington disease (HD) is caused by a CAG repeat expansion in the HTT gene, which results in an expansion of polyglutamines at the amino-terminal end of the huntingtin protein. This polyglutamine expansion results in the accumulation of cytoplasmic and nuclear aggregates in neurons, and these aggregates play a central role in the disease. There is currently no therapy available for HD. The major neuropathology in HD occurs in the striatum and cerebral cortex but degeneration is seen throughout the brain as the disease progresses.  Our research focuses on transcriptomic, metabolomic and protein biomarker discovery in biological samples from HD patients. The aim is to better  understand the molecular mechanisms of huntingtin toxicity using a genomic and proteomic approach in cell models of HD and patient material. We have optimized transcriptome sequence profiling in peripheral blood from HD patients and controls and have identified a panel of genes to follow disease progression. Using biosemantics, we are comparing transcriptome changes in blood and brain to understand the common mutant huntingtin signature. Furthermore we are investigating the regulation of huntingtin RNA and protein expression, linking this to HD pathology. 


Figure 2: The major area of the brain affected in Huntington disease is the striatum. The expanded CAG repeat in the HTT gene causes the mutant huntingtin protein to aggregate and these protein aggregates are a hallmark of the disease.

Spinocerebellar Ataxia type 1 and 3 

Spinocerebellar ataxia type 3 (SCA3), also known as Machado Joseph disease (MJD), is the second most common polyglutamine disorder. SCA3 is caused by an expanded CAG repeat in the penultimate exon of the ATXN3 gene. This mutant CAG repeat results in the translation of an expanded ataxin-3 protein. Neuropathological studies have detected widespread neuronal loss in the cerebellum, thalamus, midbrain, pons, medulla oblongata and spinal cord of SCA3 patients. The ataxin-3 protein contains an amino-terminal Josephin domain that displays ubiquitin protease activity and a carboxyl-terminal tail with 2 or 3 ubiquitin interacting motifs (UIMs), depending on the isoform. It is still not completely understood how the ataxin-3 polyglutamine expansion results in the observed pathology. In our group we study the role of the different functional domains of the ataxin-3 protein and how the functions of the ataxin-3 protein change in SCA3.  

Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant disorder caused by an expanded CAG repeat in the ATXN1 gene. The symptoms usually include ataxia, dysarthria and bulbar dysfunctions with neuropathology most prominent in cerebellum and brainstem. Aberrant interactions with transcriptional regulator capicua (CIC) and the RNA splicing factor RBM17, influenced furthermore by phosphorylation of Ser776 in the ataxin-1 protein. In our group we study disease pathology in fibroblast cells and induced pluripotent stem cells (iPSC) from patients and control.  


Figure 3: SCA3 molecular pathology 

HCHWA-D is an autosomal dominant hereditary disease caused by a point mutation in the amyloid precursor protein (APP) gene on chromosome 21. The mutation causes an amino acid substitution (Gln to Glu) at codon 693 and is called the ‘Dutch mutation’. Amyloid-β, the product after cleavage of APP, is secreted into the extracellular space. The Dutch mutation results in amyloid-β accumulation around cerebral vessel walls, causing vessel wall integrity loss leading to haemorrhages. Studying the effects of altered amyloid-β metabolism due to mutations like the Dutch mutation also provides a better understanding of amyloid-β toxicity in more common diseases like sporadic cerebral amyloid angiopathy (CAA) and Alzheimer’s disease (AD).

Developing new therapies 

The group is exploring several therapeutic approaches for neurodegenerative disorders. The first is the use of Llama-derived heavy chain antibody fragments for the prevention and/or identification of toxic huntingtin interactions. We have isolated and characterised huntingtin-specific Llama-derived heavy chain antibody fragments that can be used for in vitro imaging studies. The second therapeutic approach is to use antisense oligonucleotides to (1) reduce the levels of proteins that cause neurodegenerative disorders, or (2) to modify the mutant protein through exon skipping rendering the protein less toxic. We use this approach for HD, SCA1, SCA3 and HCHWA-D.


Figure 4: Exon skipping approach to modify the mutant ataxin-3 protein

Induced Pluripotent Neuronal Differentiation laboratory 

In neurodegenerative research, disease-specific cell models that accurately mimic the disease were very rare. This has changed with the development of induced pluripotent stem (iPS) cells. Now it is possible to transform somatic cells from a patient into a stem cell, that can then be differentiated into various neuronal-and glial cells in 2D cultures as well as in 3D cultures such as brain organoids. In close collaboration with the LUMC Stem Cell Facility, Molecular Cell Biology and Human Genetics, this laboratory will provide support and expertise for functional research using neuronally differentiated iPS cells. Link to short movie (Dutch)

 

Figuur 5: Neuronal differentiation of induced pluripotent stem cells into (A) 3D brain organoids (B) 2D neuronal (green) and glial cells (red).