NeuroD (Neurodegenerative Disorders)

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 have compared 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. Finally, we have developed an RNA targeting therapy in cell and animals models that prevents proteolytic cleavage of the huntingtin protein to reduce the toxicity of the mutant huntingtin protein.

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. Furthermore we have developed a therapeutic approach that removes the exon containing the CAG repeat that we have tested in cell and animal models.

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. We recently found that SCA1 patient-derived cells recapitulate key pathological features of SCA1 pathogenesis providing a valuable tool for the identification of novel disease-specific processes. Moreover, we are studying the 5' untranslated region (UTR) of the ATXN1 gene to understand its role in regulating the stability and translation of its mRNA. This region contains elements that influence translational efficiency, which is essential for normal gene expression in neurons. In addition, we are exploring RNA targeting strategies to lower mutant ataxin-1 levels, with the aim of developing potential therapeutic approaches for SCA1.

Spinocerebellar ataxia type 7 (SCA7) is a autosomal dominant inherited disease that is caused by a CAG repeat expansion in the ATXN7 gene (>34 CAG repeats). This results in an expanded polyglutamine tract in the ATXN7 protein which causes both gain- and loss-of-function, and the formation of protein aggregates. SCA7 is more rare compared to SCA1 and SCA3 as the prevalence is less than 1:300,000 in the general population. Similar to the other SCA’s, patients develop a variety of symptoms related to the degeneration of the cerebellum like ataxia and dysarthria. In addition, most individuals with SCA7 are affected by vision as well as several cell types in the retina degenerate over time. Symptoms can start from infancy to late adulthood. At the moment there are no treatment options available that can stop or reduce the disease progression. The main aim of our research is to learn more about the disease phenotype using SCA7 in vitro models, and to develop new therapeutic strategies.

Familial mutations in the APP gene

The first disease with an autosomal dominant mutation in the APP gene is Hereditary Cerebral Hemorrhage With Amyloidosis – Dutch type (HCHWA-D), also known as Katwijk’s disease or Dutch type CAA (D-CAA). 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 hemorrhages. 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).

The second disease we are studying is a novel variant in exon 18 of the APP gene that causes a late onset of ischemic stroke. Using 2D and 3D disease modelling with iPSC we are unravelling the disease mechanism that underlies these symptoms.

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 characterized 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, SCA7 and HCHWA-D.

All the expertise with disease modelling and development of RNA targeting therapies is applied to developing n=1/n=few RNA targeting therapies for ultra rare/unique DNA variants that cause a neurological disorder. Willeke van Roon-Mom is founding member of the Dutch Center of RNA Therapeutics (DCRT). DCRT is a nonprofit consortium of the medical centers Leiden University Medical Center (LUMC), Radboudumc (Nijmegen) and Erasmus MC (Rotterdam). The aim is to develop tailor-made RNA therapy for patients with rare genetic disorders. DCRT focuses on RNA therapy for patients in whom the affected tissue can be directly treated via local administration, i.e. the eye, the central nervous system. Through the 1 mutation 1 medicine (1M1M) network and the N of 1 collaborative (N1C) there is a world wide effort to make this approach sustainable to be able to treat as many patients as possible.