Using genetic modifiers to identify and target pathogenic mechanisms in Huntington's disease

Huntington's disease (HD) is one of almost 50 inherited 'repeat expansion disorders' known to be caused by similar short repeated sections of DNA. Many of these diseases involve progressive degeneration of the nervous system and dementia. Unfortunately, none has an effective treatment. In this project I will focus on therapeutic target identification in HD, but advances could be relevant to all repeat expansion disorders.

HD affects around 1 in 8000 and usually starts in middle age, although there is much variation. It involves progressive worsening of movement control, mental health, behaviour and memory until premature death 10-30 years later. HD is caused by a C-A-G repeat expansion in one gene called Huntingtin, or HTT. A gene is a short stretch of DNA that uses a 4 letter chemical code (A, G, C or T) as an instruction manual for making proteins in cells. HTT codes for a large protein with many jobs in organising cells. It normally contains a short run of about 20 repeated CAG triplets in its DNA code (ie CAGCAGCAG...) but in HD this section is expanded to at least 36 CAGs in a row. HTT proteins made using this faulty code are toxic to nerve cells (neurons) in the brain, ultimately leading to their degeneration and symptoms of HD. Although the CAG repeat in HTT causes HD, we have recently discovered that there are other genes in cells that can change the age at which HD symptoms start. Some of these 'genetic modifiers' can change onset by many years meaning that if we can target them with drugs we could see a dramatic clinical effect.

I aim to develop a pipeline for advancing genetic modifiers as new therapeutic targets in HD. My starting point is a list of 14 potential modifier genes that we and others have identified in large genetic studies of HD patients. To discover whether and how these genes impact on HD I will first screen them in two different but complementary experimental systems: Q109 human stem cells/neurons and fruit flies. The stem cells were donated by an HD patient who had 109 CAG repeats and are particularly useful because they can be turned into brain neurons in the lab. When we grow these cells in the lab the CAG repeat expands over time, a process thought to drive neurodegeneration in the brain. I will delete potential modifier genes from Q109 cells to test whether they affect CAG expansion rates and/or neuronal function and survival. The HD fruit flies provide an alternative system in which to screen potential modifier genes in the context of a whole organism. These flies are engineered to have a HTT gene with 128 CAGs that causes them to develop walking problems, neurodegeneration and reduced lifespan. I will use genetic tools to test the impact of reducing the levels of potential modifiers in these flies.

Next, I will select those modifiers with the biggest impact and the most therapeutic potential (i.e. where reduction of a modifier improves HD pathology) for further investigation. I have already identified one gene that is associated with stimulating CAG expansion and hence earlier onset of HD. This will be the first modifier taken forward for detailed molecular studies to work out how exactly it is affecting HD pathology and whether we can target it with drugs. A similar process will be taken with other modifiers identified in this project. I will then take promising drug targets forward to drug discovery programmes with my academic and commercial partners.

Finally, I aim to discover new drivers of HD pathology by comparing the biology of neurons made from patients with very early or very late onset HD. I will use cutting-edge technology called RNA sequencing to measure the levels of expression of all 20,000 human genes in the neurons and then analyse whether there are differences that can be linked to early or late onset disease. This experiment could identify new pathways that lead to neurodegeneration and also suggest new modifier genes for testing in my pipeline.

Huntington's disease (HD) is one of nearly 50 human diseases caused by short tandem repeat expansions in the genome. Many of these conditions have neurological phenotypes caused by neurodegeneration. None has a disease-modifying therapy. This project will focus on identifying new therapeutic targets in HD, but advances could be applicable across repeat expansion disorders as they share common underlying pathology.

HD is an autosomal dominant life-shortening neurodegenerative condition characterised by progressive worsening of movement control, mental health and cognition. HD is caused by an expansion of at least 36 CAGs in the HTT gene. Longer repeat tracts are associated with earlier onset of symptoms but there is much variation. Genetic studies have found variants elsewhere in the genome associated with significant differences in age at HD onset. These 'genetic modifiers' provide a set of potential therapeutic leads discovered in patients. In this project I aim to develop a pipeline that will use genetic modifiers to identify new drug targets. I will first screen putative modifier genes for effects in two model experimental systems of HD: human induced pluripotent stem cells (iPSCs)/neurons and Drosophila fruit flies. HD-relevant phenotypes such as somatic CAG expansion, DNA damage and neuronal survival will be tested. Identified modifier genes with therapeutic potential will then be taken forward for detailed molecular studies to determine whether a drug discovery programme would be feasible. For example, I will first characterise a modifier that we have recently shown to drive somatic CAG expansions. Finally, I aim to discover new pathogenic drivers in HD by comparing gene expression and splicing in brain neurons derived from HD patients with very early or very late onset disease compared to predictions from CAG length. New drivers of pathology would give insight into HD pathogenesis and be potential therapeutic targets, to be assessed in our pipeline.