Professor Robert Richards

Dynamic Mutation Disorders: Mechanism of Pathogenesis in Neurodegenerative Repeat Expansion Diseases

The term “dynamic mutation” was introduced to distinguish the unique properties of expanding, unstable DNA repeat sequences from other forms of mutation (Richards, 2001b). The past two decades have seen dynamic mutations uncovered as the molecular basis for a growing number of human genetic diseases (including Huntington’s Disease) in addition to all of the characterised ‘rare’ chromosomal fragile sites. The common properties of the repeats in different diseases and fragile sites have given insight into this unique form of DNA instability. While the dynamic mutation mechanism explains some of the unusual genetic characteristics of the relevant diseases, unexpected findings have raised new questions and challenged some assumptions about the pathways that lead from mutation to disease (Richards and McLeod 2005). This project is aimed at understanding the molecular mechanisms involved in the pathogenic pathways that lead from expanded repeats to the diseases with which they are associated (McLeod et al., 2005; Lawlor et al., 2011; Samaraweera et al., 2013).

Is there a Common Pathogenic Pathway in the Dynamic Mutation Diseases? – Studies in Drosophila

Several human genetic diseases are thought to be caused by polyglutamine repeats that expand in copy number beyond a threshold (e.g. Huntington’s disease). There are, however, numerous other neurodegenerative diseases that have very similar clinical symptoms to the polyglutamine diseases and are due to expanded repeats, but these repeats cannot encode polyglutamine. We have therefore used transgenic Drosophila models of these repeat expansion diseases in order to test hypotheses in regard to whether there might be common pathogenic pathways for dynamic mutation diseases.

We have specifically assessed 1) whether RNA might be the pathogenic agent [as has been shown for CUG expansion in myotonic dystrophy] 2) whether polyalanine might be the common agent (by virtue of frameshift slippage) 3) whether apoptosis initiated by DNA breakage of the expanded repeat (as is seen for rare chromosomal fragile sites) might be the common pathogenic pathway.

In each case the transgenic Drosophila analysis has been able to rule out the hypothesis being tested. Therefore the likelihood is that polyglutamine is the pathogenic agent for those diseases with the capacity to translate their expansion into polyglutamine and that a distinct pathogenic mechanism exists for those related diseases where the repeat is unable to encode polyglutamine. These results favour the view that the intrinsic properties of the polyglutamine containing proteins are likely to contribute to the pathology. Further experiments will be undertaken to validate this view.

Neurodegenerative diseases have genetic hallmarks of autoinflammatory disease

The notion that one common pathogenic pathway could account for the various clinically distinguishable, typically late-onset neurodegenerative diseases might appear unlikely given the plethora of diverse primary causes of neurodegeneration. On the contrary, an autoinflammatory pathogenic mechanism allows diverse genetic and environmental factors to converge into a common chain of causality. Inflammation has long been known to correlate with neurodegeneration. Until recently this relationship was seen as one of consequence rather than cause—with inflammatory cells and events acting to ‘clean up the mess’ after neurological injury. This explanation is demonstrably inadequate and it is now clear that inflammation is at the very least, rate-limiting for neurodegeneration (and more likely, a principal underlying cause in most if not all neurode- generative diseases), protective in its initial acute phase, but pernicious in its latter chronic phase (Richards et al., 2018)

Inflammation is activated prior to symptoms in neurodegenerative diseases, providing a plausible pathogenic mechanism for cell death. Indeed genetic and pharmacological ablation studies in animal models of several neurodegenerative diseases demonstrate that inflammation is required for pathology.  However, while there is growing evidence that inflammation-mediated cell death may be the common mechanism underlying neurodegenerative diseases, including those due to dominantly inherited expanded repeats, the proximal causal agent is unknown. Expanded CAG.CUG repeat double stranded RNA causes inflammation-mediated cell death when expressed in Drosophila. Repeat expansion mutation therefore confers ‘non-self’ recognition of endogenous RNA, thereby providing a proximal, autoinflammatory trigger for expanded repeat neurodegenerative diseases.