Lecture 1 - Diseases involving unstable expansions of polynucleotide repeats

These four lectures were devoted to this highly topical subject, which concerns important neurological and muscular genetic diseases. While Fragile X mental retardation syndrome, the first disease in which this unstable expansion mutational mechanism was identified in 1991, had been covered in various aspects in 2004, 2009 and 2011, and dystrophic myotonia in 2005, the other pathologies involving this mechanism had not been dealt with in my lectures. I mainly covered recently published major findings, such as involvement in a fairly frequent form of amyotrophic lateral sclerosis (Charcot disease), work on pathogenic mechanisms and the involvement of epigenetic mechanisms, and finally some approaches to therapeutic strategies.

The first lecture introduced the historical aspect of the notion of anticipation in dystrophic myotonia, whose biological reality had been denied for almost 70 years, and the succession of discoveries, from 1991 to 2011, of diseases linked to unstable expansions. These diseases can be classified according to the gene location, stability and functional effect of these expansions, into three broad categories: diseases with loss of function of the target gene (more or less complete loss of protein expression, in fragile X syndrome or Friedreich's ataxia), and diseases with gain of toxic property (often called gain of function) either at protein level (Huntington's disease and other polyglutamine expansion diseases) or at messenger RNA or complete transcript level (dystrophic myotonia, FXTAS syndrome or fragile X tremor ataxia syndrome). But other mechanisms could also play a role, involving antisense RNAs or protein translation abnormalities(Repeat Associated Non-ATG Translation Initiation or RAN translation, Zu et al.PNAS 2010). Instability mechanisms were briefly discussed, including the unexpected role of the mismatch-repair system, and in particular the mutator role of the MSH3 gene. A very important recent discovery is that of the expansion mutation of a non-coding hexanucleotide repeat as the cause of the most frequent genetic form of amyotrophic lateral sclerosis (motor neuron degeneration) pure or associated with fronto-temporal dementia, reported in several articles published simultaneously at the end of 2011. The mutation's target gene (C9ORF72) has an unknown function, and the mechanism (gain of toxic property or loss of function) remains controversial. Another interesting observation is the contribution of another locus (ATXN2) with a CAG/polyglutamine repeat as a risk factor (but not causal) for ALS. Two other unstable expansions were found to be responsible for very rare forms of spino-cerebellar ataxia (SCA31 and SCA36) found in the Japanese population. The second lecture focused on diseases where expansion leads to loss of protein expression: Fragile X mental retardation syndrome (FMRP protein) and Friedreich's ataxia (partial deficiency of a mitochondrial protein, frataxin). With regard to Fragile X, the focus was on therapeutic work, starting with mouse and Drosophila models and moving towards clinical trials (published in 2011 and 2012) of glutamate receptor antagonists (mGluR5) or a GABAB agonist_.Friedreich's ataxia was the subject of the end of the^2nd and beginning of the^3rd lecture, where the epigenetic mechanisms involved in the inhibition of frataxin gene expression were presented, and are being studied as potential therapeutic targets. Numerous pharmacological clinical trials are underway or proposed, but I mainly presented the gene therapy work carried out by Hélène Puccio's team at the IGBMC on a mouse model of the cardiac pathology of this disease, which looks very promising (see Abstract below).

Lectures 3 and 4 in this series were devoted to expansions leading to a gain in toxic property, either at protein level (polyglutamine repeat expansion diseases) or at RNA level. Nine neurodegenerative diseases are caused by polyglutamine expansions in different proteins, and differ in terms of the specificity of the main damage to brain regions, and therefore in their clinical manifestations, Huntington's disease being the most frequent and best known. This is a very active field of research (400 publications per year!). A wide variety of pathophysiological mechanisms, involving different cellular compartments, have been proposed on the basis of studies of cellular models or models in mice or Drosophila, to explain polyglutamine toxicity. This supposed diversity of mechanisms goes hand in hand with a number of potential therapeutic targets and some encouraging "therapeutic" results in mouse or Drosophila models, but not in the few clinical trials carried out. The difficulty lies in modeling in laboratory organisms a pathology that takes more than 30 or 40 years to produce its harmful effects, and on the other hand, in clinical trials, measuring effects on slowly and irregularly progressive pathologies. I have already mentioned the role of polyglutamine expansion aggregation, autophagy and epigenetic modulation in pathogenicity. A generic therapeutic approach for all these diseases would be the inhibition of mutant protein expression (without affecting the normal protein), and this has been achieved in model systems by the use of antisense RNA stimulating RNAi-like mechanisms (cf. e.g. Yu et al, Cell 2012). The final part of this lecture series was devoted to the mechanisms of pathologies with toxic RNA. For dystrophic myotonia, while there is general agreement on the involvement of abnormalities in the alternative splicing of several important genes, accounting for major clinical manifestations (including myotonia), recent work by Nicolas Charlet at the IGBMC (see below) has shown an involvement of abnormalities in the metabolism of a microRNA (miR-1), which may explain cardiac manifestations of the disease (Rau et al., 2011). The use of isogenic cell models (human embryonic stem cells carrying the expansion mutation in the DM1 gene) enables us to analyze pathological mechanisms in a way that is in principle faithful, and to search for corrective molecules (work at iSTEM, Evry, Marteyn et al, Cell Stem Cell 2011); and for this pathology too, an antisense-type approach to inhibiting the expression of toxic RNA has been used in various ways in cell and mouse models (work by C. Thornton and L. Garcia). The most recent topic was Fragile X tremor ataxia syndrome, linked to premutation of the FMR1 gene, which took ten years to discover its involvement in a late-onset neurodegenerative disease (by P. and R. Hagerman). Recent elegant work by Nicolas Charlet's team shows the involvement in pathological mechanisms of proteins involved in microRNA splicing or metabolism (SAM68 and DROSHA-DGCR8).

Program

Lecture

16:00 - 17:15