Mutant huntingtin mRNA silencing and DNA repair as gene therapy approaches for Huntington’s disease

Huntington’s disease is a rare neurodegenerative disease caused by a pathologic CAG expansion in the exon 1 of the huntingtin (HTT) gene. Aggregation and abnormal function of the mutant HTT (mHTT) cause motor, cognitive and psychiatric symptoms in patients, which lead to death in 15-20 years. Due to the selective death of MSN, it has been first hypothesized that only neurons are sensitive to the mHTT. However, recent results suggest that other cell populations are playing key roles in HD. Astrocytic dysfunctions have been observed in animal models and are sufficient to induce neuronal alterations classically observed in HD. Currently, there is no treatment for HD. Experimental approaches based on drug, cell or gene therapy are developed and reach progressively the clinic. Among them, RNA silencing with small-hairpin RNA has been proved to reduce mutant HTT expression and to slow down the disease progression. Due to the increasing contribution of non-neuronal cell populations to the disease, mHTT silencing in glial cells, in particular astrocytes, has to be considered. The first part of the thesis will be to optimize a lentiviral vector for astrocyte-specific gene silencing in the mouse brain.

Because HD is an autosomal dominant disease caused by a CAG expansion on a single gene, an attractive therapeutic approach should be to directly repair the huntingtin gene with genome engineering tools. Among them, the Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR) system may overcome some of limitations of previous nuclease-directed tools. The second part of the thesis will be to develop a therapeutic strategy based on the CRISPR system to definitely repair the mutant gene in both cultured cells and in the mouse brain.