CRISPR/Cas9 does not facilitate stable expression of long C9orf72 dipeptides in mice.

A C9orf72 repeat expansion is the most common cause of both frontotemporal dementia and motor neuron disease. The expansion is translated to produce dipeptide repeat proteins (DPRs), which are toxic in vivo and in vitro. However, the mechanisms underlying DPR toxicity remain unclear. Mouse models which express DPRs at repeat lengths found in human disease are urgently required to investigate this. We aimed to generate transgenic mice expressing DPRs at repeat lengths of >1000 using alternative codon sequences, to reduce the repetitive nature of the insert. We found that although these inserts did integrate into the mouse genome, the alternative codon sequences did not protect from instability between generations. Our findings suggest that stable integration of long DPR sequences may not be possible. Administration of viral vectors after birth may be a more effective delivery method for long repeats.