Background Huntington's disease (HD) is an autosomal dominant neurodegenerative disease caused by elongation of a CAG repeat within the first exon of the HD gene. This mutation leads to an elongated glutamine (Q) repeat within the huntingtin protein (Htt), giving rise to a toxic gain of function of the protein. Mutant Htt also forms intracellular inclusions but the pathogenic role of these inclusions is under debate. Current HD therapies available to patients do not target the mutant protein directly but only alleviate symptoms. One possible way to treat HD at the protein level involves the use of VHH. VHH consist of the epitope recognising domain of a unique heavy chain only antibody class found in camelids and sharks. Apart from their small size (16 kDa), VHH are thermostable, easy to produce and despite their single domain nature VHH have comparable affinities for their antigen as conventional antibody molecules. Furthermore, they can be expressed intracellularly to modulate or detect its target inside a living cell.

Aim Identify VHH that bind to mutant Htt, inhibiting its toxic function, thus alleviating HD pathogenesis.

Results VHH were selected from a llama phage display library originating from llama (Llama Glama) immunised with an N terminal fragment consisting of the first 550 amino acids of the Htt protein with an elongated repeat (Q44). Selection was performed using a smaller N terminal fragment (amino acids 14 to 378) with either a normal repeat (Q17) or an elongated repeat (Q43). Binding by the selected VHH of the N terminal Htt fragment was tested using ELISA, Western blot and immunohistochemistry. Selection resulted in 13 different VHH that were specific for the N terminal Htt fragment. There was no apparent difference in binding efficiency of the VHH to the normal fragment and the fragment with the elongated Q repeat.

Conclusion The immunised llama phage display library was successfully used to select VHH specific for the N terminal part of the Htt protein. Promising VHH will be transfected into a cell model of HD to monitor their effect on HD pathogenesis.