This study was funded by Ataxia (UK). P.F.C. is a Wellcome Trust Senior Fellow in Clinical Science who also receives funding from the United Mitochondrial Diseases Foundation, and the EU FP program EUmitocombat and MITOCIRCLE. M.Z. receives support from Fondazione Telethon-Italy (GGP030039) and Fondazione Pierfranco e Luisa Mariani, Italy.