This is the final version. It was first published by OUP at http://hmg.oxfordjournals.org/content/early/2015/06/23/hmg.ddv220.long. ; Adaptor proteins (AP 1-5) are heterotetrameric complexes that facilitate specialized cargo sorting in vesicular-mediated trafficking. Mutations in AP5Z1, encoding a subunit of the AP-5 complex, have been reported to cause hereditary spastic paraplegia (HSP), although their impact at the cellular level has not been assessed.Here we characterize three independent fibroblast lines derived from skin biopsies of patients harbouring nonsense mutations in AP5Z1 and presenting with spastic paraplegia accompanied by neuropathy, parkinsonism or and/or cognitive impairment. In all three patient-derived lines we show that there is complete loss of AP-5 ? protein and a reduction in the associated AP-5 ?5 protein. Using ultrastructural analysis we show that these patient-derived lines consistently exhibit abundant multilamellar structures that are positive for markers of endolysosomes and are filled with aberrant storage material organised as exaggerated multilamellar whorls, striated belts and 'fingerprint bodies'. This phenotype can be replicated in a HeLa cell culture model by siRNA knockdown of AP-5 ?. The cellular phenotype bears striking resemblance to features described in a number of lysosomal storage diseases.Collectively, these findings reveal an emerging picture of the role of AP-5 in endosomal and lysosomal homeostasis, illuminates a potential pathomechanism which is relevant to the role of AP-5 in neurons, and expands the understanding of recessive HSPs. Moreover, the resulting accumulation of storage material in endolysosomes leads us to propose that AP-5 deficiency represents a new type of lysosomal storage disease. ; This work was supported by theWellcome Trust (086598; J.H., J.R. E. and M.S.R.), the European Union (OMICS Call, Neuromics project; F.D., A.D., T.E. and G.S.), Verum Foundation (G.S.), Programme d?Investissement d?Avenir (ANR-10-IAIHU-06; F.D., A. D., T.E. and G.S.), ERC starting grant (311149; F.D.), the Intramural Research Program of the NINDS, National Institutes of Health (J.C., R.H.R. and C.B.) and National Institutes of Health grant NS083739 (M.M. and M.C.K.), Doris Duke Foundation (Clinical Scientist Development Award to M.C.K.) and Healthcare Research of the Italian Ministry of Health (C.M. and C.G.). C.G. also received partial research support by AriSLA (NOVALS 2010). Funding to pay the Open Access publication charges for this article was provided by the Wellcome Trust.