Adaptor protein complex 4 deficiency: a paradigm of childhood-onset hereditary spastic paraplegia caused by defective protein trafficking

Robert Behne, Julian Teinert, Miriam Wimmer, Angelica D'Amore, Alexandra K Davies, Joseph M Scarrott, Kathrin Eberhardt, Barbara Brechmann, Ivy Pin-Fang Chen, Elizabeth D Buttermore, Lee Barrett, Sean Dwyer, Teresa Chen, Jennifer Hirst, Antje Wiesener, Devorah Segal, Andrea Martinuzzi, Sofia T Duarte, James T Bennett, Thomas BourinarisHenry Houlden, Agathe Roubertie, Filippo M Santorelli, Margaret Robinson, Mimoun Azzouz, Jonathan O Lipton, Georg H H Borner, Mustafa Sahin, Darius Ebrahimi-Fakhari

Research output: Contribution to journalArticlepeer-review


Deficiency of the adaptor protein complex 4 (AP-4) leads to childhood-onset hereditary spastic paraplegia (AP-4-HSP): SPG47 (AP4B1), SPG50 (AP4M1), SPG51 (AP4E1) and SPG52 (AP4S1). This study aims to evaluate the impact of loss-of-function variants in AP-4 subunits on intracellular protein trafficking using patient-derived cells. We investigated 15 patient-derived fibroblast lines and generated six lines of induced pluripotent stem cell (iPSC)-derived neurons covering a wide range of AP-4 variants. All patient-derived fibroblasts showed reduced levels of the AP4E1 subunit, a surrogate for levels of the AP-4 complex. The autophagy protein ATG9A accumulated in the trans-Golgi network and was depleted from peripheral compartments. Western blot analysis demonstrated a 3-5-fold increase in ATG9A expression in patient lines. ATG9A was redistributed upon re-expression of AP4B1 arguing that mistrafficking of ATG9A is AP-4-dependent. Examining the downstream effects of ATG9A mislocalization, we found that autophagic flux was intact in patient-derived fibroblasts both under nutrient-rich conditions and when autophagy is stimulated. Mitochondrial metabolism and intracellular iron content remained unchanged. In iPSC-derived cortical neurons from patients with AP4B1-associated SPG47, AP-4 subunit levels were reduced while ATG9A accumulated in the trans-Golgi network. Levels of the autophagy marker LC3-II were reduced, suggesting a neuron-specific alteration in autophagosome turnover. Neurite outgrowth and branching were reduced in AP-4-HSP neurons pointing to a role of AP-4-mediated protein trafficking in neuronal development. Collectively, our results establish ATG9A mislocalization as a key marker of AP-4 deficiency in patient-derived cells, including the first human neuron model of AP-4-HSP, which will aid diagnostic and therapeutic studies.

Original languageEnglish
Pages (from-to)320-334
Number of pages15
JournalHuman Molecular Genetics
Issue number2
Publication statusPublished - Dec 5 2019


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