Mechanism of AP-4 related HSPs clarified

Applies to HSP types 47, 50, 51 and 52


The protein ATG9A has been shown to be a key marker of AP-4 deficiency that underlies HSP types 47, 50, 51 and 52. Measures of the levels and distribution of this protein will help in diagnostic and therapeutic studies of these HSP types.


Disruption of a protein ATG9A impairs the ability of brain cells (neurons) to recycle (autophagy) and to branch and grow (neuronal development). This disease-causing mechanism was established in this study of HSP types 47, 50, 51 and 52 that all share a deficiency of the adaptor protein complex 4 (AP-4). The effects of disruption to ATG9A distribution were seen in skin cells (fibroblasts) from people with these forms of HSP and also in neurons differentiated from the skin fibroblasts.



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.


SOURCE: Hum Mol Genet. 2020 Jan 15;29(2):320-334. doi: 10.1093/hmg/ddz310. © The Author(s) 2020. Published by Oxford University Press. All rights reserved. PMID: 31915823


Adaptor Protein Complex 4 Deficiency: A Paradigm of Childhood-Onset Hereditary Spastic Paraplegia Caused by Defective Protein Trafficking


Robert Behne 1 2Julian Teinert 1 3Miriam Wimmer 1Angelica D’Amore 1 4Alexandra K Davies 5 6Joseph M Scarrott 7Kathrin Eberhardt 1Barbara Brechmann 1Ivy Pin-Fang Chen 8Elizabeth D Buttermore 8Lee Barrett 8Sean Dwyer 8Teresa Chen 8Jennifer Hirst 5Antje Wiesener 9Devorah Segal 10Andrea Martinuzzi 11Sofia T Duarte 12James T Bennett 13Thomas Bourinaris 14Henry Houlden 14Agathe Roubertie 15Filippo M Santorelli 4Margaret Robinson 5Mimoun Azzouz 7Jonathan O Lipton 1 16Georg H H Borner 6Mustafa Sahin 1 8Darius Ebrahimi-Fakhari 1


1 Department of Neurology, The F.M. Kirby Neurobiology Center, Boston Children’s Hospital, Harvard Medical School, Boston, MA 02115, USA.

2 Department of Neurology, University Hospital Würzburg, 97080 Würzburg, Germany.

3 Division of Pediatric Neurology and Metabolic Medicine, Center for Child and Adolescent Medicine, University Hospital Heidelberg, 69120 Heidelberg, Germany.

4 Molecular Medicine, IRCCS Fondazione Stella Maris, 56018 Pisa, Italy.

5 Cambridge Institute for Medical Research, University of Cambridge, Cambridge CB2 0XY, UK.

6 Department of Proteomics and Signal Transduction, Max Planck Institute of Biochemistry, 82152 Martinsried, Germany.

7 Department of Neuroscience, Sheffield Institute for Translational Neuroscience (SITraN), University of Sheffield, Sheffield S10 2HQ, UK.

8 Translational Neuroscience Center, Boston Children’s Hospital, Harvard Medical School, Boston, MA 02115, USA.

9 Institute of Human Genetics, Friedrich-Alexander Universität Erlangen-Nürnberg, 91054 Erlangen, Germany.

10 Division of Pediatric Neurology, Department of Pediatrics, Weill Cornell Medicine, New York City, NY 10021, USA.

11 Scientific Institute, IRCCS E. Medea, Unità Operativa Conegliano, 31015 Treviso, Italy.

12 Department of Pediatric Neurology, Centro Hospitalar de Lisboa Central, 1169-050 Lisbon, Portugal.

13 Division of Genetic Medicine, Department of Pediatrics, University of Washington, Seattle, WA 98195, USA.

14 Department of Molecular Neuroscience, UCL Institute of Neurology, London WC1E 6BT, UK.

15 Pediatric Neurology, CHU Montpellier, 34295 Montpellier, France.

16 Division of Sleep Medicine, Harvard Medical School, Boston, MA 02115, USA.


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