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

Hum Mol Genet. 2020 Jan 15;29(2):320-334. doi: 10.1093/hmg/ddz310.


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.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adaptor Protein Complex 4 / deficiency
  • Adaptor Protein Complex 4 / genetics*
  • Adaptor Protein Complex 4 / metabolism*
  • Adaptor Protein Complex beta Subunits / metabolism
  • Adolescent
  • Autophagosomes / metabolism
  • Autophagy / genetics
  • Autophagy-Related Proteins / metabolism*
  • Cell Line
  • Child
  • Child, Preschool
  • Female
  • Fibroblasts / metabolism
  • Humans
  • Induced Pluripotent Stem Cells / metabolism
  • Iron / metabolism
  • Loss of Function Mutation
  • Male
  • Membrane Proteins / metabolism*
  • Microtubule-Associated Proteins / metabolism
  • Mitochondria / metabolism
  • Neurogenesis / genetics
  • Neurons / metabolism
  • Protein Transport / genetics*
  • Spastic Paraplegia, Hereditary / genetics
  • Spastic Paraplegia, Hereditary / metabolism*
  • Vesicular Transport Proteins / metabolism*
  • trans-Golgi Network / genetics
  • trans-Golgi Network / metabolism*


  • AP4B1 protein, human
  • Adaptor Protein Complex 4
  • Adaptor Protein Complex beta Subunits
  • Atg9a protein, human
  • Autophagy-Related Proteins
  • MAP1LC3B protein, human
  • Membrane Proteins
  • Microtubule-Associated Proteins
  • Vesicular Transport Proteins
  • Iron