Proteasome inhibitors rescue hypomyelination in iPSC based model of HIKESHI-associated leukodystrophy.

INTRODUCTION

HIKESHI Associated Leukodystrophy (HAL) is a lethal hypomyelinating genetic condition with most patients dying in childhood. Patients with HAL have developmental delay, paraparesis, and high risk of death following fevers and infections. HAL is caused by homozygous loss-of-function mutations in the HIKESHI gene leading to loss of HIKESHI protein. The most seen missense mutation changes Valine to Leucine at residue 54. HIKESHI is a nuclear import carrier for chaperone HSP70 during heat shock response (HSR). As cellular proteins misfold when exposed to stressors, cells initiate HSR with chaperones refolding damaged proteins. As the primary abnormalities in HAL appear to be related to white matter of the brain, we hypothesized that lack of HIKESHI impairs HSR resulting in abnormal oligodendrocyte development and function. To test our hypothesis, iPSCs were produced from affected homozygous HAL patients, non-symptomatic heterozygous first-degree relatives, and healthy unrelated individuals.

METHODS

iPSCs from patients with HAL were differentiated into oligodendrocytes, followed by immunofluorescent staining and various assays. RNASeq analyses were done with control, HIKESHI heterozygous, and HIKESHI homozygous cells at various stages of oligodendrocyte differentiation.

RESULTS

Immunofluorescence showed HSP70 and other proteins had altered nuclear localization in HIKESHI homozygous cells upon heat shock. We used RNASeq during oligodendrocyte differentiation and found HIKESHI homozygous cells upregulated various heat shock genes and downregulated neurodevelopmental genes. Homozygous cells also had much fewer Olig2+ cells, aberrant myelination, and enlarged and less circular nuclei. Cells were treated with proteasome inhibitors (PI) in an attempt to rescue mutant HIKESHI protein and restore cellular function. PI treatment restored some HIKESHI protein and upon heat shock, cells displayed nuclear localization of HSP70 indicating restoration of some HIKESHI functionality. PI treatment also rescued Olig2+ cell numbers, myelination, and cell morphology abnormalities.

CONCLUSION

Loss of HIKESHI results in dysfunction of oligodendrocyte development and function. Repurposed PI treatment may be considered as a novel therapeutic for patients with HAL.