Advancing the pathologic phenotype of giant axonal neuropathy: early involvement of the ocular lens

Abstract

Giant axonal neuropathy (GAN; ORPHA: 643; OMIM# 256850) is a rare, hereditary, pediatric neurodegenerative disorder associated with intracellular accumulations of intermediate filaments (IFs). GAN knockout (KO) mouse models mirror the IF dysregulation and widespread nervous system pathology seen in human GAN. Validation of therapeutic efficacy and viral vector delivery systems with these GAN KO models has provided the springboard for the development of a viral vector being delivered intrathecally in an ongoing Phase I gene therapy clinical trial for the treatment of children with GAN ( https://ichgcp.net/clinical-trials-registry/NCT02362438 ). During the course of a comprehensive pathologic characterization of the GAN KO mouse, we discovered the very early and unexpected involvement of the ocular lens. Light microscopy revealed the presence of intracytoplasmic inclusion bodies within lens epithelial cells. The inclusion bodies showed strong immunohistochemical positivity for glial fibrillary acidic protein (GFAP). We confirmed that intracytoplasmic inclusion bodies are also present within lens epithelial cells in human GAN. These IF inclusion bodies in lens epithelial cells are unique to GAN. Similar IF inclusion bodies in lens epithelial cells have not been reported previously in experimental animal models or human diseases. Since current paradigms in drug discovery and drug repurposing for IF-associated disorders are often hindered by lack of validated targets, our findings suggest that lens epithelial cells in the GAN KO mouse may provide a potential target, in vivo and in vitro, for evaluating drug efficacy and alternative therapeutic approaches in promoting the clearance of IF inclusions in GAN and other diseases characterized by intracellular IF accumulations.

Keywords: GAN KO mouse model; Giant axonal neuropathy (GAN); Gigaxonin; Human GAN; IF accumulations; Intermediate filaments (IF); Lens epithelium.

Conflict of interest statement

Ethics approval and consent to participate

All procedures were performed in accordance with the Guide for the Care and Use of Laboratory Animals [DHHS Publication No. (NIH) 85–23] and approved by the UNC–CH Institutional Animal Care and Use Committee. Human specimens were obtained through a validated autopsy consent from the legal next of kin, which explicitly stated that tissues could be used for research purposes. The consent and diagnostic autopsy report were filed in the deceased patient’s medical record.

Consent for publication

Not applicable.

Competing interests

All authors declare that they have no competing interests.

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Figures

Fig. 1

Ocular Lens. Control. Lens cells exist in two distinct forms, lens epithelial cells and lens fiber cells. The anterior surface of the lens is covered by a single layer of epithelial cells that serve as a reservoir for continual lens fiber cell formation and lens growth throughout life. The lens is unique as reflected in almost continuous cell production with negligible cell loss. On their path to becoming mature lens fiber cells, lens epithelial cells undergo extraordinary structural differentiation [10]. GAN KO mouse. Oval intracytoplasmic eosinophilic inclusion bodies within lens epithelial cells

Fig. 2

Lens epithelial cells in GAN KO mice, age-matched controls and human GAN. a. GAN KO (4-month-old) lens epithelial cells show intracytoplasmic inclusion bodies (H&E original magnification 100X). b. GAN/J KO (24-month-old) lens epithelial cell inclusion bodies (H&E original magnification 100X). c. GAN/J KO (24-month-old) lens epithelial cell inclusion bodies show strong GFAP immunoreactivity (GFAP IHC original magnification 100X). d. Control mouse (24-month-old) histologically unremarkable lens epithelial cells (H&E original magnification 100X). e. Human GAN lens epithelial cells reveal intracytoplasmic inclusion bodies (H&E original magnification 100X, formalin fixed, paraffin embedded tissue. Decedent was a young child with phenotypically typical GAN) (arrows point to one of the numerous intracytoplasmic inclusion bodies). Inset. Lens epithelial cells. Control mouse (24-month-old) (H&E); GAN/J KO (24 -month-old) lens epithelial cell inclusion body (H&E); GAN/J KO (24-month-old) lens epithelial cell inclusion body shows strong GFAP immunoreactivity (GFAP IHC). (Ndesignates nucleus, arrow points to cytoplasmic inclusion body)