Giant axonal neuropathy (GAN): cross-sectional data on phenotypes, genotypes, and proteomic signature from a German cohort

Abstract

Giant axonal neuropathy (GAN) is a progressive neurodegenerative disease affecting the peripheral and central nervous system and is caused by bi-allelic variants in the GAN gene, leading to loss of functional gigaxonin protein. A treatment does not exist, but a first clinical trial using a gene therapy approach has recently been completed. Here, we conducted the first systematic study of GAN patients treated by German-speaking child neurologists. We collected clinical, genetic, and epidemiological data from a total of 15 patients representing one of the largest cohorts described thus far. Average age of patients was 11.7 years at inclusion. The most frequently reported symptoms (HPO coded) were gait disturbance and muscle weakness, abnormality of muscle size, and abnormal reflexes. In line with the frequency of homozygous variants, in five families, parents reported being at least distantly related. In 14 patients, diagnosis was confirmed by molecular genetic testing, revealing eight different GAN variants, four being reported as pathogenic in the literature. Proteomics of white blood cells derived from four patients was conducted to obtain unbiased insights into the underlying pathophysiology and revealed dysregulation of 111 proteins implicated in diverse biological processes. Of note, diverse of these proteins is known to be crucial for proper synaptic function and transmission and affection of intermediate filament organisation and proteolysis, which is in line with the known functions of gigaxonin.

Keywords: Consanguinity; GAN Gene; Giant axonal neuropathy; Gigaxonin; Neurodegenrative; Proteomics.

Fig. 1

Clinical data of GAN patients. A Patient 1 at 12 years of age showing typical clinical signs for GAN, e.g., kyphoscoliosis, pes planus, generalised amyotrophy, contractures of interphalangeal joints and frizzy hair, as well as being fully wheelchair dependent at the time of examination due to profound muscle weakness. B T2 axial (left) and T1 coronar (right) MRI of patient 5 at 15 years of age showing typical diffuse signal hyperintensities of the white matter of the cerebrum and cerebellum. C Electron microscopy of suralis nerve biopsy of patient 15 showing enlarged axons (indicated by arrows) surrounded by relatively thin myelin sheaths in contrast to normal axons (indicated by arrowheads)

Fig. 2

Genetic data of GAN patients. All GAN variants included in this study arranged by cDNA (dark red) and protein (blue) position. All identified variants are homozygous. Gene sequence and protein domains were obtained from the ensemble and NCBI databases

Fig. 3

Proteomic data of GAN patients. A Schematic representation of the proteomic workflow applied on patient-derived white blood cells. B Volcano plot depicting and highlighting the significant upregulated proteins (red; FC ≥ 2) and downregulated proteins (blue; FC ≤ 0.5). C GO-Term analysis; bar graphs depicting upregulated and downregulated biological processes in regard to the significant upregulated and downregulated proteins. D Comparison of proteomic signatures of patients 1, 5, and 6 (grouped) versus 10 to pinpoint proteins serving as pathogenicity markers unveils 54 proteins (36 decreased and 18 increased) commonly dysregulated in GAN patients. E Box plot of HYOU1 abundances in white blood cells of patients 1, 5, 6, and 10 showing a decrease of this co-chaperone in all patients. Numbers above boxes display the respective statistical significance (p-ANOVA) of HYOU1-decrease in the individual patient (P) sample