Giant axonal neuropathy: cross-sectional analysis of a large natural history cohort

Abstract

Giant axonal neuropathy (GAN) is an ultra-rare autosomal recessive, progressive neurodegenerative disease with early childhood onset that presents as a prominent sensorimotor neuropathy and commonly progresses to affect both the PNS and CNS. The disease is caused by biallelic mutations in the GAN gene located on 16q23.2, leading to loss of functional gigaxonin, a substrate specific ubiquitin ligase adapter protein necessary for the regulation of intermediate filament turnover. Here, we report on cross-sectional data from the first study visit of a prospectively collected natural history study of 45 individuals, age range 3-21 years with genetically confirmed GAN to describe and cross-correlate baseline clinical and functional cohort characteristics. We review causative variants distributed throughout the GAN gene in this cohort and identify a recurrent founder mutation in individuals with GAN of Mexican descent as well as cases of recurrent uniparental isodisomy. Through cross-correlational analysis of measures of strength, motor function and electrophysiological markers of disease severity, we identified the Motor Function Measure 32 to have the strongest correlation across measures and age in individuals with GAN. We analysed the Motor Function Measure 32 scores as they correspond to age and ambulatory status. Importantly, we identified and characterized a subcohort of individuals with a milder form of GAN and with a presentation similar to Charcot-Marie-Tooth disease. Such a clinical presentation is distinct from the classic presentation of GAN, and we demonstrate how the two groups diverge in performance on the Motor Function Measure 32 and other functional motor scales. We further present data on the first systematic clinical analysis of autonomic impairment in GAN as performed on a subset of the natural history cohort. Our cohort of individuals with genetically confirmed GAN is the largest reported to date and highlights the clinical heterogeneity and the unique phenotypic and functional characteristics of GAN in relation to disease state. The present work is designed to serve as a foundation for a prospective natural history study and functions in concert with the ongoing gene therapy trial for children with GAN.

Keywords: Motor Function Measure 32 (MFM-32); autonomic function; giant axonal neuropathy; natural history; neuromuscular.

Figure 1

GAN mutations. Figure shows the GAN gene with mapping of the variants in our study cohort. Variants with deletions are noted with dashed lines above and below the gene map. Variants with predicted loss of function (‘null’ mutations) are shown in red. Variants that were recurrent in unrelated individuals in this study are highlighted in bold. Domains of the GAN gene are shown in colour including the BTB domain (blue), Back domain (pink) and Kelch Repeat domain (green).

Figure 2

Clinical manifestations of giant axonal neuropathy. (A) Tightly curled (also classically described as ‘kinky’ or ‘frizzy’) hair in GAN—characterized by a dull appearance and course texture with tight curls. (B) Curly, but not ‘frizzy’ hair is seen commonly in those with milder disease severity (axonal CMT-plus phenotype). (C) Severe finger flexor contractures develop as seen here in a 15-year-old male with GAN. (D and E) Rapid progression of rotational and S-shaped scoliosis in the same male with GAN at age 12 years (D) and 15 years (E). (F) Enlarged (giant) axons (asterisks) surrounded by abnormally thin myelin sheaths on electron microscopy from a prior diagnostic sural sensory nerve biopsy in a child with GAN. (G and H) Axial FLAIR brain MRI in a 3-year-old female with GAN showing no significant signal abnormalities within cerebral white matter and early hyperintense signal abnormalities within cerebellar white matter in the region surrounding cerebellar nuclei (white arrows) as compared to (I and J) axial FLAIR brain MRI in the same female at 12 years of age showing confluent hyperintense signal abnormalities within the white matter (plus signs) of the cerebrum, cerebellum and brainstem.

Figure 3

Ambulatory status compared to age. Box plot of ages in those who maintain their ability to walk independently, who walk with assistance and who have lost independent ambulation. Box plots represent the 25th and 75th percentiles within each group, and whiskers demonstrate the relationship to the upper and lower limits as calculated using the IQR [e.g. upper limit defined as Q3 + 1.5(IQR) or lower limit defined as Q1 − 1.5(IQR)]. Median age within each group is as follows: independently ambulant = 7.3 years, ambulant but requiring assistance = 10.1 years, and non-ambulant = 11.0 years.

Figure 4

Correlation matrix. Correlation plot showing Spearman correlations using a continuous colour scale with positive correlations in blue and negative correlations in red (as shown in the bar to the right), and with a bolder colour representing a larger magnitude correlation coefficient. Outcome measures are duplicated in both the x- and y-axes, but along the x-axis, the variables are labelled only by overall clinical assessment type. Correlations between pairs of variables can be read by tracing across x- and y-axes in a grid format. Non-significant correlations are denoted with an X. This figure only uses data from 37 individuals (as these individuals were >6 years old). Clinical assessment types in the correlation matrix: Function: NIS, FARS, MFM-32 percentage score. Timed Testing: supine floor-to-standing (time to arise from lying supine on the floor to standing up); the time it takes to run 10 m, climb up four steps or descend four steps is also recorded. Nerves: median motor (recorded at abductor pollicis brevis), ulnar motor (recorded at abductor digiti minimi), and peroneal motor (recorded at the tibialis anterior muscle). Of note, the peroneal motor response was recorded from the tibialis anterior (TA) muscle, given that the more distally evaluated motor response from the extensor digitorum brevis was undetectable in many of the participants. Myometry: measure of percentage predicted value for the muscle groups/movements listed. Distal: myogrip [measure of grip force (kg) as compared to normative values]; myopinch [measure of pinch force (kg) generated with pinch task], and moviplate (number of finger taps between two objects).

Figure 5

Ambulation status by MFM-32 score and subscale. Ambulation status by MFM-32 total percentage score and the MFM-32 subdomain (D1, D2 or D3) percentage scores are plotted. This plot only includes individuals over the age of 6 where MFM-32 was performed (n = 37). Eighteen individuals were independently ambulant, 10 required assistance to walk, and nine were non-ambulant (as indicated by the coloured shapes). Each individual’s total score and subscale score are plotted. The box plots highlight the MFM-32 total percentage score as well as the percentage score for the subdomains with scores as follows [median (25th percentile; 75th percentile)]: MFM-32 total, 64.5 (52.10; 79.17); D1, 35.55 (12.82; 53.85); D2, 91.21 (88.9; 100); D3, 72.46 (57.1; 90.5). Whiskers reflect relationship to the upper and lower limits as calculated using the IQR [e.g. upper limit defined as Q3 + 1.5(IQR) or lower limit defined as Q1 − 1.5(IQR)]. The MFM-32 total score, D1 score and the D3 score are distributed across the range of scores from more severe to milder, while the D2 domain appears to show a possible ceiling effect, with a lower D2 score not seen until individuals have lost ambulation.

Figure 6

Composite/functional scores compared to age. Functional measures MFM-32 total percentage score, NIS and FARS are plotted by age and ambulation status. Functional outliers, or those with the milder GAN phenotype (n = 10), are denoted with grey circles. With milder GAN individuals (or functional outliers) excluded, there appears to be a more linear and homogeneous pattern between age and MFM-32, NIS and FARS amongst individuals with the classic phenotype.

Figure 7

Impact of genetic subtype and milder GAN phenotype on functional assessments. Functional measures MFM-32 total percentage score, NIS and FARS plotted by mutation status. The box plots represent the 25th and 75th percentiles within each group, with whiskers defined as: upper limit: Q3 +1.5 (IQR) or lower limit: Q1 −1.5 (IQR). Predicted CRIM negative refers to those participants with biallelic null mutations. Functional outliers, or those with the milder GAN (‘axonal CMT-plus’) phenotype (n = 10), are denoted with grey circles. CRIM-negative participants do not ever manifest with the milder GAN phenotype, and on average tended to perform similar or slightly worse on functional scales compared to the CRIM-positive individuals, as demonstrated by a lower median MFM32 percentage score and higher median NIS and FARS score.