Introduction
Wilson’s disease (WD) is an autosomal recessive disorder caused by mutations in the gene that encodes ATPase 7B (ATP7B), the membrane-bound copper transporter [
1]. This enzyme, expressed in the liver, plays a central role in copper trafficking. It delivers copper to apoceruloplasmin and excretes excess dietary copper into the bile [
2]. In WD, dysfunction of ATP7B leads to hypoceruloplasminemia and insufficient copper removal from liver cells, which results in liver injury; consequently, copper is released into the blood in the form of non-ceruloplasmin-bound copper (NCC), which accumulates and causes damage to other tissues, particularly the brain [
2]. Thus, patients with WD present with hepatic and/or neurological symptoms and may have corneal copper deposits (Kayser-Fleisher rings) [
1,
2].
On brain magnetic resonance images (MRIs) of patients with WD, bilateral T2-hyperintensities are typically observed in the deep brain structures [
3‐
5], and widespread brain atrophy is also commonly observed [
5‐
7]. Compared to healthy controls, brains of patients with WD display reduced volumes in the caudate nuclei, globi pallidi, thalami, cerebellum, and cerebral cortex [
8]. Previous research has shown that brain volume correlates with clinical impairment in different neurological diseases. In multiple sclerosis, whole brain volume, the volumes of gray and white matter, and thalamic volume correlate with disease severity [
9]. In Alzheimer’s disease, hippocampal volume is related to cognitive function [
10], and motor impairment is related to whole brain volume and the volume of gray matter in patients with progressive supranuclear palsy [
11]. In this study, we aimed to determine whether a similar relationship is observed in WD. To that end, we correlated brain volume measures with functional and neurological impairments, assessed using the Unified Wilson’s Disease Rating Scale (UWDRS), [
12,
13] in a relatively large group of treatment-naive patients with WD. We also investigated whether brain atrophy was associated with indices of copper overload.
Discussion
This study for the first time investigated the relationship between brain volumes and disease severity and copper metabolism in WD. Our findings showed that the volumes of all the major brain structures correlated with functional and neurological impairments in treatment-naïve patients with WD. In addition, brain volume could be viewed as a marker of copper-induced neurodegeneration due to in WD, because lower brain volume was observed in patients with higher NCC concentrations.
Previous studies have attempted to investigate the relationship between neuroimaging results and the degree of neurological impairment in WD. However, they did not use validated tools, and/or they included heterogeneous patient samples (e.g., patients treated with different agents) [
7,
22,
23]. In particular, previous studies did not investigate whether the severity of brain atrophy was related to the functional and neurological states of patients with WD or to NCC. Moreover, except for the study by Stezin [
8], previous studies assessed brain atrophy in WD subjectively (presence vs. absence) [
6,
7,
23]. We addressed these issues by evaluating neurological and functional states with a disease-specific clinical scale (UWDRS) and by quantitatively measuring brain volumes (with SIENAX and FIRST tools) in treatment-naïve patients with WD.
In our study, the total brain volume and the WM and GM volumes (both PGM and DGM; Fig.
1d–j) showed comparable relevance to the functional and neurological states of patients. This finding was consistent with the diffuse nature of brain atrophy described in patients with WD [
5,
6,
9]. A likely explanation might be that copper accumulates equally in all brain regions in patients with WD [
24], which leads to damage in both WM and GM; indeed, neuronal loss, axonal disruption, and multifocal demyelination are potential causes of brain atrophy [
25,
26]. Moreover, in an earlier study, we found that greater neurological impairment severity in WD was associated with decreased retinal nerve fiber layer thickness [
27]. That finding led us to suspect that, in WD, brain volume might be correlated with retinal nerve fiber layer thickness, as it is, for instance, in multiple sclerosis [
28]. The strong correlation we found between brain volume and neurological and functional states suggested that brain volume measures could potentially become valuable endpoints in clinical trials in WD. This approach could decrease the sample size needed to demonstrate a treatment effect. However, measuring the effect would require longitudinal analyses to show that changes in brain volume over time were correlated with changes in the clinical state.
Similar to earlier studies, we found that age was associated with decreasing volumes of all the studied brain compartments [
29,
30]. Also, in our study, male sex was a risk factor for decreased DGM and increased vCSF volumes. This added to our previous findings in WD that males, more frequently than females, showed neuroimaging abnormalities [
6] and had a higher risk of developing neurological symptoms [
31]. However, in this study, we could not find a feasible explanation for the inter-sex differences regarding the two brain volumes. First, these differences were not found in healthy individuals in a previous study that employed a similar method [
32]. Second, deep brain nuclei were shown to be affected in WD to the same extent in both sexes [
6]. Thus, further research is needed to evaluate deep brain nuclei in WD.
Copper toxicity is the key pathogenic mechanism in WD, and we showed herein that it was also related to brain atrophy and neurological and functional impairments at diagnosis. Indicators of total brain volume (i.e., BPF) and vCSF volume were significantly negatively and positively correlated, respectively, with the NCC concentration, which is typically elevated in untreated patients with WD. Similarly, PGM volume tended to be lower in patients with higher NCC concentrations.
Our study had limitations. First, it was retrospective, which in principle, can lead to selection bias, data inhomogeneity, and missing data. However, we collected neuroimaging and clinical data in our cohort in a prospective manner according to a standardized protocol. Therefore, we could retrieve data relevant to this study for all consecutive patients examined during the studied period. To preserve the homogeneity of data, we did not include copper metabolism data from patients treated with anti-copper agents or any data that were not acquired in our center. Second, in statistical terms, the study sample was small; however, WD is rare, and our sample of treatment-naïve patients was relatively large in comparison to earlier studies on WD. Third, the measurement of NCC concentration—one of the serum copper overload indices—was not standardized, which could lead to different findings between different laboratories [
33]. However, all measurements of ceruloplasmin and copper in our study were consistently conducted in the same laboratory. Lastly, we did not assess cognitive function, which could be related to brain volume in WD.
In conclusion, our findings provided the first in vivo evidence that the severity of brain atrophy is an important correlate of neurological and functional impairments in patients with WD. Our findings also suggested that brain volume can serve as a marker of copper toxicity. In the future, longitudinal studies should be conducted to determine whether brain volume changes over time correlate with the changes in the clinical state.
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