ABCD1 dysfunction alters white matter microvascular perfusion

Abstract

Cerebral X-linked adrenoleukodystrophy is a devastating neurodegenerative disorder caused by mutations in the ABCD1 gene, which lead to a rapidly progressive cerebral inflammatory demyelination in up to 60% of affected males. Selective brain endothelial dysfunction and increased permeability of the blood-brain barrier suggest that white matter microvascular dysfunction contributes to the conversion to cerebral disease. Applying a vascular model to conventional dynamic susceptibility contrast magnetic resonance perfusion imaging, we demonstrate that lack of ABCD1 function causes increased capillary flow heterogeneity in asymptomatic hemizygotes predominantly in the white matter regions and developmental stages with the highest probability for conversion to cerebral disease. In subjects with ongoing inflammatory demyelination we observed a sequence of increased capillary flow heterogeneity followed by blood-brain barrier permeability changes in the perilesional white matter, which predicts lesion progression. These white matter microvascular alterations normalize within 1 year after treatment with haematopoietic stem cell transplantation. For the first time in vivo, our studies unveil a model to assess how ABCD1 alters white matter microvascular function and explores its potential as an earlier biomarker for monitoring disease progression and response to treatment.

Keywords: ABCD1; ALD; cerebral X-linked adrenoleukodystrophy; inflammatory demyelination; microvascular perfusion.

Figure 1

Cerebral adrenoleukodystrophy and proposed effects of ABCD1 deficiency upon brain microvascular system. (A) Representative T₂-weighted (T₂W) and T₁-weighted (T₁W) post-contrast images at baseline (left) and follow-up (right) in early (Loes score: 1 and 3), advanced (Loes score: 18 and 23) and end stage (Loes score: 20 and 30) of cerebral adrenoleukodystrophy (CALD). These cases illustrate the stereotypical symmetrical contiguous spread of T₂-weighted-hyperintensity during lesion progression (upper panels) and the rim of active inflammation as indicated by contrast enhancement on T₁-weighted images (lower panels). (B) Schematic illustration of the effects of ABCD1 deficiency upon endothelial function depicting increased adhesion of blood born leucocytes and increased blood–brain barrier permeability. (C) Vascular model applied to analyse tracer kinetics of dynamic susceptibility contrast perfusion MRI data. According to this model, when capillaries fail to homogenize their mean transit flow times (elevated CTH), as when slow leucocyte transit occurs, their tissue OEC and therefore the theoretical upper limit of metabolic rate of oxygen (CMRO₂^max) is compromised. (D) Schematic of flow dynamics across capillary beds where homogenous, heterogeneous and leaky microvasculature are represented. Graphed intensity curves demonstrating elevated microvascular flow heterogeneity (CTH, red) and exacerbated blood–brain barrier permeability (K_app, yellow). ROI = region of interest.

Figure 2

ABCD1-related changes in microvascular perfusion in ALD subjects without cerebral disease. (A) Representative CTH, OEC, CMRO₂^max and K_app maps of a 14-year-old male control (CON) and a 16-year-old male hemizygote without CALD (HEM) and group comparisons controls versus hemizygote subjects. Data are presented as Tukey plots and are representative of 10 matched controls and 10 hemizygote cases. *P < 0.05, **P < 0.01, two-tailed t-test. (B) Diagram displaying individual white matter perfusion mean values and standard deviation (proportional to circle size) in relationship to CTH, mean transit time (MTT) and CMRO₂^max in hemizygote subjects and controls. Deviation from the 45° line indicates a mismatch in mean transit time and CTH.

Figure 3

Spatial and temporal microvascular perfusion abnormalities in ABCD1 deficiency. (A) Statistical atlas of spatial lesion distribution based on first available MRI scans in 35 subjects with CALD. The probability map indicates percentage white matter involvement based on T₂-weighted signal abnormalities. Axial images depict regions of interest as follows: SPL = splenium of the corpus callosum; MFC = major forceps; OWM = occipital white matter; PPV = posterior periventricular white matter; ICW = internal capsule white matter; FPV = frontal periventricular white matter; LFC = lesser forceps; GEN = genu of the corpus callosum and FWM = frontal white matter; and plot ranking the probabilities of being affected by CALD for each region of interest. (B) Comparison of capillary transit time heterogeneity (CTH) within the splenium of the corpus callosum versus frontal white matter in 10 hemizygote subjects (red) without cerebral disease and 18 controls (controls = blue). Data are presented as Tukey plots, of a paired two-tailed t-test, *P ≤ 0.05. (C) Comparison of longitudinal data of rCTH (rCTH = SPL CTH/thalamic CTH) for the first and last available scans in hemizygote subjects in absence of CALD. (D) Random effects modelling association between rCTH and lesion probability based on 51 scans of eight hemizygote subjects. Regression coefficient: β = 3.8452 ± 0.2695, P < 0.001. (E) Comparison of longitudinal data of rCTH in the SPL normalized to thalamic perfusion for the first and last available scans in hemizygote subjects in absence of CALD. (F) Individual rCTH values plotted against age for n = 8 hemizygote subjects (left) and n = 22 controls (right). (G) Diagram illustrating effect of age on rCTH applying a random effects model to compare hemizygote subjects and controls. Regression coefficients: hemizygote subjects: β = −0.05185, 95% CI −0.093 to −0.010, P = 0.0176; controls: β = −0.00948, 95% CI −0.047 to 0.028, P = 0.6048.

Figure 4

Microvascular perfusion abnormalities precede T₂-weighted abnormalities. (A) CALD lesion segmentation into [A] T₂-weighted (T₂W) hyperintense necrotic core, [B] active inflammatory demyelination characterized by T₁-weighted (T₁W) imaging contrast enhancement; [C] the leading edge of the lesion (T₂-weighted-hyperintense, non-enhancing on T₁-weighted), adjacent [D] and dNAWM]and corresponding signal intensities for structural (T₂-weighted, T₁-weighted post-contrast), diffusion (ADC) and perfusion-based maps (CTH, K_app) in such segmented regions. Data are presented as Tukey plots and are representative of 22 untreated individuals with progressive CALD. *P < 0.05, **P < 0.01, ***P < 0.001 and ****P < 0.0001. One-way ANOVA with repeated measures followed by Dunnett’s multiple comparisons test for CTH and K_app and Friedman test with Dunn’s corrections for multiple comparisons for T₂-weighted, T₁-weighted and ADC. (B) Representative T₂-weighted and CTH maps of CALD illustrating areas of region D (adjacent NAWM) converting (red) versus non-converting (blue) to T₂ hyperintensity on follow-up. Comparison of CTH signal intensities are presented as Tukey plots and are representative of nine untreated individuals with progressing CALD. *P < 0.05, two-tailed t-test. (C) Representative T₁-weighted post contrast and K_app maps illustrating white matter in the outer rim of the CALD lesion progressing (red) versus a non-progressing (blue) to T₁-weighted contrast enhancement on follow-up. Comparison of K_app signal intensities are presented as Tukey plots and representative of eight untreated individuals with progressive CALD. *P < 0.05, two-tailed t-test.

Figure 5

Microvascular perfusion in a patient converting to cerebral disease. Longitudinal sequential imaging in a young male hemizygote who converted to CALD. Upper two panels show structural T₂-weighted (T₂W) and T₁-weighted post-contrast (T₁WPOST) images. Lower panels show corresponding perfusion-based maps for each time point. Slightly elevated at the baseline visit, this patient developed a substantial peak in CTH and increased permeability (K_app) within the splenium of the corpus callosum at the second visit 13 months later. While the T₂-weighted- and T₁-weighted post-contrast images showed no abnormalities at this time-point. Follow-up scan 6.5 months later revealed a corresponding T₂-weighted-hyperintense region. Areas of increased CTH moved further outward from this lesion while blood–brain barrier permeability increased in the core. T₁-weighted images show slight, hazy contrast enhancement. Low CMRO₂^max coincides with appearance of the T₂-weighted lesion. The subject underwent MRI serial screening due to confirmed hemizygote status, was successfully treated with HSCT and remained neurologically asymptomatic during the observation period.

Figure 6

Microvascular perfusion abnormalities normalize after HSCT. (A) Representative baseline and follow-up T₂-weighted (T₂W) and perfusion-based CTH scans in a subject with CALD that underwent successful HSCT. Comparison of CTH signal intensity in NAWM adjacent to the CALD lesion at baseline and follow-up within the first year after HSCT (0.6 ± 0.3 years) is presented as Tukey plots of mean of nine treated individuals with CALD. *P ≤ 0.05, two-tailed t-test. (B) Representative baseline (T₂-weighted and CTH) of a subject with progressing CALD (left) compared to a subject with self-arrested CALD (SA, right). Comparison of same regions as in (A) between progressive CALD and non-progressing CALD cases is presented as Tukey plots of means of 22 untreated CALD cases and 15 perfusion scans of three subjects with self-arrested CALD individuals with CALD. *P < 0.05, two-tailed t-test.