Neurofilament light chain and glial fibrillary acidic protein levels in metachromatic leukodystrophy

Abstract

Metachromatic leukodystrophy is a lethal metabolic leukodystrophy, with emerging treatments for early disease stages. Biomarkers to measure disease activity are required for clinical assessment and treatment follow-up. This retrospective study compared neurofilament light chain and glial fibrillary acidic protein (GFAP) levels in CSF (n = 11) and blood (n = 92) samples of 40 patients with metachromatic leukodystrophy (aged 0-42 years) with 38 neurologically healthy children (aged 0-17 years) and 38 healthy adults (aged 18-45 years), and analysed the associations between these levels with clinical phenotype and disease evolution in untreated and transplanted patients. Metachromatic leukodystrophy subtype was determined based on the (expected) age of symptom onset. Disease activity was assessed by measuring gross motor function deterioration and brain MRI. Longitudinal analyses with measurements up to 23 years after diagnosis were performed using linear mixed models. CSF and blood neurofilament light chain and GFAP levels in paediatric controls were negatively associated with age (all P < 0.001). Blood neurofilament light chain level at diagnosis (median, interquartile range; picograms per millilitre) was significantly increased in both presymptomatic (14.7, 10.6-56.7) and symptomatic patients (136, 40.8-445) compared to controls (5.6, 4.5-7.1), and highest among patients with late-infantile (456, 201-854) or early-juvenile metachromatic leukodystrophy (291.0, 104-445) and those ineligible for treatment based on best practice (291, 57.4-472). GFAP level (median, interquartile range; picogram per millilitre) was only increased in symptomatic patients (591, 224-1150) compared to controls (119, 78.2-338) and not significantly associated with treatment eligibility (P = 0.093). Higher blood neurofilament light chain and GFAP levels at diagnosis were associated with rapid disease progression in late-infantile (P = 0.006 and P = 0.051, respectively) and early-juvenile patients (P = 0.048 and P = 0.039, respectively). Finally, blood neurofilament light chain and GFAP levels decreased during follow-up in untreated and transplanted patients but remained elevated compared with controls. Only neurofilament light chain levels were associated with MRI deterioration (P < 0.001). This study indicates that both proteins may be considered as non-invasive biomarkers for clinical phenotype and disease stage at clinical assessment, and that neurofilament light chain might enable neurologists to make better informed treatment decisions. In addition, neurofilament light chain holds promise assessing treatment response. Importantly, both biomarkers require paediatric reference values, given that their levels first decrease before increasing with advancing age.

Keywords: arylsulfatase A; biomarker; glial fibrillary acidic protein; metachromatic leukodystrophy; neurofilament light.

Figure 1

NfL and GFAP levels in controls expressed on a log₁₀ scale. (A) NfL and (B) GFAP levels in blood are visualized over age with estimated LOESS regression curves (locally weighted scatter-plot smoother) and their 95% CIs (shadows). NfL and GFAP levels in blood show a non-linear decrease during childhood with a steeper slope in the first years of life, before increasing with advancing age in adulthood. NfL and GFAP levels in CSF show similar trends (data not shown because individual adult CSF control values were unavailable). (C) Median NfL level was significantly higher in CSF (100 pg/ml) compared to blood (5.8 pg/ml) and (D) median GFAP level was significantly higher in CSF (5564 pg/ml) compared to blood (370 pg/ml) in paired samples from paediatric controls (aged 0–17 years). Boxes depict median and IQR, grey lines indicate paired measurements, and upper/lower whiskers extend from the hinge towards the largest/smallest values but no farther than 1.5 times IQR from the hinge. The P-values were obtained with the non-parametric Wilcoxon signed-rank test.

Figure 2

NfL and GFAP levels in blood in patients at MLD diagnosis. (A) Levels of bNfL and (B) bGFAP expressed on a log₁₀ scale in patients with MLD compared to similar aged controls grouped by clinical phenotype and corresponding age. Boxes depict median and IQR within a group, and dots mark individual measurements. The P-values were obtained with a linear regression model adjusted for age at sampling and sex. (C) NfL levels and (D) GFAP levels in blood expressed on a log₁₀ scale in patients with slow versus rapid disease progression, and separately for the four clinical phenotypes (E and F). Boxes depict median and IQR within a group, and dots mark individual measurements for patients. Magenta-contoured dots indicate presymptomatic patients. The P-values were obtained with a linear regression model adjusted for clinical phenotype and presence of symptoms. *P < 0.05, **P < 0.01 and ***P < 0.001. y = years.

Figure 3

Longitudinal comparison of untreated and treated patients with MLD according to age (A and C) or since diagnosis (B and D). The left panels show the course of bNfL level (A) and bGFAP level (C) over time according to age as estimated by a linear mixed model for untreated (peach line) and treated patients (blue line). The 95% CIs are shown as shadows in a corresponding colour. Reference values are visualized in green with an estimated LOESS regression curve and 95% CI (shadow). Coloured dots (measurements) and shaded lines (course over time) reflect the individual patient data. The right panels show the same individual patient data at bNfL level (B) and bGFAP level (D), but over time since diagnosis.