A Review of Biomarkers of Amyotrophic Lateral Sclerosis: A Pathophysiologic Approach

Abstract

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by progressive degeneration of upper and lower motor neurons. The heterogeneous nature of ALS at the clinical, genetic, and pathological levels makes it challenging to develop diagnostic and prognostic tools that fit all disease phenotypes. Limitations associated with the functional scales and the qualitative nature of mainstay electrophysiological testing prompt the investigation of more objective quantitative assessment. Biofluid biomarkers have the potential to fill that gap by providing evidence of a disease process potentially early in the disease, its progression, and its response to therapy. In contrast to other neurodegenerative diseases, no biomarker has yet been validated in clinical use for ALS. Several fluid biomarkers have been investigated in clinical studies in ALS. Biofluid biomarkers reflect the different pathophysiological processes, from protein aggregation to muscle denervation. This review takes a pathophysiologic approach to summarizing the findings of clinical studies utilizing quantitative biofluid biomarkers in ALS, discusses the utility and shortcomings of each biomarker, and highlights the superiority of neurofilaments as biomarkers of neurodegeneration over other candidate biomarkers.

Keywords: CSF; amyotrophic lateral sclerosis; biomarker; blood; pathophysiology.

Figure 1

Pathophysiology of amyotrophic lateral sclerosis. The interaction of genetic, epigenetic, and environmental factors triggers mutations and stress conditions, which induce mislocalization or misfolding of intracellular proteins. Mislocalized and misfolded proteins are capable of self-seeding and cross-seeding other proteins into toxic aggregates. These aggregates disrupt axonal transport, mitochondrial respiration, and the clearance of damaged proteins, induce glial activation, and sequester proteins essential for RNA processing. Deregulated processing of the EAAT2 mRNA transcript reduces the expression of the transporter, leading to excessive glutamate concentration, subsequent excessive calcium influx, and overproduction of ROS, which further induce glial activation and creates a continuous cycle of mislocalization and misfolding of the proteins as well as activation of enzymatic pathways which propagate neuronal injury. Calcium overload and oxidative stress disrupt glutamate uptake by astrocytes and axonal transport and contribute to mitochondrial failure, leading to energy depletion, which induces altered proteostasis and the continuation of the cycle. Astrogliosis triggers blood-brain barrier (BBB) disruption and macrophage infiltration, leading to neuroinflammation. These converging mechanisms result in neurodegeneration, which stimulates astrogliosis through released inflammatory and cytotoxic mediators. Neurodegeneration propagates across motor neurons, leading to the disassembly of the neuromuscular junctions and the denervation of skeletal muscles. EAAT2: excitatory amino acid transporter 2; ROS: reactive oxygen species.

Figure 2

Summary of deregulated RNA profile in amyotrophic lateral sclerosis. Aberrant RNA processing in ALS is attributed to biogenesis defects and/or miRNA transcriptional changes predominantly caused by altered proteostasis as the toxic aggregates alter RNA Splicing, capping, polyadenylation, and transport and disrupt miRNA biogenesis via altering stress granule dynamics. A profile of deregulated miRNAs is illustrated here based on experimental and clinical studies of abnormal levels of several biomarkers linked to neuronal, excitotoxic, inflammatory, and muscle-related mechanisms. EAAT2: excitatory amino acid transporter 2; GLAST: glutamate aspartate transporter; SLC1A2: solute carrier family 1 member 2; Th17: T helper 17 cell.

Figure 3

Availability of candidate biomarkers for amyotrophic lateral sclerosis in biofluids. Several pathogenic mechanisms lead to the release of ALS-related biomarkers either passively or actively. Many of these are in high abundance in the CSF from ALS patients, but the invasiveness of CSF sampling limits research recruitment and subsequent longitudinal measurement. Similarly, multiple ALS-related biomarkers can be measured in the blood with less invasive sampling. The availability of biomarkers in the urine is limited by filtration restriction of proteins with high molecular weight. BBB: blood–brain barrier; BSCB: blood–spinal cord barrier; CSF: cerebrospinal fluid.