Preventing amyotrophic lateral sclerosis: insights from pre-symptomatic neurodegenerative diseases

Abstract

Abstract Significant progress has been made in understanding the pre-symptomatic phase of amyotrophic lateral sclerosis (ALS). While much is still unknown, advances in other neurodegenerative diseases offer valuable insights. Indeed, it is increasingly clear that the well-recognized clinical syndromes of Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), spinal muscular atrophy, and frontotemporal dementia are also each preceded by a pre-symptomatic or prodromal period of varying duration, during which the underlying disease process unfolds, with associated compensatory changes and loss of inherent system redundancy. Key insights from these diseases highlight opportunities for discovery in ALS. The development of biomarkers reflecting amyloid and tau has led to a shift in defining AD based on inferred underlying histopathology. PD is unique among neurodegenerative diseases in the number and diversity of non-genetic biomarkers of pre-symptomatic disease, most notably REM-sleep behavior disorder. HD benefits from an ability to predict the likely timing of clinically manifest disease based on age and CAG-repeat length alongside reliable neuroimaging markers of atrophy. SMA clinical trials have highlighted the transformational value of early therapeutic intervention, and studies in FTD illustrate the differential role of biomarkers based on genotype. Similar advances in ALS would transform our understanding of key events in pathogenesis, thereby dramatically accelerating progress towards disease prevention. Deciphering the biology of pre-symptomatic ALS relies upon a clear conceptual framework for defining the earliest stages of disease. Clinically manifest ALS may emerge abruptly, especially among those who harbor genetic mutations associated with rapidly progressive ALS. However, the disease may also evolve more gradually, revealing a prodromal period of mild motor impairment preceding phenoconversion to clinically manifest disease. Similarly, cognitive and behavioral impairment, when present, may emerge gradually, evolving through a prodromal period of mild cognitive impairment or mild behavioral impairment before progression to ALS. Biomarkers are critically important to studying pre-symptomatic ALS and essential to efforts to intervene therapeutically before clinically manifest disease emerges. The use of non-genetic biomarkers, however, presents challenges related to counseling, informed consent, communication of results, and limited protections afforded by existing legislation. Experiences from pre-symptomatic genetic testing and counseling, and the legal protections against discrimination based on genetic data, may serve as a guide. Building upon what we have learned—more broadly from other pre-symptomatic neurodegenerative diseases and specifically from ALS gene mutation carriers—we present a roadmap to early intervention, and perhaps even disease prevention, for all forms of ALS.