Etiology of Retinal and Cerebellar Pathology in Western Pacific Amyotrophic Lateral Sclerosis and Parkinsonism-Dementia Complex

Brain health: Pathway to primary prevention of neurodegenerative disorders of environmental origin

While rising global rates of neurodegenerative disease encourage early diagnosis and therapeutic intervention to block clinical expression (secondary prevention), a more powerful approach is to identify and remove environmental factors that trigger long-latencybrain disease (primary prevention) by acting on a susceptible genotype or acting alone. The latter is illustrated by the post-World War II decline and disappearance of Amyotrophic Lateral Sclerosis and Parkinsonism-Dementia Complex (ALS/PDC), a prototypical often-familial neurodegenerative disease formerly present in very high incidence on the island of Guam. Lessons learned from 75 years of investigation on the etiology of ALS/PDC include: the importance of focusing field research on the disease epicenter and patients with early-onset disease; soliciting exposure history from patients, family, and community to guide multidisciplinary biomedical investigation; recognition that disease phenotype may vary with exposure history, and that familial brain disease may have a primarily environmental origin. Furthermore, removal from exposure to the environmental trigger effects primary disease prevention.

An introduction to environmental neurotoxicology: Lessons from a clinical perspective

In 1992, the Committee on Neurotoxicology and Models for Assessing Risk of the National Academy of Sciences in Washington DC focused with a scientific perspective on the identification of substances with neurotoxic potential, studies of exposed populations, risk assessment, and biologic markers of disease. This Committee recommended: "all physicians should be trained to take a thorough occupational-exposure history and to be aware of other possible sources of toxic exposure". Although convened after several outbreaks of neurotoxic syndromes, clinical neurological considerations were lacking. After defining keys words, namely Environment, Neurotoxicology and Neurotoxicants, we present some demonstrative cases; e.g., the Epidemic Neuropathy in Cuba, Minamata disease, ALS/PDC on Guam, and the ALS hot spot in the French Alps. Always with a clinical and practical approach, we will then review the milieux that contain and convey potential neurotoxicants, the different exposure routes and the clinical presentations. Drawing lessons from clinical cases, we offer some thoughts concerning the future of Environmental Neurotoxicology (ENT). Pointing notably to the diffuse chemical contamination of ecosystems and living beings, including Homo sapiens, we question the real impact of agents with neurotoxic potential on the human brain, considering the effects, for example, of air pollution, endocrine disruptors and nanoparticles. Concern is expressed over the lack of knowledge of the non-monotonic kinetics of many of these chemicals, the major concern being related to mixtures and low-dose exposures, as well as the delayed appearance in clinical expression of prevalent neurodegenerative diseases.

Introducing the Role of Genotoxicity in Neurodegenerative Diseases and Neuropsychiatric Disorders

Decades of research have identified genetic and environmental factors involved in age-related neurodegenerative diseases and, to a lesser extent, neuropsychiatric disorders. Genomic instability, i.e., the loss of genome integrity, is a common feature among both neurodegenerative (mayo-trophic lateral sclerosis, Parkinson's disease, Alzheimer's disease) and psychiatric (schizophrenia, autism, bipolar depression) disorders. Genomic instability is associated with the accumulation of persistent DNA damage and the activation of DNA damage response (DDR) pathways, as well as pathologic neuronal cell loss or senescence. Typically, DDR signaling ensures that genomic and proteomic homeostasis are maintained in both dividing cells, including neural progenitors, and post-mitotic neurons. However, dysregulation of these protective responses, in part due to aging or environmental insults, contributes to the progressive development of neurodegenerative and/or psychiatric disorders. In this Special Issue, we introduce and highlight the overlap between neurodegenerative diseases and neuropsychiatric disorders, as well as the emerging clinical, genomic, and molecular evidence for the contributions of DNA damage and aberrant DNA repair. Our goal is to illuminate the importance of this subject to uncover possible treatment and prevention strategies for relevant devastating brain diseases.

Endemic parkinsonism: clusters, biology and clinical features

The term 'endemic parkinsonism' refers to diseases that manifest with a dominant parkinsonian syndrome, which can be typical or atypical, and are present only in a particular geographically defined location or population. Ten phenotypes of endemic parkinsonism are currently known: three in the Western Pacific region; two in the Asian-Oceanic region; one in the Caribbean islands of Guadeloupe and Martinique; and four in Europe. Some of these disease entities seem to be disappearing over time and therefore are probably triggered by unique environmental factors. By contrast, other types persist because they are exclusively genetically determined. Given the geographical clustering and potential overlap in biological and clinical features of these exceptionally interesting diseases, this Review provides a historical reference text and offers current perspectives on each of the 10 phenotypes of endemic parkinsonism. Knowledge obtained from the study of these disease entities supports the hypothesis that both genetic and environmental factors contribute to the development of neurodegenerative diseases, not only in endemic parkinsonism but also in general. At the same time, this understanding suggests useful directions for further research in this area.

Early-onset, conjugal, twin-discordant, and clusters of sporadic ALS: Pathway to discovery of etiology via lifetime exposome research

The identity and role of environmental factors in the etiology of sporadic amyotrophic lateral sclerosis (sALS) is poorly understood outside of three former high-incidence foci of Western Pacific ALS and a hotspot of sALS in the French Alps. In both instances, there is a strong association with exposure to DNA-damaging (genotoxic) chemicals years or decades prior to clinical onset of motor neuron disease. In light of this recent understanding, we discuss published geographic clusters of ALS, conjugal cases, single-affected twins, and young-onset cases in relation to their demographic, geographic and environmental associations but also whether, in theory, there was the possibility of exposure to genotoxic chemicals of natural or synthetic origin. Special opportunities to test for such exposures in sALS exist in southeast France, northwest Italy, Finland, the U.S. East North Central States, and in the U.S. Air Force and Space Force. Given the degree and timing of exposure to an environmental trigger of ALS may be related to the age at which the disease is expressed, research should focus on the lifetime exposome (from conception to clinical onset) of young sALS cases. Multidisciplinary research of this type may lead to the identification of ALS causation, mechanism, and primary prevention, as well as to early detection of impending ALS and pre-clinical treatment to slow development of this fatal neurological disease.

Genotoxic Damage During Brain Development Presages Prototypical Neurodegenerative Disease

Western Pacific Amyotrophic Lateral Sclerosis and Parkinsonism-Dementia Complex (ALS/PDC) is a disappearing prototypical neurodegenerative disorder (tau-dominated polyproteinopathy) linked with prior exposure to phytogenotoxins in cycad seed used for medicine and/or food. The principal cycad genotoxin, methylazoxymethanol (MAM), forms reactive carbon-centered ions that alkylate nucleic acids in fetal rodent brain and, depending on the timing of systemic administration, induces persistent developmental abnormalities of the cortex, hippocampus, cerebellum, and retina. Whereas administration of MAM prenatally or postnatally can produce animal models of epilepsy, schizophrenia or ataxia, administration to adult animals produces little effect on brain structure or function. The neurotoxic effects of MAM administered to rats during cortical brain development (specifically, gestation day 17) are used to model the histological, neurophysiological and behavioral deficits of human schizophrenia, a condition that may precede or follow clinical onset of motor neuron disease in subjects with sporadic ALS and ALS/PDC. While studies of migrants to and from communities impacted by ALS/PDC indicate the degenerative brain disorder may be acquired in juvenile and adult life, a proportion of indigenous cases shows neurodevelopmental aberrations in the cerebellum and retina consistent with MAM exposure in utero. MAM induces specific patterns of DNA damage and repair that associate with increased tau expression in primary rat neuronal cultures and with brain transcriptional changes that parallel those associated with human ALS and Alzheimer's disease. We examine MAM in relation to neurodevelopment, epigenetic modification, DNA damage/replicative stress, genomic instability, somatic mutation, cell-cycle reentry and cellular senescence. Since the majority of neurodegenerative disease lacks a solely inherited genetic basis, research is needed to explore the hypothesis that early-life exposure to genotoxic agents may trigger or promote molecular events that culminate in neurodegeneration.

Parkinsonism and motor neuron disorders: Lessons from Western Pacific ALS/PDC

Recognized worldwide as an unusual "overlap" syndrome, Parkinsonism and motor neuron disease, with or without dementia, is best exemplified by the former high-incidence clusters of Amyotrophic Lateral Sclerosis and Parkinsonism-Dementia Complex (ALS/PDC) in Guam, USA, in the Kii Peninsula of Honshu Island, Japan, and in Papua, Indonesia, on the western side of New Guinea. Western Pacific ALS/PDC is a disappearing neurodegenerative disorder with multiple and sometime overlapping phenotypes (ALS, atypical parkinsonism, dementia) that appear to constitute a single disease of environmental origin, in particular from exposure to genotoxins/neurotoxins in seed of cycad plants (Cycas spp.) formerly used as a traditional source of food (Guam) and/or medicine (Guam, Kii-Japan, Papua-Indonesia). Seed compounds include the principal cycad toxin cycasin, its active metabolite methylazoxymethanol (MAM) and a non-protein amino acid β-N-methylamino-L-alanine (L-BMAA); each reproduces components of ALS/PDC neuropathology when individually administered to laboratory species in single doses perinatally (MAM, L-BMAA) or repeatedly for prolonged periods to young adult animals (L-BMAA). Human exposure to MAM, a potent DNA-alkylating mutagen, also has potential relevance to the high incidence of diverse mutations found among Guamanians with/without ALS/PDC. In sum, seven decades of intensive study of ALS/PDC has revealed field and laboratory approaches leading to discovery of disease etiology that are now being applied to sporadic neurodegenerative disorders such as ALS beyond the Western Pacific region. This article is part of the Special Issue "Parkinsonism across the spectrum of movement disorders and beyond" edited by Joseph Jankovic, Daniel D. Truong and Matteo Bologna.

Lytico-bodig in Guam: Historical links between diet and illness during and after Spanish colonization

This paper analyses documents on health and disease among Chamorro people during and after 333 years (1565-1898) of the Spanish claim to and occupation of Guam. Here, a complex neurodegenerative disease-known locally as lytico-bodig and medically as amyotrophic lateral sclerosis and Parkinsonism-dementia complex (ALS/PDC)-reached hyperendemic proportions in the mid-twentieth century but then declined and is now disappearing. A tau-dominated polyproteinopathy, clinical phenotypes included amyotrophic lateral sclerosis (ALS or lytico), atypical parkinsonism with dementia (P-D or bodig), and dementia alone. A plausible etiology for lytico-bodig is consumption of flour derived from the incompletely detoxified seed of Cycas micronesica (fadang in Chamorro; Federico in Spanish), a poisonous gymnosperm that survives climatic extremes that can affect the island. Traditional methods for safe consumption appear to have been lost over the course of time since governors Francisco de Villalobos (1796-1862) and Felipe de la Corte (1855-1866) proposed banning consumption in view of its acute toxic effects. A death certificate issued in 1823 might suggest ALS/PDC in people dying with disability or impedidos, and premature aging and a short life was linked to food use of fadang in the mid-1850s (Guam Vital Statistics Report, 1823). During the Japanese occupation of Guam (1941-1944), Chamorro people took refuge in the jungle for months, where they relied on insufficiently processed fadang as a staple food. After World War II, traditional foods and medicines were subsequently replaced as islanders rapidly acculturated to North American life.

Role of Hydrazine-Related Chemicals in Cancer and Neurodegenerative Disease

Hydrazine-related chemicals (HRCs) with carcinogenic and neurotoxic potential are found in certain mushrooms and plants used for food and in products employed in various industries, including aerospace. Their propensity to induce DNA damage (mostly O⁶-, N⁷- and 8-oxo-guanine lesions) resulting in multiple downstream effects is linked with both cancer and neurological disease. For cycling cells, unrepaired DNA damage leads to mutation and uncontrolled mitosis. By contrast, postmitotic neurons attempt to re-enter the cell cycle but undergo apoptosis or nonapoptotic cell death. Biomarkers of exposure to HRCs can be used to explore whether these substances are risk factors for sporadic amyotrophic laterals sclerosis and other noninherited neurodegenerative diseases, which is the focus of this paper.

Retinal Damage in Amyotrophic Lateral Sclerosis: Underlying Mechanisms

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease resulting in a gradual loss of motor neuron function. Although ophthalmic complaints are not presently considered a classic symptom of ALS, retinal changes such as thinning, axonal degeneration and inclusion bodies have been found in many patients. Retinal abnormalities observed in postmortem human tissues and animal models are similar to spinal cord changes in ALS. These findings are not dramatically unexpected because retina shares an ontogenetic relationship with the brain, and many genes are associated both with neurodegeneration and retinal diseases. Experimental studies have demonstrated that ALS affects many “vulnerable points” of the retina. Aggregate deposition, impaired nuclear protein import, endoplasmic reticulum stress, glutamate excitotoxicity, vascular regression, and mitochondrial dysfunction are factors suspected as being the main cause of motor neuron damage in ALS. Herein, we show that all of these pathways can affect retinal cells in the same way as motor neurons. Furthermore, we suppose that understanding the patterns of neuro-ophthalmic interaction in ALS can help in the diagnosis and treatment of this disease.

Western Pacific ALS-PDC: Evidence implicating cycad genotoxins

Amyotrophic Lateral Sclerosis and Parkinsonism-Dementia Complex (ALS-PDC) is a disappearing neurodegenerative disorder of apparent environmental origin formerly hyperendemic among Chamorros of Guam-USA, Japanese residents of the Kii Peninsula, Honshu Island, Japan and Auyu-Jakai linguistic groups of Papua-Indonesia on the island of New Guinea. The most plausible etiology is exposure to genotoxins in seed of neurotoxic cycad plants formerly used for food and/or medicine. Primary suspicion falls on methylazoxymethanol (MAM), the aglycone of cycasin and on the non-protein amino acid β-N-methylamino-L-alanine, both of which are metabolized to formaldehyde. Human and animal studies suggest: (a) exposures occurred early in life and sometimes during late fetal brain development, (b) clinical expression of neurodegenerative disease appeared years or decades later, and (c) pathological changes in various tissues indicate the disease was not confined to the CNS. Experimental evidence points to toxic molecular mechanisms involving DNA damage, epigenetic changes, transcriptional mutagenesis, neuronal cell-cycle reactivation and perturbation of the ubiquitin-proteasome system that led to polyproteinopathy and culminated in neuronal degeneration. Lessons learned from research on ALS-PDC include: (a) familial disease may reflect common toxic exposures across generations, (b) primary disease prevention follows cessation of exposure to culpable environmental triggers; and (c) disease latency provides a prolonged period during which to intervene therapeutically. Exposure to genotoxic chemicals ("slow toxins") in the early stages of life should be considered in the search for the etiology of ALS-PDC-related neurodegenerative disorders, including sporadic forms of ALS, progressive supranuclear palsy and Alzheimer's disease.