Return of the cycad hypothesis – does the amyotrophic lateral sclerosis/parkinsonism dementia complex (ALS/PDC) of Guam have new implications for global health?

Recently published work provides evidence in support of the cycad hypothesis for Lytico–Bodig, the Guamanian amyotrophic lateral sclerosis/parkinsonism dementia complex (ALS/PDC), based on a new understanding of Chamorro food practices, a cyanobacterial origin of β‐methylaminoalanine (BMAA) in cycad tissue, and a possible mechanism of biomagnification of this neurotoxic amino acid in the food chain. BMAA is one of two cycad chemicals with known neurotoxic properties (the other is cycasin, a proven developmental neurotoxin) among the many substances that exist in these highly poisonous plants, the seeds of which are used by Chamorros for food and medicine. The traditional diet includes the fruit bat, a species that feeds on cycad seed components and reportedly bioaccumulates BMAA. Plant and animal proteins provide a previously unrecognized reservoir for the slow release of this toxin. BMAA is reported in the brain tissue of Guam patients and early data suggest that some Northern American patients dying of Alzheimer's disease (AD) have detectable brain levels of BMAA. The possible role of cyanobacterial toxicity in sporadic neurodegenerative disease is therefore worthy of consideration. Recent neuropathology studies of ALS/PDC confirm understanding of this disorder as a ‘tangle’ disease, based on variable anatomical burden, and showing biochemical characteristics of ‘AD‐like’ combined 3R and 4R tau species. This model mirrors the emerging view that other neurodegenerative disease spectra comprise clusters of related syndromes, owing to common molecular pathology, with variable anatomical distribution in the nervous system giving rise to different clinical phenotypes. Evidence for ‘ubiquitin‐only’ inclusions in ALS/PDC is weak. Similarly, although there is evidence for α‐synucleinopathy in ALS/PDC, the parkinsonian component of the disease is not caused by Lewy body disease. The spectrum of sporadic AD includes involvement of the substantia nigra and a high prevalence of ‘incidental’α‐synucleinopathy in sporadic AD is reported. Therefore the pathogenesis of Lytico–Bodig appears still to have most pertinence to the ongoing investigation of the pathogenesis of AD and other tauopathies.

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