commentary on: return of the cycad hypothesis – does the amyotrophic lateral...

Neuropathology and Applied Neurobiology

(2006),

32

, 679–682 doi: 10.1111/j.1365-2990.2006.00796.x

© 2006 Blackwell Publishing Ltd

679

Blackwell Publishing LtdOxford, UKNANNeuropathology and Applied Neurobiology0305-1846Blackwell Publishing Ltd, 2006

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Miscellaneous)

Scientific correspondenceScientific correspondence

Scientific correspondence

Commentary on: return of the cycad hypothesis

– does the amyotrophic lateral sclerosis/

Parkinsonism dementia complex (ALS/PDC) of

Guam have new implications for global health?

Ince and Codd [1] published an excellent review of the pos-sible linkage of an unusual amino acid,

β

-N-methylamino-L-alanine (BMAA), originally discovered in cycad seeds[2], to the prevalence of amyotrophic lateral sclerosis/Parkinsonism dementia complex (ALS/PDC) among theChamorro people of Guam. The statistical correlation [3]of risk for ALS/PDC with the consumption of a traditionalChamorro diet implicated cycads as a possible source ofneurotoxin. As the concentration of cycad toxins, such asBMAA, were subsequently claimed to be low in washedcycad flour, the cycad hypothesis fell out of favour. How-ever, further consideration of the possible link betweentraditional Chamorro diet and ALS/PDC led to a variant ofthe cycad hypothesis. If animals that forage on cycad seedsbiomagnify cycad toxins, then Chamorros who consumethose animals may be exposed to high concentrations ofthose neurotoxins [4]. Thus Chamorros who eat wildgame, such as flying foxes, which forage in part on cycadseeds, may ingest a far larger dose than what they wouldotherwise receive from their consumption of tortillas,dumplings and soups prepared from washed cycad flour[5]. Although a variety of different cycad toxins might bebiomagnified in such a manner, initial interest focused onBMAA.

BMAA was recently discovered to be produced bycyanobacterial symbionts of the genus

Nostoc

thatinhabit unusual, positively geotropic roots in

Cycas micro-nesica

Hill and all other known cycad species [6–8].BMAA production is not unique to endosymbionts ofcycads: diverse taxa of free-living cyanobacteria isolatedfrom disparate geographical areas were also recently dis-covered to produce the neurotoxin [9], which suggeststhat human exposure could occur outside of Guam. Evi-dence for such human exposure to BMAA comes fromdiscovery of the amino acid in brain tissues from blindedanalysis of eight Chamorro patients from Guam and eightneurodegenerative patients from outside of Guam, but

not in 13 healthy control patients from North America[10,11]. We had access only to preserved Chamorrotissues in the initial study, and later found that BMAAconcentrations are significantly higher in frozen tissuesby comparison of matched fixed and frozen tissues fromthe same organs [12]. Using completely different meth-ods, Montine

et al

. failed to detect to free BMAA in frozenChamorro ALS/PDC brain tissues [13], but they did notsearch for protein-bound BMAA. BMAA occurs as a freeamino acid, but is typically more abundant in the proteinfraction [10]. Montine

et al

. did not repeat the validatedAQC (6-aminoquinolyl-N-hydroxy succinimidyl carbam-ate) derivatization procedure used in the original studies,nor did they verify their results with a second analyticaltechnique such as liquid chromatography-mass spec-trometry or thin-layer chromatography [6,10–12]. Incontrast, a team led by Deborah Mash at the Miami BrainBank detected protein-bound BMAA in brain tissues ofneurodegenerative patients from outside of Guam withthe more modern procedures [12].

Despite these interesting results, the link betweenBMAA and neurodegenerative disease is far from proven[1,10,14]. Any putative environmental cause of ALS/PDC must satisfy the following criteria: (i) it must differ-entially impact the Chamorro people of Guam but not theother cultures resident on the island; (ii) specific familiesand villages must have a higher rate of exposure; (iii)motor neurones must be especially vulnerable and expo-sure should result in tauopathies; (iv) there should be along latency period between last possible exposure andonset of disease as experienced by Chamorros who havebeen gone from Guam years or even decades after theyhave left the island [15]; (v) the environmental factormust occur not only in Guam, but also in other knownfoci of ALS/PDC including the Kii peninsula of Japan andcertain regions of West Papua; (vi) if there is a linkbetween the Guam disease and other forms of neurode-generative disease outside of the known ALS/PDC foci,the environmental factor should be relatively ubiquitous,albeit at a low level; and (vii) gene/environment interac-tions should result in differential human vulnerability toexposure.

680

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Neuropathology and Applied Neurobiology,

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, 679–682

By these criteria, far more work remains to make a con-vincing argument that BMAA plays a role in ALS/PDC.Additionally, the weaknesses in the BMAA argumentpointed out by Ince and Codd [1] are quite valid. Theserange from questions concerning the putative mechanismof BMAA neurotoxicity to lack of information concerningcharacteristics of ‘exposure to the toxin, and the geneticsof susceptibility’. High on this list of potential problemswith the BMAA hypothesis should also be the absence ofa published animal model which demonstrates BMAA-inducible progressive neurodegeneration. Short-term ani-mal experiments may show acute toxicity, but are unlikelyto reveal chronic effects comparable to the decade or morelatency period of ALS/PDC [15–18]. A third major weak-ness is the apparent lack of applicability of the BMAAhypothesis to the other known geographical foci of ALS/PDC. Human consumption of cycads or animals whichfeed upon cycads does not currently occur in the Kii pen-insula of Japan. Dietary habits in the West Papua ALS/PDC focus remain unstudied, although Spencer hasreported evidence for the medical use of cycad seeds inboth Japan and West Papua [19,20]. The BMAA hypoth-esis would be supported if this unusual amino acid werefound to be biomagnified in the Kii ecosystem within acyanobacterial-based food chain independent of cycadsor bats.

We have previously noted that ‘other neurotoxicmolecules, including cycasin, sterol

β

-D-glucoside, orother unknown compounds, might be similarly biomag-nified and should also be considered as possible environ-mental neurotoxins related to ALS-PDC’ [21, p. 388]. Asterol glucoside is being commercially developed by ShawNeuronal Dynamics (SND), a subsidiary of Dynamikos, aprivately held Nevada, USA corporation, having its corpo-rate headquarters in Vancouver, British Columbia, Can-ada. SND is ‘dedicated to the prevention of cytoskeletaldisorders which include Alzheimers (AD), Parkinson’s(PD) and Lou Gehrig’s disease (ALS)’ [22]. ‘SND has iden-tified an important molecule, a sterol glucoside, whichplays a pivotal role in the development of cytoskeletal dis-orders’ [22,23]. Evidence for neurotoxicity of the sterolglucoside includes the results of feeding mice cycad chipswhich have been repeatedly washed with water in the lab-oratory [24]. As water washing does not remove protein-bound BMAA from the cycad chip flour, these experimentsdo not exclude the BMAA hypothesis [10]. Natural inser-tion into polypeptides of ‘non-protein’ amino acids such ascanavanine, which is transported to the ribosome for pro-

tein synthesis by tRNA

ARG

, has long been known [25]. Asimilar process might account for BMAA detected in theproteins of cyanobacteria, cycads, flying foxes, and theneuroproteins of ALS/PDC patients [5,10]. These newfindings indicate multiple sources of BMAA in the tradi-tional Chamorro diet including cycad flour, flying foxesand other animals that feed on cycad seeds. Failure toquantify protein-bound BMAA has resulted in significantunderestimates of total BMAA in washed cycad flour[10,24].

Although it has been claimed that BMAA is a ‘weak’ N-methyl-D-aspartate agonist [26], BMAA is in fact a potentexcitotoxin that activates AMPA (

α

-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid)/kainate receptors,with motor neurones being damaged by BMAA concen-trations as low as 30

µ

M [27,28]. The role of bicarbonatein potentiating the neurotoxic effects of BMAA suggeststhe possibility that other BMAA-related carbamates maybe involved [27–30].

In our original medical hypothesis we simply suggestedthat ‘the decline of ALS/PDC among the Chamorro mir-rors the decline of flying foxes in Guam’ [4] and separatelypublished complete details of our illustrative figure [31].The claim that our original medical hypothesis paper‘allegedly shows a close correlation between the bat pop-ulation of Guam and ALS-PDC’ [26] is inaccurate as wemade no statistical assertions [4]. Data documenting thedramatic decline of Guam’s flying fox population led toplacement of

Pteropus mariannus

on the US EndangeredSpecies list and on CITES Appendix I [32,33]. There areabundant data [33–46] documenting the shift fromlocally obtained flying foxes to imported flying foxes asGuam’s bat population continued to disappear.

In summary, the BMAA hypothesis is far from provenbut remains a viable possibility. We think it worthwhile tocontinue multidisciplinary investigations on this neuro-toxin and its possible link to neurodegenerative illness inGuam and other locales.

P. A. Cox

1

,S. Banack

2

,S. Murch

3

andO. Sacks

4

1

Institute for Ethnomedicine, Jackson, Wyoming, USA

2

California State University-Fullerton, Department of BiologicalScience, Fullerton, CA, USA

3

University of British Columbia Okanagan, ChemistryDepartment, Kelowna, British Columbia, Canada

4

Albert Einstein College of Medicine, Bronx, NY, USA

Scientific correspondence

681

© 2006 Blackwell Publishing Ltd,

Neuropathology and Applied Neurobiology,

32

, 679–682

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Received 3 Auguest 2006Accepted after revision 20 September 2006