The "Cobalaminergic" hypothesis
by Andrew McCaddon, October 2000
Abstract: An association between Alzheimer's Disease (AD) and low CSF
and serum vitamin B12 (B12) has recently been described (Van Tiggelen 1983;
& Prchal 1984; Karnaze
& Carmel 1987). This is apparently independent of nutritional intake
(Renvall, Spindler, et al. 1989). It has been suggested that such patients
may exhibit an atypical form of cobalamin deficiency (Karnaze
& Carmel 1987; Renvall,
Spindler, et al. 1989). It is therefore proposed that these deficiencies
may be aetiologically important, at least in sub-groups of AD, and a mechanism
is described whereby B12 deficiency may result in the characteristic neurotransmitter
changes of the disease. The hypothesis generates predictions regarding biochemical
evaluation of such patients and suggests associations between the neurochemical
disturbances and structural abnormalities of AD.
The metabolic inter-relations of B12 and folate have been extensively
documented with regard to the pathogenesis of megaloblastic anaemia. The
'methyl-folate' trap, proposed nearly 30 years ago to explain the biological
response to B12 and folate deficiency, is now generally accepted. It is
believed that the trap is a physiological response to impending methyl group
deficiency due to a low supply of dietary methionine, but results in an
inappropriate response to B12 deficiency and to the development of a potentially
lethal anaemia. Essentially, the trap revolves around the B12 dependent
conversion of homocysteine to methionine by methionine synthetase (MS),
with the associated formation of free tetrahydrofolate (THF) from 5-methyl
THF (see Fig.1). Free THF is the form from which various polyglutamate folate
co-enzymes are subsequently derived via the synthesis of THF-polyglutamate.
In the absence of B12, the MS reaction is impaired and a critical lack of
free THF results.
This has two important effects:
- Various methylation reactions are compromised, due to reduced levels
of S-adenosyl methionine (SAM), a methionine derivative and important intracellular
methyl group donor.
- Purine (and pyrimidine) synthesis is impaired, due to a lack of formyl
polyglutamate forms of folate, which serve as essential one-carbon donors
in this pathway . This occurs, not only due to blocked THF synthesis, but
also by a release of inhibition of 5-10 methylene THF polyglutamate reductase
activity, secondary to falling SAM levels.
In essence, B12 deficiency results in the diversion of folate from nucleotide
synthesis towards essential cellular methylation reactions. This response
is ineffective, however, due to impaired B12 dependent MS activity and folate
becomes trapped in the non-usable 5-methyl form.
It is hypothesised that these two major consequences of B12 deficiency
can result in the characteristic cholinergic and monoaminergic deficits
B12 deficiency and the subsequent sequestration of folate in the 5-methyl
form results in deficient supplies of formyl groups necessary for nucleotide
biosynthesis . The de novo synthesis of ATP and GTP are both dependent
upon the intracellular supply of inosine monophosphate (IMP) . Folate, in
the formyl polyglutamate form, is necessary at two stages of IMP synthesis.
B12 deficiency results in reduced intracellular IMP and, hence, ATP and
GTP. GTP is an essential precursor of tetrahydrobiopterin (BH4) via dihyroneopterin
triphosphate. By way of the methyl-folate trap, B12 deficiency will therefore
lead to reduced supplies of BH4, an essential and regulatory co-factor for
biosynthesis of the monoamine neurotransmitters, dopamine, noradrenaline,
and serotonin . Such an effect on monoamine synthesis has already been reported
in B12 deficiency .
Choline in cholinergic neurones is derived from three main sources :
Intrasynaptic choline, via degradation of acetylcholine by acetylcholinesterase;
extracellular choline, via a low affinity transport mechanism;
intraneuronal choline, via sequential methylation of membrane phosphatidylethanolamine
(PE) or ethanolamine plasmalogens.
It is hypothesised that B12 deficiency may reduce both extracellular
choline supplies and intraneuronal synthesis and thereby result in the characteristic
cholinergic deficit of AD.
As discussed previously, one effect of B12 deficiency is impairment of
essential methylation reactions, due to reduced formation of SAM, and important
source of intracellular methyl-groups . Nitrous oxide induced cobalamin
inactivation in the rat results in lowered levels of hepatic SAM and methylation
reactions are compromised . These animals remain well, however, and it has
been shown that an alternative B12 independent pathway for methylation of
homocysteine to methionine is induced . This is the betaine homocysteine
methyltransferase pathway, betaine supplying the methyl group instead of
methyl-folate. Betaine is derived from the oxidation of choline via betaine
aldehyde. Hence, B12 deficiency results in the diversion of endogenous and
dietary choline to overcome the MS block, with consequent reduction in plasma
and extraneuronal supplies.
It is also proposed that intraneuronal choline synthesis is compromised
in B12 deficiency. Choline may be released from phosphatidylcholine (PC)
by base exchange of phospho-lipase mediated hydrolyses . PC is formed either
by the incorporation of existing choline, or by de novo synthesis. The latter
process involves three sequential methylations of PC by SAM . B12 deficiency
should therefore result in a decreased supply of neuronal SAM. Furthermore,
betaine homocysteine methyltransferase is absent in the brain , which will
further exacerbate this process. Sequential methylation of PE to PC will
therefore be inhibited in B12 deficiency, resulting in impaired de novo
synthesis of intraneuronal choline. There will also be an inversion of the
SAM/SAH ratio, thereby inhibiting all transmethylation reactions .
This hypothesis outlines established biochemical pathways by which B12
deficiency may result in the cholinergic and monoaminergic deficits observed
in AD. The hypothesis lends itself to certain neurochemical predictions
and suggests possible mechanisms of structural change in AD which are now
discussed more fully.
The B12 dependent MS block should result in an elevation of serum homocysteine,
a potentially useful indicator of subtle and atypical B12 deficient states
Impaired monoaminergic synthesis should be apparent by increased urinary
excretion of amino-imidazole carboxamide, due to blocked IMP synthesis .
Furthermore, the concentrations of total biopterin in brains of patients
with AD should be reduced. This has already been demonstrated in one recent
Three further neurochemical implications have already been established.
Firstly, as nucleotide synthesis is of prime importance for the formation
of nucleic acid and protein, some derangement of this process would be expected
in AD if B12 or folate deficiencies are indeed pathogenic. In fact, a 30%
reduction in RNA synthesis has been described and protein formation is known
to be similary deranged . In addition, the amount of RNA depletion exceeds
that predicted solely on the basis of neuronal loss or neurofibrillary tangle
formation . Secondly, and more specifically, reductions in adenine nucleotide
content of neocortical samples in AD have been shown . Thirdly, as predicted,
levels of CSF SAM are reduced in AD patients suggesting a disturbance of
methylation reactions in the disease process .
The hypothesis that B12 deficiency is of primary aetiological importance
for sub-groups of AD presupposes that neuropathological changes of the disease
occur secondary to metabolic derangement. The hypothesis raises interesting
possibilities regarding this.
Reduced supplies of free choline, precipitated by B12 deficiency, could
be the trigger factor for membrane disruption or "autocannabilism"
which may occur when neuronal tissue resorts to this alternative choline
supply . This membrane disruption could result in increased permeability,
with subsequent leakage of protein and enzymes. There are two major consequences
of such a process, which may correlate with recent findings. Membrane disruption
may facilitate proteolytic cleavage of the putative A4 precursor (A4CT)
into amyloid A4 protein, which would then aggregate into pathological fibrils,
fibril bundles and amyloid . Secondly, altered membrane permeability, with
protein and enzyme leakage, may account for cholinergic auto-antibody formation
observed in AD, which may then further exacerbate this process .
Any hypothesis of the pathogenesis of AD must also account for the selective
destruction of central cholinergic neurones. Interestingly, homocysteic
acid is an endogenous agonist of the NMDA receptor which has an anatomical
distribution correlating with the distribution of neurofibrillary tangles
(NFT) and senile plaques (SP) seen in AD . It has been suggested that hyper-activation
of this receptor may result in neuronal death . Elevated homocysteine, as
predicted by this cobalaminergic hypothesis, could therefore account for
the characteristic NFT and SP distribution of AD.
Although the effects of B12 deficiency have been discussed throughout
it is interesting to note the predicted effects of a pure folate deficiency
with regard to this hypothesis. Folate deficiency will result in reduced
levels of SAM and the diversion of folate from purine synthesis to methylation
reactions. B12 dependent MS activity remains intact and the strategy is
successful with regard to methylation. Purine synthesis declines, however,
with subsequent monoaminergic deficit. An affective disorder should therefore
predominate which is, in fact, the commonest neuropsychiatric feature of
this deficiency . Furthermore, the efficacy of SAM in the treatment of depression
may be explained by its inhibition of 5-10 methylene THF reductase, resulting
in an increased availability of folate co-enzymes necessary for biosynthesis
of purines and, hence, monoamines.
It would be interesting to observe the effects of folinic acid supplementation
for folate deficiency depression (and indeed for the emotional lability
of AD) as this is a more direct substrate for formyl-folate co-enzyme synthesis
This hypothesis delineates several mechanisms whereby B12 deficiency
could result in some of the characteristic neurotransmitter deficits and
structural abnormalities of AD (see Fig.3) Although it is not proposed that
B12 deficiency is the primary cause of neurotransmitter and structural changes
in all patients with AD, evidence is accumulating that a significant sub-group
exists in which such deficits may be of primary aetiological importance
. Furthemore, the 30% prevalence of sub-normal B12 levels observed in these
studies may not reflect the true prevalence of low B12 in AD. A recent model
of B12 deficiency suggests that subtle B12 deficient states could exist
in the early stages of B12 malabsorption, before a measurable decline in
serum B12 levels . It is also possible that B12 may be functionally inactive
(although present in normal serum concentration), as a result of increased
oxidative damage known to occur in AD . Nitrous oxide is known to inactivate
B12 by the oxidation of Cob(I)alamin to Cob(III)alamin . The age-related
increase in free radical formation could exert a similar oxidative effect
on B12 resulting in the development of a "cryptic" cerebral B12
deficient state, not unlike that observed after prolonged N2O exposure .
Indeed, such a mechanism may account for the well-established association
between Down's syndrome and AD, as the gene dosage effect of superoxide
dismutase (SOD I) located on chromosome 21 could also result in oxidative
inactivation of B12 . Furthermore, the effects of aluminium on cellular
redox potential and subsequent inactivation of cerebral B12 may explain
the link between aluminium and AD . In these situations, serum homocysteine
concentration may prove to be a more accurate indicator of B12 tissue status
The effects of B12 supplementation on restoration of cognitive function
in B12 deficient subjects with AD remains to be seen. By virtue of the pivotal
status of B12/folate inter-relationships, early supplementation may well
lead to a restoration of balanced neurotransmission, but correction of subsequent
structural abnormalities, namely senile plaques and neurofibrillary tangles,
would seem unlikely.
Finally, if, as is suspected, B12 deficiency is found to be of primary
importance, the possibility of a simple screening procedure may be realised.
Ideally, estimation of serum homocysteine or transcobalamin II saturation
may provide the earliest evidence of a deficiency state. Appropriate intervention
could then be provided for such patients before the onset of deteriorating
cognitive function so characteristic of this devastating disease.
Adolffson, R., Gottfries, C.G., Roos, B.E. and Winblad, B. Changes in
the brain catecholamines in patients with dementia of Alzheimer type. Br.J.Psych.
1979 Sep;135, 216-223. Abstract.
Amess, J.A.L., Burman, J.F., Rees, G.M., Nancekievill, D.G. and Mollin,
D.L. Megaloblastic haemopoiesis in patients recieving nitrous oxide. Lancet.
1978 Aug 12; (ii), 339-342. Abstract.
Blusztajn, J.K. and Wurtman, R.J. Choline and cholinergic neurons. Science.
1983 Aug 12;221, 614-619. Abstract.
Bottiglieri, T., Godfrey, P., Flynn, T., Carney, M.W.P., Toone, B.K.
and Reynolds, E.H. Cerebrospinal fluid S-adenosylmethionine in depression
and dementia: effects of treatment with parenteral and oral s- adenosylmethionine.
J.Neurol.Neurosurg.Psych. 1990 Dec;53, 1096-1098. Abstract.
Chanarin, I., Deacon, R., Lumb, M., Muir, M. and Perry, J. Cobalamin-folate
interrelations - a critical review. Blood. 1985 Sep;66, 479-489.
No abstract available.
Cole, M.G. and Prchal, J.F. Low serum vitamin B12 in Alzheimer-type dementia.
Age Ageing. 1984 Mar;13, 101-105. Abstract.
Davies, P. and Maloney, A.J.F. Selective loss of cholinergic neurons
in Alzheimer's disease. Lancet 1976 Dec 25;2(8000) 1403-1403. Abstract.
Deacon, R., Lumb, M., Perry, J., Chanarin, I., Minty, B., Halsey, M.J.
and Nunn, J.F. (1978) Selective inactivation of vitamin B12 in rats by nitrous
oxide. Lancet. 1978 Nov 11; 2(8098):1023-1024. Abstract.
Do, K.Q. and et al. (1988) Release of neuroactive substances: Homocysteic
acid as an endogenous agonist of the NMDA receptor. J.Neural.Transm.
72 (3), 185-190. Abstract.
Doebler, J.A., Markesbery, W.R., Anthony, A. and et al. (1987) Neuronal
RNA in relation to neuronal loss and neurofibrillary pathology in the hippocampus
in Alzheimer's Disease. J.Neuropath.Exp.Neurol. 46, 28-39.
Foley, P., Bradford, H.F., Docherty, M. and et al. (1988) Evidence for
the presence of antibodies to cholinergic neurones in the serum of patients
with Alzheimer's disease. J.Neurol. 235, 466-471. Abstract.
Halliwell, B. (1989) Oxidants and the central nervous system : some fundamental
questions. Is oxidant damage relevant to Parkinson's disease, Alzheimer's
disease, traumatic injury or stroke? Acta.Neurol.Scand. 126,
Hamon, C.G.B., Blair, J.A. and Barford, P.A. (1986) The effect of tetrahydrofolate
on tetrahydrobiopterin metabolism. J.Ment.Defic.Res. 30, 179-183.
Harman, D. (1981) The aging process. Proc.Natl.Acad.Sci.(USA)
78, 7124-7128. Abstract.
Herbert, V. (1988) Don't ignore low serum cobalamin(vitamin B12) levels
[editorial]. Arch.Intern.Med. 148, 1705-1707. No abstract
Hoyer, S. and Nitsch, R. (1989) Cerebral excess release of neurotransmitter
amino-acids subsequent to reduced cerebral glucose metabolism in early-onset
dementia of Alzheimer type. J.Neural.Transm. 75, 227-232.
Karnaze, D.S. and Carmel, R. (1987) Low serum cobalamin levels in primary
degenerative dementia. Arch.Intern.Med. 147, 429-431. Abstract.
Kshitish, C.D. and Herbert, V. (1976) Vitamin B12-folate inter-relations.
Clin.Haem. 5, 697-725. No abstract available.
Mann, D.M.A. (1985) The neuropathology of Alzheimer's disease. Mech.Age.Develop.
31, 213-255. No abstract available.
Martins, R.N., Harper, C.G., Stokes, G.B. and Masters, C.L. (1986) Increased
cerebral glucose-6-phosphate dehydrogenase activity in Alzheimer's disease
may reflect oxidative stress. J.Neurochem. 46, 1042-1045.
Middleton, J.R., Coward, R.F. and Smith, P. (1969) Urinary excretion
of AIC in Vitamin B12 and folic acid deficiencies. Lancet 1,
Muller-Hill, B. and Beyreuther, K. (1989) Molecular biology of Alzheimer's
disease. Ann.Rev.Biochem. 58, 287-307. No abstract available.
Nagatsu, T., Matsuura, S. and Sugimoto, T. (1989) Physiological and clinical
chemistry of biopterin. Med.Res.Rev 9, 25-44. No
Nijst, T.Q., Wevers, R.A., Schoonderwaldt, H.C., Hommes, O.R. and de
Haan, A.F.J. (1990) Vitamin B12 and folate concentrations in serum and cerebrospinal
fluid of neurological patients with special reference to multiple sclerosis
and dementia. J.Neurol.neurosurg.Psychiatry. 53, 951-954.
Regland, B., Gottfries, C.G. and Oreland, L. (1988) Low B12 levels related
to high activity of platelet MAO in patients with dementia disorders. Acta.Psych.Scand.
78, 451-457. Abstract.
Renvall, M.J., Spindler, A.A., Ramsdell, J.W. and Paskvan, M. (1989)
Nutritional status of free-living Alzheimer's patients. Am.J.Med.Sci.
298, 20-27. Abstract.
Reynolds, E.H. and Stramentinoli, G. (1983) Folic acid, S-adenosylmethionine
and affective disorder. Psycholog.Med. 13, 705-710. Abstract.
Sawada, M., Hirata, Y., Arai, H., Iizuka, R. and Nagatsu, T. (1987) Tyrosine
hydroxylase, tryptophan hydroxylase, biopterin and neopterin in the brains
of normal controls and patients with senile dementia of Alzheimer type.
J.Neurochem. 48, 760-764. Abstract.
Schatz, R.A., Vunnam, C.R. and Sellinger, O.Z. (1977) S-adenosyl-L-homocysteine
in brain. Regional concentrations, catabolism and the effects of methionine
sulfoximine. Neurochem.Res. 2, 27-38. Abstract not available.
Scott, J.M. and Weir, D.G. (1981) The methyl-folate trap. Lancet
1, 337-340. Abstract.
Shorvon, S.D., Carney, M.W.P., Chanarin, I. and Reynolds, E.H. (1980)
The neuropsychiatry of megaloblastic anaemia. BMJ 281, 1036-1038.
Sims, N.R., Bowen, D.M., Neary, D. and Davison, A.N. (1983) Metabolic
processes in Alzheimer's disease. J.Neurochem. 41, 1329-1334.
Sinet, P.M. (1982) Metabolism of oxygen derivatives in Down's syndrome.
Ann.N.Y.Acad.Sci. 396, 83-94. Abstract.
Stabler, S.F., Marcell, P.D., Podell, E.R., Allen, R.H., Savage, D.G.
and Lindenbaum, J. (1988) Elevation of total homocysteine in the serum of
patients with cobalamin or folate deficiency detected by capillary gas chromatography-mass
spectrometry. J.Clin.Invest. 81, 466-474. Abstract.
Van Tiggelen, C.J.M. (1983) Alzheimer's disease/alcohol dementia: association
with zinc deficiency and cerebral vitamin B12 deficiency. J.Orthomolecular.Psychiatry.
13, 97-104. Abstract not available.
Wurtman, R.J., Blusztajn, J.K. and Marie, J.C. (1985) The 'autocannabilism'
of choline-containing membrane phospholipids in the pathogenesis of Alzheimer's
disease. In: Briley, M. and Kato, A., (Eds.) New concepts in Alzheimer's
disease, pp. 17-22. Plenum Press New York].
Reprinted from Medical Hypotheses, 37,(3), McCaddon A, and Kelly C,
"Alzheimer's Disease: A "cobalaminergic" hypothesis, 161-165,
1992, by permission of the publisher Churchill Livingstone. www.harcourt-international.com/journals/mehy