Scientists raise possibility of vaccine for Alzheimer's disease 

Deborah Josefson , San Francisco

 An experimental vaccine directed against amyloid, a protein implicated in Alzheimer's disease, has shown promise in animal trials (Nature 1999;400:173-7). The vaccine may lead to an effective treatment.

 Scientists at Elan Pharmaceuticals developed a transgenic mouse model of Alzheimer's disease by injecting the rodent with a mutant form of the human amyloid precursor protein. The mutant amyloid precursor protein gene, which occurs in a number of familial forms of Alzheimer's disease, leads to an overexpression of the amyloid <Immagine: beta > peptide, the principal constituent of amyloid plaque in the disease.

Amyloid plaques consist of insoluble aggregates of amyloid protein and are thought to be involved in neuronal cell death.

 The transgenic mice developed amyloid plaques in their brains in a manner specific to age and brain region, mimicking the changes seen in human forms of Alzheimer's disease.

 The researchers then sought to see if immunisation with a fragment of amyloid protein would modify the disease in their affected mice. Accordingly, mice were immunised with a 42 amino acid segment of the amyloid B protein.

One group of mice was immunised at 6 weeks of age, when the neuropathological hallmarks of Alzheimer's disease are not yet present, and a second group at 11 months of age, when amyloid deposition is already prominent. Two main experiments were then conducted. In the first experiment (with the mice immunised at 6 weeks old), three control groups were used for comparison: one group received saline vaccinations, a second was left untreated, and the third was immunised with another plaque associated protein (serum amyloid protein).

 At the age of 13 weeks, the groups vaccinated at 6 weeks were killed and their brains examined. Seven out of the nine mice treated with the b amyloid protein had no detectable plaques, whereas the other groups showed age related plaque accumulation.

 In the second experiment (with the mice immunised at 11 months old) two control groups from the same litter were left untreated. At 18 months the vaccinated mice showed significantly less plaque formation than their 18 month old controls. They also had less gliosis and neuritic dystrophy. Even more striking, however, was the finding that these mice also had less plaque formation than younger untreated control mice, those at 12 months old.

This suggests that immunisation with the amyloid b protein facilitated the removal of amyloid plaque, probably by an antibody mediated immune attack.

 Commenting on the study, Dale Schenk, the chief investigator on the project, said: "When we examined that group at 18 months ... we expected to see widespread brain pathology [but] it had been halted in its tracks.

 The brain tissue looked essentially like [that of] the original 11 month old animal and in fact looked somewhat better. This suggested to us that the vaccine had potential for treatment."

 The researchers said that the vaccine did not have any detectable side effects in mice.

 Although the results are promising, it is not known if the findings are applicable to humans. Whether amyloid deposition is the cause or the effect of Alzheimer's disease is still widely debated.

 Furthermore, although the mice developed plaques, they lacked other features of the disease, such as neurofibrillary

tangles, and cognitive decline.

A spokesman for Elan Pharmaceuticals said that the company plans to submit an application to the US Food and Drug Administration and hopes to begin human clinical trials by the end of next year.

Science, medicine, and the future: Alzheimer's disease Colin L Masters, head,a Konrad Beyreuther, director

There is a noticeable air of optimism in the research community studying Alzheimer's disease. This because the molecular basis of Alzheimer's disease and other neurodegenerative conditions, such as Parkinson's and Huntington's diseases, is rapidly being elucidated. From these molecular insights, it is likely that effective therapeutic strategies will be developed within the next 10 years. Future treatment will probably be based on combination therapies—such as neurotransmitter replacement combined with a drug to protect against the toxic effect of Aß amyloid—tailored to the genetic profile of an individual. Assuming proved efficacy and safety, these forms of treatment are likely to be as widespread and acceptable as cholesterol lowering treatment is today.

In the past few years there has been an avalanche of knowledge surrounding the genesis of Aß amyloid plaques, one of the principal pathological hallmarks of this disease (see fig 1). In the absence of a serious candidate for the pathway leading to the neurofibrillary tangle, the other major pathological lesion in Alzheimer's disease (fig 1), the focus will probably remain on amyloid plaques. Strong genetic risk factors have been identified for Alzheimer's disease, all of which interact directly or indirectly with the Aß amyloid pathway. Undoubtedly, other genetic factors remain to be discovered, some of which might open the door to the neurofibrillary tangle. More importantly, the major environmental risk factors for Alzheimer's disease remain elusive. This is not surprising, given the relatively few analytical epidemiological studies that have been conducted and the difficulty of case ascertainment, particularly in the early stages of the disease.

  Fig 1 The central pathway leading to Alzheimer's disease involves the processing of amyloid precursor protein into Aß amyloid, which accumulates as amyloid plaques or perivascular amyloid. Concomitantly, degeneration occurs in neurons and their processes, leading to neurofibrillary tangles. (Images are from Spielmeyer's Histopathology of the Nervous System, 1922.) Mutations in the gene encoding amyloid precursor protein can cause Alzheimer's disease, as do mutations in the presenilin genes. Inheritance of particular polymorphisms in genes such as ApoE also can increase susceptibility for Alzheimer's disease. Major environmental risk factors for Alzheimer's disease remain to be determined

We review briefly where the research is heading and give some predictions on where our concepts might lie a decade from now (for more details see recent reviews1 2 3).

Many lines of evidence confirm that the generation of Aß amyloid from the amyloid precursor protein is the central pathway in Alzheimer's disease (see fig 2). The clinching evidence has come from the recognition that rare genetic mutations in the gene encoding amyloid precursor protein actually cause Alzheimer's disease at an early age (onset before 65 years).

 
   Fig 2  (a) Cleavage of amyloid precursor protein (APP) by enzymes (secretases) release the Aß amyloidogenic fragment. (b) The critical region of amyloid precursor protein shown schematically in the one letter amino acid code. The secretases act at three principal sites (<Immagine: {alpha}>, ß, and <Immagine: {gamma}>). Mutations in the gene for amyloid precursor protein at these sites can adversely affect the action of secretases: mutations towards the NH2 terminus increase the absolute rate of ß-secretion, while mutations near the COOH terminus affect the ratio of Aß42 to Aß40. The Aß42 forms are more damaging for nerve cells

 Possible futures

 Genotype screening, analysis, and counselling

 Presymptomatic diagnosis

 Rational preventive treatment—drug based or gene based

Advice on preventive measures—changes to lifestyle to avoid known environmental risk factor(s)

 Drugs targeting the amyloidogenic pathway to modify the course of the disease

 Amyloid precursor protein is a normal transmembrane glycoprotein that is widely expressed in the body, but particularly in brain and platelets. Its function remains uncertain: mice lacking the protein seem largely normal but have subtle defects in synaptic function. Expression of the gene for the protein is closely regulated and responds quickly to a wide variety of cellular stresses and exogenous factors (including trauma, oestrogens, and certain metal ions). Generated in the endoplasmic reticulum, amyloid precursor protein is sent to the Golgi apparatus for glycation before export to the cell surface.

 At critical points of its biogenesis, amyloid precursor protein is subjected to enzymatic proteolytic cleavages, which in concert generate the Aß peptides (fig 2). These enzymes, termed secretases, release the amyloid precursor protein from the cell membrane and thereby affect the proportion of the protein that remains on the cell surface or is released into the extracellular milieu. The Aß peptides encompass part of the hydrophobic transmembrane domain. The exact cleavage sites of the <Immagine: {gamma}>-secretases are important, since the length of the hydrophobic tail of the Aß peptide may be a crucial factor determining its aggregation and toxicity. Thus, the shorter Aß40 is the species most often identified in non-neuronal cells and has less tendency to aggregate than the longer Aß42: it is this longer Aß42 that is found at the centre of amyloid plaques. Neuronal cells have a propensity to make the longer forms, probably in a different cellular compartment (the endoplasmic reticulum). How either form is released from the cell remains uncertain.

 Once released from the cell, Aß peptides aggregate into amyloid fibrils. The rates of deposition and clearance of Aß from the brain may be critical determinants in establishing disease. The exact mechanisms by which Aß exerts its toxicity or adverse "gain of function" is under intense scrutiny.

Other chronic degenerative diseases of aging nervous system

Recent elucidation of a variety of gene mutations causing diverse chronic neurodegenerative diseases point to a common mechanism—the toxic gain of function of small, relatively insoluble, protein polymers (see box). If further research confirms and extends this line of reasoning, Alzheimer's disease may eventually be seen as only one example of a process in which an abnormally shaped molecule accumulates in the brain and causes neuronal damage. In that case, a treatment developed for the toxic effect of the polyglutamine expression of the abnormal gene in Huntington's disease might be relevant for one or all of the other neurodegenerative diseases.

Neurodegenerative diseases associated with abnormal protein conformations (toxic gain of function)  Disease

 •Alzheimer's disease

 •Creutzfeldt-Jakob disease

 •Amyotrophic lateral sclerosis

 •Parkinson's disease

 •Huntington's disease

 •Machado-Joseph disease

Gene product

 Amyloid precursor protein and Aß amyloid

 Prion protein

 Superoxide dismutase

{alpha}>-synuclein

Huntingtin

Ataxin-3

 The discovery of the presenilin family of genes has been a major breakthrough for research.4 Together with mutations in amyloid precursor protein, mutations in these presenilin genes also cause early onset of Alzheimer's disease and probably act directly through the amyloidogenic pathway. We have now identified about half of all the causative genes (responsible for possibly 10-20% of all cases of Alzheimer's disease). Over the next decade, it is highly likely that the remaining genes will be discovered, particularly in view of the rapid progress in mapping and sequencing the human genome.

 In contrast with the causative genetic mutations, genetic risk factors are emerging as important contributors to the occurrence of sporadic Alzheimer's disease (responsible for 80-90% of all cases). The first to be identified, the ApoE gene on chromosome 19, has provided clues to the likely size of effect of these "public" genetic polymorphisms in a complex disease. Thus, inheritance of the ApoE-<Immagine: {epsilon}>4 allele may increase the risk for Alzheimer's disease by up to eightfold. In the near future other genetic loci that act as susceptibility factors for Alzheimer's disease will undoubtedly be discovered. For example, there is much current interest in loci on chromosome 12. These discoveries will bring forward the emerging field of pharmacogenetics, in which treatments and preventive strategies will be tailored to an individual's genetic profile.

 Environmental factors might be expected to have a role in causing Alzheimer's disease, in common with all multifactorial complex diseases, but, surprisingly, none has yet been convincingly identified. Estimates of relative risk indicate that factors such as low education, head trauma, smoking, concomitant vascular disease, diabetes, and the menopause have modest or inconsequential effects. Is there a major environmental risk factor still waiting to be discovered by some enterprising epidemiologist? Could some subtle factor in the Western diet or lifestyle be uncovered through a more thorough understanding of the amyloidogenic pathway? For example, we know that metal ions (such as copper and zinc) interact adversely with amyloid precursor protein and Aß, and evidence is emerging that oxidative stress mediated by hydroxyl radicals could underlie the basis of Aß toxicity. These clues may provide the impetus for future epidemiological studies.

 A major impediment to the development of rational treatments has been the lack of an authentic and practical small animal model of Alzheimer's disease. Fortunately, this seems to have been solved by the development of various transgenic mouse models, which are progressively looking more like the human disease. The latest are based on the overexpression of amyloid precursor protein combined with the effects of the causative human mutations.5 The next step may be to modulate the strain background of the mice or to introduce another transgene to replicate the full human phenotype. Progress in this area has been so rapid that there is every reason to believe that an effective mouse model will soon be available.

 In the past decade, much has been learned about the conduct of clinical trials by which the efficacy of any proposed treatment for Alzheimer's disease can be assessed. The licensing of compounds such as tacrine, donepezil, and rivastigmin have set standards by which all future drugs will be judged. There are currently four drugs awaiting approval and more than 16 drugs undergoing phase III clinical evaluation. Most are directed at the cholinergic system. Drugs specifically targeting the amyloidogenic pathway (see box) are only now beginning to emerge in a preclinical setting.

Therapeutic targets in the amyloidogenic pathway

 •Inhibit Aß forming enzymes

 •Redirect processing of amyloid precursor protein away from Aß42

 •Inhibit aggregation or promote dissolution of Aß

 •Ameliorate toxicity of Aß

 •Suppress reactive responses to Aß toxicity

Looking ahead to the next decade, it is likely that a comprehensive package of genotypic analysis, presymptomatic diagnosis, and advice on preventive measures will be advocated (see box), with the use of a combination of drugs that effectively modify the course of the disease. Perhaps it is too much to expect any form of curative treatment by the year 2008, but the underlying concepts and principles for preventing the amyloidogenic processes from damaging neurons is straightforward and eminently amenable to intervention. Current estimates of the economic and social costs of Alzheimer's disease vary widely within and between countries, but all agree on the immense size of the problem and that it will increase dramatically over the next decade. The cost effectiveness of any preventive treatment is potentially enormous. In contrast with the small effect of today's symptomatic treatments,6 future strategies may alleviate a burden that threatens most families; up to 25% of a family's annual income is required to care for a member with Alzheimer's disease.7 As the average duration of the illness is 10 years, it is relatively easy to derive a rough estimate of the economic impact of the disease. Since a large proportion of the population is at risk of developing Alzheimer's disease (possibly over half), an effective drug based preventive treatment would justify universal screening (probably beginning at ages 40-50 years, when amyloid plaques are starting to appear in the temporal cortex).

Managing Alzheimer's disease in the year 2008

 •Genotype screening, analysis, and counselling

 •Presymptomatic diagnosis

 •Rational preventive treatment—drug based or gene based

 •Changes to lifestyle—avoiding the environmental risk factor(s)

 References 

1.Hardy J. Amyloid, the presenilins and Alzheimer's disease. Trends Neurosci 1997;20:154-9. [Medline] 2.Selkoe DJ. Amyloid ß-protein and the genetics of Alzheimer's disease. J Biol Chem 1996;271:18295-8. [Full Text] 3.Yankner BA. Mechanisms of neuronal degeneration in Alzheimer's disease. Neuron 1996;16:921-32. [Medline] 4.Kim T-W, Tanzi RE. Presenilins and Alzheimer's disease. Curr Opin Neurobiol 1997;7:683-8. [Medline] 5.Struchler-Pierrat C, Abramowski D, Duke M, Wiederhold K-H, Mistl C, Rothacher S, et al. Two amyloid precursor protein transgenic mouse models with Alzheimer disease-like pathology. Proc Natl Acad Sci USA 1997;94:13287-92. [Full Text] 6.Wimo A, Karlsson G, Nordberg A, Winblad B. Treatment of Alzheimer disease with tacrine: a cost-analysis model. Alzheimer Dis Assoc Disord 1997;11:191-200. [Medline] 7.Cavallo MC, Fattore G. The economic burden of Alzheimer disease on families in the Lombardy region of Italy. Alzheimer Dis Assoc Disord 1997;11:184-90

Please follow the links or contact us for further information.