The clinical features of Alzheimer's disease (AD) are, loss of short-term memory, deficient in praxis (ability to perform skilled movements) and the skill of reasoning and judgment (Doraiswamy PM, 1997). These symptoms arise from involvement of the temporal lobe, hippocampus, and the parietal association cortices, with lesser involvement of frontal lobes, until the disease is in its later stage. A second most prominent neuropathological feature which is also present in AD is the complex, fibrilar deposits in the cortex of the brain; this is known as senile and amyloid plaques. Amyloidal plaques have been the subject of AD research in recent times. These plaques contain a number of proteins, including apolipoprotein E, and 1-anti-chymotrypsin (Carlos Morgana, 2004). The principal component amyloidal plaques are amyloidal-beta peptide that is derived from a beta-amyloidal precursor protein. The presence of another distinct characteristic, that is also present in other dementias like Lewy Body Variant of AD and Fronto-temporal dementia, is the incidence of neurofibrillary tangles. These tangles are intraneural inclusions that are composed of hyperphosphorylated forms of a microtubule associated protein known as tau (Peter H. St George-Hyslop, 2004). In other words neurofibrillary tangles are the pathological neuron aggregates present in the neurons of the patiets suffering from AD.
In the last several years, due to the increasing incidence of AD, researchers have been focussing on the main causes of this disease in the general population. Much research has been carried out, and it generally confirms that the overall life time risk of AD in a first degree relative of someone with AD is about 38%, by the age of 85 years. Three main reasons seem to reflect a complex mode of transmission (1) single autosomal gene defects, (2) multi-gene traits, (3) a mode of transfer in which both genetic and environmenal factors connect. The research also suggested that only a smal part of human poulation reflected, the transmission as a pure autosomal dominant mendelian trait. Recent genetic studies have identified four genes associated with inherited risk for AD (presenilin 1, presenilin 2, amyloid precursor protein, and apolipoprotein E) (Peter H. St George-Hyslop, 2004). The first gene to be identified as a precursor for AD is the amyloid precursor protein (APP). This gene is responsible for encoding an alternatively spliced transcript which, in its longest isoform encodes a singe transmembrane that spans a polypeptide of 770 amino acids (J. Kang, 325). This protein undergoes a series of endoproteolytic cleavages. This is mediated by a membrane-associated alpha-secretase that cleaves this protein in the middle of the A peptide domain, and liberates the extracellular N-terminus APP. The other cleavage pathway involves sequential cleavages by the - and -secretases, thus generating a 40-42 amino acid A peptide. The second protein responsible is Apolipoprotein E. This protein in humans contains 3 common polymorphisms. Analysis of these polymorphisms in AD unaffected persons has shown that there is an increased frequency of the delta 4 allele in people with AD. The third gene responsible for AD is Presnilin 1 (R. Sherrington, 1995). Presenilin 2 is the fourth gene that was identified during the cloning of Presenilin 1 on chromosome 14. This gene encoded a polypeptide whose open reading frame