Alzheimer's disease is the most common form of dementia that attacks the brain and accounts for 60 to 80 percent of cases. The more gradual form, with slow progress of symptoms, begins around ages 65 to 70 and affects approximately 1%-6% of the AD patients. See Table 1.1. About 60% of early-onset AD is familial, with 13% being inherited due to family history [Rocca et al 1991, Campion et al 1999]. A rapidly progressive form begins around ages 36 to 45. Both sexes are affected usually beginning in middle age and older adults, but women are more likely to develop Alzheimer's than men, as women live longer, on average, than men.
According to the research from the Aging, Demographics and Memory Study (ADAMS), 14% of all people aged 71 and older have dementia. The estimate done in 2008, states that 2.4 million women and 1 million men aged 71 and older have dementia. As many as 5.3 million people in the United States are living with Alzheimer's. This figure includes 5.1 million people aged 65 and older and 200,000 individuals under age 65 who have younger-onset Alzheimer's. Based on these estimates, approximately 500,000 Americans under age 65 have Alzheimer's or other dementia. Of these, about more than 46% are estimated to have Alzheimer's disease. See Table 1.2
AD is a genetically complex and heterogeneous disorder. Particularly for genetic studies, Alzheimer's disease is often categorized according to age. ...
Commonly knowm as the "Mild Cognitive impairment". (Petersen et al. 1999) Mild cognitive impairment is a major focus of research to facilitate early intervention while functional status is preserved. It can be further classified into Psychiatric Genetics or Molecular Genetics.
Late on-set Familial AD:
According to Smoller, Sheidley and tsuang, the research evidence proves the presence of additional AD genes. For example, the family history (Payami et al, 1997) or monozygotic twin status (Bergem et al, 1997) remains after APOE4 is controlled for. Also, a segregation analysis points to the existence if many additional late on-set AD genes (Daw et al, 2000) and the peak age at onset is in the 60's. Linkage analysis has provided several clues to where such AD genes may reside (reviewed in Bertram and tanzi 2004; Kamboh 2004). Other than the APOE region on chromosomes 9, 10 and 12. (Blacker et al 2003); Myers et al 2002). More than 200 genes have been tested for association with AD, with discouraging results (Alzheimer research Forum 2005; Kamboh 2004). There is a possibility of huge understanding in AD genetics. The discovery of the APP, PSEN1, and PSEN2 has contributed greatly to the AD, and at least 20 drugs are in the process of development in genetics research. These may help in early detection and intervention and further prevention. In addition, the study can help in providing genetic counseling and genetic testing for patients with family history.
Early-onset familial AD (EOFAD):
Early-onset cases can occur in families with generally late-onset disease (Brickell et al 2006). At least three subtypes of molecular genetics EOFAD (AD1, AD3, and AD4) have been identified based on the causative gene.