A degenerative brain disease characterized primarily by progressive dementia. Type 4 has an early onset (starts before the age of 65). It is caused by mutations in the PSEN2 gene which results in the production of a toxic protein (amyloid beta peptide) in the brain which collects into clumps (amyloid plaques) in the brain. These plaques cause damage to nerve cells in the brain. See also Alzheimer disease type 4