MFN2 and Alzheimer disease: Although no specific mutations in fusion genes were associated with AD, studies from several groups consistently demonstrated a significantly reduced expression of large GTPases involved in fusion (i.e., Mfn1, Mfn2 and OPA1) in the brain of AD patients, implicating that mitochondrial fragmentation, largely due to disrupted mitochondrial fusion, likely occurs in the susceptible neurons in AD brain [12, 22, 23].