Brain derived neurotrophic factor Val66Met polymorphism and psychotic symptoms in Alzheimer's disease

Alzheimer's disease (AD) is an irreversible, progressive neurodegenerative disorder with a high prevalence. Since behavioral disturbances, such as psychotic symptoms, represent a key feature of AD, genes related to dopamine, serotonin and brain derived neurotrophic factor (BDNF), are considered as candidate genes for AD. BDNF is a neurotrophin that regulates neurodevelopment, neuroplasticity, and neuronal functions. BDNF is involved in the etiopathogenesis of psychiatric and neurodegenerative disorders. A single base pair polymorphism (BDNF Val66Met) was reported to be associated with AD and/or schizophrenia, as well as other psychoses, although some studies failed to replicate these findings. The aim of the study was to evaluate the association between BDNF Val66Met variants and AD, as well as onset of AD or presence of psychotic symptoms in AD.

BDNF Val66Met was analyzed in 211 patients with AD and in 402 aged healthy control subjects. All subjects were ethnically homogenous Caucasians from Croatia, and were subdivided according to the gender, onset of AD, and presence of psychotic symptoms. A χ(2) test, with Bonferroni correction and standardized residuals were used to evaluate the data.

Distribution of the BDNF Val66Met genotypes differed significantly between male and female AD patients with or without psychotic symptoms. This difference was due to the significant contribution of the Met/Val genotype and the combined Met/Met and Met/Val genotypes between psychotic and non-psychotic symptoms in male, but not in female patients with AD. The frequency of the gene variants of the BDNF Val66Met did not differ significantly among male and female patients with AD and control subjects, or between male and female patients with early or late onset AD. There were significant sex related differences in age, duration of illness and scores of dementia between patients with AD.

Our male patients were younger, had shorter duration of illness, and had less severe dementia and higher cognitive performance than female AD patients. The gene variants of the BDNF Val66Met polymorphism were significantly associated with the presence of psychotic symptoms in male, but not in female patients with AD. The results had adequate statistical power to suggest that BDNF Val66Met was not related to susceptibility to AD or the onset of AD, but that presence of one or two Met alleles of BDNF Val66Met polymorphism might present a risk factor for psychosis in AD.