The present invention pertains to the domain of brain diseases, and provides novel markers and methods for diagnosing a brain alteration in an individual, especially in patients suffering from neurodegenerative diseases such as Alzheimer's disease. The present invention also provides tools for evaluating the probability, for an individual, of developing the disease, as well as a target for identifying new drugs for treating neurodegenerative diseases such as Alzheimer's disease. In particular, the invention provides a genetic marker based on combination of two single nucleotide polymorphism, at positions -389 and -241 of the ornithine transcarbamylase (OTC) gene.