뇌암의 원인은 유전자 돌연변이

But recent research has suggested that more than 50 types of human cancers may be caused not by a defective TERT gene itself, but by mutations in the TERTpromoter—... ... These mutations enable a tranion factor called GABP to bind to the TERT promoter and activate it. 

https://m.medicalxpress.com/news/2018-09-secret-deadly-brain-cancer-immortality.html