Generally it is not safe to remove samples of brain tissue from living individuals - it can lead to brain death (and actual death) far too often. Plus, it doesn't matter that these tests were done on deceased brains as RNA doesn't continue to be produced after death, nor does it alter, just like dead DNA doesn't alter (it can decay, but these brains were preserved for science). As RNA expression is different in different organs of the body (based on which genes are needed to carry out different functions), it's not quite the same as being able to take a swab of saliva to do a DNA test. What seems to be happening is not that those of us with bipolar disorder (or other disorders) have gene mutations (they say subtle differences), but rather that something happens in our brains to read the genes differently to "normal" individuals meaning the RNA in the brain looks different from what would normally be expected. RNA is what reads the genes off a DNA strand and then "copies" the relevant instructions to produce proteins. For some reason, some of these instructions are acted on too much and others too little in the brains of bipolar individuals compared to "normal" individuals.
It would be nice to see if there is a way to test live individuals for this difference, perhaps DNA testing might work to aid diagnosis. Though scientists have already highlighted gene abnormalities, and are currently trying to study what the abnormalities do and why. This article is obviously an early step in the process. There are bound to be further experiments using lab rats/mice where they purposely introduce these abnormal genes and see if they result in the same pattern of RNA expression or if there is something else going on as well. I'm not really sure how they can use us live bipolar patients to further this branch of investigation at this stage. Better to study us in a less invasive way for now!