This is an interesting article! Bipolar disorder has always been thought to be a genetic disorder, that is due to mutations in genes. One of the reasons bring it is inherited and runs in families. However, here the authors make a case for it not being genetic but physiological in nature. I’ve put the most important passage in the article in quotes below. If you’d love to read the whole article, there’s, as always, a link below too. CRMP2 is a protein that is involved in neural networking, i.e in arranging neurons from soma to axon and making networks between neurons. In bipolar disorder, the activity of this protein is low. When Lithium is given to Lithium responsive neurons, the activity of CRMP2 is brought back up to normal, thereby correcting the underlying cause of the disorder. That’s what the authors say. Just have to wait and see if this is truly the underlying cause of bipolar disorder. 
https://m.medicalxpress.com/news/2017-05-underlying-molecular-mechanism-bipolar-disorder.html

“In hiPS cells created from lithium-responsive and non-responsive patients, researchers observed a physiological difference in the regulation of CRMP2, which rendered the protein to be in a much more inactive state in responsive patients. However, the research showed that when lithium was administered to these cells, their regulatory mechanisms were corrected, restoring normal activity of CRMP2 and correcting the underlying cause of their disorder. Thus, the study demonstrated that bipolar disorder can be rooted in physiological—not necessarily genetic—mechanisms. The insights derived from the hiPS cells were validated in actual brain specimens from patients with bipolar disorder (on and off lithium), in animal models, and in the actions of living neurons.”