The article mentions Serotonin, but is it possible some other chemical is responsible? Coz i can't think of any other reason apart from a biochemical one for some types of depression that aren't caused by circumstances.
It's not necessarily likely that a single chemical is the cause (as is suggested in the serotonin hypothesis).
I know there are people who are chronically depressed but I don't know how to untangle them from the context in which their disease exists. The only way to really know would be to try adapting them to wholly different circumstances and see whether or not the way the brain responded changed as a result.
I think that's the point: depression is not independent of circumstances (in the wider sense).