Depression is not a barrier to doing a startup. Indeed, given that a large number of people (statistics vary widely as to the exact %) suffer from depression at some point in their lives, it is likely that there are far more happy outcomes of depressed people (or people prone to depression) doing startups than there are tragic cases where it has led to suicide.

There is some evidence that depressive spells can lead to more clarity - this is wrapped up in the debate as to why depression exists in the first place. Additionally, the severity of the depression is a major factor - I have seen people entirely incapacitated by it, whereas others are able to effectively manage it using therapy and/or meds.

Also relevant is the nature/cause of the depression - it is possible to suffer from symptoms of depression (i.e. scoring highly on, say, the Beck depression inventory) that are largely situational - i.e. they are brought on by a job you hate, a bad relationship, etc. In these cases, doing a startup might actually be something that enables you to focus your energy and actually reduce or eliminate the symptoms entirely.