People have to be totally selfish or selfless says a false dichotomy. People seem to like those. I see people's actions to be along a spectrum between where the motivations vary depending on what they want to do. Some people seem to lean almost entirely toward one or the other but I'm not prescriptive about that.

We don't have to guess about his motivations, though. It's in the article:

"For Johnson, the quest is innately personal. His step-father has early signs of Alzheimer’s."

He and his step-father are suffering through the effects of Alzheimer's. He definitely has personal gain from a treatment that would work. There's a clear selfless angle on helping the step-father. He probably, after personally experiencing it, also would like to prevent others from going through the same thing. That's my guess. Respectable motivations for sure.