Hi,
I like the idea but have a few questions. The first is around the disease. As Huntingtons is a degenerative disease I'm not sure how he could overcome it. Usually it progresses in stages, starting out subtly which wouldn't prevent your character from helping his friend and then growing in severity to physical and mental impairment, which unfortunately can't be reversed (i believe). You may know more about the disease/studied it more than me so please ignore me if I'm wrong.
If he had a psychological impairment however that he had to force himself to overcome that might provide a challenge that your hero has the ability to defeat. Maybe he hasn't spoken in ten years. Then again, as I write this I realise that would make for a rather boring script because your hero wouldn't speak until Act 3...but you get the idea.
Also keen to know if he's a doctor or in the field of science. Maybe he was once brilliant and has started to lose his esteem. This would make for an interesting character - in a similar fashion to A Beautiful Mind
Good luck :)