English speakers: Can you solve this grammatical doubt?

The first one, either way is okay, but we don't say "heart's disease," it's just "heart disease." That's just the way we do it. In most cases, it doesn't matter which way you do it, but in cases related to a disease, we usually say "eye disease" or "kidney disease" instead of "eye's disease," etc. "Disease of the eye" or "disease of the heart" is also correct.