Polycystic kidney is wide spread disease in some cat breeds.
You can read more on NCBI. I'll describe only "in game" mutation using the source + deaf lion description from wiki.

It can be adult lethal mutation (like deaf) and AMP (it usually appears between 3-10 years in domestic cats).

Polycystic kidney (PK) is a lethal mutation that leads to severe and irreversible kidney failure. It is lethal mutation that does not change the way a lion looks; however, it does change many mechanisms connected to lions.

Polycystic kidney lions cannot hunt, patrol, or be a king. If a PK lioness is pregnant, she can die while pregnant; a deaf nursing lioness, however, cannot die while nursing.

What's also unique about PK lions is that they give updates about your lion's health, as well as unique flavour texts specific to the PK mutation!

PK lions can die randomly between 2 years 1 month to 7 (or less) years of age. If they live to be 7 years old, they will pass away upon the next rollover.

If your adolescent lion with hidden PK ages up, you'll receive the following health message:
I'm really bad in flavour texts and so on, something about uncomfortable symptoms, apathy

You will also receive other health messages and flavour texts during the lion's life that are specific for PK mutation, as well as specific death message.

PK mutation can be passed to cubs with Lion Scrotum (25% chance). But it's risky because pregnant lioness can die.

Please, share flavour texts and health messages if you have any ideas