'Notebook' Star Gena Rowlands Battling Alzheimer's After Onscreen Portrayal

Gena Rowlands -- most famous from "The Notebook" -- is sadly battling Alzheimer's disease ... a heartbreaking update after she portrayed a key character with the same disease.

The actress' son and 'Notebook' director Nick Cassavetes shared the news Tuesday with Entertainment Weekly -- this while celebrating the 20th anniversary of his beloved and iconic romance flick ... which he directed and his mother starred in as Allie in old age.

NC says ... "I got my mom to play older Allie, and we spent a lot of time talking about Alzheimer's and wanting to be authentic with it, and now, for the last five years, she's had Alzheimer's. She's in full dementia. And it's so crazy -- we lived it, she acted it, and now it's on us."

Remember ... the big screen adaptation of the Nicholas Sparks' romance novel starred James Garner as Duke, a resident in a nursing home telling Gena's Allie a love story ... which of course also starred Ryan Gosling and Rachel McAdams.

By the end of the film, it's revealed that Duke is actually the older version of Ryan's Noah character ... and he's trying to help older Allie regain her memories.

It's a tearjerker, to say the least ... and with this latest real-life twist, doubly so.

While "The Notebook" may be hitting a bit too close to home for Gena and her loved ones, Nick told the outlet he's proud of their flick ... and that it still holds up 2 decades later.

He continued ... "It's always a shock to hear that as much time has gone by as it has, but it makes sense. I'm just happy that it exists. It seems to have worked and I'm very proud of it."

Gena -- the daughter of actress Lady Rowlands, who was also diagnosed with Alzheimer's before her death -- earned 4 Emmy awards, 2 Golden Globes, and 2 Oscar nominations before retiring in 2015.

She collaborated with her filmmaker husband, John Cassavetes, on a number of occasions -- including on "A Woman Under the Influence" and "Gloria."