Found: mystery based on a bone marrow transplant

English or American mystery/thriller about a woman who gives a bone marrow transplant against her boyfriend's wishes. He becomes abusive, she escapes. There are a few story threads that intertwine - a man who deliberately become homeless when his family dies, a shih tzu, and the bone marrow recipient/brother. I think the word "gate" is in the title (something to do with a park in London (I think)). I believe the author was a relatively famous woman author.

The Keys to the Street by Ruth Rendell?

Yes! I think the paperback I read had a gate on the cover. I thought it was Ruth Rendell, but couldn't find a title by her with "gate." Thank you.

You're welcome.