When an author has a manuscript with graphic scenes (abuse) and it is not in the Horror or Thriller genre where such events may be expected, what is the best way to mention this in a query letter as to be sensitive to agents / editors who may have certain “trigger” issues?
Your job is not to protect me from anything.
Your job is to tell your story as well, as compelling, as possible.
If that involves graphic scenes of abuse, don't pussy foot around and warn people. Tell your story in a way that makes me compelled to read on.
[I'm going to spare everyone a rant about the recent move to put warning signs on reading matter for adults.]
I will say this however: agents and readers may elect not to read things that have graphic scenes of abuse and they don't have to explain why that is to you either.
For example, I don't take on any projects about child abuse. I read Bastard Out of Carolina, and I Know Why The Caged Bird Sings, and The Bluest Eye, and the was pretty much it for me.
That said, I think Spotlight is one of the best movies I've ever seen and in fact use it as an example of when/how to effectively include a prologue in your work.
If this is the novel that you must write, do it.
Let the chips fall where they may.