I don't believe this is directly addressed in the movie, but the early script did (November 29th, 1992).
In the original script, there is a conversation between Schreber and John where Schreber says that he had been experimenting on him.
The doctor removes a SMALL SYRINGE from his black bag, places it on the table in front of the glass, blocks it from the guard's view.
If you would only take this, inject
it in your brain, everything would be
Not that again...
Everyone get's one - very much like
(points to syringe)
But this one's special. It will help
you understand, everything...
WHITE (cuts in)
I'm not injecting anything into my
Anton - I mean, John, there
has been an experiment, a dangerous
experiment. I arranged it. It was a
lie from the beginning. It almost
worked, but things did not go as they
should have. You have been left -
Schreber waves frantically at the air surrounding him.
You can change this, all this.
(points at syringe again)
But you must take it.
But why John? Was John given the power from scratch? What made him different?
This earlier conversation, which Schreber passes off as his own rambling, leads me to believe that he was chosen due to scar tissue on John's brain.
INT. DOCTOR SCHREBER'S OFFICE - NIGHT
SCHREBER is showing WHITE a series of CHARTS. The doctor seems nervous about White's potential for further violence.
Put simply... a penetration of the
left parieto-occipital area of the
cranium... complicated by
inflammation that resulted in
adhesions of the brain to the
meninges. Without going into
detail... the formation of scar
tissue altered the configuration of
the lateral ventricles, producing
incipient atrophy of the medulla...
In the final cut of the movie, Schreber explains Dark City, the Strangers and John's ability to Tune to John. While not definitive, Schreber talks like John had or developed the ability independently. At this point, he has no reason to lie, so it would seem odd for him to change that one fact, or otherwise leave it out. From the transcript (Note that I added formatting and identified speakers):
Where do I fit in?
You're different. You resisted my attempt to imprint you. Somehow, you have developed their ability to Tune. That is how they change things, how they built this city. They have machines buried beneath the surface... that allow them to focus their telepathic energies. They control everything here, even the sun. That's why it's always dark. They can't stand the light.
Why do they need you?
When they first brought us here, they extracted what was in us and stored the information, remixed it like paint... and gave us back new memories of their choosing. But they still needed an artist to help them. I understood the human mind better than they ever could, so they allowed me to keep my skills as a scientist... because they needed them. They made me delete everything else.