Knock-Out stable cell lines

[Joven99]

Hey there!

We've created in our lab a stable cell line knock-out for a gene (with CRISPR Cas9). By the way, down-regulation (by RNAi) of this gene was able to up-regulate RB phosphorylation on S807/811 (inhibitory phosphorylations). Unfortunately, once the stable cell line was formed, this phosphorylation was not detectable anymore comparing WT e KO. 

When KO was reconstituted (KO + overexpression of the gene), pRB S807/811 dropped. 

Does anyone know how this could be explained? I could not find anything in the literature do you know some papers I could read about?

Edited May 29 by Joven99
missing a point