Netrin-1 limits the number of cells, since those which grow too far from the crypts will lose the signal (Netrin-1) which prevents cell death. Without the receptor, there is unregulated cell growth/division, which is the major part of tumor formation.
Work Step by Step
Consider first what makes a tumor: uncontrolled cell growth. Then, ask yourself what about this example has to do with the control of cell growth/division: Netrin-1 abundance perceived via its receptor. Thus, no receptor means no control, which sets the stage for tumor formation.