Debra was first diagnosed with the disease 17 years ago. It went into remission but returned and spread throughout her body, including her brain, in 2010, the celebrity news website said.
Where she died Friday and how old she was were not reported.
Jennette, 21, is best known for her work on the Nickelodeon shows "Sam & Cat" and "iCarly."
She has canceled public appearances this weekend so she can grieve for her mom, TMZ said.
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