Which of the following statements are $CORRECT$?
- Viruses can play a role in causing human cancer.
- A tumor suppressor gene can be turned off without any change in its $DNA$ sequence.
- Alteration in $miRNA$ expression levels contributes to the development of cancer.
- $P$ and $Q$ only
- $Q$ and $R$ only
- $P$ and $R$ only
- $P$, $Q$ and $R$