Which of the following statements are $CORRECT$?

1.  Viruses can play a role in causing human cancer.
1. A tumor suppressor gene can be turned off without any change in its $DNA$ sequence.
1. Alteration in $miRNA$ expression levels contributes to the development of cancer.
1. $P$ and $Q$ only
2. $Q$ and $R$ only
3. $P$ and $R$ only
4. $P$, $Q$ and $R$