What happened to Snapes Mum and Dad?

[Tonks]

No: HPFGUIDX 149843

I don't think we have ever discussed this before.  Snape is a young 
man, his parents should still be alive. Where are they?  I get the 
impression that they are dead and that he is living in what was once 
their house.  

What happened to them? Did LV kill them? Did Bella? Did Snape come 
home from Hogwarts to find his father abusing his mother and he 
couldn't take it any more. His father is a Muggle. Snape picked up the 
pure-blood ideology of Slytherin house. Is it possible that our dear 
beloved Snape has something in common with Tom Riddle?

Thoughts anyone??