What is the major cause of retinitis pigmentosa?

It depends..: RP is a cause blindness. Its causes are multifactorial and exact cause depends on lots of other factors--like whether it occurs alone, or if it's associated with other symptoms or syndromes, or if it was secondary to a systemic disease. It's almost always genetic (there are >50 genes associated with RP), and it's often part of a syndrome, like Usher, Biedl-Bardet, Kearns-Sayer, McLeod, and others.