What other approaches are there for abstractive summarization, other then seq2seq?

I'm researching on abstractive text summarization, and has come across many recent papers. They all seem to be focusing on Sequence to Sequence models based on RNNs. Apart from RNNs, what other approaches are there when it comes to abstractive text summarization? Does ontology-based summarization revolve around the same seq2seq model?

Most of the material I have come across are research papers on this subject; what better sources are there to get an understanding of the underlining concepts of abstractive summarization?