I don't know anything about this one particularly, but I do know that all mutations that have the X at the end are considered stop mutations. They are class I mutations that keeps the protein from being produced because the mutation can't read through.
The combination of a class I (Q493X) and a class II (DF508) would be a classic CF.
The good news that you know this is that in addition to the Vertex drugs that seek to fix the DF508 and are very promising right now, the other one is the target of the drug atalauren (formerly PTC124) and seeks to fix the X stop mutations.
So there is lots of hope to be had here.