GraphX works by loading an entire graph into a combination of VertexRDDs and EdgeRDDs, so the underlying database's capabilities are not really relevant to the graph computation, since GraphX won't touch it beyond initial load.
On that basis you can really use any thing that will effectively store and scan a list of paired tuples, and a list of ids and other properties. From this perspective HBase or Accumulo would seem like a good bet to attach Spark to, but of course any file in HDFS would do.
For the ability to modify a graph prior to analysing it in GraphX, it's more useful to pick a 'proper' graph database. For this it's worth looking at something like Accumulo Graph which provides a graph database hosted on Accumulo, or possibly another very exciting new project, Gaffer (https://github.com/GovernmentCommunicationsHeadquarters/Gaffer) also hosted on Accumulo. Tinkerpop (https://tinkerpop.apache.org/) provides some other options focussed around the common Gremlin APIs, which are generally well understood in graph world. Other options might include something like TitanDB hosted on HBase. These will provide you with an interface API for modifying graphs effectively.
