The missing indexes DMVs are reset when the server starts. They can't be modified by a user.

One thing I would like to add is that the missing indexes DMVs should (like the Database Tuning Advisor) be taken as a suggestion and tested carefully before been applied. The missing indexes are less accurate than the DTA, as only single queries are considered when entries are added to that (the query optimiser adds the entries as it's optimising queries).

It doesn't take into account existing indexes, it doesn't compare with other similar suggestions already in the DMV and it doesn't considered clustered/nonclustered indexes.

They are very, very useful for index tuning, they just shouldn't be applies without consideration.