In my experience, the mmthr thing is not fully embraced, understood and recognized by geneticists. There is an integrative Dr. at GWU Center for Integrative Medicine, Dr. Kogan, who is into it. He would probably have you do 23andme.
I did have a doctor -- not in this area -- test my son for this using labcorp; it found two mutations.
From what I have seen, the way this mutation is addressed, outside of supplements, is with rx meds like deplin or leuvoricin and b12 shots.
There is a neurogentist at children's, Dr. Schreiber. I think he has a very long waiting list.