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This is a landmark finding. Even though it is only one family, it's a large pedigree and it seems to be a bonafide genetic mutation linked with Tourettes. Some of the family members have OCD.
I (and most doctors) think of Tourette/OCD as a spectrum.
The mutated gene, histidine decarboxylase (HDC) is an very interesting protein. First, it makes histamine in the brain and body (from another amino acid called histidine) and needs Vitamin B6 to do so,
Obviously, histamine is a key mediator in the immune system as well (that's what Benadryl and Claritin, Allegra all block) . These are germ line mutations so the mutations would apply to all cells in the body. So another message of this paper is that there are immune problems in Tourette syndrome! The mast cell outside the body must also be deficient in making histamine in these patients. This is a major problem in the innate immune system - which is what I am thinking is behind PANDAS/PITANDS.
This might also explain the finding of improvement of tics with Vitamin B6, and perhaps the modulation of tics with allergies.
Clearly, this finding would add biological plausibility to an infectious/immune cause of Tourettes - i.e. PANDAS.