Multimodal assessment of the GABA system in patients with fragile-X syndrome and neurofibromatosis of type 1.

Fragile-X syndrome (FXS) and Neurofibromatosis of type 1 (NF-1) are two monogenic disorders sharing neurobehavioral symptoms and pathophysiological mechanisms. Namely, preclinical models of both conditions show overactivity of the mTOR signaling pathway as well as GABAergic alterations. However, despite its potential clinical relevance for these disorders, the GABAergic system has not been systematically studied in humans. In the present study, we used an extensive transcranial magnetic stimulation (TMS) assessment battery in combination with magnetic resonance spectroscopy (MRS) to provide a comprehensive picture of the main inhibitory neurotransmitter system in patients with FXS and NF1. Forty-three participants took part in the TMS session (15 FXS, 10 NF1, 18 controls) and 36 in the MRS session (11 FXS, 14 NF1, 11 controls). Results show that, in comparison to healthy control participants, individuals with FXS and NF1 display lower GABA concentration levels as measured with MRS. TMS result show that FXS patients present increased GABAB-mediated inhibition compared to controls and NF1 patients, and that GABAA-mediated intracortical inhibition was associated with increased excitability specifically in the FXS groups. In line with previous reports, correlational analyses between MRS and TMS measures did not show significant relationships between GABA-related metrics, but several TMS measures correlated with glutamate+glutamine (Glx) levels assessed with MRS. Overall, these results suggest a partial overlap in neurophysiological alterations involving the GABA system in NF1 and FXS, and support the hypothesis that MRS and TMS assess different aspects of the neurotransmitter systems.