Cerebral 3-[18F]fluoro-5-(2-pyridinylethynyl)benzonitrile ([18F]FPEB) uptake and Fragile X Mental Retardation Protein in men with fragile X syndrome

Introduction: Although glutamatergic agents have ameliorated the behavioral phenotype in animal models of fragile X syndrome (FXS), the most common single-gene cause of intellectual disability (ID) and autism spectrum disorder(ASD), clinical trials of glutamatergic agents have not produced similar beneficial effects in humans with FXS. Marked reductions in the uptake of (2-pyridinylethynyl)benzonitrile ([¹⁸F]FPEB), a potent inhibitor of metabotropic glutamate receptor subtype 5 (mGluR₅), throughout cortical and subcortical regions of men with FXS provide a clue to develop effective interventions. We sought to determine if [¹⁸F]FPEB uptake was associated with the levels ofFragile X Mental Retardation (FMRP), a key endogenous component of normal development, in men with FXS.

Methods: Six men with the full mutation of FXS based on FMR1 DNA gene testing by polymerase chain reaction (PCR)/Southern Blot on peripheral venous blood samples confirmed with clinical neurobehavioral assessments (N = 6, mean age 27.8 ± 4.7, range (22.3, 33.6) years) underwent (A) FMRP measurement by enzyme-linked immunoassay (ELISA) with Luminex technology and (B) mock scanner training followed by positron emission tomography (PET) with head stabilization by a gauze strip taped across the forehaed and a rounded head holder after an intravenous bolus injection of 185 MBqs (5 mCis) of [¹⁸F]FPEB at 1 PM followed by scans on an ECAT EXACT HR+ PET manufactured by Siemens/CTI (Knoxville, TN) attaining an axial resolution of approaching y = 4–5 mm, with consecutive 6 × 300 s frames performed for 90 to 120 min after the injection time. Time activity (radioactivity) curves (TACs) by Statistical Parametric Mapping (SPM) provided the data for standard uptake values (SUVs) of ratios of radiotracer uptake in the region of interest to the cerebellum. FMRP in nanogram per microgram total protein and ratios of ([¹⁸F]FPEB) uptake in the regions of interest (ROIs) to uptake in the cerebellum underwent descriptive analysis and Pearson correlation coefficient R.

Results: Representation of the values of FMRP values for cohorts of numbers (N) of 1 or 2 and the corresponding measurements of [¹⁸F]FPEB) uptake in the anterior cingulate cortex (ACC), posterior cingulate cortex (PCC), parietal cortex (PC), temporal cortex (TC), parietal cortex (PC), striatum (S), and thalamus (T) (figure) suggest a possible direct relationship between FMRP and [¹⁸F]FPEB) uptake (Weissgerber, T.L.; Savic, M.; Winham, S.J.; Stanisavljevic, D.; Garovic, V.D.; Milic, N.M. Data visualization, bar naked: A free tool for creating interactive graphics. J. Biol. Chem. 2017, 292, 20592–20598) that was not confirmed by Pearson correlation coefficient R (Stangroom, J. Social Sciences Statstics. 2021. Available online: https://www.socscistatistics.com/tests/pearson/default2.aspx).

Visual inspection of the tabulation of FMRP and [¹⁸F]FPEB) uptake suggests the presence of a direct relationship particularly in the ACC.

Conclusions: A pilot investigation of FMRP levels and [¹⁸F]FPEB) uptake in selected cerebral regions of men with the full mutation of FXS suggests a possible direct association in the ACC and other regions that is not confirmed with statistical analysis. Training of participants with FXS by behavioral psychologists to facilitate completion of mock scanner sessions enhances the conduct of scans in a pleasant evironment without stress for the participants, parents, providers, and technologists. We anticipate that futures studies with adequate sample sizes will generate statistical significance. The proposed protocol may then provide the foundations for measurements of key variables for meaningful clinical trials of glutamatergic agents for FXS and other conditions. The proposed portocol may provide the means to conduct rigorous clinical trials of glutamatergic agents for FXS generating data to facilitiate the application of the principles of precision medicine to tailor interventions to the specific needs of each individual with FXS.

• Download figure
• Open in new tab
• Download powerpoint