Table 4.

Promising Biomarkers

Domain	Potential Outcome Measure	Pros	Cons
FMR1/FMRP expression measures in blood	FMR1 methylation	Predicted response to AFQ056 in small study, stable, easy to store/ship	True predictive effect unknown, variability between assays
	FMRP levels	Key factor in severity of molecular deficit being targeted	No easy and accurate assay, no association with outcome of treatment yet, not reflect downstream events from FMRP deficit
Signaling Pathway Measures proposed as biomarkers in blood	ERK activation rate	Abnormal in FXS blood, responsive to lithium tx	Not responsive to tx in some trials, not stable, assay difficult, requires high blood volume
	APP and metabolite levels	Abnormal in FXS blood, protein regulated by FMRP, stable in serum. easy to collect, store and process samples,	Treatment response not yet shown
	PI3K activity		Treatment response not yet shown
	mTOR activity	One study showed abnormal in FXS	Treatment response not yet shown
	S6 Kinase activity	One study showed abnormal in FXS	Treatment response not yet shown
	MMP activity	Levels abnormal, normalized in minocycline trial	Variable results between laboratories for levels in individuals with FXS
Biophysical Measures	Prepulse Inihibtion (PPI)	Good reproducibility, treatment responsive in fenobam trial	Lack of demonstration of clinical validity, expensive and difficult to standardize across sites, younger patients don’t tolerate it
	Eye Tracking and Pupillometry	Easy to do, feasible for most patients, good reproducibility	Clinical validity not clear, not yet shown to be treatment responsive, expensive
	Heart Rate Variability	Feasible for most patients	Not treatment responsive in lithium study, inconsistent methods across labs, reproducibility not defined, expensive
Neuroimaging	Structural MRI DTI MRS rsfMRI fMRI PET		Some procedures may require behavioral preparation or sedation in lower functioning affected males Functional MRI is not feasible in many affected males