Quantitative measurement of FMRP in blood platelets as a new screening test for fragile X syndrome.

The fragile X syndrome usually results from CGG repeats expansion and methylation of the FMR1 gene leading to the absence of expression of its encoded protein, fragile X mental retardation protein (FMRP). Therefore, its diagnosis is traditionally based on the detection of these molecular alterations. As an alternative, FMRP-based screening methods have been proposed over the years. Most of them are based on immunohistochemistry analyses applied to a restricted number of lymphocytes (100) or hair roots (10-20) with limited diagnosis potential. In this study, we describe a truly quantitative approach using a new model, the blood platelet, which can be recovered easily with very high purity (99.9%). FMRP levels in platelets were first measured in a control population (n = 124) and reference values were established. FMRP measurements were also performed in confirmed fragile X subjects. Receiver operating characteristic curve analysis has shown that our test can easily discriminate fragile X males and females from controls (area under curve, AUC = 0.948). Cognitive functions were also assessed in these individuals using age-specific Wechsler Intelligence Scales for Children and the Vineland Adaptive Behavior Scales. A proportional relationship between FMRP levels, intelligence quotient and adaptive behavior was observed among fragile X individuals, suggesting that our test would be able to detect fragile X cases and may predict cognitive functions.