Fragile X (FRAX) syndrome is a commonly inherited form of mental retardation resulting from the lack of expression of the fragile X mental retardation protein (FMRP). It is caused by a stretch of CGG repeats within the fragile X gene, which can be unstable in length as it is transmitted from generation to generation. Once the repeat exceeds a threshold length, the FMR1 gene is methylated and no protein is produced resulting in the fragile X phenotype. The consequences of FMRP absence in the mechanisms underlying mental retardation are unknown. We have identified a male patient in a classical FRAX family without the characteristic FRAX phenotype. His intelligence quotient (IQ) is borderline normal despite the presence of a mosaic pattern of a pre-mutation (25%), full mutation (60%) and a deletion (15%) in the FMR1 gene. The cognitive performance was determined at the age of 28 by the Raven test and his IQ was 81. However, FMRP expression studies in both hair roots and lymphocytes, determined at the same time as the IQ test, were within the affected male range. The percentage of conditioned responses after delay eyeblink conditioning was much higher than the average percentage measured in FRAX studies. Moreover, this patient showed no correlation between FMRP expression and phenotype and no correlation between DNA diagnostics and phenotype.

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Keywords Delay eyeblink conditioning, Exceptional phenotype, Fragile X syndrome
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Journal Clinical Genetics: an international journal of genetics and molecular medicine
Govaerts, L.C, Smit, A.E, Saris, J.J, VanderWerf, F, Willemsen, R, Bakker, C.E, … Oostra, B.A. (2007). Exceptional good cognitive and phenotypic profile in a male carrying a mosaic mutation in the FMR1 gene. Clinical Genetics: an international journal of genetics and molecular medicine, 72(2), 138–144. doi:10.1111/j.1399-0004.2007.00829.x