© 2002 Journal of Medical Genetics
LETTER TO JMG
Mosaicism for FMR1 and FMR2 deletion: a new case
Institute of Human Genetics, Johann Wolfgang Goethe University Hospital, Theodor-Stern-Kai 7, D-60590 Frankfurt/Main, Germany
Correspondence to:
Correspondence to:
Dr J Arnemann, Institute of Human Genetics, Johann Wolfgang Goethe University Hospital, Theodor-Stern-Kai 7, D-60590 Frankfurt/Main, Germany;
Arnemann@em.uni-frankfurt.de
Keywords: FMR1; FMR2; mosaicism; fragile X syndrome
Fragile X syndrome is the most common disorder causing mental retardation with an estimated prevalence of between 1 in 4000 to 1 in 6000 males.1, 2 In nearly all cases, the molecular basis of this X linked mental impairment is an amplification in excess of 200 copies of an unstable CGG trinucleotide repeat sequence within the 5` untranslated region of the FMR1 gene (FRAXA, MIM 309550) at Xq27.3.3 This causes an inhibition of the transcription leading to a loss of function effect in the male. In the female, the amplified gene is preferentially X inactivated by methylation.4 However, the X inactivation pattern can vary between different cell types and during development and thus, usually, causes a milder phenotype in female patients.5 In close distal proximity to the FMR1 gene, a second gene, FMR2 (FRAXE, MIM 309548) involved in mental retardation, has been described with amplification of an unstable GCC trinucleotide repeat
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