Transcription of the FMR1 gene in individuals with fragile X syndrome

Fragile X syndrome generally arises as a consequence of a large expansion o f a CGG trinucleotide repeat element that is located in the GC-rich promote r region of the fragile X mental retardation gene (FMR1). In the convention al model for fragile X, clinical involvement arises as a consequence of sil encing of the FMR1 gene, with the attendant loss of FMR1 protein (FMRP). Ho wever, it has recently been demonstrated that most males with large premuta tion alleles (100-200 repeats), or with unmethylated full mutation alleles, have FMR1 mRNA levels that are higher than normal, despite reduced levels of FMRP. In the current work, we extend and confirm these observations usin g quantitative (fluorescent) reverse transcription polymerase chain reactio n on larger sample populations, establishing that even for smaller premutat ion alleles (55-100 repeats) the mRNA levels are significantly elevated (me an 2.1-fold elevation; P = 3.9 x 10(-3)), relative to normal controls. Thus , an abnormal molecular phenotype is established close to the upper end of the normal range. We also demonstrate that the levels of FMR1 mRNA are elev ated in females with premutation alleles; however, the mRNA levels are more varied than in the males, and are attenuated in a manner that is consisten t with the fraction of normal alleles that are active in any given individu al. Finally, we demonstrate that in lymphoblastoid cells derived from a pat ient with a severe form of fragile X caused by a point mutation in the seco nd KH domain of the gene, but with a normal CGG element (25 repeats), the F MR1 mRNA level is normal. Thus, although models in which FMRP level (or lev el of function) modulates transcriptional activity remain viable, other exp lanations for the elevated message levels, including direct (cis) effects o f the CGG element on transcription, must also be considered. Am. J. Med. Ge net. (Semin. Med. Genet.) 97:195-203, 2000. (C) 2000 Wiley-Liss, Inc.