Loss of FMR1 hypermethylation in somatic cell heterokaryons.

Fragile X syndrome is associated with a trinucleotide (CGG) repeat expansion in the 5'-untranslated region of the FMR1 gene and hypermethylation of the FMR1 promoter. Rare cases of clinically normal males (HFM) have been identified with an expanded CGG repeat; however, here, the FMR1 promoter is not methylated. Using classical complementation (cell fusion) studies, we analyzed if possible differences in the genetic background between HFM and cells from individuals with fragile X syndrome (FX cells) could have an influence on the methylation status of the FMR1 promoter. We observed that demethylation of the hypermethylated FMR1 promoter can occur when FX cells are complemented (by cell fusion) with cells from HFM as well as with cells from control individuals. The observed demethylation is specific and can happen without DNA replication. In contrast, demethylation was not observed when cells from unrelated individuals with fragile X syndrome were fused, indicating that FX cells have lost the necessary factor(s) to demethylate the aberrantly methylated FMR1 promoter.