Abstract
Although the majority of fragile-X patients demonstrate methylation and a much-expanded CGG repeat region in the 5′-untranslated region of exon 1 of the FMR1 gene, exceptional cases have been reported to be due to deletions. However, fine mapping of the deletion breakpoints is still lacking and so far the underlying mechanism is unknown. We identified a fragile-X patient mosaic for a full mutation and a microdeletion. The microdeletion spans 486 bp, involving 168 bp upstream from the CGG repeat region, the entire CGG repeat region, exon 1, and 138 bp of the first intron of the FMR1 gene. In contrast to previous reports, the 5′ breakpoint does not fall into the hotspot region. The proximal breakpoint, 5′-GTGGTT/T-3′, and the distal breakpoint, 5′-GTTGTT/GG-3′, can be characterized as chi-like elements and are flanked by direct tandem repeats. Mosaicism of a full mutation and the microdeletion in the DNA of the patient’s leukocytes indicates the mitotic origin of the microdeletion. Since the microdeletion allele is unmethylated, it can be concluded that it is not derived from the methylated full mutation but from an unmethylated premutational allele.
Similar content being viewed by others
Author information
Authors and Affiliations
Additional information
Received: 18 March 1996 / Revised: 15 May 1996
Rights and permissions
About this article
Cite this article
Schmucker, B., Ballhausen, W. & Pfeiffer, R. Mosaicism of a microdeletion of 486 bp involving the CGG repeat of the FMR1 gene due to misalignment of GTT tandem repeats at chi-like elements flanking both breakpoints and a full mutation. Hum Genet 98, 409–414 (1996). https://doi.org/10.1007/s004390050230
Issue Date:
DOI: https://doi.org/10.1007/s004390050230