Fragile X syndrome in a male with methylated premutation alleles and no detectable methylated full mutation alleles.

Am J Med Genet A

Section on Neuroadaptation and Protein Metabolism, National Institute of Mental Health, National Institutes of Health, Bethesda, Maryland.

Published: October 2019

Most cases of fragile X syndrome (FXS) result from aberrant methylation of the FMR1 gene. Methylation occurs when the number of tandemly arranged cytosine guanine guanine (CGG)-repeats in the 5' end of the transcriptional unit of FMR1 exceeds a certain critical threshold, thought to be between 200 and 400 repeats. Such alleles are referred to as full mutation (FM) alleles. Premutation (PM) alleles, alleles with 55-200 repeats, are generally not aberrantly methylated and in fact may have hyperexpression of the FMR1 mRNA. We describe here a male who meets the diagnostic criteria for FXS, who is highly mosaic with a mixture of multiple PM and FM alleles and 50% methylation. However, the methylated alleles are limited to two alleles in the PM range, ~165 and ~175 repeats respectively, with the FM alleles being unmethylated. This finding has implications for FXS diagnosis as well as for efforts to delete the repeat in individuals with FXS using a CRISPR-Cas9 approach.

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Source
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC9423038PMC
http://dx.doi.org/10.1002/ajmg.a.61286DOI Listing

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