An assay for X inactivation based on differential methylation at the fragile X locus, FMR1.


Journal Article

We describe an assay analyzing methylation at the fragile X mental retardation gene, FMR1, to examine patterns of random or non-random X chromosome inactivation. Digestion of genomic DNA with the methylation-sensitive enzyme HpaII cleaves two restriction sites near the CGG repeat of the FMR1 gene if they are unmethylated on the active X chromosome, but fails to digest these sites on the inactive chromosome. Subsequent PCR using primers that flank the sites and the variable CGG repeat within the FMR1 gene amplifies alleles only on undigested, methylated inactive X chromosomes. Amplification of the hypervariable CGG repeat distinguishes alleles in heterozygous samples, while the relative ratio of alleles within a HpaII-digested sample reflects the randomness or non-randomness of inactivation. To demonstrate that methylation of the HpaII sites within the amplified FMR1 fragment correlates strictly with the activity state of the X chromosome, we have tested the validity of this assay by comparing DNA from normal males and females, as well as DNA from mouse/human somatic cell hybrids carrying either active or inactive human X chromosomes. The data demonstrate that this assay provides a reliable means of assessing the inactivation status of X chromosomes in individuals with X-linked disorders or X chromosome abnormalities.

Full Text

Cited Authors

  • Carrel, L; Willard, HF

Published Date

  • July 1996

Published In

Volume / Issue

  • 64 / 1

Start / End Page

  • 27 - 30

PubMed ID

  • 8826444

Pubmed Central ID

  • 8826444

Electronic International Standard Serial Number (EISSN)

  • 1096-8628

International Standard Serial Number (ISSN)

  • 0148-7299

Digital Object Identifier (DOI)

  • 10.1002/(sici)1096-8628(19960712)64:1<27::aid-ajmg3>;2-o


  • eng