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Severe phenotypes associated with inactive ring X chromosomes.

B R Migeon¹, M Ausems, J Giltay

¹McKusick-Nathans Institute of Genetic Medicine, Johns Hopkins University, Baltimore, Maryland 21287-3914, USA. bmigeon@welch.jhu.edu

American Journal of Medical Genetics

|June 22, 2000

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Individuals with ring X chromosomes may have intellectual disability due to X chromosome disomy. This study found that even with an intact XIST locus, ring X chromosomes can be inactive, suggesting a second ring caused the severe phenotype.

Area of Science:

Genetics
Developmental Biology
Molecular Biology

Background:

Ring X chromosomes can lead to X chromosome disomy, causing developmental abnormalities.
X inactivation, regulated by the XIST gene, normally prevents disomy.
Failure of X inactivation in ring X chromosomes often results in severe phenotypes.

Observation:

Two girls with 45,X/46,X,r(X) karyotype exhibited severe phenotypes, including intellectual disability and congenital malformations.
Their ring X chromosomes possessed an intact and expressed XIST locus.
Androgen receptor gene analysis confirmed the transcriptional inactivation of these ring X chromosomes.

Findings:

Ring X chromosomes with intact, expressed XIST loci can still be transcriptionally inactive.

Related Experiment Videos

The breakpoints of the analyzed ring X chromosomes were outside the critical X inactivation center (XIC).

A subset of cells contained a second ring X chromosome.

Implications:

The severe phenotype associated with an apparently inactive ring X chromosome may be due to the transient activity of a second ring X chromosome during early development.
This suggests complex mechanisms regulating X inactivation in ring X chromosome syndromes.
Further research is needed to understand the role of mosaicism and secondary ring structures in developmental outcomes.