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Seven ring (X) chromosomes lacking the XIST locus, six with an unexpectedly mild phenotype
C Turner1, N R Dennis, D H Skuse
1Wessex Regional Genetics Laboratory, Salisbury District Hospital, Salisbury, Wiltshire, UK.
Human Genetics
|September 12, 2000
Summary
Ring (X) chromosomes lacking the XIST gene can cause severe phenotypes. However, most females with XIST-negative rings show milder Turner syndrome symptoms, suggesting factors beyond XIST absence influence severity.
Area of Science:
- Genetics
- Human genetics
- Molecular genetics
Background:
- Small ring X chromosomes lacking the XIST gene are linked to severe developmental issues.
- The loss of XIST is hypothesized to cause functional disomy for genes on the ring chromosome.
Purpose of the Study:
- To investigate the phenotype associated with XIST-negative ring X chromosomes.
- To understand the genetic and molecular basis of varying phenotypes in females with ring X chromosomes.
Main Methods:
- Karyotyping of 47 females with 45,X/46,r(X) karyotype.
- Cytogenetic and molecular characterization of XIST-negative ring X chromosomes.
- Phenotypic evaluation of patients with XIST-negative ring X chromosomes.
Main Results:
- Seven out of 47 patients had XIST-negative ring X chromosomes.
- Only one patient exhibited the severe phenotype; the other six showed Turner syndrome features.
- Molecular and cytogenetic analysis of the rings was performed.
Conclusions:
- The severe phenotype in one patient is likely due to XIST absence, significant Xp material, and potential maternal uniparental isodisomy.
- Mild phenotypes in other patients may result from limited ring material, tissue-specific mosaicism, or XIST expression in some tissues.