Mosaic ring 20 with no detectable deletion by FISH analysis: Characteristic seizure disorder and literature review

Ying S. Zon, Daniel L. Van Dyke, Erik C. Thorland, Harinderpal S. Chhabra, Virginia V. Michels, Jeannette G. Keefe, Melanie A. Lega, Molly A. Feely, Timothy S. Uphoff, Syed M. Jalal

Research output: Contribution to journalReview articlepeer-review

35 Scopus citations


Ring chromosome 20 is a rare chromosome disorder characterized by a typical seizure phenotype consisting of complex partial seizures, frequent progression to generalized tonic or tonic-clonic seizures, and nocturnal frontal lobe seizures with frequent episodes of non-convulsive status epilepticus. Development may be normal or mildly delayed, followed by cognitive and behavioral decline after seizure onset. Here, we describe a patient with a typical severe seizure phenotype and a mosaic ring chromosome 20 without loss of p or q subtelomere regions or telomeric sequences. The ring had a longer telomere length than either of the telomere ends of its homologous chromosome 20 by quantitative fluorescence in situ hybridization analysis, suggesting that it might be derived from telomere-telomere fusion. The phenotypic comparison of this patient and other chromosome 20 cases with terminal deletions of 20qter (n = 1) and 20pter (n = 7), shows that the epilepsy phenotype and electroencephalographic abnormalities are characteristic in patients with ring chromosome 20. Several hypotheses have been proposed to address the elusive mechanisms underlying the seizure disorder in ring chromosome 20. These possibilities include haploinsufficiency of two epilepsy genes CHRNA4 and KCNQ2 located at 20qter, silencing of these genes by a telomere position effect, or microdeletions or rearrangements of genetic material during the ring formation.

Original languageEnglish (US)
Pages (from-to)1696-1706
Number of pages11
JournalAmerican Journal of Medical Genetics, Part A
Issue number15
StatePublished - Aug 1 2006


  • Epilepsy
  • Mosaicism
  • Ring chromosome 20
  • Subtelomere
  • Telomere

ASJC Scopus subject areas

  • Genetics
  • Genetics(clinical)


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