Ring chromosome 20 syndrome – the genetics of inheritance and the main symptom: seizures.
Ring autosomes are relatively rare, but ring chromosome 20 may be the most common type of all autosomal rings. Based on the analysis of the literature and their own experience, the authors showed that ring chromosome 20 may exist both in mosaic form (> 150 known cases) and in non-mosaic form (26 known cases). There are different mechanisms of formation of the ring chromosome 20: breaks on both arms with fusion of remaining parts [this variant presumes the loss of several genes], telomere fusion without a loss of genes [such a ring chromosome remains undetected upon molecular cytogenetic examination], and complex rearrangements, which include not only deletions but also duplications of some chromosomal segments. Non-mosaic rings are always sporadic, mosaic r(20) may be inherited. Mechanisms leading to inheritance of the mosaic r(20) may include loss of the ring chromosome in a subset of cells with consequent monosomy rescue by uniparental disomy (UPD) or the opening of unstable r(20) in postzygotic division. UPD was confirmed in several cases, but not always.
Children with r(20) are born usually at term, their height and weight are normal. As a rule they have no dysmorphic features. The main clinical manifestations of r(20) patients are seizures, which may develop by 5-7 years. Genetically seizures may be caused by the loss of several genes on 20q, but they may develop also in children who did not lose these genes. Gene silencing in telomere sequences may explain the occurrence of seizures in patients without an evident loss of the genetic material. The type of seizures in r(20) patients is different from seizures in other chromosomal disorders such as r(14), r(17) or del 22q13.3. Following the onset of seizures, patients develop cognitive decline and behavioral problems. Sometimes children experience hallucinations even before seizures. Several patients may develop non-convulsive status epilepticus – a state of confusion of variable duration. Some patients show good outcomes after treatment by anti-convulsive drugs, in others seizures are resistant to medications.
Peron A. et al. “Ring chromosome 20 syndrome: genetics, clinical characteristics, and overlapping phenotypes”. “Front. Neurology” 2020, v. 11:613035.
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