Abstracts

Rationale: The Ring Chromosome 20 (R(20)) syndrome is a rare genetic disorder associated with intractable seizures. The etiology of the seizures is not well understood, with several potential mechanisms including deletion of sequences involved in ring formation, instability of the ring, or silencing of genes due to altered chromatin conformation of the ring chromosome. We analyzed 6 cases of R(20) by genomic analysis and molecular cytogenetic techniques to analyze the ring chromosomes. Methods: We studied 6 patients with a R(20) by standard cytogenetics, fluorescence in situ hybridization (FISH) using chromosome 20 specific subtelomere probes, and whole genome single nucleotide polymorphism (SNP) array analysis (Illumina HumanHap550). Results: By cytogenetic analysis, four patients (1-4) were mosaic for the R(20). The percentages of cells with the ring varied with 5, 36, 42 and 50% of cells affected. In these patients, FISH using subtelomeric probes for both the short and long arms of chromosome 20 did not reveal a deletion, indicating there is very little if any genetic material lost in the formation of these rings. Genomic analysis using the SNP array platform confirmed that the ring chromosomes in these patients were not associated with any genomic imbalance on chromosome 20. The mosaic nature of these rings suggests a postzygotic origin and formation appears to be occurring by fusion of the telomeric regions with no loss of subtelomeric DNA. Two patients (5, 6) had non-mosaic rings. FISH analysis in these patients revealed that patient 5 had deletion of the 20q subtelomeric region. SNP array analysis in patient 5 demonstrated a 1.6 Mb deletion containing multiple genes, including KCNQ2 and CHRNA4, previously proposed as candidate genes for the R(20) associated seizures. Patient 6 had deletion of both 20p and 20q subtelomere sequences by FISH, and SNP array analysis revealed a 2 Mb deletion of the short arm of chromosome 20 (20p), which includes more than 15 genes, and a 704 kb deletion of the long arm (20q), which also contains more than 15 genes. The candidate genes KCNQ2 and CHRNA4 are NOT deleted in patient 6. The non-mosaic nature of these rings is consistent with a meiotic origin. Patient 6, with the non-mosaic ring and deletion of both 20p and 20q had the most severe developmental delay, as expected from the large number of genes deleted. We do not observe a difference in seizures type between these two groups, although sample size is too small to draw conclusions at this time. Conclusions: These studies indicate 1) ring chromosome 20 is heterogeneous, with some rings associated with loss of genomic material, while others are not; 2) mosaic rings were consistently not associated with deletions, while the two non-mosaic cases both had deletions, suggesting the mechanism of ring formation is different in these two classes; 3) the chromosome 20 candidate genes, KCNQ2 and CHRNA4 were only deleted in 1 of the 6 patients studied, indicating that deletion of these genes in not likely to explain the etiology of seizures in most ring chromosome 20 patients.