Ring chromosome 20.

Area of Science:

• Genetics and Molecular Biology
• Neurology
• Clinical Diagnostics

Background:

• Ring Chromosome 20 syndrome is a rare chromosomal disorder.
• It is characterized by refractory epilepsy, behavioral problems, and cognitive impairment.
• Clinical diagnosis is challenging due to infrequent facial dysmorphism or congenital malformations.

Purpose of the Study:

• To review the characteristics of Ring Chromosome 20 syndrome.
• To discuss diagnostic approaches and molecular insights.
• To highlight the importance of cytogenetic analysis for diagnosis.

Main Methods:

• Review of published cases since 1972.
• Analysis of cytogenetic data, including fluorescence in situ hybridization (FISH) and array-based comparative genomic hybridization (aCGH).
• Correlation of seizure onset with the percentage of cells exhibiting the ring chromosome in mosaic patients.

Main Results:

• Non-mosaic r(20) cases often involve deletions of 20pter and/or 20qter.
• Mosaic r(20) cases, where the ring is present in a proportion of cells, show no deletions.
• Seizure onset in mosaic patients correlates with the percentage of cells with the ring chromosome.

Conclusions:

• Cytogenetic analysis is recommended for diagnosing Ring Chromosome 20 syndrome.
• Chromosomal microarray analysis (CMA) is insufficient as mosaic cases lack copy number alterations.
• The underlying etiology remains unclear, and gene deletions on chromosome 20 are unlikely causes.