Multi-Dimensional Genomic Dissection of Ring Chromosome 14 Syndrome

Chromosome 14 ring syndrome [r(14)S] is a rare genetic disorder mainly characterized by complex and severe neurodevelopmental disorders, ranging from intellectual disability to aggressive/hyperactive behavior and drug-resistant epilepsy. Indeed, epilepsy is the most important clinical challenge in r(14)S, with enormous difficulty in controlling severe seizures and a strong need for innovative and effective treatments. Precision medicine for r(14)S patients would be greatly facilitated by knowledge of specific genes involved in pathogenesis. However, the pathophysiology of r(14)S is still largely unknown, and the identification of genotype/phenotype correlations is complicated by the co-occurrence of chromosome 14 rearrangements and the unknown degree of r(14) mosaicism in tissues most relevant to the disease. On the one hand, deletions of different sizes, from a few hundred Kbs to several Mbs, are often found in the terminal 14q region. These are an unlikely explanation for the severity and expressiveness of r(14) disease, since it has been observed that carriers of similar linear deletions of chromosome 14q rarely suffer from epilepsy. On the other hand, ring instability may promote increased monosomy of chromosome 14 in epilepsy-related areas of the brain (it is about 20% in peripheral blood).

Another fascinating hypothesis is that the mechanism and expressiveness of r(14) disease are driven primarily by the disruption of chromosome 14 conformation and positioning within the nucleus caused by its circularization, with dramatic effects on physiological interactions between genetic loci and, consequently, on gene regulation . This idea revives an unresolved question in cytogenetic disorders, whether the chromosomal abnormality itself produces a clinical phenotype beyond the pathogenic effects of the altered gene dosage. Rings can form from any chromosome, and most ring syndromes share largely similar clinical phenotypes. Severe epilepsy, for example, has been described in r(7), r(17), r(18), r(20), r(21) and r(22) syndromes in addition to r(14)S . This observation suggests that the presence of a loop within the nucleus may itself disrupt the balance of gene expression.