Clinical, cytogenetic and molecular study in a case of r(3) with 3p deletion and review of the literature

Ring chromosomes: from formation to clinical potential

Ring chromosomes (RCs) are circular DNA molecules, which occur rarely in eukaryotic nuclear genomes. Lilian Vaughan Morgan first described them in the fruit fly. Human embryos very seldom have RCs, about 1:50,000. Carriers of RCs may have varying degrees of symptoms, from healthy phenotype to serious pathologies in physical and intellectual development. Many authors describe common symptoms of RC presence: short stature and some developmental delay that could be described as a "ring chromosome syndrome." As a rule, RCs arise de novo through the end-joining of two DNA double-strand breaks, telomere-subtelomere junction, or inv dup del rearrangement in both meiosis and mitosis. There are family cases of RC inheritance. The presence of RCs causes numerous secondary chromosome rearrangements in vivo and in vitro. RCs can change their size, become lost, or increase their copy number and cause additional deletions, duplication, and translocations, affecting both RCs and other chromosomes. In this review, we examine RC inheritance, instability, mechanisms of formation, and potential clinical applications of artificially created RCs for large-scale chromosome rearrangement treatment.

Chromosome r(3)(p25.3q29) in a Patient with Developmental Delay and Congenital Heart Defects: A Case Report and a Brief Literature Review

Ring chromosome 3, r(3), is an extremely rare cytogenetic abnormality with clinical heterogeneity and only 12 cases reported in the literature. Here, we report a 1-year-old girl presenting distinctive facial features, developmental delay, and congenital heart defects with r(3) and a ∼10-Mb deletion of chromosome 3pterp25.3 (61,891-9,979,408) involving 42 known genes which was detected using G-banding karyotyping and CytoScan 750K-Array. The breakpoints in r(3) were mapped at 3p25.3 and 3q29. We also analyzed the available information on the clinical features of the reported cases with r(3) and 3p deletion syndrome in order to provide more valuable information of genotype-phenotype correlations. To our knowledge, this is the largest detected fragment described in r(3) cases and the second r(3) study using whole-genome microarray.

Human ring chromosomes and small supernumerary marker chromosomes-do they have telomeres?

Ring chromosomes and small supernumerary marker chromosomes (sSMC) are enigmatic types of derivative chromosomes, in which the telomeres are thought to play a crucial role in their formation and stabilization. Considering that there are only a few studies that evaluate the presence of telomeric sequences in ring chromosomes and on sSMC, here, we analyzed 14 ring chromosomes and 29 sSMC for the presence of telomeric sequences through fluorescence in situ hybridization (FISH). The results showed that ring chromosomes can actually fall into two groups: the ones with or without telomeres. Additionally, telomeric signals were detectable at both ends of centric and neocentric sSMC with inverted duplication shape, as well as in complex sSMC. Apart from that, generally both ring- and centric minute-shaped sSMC did not present telomeric sequences neither detectable by FISH nor by a second protein-directed immunohistochemical approach. However, the fact that telomeres are absent does not automatically mean that the sSMC has a ring shape, as often deduced in the previous literature. Overall, the results obtained by FISH studies directed against telomeres need to be checked carefully by other approaches.

Bilateral anterior segment dysgenesis in an infant with partial trisomy 16q and partial monosomy 3p

Anterior segment dysgenesis comprises a spectrum of malformations arising from faulty neural crest cell migration. We report a newborn boy with partial trisomy 16q and partial monosomy 3p who presented with anterior segment dysgenesis with iris hypoplasia on the right and glaucoma on the left in association with systemic anomalies. The anterior segment dysgenesis features observed in this case have not been previously associated with partial trisomy 16q or partial monosomy 3p. Our findings support the hypothesis that an additional anterior segment dysgenesis gene may reside on chromosome 3p or 16q.

Mechanisms of ring chromosome formation, ring instability and clinical consequences

Background

The breakpoints and mechanisms of ring chromosome formation were studied and mapped in 14 patients.

Methods

Several techniques were performed such as genome-wide array, MLPA (Multiplex Ligation-Dependent Probe Amplification) and FISH (Fluorescent in situ Hybridization).

Results

The ring chromosomes of patients I to XIV were determined to be, respectively: r(3)(p26.1q29), r(4)(p16.3q35.2), r(10)(p15.3q26.2), r(10)(p15.3q26.13), r(13)(p13q31.1), r(13)(p13q34), r(14)(p13q32.33), r(15)(p13q26.2), r(18)(p11.32q22.2), r(18)(p11.32q21.33), r(18)(p11.21q23), r(22)(p13q13.33), r(22)(p13q13.2), and r(22)(p13q13.2). These rings were found to have been formed by different mechanisms, such as: breaks in both chromosome arms followed by end-to-end reunion (patients IV, VIII, IX, XI, XIII and XIV); a break in one chromosome arm followed by fusion with the subtelomeric region of the other (patients I and II); a break in one chromosome arm followed by fusion with the opposite telomeric region (patients III and X); fusion of two subtelomeric regions (patient VII); and telomere-telomere fusion (patient XII). Thus, the r(14) and one r(22) can be considered complete rings, since there was no loss of relevant genetic material. Two patients (V and VI) with r(13) showed duplication along with terminal deletion of 13q, one of them proved to be inverted, a mechanism known as inv-dup-del. Ring instability was detected by ring loss and secondary aberrations in all but three patients, who presented stable ring chromosomes (II, XIII and XIV).

Conclusions

We concluded that the clinical phenotype of patients with ring chromosomes may be related with different factors, including gene haploinsufficiency, gene duplications and ring instability. Epigenetic factors due to the circular architecture of ring chromosomes must also be considered, since even complete ring chromosomes can result in phenotypic alterations, as observed in our patients with complete r(14) and r(22).