Position effect modifying gene expression in a patient with ring chromosome 14.

The clinical phenotype of patients with ring chromosomes usually reflects the loss of genomic material during ring formation. However, phenotypic alterations can also be found in the presence of complete ring chromosomes, in which the breakage and rejoining in terminal regions of both chromosome arms result in no gene loss. Here, we present a patient with a ring chromosome 14 that lost nothing but the telomeres. Since he and other patients with a similar chromosome abnormality present certain abnormal characteristics, we investigated the gene expression of eight chromosome 14 genes to find out whether the configuration of the ring had changed it, possibly producing some of these clinical features. The expression of these eight genes was studied by quantitative real-time polymerase chain reaction (qPCR) in the patient and in seven controls matched for gender and age. Two of them were found to be downregulated in the patient compared to the controls, indicating that his phenotype might be related to alterations in the expression of genes located in the abnormal chromosome, even when the copy number is normal. Thus, the phenotypic alterations found in the presence of complete ring chromosomes may be related to changes in the chromatin architecture, bringing about a change of expression by position effect. These results may explain some of the characteristics presented by our patient.

X-linked intellectual disability related genes disrupted by balanced X-autosome translocations.

Detailed molecular characterization of chromosomal rearrangements involving X-chromosome has been a key strategy in identifying X-linked intellectual disability-causing genes. We fine-mapped the breakpoints in four women with balanced X-autosome translocations and variable phenotypes, in order to investigate the corresponding genetic contribution to intellectual disability. We addressed the impact of the gene interruptions in transcription and discussed the consequences of their functional impairment in neurodevelopment. Three patients presented with cognitive impairment, reinforcing the association between the disrupted genes (TSPAN7-MRX58, KIAA2022-MRX98, and IL1RAPL1-MRX21/34) and intellectual disability. While gene expression analysis showed absence of TSPAN7 and KIAA2022 expression in the patients, the unexpected expression of IL1RAPL1 suggested a fusion transcript ZNF611-IL1RAPL1 under the control of the ZNF611 promoter, gene disrupted at the autosomal breakpoint. The X-chromosomal breakpoint definition in the fourth patient, a woman with normal intellectual abilities, revealed disruption of the ZDHHC15 gene (MRX91). The expression assays did not detect ZDHHC15 gene expression in the patient, thus questioning its involvement in intellectual disability. Revealing the disruption of an X-linked intellectual disability-related gene in patients with balanced X-autosome translocation is a useful tool for a better characterization of critical genes in neurodevelopment. © 2015 Wiley Periodicals, Inc.

Terminal 18q deletions are stabilized by neotelomeres.

BACKGROUND: All human chromosomes are capped by tandem repeat (TTAGGG)n sequences that protect them against end-to-end fusion and are essential to chromosomal replication and integrity. Therefore, after a chromosomal breakage, the deleted chromosomes must be stabilized by retaining the telomere or acquiring a new cap, by telomere healing or telomere capture. There are few reports with molecular approaches on the mechanisms involved in stabilization of 18q terminal deletions. RESULTS: In this study we analyzed nine patients with 18q terminal deletion identified by G-banding and genomic array. FISH using PNA probe revealed telomeric signals in all deleted chromosomes tested. We fine-mapped breakpoints with customized arrays and sequenced six terminal deletion junctions. In all six deleted chromosomes sequenced, telomeric sequences were found directly attached to the breakpoints. Little or no microhomology was found at the breakpoints and none of the breaks sequenced were located in low copy repeat (LCR) regions, though repetitive elements were found around the breakpoints in five patients. One patient presented a more complex rearrangement with two deleted segments and an addition of 17 base pairs (bp). CONCLUSIONS: We found that all six deleted chromosomes sequenced were probably stabilized by the healing mechanism leading to a neotelomere formation.

Duplication 9p and their implication to phenotype.

BACKGROUND: Trisomy 9p is one of the most common partial trisomies found in newborns. We report the clinical features and cytogenomic findings in five patients with different chromosome rearrangements resulting in complete 9p duplication, three of them involving 9p centromere alterations. METHODS: The rearrangements in the patients were characterized by G-banding, SNP-array and fluorescent in situ hybridization (FISH) with different probes. RESULTS: Two patients presented de novo dicentric chromosomes: der(9;15)t(9;15)(p11.2;p13) and der(9;21)t(9;21)(p13.1;p13.1). One patient presented two concomitant rearranged chromosomes: a der(12)t(9;12)(q21.13;p13.33) and an psu i(9)(p10) which showed FISH centromeric signal smaller than in the normal chromosome 9. Besides the duplication 9p24.3p13.1, array revealed a 7.3 Mb deletion in 9q13q21.13 in this patient. The break in the psu i(9)(p10) probably occurred in the centromere resulting in a smaller centromere and with part of the 9q translocated to the distal 12p with the deletion 9q occurring during this rearrangement. Two patients, brother and sister, present 9p duplication concomitant to 18p deletion due to an inherited der(18)t(9;18)(p11.2;p11.31)mat. CONCLUSIONS: The patients with trisomy 9p present a well-recognizable phenotype due to facial appearance, although the genotype-phenotype correlation can be difficult due to concomitant partial monosomy of other chromosomes. The chromosome 9 is rich in segmental duplication, especially in pericentromeric region, with high degree of sequence identity to sequences in 15p, 18p and 21p, chromosomes involved in our rearrangements. Thus, we suggest that chromosome 9 is prone to illegitimate recombination, either intrachromosomal or interchromosomal, which predisposes it to rearrangements, frequently involving pericentromeric regions.

Cytogenomic delineation and clinical follow-up of two siblings with an 8.5 Mb 6q24.2-q25.2 deletion inherited from a paternal insertion.

The chromosomal segment 6q24-q25 comprises a contiguous gene microdeletion syndrome characterized by intrauterine growth retardation, growth delay, intellectual disability, cardiac anomalies, and a dysmorphic facial phenotype. We describe here a 10-year follow-up with detailed clinical, neuropsychological, and cytomolecular data of two siblings, male and female, who presented with developmental delay, microcephaly, short stature, characteristic facial dysmorphisms, multiple organ anomalies, and intellectual disability. Microarray analysis showed an 8.5 Mb 6q24.2-q25.2 interstitial deletion. Fluorescence in situ hybridization analyses confirmed the deletions and identified an insertion of 6q into 8q13 in their father, resulting in a high recurrence risk. This is the first report in sibs with distinct neuropsychological involvement, one of them with stenosis of the descending branch of the aorta.

Clinical, cytogenetic, and molecular characterization of six patients with ring chromosomes 22, including one with concomitant 22q11.2 deletion.

We report here on six patients with a ring chromosome 22 and the range of cytogenetic and phenotypic features presented by them. Genomic analysis was carried out using classical and molecular cytogenetics, MLPA (Multiplex Ligation-dependent Probe Amplification) and genome-wide SNP-array analysis. The ring was found in all patients, but Patient 6 displayed constitutional mosaicism with a normal cell line. Five patients had deletions in the ring chromosome 22, and in four of them the breakpoints--unique for each patient--could be identified by genome-wide SNP-array analysis. One patient presented with a 22q11.2 deletion concomitant with the deletion caused by the ring formation. Common phenotypic features included autism, speech delay and seizures, as previously reported for individuals with r(22) and/or 22q13.3 deletions. Investigation of the genes within the deletions revealed multiple genes related to development of the central nervous system, psychomotor delay, severe language impairment, hypotonia, and autistic symptoms. There was no clear correlation between the severity of clinical features and the size of the deleted segment. This study underscores the variability in ring structure and clinical presentation of the r(22) and adds information to the limited literature on this rare disorder.

Ring chromosome 10: report on two patients and review of the literature.

Ring chromosome 10--r(10)--is a rare disorder, with 14 cases reported in the literature, but only two with breakpoint determination by high-resolution techniques. We report here on two patients presenting a ring chromosome 10, studied by G-banding, fluorescent in situ hybridization (FISH), multiplex ligation-dependent probe amplification (MLPA) and SNP-array techniques, in order to investigate ring instability and determine breakpoints. Patient 1 showed a r(10)(p15.3q26.2) with a 7.9 Mb deletion in 10q26.2-q26.2, while patient 2 showed a r(10)(p15.3q26.13) with a 1.0 Mb deletion in 10p15.3 and a 8.8 Mb deletion in 10q26.13-q26.3, both unstable. While patient 1 presented with clinical features usually found in patients with r(10) and terminal 10q deletion, patient 2 presented characteristics so far not described in other patients with r(10), such as Dandy-Walker variant, osteopenia, semi-flexed legs, and dermal pigmentation regions. Our data and the data from literature show that there are no specific clinical findings to define a r(10) syndrome.

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.

How to narrow down chromosomal breakpoints in small and large derivative chromosomes--a new probe set.

Here a new fluorescence in situ hybridization (FISH-) based probe set is presented and its possible applications are highlighted in 34 exemplary clinical cases. The so-called pericentric-ladder-FISH (PCL-FISH) probe set enables a characterization of chromosomal breakpoints especially in small supernumerary marker chromosomes (sSMC), but can also be applied successfully in large inborn or acquired derivative chromosomes. PCL-FISH was established as 24 different chromosome-specific probe sets and can be used in two- up multicolor-FISH approaches. PCL-FISH enables the determination of a chromosomal breakpoint with a resolution between 1 and ∼10 megabasepairs and is based on locus-specific bacterial artificial chromosome (BAC) probes. Results obtained on 29 sSMC cases and five larger derivative chromosomes are presented and discussed. To confirm the reliability of PCL-FISH, eight of the 29 sSMC cases were studied by array-comparative genomic hybridization (aCGH); the used sSMC-specific DNA was obtained by glass-needle based microdissection and DOP-PCR-amplification. Overall, PCL-FISH leads to a better resolution than most FISH-banding approaches and is a good tool to narrow down chromosomal breakpoints.

Centromeric association of small supernumerary marker chromosomes with their sister-chromosomes detected by three dimensional molecular cytogenetics.

BACKGROUND: Small supernumerary marker chromosomes (sSMC) are detected in 0.043% of general population and can be characterized for their chromosomal origin, genetic content and shape by molecular cytogenetic approaches. Even though recently progress was achieved towards genotype-phenotype-correlations of sSMC, nothing is known on the influence that an additional derivative extra chromosome has on the nuclear architecture. RESULTS: Here we present the first three-dimensional interphase fluorescence in situ hybridization (FISH) studies for the nuclear architecture of sSMC. It could be shown that sSMC derived from chromosomes 15, 16 or 18 preferentially colocalized with one of their corresponding sister chromosomes. This was true in B- and T-lymphocytes as well as in skin fibroblasts. Additionally, a case with a complex sSMC with a karyotype 47,XY,+der(18)t(8;18)(8p23.2 ~ 23.1;18q11.1) was studied. Here the sSMC co-localized with one homologous chromosome 8 instead of 18. CONCLUSION: Overall, there is a kind of "attraction" between an sSMC and one of its homologous sister chromosomes. This seems to be transmitted by the euchromatic part of the sSMC rather than its heterochromatic one.

Cytogenetic and molecular evaluation and 20-year follow-up of a patient with ring chromosome 14.

We present a 20-year follow-up on a patient with a ring chromosome 14. The ring chromosome was studied by fluorescence in-situ hybridization (FISH), multiplex-ligation probe amplification (MLPA), and genome wide SNP array, and no deletions of chromosome 14 were detected, although the telomeric repeat sequence was absent from the ring chromosome. The patient had skeletal abnormalities, and susceptibility to infections, as well as seizures and retinal pigmentation, which are commonly found in individuals with a ring 14. Our patient corroborates the idea that even when no genes are lost during ring formation, a complete ring chromosome can produce phenotypic alterations, which presumably result from ring instability or gene silencing due to the new chromosomal architecture.