FISH-mapping of telomeric 14q32 deletions: Search for the cause of seizures

Chromosome 14 deletions, rings, and epilepsy genes: A riddle wrapped in a mystery inside an enigma

The ring 14 syndrome is a rare condition caused by the rearrangement of one chromosome 14 into a ring-like structure. The formation of the ring requires two breakpoints and loss of material from the short and long arms of the chromosome. Like many other chromosome syndromes, it is characterized by multiple congenital anomalies and developmental delays. Typical of the condition are retinal anomalies and drug-resistant epilepsy. These latter manifestations are not found in individuals who are carriers of comparable 14q deletions without formation of a ring (linear deletions). To find an explanation for this apparent discrepancy and gain insight into the mechanisms leading to seizures, we reviewed and compared literature cases of both ring and linear deletion syndrome with respect to both their clinical manifestations and the role and function of potentially epileptogenic genes. Knowledge of the epilepsy-related genes in chromosome 14 is an important premise for the search of new and effective drugs to combat seizures. Current clinical and molecular evidence is not sufficient to explain the known discrepancies between ring and linear deletions.

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.

Epilepsy in ring 14 syndrome: a clinical and EEG study of 22 patients

PURPOSE

To characterize epileptic phenotype, electroencephalography (EEG) features, and epileptic evolution in patients with ring 14 r(14) syndrome.

METHODS

Twenty-two patients with ring chromosome 14 were enrolled in the study. We examined age at onset, seizure semiology and frequency at onset and at follow-up, drug responsiveness/resistance, and interictal/ictal EEG data. The degree of severity of the epileptic phenotype negatively influences child cognitive development.

KEY FINDINGS

The incidence of epilepsy in patients with r(14) syndrome is virtually 100%, characterized by early onset, polymorphic seizures, and drug-resistant seizures. In addition, we ascertained focal secondarily generalized epilepsy, seizure cluster tendency, frequent status epilepticus, and a rather typical epilepsy evolution. EEG abnormalities consisted of slow background activity with pseudoperiodic bursts of generalized slow waves in the early stage, focal frontotemporal or temporoposterior slow waves with multifocal spikes interposed, and unusual rhythmic fast recruiting posterior spikes followed by secondary generalization. The degree of severity of the epileptic phenotype negatively influences child cognitive development.

SIGNIFICANCE

This study provides a more precise definition of seizure types, natural history, and drug responsiveness of r(14) syndrome, a highly epileptogenic chromosomal condition.

Epilepsy in ring 14 chromosome syndrome

Ring chromosome 14 [r(14)] is a rare disorder. The aim of this study was to describe two new cases of r(14) drug-resistant epilepsy, and, through an extensive review of literature, highlight those epileptological features which are more commonly found and which may help in early diagnosis, genetic counseling, and treatment. Epilepsy onset in r(14) syndrome takes place during the first year of life; seizures are generalized or focal and less frequently myoclonic. Seizures might be induced by fever. Focal seizures are characterized by staring, eye or head deviation, respiratory arrest, swallowing, and hypertonia/hypotonia or clonic movements. Ictal EEG might show both focal and diffuse discharges. Interictal EEG reveals mainly focal abnormalities. Mental retardation represents a constant feature. Neurological assessment yields a delay in motor skill acquisition and less frequently both pyramidal and cerebellar signs. Dysmorphic features are evident in the majority of cases. Epilepsy associated with r(14) has many features that entail a challenging diagnostic process. The reported cases of r(14)-related epilepsy seem to highlight a series of common elements which may be helpful in pointing the clinician towards a correct diagnosis.

A phenotype map for 14q32.3 terminal deletions

Detailed molecular-cytogenetic studies combined with thorough clinical characterization are needed to establish genotype-phenotype correlations for specific chromosome deletion syndromes. Although many patients with subtelomeric deletions have been reported, the phenotype maps for many of the corresponding syndromes, including the terminal deletion 14q syndrome, are only slowly emerging. Here, we report on five patients with terminal partial monosomy of 14q32.3 and characteristic features of terminal deletion 14q syndrome. Four of the patients carry de novo terminal deletions of 14q, three of which have not yet been reported. One patient carries an unbalanced translocation der(14)t(9;14)(q34.3;q32.3). Minimum deletion sizes as determined by molecular karyotyping and FISH are 5.82, 5.56, 4.17, 3.54, and 3.29 Mb, respectively. Based on our findings and a comprehensive review of the literature, we refine the phenotype map for typical clinical findings of the terminal deletion 14q syndrome (i.e., intellectual disability/developmental delay, muscular hypotonia, postnatal growth retardation, microcephaly, congenital heart defects, genitourinary malformations, ocular coloboma, and several dysmorphic signs). Combining this phenotype map with benign copy-number variation data available from the Database of Genomic Variants, we propose a small region critical for certain features of the terminal deletion 14q syndrome which contains only seven RefSeq genes.

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).

Interstitial deletion of 14q24.3-q32.2 in a male patient with plagiocephaly, BPES features, developmental delay, and congenital heart defects

Distal interstitial deletions of chromosome 14 involving the 14q24-q23.2 region are rare, and only been reported so far in 20 patients. Ten of these patients were analyzed both clinically and genetically. Here we present a de novo interstitial deletion of chromosome 14q24.3-q32.2 in a male patient with developmental delay, language impairment, plagiocephaly, BPES features (blepharophimosis, ptosis, epicanthus), and congenital heart defect. The deletion breakpoints were fine mapped using fluorescence in situ hybridization (FISH) and the size of the deletion is estimated to be approximately 23 Mb. Based on genotype-phenotype comparisons of the 10 previously published patients and the present case, we suggest that the shortest regions for deletion overlap may include candidate genes for speech impairment, mental retardation, and hypotonia.

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.

Array CGH defined interstitial deletion on chromosome 14: a new case

Interstitial deletions of the long arm of chromosome 14 are relatively rare. We report a 8.5-year-old girl with dysmorphic facial features and mental retardation associated with a de novo interstitial deletion of chromosome 14. The comparison between our patient and all published patients is reviewed. The genetic investigations have allowed us to define the critical chromosomal region and to start an accurate follow-up.

Submicroscopic subtelomeric aberrations in Chinese patients with unexplained developmental delay/mental retardation

BACKGROUND

Subtelomeric imbalance is widely accepted as related to developmental delay/mental retardation (DD/MR). Fine mapping of aberrations in gene-enriched subtelomeric regions provides essential clues for localizing critical regions, and provides a strategy for identifying new candidate genes. To date, no large-scale study has been conducted on subtelomeric aberrations in DD/MR patients in mainland China.

METHODS

This study included 451 Chinese children with moderate to severe clinically unexplained DD/MR. The subtelomere-MLPA (multiplex ligation dependent probe amplification) and Affymetrix human SNP array 6.0 were used to determine the subtelomeric copy number variations. The exact size and the breakpoint of each identified aberration were well defined.

RESULTS

The submicroscopic subtelomeric aberrations were identified in 23 patients, with a detection rate of 5.1%. 16 patients had simple deletions, 2 had simple duplications and 5 with both deletions and duplications. The deletions involved 14 different subtelomeric regions (1p, 2p, 4p, 6p, 7p, 7q, 8p, 9p, 10p, 11q, 14q, 15q, 16p and 22q), and duplications involved 7 subtelomeric regions (3q, 4p, 6q, 7p, 8p, 12p and 22q). Of all the subtelomeric aberrations found in Chinese subjects, the most common was 4p16.3 deletion. The sizes of the deletions varied from 0.6 Mb to 12 Mb, with 5-143 genes inside. Duplicated regions were 0.26 Mb to 11 Mb, with 6-202 genes inside. In this study, four deleted subtelomeric regions and one duplicated region were smaller than any other previously reported, specifically the deletions in 11q25, 8p23.3, 7q36.3, 14q32.33, and the duplication in 22q13. Candidate genes inside each region were proposed.

CONCLUSIONS

Submicroscopic subtelomeric aberrations were detected in 5.1% of Chinese children with clinically unexplained DD/MR. Four deleted subtelomeric regions and one duplicated region found in this study were smaller than any previously reported, which will be helpful for further defining the candidate dosage sensitive gene associated with DD/MR.

Ring 14 chromosome presenting as early-onset isolated partial epilepsy

We report four infants (two males, two females) with ring 14 chromosome presenting with early-onset partial epilepsy. The first seizure occurred between 3 and 6 months (3, 3, 4, and 6mo respectively). In all four cases, diagnosis was based on early focal seizures, rather than on psychomotor retardation or morphological features, which were not prominent at seizure onset. Moreover, despite the young age of the patients and the high frequency of seizures, neither epileptic spasms nor progression to 'epileptic encephalopathy', such as hypsarrhythmia, were observed. Epilepsy remained partial in these patients. At the most recent follow-up, all four children had slight or mild psychomotor delay, and two of them had moderate non-specific dysmorphic traits. Data from the literature about epilepsy in ring 14 chromosome syndrome were also reviewed. Ring 14 chromosome syndrome may be revealed by isolated, early-onset focal epilepsy suggestive of focal lesions with only mild mental retardation and morphological features at the time of diagnosis. The characteristics of these observations differ from classic ring 14 syndrome, and may enlarge this clinical spectrum. Many unanswered questions remain concerning phenotype-genotype correlation and identification of the potential genes and molecular mechanisms responsible for epilepsy in patients with ring 14 syndrome.

The ring 14 syndrome: clinical and molecular definition

The ring 14 (r14) syndrome is a rare condition, whose precise clinical and genetic characterization is still lacking. We analyzed a total of 20 patients with r14 and another 9 patients with a linear 14q deletion. The ring was complete, with no apparent loss of chromosome material, in 6 cases; a terminal 14q deletion, varying in size from 0.65 to 5 Mb, was detected in the remaining 14 cases. Deleted ring chromosomes were 70% paternal and 30% maternal. UPD (14) was never detected. With respect to the linear 14q deletions, three were proximal, varying in size from 4 to 7.2 Mb, and six distal, varying in size from 4.8 to 20 Mb. The majority of the linear deletions were also of paternal origin, and UPD (14) was excluded in all cases. Clinically, the r14 syndrome was characterized by a recognizable phenotype, consisting of shortness of stature, a distinctive facial appearance, microcephaly, scoliosis, and ocular abnormalities, which included abnormal retinal pigmentation, strabismus, glaucoma, and abnormal macula. All patients except one had mental retardation. Drug-resistant epilepsy was another highly consistent finding. Aggressive and hyperactive behavior was noted in about half of the patients. Based on genotype-phenotype correlations, we could deduce that retinal abnormalities, epilepsy, microcephaly, and mental retardation map within the proximal 14q11.2-q12 region. Likewise, behavior disorders and scoliosis could be assigned to the 14q32 region.

A child with terminal 14q deletion syndrome: consideration of genotype-phenotype correlations

Patients with terminal deletions of chromosome 14 usually share a number of clinical features. The syndrome is thought not to be associated with multiple congenital anomalies. We report on a patient having a terminal deletion of about 3.2 Mb, with the breakpoint in 14q32.32. Multiple health problems led to his early death. By molecular techniques (array comparative genomic hybridization (CGH) and fluorescence in situ hybridization (FISH)), we identified two previously reported patients with deletions in the terminal part of chromosome 14 of almost exactly the same size and compare the phenotypes of all three children. The phenotype of the current patient is much more severe than the phenotypes of the two patients reported previously. The patients also present different sets of dysmorphic features described previously as characteristic for 14q deletion syndrome. Molecular cytogenetic mapping showed that the breakpoints in all three patients were clustered within a 240 kb interval. The possibility of recurrent breakpoint location in terminal 14q deletion syndrome, as well as detailed characterization of the spectrum of phenotypes associated with the syndrome, will require the investigation of multiple patients with similar deletions in 14q.

Molecular cytogenetic characterization of terminal 14q32 deletions in two children with an abnormal phenotype and corpus callosum hypoplasia

Among previously reported cases of 14q terminal deletions, only 11 have dealt with pure terminal deletion of 14q (14q3-14qter) and the break points were mapped by fluorescent in situ hybridisation (FISH) or genotyping in only four of them. Thanks to a collaborative study on behalf of the 'Association des Cytogeneticiens de langue Française'(ACLF), we report two patients with terminal deletion of the long arm of chromosome 14, del(14)(q32.2) and del(14)(q32.32), diagnosed by subtelomere screening. In the two cases, a thick nuchal skinfold was detected by early ultrasound with normal prenatal karyotype. Their postnatal phenotype included large forehead, narrow palpebral fissures, epicanthic folds, upturned tip of the nose, narrow mouth and thin upper lip, microretrognathia, prominent earlobes, hypotonia, delayed psychomotor development and hypoplastic corpus callosum. By physical mapping using FISH, the size of the deletions was measured for patients 1 and 2: 6.55+/-1.05 and 4.67+/-0.10 Mb, respectively. The paternal origin of the deleted chromosome 14 was established by genotyping of microsatellites for patient 1 and the phenotype of terminal del(14)(q32) was compared to maternal uniparental disomy 14.

Mosaic trisomy r(14) associated with epilepsy and mental retardation

We report a patient with moderate mental retardation, benign clinical course of epilepsy, and type 2 diabetes mellitus. The patient has a mosaic karyotype with 2 cell lines: 1 with a ring chromosome 14 [r(14)], and 1 with an apparently duplicated r(14) chromosome.

Terminal 14q32.33 deletion: genotype-phenotype correlation

We report on a female infant presenting with psychomotor retardation and facial dysmorphism. Cytogenetic studies showed an abnormal chromosome 14 with ectopic NOR sequences at the extremity of the long arm with a terminal 14q32.33 deletion. Review of the eight cases with pure terminal 14q32.3 deletions described to date documented that our observation is the smallest terminal 14q deletion ever reported. Thus, genotype-phenotype correlation allows us to delimit the critical region for mental retardation, hypotonia, epi-telecanthus, short bulbous nose, long philtrum, thin upper lip, and small mouth observed in 14 qter deletions to the subtelomeric 1.6 Mb of chromosome 14.