Genetic investigations on 8 patients affected by ring 20 chromosome syndrome

Ring Chromosome 20 Syndrome: Genetics, Clinical Characteristics, and Overlapping Phenotypes

Ring chromosome 20 [r(20)] syndrome is a rare condition characterized by a non-supernumerary ring chromosome 20 replacing a normal chromosome 20. It is commonly seen in a mosaic state and is diagnosed by means of karyotyping. r(20) syndrome is characterized by a recognizable epileptic phenotype with typical EEG pattern, intellectual disability manifesting after seizure onset in otherwise normally developing children, and behavioral changes. Despite the distinctive phenotype, many patients still lack a diagnosis-especially in the genomic era-and the pathomechanisms of ring formation are poorly understood. In this review we address the genetic and clinical aspects of r(20) syndrome, and discuss differential diagnoses and overlapping phenotypes, providing the reader with useful tools for clinical and laboratory practice. We also discuss the current issues in understanding the mechanisms through which ring 20 chromosome causes the typical manifestations, and present unpublished data about methylation studies. Ultimately, we explore future perspectives of r(20) research. Our intended audience is clinical and laboratory geneticists, child and adult neurologists, and genetic counselors.

A further case of familial ring chromosome 20 mosaicism - molecular characterization of the ring and review of the literature

Ring chromosome 20 syndrome is a rare chromosomal disorder characterized by childhood-onset drug-resistant epilepsy, behavioral problems and variable cognitive impairment. While most cases occur sporadically, parent-to-child transmission of ring 20 mosaicism has only been reported in a few exceptional families. We identified a further family with mother-to-child transmission of ring 20 mosaicism. Detailed characterization of the ring chromosome showed a complete ring with preserved telomere repetitive sequences. SNP genotyping excluded mosaic uniparental disomy and indicated that the chromosome was transmitted without recombination from mother to child. These results corroborate the findings of a previous study and support the hypothesis that inherited mosaicism is due to transmission of an unstable chromosome either prone to ring opening or to ring re-formation.

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.

Specificity of electroclinical features in the diagnosis of ring chromosome 20

BACKGROUND

Ring chromosome 20 (R20) syndrome is a chromosomal disorder characterized mainly by drug-resistant frontal lobe seizures, recurrent nonconvulsive status epilepticus (NCSE), and typical EEG features. The aim of this study was to investigate if this triad is common and specific to all patients with R20.

METHODS

In this cross-sectional study (from 2000 to 2011), we selected patients who fulfilled at least two out of three criteria: drug-resistant frontal lobe seizures, recurrent NCSE, and characteristic electroencephalography (EEG) features. In all patients, diagnosis was based on karyotype analysis of at least 100 metaphases.

RESULTS

We identified 36 patients who met at least two of the selected criteria: six patients (16.7%) with R20 and 30 (83.3%) without R20 (non-R20). All patients with R20 met all three criteria. Eleven (36.7%) patients without R20, however, also displayed the full triad. In 19 patients without R20 (63.3%), one of the three clinical features was missing: frontal lobe seizures were not resistant to antiepileptic drugs (AED) in four (13.3%), recurrent NCSE was missing in six (20%), and nine (30%) patients did not have typical EEG features. Based on this data, specificity was 63.3%, positive predictive value was 35.3%, and sensitivity and negative predictive values were 100%. Additionally, a review of all publications describing the R20 phenotype revealed that 81.98% of patients with R20 display the full electroclinical triad.

CONCLUSIONS

In our study, all patients with R20 displayed the three electroclinical characteristics. This is in line with previous reports (presenting high sensitivity and negative predictive value). However, these features can also be observed in other epilepsies and are not specific to R20. Our findings suggest that in the presence of the full triad of symptoms, karyotype analysis focused on chromosome 20 should be conducted.

Dysregulation of FOXG1 by ring chromosome 14

In this study we performed molecular characterization of a patient with an extra ring chromosome derived from chromosome 14, with severe intellectual disability, epilepsy, cerebral paresis, tetraplegia, osteoporosis and severe thoraco-lumbal scoliosis. Array CGH analysis did not show any genomic imbalance but conventional karyotyping and FISH analysis revealed the presence of an interstitial 14q12q24.3 deletion and an extra ring chromosome derived from the deleted material. The deletion and ring chromosome breakpoints were identified at base-pair level by mate-pair and Sanger sequencing. Both breakpoints disrupted putative long non-coding RNA genes (TCONS00022561;RP11-148E17.1) of unknown function. However, the proximal breakpoint was 225 kb downstream of the forkhead box G1 gene (FOXG1), within the known regulatory landscape of FOXG1. The patient represents the first case of a r(14) arising from an interstitial excision where the phenotype is compatible with dysregulation of FOXG1. In turn, the phenotypic overlap between the present case, the FOXG1 syndrome and the r(14) syndrome supports that dysregulation of FOXG1 may contribute to the classical r(14)-syndrome, likely mediated by dynamic mosaicism.

Autosomal ring chromosomes in human genetic disorders

Ring chromosomes arise following breakage and rejoining in both chromosome arms. They are heterogeneous with variable size and genetic content and can originate from any chromosome. Phenotypes associated with ring chromosomes are highly variable as apart from any deletion caused by ring formation, imbalances from ring instability can also occur. Of interest is ring chromosome 20 which has a significant association with epilepsy with seizure onset in early childhood. Severe growth deficiency without major malformations is a common finding in the ring chromosome carrier. This phenotype associated with ring behaviour and mitotic instability and independent of the chromosome involved has been termed the "ring syndrome". Precise genotype-phenotype correlations for ring chromosomes may not be possible as influencing factors vary depending on the extent of deletion in ring formation, ring instability and the level of mosaicism. Although ring chromosomes usually arise as de novo events, familial transmission of rings from carrier to offspring has been described and prenatal diagnosis for any pregnancies should always be considered.

Emerging neuroimaging contribution to the diagnosis and management of the ring chromosome 20 syndrome

Ring chromosome 20 [r(20)] syndrome is an underdiagnosed chromosomal anomaly characterized by severe epilepsy, behavioral problems, and mild-to-moderate cognitive deficits. Since the cognitive and behavioral decline follows seizure onset, this syndrome has been proposed as an epileptic encephalopathy (EE). The recent overwhelming development of advanced neuroimaging techniques has opened a new era in the investigation of the brain networks subserving the EEs. In particular, functional neuroimaging tools are well suited to show alterations related to epileptiform discharges at the network level and to build hypotheses about the mechanisms underlying the cognitive disruption observed in these conditions. This paper reviews the brain circuits and their disruption as revealed by functional neuroimaging studies in patients with [r(20)] syndrome. It discusses the clinical consequences of the neuroimaging findings on the management of patients with [r(20)] syndrome, including their impact to an earlier diagnosis of this disorder. Based on the available lines of evidences, [r(20)] syndrome is characterized by interictal and ictal dysfunctions within basal ganglia-prefrontal lobe networks and by long-lasting effects of the peculiar theta-delta rhythm, which represents an EEG marker of the syndrome on integrated brain networks that subserve cognitive functions.

Low frequency mu-like activity characterizes cortical rhythms in epilepsy due to ring chromosome 20

OBJECTIVES

To evaluate the spectral and spatial features of the cortical rhythms in patients affected by ring chromosome 20 - [r(20)]-syndrome.

METHODS

Twelve patients with [r(20)] syndrome were studied. As controls we enrolled 12 patients with idiopathic generalized epilepsy (IGE) and 12 healthy volunteers (HV). Blind source separation, spectral analyses and source reconstruction were applied in all cases in order to identify reliable spatio-temporal patterns of cortical activity.

RESULTS

A theta-delta EEG rhythm was identified in [r(20)] patients, with spectral peak ranging between 3 and 7Hz and whose generators mapped over the sensory-motor cortices. A second peak laying at a frequency about double with respect to the first one was present in 6 cases. Analogue methodological approach in HV and IGE groups failed to show similar findings.

CONCLUSIONS

EEG of [r(20)] patients reveals the existence of a highly reproducible EEG pattern arising from the sensory-motor system.

SIGNIFICANCE

The recognition of this peculiar EEG pattern could help the diagnostic work-up. Additionally, our findings supports the existence of a parallelism between this EEG trait and the physiological "mu" rhythm which is generate by the sensory-motor system. Such link suggests a sensory-motor system dysfunction in [r(20)] patients.

Prenatal diagnosis of a fetus with a ring chromosome 20 characterized by array-CGH

OBJECTIVE

a fetus with a ring chromosome 20 is presented.

METHODS

at 16 weeks' gestation, ultrasound examination evidenced no apparent structural malformation. Amniocentesis was performed for maternal anxiety.

RESULTS

chromosome analysis identified a ring chromosome 20 and array-CGH demonstrated that the ring including micro-deletion of the short arm in 20p13, that was extended for about 632.2 kb and a micro-deletion of the long arm in 20q13.33 region.

CONCLUSION

this is the first case of a ring chromosome 20 diagnosed prenatally. This reinforces the importance of offering amniocentesis with a-CGH to make more accurate prenatal diagnosis.

A new structural rearrangement associated to Wolfram syndrome in a child with a partial phenotype

Wolfram syndrome (WS) is a rare autosomal recessive disorder characterized by diabetes insipidus (DI), insulin-dependent diabetes mellitus (DM), optic atrophy (OA) and deafness caused by mutations in WFS1 gene (4p16.1), which encodes an endoplasmic reticulum protein, called Wolframin. We describe the case of an infant who presented hypernatremia and severe hypoplasia of the left eyeball with alteration of visual evoked potentials. Persistent hypernatremia, iposmolar polyuria and high plasma osmolality suggested DI, confirmed by a normal urine concentration after vasopressin test. Treatment with vasopressin allowed a normalization of sodium levels and urine output. Brain magnetic resonance imaging showed absence of the neurohypophysis hyperintense signal, normal adenohypophysis and optic tracts hypoplasia. The concomitant presence of DI and OA, even in the absence of DM and deafness, prompted the suspicion of WS and complete genetic analysis was performed. Genomic DNA sequencing of WFS1 showed no inactivating mutations described to date, but suggested a structural mutation as markers genotyping revealed a segmental paternal heterodisomy involving the upstream regulatory region (promoter and 5'UTR). cDNA sequencing revealed the coexistence of the wild-type transcript and two splice variants; one variant, probably benign, is known in literature and the other one causes the loss of exon 2, containing the translation initiation site. Western blot confirmed a marked protein reduction. During the clinical follow-up child's condition remained stable and glucose metabolism is still in the standard. In conclusion, the phenotype associated with this structural rearrangement, which substantially reduces the synthesis of Wolframin, confirms a tissue-specific pattern of expression of WFS1, suggests the presence of a different protein dosage sensitivity in different tissues and could be causative of DI and OA in our patient. The "incomplete" phenotype here described, usually absent in typical WS cases, is explained by the residual Wolframin expression that would preserve other organs, i.e. pancreatic islets. A careful longitudinal clinical follow-up will assess any changes in the phenotypic penetrance in our patient.

The evolving electroclinical syndrome of "epilepsy with ring chromosome 20"

PURPOSE

Ring chromosome 20 {r(20)}--manifests as a refractory epilepsy syndrome with complex partial seizures (CPS), nocturnal frontal lobe seizures and non-convulsive status epilepticus (NCSE) in the majority of cases. r20 lacks a specific phenotypic expression or dysmorphic features. Psychomotor development may be normal, making the diagnosis difficult unless there is a high index of suspicion. This description of further cases is intended to improve recognition of this syndrome.

METHODS

Karyotyping for r20 was done in patients presenting with cryptogenic refractory epilepsy suggestive of r20 from August 2008 to September 2010. We identified three patients with r20 associated with refractory epilepsy. All were investigated with long-term video EEG. Their unique electro clinical features; epilepsy syndrome and outcome are discussed and compared to the available literature.

RESULTS

Karyotyping revealed ring 20 in 6-40% of lymphocytes. All manifested with seizures beginning at 2-10years of age. All had CPS; two had NCSE and one a seizure disorder resembling Lennox-Gastaut syndrome. The interictal EEG showed epileptiform abnormalities predominantly over the frontotemporal regions. Two patients had NCSE with generalized, frontally dominant theta and spike and wave discharges. None had facial dysmorphism or imaging abnormalities. They remained resistant to antiepileptic drugs.

CONCLUSIONS

The use of routine karyotyping can easily pick up r(20); this information is especially useful in resource-poor countries. We have evolved an algorithm stating the indications to attempt r(20) karyotyping in a given patient in the light of the results of the present study and the existing literature.