Detection of a deletion of exons 8-16 of the UBE3A gene in familial Angelman syndrome using a semi-quantitative dosage PCR based assay

Novel intragenic deletions within the UBE3A gene in two unrelated patients with Angelman syndrome: case report and review of the literature

BACKGROUND

Patients with Angelman syndrome (AS) are affected by severe intellectual disability with absence of speech, distinctive dysmorphic craniofacial features, ataxia and a characteristic behavioral phenotype. AS is caused by the lack of expression in neurons of the UBE3A gene, which is located in the 15q11.2-q13 imprinted region. Functional loss of UBE3A is due to 15q11.2-q13 deletion, mutations in the UBE3A gene, paternal uniparental disomy and genomic imprinting defects.

CASE PRESENTATION

We report here two patients with clinical features of AS referred to our hospital for clinical follow-up and genetic diagnosis. Methylation Specific-Multiplex Ligation-Dependent Probe Amplification (MS-MLPA) of the 15q11.2-q13 region was carried out in our laboratory as the first diagnostic tool detecting two novel UBE3A intragenic deletions. Subsequently, the MLPA P336-A2 kit was used to confirm and determine the size of the UBE3A deletion in the two patients. A review of the clinical features of previously reported patients with whole UBE3A gene or partial intragenic deletions is presented here together with these two new patients.

CONCLUSION

Although rare, UBE3A intragenic deletions may represent a small fraction of AS patients without a genetic diagnosis. Testing for UBE3A intragenic exonic deletions should be performed in those AS patients with a normal methylation pattern and no mutations in the UBE3A gene.

Angelman syndrome: review of clinical and molecular aspects

"Angelman syndrome" (AS) is a neurodevelopmental disorder whose main features are intellectual disability, lack of speech, seizures, and a characteristic behavioral profile. The behavioral features of AS include a happy demeanor, easily provoked laughter, short attention span, hypermotoric behavior, mouthing of objects, sleep disturbance, and an affinity for water. Microcephaly and subtle dysmorphic features, as well as ataxia and other movement disturbances, are additional features seen in most affected individuals. AS is due to deficient expression of the ubiquitin protein ligase E3A (UBE3A) gene, which displays paternal imprinting. There are four molecular classes of AS, and some genotype-phenotype correlations have emerged. Much remains to be understood regarding how insufficiency of E6-AP, the protein product of UBE3A, results in the observed neurodevelopmental deficits. Studies of mouse models of AS have implicated UBE3A in experience-dependent synaptic remodeling.

Screening of UBE3A gene in patients referred for Angelman Syndrome

Angelman Syndrome (AS) is a neurodevelopmental disorder characterized by severe developmental delay, speech impairment and unique behaviors including inappropriate laughter and happy disposition. AS is related to deficient maternal UBE3A gene expression caused either by chromosomal deletions, uniparental disomy, molecular defects of the imprinted 15q11-q13 critical region or by loss of function mutations in the maternally inherited UBE3A. In the present study, screening UBE3A was performed in 43 patients who were referred for AS but whom previous molecular diagnostic tests failed to provide a diagnosis. Two causative mutations--one of them novel--and four polymorphic variants one of which is also novel were revealed. Further investigation of 7 patients disclosed defects in other genes involved in clinical phenotypes mimicking AS. A typical EEG pattern and microcephaly in patients with developmental delay prompt for AS investigation while wide genetic screening should be applied to help resolution of the complex phenotypes characterized by developmental delay.

Novel deletion encompassing exons 5-12 of the UBE3A gene in a girl with Angelman syndrome

Angelman syndrome (AS) is characterised by severe developmental delay, severe speech impairment, gait ataxia and/or limb tremor and a unique behavioural phenotype. The diagnosis of AS is based on a combination of clinical features and molecular genetic testing. Currently, molecular genetic testing (methylation analysis and UBE3A sequence analysis) identifies anomalies in about 90% of individuals. The aetiology of the remaining 10% is still unknown. We report a novel deletion encompassing the exons 5-12 of the UBE3A gene in a girl with AS, identified by MLPA (Multiplex Ligation-dependent Probe Amplification), which was not detected by the conventional diagnostic protocol. We propose that copy number analysis of the UBE3A gene should be considered in individuals whose clinical examination is strongly suggestive of AS, after more common mechanisms have been excluded.

Novel deletion of the E3A ubiquitin protein ligase gene detected by multiplex ligation-dependent probe amplification in a patient with Angelman syndrome

Angelman syndrome (AS) is a severe neurobehavioural disorder caused by failure of expression of the maternal copy of the imprinted domain located on 15q11-q13. There are different mechanisms leading to AS: maternal microdeletion, uniparental disomy, defects in a putative imprinting centre, mutations of the E3 ubiquitin protein ligase (UBE3A) gene. However, some of suspected cases of AS are still scored negative to all the latter mutations. Recently, it has been shown that a proportion of negative cases bear large deletions overlapping one or more exons of the UBE3A gene. These deletions are difficult to detect by conventional gene-scanning methods due to the masking effect by the non-deleted allele. In this study, we have used for the first time multiplex ligation-dependent probe amplification (MLPA) and comparative multiplex dosage analysis (CMDA) to search for large deletions affecting the UBE3A gene. Using this approach, we identified a novel causative deletion involving exon 8 in an affected sibling. Based on our results, we propose the use of MLPA as a fast, accurate and inexpensive test to detect large deletions in the UBE3A gene in a small but significant percentage of AS patients.

Clinical and genetic aspects of Angelman syndrome

Angelman syndrome is characterized by severe developmental delay, speech impairment, gait ataxia and/or tremulousness of the limbs, and a unique behavioral phenotype that includes happy demeanor and excessive laughter. Microcephaly and seizures are common. Developmental delays are first noted at 3 to 6 months age, but the unique clinical features of the syndrome do not become manifest until after age 1 year. Management includes treatment of gastrointestinal symptoms, use of antiepileptic drugs for seizures, and provision of physical, occupational, and speech therapy with an emphasis on nonverbal methods of communication. The diagnosis rests on a combination of clinical criteria and molecular and/or cytogenetic testing. Analysis of parent-specific DNA methylation imprints in the 15q11.2-q13 chromosome region detects approximately 78% of individuals with lack of maternal contribution. Less than 1% of individuals have a visible chromosome rearrangement. UBE3A sequence analysis detects mutations in an additional 11% of individuals. The remaining 10% of individuals with classic phenotypic features of Angelman syndrome have a presently unidentified genetic mechanism and thus are not amenable to diagnostic testing. The risk to sibs of a proband depends on the genetic mechanism of the loss of the maternally contributed Angelman syndrome/Prader-Willi syndrome region: typically <1% for probands with a deletion or uniparental disomy; as high as 50% for probands with an imprinting defect or a mutation of UBE3A. Members of the mother's extended family are also at increased risk when an imprinting defect or a UBE3A mutation is present. Chromosome rearrangements may be inherited or de novo. Prenatal testing is possible for certain genetic mechanisms.

Novel UBE3A mutations causing Angelman syndrome: different parental origin for single nucleotide changes and multiple nucleotide deletions or insertions

Angelman syndrome (AS) is a genetic disorder caused by a deficiency of UBE3A imprinted gene expression from the maternal chromosome 15. In 10% of AS cases the genetic cause is a mutation affecting the maternal copy of the UBE3A gene. In two large Spanish series of clinically stringently selected and nonstringently selected patients, we have identified 11 pathological mutations--eight of them novel mutations--and 14 sequence changes considered polymorphic variants. Remarkably, single nucleotide substitutions are more likely to be inherited, while multiple nucleotide deletions or insertions are less frequently inherited, thus indicating that single nucleotide substitutions are more likely to originate from the paternal germline. Additionally, there seems to be a different distribution of nucleotide changes and multiple nucleotide deletions or insertions along the UBE3A gene sequence.