Information about:
Angelman syndrome
(Angelman syndrome, OMIM 105830)
by Karen Grønskov, Zeynep Tümer
Clinical characteristics:
A clinical diagnosis of Angelman syndrome (AS) demands fulfillment of four major criteria and minimum three of the six minor criteria. The major criteria are severe developmental delay, movement or balance disorder, severe limitations in speech and language and typical abnormal behavior including happy demeanor and excessive laughter. The six minor criteria are postnatal microcephaly, seizures, abnormal EEG, sleep disturbance, attraction to or fascination with water, and drooling (Tan et al. 2011). The unique clinical features do not usually manifest within the first year of life, but developmental delay is noticed around 6 months of age. The diagnosis of AS is a combination of clinical and molecular genetic diagnosis. In about 10% of the individuals with a clinical diagnosis of AS it is not possible to find the underlying genetic mechanism and other diagnoses should be considered.
Genetic background: ↓ The 15q11-q13 chromosomal region contains imprinted genes, some of which are exclusively expressed from either of the parental alleles (Figure 1). Two exclusively maternally expressed genes, UBE3A and ATP10A, are located with this region: UBE3A encodes an E3 ubiquitin-protein ligase which is expressed exclusively from the maternal allele in human fetal brain and in adult frontal cortex (Rougeulle et al. 1997; Vu et al. 1997). AS can be caused either by lack of UBE3A expression or by mutations in UBE3A. The role of the other imprinted gene, ATP10A, is however unclear. In individuals with deletions, uniparental disomy (UPD) or imprinting defects, ATP10A expression is lacking, but in individuals with point mutations in UBE3A it is left unaffected. Of note, lack of expression of the paternally expressed genes in the same region causes Prader-Willi syndrome (PWS; OMIM 176270).
Molecular mechanisms: ↓ AS can be caused by maternally derived de novo deletion of 15q11-q13, paternal UPD of chromosome 15, or an imprinting defect all of which lead to lack of expression of maternally expressed 15q11-q13 genes. Furthermore, mutations in UBE3A also cause Angelman syndrome. The deletions typically have common breakpoints, extending from the distal breakpoint BP3 to the proximal breakpoints BP1 or BP2, however unique breakpoints have also been reported (Sahoo et al. 2007). Deletions can in very rare occasions be associated with maternal chromosomal rearrangements, and the recurrence risk is increased in these cases. Imprinting defects can either be due to primary imprinting epimutations without DNA sequence alterations, or due to deletions in the imprinting center IC critical region (AS-SRO) (Buiting et al. 2001; Buiting et al. 2003). Recurrence risk depends on the underlying genetic mechanism, which should be established for optimal genetic counseling of the families. Prenatal diagnosis is possible in pregnancies with increased risk if the genetic mechanism is known. A summary of genetic mechanisms, their frequencies and the recurrence risks are given in table 1. Molecular genetic testing: ↓ Guidelines for molecular analysis of AS are available (Ramsden et al. 2010). DNA methylation analysis will enable AS diagnosis in approximately 80% of the cases. Around 10% of the cases have UBE3A mutations while the genetic mechanism is unknown in the remaining 10%. For DNA methylation analysis most diagnostic laboratories in Europe use MS-MLPA which will detect both the methylation status of 15q11-q13 and a deletion if present. For individuals with methylation abnormalities, without an accompanying deletion, microsatellite analysis using polymorphic markers will distinguish between UPD and another defect. If microsatellite analysis shows biparental inheritance of chromosome 15 analysis of the IC region should be performed to distinguish between an isolated epimutation and a deletion in the IC domain. Individuals with normal methylation of 15q11-q13 and high clinical suspicion of AS should be investigated for mutations in UBE3A. Some laboratories perform MS-PCR for methylation analysis. If abnormal methylation is detected, presence of a deletion should be investigated with FISH or MLPA analyses. In the absence of a deletion the testing strategy is similar to that described for MS-MLPA. In cases of de novo deletions the mother should be further investigated to rule out maternal chromosomal rearrangements. A testing strategy is shown in figure 2.
Perspectives: ↓ Several differential diagnoses of individuals referred for AS should be considered upon negative molecular genetic testing. It is important to establish a molecular genetic diagnosis for proper genetic counseling of the family and to be able to determine the recurrence risk.
Tables & figures: ↓ Table 1. Frequencies of (epi)genetic mechanisms and recurrence risks.
Underlying defect
Frequency
Recurrence risk
Deletion
70-75%
<1%
<1%
up to 50%
UPD
3-7%<1%
<1%Increased (varies 1-100%)
Imprinting defects
2-3%
<1%
<0.5%
Up to 50%
UBE3A mutation
10 %
50% if present in mother
No identifiable molecular abnormality
10%
Unknown
MS-MLPA or MS_PCR are the two preferred strategies. Blue colored circles show either confirmation or discarding of AS diagnosis. Pink circles show determination of the genetic mechanism. *Testing with FISH or MLPA is only necessary if initial analysis method is MS-PCR.
Genes in blue are expressed only from the paternal allele, genes in red only from the maternal allele and genes in green show biallelic expression. SNORD116 and SNORD115 are present in more copies than indicated on the figure. The imprinting center (IC) is shown below as a horizontal line located between C15orf2 and SNURF-SNRPN, PWS-SRO in blue and AS-SRO in red. BP1 to BP5 indicates positions of the breakpoints. The bottom solid black lines indicate extension of class I and class II deletions. Only in rare cases the deletions extend to BP4 or BP5. Not all genes with biallelic expression are shown on the map. The figure is not drawn to scale.
References: ↓ Buiting K, Barnicoat A, Lich C, Pembrey M, Malcolm S, Horsthemke B. Disruption of the bipartite imprinting center in a family with Angelman syndrome. Am J Hum Genet. 2001 May;68(5):1290-4. Epub 2001 Mar 23. PubMed PMID: 11283796; PubMed Central PMCID: PMC1226110. Buiting K, Gross S, Lich C, Gillessen-Kaesbach G, el-Maarri O, Horsthemke B. Epimutations in Prader-Willi and Angelman syndromes: a molecular study of 136 patients with an imprinting defect. Am J Hum Genet. 2003 Mar;72(3):571-7. Epub 2003 Jan 23. PubMed PMID: 12545427; PubMed Central PMCID: PMC1180233. Ramsden SC, Clayton-Smith J, Birch R, Buiting K. Practice guidelines for the molecular analysis of Prader-Willi and Angelman syndromes. BMC Med Genet. 2010 May 11;11:70. doi: 10.1186/1471-2350-11-70. PubMed PMID: 20459762; PubMed Central PMCID: PMC2877670. Rougeulle C, Glatt H, Lalande M. The Angelman syndrome candidate gene, UBE3A/E6-AP, is imprinted in brain. Nat Genet. 1997 Sep;17(1):14-5. PubMed PMID: 9288088. Sahoo T, Bacino CA, German JR, Shaw CA, Bird LM, Kimonis V, Anselm I, Waisbren S, Beaudet AL, Peters SU. Identification of novel deletions of 15q11q13 in Angelman syndrome by array-CGH: molecular characterization and genotype-phenotype correlations. Eur J Hum Genet. 2007 Sep;15(9):943-9. Epub 2007 May 23. PubMed PMID: 17522620. Tan WH, Bacino CA, Skinner SA, Anselm I, Barbieri-Welge R, Bauer-Carlin A, Beaudet AL, Bichell TJ, Gentile JK, Glaze DG, Horowitz LT, Kothare SV, Lee HS, Nespeca MP, Peters SU, Sahoo T, Sarco D, Waisbren SE, Bird LM. Angelman syndrome: Mutations influence features in early childhood. Am J Med Genet A. 2011 Jan;155A(1):81-90. doi: 10.1002/ajmg.a.33775. PubMed PMID: 21204213; PubMed Central PMCID: PMC3563320. Vu TH, Hoffman AR. Imprinting of the Angelman syndrome gene, UBE3A, is restricted to brain. Nat Genet. 1997 Sep;17(1):12-3. PubMed PMID: 9288087.