Molecular Genetic Study on Angelman Syndrome Patients without a Chromosomal Deletion

Abstract

Purpose: Angelman syndrome (AS) is a ncurobehavioral disorder characterized by severe mental retardation, easily cvoked laughter, ataxic gait, and epilepsy. Epilepsy associated with AS is characterized by early childhood onset gencralized seizures with profound EEG abnormalities. Therefore, AS is a good human model for genetic epilepsy syndromes. Approximately 70% of AS cases are caused by maternal deletions of chromosomc 15q I I‐qI3; whereas, 30% are not associated with a chromosomal dcletion. Thcse non‐deletion AS patients are caused by paternal uniparental disomy (UPD), imprinting mutation (IM), or loss‐or‐function mutations of the UBE3A gene, cach of which predisposes different recurrence risk. To elucidate molecular etiology of non‐dclction AS patients, we investigated 34 AS patients without a chromosomal deletion. Methods: Thirty sporadic AS patients, and 4 familial AS patients (2 families of 2 sibs) were enrolled to the study. The diagnosis of AS was based on Williams’ criteria (Williams et al., Am J Med Genet 1995, 56: 237). Genomic DNA was extracted from peripheral blood by a standard procedure. DNA mcthylation tcst at SNRPN locus and genotyping using 7 highly informative PCR‐based polymorphisms within 15q I I ‐ q I3 were carried out to identify UPD and IM. When both UPD and IM were ruled out, the patients were classified :LS non‐UPD, non‐IM. For thcsc non‐UPD, non‐1M paticnts, UBE3A mutations were screened by PCR‐SSCP analysis using 10 sets ofprimcrs covering all coding exons. Results: Among 30 sporadic patients, I UPD and 3 IM patients were identified, and the remaining 26 patients were classified as non‐UPD, non‐IM. Among 4 familial patients, 2 sibs from I family were detected as IM, whcrcas 2 sibs from another family were classified as non‐UPD, non‐IM. No UBE3A mutations were identified within 26 sporadic and 2 familial non‐UPD, non‐IM patients. Conclusion: Threc molecular classes were identified for noindeletion AS patients. Therefore, the underlying genetic mechanism was dcmonstratcd to be complex for AS patients without a chromosomal deletion. Combination of the DNA methylation test and PCR‐based polymorphisms was sufficient to detect UPD and IM patients. Because recurrence risk is low for UPD and high lor IM, systematic molecular investigation including the DNA methylation test and PCR‐based polymorphisms should bc donc for non‐delction AS paticnts for genetic counscling purpose. A majority of non‐deletion patients were classified as noii‐UPD, non‐1M. Although, approximate 30% of non‐UPD, nonIM patients arc rcportcd to have UBE3A mutations, no such mutations were identified in our study. An underlying molecular mechanism was not rcvealcd for this group of patients, and therefore, assessment of recurrence risk was difficult. Further investigation is necessary for noii‐UPD, non‐1M paticnts.