Somatic and germ-line mosaicism of deletion 15q11.2-q13 in a mother of dyzigotic twins with Angelman syndrome

Angelman syndrome (AS, OMIM105830) is a neurogenetic disorder caused by different genetic mechanisms. Determining the genetic mechanism is essential to establish the recurrence risk and the accuracy of genetic/reproductive counseling. The majority of AS patients present with a deletion of the 15q11.2-q13 region on the maternally derived chromosome. The other genetic mechanisms are: paternal disomy of chromosome 15, imprinting center defects, and mutations in the ubiquitin-protein ligase E3A gene (UBE3A). Different recurrence risks are associated with each specific genetic mechanism involved. We report on the study of dizygotic twins with classic phenotypic AS due to deletion of the same maternally derived chromosome 15. The mother presented with hypopigmented macular lesions on the inner side of both arms. Fibroblast culture studies of the maternal hypopigmented skin areas from both arms showed mosaicism for a normal cell line and for a second cell line with a 15q11.2-q13 deletion. This family represents the first demonstrated case of maternal somatic and germ line mosaicism for 15q11.2-q13 deletion as the cause of AS.