In all the cells in our bodies, we have 22 pairs of chromosomes and two extra chromosomes that determine our biological gender (females are XX, males are XY). For each pair, we acquire one copy from our mother (the maternal copy) and one from our father (the paternal copy).
Under the microscope, each chromosome looks like two cylinders joined together. One cylinder is shorter (called the ‘p’ arm) and one is longer (called the ‘q’ arm). Each arm can be seen to have separate regions (appearing as bands under the microscope) which are numbered. Each chromosome is home to thousands of different genes, small packets of information that contribute together to make each unique person. Angelman Syndrome is caused by problems with a single gene, UBE3A, situated on chromosome 15, in the region 11-13 of the ‘q’ arm – referred to as 15q11-13.
UBE3A makes a protein called E6-AP ubiquitin protein ligase. It is a “housekeeping” gene, responsible for the basic maintenance of cells. In the brain, only the maternal copy is “switched on”. The paternal copy is present but is “switched off”. This is known as imprinting. For most genes, where both copies are present and “switched on”, if one copy becomes damaged, the other copy simply takes up the slack and there are no issues. However, when a gene is imprinted in this way, any disruption to the “switched on“copy causes issues as there is no back-up version.
