Angelman syndrome is due to?
**Core Concept:** Angelman syndrome is a genetic disorder characterized by severe developmental delay, hypotonia, ataxia, and a distinctive happy demeanor in affected individuals. It is caused by a deficiency in the expression of a specific gene, usually due to a deletion, mutation, or imprinting defect on chromosome 15q11-q13.
**Why the Correct Answer is Right:** Angelman syndrome is caused by a deficiency in the expression of the UBE3A gene, which is imprinted, meaning that it is expressed only from the maternal allele. This gene is located on chromosome 15q11-q13 and codes for an E3 ubiquitin ligase (UBE3A). The loss of its function leads to the clinical features seen in patients with Angelman syndrome.
**Why Each Wrong Option is Incorrect:**
A. This option is incorrect because Angelman syndrome is primarily caused by a deficiency in the expression of the UBE3A gene, not the UBE3B gene. They are different genes located on different chromosomes and have distinct functions.
B. This option is incorrect because a mutation in the UBE3A gene itself is not the primary cause of Angelman syndrome. The deficiency arises from the imprinting defect, which is responsible for the loss of gene expression, not a mutation in the gene itself.
C. This option is incorrect because a deletion of chromosome 15q11-q13 is a common cause of Angelman syndrome, not an insertion or duplication of the same region. A deletion leads to the loss of expression of several genes, including UBE3A, resulting in the syndrome's clinical features.
D. This option is incorrect because a mutation in the UBE3A gene itself is not the primary cause of Angelman syndrome. The deficiency arises from the imprinting defect, which is responsible for the loss of gene expression, not a mutation in the gene itself.
**Clinical Pearl:** The imprinting defect in Angelman syndrome is a critical aspect to understand, as it explains the molecular basis of the syndrome and distinguishes it from other genetic disorders with overlapping features. The identification of the underlying cause aids in accurate genetic counseling, diagnosis, and potential treatment strategies.