Genes of Interest in Duplication 15q Syndrome

Chromosomes carry genes that control the physical development and behavior of every individual. To understand the developmental problems caused by duplication 15q syndrome, it is helpful to be aware of some of the genes of interest on this chromosome.

What do we know about the genes involved in duplications of 15q11-q13? The most commonly duplicated region is gene rich, including genes that are expressed from both paternal and maternal chromosomes as well as genes that are expressed differentially based on parent of origin of the chromosome.

Because maternally derived 15q duplications are most often associated with developmental problems, including autism, cognitive impairments, and seizures there is a lot of scientific interest into the two known maternally expressed genes UBE3A and ATP10A (aka ATP10C). It is likely that other genes within the duplication and beyond are contributing to the symptoms of duplication 15q syndrome.

The UBE3A gene is present in 4 copies in most children with idic(15) and 3 copies in most patients with interstitial duplications. Studies have shown that the extra copies of the gene are active in patients with idic(15) chromosomes . The UBE3A gene provides instructions for making an enzyme called ubiquitin protein ligase E3A. This enzyme is involved in targeting other proteins to be broken down (degraded) within cells. Protein degradation is a normal process that removes damaged or unnecessary proteins and helps maintain the normal functions of cells. Both copies of the UBE3A gene are active in most of the body's tissues. In the brain, however, only the copy inherited from a person's mother (the maternal copy) is normally active.

The ATP10A (also referred to as ATP10C) gene is currently being studied to determine if it plays a role in the development of the autism spectrum disorders associated with maternal rearrangements of chromosome 15. The exact function of this gene is not known, but it is believed to produce a protein that helps transmit molecules called ions (such as calcium) between cells in the body. In addition to its location on chromosome 15 and imprint status, the assumed function would involve it in maintenance of cell membrane integrity and it may therefore be critical for cell signaling in the central nervous system.

There are also 3 GABA_A receptor genes in the region that is commonly duplicated. These three GABA receptor subunit genes are called GABRB3, GABRA5 and GABRG3. GABA (g-aminobutyric acid) is a neurotransmitter (a chemical that carries messages between nerve cells). When GABA communicates to nerve cells, it causes them not to respond to other stimulatory signals in brain. Thus, GABA is largely considered an inhibitory neurotransmitter, although its activity actually evolves with age. After infancy, the overall effect of GABA and GABA_A receptors is to stabilize the activity of nerve cells.

Impaired GABA function, especially GABAA receptor function, has been proposed to play an important role in autism and Angelman syndrome. It is hypothesized that the role of GABA receptor subunit genes in autism most likely comes via complex gene-gene interactions. GABA also plays an important role in seizures. Because GABA inhibits neurons from firing, and seizures are caused by inappropriate or unregulated firing of nerve cells, increasing GABA activity through its receptors can cause the system to stabilize and decrease seizures.

The 3 GABA_A receptor genes contain instructions for making proteins that form GABA receptors. A functional GABA receptor is composed of several parts, and it is believed that expressing extra copies of some components will actually lead to fewer functional receptors. Diane C. Chugani, PhD, at Wayne State University in Detroit, Michigan is currently conducting a pilot study of Brain GABA_A Receptor Abnormalities in Children with Chromosome 15q11-13 Mutations to investigate this further.

Carolyn Schanen, MD, PhD, IDEAS Professional Advisory Board contributed to this article.

Information Last Updated: August 13, 2006

References

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Herzing LB, Kim SJ, Cook, Jr. EH, Ledbetter DH.: The Human Aminophospholipid-Transporting ATPase Gene ATP10C Maps Adjacent to UBE3A and Exhibits Similar Imprinted Expression. Am J Hum Genet. 2001 Jun; 68(6): 1501-1505.

Cook EH Jr, Lindgren V, Leventhal BL, Courchesne R, Lincoln A, Shulman C, Lord C, Courchesne E. Autism or atypical autism in maternally but not paternally derived proximal 15q duplication. Am J Hum Genet 1997;4:928-934.

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