Genetics 201

For the basics of Angelman syndrome genetics, start with Genetics 101. If you want to dive deeper into AS genetics, you're in the right place. 

Angelman syndrome is caused by the lack of functional UBE3A protein in the brain. To understand this, we first need to understand the basics of chromosomes, genes and proteins. 

Back to genetics basics

The human body is made up of cells. Generally, humans have 46 chromosomes, which come in 23 pairs, in most cells of the body.  The reproductive cells, the egg and sperm, are meant to pass along one copy of each pair of chromosomes.  That way we inherit one copy of each chromosome from the egg (maternal copy) and one copy from the sperm (paternal copy).  

In this karyotype (picture of a set of chromosomes from one cell), you can see a set of chromosomes arranged in their pairs from biggest (chromosome 1) to smallest (chromosome 22), and then the sex chromosomes.  Most males have XY sex chromosomes while most females have XX sex chromosomes. 

Chromosomes package the genetic information inside our cells. Each chromosome is made up of a string of DNA. Some sections of the DNA are called genes, because that DNA section provides the instruction to make a protein. Proteins perform most functions within our cells and our body.   

Each gene provides the specific instruction to make a specific protein. DNA is made up of a long series of 4 chemicals, also called nucleotides. The chemicals are typically referred to by their abbreviations, A (adenine), C (cytosine), G (guanine), and T (thymine).  The nucleotides form pairs with each other, called basepairs, and are supposed to be arranged in a specific order for a specific gene. The nucleotides code for the building blocks of the protein, called amino acids.   

There are around 20,000 different genes.  Because most individuals have two copies of each chromosome, most individuals also have two copies of each gene, one from the egg (maternal) and one from the sperm (paternal).  The copies of the genes may not be completely identical, as they may have slight variations that do not affect the function of the protein.  Those types of variations just make us all unique individuals.   

Each gene has a specific “correct” sequence, with all the nucleotides in a specific order.  Some differences, including duplications (extra pieces), deletions (missing pieces), or switches in the nucleotides, can result in a protein that is non-functional.  

DNA to RNA to Protein 

The process of making a protein requires multiple steps, but the middle step (RNA) is often left out for simplicity. Each gene is read (or expressed) to produce messenger RNA (mRNA), which is a one-to-one copy of the DNA using slightly different chemicals. Essentially, the cell is translating the DNA into mRNA. The DNA is located in the nucleus or center of the cell, but machinery for making proteins is outside the nucleus. To get the instructions to the protein-assembly machinery, the mRNA needs to leave the nucleus, moving the translated message to the protein-assembly machinery in the cell. The protein-assembly machinery then builds the protein.  Every three nucleotides of the DNA (and therefore every three nucleotides of mRNA) also called a codon, codes for one amino acid. The complete string of amino acids makes up the protein.   

UBE3A and Imprinting 

The causative gene in AS, the UBE3A gene, is located on chromosome 15.  The UBE3A gene contains 2700 basepairs of DNA and codes for a protein that contains approximately 875 amino acids. In 1987, researchers recognized that AS is due to a difference on chromosome 15, and in 1997, UBE3A was identified to be the causative gene for AS. 

In most individuals, the UBE3A gene is present on each copy of chromosome 15.  The vast majority of cells in the body make UBE3A protein using both the maternal and paternal copy of the UBE3A gene. However, in neurons, this is different. In neurons, the cells of the brain and spinal cord, the UBE3A gene is silenced or “turned off” on the paternal chromosome 15 and is active or “turned on” on the maternal chromosome 15. This is a very unique phenomenon called imprinting, which is a form of gene regulation where one parental copy of a gene is silenced.  In human neurons, only the maternal copy of the UBE3A gene is producing the needed UBE3A protein. All other cells of the body have both copies of the UBE3A gene producing this protein. 

UBE3A-ATS 

The paternal UBE3A gene is silenced by a strand of RNA made from the DNA on the paternal chromosome 15. This strand is known as the “antisense transcript (ATS)” or UBE3A-ATS. This is also sometimes referred to as a long non-coding strand of RNA (lnc-RNA). The term “antisense” is used because the instructions for the UBE3A-ATS are produced by DNA instructions that are read in the opposite direction, essentially backwards from the instructions that would produce UBE3A. 

 The UBE3A-ATS prevents the reading of the paternal UBE3A gene on the paternal chromosome 15, leaving only the maternal UBE3A gene active. Exactly how the production of the UBE3A-ATS blocks this reading of the paternal UBE3A is still being studied, but we know we can manipulate it. 

If an individual has a problem with the maternal UBE3A gene, then neurons do not have the ability to make the UBE3A protein– remember, the paternal UBE3A gene is off.  The lack of functional UBE3A protein in the brain causes Angelman syndrome.   

Genotypes 

There are 5 main mechanisms or genetic causes that result in maternal UBE3A deficiency, shown here in this diagram. The different genetic mechanisms that cause AS are referred to as “genotypes.” 

For more information on the different genotypes, see this page. For information on testing for them, go here.)

How does AS occur? 

AS is never caused by anything the mother did (or did not do) before or during the pregnancy of the child living with AS.  AS is often due to a random event that occurred during the development of the egg that became that child, which actually happened before the mother was even born! More rarely, AS can be inherited from a mother who is considered a carrier, meaning that she herself does not have any symptoms. The chance that a mother is a carrier varies depending on the genotype. More information about heredity and chances for family members can be found under the specific genotype.

Regardless of the genetics behind the AS genotype, all individuals living with AS are missing functional UBE3A protein in neurons. The loss of neuronal UBE3A expression results in a plethora of symptoms because the UBE3A protein is incredibly important in neurologic function. The characteristics of AS are relatively consistent across all the genotypes. 

 What does UBE3A do? 

The UBE3A protein has multiple functions, many of which are not yet fully understood.  Proteins perform the jobs in the body; and each protein has one or more jobs to do.   

UBE3A’s most understood role is to act in a complex with other proteins to keep other proteins in balance both inside and outside of the neuron.  UBE3A does this by working together with the other proteins to perform ubiquitination.  

Ubiquitination is the “tagging” of other proteins in the cell, either for breaking down and recycling or to be protected and continue functioning. The tag itself is a small protein called ubiquitin (named so because of its abundance in cells).  Since UBE3A adds, or ligates, ubiquitin to proteins, UBE3A is also called ubiquitin ligase. In the science literature, UBE3A is also called  ubiquitin protein ligase E3A or E3 ligase E6-associated protein (E6AP).  

When neurons do not have UBE3A, other proteins can accumulate to high levels or become unstable to low levels, both of which have a negative effect on neuron function and coordination.  This results in neurons having too much excitation and limited amounts of inhibition, or calmness. This calmness is something our brains need to coordinate movement, brain activity, sleep, and most other functions. All these functions are impacted in AS. 

Watch a video on what UBE3A does.

UBE3A protein has other jobs. Adding ubiquitin to a protein can also regulate how that protein works.  UBE3A can tag other proteins and potentially make them more or less effective at their jobs depending on the needs of the cell.  

We also know UBE3A can work with other proteins to control the expression of other genes on other chromosomes, which may be independent of its work with ubiquitin. Ultimately, understanding the critical functions of UBE3A will help us understand why neurons function poorly in the absence of UBE3A.  

Scientists and clinicians are working to correlate protein function to the symptoms of AS. We know that UBE3A is vital to how the brain develops and controls speech, movement and learning. The large impact UBE3A has in neurons as well as at the junction between neurons (synapse) explains why individuals living with AS have challenges coordinating their muscles for controlled verbal speech, coordinated gait, and prolonged focus. The neurons are overactive, making sleep challenging and increasing the risk for seizures. 

By understanding the genetics and physiology of AS, we can understand the potential approaches to treatments for Angelman syndrome.  Increasing UBE3A protein in the brain and/or to decreasing the excitation that neurons experience is likely to have significant benefits for individuals living with AS.  Because Angelman syndrome is caused by an imprinted gene, AS is unique– the fact that individuals living with AS have a functional but silenced copy of the UBE3A gene provides an opportunity for treatment that does not exist for most other genetic conditions.   

Approaches to treatment in AS 

To organize how AS could be treated, we have broken the treatment approaches into three main therapeutic pillars.  

Pillar 1: Replace mom’s UBE3A gene or protein:  Potential mechanisms include gene replacement therapy (e.g. AAV-GT, HSC-GT) or enzyme replacement therapy (ERT). 

Pillar 2: Turn on dad’s UBE3A. This can be done via various different mechanisms, all aiming for the same result. (e.g. ASO, CRISPR, Artificial Transcription Factors, shRNA/miRNA). 

Pillar 3: Improve the symptoms of AS by targeting the downstream impact of UBE3A loss.  Examples include small molecules that could  like improve the coordination and communication between neurons at the synapse.  

For more information on these potential treatment approaches, visit our Angelman Syndrome Drug Development Pipeline.

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