USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 4-year-old girl with severe intellectual disability is brought to the physician for follow-up. The patient is nonverbal, and she did not begin walking until the age of two. Last year, she was initiated on valproate due to recurrent seizures. During the interview, the physician notices that the patient smiles constantly and laughs most of the time. She claps her hands and laughs hysterically without prompting. Family history is negative for neurological disorders. At the end of the interview, the mother asks the physician about the possibility of having additional children with the same condition. Which of the following is the most appropriate response by the physician?
Content Reviewers:Rishi Desai, MD, MPH
Now, it happens when a gene on chromosome 15 called UBE3A is not expressed, or transcribed into messenger RNA.
UBE3A stands for ubiquitin-protein ligase E3A, and the protein it codes for is called E6AP or E6-associated protein.
The job of E6AP is to go around tagging, or attaching, a tiny protein called ubiquitin to other proteins, a process called ubiquitination.
Once that happens, the ubiquitinated protein is degraded by the proteasome, a part of the cell’s recycling machinery.
It’s kind of painting an orange U on a tree so that a lumberjack knows to chop it down.
So E6 associated protein has an important job, and it turns out that the region of chromosome 15 around UBE3A is imprinted, imprinting refers to gene expression that’s dependent on the parent of origin of a gene.
This means that either the maternally derived or paternally derived copy of the gene is silenced.
This differs from most genes in the genome, where both the maternal and paternal copies are expressed.
Normally, in the brain, only the maternally derived copy of UBE3A is expressed, while the paternal copy is silenced, unfortunately this process of imprinting leaves the maternal copy of UBE3A vulnerable.
So with the paternal copy of the gene imprinted, and epigenetically silenced, you’ve only got the maternal copy left.
So this means that if anything happens to the maternal copy, the result is Angelman syndrome.
There are a few different types of mutations that can cause Angelman syndrome.
The most common one is a deletion of a couple million base pairs of DNA on the maternal copy of chromosome 15 which includes UBE3A.
Sometimes the deletion overlaps a nearby gene called OCA2, which codes for a pigment that gives color to eye, hair, and skin.
As a result of this, these Angelman syndrome patients can have a light complexion.
A second way is a mutation within the maternal copy of UBE3A, making the protein ineffective.
A third way to get Angelman syndrome is when the entire maternal chromosome 15 is absent and instead there’s an extra copy of the paternal chromosome 15.
This scenario is called paternal uniparental disomy, which means that one parent – the father – contributed two of the same chromosome while the mother contributed none.