AssessmentsGlycogen storage disease type II (NORD)
Glycogen storage disease type II (NORD)
USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 3500-g (7.7-lb) female newborn delivered at term is brought to her four month well-child checkup by her parents who report that she has had problems feeding. Prenatal examinations and assays identified no abnormalities but in the time since her birth she has failed to meet developmental milestones. Physical examination is remarkable for diminished muscle tone. Echocardiography shows cardiomegaly. Laboratory studies show the following results:
This child most likely has a deficiency of which of the following enzymes?
Content Reviewers:Rishi Desai, MD, MPH
Glycogen storage disease type II, or Pompe’s disease, is a genetic disorder where there’s a mutation on a gene on chromosome 17.
This results in deficiency of the lysosomal enzyme alpha acid glucosidase, or acid maltase. Without this enzyme, muscle cells cannot use their glycogen stores for energy.
Glycogen is basically an enormous molecule or polymer, that’s made up of glucose molecules linked together by glycosidic bonds.
You can think of glycogen having a main chain, where glucose molecules are linked together by alpha 1-4 glycosidic bonds.
Then there are multiple branches sprouting off of the main chain, and each branch is connected to the main chain by alpha 1-6 glycosidic bonds.
These branches allow glycogen to be compact and also allow it to rapidly add and remove glucose to and from the big glycogen molecule.
Glucose molecules are usually added to glycogen in response to insulin.
And the pancreas secretes insulin after meals, in response to high blood sugar - so when there’s plenty of glucose floating around in the bloodstream.
It makes sense for some of this glucose to be stored as glycogen, right?
Now when it’s been a while after a meal, so when you’re fasting, blood sugar levels take a dip.
It turns out that glucagon tells the liver cells to break glycogen down into individual glucose molecules, and epinephrine tells skeletal muscle cells to do the same.
Now in muscle cells, glycogen degradation can take place inside of a lysosome, which acts like the digestive center of a cell.
A lysosomal enzyme called acid maltase that has both α-1,4- glucosidase with α-1,6- glucosidase activity, so it can chop off glucose molecules off glycogen, which the muscles cells then use for energy.
Now glycogen storage disease type II, or Pompe's disease, is inherited in an autosomal recessive fashion - so one mutated copy from each parent is required to develop the condition.
This results in a deficiency of acid maltase, so glycogen accumulates the lysosomes of skeletal muscle, smooth muscle, and cardiac muscle cells.
This results in a two-fold pathologic process.
First, glycogen accumulation leads to lysis, or rupture, of lysosomes.