biology

Glycogen storage disease type II

Glycogen storage disease type II is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Glycogen storage disease type II rather than just read about it. In short: Glycogen storage disease type II (GSD-II), also called Pompe disease, and formerly known as GSD-IIa or Limb–girdle muscular dystrophy 2V, is an autosomal recessive metabolic disorder which damages muscle and nerve cells throughout the body. It is caused by an accumulation of glycogen in the lysosome due to a deficiency of the lysosomal acid alpha-glucosidase enzyme (GAA).

Key takeaways

Glycogen storage disease type II belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
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Reproduce the core statement of Glycogen storage disease type II from memory before moving on to harder problems.

Reference excerpt

Glycogen storage disease type II (GSD-II), also called Pompe disease, and formerly known as GSD-IIa or Limb–girdle muscular dystrophy 2V, is an autosomal recessive metabolic disorder which damages muscle and nerve cells throughout the body. It is caused by an accumulation of glycogen in the lysosome due to a deficiency of the lysosomal acid alpha-glucosidase enzyme (GAA). The inability to break down glycogen within the lysosomes of cells leads to progressive muscle weakness throughout the body and affects various body tissues, particularly in the heart, skeletal muscles, liver, and the nervous system. GSD-II and Danon disease are the only glycogen storage diseases characterised by a defect in lysosomal metabolism. It was first identified in 1932 by Dutch pathologist Joannes Cassianus Pompe, making it the first glycogen storage disease to be discovered.

Signs and symptoms

Infantile-Onset (IOPD) The infantile-onset (IOPD) form usually comes to medical attention within the first few months of life, either clinically or through newborn screening. The usual presenting features are cardiomyopathy, cardiomegaly, hypotonia, respiratory distress, muscle weakness, feeding difficulties, and failure to thrive. IOPD patients can be further classified by Cross-Reactive Immunological Material (CRIM) status which is an important predictor of clinical response. Patients that produce no GAA protein are referred to as CRIM negative. Therefore, they can develop highly sustained antibody titers for enzyme replacement therapy (ERT). Immunomodulation or immunotherapy is an effective treatment to prevent an immune response to ERT. The main clinical findings include floppy baby appearance, delayed motor milestones, and feeding difficulties. Moderate hepatomegaly may or may not be present. Facial features include macroglossia, hypernasal speech, hearing loss, and myopathic facies. Cardiopulmonary involvement is manifested by increased respiratory rate, use of accessory muscles for respiration, recurrent chest infections, decreased air entry in the left lower zone (due to cardiomegaly), arrhythmias, and evidence of heart failure. Before the development of a treatment, the median age at death in untreated cases was 8.7 months, usually due to cardiorespiratory failure. However, this outcome has drastically changed since enzyme replacement therapy became available, improving with early initiation of treatment.

Late onset form The Late-onset (LOPD) form differs from the infantile-onset principally in the relative lack of cardiac involvement. The onset has a slower progression and can present at any decade of life. Cardiac involvement may occur but is milder than in the infantile form. Skeletal involvement is more prominent with a predilection for the lower limbs. Late-onset features include impaired cough, recurrent chest infections, hypotonia, progressive muscle weakness, delayed motor milestones, difficulty swallowing or chewing, and reduced vital capacity. Variability in the age of onset, severity, and symptoms, between those as genetically similar as identical twins, indicates that there may be epigenetic - or secondary factors - influencing disease presentation. Prognosis depends on the age of onset of symptoms with a better prognosis being associated with later onset disease.

Cause

Pompe disease has an autosomal recessive inheritance pattern. This means the defective gene is located on an autosome, and two faulty copies of the gene—one from each parent—are required to be born with the disorder. As with all cases of autosomal recessive inheritance, children have a one in four chance of inheriting the disorder when both parents carry the defective gene, and although both parents carry one copy of the defective gene, they are usually unaffected by the disorder. The disease is caused by a mutation in a gene (acid alpha-glucosidase: also known as acid maltase) on the long arm of chromosome 17 at 17q25.2-q25.3 (base pair 75,689,876 to 75,708,272). The number of mutations described is currently (in 2010) 289 with 67 being non-pathogenic mutations and 197 pathogenic mutations. The remainder are still being evaluated for their association with disease. The gene spans approximately 20 kb and contains 20 exons with the first exon being noncoding. The coding sequence of the putative catalytic site domain is interrupted in the middle by an intron of 101 bp. The promoter has features characteristic of a housekeeping gene. The GC content is high (80%) and distinct TATA and CCAAT motifs are lacking. Most cases appear to be due to three mutations. A transversion (T → G) mutation is the most common among adults with this disorder. This mutation interrupts a site of RNA splicing. The gene encodes a protein—acid alpha-glucosidase (EC 3.2.1.20)—which is a lysosomal hydrolase. The protein is an enzyme that normally degrades the alpha -1,4 and alpha -1,6 linkages in glycogen, maltose and isomaltose and is required for the degradation of 1–3% of cellular glycogen. The deficiency of this enzyme results in the accumulation of structurally normal glycogen in lysosomes and cytoplasm in affected individuals. Excessive glycogen storage within lysosomes may interrupt the normal functioning of other organelles and lead to cellular injury. A putative homologue—acid alpha-glucosidase-related gene 1—has been identified in the nematode Caenorhabditis elegans.

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Illustrations

Worked examples

Example 1 — a first encounter with Glycogen storage disease type II

Start with the simplest possible case. Write down what Glycogen storage disease type II claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Glycogen storage disease type II before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Glycogen storage disease type II ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Glycogen storage disease type II

In research: Glycogen storage disease type II appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Glycogen storage disease type II in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Glycogen storage disease type II is common in secondary-school and first-year university syllabi. It links to neighbouring topics Autosomal recessive disorders, Hepatology, Inborn errors of carbohydrate metabolism, so understanding it makes those chapters shorter.
In everyday life: Look for Glycogen storage disease type II outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Glycogen storage disease type II in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Glycogen storage disease type II means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
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Frequently asked questions

What is Glycogen storage disease type II in simple terms?

Why does Glycogen storage disease type II matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Glycogen storage disease type II?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Glycogen storage disease type II.