APBDRF Glossary of Terms

Last update: 3/20/2026

Adult Polyglucosan Body Disease (APBD)

A rare genetic glycogen storage disease that results from mutations in the GBE1 gene (glycogen branching enzyme; see also GBE1) and results in the accumulation of misformed glycogen in cells of the body (see also polyglucosan bodies). Symptoms can include bladder dysfunction, leg stiffness and weakness, mobility issues, numbness and tingling in hands and feet, fatigue, and cognitive impairment. Please see the What is APBD? page on the foundation’s website for a comprehensive explanation of the condition.

Amyotrophic Lateral Sclerosis (ALS)

Also called Lou Gherig’s disease, ALS is a condition that affects the central nervous system. The symptoms of APBD can appear similar to the symptoms of ALS, resulting in misdiagnosis.

Antisense oligonucleotide 

A type of therapy in which short strands of DNA or RNA are designed to specifically target the intermediate mRNA product of a gene. This can be to correct the specific mutation in an affected gene copy or to change the activity of a targeted gene.

Biobank/biorepository 

A collection of biological samples (often blood) used to help researchers study how genes, the environment, and lifestyle can affect a person’s health.

Biomarker 

A biological molecule that serves as a characteristic that is measured as an indicator of a person’s health in regard to a specific condition.

Biosample

Small samples collected from the body like blood, urine, saliva, cells, biopsies, etc. Also called biological samples. They are routinely collected from individuals for medical testing, as well as for research studies. 

Blood enzyme assay

A type of laboratory test of blood to measure the levels of certain enzymes in the body. Certain enzyme assays can measure how much glycogen branching enzyme the body is making or how functional (enzyme activity) the enzyme is.

Carrier

An individual that has one copy of a mutation in a gene that is associated with a genetic condition. In recessive conditions, carriers do not have symptoms of the genetic disease.

Carrier screening 

A type of genetic test that determines if someone is a carrier for certain genetic conditions, specifically recessive conditions. Carrier screening is most often done when a person is pregnant to see if they have a chance of having a child with a genetic disorder. See also: recessive condition.

Central Nervous System (CNS)

The part of the nervous system consisting primarily of the brain and spinal cord, regulating everything the body does. The CNS collects sensory information from different parts of the body and responds to them accordingly.

Cerebral small vessel disease

A grouping of diseases that affect the blood vessels in the brain. The symptoms of APBD can appear similar to the symptoms of some cerebral small vessel diseases, resulting in misdiagnosis.

Clinical Research ID (CRID)

A unique code created by individuals to protect their privacy when participating in clinical research studies and connect their research data across different studies. Please see the Create your Clinical Research ID page on the foundation’s website for a comprehensive explanation of this valuable tool for assisting research efforts.

Clinical trial 

A type of research study that tests potential new medical treatments on people to see if the treatments are safe and effective.

Diagnostic odyssey

The long and often difficult process of receiving a rare disease diagnosis. 

DNA (deoxyribonucleic acid) 

The most basic/fundamental component of our genetic information. DNA contains the instructions that allow us to grow, develop, and function. Genes are made up of DNA and permanently store the information for making proteins and biological molecules. 

Enzyme

A type of protein that works to accelerate chemical reactions and biological processes.

Exon

The section of a gene that contains the information or code to make proteins and other important biological molecules. Can be thought of as the “functional” part of the gene.

GBE1

The gene associated with APBD and GSD Type IV. It provides instructions for making glycogen branching enzyme. When the gene does not work properly, not enough glycogen branching enzyme is made. As a result, glycogen molecules do not take their proper shape, and can accumulate in cells in abnormal ways, causing damage that results in the health problems seen in APBD and/or GSD Type IV.

Gene

The basic unit of heredity passed on from parents to children. Genes are made up of DNA and act as instructions for specific traits and/or body functions. Every cell in the body contains two copies of all 20,000 genes known to exist, with one copy being inherited from a person’s mother, and the other copy being inherited from a person’s father.

Genetic counseling 

A type of healthcare that involves helping people understand and adapt to the medical, emotional, psychological, and familial implications of genetic conditions and/or the risk of being diagnosed with a genetic condition.

Genetic prevalence 

The estimated number of individuals in a population (i.e. global, country, ethnicity, etc) that are likely to be affected by a specific genetic disorder and/or gene variant. 

Genetic testing 

The process of examining a person’s genetic information (including DNA, genes, and chromosomes) to see if there are any changes that may affect their health.

Gene therapy

A type of treatment that is focused on a person’s genes to cure a disease.  Can involve directly repairing or editing the patients’ genetic code (see also gene) or introducing a functional copy of a gene to replace the mutated one.

GINA (Genetic Information Non-Discrimination Act) 

A law that prevents someone from being denied health insurance or employment opportunities based on their genetic information or having a genetic condition.

Glycogen

A complex sugar that is a major source of stored energy in the body. Glycogen is made up of many molecules of glucose (a simple sugar) and is broken down into these smaller molecules when the body is ready to use the stored energy.

Glycogen branching enzyme

An enzyme that helps make growing glycogen molecules more compact. This in turn makes storage and breakdown of glycogen easier.

Glycogen synthase

An enzyme that helps make glycogen molecules by attaching glucose onto growing chains. Glycogen synthase controls the step before glycogen branching enzyme. Encoded by the GYS1 (in muscle and other tissues)and GYS2 (in liver).

Glycogen Storage Disease (GSD)

A group of rare, genetic metabolic disorders that result in the body not properly making, breaking down, or storing glycogen. 

Glycogen Storage Disease Type IV (GSD IV) 

A larger classification of the genetic condition caused by pathogenic variants (mutations) in the GBE1 gene. It includes early-onset GSD IV (infantile, pediatric, and adolescent) forms that typically impact liver and/or muscle function, as well as adult-onset GSD IV or APBD.  “GSD IV continuum” refers collectively to all types of GSD IV.

Heterozygous

Having two different copies of a given gene. People who are heterozygous for a recessive genetic condition can be carriers for that condition (one variant/mutant copy and one healthy copy) or have the condition by carrying two different disease-causing variants (mutations).

Homozygous 

Having two of the same copy of a given gene. People who are homozygous for a recessive genetic condition will be affected by the condition.

Inheritance 

The way in which genetic information - including traits, variants, gene copies, chromosomes, etc. - is passed from parents to their children.

Institutional review board (IRB) 

An administrative group that oversees and regulates the ethics and conduct of research involving human subjects.

Intron

The section of a gene that contains information that does not result in the production of proteins or other important biological molecules. Can be thought of as the “non-coding” part of a gene. Comparatively little is known about introns in relation to exons.

Intronic mutation

A variant (see: variant) located within an intron, rather than the exon part of a gene, which is where the vast majority of known variants are found. Intronic mutations can result in health problems in similar ways as variants in exons, but they function differently and necessitate different research and treatment from “normal” exonic variants.

Magnetic resonance imaging (MRI)

A noninvasive medical imaging technique that uses strong magnets to generate high-resolution 3-dimensional images of the body. Often used to get clear images of the brain and spinal cord in neurological and neurodegenerative diseases.

Manifesting heterozygotes 

Individuals who have symptoms or health problems associated with a recessive genetic condition but only have one identified disease-causing variant in the related gene. This term was originally used to describe patients with the deep intronic mutation in GBE1 before the variant was identified by more advanced genetic testing.

Metabolism/Metabolic 

The collection/sum of all chemical and biological reactions that allow the body to turn food and drink into usable energy for normal growth, function, and development. Also describes the chemical and biological reactions that occur within individual cells of the body that allow them to function.

Multiple Sclerosis (MS)

A neurological disease in which the body’s immune system attacks the central nervous system, interrupting communication between the brain and the rest of the body. The symptoms of APBD can appear similar to the symptoms of MS, resulting in misdiagnosis.

Mutation/pathogenic variant 

A change to the DNA sequence of a gene that is known to cause disease or health problems. Mutations/pathogenic variants in GBE1cause APBD/GSD Type IV. Also referred to as disease-causing mutations.

Natural history study 

A type of research study that collects information about how a disease or condition develops/progresses in people in order to better understand the condition and how to treat or manage it.

Neurodegenerative

A type of disease that causes gradual damage and destruction of parts of the nervous system. APBD is a neurodegenerative disease.

Neurological glycogen storage disorders

A collection of allied diseases involving issues with glycogen processing that largely present with neurological symptoms. Includes APBD, Lafora Disease, and Pompe Disease.

Peripheral Nervous System (PNS)

The part of the nervous system that consists of everything else besides the brain and spinal cord. The PNS sends information to different parts of the body back to the CNS, and then carries out the commands sent from the brain in response to this sensory information.  Includes the nerves that run from the brain and spinal cord to the rest of the body.

Peripheral neuropathies

A grouping of diseases that affect the function of the nerves of the peripheral nervous system. The symptoms of APBD can appear similar to the symptoms of some peripheral neuropathies, resulting in misdiagnosis.

Polyglucosan bodies

Abnormal glycogen molecules that do not have branches in the normal way and are larger, less compact, and difficult for the body to break down for energy. These molecules can build up in cells and cause damage, particularly nerve cells, resulting in the health problems seen in APBD and/or GSD Type IV

Recessive condition 

A type of genetic condition in which a person must have two abnormal copies of a gene in order to be affected with health problems. A person who has only one abnormal copy of a gene associated with a recessive condition is considered to be a carrier for that condition, and will not have health problems. See also: carrier and carrier screening.

RNA (Riboneucleic acid) 

A type of biological molecule that is made from DNA and is an intermediate in the process of making proteins and important biological molecules in the body.  RNA is a temporary template of the DNA code and is broken down once used - this is one way the body controls the levels of proteins and biological molecules in the system. 

Tissue analysis 

A type of medical test that examines the function and/or health of a certain part of the body in order to help make a diagnosis for a specific condition.

Variant

A permanent change in the DNA sequence that makes up a gene. Everyone has all kinds of variants in their DNA and most do not cause disease (benign).

Whole exome sequencing

A type of genetic testing that screens all exons in all of the known genes of the human body. Can miss intronic mutations.

Whole genome sequencing

A type of genetic testing that screens all of the known genes of the human body, including exons and introns. This type of testing is often recommended for genetic conditions with known disease-causing intronic mutations.