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Glossary of Terms


  • Acyl-Coenzyme A:

Cholesterol Acyltransferase: The gene for this enzyme is on chromosome 1. It is an enzyme located in the membrane of the endoplasmic reticulum (ER) and acts to join cholesterol molecules to long chain fatty acids for storage purposes.

  • Acid Sphingomyelinase (ASM):

This is a lysosomal enzyme that breaks down a substance called Sphingomyelin. This enzyme is defective to a greater or lesser extent in NPA/B disease.

  • Allele:

This is a particular form of gene that occur in pairs; one on each chromosome inherited from each parent.

  • Amino Acids:

These are organic molecules that link together in order to form proteins.

  • Ataxia:

This refers to the failure of muscular coordination, which can lead to irregular muscle action.

  • Autosomal:

Refers to chromosomes 1 to 22; i.e. any chromosome other than the sex chromosomes.



  • Bases:

In the context of this site, and in scientific discourse at large – the term ‘bases’ is used to describe a number of chemicals that are used as introduction code by DNA and RNA. These are abbreviated as; A, T, C, and G in DNA, and as A, U, C, G in RNA.

  • Blood Brain Barrier:

This term refers to a highly selective barrier formed by blood vessels and a type of brain cell (called astrocytes) that allow only appropriate molecules to cross from blood to brain; e.g. oxygen and sucrose. The blood brain barrier aims to keep potentially harmful molecules out of the brain.

  • Brainstem:

The ‘stalk’ of brain tissue which connects the spinal cord, cerebellum, and cerebral hemispheres. Breathing, heart-rate, blood pressure, control of eye movements, and nerves of the head/neck reside in the brainstem. As a result ‘brain death’ is defined by irreversible loss of the brainstem function.


  • Calcium:

An essential element of all animals and plants – as it is vital in all cells.

  • Catalysts:

A substance that accelerates a chemical reaction without being changed itself. An example of a catalyst can be found with enzymes.

  • Cataplexy:

This refers to the sudden loss of muscle tone, usually brought on by a strong stimulus such as laughter or anger. Cataplexy is believed to represent a fragment of rapid eye movement (REM) sleep that intrudes into the otherwise normal consciousness. Cataplexy correlates with the dysfunction of the upper brainstem.

  • Cells:

The basic unit for life! Cells are bound by a protective membrane and contain all the information; lysosome, mitochondria, nucleus, DNA, production mechanisms and material, which allows for communication links with other cells…which consequently allows for other cells to replicate and grow into a living creature, whose cells can replicate and in turn, function.

  • Cerebellum:

This is the part of the brain located behind the brain stem under the main mass of the brain. It is known to be responsible for the coordination of movement and contains Purkinje cells (which are badly affected by NP-C).

  • Cholesterol:

A fatty substance known as sterol that is a major component of cell membranes; especially the main (plasma) membrane. It is also required for digestive bile acids and sterol hormones. Some cholesterol is taken in from certain foods but most is produced by the cells of the body, notably the liver. A constant supply of cholesterol is needed for cell growth and maintenance and is distributed in the blood stream by LDL.

  • Cholesteryl Esterase:

A lysosomal enzyme that cuts (hydrolyses) LDL cholesterol molecules free from their fatty acid tails. Wolman’s disease results from defects within this enzyme.

  • Chromosomes:

Located in the nucleus of the cell, these are very long super-coiled DNA molecules that carry the information template which enable the replication of cells in the associated living creature.

  • Coat Protein:

Proteins that coat vesicles, and direct their movement in cells.

  • Common Mutation:

A mutation in a gene that occurs in a high percentage of patients affected by a specific disease: e.g. I1061T in Niemann-Pick Type C.

  • Cortex:

The outer layer of any organ; e.g. cerebral cortex, or cerebellar cortex. In the brain the cortex consists of grey matter (also referred to as neurons).

  • Cytosol:

The internal liquid/gel content of a cell that surrounds the sub-cellular organelles and contains many of the simple and complex molecules required for the cell to function. The cytosol is not uniform but varies throughout its extent, and is organised by the internal skeleton of the cell.


  • Diploid:

These are cells containing two sets of chromosomes, and as a result: two copies of genes. Most cells of an organism (except sex cells and red blood cells) are diploid.

  • DNA:

The term ‘DNA’ stands for ‘de-oxy ribose nucleic acid’. It is a very long molecule and is the main component of chromosomes. DNA contains a code which, when processed by the cells’ production machinery, produces proteins. It also contains information relating to the timing and life cycle of the cell.

  • DNA Sequence:

Refers to the relative order of base pairs in a fragment of DNA, a chromosome, a gene, or even an entire genome.

  • Dominant (Allele):

Only one copy of the allele is required in order to produce a certain characteristic. For example, if you have one allele for brown eyes and one for blue – brown is dominant and you will have brown eyes.

  • Dystonia:

This is a state of abnormal muscle tone which results in both abnormal posture and muscular spasm. Dystonia typically occurs due to the presence of a neurological disease or as a side effect of drug therapy.


  • Endogenous:

Coming from within, or developing within an organism, tissue, or cell.

  • Endoplasmic Reticulum:

Series of interconnected flattened cavities lined with a thin membrane – in the nuclear membrane this is continuous.

  • Endosomes:

Membrane-bound compartments of the cell – endosomes are the destination for internalised plasma membrane components and external molecules.

  • Enzyme:

This is essentially a protein that acts as a catalyst which accelerates a chemical reaction.

  • Esterification:

A type of chemical reaction carried out by an enzyme. In Niemann-Pick Type C it is related to the attaching of long chain fatty acids to cholesterol and renders the cholesterol molecule chemically inactive. During this reaction a molecule of water is given off – the reverse of this process is known as hydrolysis, in which cholesterol is separated from the fatty acid.

  • Exogenous:

Developed or originating outside the organism, tissue, or cell.


  • Fatty Acid:

This is a long carbon chain that ends in a carboxylic acid – and is a major building block of cellular lipids.

  • Fibroblasts:

Cells found in connective tissue associated with healing wounds. Skin fibroblasts are often grown (and used) extensively during laboratory testing.

  • Filipin:

This is a blue substance used to stain fibroblast cells. It appears fluorescent when under a microscope.


  • Gametes:

These are sex cells; that are either mature haploid male (sperm), or female germ cells (ova) which are able to unite with another of the opposite sex during sexual reproduction in order to form a zygote.

  • Gangliosides:

A form of glycolipid, this serves as a component of cell membranes which is especially abundant in nerve cells. They are found to be abnormal in those affected by NPC.

  • Genes:

Regions of DNA that are copied in order to make proteins.

  • Gene Product:

The biochemical material (either RNA or protein) which results from the expression of a gene. The amount of gene product is used to measure how active a gene is as abnormal amounts can be correlated with disease-causing alleles.

  • Genome:

The total complement of genes on all the chromosomes that define a person or animal’s life cycle.

  • Genotype:

Simply put, this is the genetic make-up of an individual.

  • Glycolipids:

A group of lipids which contain a carbohydrate.

  • Glycosphingolipids:

These are sphingolipids with one or more carbohydrate attached to them.

  • GM2 and GM3:

Both are simple gangliosides which serves as building blocks for complex ones.

  • Golgi Apparatus:

A membrane bound compartment of the cell which acts a branch point between proteins destined for the lysosomes via the plasma membrane.


  • HMGCoA Reductase:

An enzyme that operates within the cells own cholesterol manufacturing pathway. This pathway is known as the de-novo pathway, or as new. This is a separate pathway to the LDL method of obtaining cholesterol.

  • Haploid:

Cells that contain one single set of chromosomes, such as; the sex cells, gametes, and red blood cells.

  • Heterozygous:

A heterozygous condition exists if the corresponding genes on each of the related pair of chromosomes are different to each other. The different genes may be normal, or disease causing.

  • Homeostasis:

In the context of NPC, homeostasis is a state of balance of the cholesterol content of a cell. This takes place when the supply of cholesterol has met the demanded amount, so as a result the cell has stopped importing via LDL and manufacturing.

  • Homologies:

Similarities in DNA or protein sequences between individuals of the same species (or among that of different species).

  • Homozygous:

A homozygous condition exists if the corresponding genes on each of the related pair of chromosomes are the same. The genes may both be normal, or disease causing.


  • LDL:

This stands for low density lipoprotein’. Manufactured in the liver and distributed to the other organs and cells of the body via the blood stream. The LDL particles contain esterified cholesterol within a membrane and also have a protein called ‘Apo B100’ which forms a strong bond with the LDL receptors on the surface of the cells. Too much LDL in the blood is considered to be a bad sign as it is an indicator of heart disease, and can consequently lead to strokes and other health problems.

  • LDL Receptor:

A receptor on the surface of a cell that binds LDL (low density lipoprotein) particles prior to transferring them to the interior of the cell. The receptor appears in abundance during cellular cholesterol demand, and then diminishes to a low level once the demand has been satisfied.

  • Lipid:

An organic molecule that is insoluble in water. This property is important for the formation of membranes (very high lipid content) which allows for the separation of different biologically active solutions.

  • Lysosomes:

These are membrane-bound organelles within the cell whose task is to recycle biochemical molecules for re-use. They contain a variety of enzymes for this purpose.


  • Meiosis:

A special type of cell division that produces sex cells (ova, or sperm) each containing half or a haploid set of chromosomes.

  • Membranes:

Two layers of lipids made up of phospholipid bilayers which contain cholesterol and proteins.

  • Metabolism:

This defines all of the processes taking place within living cells. The main processes are anabolic and entail either the building up of complex molecules from simpler ones, and others are catabolic and involves the breaking down of complex molecules into simpler ones.

  • Mitochondria:

Cellular organelles that produce a large amount of the cells’ energy requirements. Mitochondria contain their own DNA which is inherited only through the female line.

  • Mitosis:

A complex process of cell division whereby two cells are produced from one, each with its own complement of chromosomes, membranes, organelles and cytosol.

  • Mutagen:

This is an agent that is capable of increasing mutation rate in an organism – an example of this can be found in excessive X-rays.

  • Mutations:

Heritable changes in the DNA of cells. Often these changes are potentially detrimental to the operation of the cell.


  • Organelle:

A sub-unit of the cell which is usually bound by a membrane, and is distinct in its composition and functional behaviour. Examples of an organelle include; the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, lysosomes, and endosomes.

  • Ova:

Plural of ‘ovum’, the female sex cell or egg. The ova are haploid cells.


  • Permease:

A protein pump that permits efficient movement of molecules through membranes – this protein helps molecules permeate (cross) the membrane.

  • Phenotype:

The outward physical symptoms (appearance) of an individual.

  • Phospholipid:

This is another building block of membranes which, along with cholesterol, forms a barrier for the cell from the outside and also for smaller sub-compartments of the cell. Gangliosides are an example of phospholipids.

  • Plasma Membrane:

This is the outer boundary of cells, also known as the ‘cell membrane’.

  • Polymorphisms:

Variations of genes (DNA) that may improve, degrade, or leave unchanged, the capability of the associated protein. They contribute towards the differences between individuals in a population.

  • Proteins:

Large organic molecules that perform many of the metabolic activities taking place within a cell. Typical roles of these proteins are as enzymes, transporters, communications, sensors, activators, and structural elements. The proteins are constructed from chains of amino acids which are linked together in a sequence defined by the DNA of genes.

  • Purkinje Cells:

These are thought to be the only cells of the cerebellum (where they form a single layer of large neurons) with outgoing connections to the rest of the brain. These cells are vulnerable to damage and death in those affected by NPC.


  • RNA:

This stands for ‘ribonucleic acid’which is a chemical found in the nucleus and cytoplasm of cells. It plays an important role in the formation of proteins and other chemical activities of the cell. The structure of RNA is similar to that of DNA.

  • Recessive:

The effects of a gene that are masked by the activity of the same gene on the other chromosome (see dominant).

  • Ribosomes:

These are small cellular components which serve as the site of protein synthesis, and are composed of specialised ribosomal RNA and protein.


  • Sequencing:

The determination of the order of base sequences (also known as ‘nucleotides’) in a DNA or RNA molecule. Sequencing can also refer to the order of amino acids in a protein.

  • Sex-chromosomes:

Non-autosomal chromosomes; X and Y inherited from parents. XX is female, whereas XY is male.

  • Sex-linked:

These are traits (which may be either normal or disease causing) that are derived from the genes or DNA of the X, Y chromosomes, or mitochondrion (as opposed to autosomal).

  • Sperm:

A male sex cell (or gamete) which contains one haploid set of chromosomes.

  • Sphingolipids:

This is a type of molecule found in all plant and animal cells. They are particularly abundant in the tissues of the nervous system.

  • Sphingomyelin:

A complex organic molecule that is present in membranes in association with cholesterol. Especially found in neurons, this is not processed correctly by those individuals affected by both Niemann-Pick type A, and type B.

  • Steroids:

These are any of a large class of organic compounds with a characteristic molecular structure containing four rings of carbon atoms, examples of which include cholesterol, bile acids, some vitamins (including Vitamin D) and steroid hormones.

  • Substrate:

A substance on which an enzyme acts to form either a more complex substance or a simpler substance.

  • Systematic:

Affecting or pertaining to the body as a whole.


  • Transcribe:

In terms of the cell, ‘transcribe’ refers to the process of turning the information contained on a RNA template into a protein. This task is conducted by organelles called ribosomes.

  • Translate:

The process of using RNA to produce a protein.

  • Transporter:

A protein that permits/promotes movement within the cell or across membranes; this can act as both a permease or as a mediator of transport vesicle fusion events.



This is a small structure within a cell which consists of fluid enclosed by a lipid bilayer which form naturally during the processes of secretion, uptake, and transport of materials within the cytoplasm. Vesicles perform a variety of functions; including metabolism, buoyancy control, and enzyme storage (to name but a few).


  • X-linked:

These are traits associated with the X chromosome which may be normal or disease causing.


  • Zygote:

A fertilised ovum, which contains two haploid cells (sperm and ovum), which combine to form a diploid cell.

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