HD Glossary

Note: Some words can have differing definitions depending on the context in which they are used. The definitions presented here are the ones that best apply to the words as they are used. HDSA is grateful to the Stanford Hopes website for their contribution and maintenance of this glossary.

 

 

M
  • MAC - Abbreviation for membrane attack complex.
  • macrophage - An immune cell found throughout the body. Macrophages act as scavengers that engulf dead cells, foreign substances, and other debris.
  • mad cow disease - Another term for bovine spongiform encephalopathy (BSE).
  • magnesium ion (Mg2+) - A single magnesium ion blocks the opening of the NMDA receptor, preventing the influx of calcium ions into the cell. This magnesium ion is removed only when the electrical charge inside the cell rises above a certain value. This increase in charge is mediated by the binding of glutamate to non-NMDA receptors.
  • mammal - Any of various warm-blooded vertebrate animals, including humans, characterized by a covering of hair on the skin and, in the female, milk-producing mammary glands for nourishing the young.
  • malonate - An inhibitor of complex II of the electron transport chain.
  • marker - A variable segment of DNA, often located near a gene. An allele at the marker is typically inherited along with the corresponding allele at the nearby disease gene.
  • mass - The amount of matter in an object; often used interchangeably with weight. (Weight is actually the force with which an object is attracted toward the center of the earth.)
  • maximum tolerated dose (MTD) - The highest daily dose of a drug that the average human body can tolerate before passing a threshold level of excessive toxicity
  • median - The middle value in a set of numbers arranged in increasing order. If there is an even number of values, then the median is the average of the middle two values. For example, the median of the set {10, 12, 14, 19, 20} is 14. The median of the set {2, 3, 4, 6, 8, 9} is 5, which is the average of 4 and 6.
  • medium - A nutrient-rich liquid used in tissue cultures.
  • medulla - Also known as the medulla oblongata, this region of the brain is concerned with vital functions like breathing, blood circulation, vomiting, and swallowing.
  • meiosis - A reproductive process involving two successive divisions of a cell, resulting in four daughter cells. Unlike what occurs in mitosis, the daughter cells produced in meiosis are not identical to each other. Meiosis is the process by which sperm and egg cells are made.
  • MELAS - Mitochondrial encephalomyopathy lactic acidosis and stroke-like episodes; a common type of mitochondrial myopathy (MM). MELAS is one of a group of rare muscular disorders called mitochondrial myopathies. It is caused by a defect in the gene encoding for certain proteins/components of the mitochondria and results in disorders in the brain and muscles. Characteristics of MELAS include stroke-like episodes, seizures, vomiting, hearing loss, and elevated lactate levels. Also referred to as mitochondrial myopathy, encephalomyopathy lactic acidosis, and stroke-like episodes.
  • melatonin - A compound that is believed to decrease the presence of beta-amyloid fibrils, but has little or no success in inhibiting huntingtin protein aggregation.
  • memantine - A well-tolerated drug that acts as an NMDA receptor antagonist, preventing glutamate toxicity. It may slow the progression of HD.
  • membrane - A flexible layer surrounding a cell, organelle (such as the nucleus), or other bodily structure. The movement of molecules across a membrane is strictly regulated in both directions.
  • membrane attack complex (MAC) - A complex of complement proteins that assemble together to form a pore across the membrane of the cell. The MAC allows the entrance and exit of various ions and substances, resulting in the death of the cell.
  • membrane fluidity - The motion of the phospholipids within the cell membrane, dependent on the membrane´s fatty acid composition and cholesterol content.
  • menstruation - The discharging of blood, secretions, and tissue debris from the uterus that occurs in non-pregnant females of childbearing age at approximately monthly intervals.
  • MERRF - Myoclonus epilepsy with ragged-red fibers; a common type of mitochondrial myopathy (MM).
  • messenger cascade - A process in which an initial message is greatly amplified in a cascade of ensuing messages, resulting in cellular change.
  • messenger RNA (mRNA) - The mediating molecule between DNA and protein synthesis.
  • metabolic - Pertaining to metabolism, the process of breaking down food and producing energy.
  • metabolism - The process of cells burning food to produce energy. This is similar to a car's engine burning gasoline to produce the energy that is used to rotate the car's wheels. The cell’s mitochondria acts as the car’s engine, the food we eat acts as the gasoline, and the energy we need to move and think is similar to the energy used to move the car. Also referred to as energy metabolism.
  • methionine - An amino acid that must be obtained through the diet because it cannot be produced by humans.
  • methionine cycle - The cycling of methionine to SAM, homocysteine, and back to methionine.
  • methionine synthase (MS) - Along with methyltetrahydrofolate, methionoine synthase is an enzyme responsible for the addition of a methyl group to the sulfur atom of homocysteine to form methionine.
  • methyl group - A chemical group made up of one carbon atom and three hydrogen atoms that is added to histones to silence genes by causing the DNA to coil up. Methylation of histones prevents further transcription.
  • methylation - A way to silence genes by adding a methyl group to a histone, causing the DNA to coil up into chromosomes and preventing further transcription.
  • methyltetrahydrofolate (methylTHF) - Along with methionoine synthase, methyltetrahydrofolate is an enzyme responsible for the addition of a methyl group to the sulfur atom of homocysteine to form methionine.
  • Mg2+ - See Magnesium ion.
  • MHC II - Proteins that bind to and “present” proteins of foreign substances on the surface of cells for recognition by other immune cells.
  • microglia - A type of glial cell that is activated in the inflammatory response. Sometimes called “brain macrophages.”
  • microgram (mcg) - A measurement of mass; commonly used in measuring dietary supplements (1 mcg= 10^-6 grams).
  • microRNA (miRNA) - A group of small RNA molecules, distinct from but related to small interference RNA (siRNA), that can prevent the expression of certain genes.
  • microtubule - An intracellular component of the cytoskeleton which aids in cell support, intracellular transport, and cell proliferation.
  • midbrain - A part of the brain that is mainly concerned with the control of eye movement. Also relays signals for auditory and visual reflexes.
  • mineralcorticoids - Steroids released from the adrenal cortex that maintain salt and fluid balance in the body.
  • mini mental state examination (MMSE) - A screening test designed to measure the cognitive (thinking) abilities in adults. It can be used to test if a person is cognitively impaired, to track the deterioration or improvement in cognitive functioning over time, or to rate the degree of cognitive impairment. It is scored on a scale of 30, with 0 being profoundly impaired and 24-30 normal for most adults. As a screening test, the MMSE is designed to yield falsely positive results (i.e. cognitive problems when there are none) as opposed to falsely negative results (i.e. no problems when problems do, in fact, exist). Because of this, low MMSE scores are usually followed up with more in-depth cognitive assessments for clinical diagnosis purposes.
  • minocycline - An antibiotic commonly used to treat arthritis and acne. Studies indicate that minocycline may have beneficial effects on people with HD due to the decreased caspase activity that results from minocycline treatment.
  • Miraxion - A novel compound that may function as a neuroprotectant by inhibiting harmful enzymes known as phospholipases and stabilizing the phospholipid components of cell membranes and mitochondria.
  • misfolded protein - proteins have a certain shape, or structure that they should have in the cell, and this structure is often very important to a protein’s correct function. Under various conditions, a protein can misfold or take the wrong shape, which can impair the function of the protein, render it useless or even harmful.
  • mismatch repair enzyme - An enzyme that recognizes and repairs incorrect pairings of nitrogenous bases in DNA. For instance, a mismatch repair enzyme would recognize when Cytosine (C) is mistakenly paired with Thymine (T). The enzyme would then replace Thymine with Guanine (G), resulting in a correct pair between Cytosine (C) and Guanine (G).
  • mithramycin - An antibiotic that binds to specific regions of DNA rich in guanine and cytosine to regulate transcription. While it is currently prescribed to treat certain types of cancer and a few other conditions, recent research shows that it is also helpful in treating motor symptoms and prolonging life in a mouse model of HD. Also known as MIT and plicamycin.
  • mitochondria - Plural form of mitochondrion.
  • mitochondrial creatine kinase (mtCK) - A protein found in the mitochondria that exists in two forms. The first form prevents the mitochondrial membrane from destabilizing. When mtCK is inactivated by free radicals, it transforms to the second form, making the membrane less stable.
  • mitochondrial myopathies (mm) - A group of neuromuscular diseases caused by damage to the mitochondria in cells.
  • mitochondrion - A part of the cell (organelle) that is responsible for energy production. The mitochondria take care of many specialized tasks, including converting nutrients into energy. (Plural form: mitochondria.)
  • mitogen-activated protein (MAP) kinase - An enzyme that is involved in numerous aspects of signal transduction.
  • mitosis - A reproductive process in cells that ultimately results in one cell dividing into two new cells, both identical to the original. The two new cells are called daughter cells.
  • mitotic spindle - Array of microtubules that forms during mitosis and physically separates duplicated chromosomes.
  • MLK2 - Abbreviation for mixed-lineage kinase 2, a protein that interacts with the normal huntingtin protein.
  • models - There are many parallels between biological systems in many types of creatures, so this has led to the establishment of a number of "model organisms". Scientists reproduce features of a disease of interest in an organism and then study it. For example, inserting the gene for HD into a mouse means that it will produce the altered HD protein in the brain. This creates an HD mouse model. To do the same experiments with people would be very time-consuming, not to mention unethical, but the consistent use of these models allows us to test ideas about biology in a reproducible way.
  • modifier - A gene that alters the effect of another.
  • molecular chaperones - Substances inside the cell that bind and stabilize proteins at intermediate stages of folding, assembly, movement across membranes, and degradation.
  • molecular pathways - A system where one molecule affects another, either by activating it or deactivating it. This molecule, in turn, affects another molecule, and so on. Any mutation or change in one part of the pathway has a cascading effect, changing everything that happens after the mutation.
  • molecule - The smallest unit of matter of a substance that retains all the physical and chemical properties of that substance, consisting of a single atom or a group of atoms bonded together. Example: A single water molecule (H2O) consists of just one single oxygen atom bonded to just two hydrogen atoms. There are thousands of water molecules in a single droplet of water. Molecules are too small to be seen without the assistance of powerful microscopes.
  • molecular compounds - A substance formed when two or more non-metal molecules combine
  • molecular structure - The size and shape of a molecule, determined by the atoms that make up the molecule. The structure of a molecule usually determines its function, which other molecules it interacts with, and how it affects those molecules.
  • monoamine oxidase (MAO) - An enzyme found primarily in the liver and nervous system that generates free radicals.
  • monomer - A single molecule or molecular unit that has the potential to be chemically bound to more molecules or units.
  • monounsaturated fat - A type of unsaturated fat in which there is only one double bond.
  • motor cortex - A region of the cortex that carries out the initial processing of motor information in the brain; involved in the control of movement.
  • motor neuron - A neuron that has the specific job of controlling muscle movement.
  • MPTP - A toxic molecule that causes damage to the nervous system, resulting in Parkinson’s Disease.
  • mRNA - Abbreviation for messenger RNA.
  • MSH2 - A mismatch repair enzyme that also repairs unpaired regions of DNA (such as bubbles resulting from polymerase slippage) and stops hairpins from forming. It is possible that this enzyme becomes altered and unable to function in the cells of people with Huntington’s disease.
  • mtCK - See mitochondrial creatine kinase.
  • mTOR - Stands for “target of rapamycin”; a protein that inhibits the process of autophagy.
  • multipotent - The ability to give rise to a number of limited cell types.
  • muscular system - The muscular system is the biological system of animals that allows them to move internally and externally.
  • mutagen - Something in the environment that causes mutations.
  • mutagenic - See mutation
  • mutant huntingtin - The altered form of the huntingtin protein caused by having the HD gene.
  • mutation - Any heritable change in DNA. There are many types of mutations, such as chromosomal mutations, point mutations, contractions, and expansions.
  • myelin - The fatty sheath coating the axons of the nerves; it allows for the efficient conduction of nerve impulses.
  • myoclonic seizure - A seizure characterized by myoclonus (sudden, brief muscle contractions).
  • myoclonus - Twitching or contraction of a muscle or group of muscles.
  • myorelaxation - Relaxation of the muscle.
  • myotonia - Frequent spasms of the muscles.