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Term Definition
Hairy Cell Leukemia A rare version of chronic leukemia in which abnormal white blood cells, under the microscope, appear to be covered with tiny hairs. "
Haplotype (1) A set of genes that code for different proteins but are in close enough physical proximity along a chromosome that they tend to be inherited as a unit. An individual’s gene sequence with respect to one member of a pair of allelic gene sequences; two people are of the same haplotype (but different genotype) if alike with respect to one allele of a pair but different with respect to the other allele of a pair. (2) A series of DNA sequences that are in close enough physical proximity along a chromosome that they tend to be inherited as a unit (3) The phenotype(s) determined by (1) or (2). Derived from haploid genotype.
HbA1c Short for hemoglobin A1c, it is a form of hemoglobin that has glucose attached to it. Because the glucose tends to remain attached for the life of the cell, approximately 4 months, measurement of HbA1c in the blood tends to reflect a person's average blood glucose level for that period of time. Thus, it is the classical test measurement used to follow the effectiveness of diabetes mellitus control. The recommended frequency of measurement is 3 to 6 months. Normal HbA1c is less than 7%; diabetics rarely achieve this target. Above 9% is considered poor control, and above 12% is extremely poor control.
HDL Short for “high density lipoprotein,” HDL cholesterol is the “good” cholesterol, as elevated HDL levels are associated with a decreased risk of coronary heart disease. See “cholesterol” entry.
Head and Neck Cancer (1) Colloquialism for squamous cell carcinomas arising from structures of the head and/or neck (2) Any malignant neoplasm arising in the head and/or neck
Health-Related Quality of Life (HRQOL) The degree to which aspects of optimal quality of life (QOL) are related to optimal health (typically mental, perceived physical, social, sexual, and role functioning). As a goal of much of health care is stated to be improved QOL, a significant challenge becomes its measurement such that (1) improvement can be identified and (2) cost-benefit and cost-utility analyses can be conducted. On an individual level, HRQOL is determined from standardized, questionnaire-derived measurements of an individual’s cognitive and affective response to their health status and, among other things, positively correlates with increased compliance with prescribed drug regimens. Such compliance is known to increase both prescription filling and sales. On a population level HRQOL, often measured in part by quality adjusted life years (QALYs) is often used in analyses of cost-benefit or cost-utility. A QALY is a year of life adjusted for its quality or value. A year of perfect health is equal to 1.0 QALY. The value of a year in ill health is discounted depending upon its severity.
Hedgehog A family of proteins that signals cells, best characterized in the nervous system, to progress down specific developmental pathways. Members of the hedgehog family include sonic hedgehog, patched, smoothened, and the cubitus interruptus / Gli family of transcription factors. Sonic hedgehog was originally isolated as a vertebrate homologue to the fruit fly protein hedgehog. Hedgehog proteins are hyperactive in certain proliferative disorders of both the central nervous system (CNS) (such as medulloblastoma) and periphery (such as various tumors) and are underactive in certain neurodegenerative disorders or CNS developmental anomalies such as holoprosencephaly (the complete or partial absence of a forebrain and related structures). Interfering with hedgehog activity has therapeutic potential in proliferative disorders whereas augmenting its activity has therapeutic potential in degenerative and developmental disorders.
Hematology The study of the blood and the organs that form its components.
Hemodynamics (1) The branch of physiology that deals with blood circulation. (2) The forces or mechanisms involved in blood circulation.
Hemoglobin The oxygen-carrying pigment and primary protein of red blood cells.
Hemophilia (1) An X-linked single gene bleeding disorder. Blood does not clot efficiently and abnormal bleeding occurs. Hemophilia A is caused by lack of blood clotting factor VIII while hemophilia B is caused by lack of factor IX. (2) Acquired hemophilia is due to a molecular inhibitor of either of these factors.
Hemorrhage Bleeding.
Heparin A type of anticoagulant (anti-clotting) medication. The natural mucopolysaccharide found most prominently in the liver.
Hepatectomy Surgical removal of the liver.
Hepatitis B A specific type of infectious inflammation of the liver caused by the Hepatitis B Virus.
Hepatitis C Liver inflammation caused by the Hepatitis C Virus (HCV).
Hepatocellular carcinoma The most common sort of liver cancer.
HER2 Gene A gene overexpressed in approximately 30% of metastatic breast cancers. Its overexpression is associated with poor clinical outcomes such as metastasis, resistance to chemotherapy, and short survival. This gene also tends to be overexpressed in a substantial proportion of patients with poor-outcome ovarian cancer and non small cell lung cancer. Its expression in normal, healthy adult tissue is low. The gene encodes a specific receptor in the epidermal growth factor receptor (EGFR) family, a cadre of receptors known to be overexpressed in cancers of the head and neck, kidney, bladder, prostate, and colon.
Herceptin Genentech’s monoclonal antibody formulation directed against the HER 2 gene, for the treatment of breast cancer.
Herzyme (Élan / Ribosome) Developmental ribozyme formulation for the treatment of metastatic breast cancers where the gene HER2 is overexpressed. The ribozyme recognizes, binds, and can enzymatically digest specific mRNA sequences.
Hippocampus A deep forebrain structure that helps regulate emotion and memory.
Histamine A substance with a fundamental role in the biochemistry of allergy. It dilates blood vessels and makes blood vessel walls leaky. Pharmacologically antagonizing its effects with "antihistamines" tends to provide relief from many allergic symptoms.
Histones A group of proteins found in the nucleus of every cell. DNA wraps around histones, which allows compression of the DNA into the small space available in the nucleus. Histones have several other functions such as regulating gene expression.
HIV (Human Immunodeficiency Virus) The retrovirus that causes AIDS (Acquired Immunodeficiency Syndrome).
HLA-DR Antigen A major histocompatibility complex (MHC) protein. MHC proteins present antigens to T cells. Their sequence varies widely between individuals, making them key factors in graft rejection.
HMG CoA Reductase Abbreviated name of 3-Hydroxy-3-methylgluatryl coenzyme A (CoA) reductase (HMGR). This enzyme functions in cholesterol metabolism to catalyze the conversion of HMG-CoA into CoA and mevalonate, the rate limiting step of cholesterol synthesis. Drugs that inhibit this chemical reaction, “statins,” have been effective in lowering blood cholesterol levels to provide cardiovascular event risk reduction.
Homocysteine A type of amino acid typically formed in the body as a byproduct of meat consumption. Homocysteine can damage blood vessels by injuring their lining cells and by stimulating the growth of smooth muscle cells. Elevated blood levels of homocysteine have been associated with elevated risks of coardiovascular disease; cardiovascular events including heart attacks, strokes, and peripheral vascular disease; venous thromboembolism; Alzheimer disease; and poor obstetrical outcomes. The most studied ways to decrease blood levels of homocysteine are eating less meat and taking folate, B6, and B12 vitamin supplements. Though homocysteine was found to be a cardiovascular risk factor prior to cholesterol being found to be one, scientific and clinical attention has only recently focused on this crucial amino acid.
Hormone A substance secreted in the body and carried through the bloodstream to various tissues, where it serves a regulatory function.
Hormone Replacement Therapy (HRT) (1) Combination therapy of estrogen plus a progestogen. Estrogen replacement therapy (ERT) treats menopause by minimizing its short-term changes such as hot flashes, disturbed sleep, and vaginal dryness. ERT can also prevent osteoporosis when it is associated with low, circulating estrogen levels, as well as reduce the risk of heart attacks or sudden death by up to 50%. Vaginal ERT can alleviate vaginal dryness, more severe vaginal changes, and bladder effects. When ERT is unopposed (used without a progetogen), it is associated with an increased risk of endometrial cancer (cancer of the lining of the uterus). Progestogen significantly reduces this risk. (2) Replacing any hormone of low circulating levels.
HPC2 Gene A gene whose mutations contribute to a susceptibility to familial prostate cancer.
H-ras A small signaling protein in the G protein family involved in intracellular signal transduction. Overactive H-ras activity is associated with tumor formation.
HuMab Mouse The proprietary mouse strain from Medarex that produces fully human antibodies.
Human Anti-Mouse Antibodies (HAMA) Antibodies produced in humans during an immune response against transplanted mouse proteins. HAMA responses are often seen when mouse monoclonals are used as therapeutics, and can lead to the immune rejection of the mouse monoclonal.
Human Genome Project Begun as the Human Genome Initiative, it is the collective name for several projects begun in 1986 by The United States Department of Energy (DOE). The goals of the Initiative were to (1) create an ordered set of DNA segments from known chromosomal locations, (2) develop new computational methods for analyzing genetic map and DNA sequence data, and (3) develop new techniques and instruments for detecting and analyzing DNA. The DOE initiative is presently known as the Human Genome Program, and the national combined effort, led by DOE and the National Institutes of Health (NIH), is known as the Human Genome Project. The United States Human Genome Project officially began October 1, 1990 as part of an international effort to determine the location and structure of the entire set of human genes. It continues in earnest, having a completed, published first draft in Feb., 2001, and with intermediate and final sequence milestones to be completed by June 2001, and no later than the year 2003, respectively.
Human Papilloma Virus A group of more than 60 viruses responsible for causing warts. Most produce warts on the hands, fingers, and face. However, the notorious, sexually-transmitted versions are confined primarily to the moist skin of the genitals, producing genital warts and increasing the risk for cervical cancer. "
Humanized A technology that assembles monoclonal antibodies by attaching several small pieces of the antibody derived from a mouse to the basic framework of a human antibody. The goal is to decrease the chance of an adverse immune response.
Huntington Disease A progressive neurological disorder caused by a mutation in a gene on chromosome 4 and characterized by abnormal body movements and early onset dementia. It is one of several neuropsychiatric conditions caused by an expanded set of repeated elements along the length of a chromosome (trinucleotide repeats). Trinucleotide repeat expansion in Huntington disease is along a specific stretch of chromosome 4.
Hybridoma An immortal cell with unique properties made in the laboratory from the fusion of a tumor cell and a normal cell.
Hyperglycemic (1) Relating to a state of hyperglycemia — excess of sugar in the blood. (2) An agent designed to raise blood sugar.
Hyperglycosylation Related to having abnormally abundant attached carbohydrates.
Hyperparathyroidism Excessive parathyroid hormone (PTH) production by the parathyroid glands. PTH regulates calcium and phosphorus balance in the body. The disorder can be of a primary parathyroid gland etiology or secondary to other primary pathologies. A tertiary form also exists.

Primary hyperparathyroidism involves excessive PTH production due to enlargement of one or more of the 4 tiny parathyroid glands located at the front and base of the neck. Effects of increased calcium manifest in several body systems including the skeletal, gastrointestinal, renal (kidney), muscular, cardiovascular, and central nervous system. Most commonly, primary hyperparathyroidism is seen in people over 60 years of age with women more likely to be affected than men. Radiation to the head and neck increases risk as does the rare parathyroid carcinoma.

Secondary hyperparathyroidism, from disorders associated with hypocalcemia (low levels of calcium in the blood), can arise in many settings and can also involve significant disorders of phosphate balance. Contributors can be disorders of vitamin D (such as osteomalacia [rickets], vitamin D deficiency, vitamin D malabsorption, and abnormal vitamin D metabolism induced by drugs), disorders of phosphate metabolism (such as kidney disease, malnutrition, malabsorption, aluminum toxicity, cancers, and phosphate depletion), calcium deficiency (not enough in the diet, too much lost in the urine), and especially chronic renal failure.

Chronic renal failure-induced secondary hyperparathyroidism is especially important and complex. It is a disorder in which phosphate clearance is impaired, phosphate is released from bone, vitamin D production is diminished or absent, intestinal calcium absorption is low, and blood levels of calcium are diminished. A feedback loop accentuates increased bone resorption (bone breaks down in an attempt to regulate abnormal levels of calcium, phosphate, and other entities) and hyperphosphatemia (high levels of phosphates in the blood) ensues which causes accelerated secondary hyperparathyroidism. Various therapies can contribute to high levels of calcium and phosphorous in the blood that can have deleterious effects on multiple organ systems.

Hypertension Persistently high arterial blood pressure, from one of a number of different mechanisms. Hypertension is a risk factor for the development of heart disease, peripheral vascular disease, stroke, and kidney disease.
Hypoglycemic (1) Relating to a state of hypoglycemia — deficiency of sugar in the blood. (2) An agent designed to lower blood sugar.
Hypoperfusion Underperfusion. Not enough blood, and therefore oxygen, getting to tissues or organs.
Hypotension Low blood pressure.
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