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What You Need to Know About Medical and Genetic Research
Emerging Ethical, Legal, and Societal Issues Arising From Genetic Research
What You Need to Know About Informed Consent




Patient Education


What You Need to Know About Genetics

Written by Ruth Papazian, MS
Reviewed by Gerry Klein, MD


Introduction

The Human Genome Project (HGP) is an international research program initiated by the U.S. Department of Energy and the National Institutes of Health in 1987 to understand genetic heredity from one generation to the next.1 The main mission of the HGP is to characterize the structure, organization and function of human DNA.

This is being accomplished with the creation of a map of the human genome that details the nucleotide sequence of all 46 chromosomes, as well as the estimated 30,000 to 100,000 genes comprising our DNA.2

Over the past decade, genetic research has shown that most diseases arise from genetic mutations or polymorphisms passed along from parents to children that can result in a serious inherited disorder (such as Huntington's Disease), predispose to a chronic condition (such as high blood pressure) or increase vulnerability to infectious disease or to environmental toxins.

Genetic variations from one person to the next also affect responsiveness to medication and susceptibility to treatment-limiting side effects - and help explain why the same medication at the same dose improves one patient's symptoms, has no effect on another patient's symptoms and may cause a severe adverse effect in a third patient.



Table of Contents
1. Introduction
2. The ABCs of DNA and SNPs
DNA Genetic researchers are examining the genomes of people who are helped and who are harmed by particular medications to determine what the differences are between these two groups at the molecular level. This type of research is called "pharmacogenetics."3

For instance, consider variation in a gene that codes for the production of an enzyme that metabolizes (digests) a medication to treat high blood pressure. If someone inherits a version of the gene that causes the body to pump out too much of this enzyme, the medication would be metabolized and cleared from the body too quickly to adequately lower blood pressure.

Doctors now use trial-and-error to determine which drug is the safest and most effective for a particular patient, and some patients are switched from one medication to another until they and their doctors are satisfied. The goal of pharmacogenetic research is to enable doctors to practice "personalized medicine" - disease prevention strategies and treatments devised for each individual patient based on his or her one-of-a-kind genetic profile.


The ABCs of DNA and SNPs

To help you understand the basic concepts involved in genetic and pharmacogenetic research, here's a list of terms you are likely to see in newspaper and magazine articles on this Web site and elsewhere announcing new breakthroughs.

What is DNA?
DNA (deoxyribonucleic acid) is the chemical of which genes and chromosomes are made. DNA, which is found in every cell of your body, is a molecule made up of two strands to which nitrogenous bases, known as nucleotides, are attached. There are four nucleotides: adenine (A), thymine (T), guanine (G) and cytosine (C). Like magnets that are attracted to each other, these bases pair up - A with T and G with C - to hold the two strands together and cause them to wind around each other like a twisted ladder.4 About five percent of DNA in a chromosome consists of genes; scientists aren't yet sure what the rest of the DNA does.
Slideshow: What is DNA?

What is a gene?
Genes are found on chromosomes. Each gene is a particle of DNA that determines the characteristics, or traits, of an individual and are passed on from parent to child.5 We have two copies of every gene, one from each parent. A gene is a single component of a person's genome. The human genome is estimated to comprise 30,000 to 100,000 genes, with each gene ranging in size from fewer than one thousand to several million nucleotide bases.6

What do genes do?
Each gene consists of a sequence of nucleotides that encode instructions for making a specific protein. Different sequences of adenine (A), thymine (T), guanine (G) and cytosine (C) result in different proteins.

nucleotides Proteins are essential to the structure and function of every cell in your body. For instance, insulin regulates blood sugar, collagen keeps blood vessels elastic and melanin gives hair and skin its color. Everyone has slightly different versions of some genes that code for these and other proteins. In some cases, genetic variations are linked to a disease or disorder - a mutation or polymorphism in a gene involved in melanin production could cause albinism (lack of pigment in the skin and hair). In other cases, polymorphisms are not linked to a disease, as when different versions of a gene involved in melanin production result in one person having blond hair, another having black hair and a third having auburn hair.

What is a chromosome?
Most cells in your body contains a nucleus, which is where your chromosomes are found. Each chromosome, which looks something like a Gummi worm, is made up of protein and a single molecule of DNA. The normal human genome comprises two sets of 23 chromosomes - one set from each parent. Among these 46 chromosomes is one pair of sex chromosomes that determine gender (your mother can only pass on X chromosomes so if you got a second X chromosome from your father, you're female and if you got a Y chromosome from your father, you're male). The other 44 chromosomes are called autosomes.

What do chromosomes do?
DNA molecules are very long - consisting of anywhere from 50 to 250 million bases- and chromosomes keep DNA tightly bundled in a neat package.

What is a polymorphism?
Polymorphisms are sequence variations in DNA.7 The simplest type of sequence polymorphism is a Single Nucleotide Polymorphism (or SNP, pronounced "snip"), in which a nucleotide in a base pair is substituted for another nucleotide. When this happens, a nucleotide sequence that might look like AACCAAG instead looks like AACTAAG. Each of the two variations of the gene is called an allele.8 Polymorphisms involve alleles that occur in at least one percent of the population.
Slideshow: Polymorphism and Mutations

What is a mutation?
Most people use the term "mutation" to refer to a harmful genetic variation associated with a specific human disease or disorder. Like a polymorphism, a mutation is a variation in DNA sequence - but mutations are rarer, occurring in less than one percent of the population. Mutations cause cells to produce proteins that function abnormally or are non-functional, which can result in disease or adverse reaction to medicine.

What is a genome?
genome The word "genome" was coined around 1930, and is a combination of "gene" and "chromosome." A genome is the total amount of DNA in the genes and chromosomes of a particular organism, typically expressed in the number of base pairs. The normal human genome consists of three billion base pairs. Since every cell in your body contains all 46 chromosomes, every cell also contains your entire genome.9

What is a genotype?
Except for identical twins, everyone has his or her own unique genotype. One copy of every chromosome - and all of the genes on it - is inherited from your mother (via an egg cell), and the other copy is inherited from your father (via a sperm cell). The gene inherited from Mom can have the identical base pair sequence as the one inherited from Dad. Or each copy of the gene can have a slightly different nucleotide sequence. It's the unique combination of genes that are exact duplicates of each other and those that are slightly different at a specific location on each set of chromosomes that make up a person's one-of-a-kind genotype.

What is a phenotype?
The physical characteristics of a person make up his or her phenotype - which are often, but not always, determined by his or her genotype.10 For instance, hair color can be inherited or can be store-bought.

What is genome mapping and genome sequencing?
Genome maps and genome sequences depict an individual's DNA in different ways. A genome map identifies landmarks (such as short base sequences or regulatory sites that turn genes on and off) that help scientists find specific genes on your chromosomes.11 A genome sequence specifies the order of every nucleotide base in every gene on every chromosome.12

There are two kinds of genome maps - genetic maps and physical maps. Genetic maps show the order of genes on a chromosome, and the relative distances between those genes (the closer the genes, the more likely they will be inherited together, or linked).13 Physical maps show the base pair distances from one landmark to another.

How will genome sequencing and mapping help scientists figure out better ways of preventing, diagnosing and treating disease?

Genome mapping and sequencing are complementary techniques that help scientists find the locations of genes and polymorphisms. In particular, scientists are looking for SNPs that:
  • Help elucidate disease pathways and processes;
  • Are associated with susceptibility to such complex, chronic diseases as diabetes and heart disease - either as predictive indicators, or as drug targets; and
  • Hamper the efficacy of medications, or cause adverse side effects.
Scientists are looking for genes and polymorphisms that determine individual response to medication by affecting absorption, distribution, metabolism, excretion (ADME) or toxicity. There may be as many as 100 such genes or polymorphisms, and patients may one day be able to get screened for them the way they are screened for high cholesterol. And doctors may be able to check a patient's genetic profile to predict his or her response to a drug before it is prescribed.


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4Moneger F: Molecular and evolutionary analysis of a plant Y chromosome. C R Acad Sci III 2001, 324:531-5
5 Lalande Met al: Parental imprinting and human disease. Annu Rev Genet. 1996, 30:173-95
6 Gassterland T et al: Whole-genome analysis: annotations and updates. Curr Opin Struct Biol 2001 11: 377-81
7Merck Manual Home Edition, Merck & Co. Inc. 2001.
8Hickman WD et al: Elite swimmers and the D allele of the ACE I/D polymorphism. Hum Genet 2001, 108:230-2
9Mahner M: What exactly are genomes, genotypes and phenotypes? And what about phenomes? J Theor Biol 1997, 186:55-63
10 Summers KM: Relationship between genotype and phenotype in monogenic diseases: relevance to polygenic diseases. Hum Mutat 1996, 7:283-93
11 The human genome. Science genome map. Science 2001, 291:1218
12 Guigo WT et al: Genome sequence comparisons: hurdles in the fast lane to functional genomics. Brief Bioinform 2000, 1:381-8
13 Rojas K et al: Integration of the 1993-94 Genethon genetic linkage map for chromosome 18 with the physical map using a somatic cell hybrid mapping panel.



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