Gale Encyclopedia of Genetic Disorder / Gale Encyclopedia of Genetic Disorders, Two Volume Set - Volume 1 - A-L - I

Grabowski, Gregory A. “Current Issues in Enzyme Therapy for Gaucher Disease.” Drugs 52 (August 1996): 159-167.

NIH Technology Assessment Conference. “Gaucher Disease: Current Issues in Diagnosis and Treatment.” JAMA 275 (February 12, 1996): 548-553.

ORGANIZATIONS

Alliance of Genetic Support Groups. 4301 Connecticut Ave. NW, Suite 404, Washington, DC 20008. (202) 966-5557. Fax: (202) 966-8553. http://www.geneticalliance.org .

Children’s Gaucher Research Fund. PO Box 2123, Granite Bay, CA 95746-2123. (916) 797-3700. Fax: (916) 797-3707.http://www.childrensgaucher.org .

National Gaucher Foundation. 11140 Rockville Pike, Suite 350, Rockville, MD 20852-3106. (800) 925-8885. http://www

.gaucherdisease.org .

National Organization for Rare Disorders (NORD). PO Box 8923, New Fairfield, CT 06812-8923. (203) 746-6518 or (800) 999-6673. Fax: (203) 746-6481. http://www

.rarediseases.org .

WEBSITES

“Cerezyme.” Genzyme Therapeutics.

http://www.cerezyme.com .

“Gaucher Disease: Current Issues in Diagnosis and Treatment.”http://text.nlm.nih.gov/nih/ta/www/16.html .

“Living with Gaucher Disease: A Guide for Patients, Parents, Relatives, and Friends.” http://neuro-www3.mgh

.harvard.edu/gaucher/living.html .

National Foundation for Jewish Genetic Diseases (NFJGD).http://www.nfjgd.org/ .

Amy Vance, MS, CGC

I Gene

A gene is the fundamental physical and functional unit of heredity. It is an individual element of an organism’s genome and determines a trait or characteristic by regulating biochemical structure or metabolic process.

Genes are segments of nucleic acid, consisting of a specific sequence and number of the chemical units of nucleic acids, the nucleotides. In most organisms the nucleic acid is deoxyribonucleic acid (DNA), although in retroviruses the genetic material is composed of ribonucleic acid (RNA). Some genes in a cell are active more or less all the time, which means that they are continuously transcribed and provide a constant supply of their protein product. These are the “housekeeping” genes that are always needed for basic cellular reactions. Others may be rendered active or inactive depending on the needs and functions of the organism under particular conditions. The signal that masks or unmasks a gene can come from

outside the cell, for example, from a steroid hormone or a nutrient, or it can come from within the cell itself as a result of the activity of other genes. In both cases, regulatory substances can bind to the specific DNA sequences of the target genes to control the synthesis of transcripts.

In a paper published in 1865, Gregor Mendel (1823–1884) advanced a theory of inheritance dependent on material elements that segregate independently from each other in sex cells. Before Mendel’s findings, inherited traits were thought to be passed on through a blending of the mother and father’s characteristics, much like a blending of two liquids. The term “gene” was coined later by the Danish botanist Wilhelm Johannsen (1857–1927), to replace the variety of terms used up until then to describe hereditary factors. His definition of the gene led him to distinguish between genotype (an organism’s genetic makeup) and phenotype (an organism’s appearance). Before the chemical and physical nature of genes were discovered they were defined on the basis of phenotypic expression and algebraic symbols were used to record their distribution and segregation. Because sexually reproducing, eukaryotic organisms possess two copies of an inherited factor (or gene), one acquired from each parent, the genotype of an individual for a particular trait is expressed by a pair of letters or symbols. Each of the alternative forms of a gene is also known as alleles. Dominant and recessive alleles are denoted by the use of higher and lower case letters. It can be predicted mathematically, for example, that a single allele pair will always segregate to give a genotype ratio 1AA:2Aa:1aa, and the phenotype ratio 2A:1aa (where A represents both AA and Aa since these cannot be distinguished phenotypically if dominance is complete).

The molecular structure and activity of genes can be modified by mutations and the smallest mutational unit is now known to be a single pair of nucleotides, also known as a muton. To indicate that a gene is functionally normal, it is assigned a plus ( ) sign, whereas a damaged or mutated gene is indicated by a minus ( ) sign. A wild type Escherichia coli able to synthesize its own arginine would thus be symbolized as arg , and strains that have lost this ability by mutation of one of the genes for arginine utilization would be arg . Such strains, known as arginine auxotrophs, would not be able to grow without a supplement of arginine. At this level of definition, the plus or minus actually refer to an operon rather than a single gene, and finer genetic analysis can be used to reveal the exact location of the mutated gene.

The use of mutations in studying genes is well-illus- trated in a traditional genetic test called the “cis-trans test” which also gave the gene the alternative name, cistron. This is a complemetntation test that can be used to determine whether two different mutations (m1 and m2) occur

Gene

Genetic mapping

in the same functional unit, i.e., within the same gene or cistron. It demonstrates well how genes can be defined phenomenologically and has been performed successfully in microrganisms such as yeasts. It works on the principle that pairs of homologous chromosomes containing similar genes can complement their action. Two types of heterozygotes of the test organism are prepared. Heterozygotes are organisms with different alleles in the two homologous chromosomes, each of which was inherited from one parent. One heterozygote contains the mutations under investigation within the same chromosome, that is in the cis-configuration, which is symbolically designated /m1m2 (m1 and m2 are the two mutations under investigation and the symbol “ ” indicates the same position on the homologous chromosome in the unmutated, wild type state). The second mutant is constructed to contain the mutations in such a way that one appears on each of the homologous chromosomes. This is called the trans-configuration and is designated, for example, by m2 / m1. If two recessive mutations are present in the same cistron, the heterozygous trans-configuration displays the mutant phenotype, whereas the cis-configu- ration displays the normal, wild type phenotype. This is because in the cis-configuration, there is one completely functional, unmutated, cistron ( ) within the system that masks the two mutations on the other chromosome and allows for the expression of the wild type phenotype. If one or both mutations are dominant, and the cisand trans-heterozygotes are phenotypically different, then both mutations must be present in the same cistron. Conversely, if the cisand trans-heterozygotes are phenotypically identical, this is taken as evidence that the mutations are present in different cistrons.

In 1910, the American geneticist Thomas Hunt Morgan (1866–1945) began to uncover the relationship between genes and chromosomes. He discovered that genes were located on chromosomes and that they were arranged linearly and associated in linkage groups, with all the genes on one chromosome being linked. For example, the genes on the X and Y chromosomes are said to be sex-linked because the X and Y chromosomes determine the sex of the organisms, (in humans, X determines femaleness and Y determines maleness). Nonhomologous chromosomes possess different linkage groups, whereas homologous chromosomes have identical linkage groups in identical sequences. The distance between two genes of the same linkage group is the sum of the distances between all the intervening genes. A schematic representation of the linear arrangement of linked genes, with their relative distances of separation, is known as a genetic map. In the construction of such maps the frequency of recombination during crossing over is used as an index of the distance between two linked genes.

Advances in molecular genetics have allowed analysis of the structure and biochemistry of genes in greater detail. They are no longer the nebulous units described by Mendel purely in terms of their visible expression (phenotypic expression). It is now possible to understand their molecular structure and function in considerable detail. The biological role of genes is to carry, encode, or control information on the composition of proteins. The proteins, together with their timing of expression and amount of production, are possibly the most important determinants of the structure and physiology of organisms. Each structural gene is responsible for one specific protein or part of a protein and codes for a single polypeptide chain via messenger RNA (mRNA). Some genes code specifically for transfer RNA (tRNA) or ribosomal RNA (rRNA) and some are merely sequence that are recognized by regulatory proteins. The latter are termed regulator genes. In higher organisms, or eukaryotes, genes are organized in such a way that at one end there is a region to which various regulatory proteins can bind, for example, RNA polymerase during transcription, and at the opposite end there are sequences encoding the termination of transcription. In between lies the protein encoding sequence. In the genes of many eukaryotes, this sequence may be interrupted by intervening non-coding sequence segments called introns, which can range in number from one to many. Transcription of eukaryotic DNA produces pre-mRNA containing complementary sequences of both introns and the information carrying sections of the gene called exons. The pre-mRNA then undergoes post-transcriptional modification or processing in which the introns are excised and exons are spliced together, leaving the complete coding transcript of connected exons ready to code directly for the protein. When the central dogma of genetics was first established, a “one gene-one enzyme” hypothesis was proposed, but today it is more accurate to restate this as a one-to-one correspondence between a gene and the polypeptide for which it codes. This is because a number of proteins are now known to be constituted of multiple polypeptide subunits coded by different genes.

Judyth Sassoon, ARCS, PhD

I Genetic mapping

The aim of genetic mapping is to determine the linear sequence of genes in genetic material. The mapping can be performed at several levels of detail (resolution) that fall into two broad types: traditional genetic or linkage mapping and more detailed, physical mapping.

Linkage mapping shows the relative rather than absolute positions of genes along a chromosome and is a technique that has been used since the early 1900s. Early geneticists determined that genes were found on chromosomes. They also reasoned that because the various forms of genes, or alleles, could be precisely exchanged during meiosis through crossovers between homologous chromosomes, the genes for specific characteristics must lie at precise points along each chromosome. It followed that the mapping of chromosomes could, therefore, be made from the observation of crossovers. Between 1912 and 1915, the American scientist Thomas Hunt Morgan (1866–1945) hypothesized that if genes were arranged linearly along chromosomes, then those genes lying closer together would be separated by crossovers less often than those lying further apart. Genes lying closer together would thus have a greater probability of being passed along as a unit. It follows that the percentage of crossovers would be proportional to the distance between two genes on a chromosome. The percentage crossover can be expressed as the number of crossovers between two genes in meiosis. One genetic map unit (m.u.) is defined as the distance between gene pairs for which one product out of 100 is recombinant (a product of crossover). S recombinant frequency (R.F.) of 0.01 (1%) is defined as 1 m.u., and a map unit is sometimes referred to as a centimorgan (cM) in honor of Thomas Hunt Morgan.

As an example of how linkage mapping might work, suppose two characteristics, A and B, show a 26% crossover. Assign 26 crossover units to the distance between these two genes. If a characteristic C turns out in breeding experiments to have 9% crossover with B and 17% crossover with A, it would then be located between A and B at a point 9 units from B and 17 units from A. Compiling the information from many such breeding experiments creates a chromosome map that indicates the relative positions of the genes that code for certain characteristics. Accordingly, the further apart any two genes are on the same chromosome, the greater the incidence of crossing over between them.

A linkage map is limited because recombination frequencies can be distorted relative to the physical distance between sites. As a result, the linkage map is not always the best possible representation of genetic material.

While linkage maps only indicate relative positions of genes, physical maps are more accurate and aim to show the actual number of nucleotides between each gene. Restriction maps are constructed by cleaving DNA into fragments with restriction enzymes. These enzymes recognize specific short DNA sequences and cut the duplex. The distances between the sites of cleavage are then measured. The positions of the target restriction sites for these enzymes along the chromosome can be used as

DNA markers. Restriction sites generally exist in the same positions on homologous chromosomes so the positions of these target sites can be used rather like milestones along a road and can act as reference points for locating significant features in the chromosome.

A map of the positions of restriction sites can be made for a localized region of a chromosome. It is made by comparing the sizes of single enzyme breakages (digests) of the region of interest with double digests of the same region. This means that two different restriction enzymes are applied, one to each of two separate chromosome extracts of the region of interest, and subsequently the two enzymes are used together in a third digestion with the chromosome extract. The chromosome fragments resulting from the three digestions are then subjected to a biochemical procedure known as gel electrophoresis, which separates them and gives an estimation of their size. Comparison of the sizes of the chromosome fragments resulting from single and double restriction enzyme digestions allows for an approximate location of the target restriction sites. Thus, such maps represent linear sequences of restriction sites. As this procedure determines the sizes of digested chromosome fragments, the distances between sites in terms of the length of DNA can be calculated, because the size of a fragment estimated from an electrophoresis experiment is proportional to the number of base pairs in that fragment.

A restriction map does not intrinsically identify sites of genetic interest. For it to be of practical use, mutations have to be characterized in terms of their effects upon the restriction sites. In the 1980s, it was shown how restriction fragment length polymorphisms (RFLPs) could be used to map human disease genes. RFLPs are inherited by Mendelian segregation and are distributed in populations as classical examples of common genetic polymorphisms. If such a DNA variant is located close to a defective gene (which cannot be tested directly), the DNA variant can be used as a marker to detect the presence of the disease-causing gene. The prenatal examination of DNA for particular enzyme sites associated with certain hereditary diseases has proved to be an important method of diagnosis. Clinically useful polymorphic restriction enzyme sites have been detected within the Beta-like globin gene cluster. For example, the absence of a recognition site for the restriction enzyme HpaI is frequently associated with the allele for sickle-cell anemia, and this association has been useful in prenatal diagnosis of this disease.

The ultimate genetic map is the complete nucleotide sequence of the DNA in the whole chromosome complement, or genome, of an organism. Today, several completed genome maps already exist. Simple prokaryotic organisms, e.g., bacteria, with their relatively small

mapping Genetic

Gene mutations

chromosomes of one to two million base pairs were the first to be mapped. Later, eukaryotic organisms such as the yeast, Saccharomyces cerevisiae, and the nematode worm, Caenorhabditis elegans, were mapped. In 2000, the Human Genome Project produced the first draft of the human genome. The project adopted two methods for mapping the three billion nucleotides. The earlier approach was a “clone by clone” method. In this, the entire genome was cut into fragments up to several thousand base pairs long, and inserted into synthetic chromosomes known as bacterial artificial chromosomes (BACs). The subsequent mapping step involved positioning the BACs on the genome’s chromosomes by looking for distinctive marker sequences called sequence tagged sites (STSs), whose location had already been pinpointed. Clones of the BACs are then broken into smaller fragments in a process known as shotgun cloning. Each small fragment was then sequenced and computer algorithms, that recognize matching sequence information from overlapping fragments, were used to reconstruct the complete sequence inserted into each BAC. It was later argued that the first mapping step was unnecessary and that the algorithms used to reassemble the shotgunned DNA fragments could be applied to cloned random fragments taken directly from the whole genome. In this whole genome shotgun strategy, fragments were first assembled by algorithms into larger scaffolds and the correct position of these scaffolds on the genome was worked out by STSs. The latter method speeded up the whole procedure considerably and is currently being used to sequence genomes from other organisms.

Judyth Sassoon, ARCS, PhD

I Gene mutations

In a strict sense, mutations are changes in genes not caused by genetic recombination. A change in the base sequence of DNA, for example, represents a mutational change. Spontaneous mutations are mutations that occur at a given frequency without the need for an inducing agent of change (mutagenic agent). The term mutation is also used in a less technical sense to describe changes in the human genome (i.e., evolution) that result from a broad spectrum of processes that act to increase or decrease genetic variation within a population.

By definition, a gene is a hereditary unit that carries information used to construct proteins via the processes of transcription and translation. The human gene pool is the set of all genes carried within the human population.

Genetic changes, including mutations, can be beneficial, neutral or deleterious. In general, mutations, along with recombination and gene flow, act to increase genetic variation (i.e., the number of types of genes or alleles) within the human species.

The term mutation was originally used by Dutch botanist Hugo De Vries (1848–1935) to describe rapid changes in phenotype from one generation to the next. Subsequently, scientists used the term mutation to describe long-term, multi-generational, and heritable physical changes to genes.

Mutations generally occur via chromosomal mutations, point mutations, frame shifts, and breakdowns in DNA repair mechanisms. Chromosomal mutations include translocations, inversions, deletions and chromosome non-disjunction. Essentially there are five types of genetic rearrangements: deletions, duplications, inversions, translocations, and transposition.

Mutational deletions physically remove portions of genes (e.g., a portion of the DNA comprising the gene). Deletional mutations range from the single base point mutations to mutations that can span many functional genes. Chemical and radioactive agents account for the majority of induced point mutations. Scientists currently argue that most cancers and other degenerative diseases result from acquired genetic mutations due to environmental exposure, and not as an outcome of inherited traits. Chemicals capable of inducing genetic mutation (i.e., chemical mutagenesis or genotoxic compounds) are present a wide variety of natural and man-made products.

Point mutations may be nonsense mutations leading to the early termination of protein synthesis, missense mutations (a mutation that results an a substitution of one amino acid for another in a protein), or silent mutations that cause no detectable change. Accordingly, the effects of point mutational changes range from 100% lethality (all individuals die, usually early in fetal development) to no observable (phenotypic) change.

Duplications result in multiple copies of genes, and can occur as a result of unequal crossover or chromosome breaks. In addition, because some alteration of DNA is inevitable in the replication process, any mutation that hinders DNA repair mechanism will also increase the chance that a mutation will go uncorrected. Duplications also manifest a range of deleterious effects.

Inversions, which are changes in the orientation of gene bearing chromosomal regions, may cause deleterious effects if the inversion breaks through a gene critical for a particular protein or enzyme.

Translocations occur when one a portion of one chromosome becomes linked to a non-homologous chromosome (a chromosome outside its normal pairing) or

when portions of non-homologous chromosomes make a reciprocal exchange. Once again, the effect of such genetic change is a result of whether such translocations physically or functionally alter vital genes.

Recombination involves the reassortment of genes through new chromosome combinations. Recombination occurs via an exchange of DNA between homologous chromosomes (crossing over) during meiosis. Recombination also includes linkage disequilibrium. With linkage disequilibrium,variations of the same gene (alleles) occur in different combinations in the gametes (sexual reproductive cells) than should occur according to the rules of probability.

Gene flow occurs when individuals change their local genetic group by moving from one place to another. These migrations allow the introduction of new variations of the same gene (alleles) when they mate and produce offspring with members of their new group. In effect, gene flow acts to increase the gene pool in the new group. Because genes are usually carried by many members of a large population that has undergone random mating for several generations, random migrations of individuals away from the population or group usually do not significantly decrease the gene pool of the group left behind.

In contrast to mechanisms that operate to increase genetic variation, there are fewer mechanisms that operate to decrease genetic variation. Mechanisms that decrease genetic variation include genetic drift and natural selection.

Genetic drift results form the changes in the numbers of different forms of a gene (allelic frequency) that result from sexual reproduction. Genetic drift can occur as a result of random mating (random genetic drift) or be profoundly affected by geographical barriers, catastrophic events (e.g., natural disasters or wars that significantly affect the reproductive availability of selected members of a population) and other political-social factors.

Natural selection is based upon the differences in the viability and reproductive success of different genotypes with a population (differential reproductive success). Natural selection can only act on those differences in genotype that appear as visible (phenotypic) differences that affect the ability to attract a mate and produce viable offspring that are, in turn, able to live, mate and continue the species. The term evolutionary fitness describes the success of an entity in reproducing (i.e., contributing alleles to the next generation).

There are three basic types of natural selection. With directional selection, an extreme phenotype is favored (high or low body fat). Stabilizing selection occurs when an intermediate phenotype is fittest (e.g., body fat content

Polydactyly, which results in extra fingers or toes, is one type of genetic mutation. (Custom Medical Stock Photo, Inc.)

is neither too high nor low) and for this reason it is often referred to a normalizing selection. Disruptive selection occurs when two extreme phenotypes are fitter than an intermediate phenotype. In studying changes in the human genome, the operation of natural evolutionary mechanisms is complicated by geographic, ethnic, religious, and social groups and customs. Accordingly, the effects of various evolution mechanisms on human populations are not as easy to predict. Increasingly sophisticated statistical studies are carried out by population geneticists to characterize changes in the human genome.

K. Lee Lerner

I Gene pool

Definition

The term gene pool refers to the total sum of genetic information present in a population at any given time. A gene pool can be assigned to any set group or population. This is true for plants, animals, and humans alike. Each gene pool contains all of the inherited information for all of the traits of the members of the population.

Genetic information

Genetic information, in the form of deoxyribonucleic acid (DNA), is passed down from generation to generation. DNA tells a person’s body how to work and how to grow. It provides instructions that assign features to each individual, such as giving one person brown hair and another person blonde hair, and one person brown eyes and another person green eyes.

pool Gene

Gene pool

K E Y T E R M S

Allele—One of two or more alternate forms of a gene.

Chromosome—A microscopic thread-like structure found within each cell of the body and consists of a complex of proteins and DNA. Humans have 46 chromosomes arranged into 23 pairs. Changes in either the total number of chromosomes or their shape and size (structure) may lead to physical or mental abnormalities.

Deoxyribonucleic acid (DNA)—The genetic material in cells that holds the inherited instructions for growth, development, and cellular functioning.

Gene—A building block of inheritance, which contains the instructions for the production of a particular protein, and is made up of a molecular sequence found on a section of DNA. Each gene is found on a precise location on a chromosome.

Genome—A term used to describe a complete representation of all of the genes in a species.

DNA is much like a linear string, with individual segments along the string known as genes. Genes provide the specific directions for the body. Each gene is a segment of DNA, and sequencing of the four base molecules of DNA create the gene. Variations in the sequence account for variations in genes. A gene is the equivalent of an allele, and each particular gene is found on the same chromosome in each individual. The long, linear strings of DNA are arranged into smaller packages known as chromosomes. In general, there are 46 chromosomes in each cell of a person’s body. The 46 chromosomes can be matched into 23 pairs. One of each pair is inherited from the mother’s egg and one of each pair is inherited from the father’s sperm. Most animals, including humans, contain two copies of each chromosome and likewise two copies of each gene. Each individual receives one allele from each parent because they receive one of each of the 23 chromosomes from each parent.

Although each person has 46 chromosomes, the DNA that makes up those chromosomes is slightly different from individual to individual. It is this variation within specific genes that gives the diversity observed throughout populations around the world.

Alleles

Different versions of the same gene are referred to as alleles. Blood types are examples of alleles. In humans

there are several different blood types, including A, B, O, and AB. These arise by various combinations of the three blood-type alleles; the A-allele, the B-allele, and the O- allele. The specific blood type a person has depends on the exact blood type alleles they inherited from their parents. For example, a person may inherit two O-alleles, in which case they would have type O blood, or they may inherit an A- and a B-allele, in which case they would have type AB blood, and so on.

Population genetics

Population genetics is the study of genetic variation within a population. This includes the subtle changes in DNA sequences and the frequencies of these different forms. Changes within the DNA sequences may arise through several pathways. Mechanisms commonly studied by population geneticists include mutation, natural selection, and genetic drift.

Mutations are changes within the DNA sequence that alter the original directions encoded within DNA. Mutation may result from damage to DNA, or a mistake in the replication of DNA resulting in a sequence change. The majority of mutations arise by chance, although some may be caused by environmental factors, such as toxins that penetrate the cells of the body and attack the DNA. Natural selection is the difference in mortality (death rates) and fertility (birth rates) between different genetic types. The interplay of the expressed phenotype and the environment influences natural selection. If the phenotype is favorable, the individual survives and perpetuates his or her genetic profile in the gene pool. Genetic drift is a process by which the frequencies of specific alleles change, by chance, within a population.

Each gene pool accounts for all of the alleles for all of the traits of the members of a population. Within a population, different alleles will occur at different frequencies. For instance, approximately 44% of the population has type O blood, 42% of the population has type A blood, 10% of the population has type B blood, and 4% of the population has type AB blood. The percentages of each blood type are directly related to the frequency of each blood type allele. The more frequent the A-allele, the more frequent type A blood would be seen in the population.

The gene frequency of an allele is equal to the number of times the allele occurs compared to the total number of alleles for that trait.

Gene frequency equals the number of a specific type of allele, or the total number of alleles in the gene pool

DNA changes and genetic disorders

Genetic disorders are caused by changes in the DNA sequence. In general, there is a non-disease causing

allele and a disease-causing allele. Some genetic disorders arise by sporadic mutations in the DNA sequence. Others are inherited from one or both of the parents.

There are several different inheritance patterns associated with genetic disorders. Autosomal dominant and autosomal recessive are two of the most common. Chromosomes come in pairs, one from the egg and one from the sperm. Autosomal dominant disorders require that a person inherit only one disease-causing allele in order to be affected. Even though the corresponding gene on the other chromosome in the pair may be the non-dis- ease-causing allele, having one disease-causing allele is enough to cause the disorder to be present. Autosomal recessive disorders require that a person inherit two dis- ease-causing alleles, one on each chromosome of the pair, for the individual to be affected. If a person inherits only one disease-causing allele of a recessive disorder they are called a carrier. Carriers are not affected by disease; however, they carry the possibility of passing that disease on to a future child.

Hardy-Weinberg equilibrium

The frequency of disease-causing and non-disease- causing alleles along with the frequency of affected indi-

viduals, carriers, and unaffected individuals are related within a mathematical equation known as the HardyWeinberg equation.

The equation itself is written as p2 2pq q2 1. For autosomal recessive disorders, p2 represents the people within the population that have two non-disease-causing alleles (unaffected), 2pq represents the people within the population with one disease-causing allele and one non- disease-causing allele (carriers), and q2 represents the people within the population that have two disease-caus- ing alleles (affected). Because the Hardy-Weinberg equation deals with allele frequencies, the equation p q 1 may also be used. In this case, p represents the frequency of the non-disease-causing allele within the population and q represents the frequency of the disease-causing allele within the population.

The Hardy-Weinberg equation is based on the work of Drs. Hardy and Weinberg. Independently, they suggested that there should exist an equilibrium, or balance, between different allele frequencies. They devised a list of conditions that must be true for this balance, known as the Hardy-Weinberg equilibrium, to occur. These include:

• no evolutionary forces acting upon the population

Gene therapy

•the population is “infinitely” large (meaning it is so large that it may be assumed to be infinitely large)

•individuals have two copies of each gene

•there is random mating between individuals within the group

•the frequencies of the alleles are the same in both males and females

•generations are non-overlapping

The Hardy-Weinberg equation has several applications including use by population geneticists to study the characteristics of certain populations and use by genetic counselors to calculate recurrence risks for individual families affected by genetic disease.

The future

There are several projects underway at this time in an effort to further understand the gene pool, population genetics, and the human genome. The Human Genome Diversity Project (HGDP) is an international project that seeks to understand the diversity and unity of the entire human species.

The Human Genome Project, a separate venture from HGDP, made the news in 2000 when scientists announced they had elucidated a working draft of the human genome sequence.

Resources

WEBSITES

Bioethics and Human Population Genetics Research.

http://www.biol.tsukuba.ac.jp/~macer/PG.html .

Biology Website References for Students and Teachers.

http://www.hoflink.com/~house/evolution.html#anchor 25392 .

Evolution—Population Genetics.

http://www.nearctica.com/evolve/popgen.htm .

Human Genome Diversity Project.

http://www.standford.edu/group/morrinst/hgdp.html . Human Genome Project. http://www.ornl.gov/hgmis . Talk Origins. http://www.talkorigins.org .

Java O. Solis, MS

I Gene therapy

Gene therapy is a rapidly growing field of medicine in which genes are introduced into the body to treat diseases. Genes control heredity and provide the basic biological code for determining a cell’s specific functions.

Gene therapy seeks to provide genes that correct or supplant the disease-controlling functions of cells that are not, in essence, doing their job. Somatic gene therapy introduces therapeutic genes at the tissue or cellular level to treat a specific individual. Germ-line gene therapy inserts genes into reproductive cells or possibly into embryos to correct genetic defects that could be passed on to future generations. Initially conceived as an approach for treating inherited diseases, like cystic fibrosis and Huntington’s disease, the scope of potential gene therapies has grown to include treatments for cancers, arthritis, and infectious diseases. Although gene therapy testing in humans has advanced rapidly, many questions surround its use. For example, some scientists are concerned that the therapeutic genes themselves may cause disease. Others fear that germ-line gene therapy may be used to control human development in ways not connected with disease, like intelligence or appearance.

The biological basis of gene therapy

Gene therapy has grown out of the science of genetics or how heredity works. Scientists know that life begins in a cell, the basic building block of all multicellular organisms. Humans, for instance, are made up of trillions of cells, each performing a specific function. Within the cell’s nucleus (the center part of a cell that regulates its chemical functions) are pairs of chromosomes. These threadlike structures are made up of a single molecule of DNA (deoxyribonucleic acid), which carries the blueprint of life in the form of codes, or genes, that determine inherited characteristics.

A DNA molecule looks like two ladders with one of the sides taken off both and then twisted around each other. The rungs of these ladders meet (resulting in a spiral staircase-like structure) and are called base pairs. Base pairs are made up of nitrogen molecules and arranged in specific sequences. Millions of these base pairs, or sequences, can make up a single gene, specifically defined as a segment of the chromosome and DNA that contains certain hereditary information. The gene, or combination of genes formed by these base pairs ultimately direct an organism’s growth and characteristics through the production of certain chemicals, primarily proteins, which carry out most of the body’s chemical functions and biological reactions.

Scientists have long known that alterations in genes present within cells can cause inherited diseases like cystic fibrosis, sickle-cell anemia, and hemophilia. Similarly, errors in the total number of chromosomes can cause conditions such as Down syndrome or Turner syndrome. As the study of genetics advanced, however, scientists learned that an altered genetic sequence can also make people more susceptible to diseases, like ather-

therapy Gene

Gene therapy

Viral vectors

In both types of therapy, scientists need something to transport either the entire gene or a recombinant DNA to the cell’s nucleus, where the chromosomes and DNA reside. In essence, vectors are molecular delivery trucks. One of the first and most popular vectors developed were viruses because they invade cells as part of the natural infection process. Viruses have the potential to be excellent vectors because they have a specific relationship with the host in that they colonize certain cell types and tissues in specific organs. As a result, vectors are chosen according to their attraction to certain cells and areas of the body.

One of the first vectors used was the retrovirus. Because these viruses are easily cloned (artificially reproduced) in the laboratory, scientists have studied them extensively and learned a great deal about their biological action. They have also learned how to remove the genetic information which governs viral replication, thus reducing the chances of infection.

Retroviruses work best in actively dividing cells, but cells in the body are relatively stable and do not divide often. As a result, these cells are used primarily for ex vivo (outside the body) manipulation. First, the cells are removed from the patient’s body, and the virus, or vector, carrying the gene is inserted into them. Next, the cells are placed into a nutrient culture where they grow and replicate. Once enough cells are gathered, they are returned to the body, usually by injection into the blood stream. Theoretically, as long as these cells survive, they will provide the desired therapy.

Another class of viruses, called the adenoviruses, may also prove to be good gene vectors. These viruses can effectively infect nondividing cells in the body, where the desired gene product is then expressed naturally. In addition to being a more efficient approach to gene transportation, these viruses, which cause respiratory infections, are more easily purified and made stable than retroviruses, resulting in less chance of an unwanted viral infection. However, these viruses live for several days in the body, and some concern surrounds the possibility of infecting others with the viruses through sneezing or coughing. Other viral vectors include influenza viruses, Sindbis virus, and a herpes virus that infects nerve cells.

Scientists have also delved into nonviral vectors. These vectors rely on the natural biological process in which cells uptake (or gather) macromolecules. One approach is to use liposomes, globules of fat produced by the body and taken up by cells. Scientists are also investigating the introduction of raw recombinant DNA by injecting it into the bloodstream or placing it on micro-

scopic beads of gold shot into the skin with a “gene-gun.” Another possible vector under development is based on dendrimer molecules. A class of polymers (naturally occurring or artificial substances that have a high molecular weight and formed by smaller molecules of the same or similar substances), is “constructed” in the laboratory by combining these smaller molecules. They have been used in manufacturing Styrofoam, polyethylene cartons, and Plexiglass. In the laboratory, dendrimers have shown the ability to transport genetic material into human cells. They can also be designed to form an affinity for particular cell membranes by attaching to certain sugars and protein groups.

The history of gene therapy

In the early 1970s, scientists proposed “gene surgery” for treating inherited diseases caused by faulty genes. The idea was to take out the disease-causing gene and surgically implant a gene that functioned properly. Although sound in theory, scientists, then and now, lack the biological knowledge or technical expertise needed to perform such a precise surgery in the human body.

However, in 1983, a group of scientists from Baylor College of Medicine in Houston, Texas, proposed that gene therapy could one day be a viable approach for treating Lesch-Nyhan disease, a rare neurological disorder. The scientists conducted experiments in which an enzyme-producing gene (a specific type of protein) for correcting the disease was injected into a group of cells for replication. The scientists theorized the cells could then be injected into people with Lesch-Nyhan disease, thus correcting the genetic defect that caused the disease.

As the science of genetics advanced throughout the 1980s, gene therapy gained an established foothold in the minds of medical scientists as a promising approach to treatments for specific diseases. One of the major reasons for the growth of gene therapy was scientists’ increasing ability to identify the specific genetic malfunctions that caused inherited diseases. Interest grew as further studies of DNA and chromosomes (where genes reside) showed that specific genetic abnormalities in one or more genes occurred in successive generations of certain family members who suffered from diseases like intestinal cancer, manic-depression, Alzheimer’s disease, heart disease, diabetes, and many more. Although the genes may not be the only cause of the disease in all cases, they may make certain individuals more susceptible to developing the disease because of environmental influences, like smoking, pollution, and stress. In fact, some scientists theorize that all diseases may have a genetic component.

On September 14, 1990, a four-year-old girl with a genetic disorder that prevented her body from produc-