Part 1 – GENETICS and OCULAR EMBRYOLOGY
Janey L. Wiggs
Chapter 1 – Fundamentals of Human Genetics
JANEY L. WIGGS
DNA AND THE CENTRAL DOGMA OF HUMAN GENETICS
The regulation of cellular growth and function in all human tissue is dependent on the activities of specific protein molecules. In turn, protein activity is dependent on the expression of the genes that contain the correct DNA sequence for protein synthesis. The DNA molecule is a double-stranded helix. Each strand is composed of a sequence of four nucleotide bases—adenine (A), guanine (G), cytosine (C), and thymine (T)—joined to a sugar and a phosphate. The order of the bases in the DNA sequence forms the genetic code that directs the expression of genes. The double-stranded helix is formed as a result of hydrogen bonding between the nucleotide bases of opposite strands. The bonding is specific, such that A always pairs with T, and G always pairs with C. The specificity of the hydrogen bonding is the molecular basis of the accurate copying of the DNA sequence that is required during the processes of DNA replication (necessary for cell division) and transcription of DNA into RNA (necessary for gene expression and protein synthesis; Fig. 1-1 ). 
Gene expression begins with the recognition of a particular DNA sequence, called the promoter sequence, as a start site for RNA synthesis by the enzyme RNA polymerase. The RNA polymerase “reads” the DNA sequence and assembles a strand of RNA that is complementary to the DNA sequence. RNA is a single-stranded nucleic acid composed of the same nucleotide bases as DNA, except that uracil takes the place of thymine. Human genes (and genes found in other eukaryotic organisms) contain DNA sequences that are not translated into polypeptides and proteins. These sequences are called intervening sequences or introns. Introns do not have a specific function, and although they are transcribed into RNA by RNA polymerase, they are spliced out of the initial RNA product (termed heteronuclear RNA, or hnRNA) to form the completed messenger RNA (mRNA). The mRNA is the template for protein synthesis. Proteins consist of one or more polypeptide chains, which are sequences of specific amino acids. The sequence of bases in the mRNA directs the order of amino acids that make up the polypeptide chain. Individual amino acids are encoded by units of three mRNA bases, termed codons. Transfer RNA (tRNA) molecules bind specific amino acids and recognize the corresponding three-base codon in the mRNA. Cellular organelles called ribosomes bind the mRNA in such a configuration that the RNA sequence is accessible to tRNA molecules and the amino acids are aligned to form the polypeptide. The polypeptide chain may be processed by a number of other chemical reactions to form the mature protein ( Fig. 1-2 ).
Human DNA is packaged as chromosomes located in the nuclei of cells. Chromosomes are composed of individual strands of DNA wound about proteins called histones. The complex winding and coiling process culminates in the formation of a chromosome. The entire collection of human chromosomes, called the human genome, includes 22 paired autosomes and two sex chromosomes. Women have two copies of the X chromosome, and men have one X and one Y chromosome ( Fig. 1-3 ). 
Figure 1-1 Structure of the DNA double helix. The sugar-phosphate backbone and nitrogenous bases of each individual strand are arranged as shown. The two strands of DNA pair by hydrogen bonding between the appropriate bases to form the double-helical structure. Separation of individual strands of the DNA molecule allows DNA replication, catalyzed by DNA polymerase. As the new complementary strands of DNA are synthesized, hydrogen bonds are formed between the appropriate nitrogenous bases.
Mitosis and Meiosis
In order for cells to divide, the entire DNA sequence must be copied so that each daughter cell can receive a complete complement of DNA. The growth phase of the cell cycle terminates with the separation of the two sister chromatids of each chromosome, and the cell divides during mitosis. Prior to cell division, the complete DNA sequence, which comprises the entire human genome, is copied by the enzyme DNA polymerase in a process called DNA replication. DNA polymerase is an enzyme capable of the synthesis of new strands of DNA according to the exact sequence of the original DNA. Once the DNA is copied, the old and new copies of the chromosomes pair, and the cell divides such that one copy of each chromosome pair belongs to each cell ( Fig. 1-4 ). Mitotic cell division produces a daughter cell that is an exact replica of the dividing cell.
Figure 1-2 The central dogma of molecular genetics. Transcription of DNA into RNA occurs in the nucleus of the cell, catalyzed by the enzyme RNA polymerase. Mature mRNA is transported to the cytoplasm, where translation of the code produces amino acids linked to form a polypeptide chain, and ultimately a mature protein is produced.
Meiotic cell division is a special type of cell division that results in a reduction of the genetic material in the daughter cells, which become the reproductive cells—eggs (women) and sperm (men). Meiosis begins with DNA replication, followed by a pairing of the maternal and paternal chromosomes (homologous pairing) and an exchange of genetic material between chromosomes by recombination ( Fig. 1-5 ). The homologous chromosome pairs line up on the microtubule spindle and divide such that the maternal and paternal copies of the doubled chromosomes are distributed to separate daughter cells. A second cell division occurs, and the doubled chromosomes divide, which results in daughter cells that have half the genetic material of somatic (tissue) cells.
BASIC MENDELIAN PRINCIPLES
Two important rules central to human genetics emerged from the work of Gregor Mendel, a nineteenth-century Austrian monk. The first is the principle of segregation, which states that genes exist in pairs and that only one member of each pair is transmitted to the offspring of a mating couple. The principle of segregation describes the behavior of chromosomes in meiosis. Mendel’s second rule is the law of independent assortment, which states that genes at different loci are transmitted independently. This work also demonstrated the concepts of dominant and recessive traits. Mendel found that certain traits were dominant and could mask the presence of a recessive gene.
A practical example of Mendel’s two laws is seen in the inheritance of human eye and hair color. Blue eyes and blond hair are recessive traits, while brown eyes and hair are dominant traits. This means that for an individual to have blond hair and blue eyes, he or she must have two genes for blond hair and two genes for blue eyes (one from the mother and one from the father). An individual with brown eyes and brown hair may have two genes for brown eye color and two genes for brown hair color; however, because the brown genes are dominant, brown eyes may occur when an individual has one gene for brown eye color and one gene for blue eye color. A homozygous individual has two of the same genes (i.e., two blue eye-color genes or two brown eye-color
Figure 1-3 The packaging of DNA into chromosomes. Strands of DNA are wound tightly around proteins called histones. The DNA-histone complex becomes further coiled to form a nucleosome, which in turn coils to form a solenoid. Solenoids then form complexes with additional proteins to become the chromatin that ultimately forms the chromosome.
genes), whereas a heterozygous individual has two different genes (i.e., one blue eye-color gene and one brown eye-color gene).
Mendel’s rules on segregation and independent assortment are evident when the possible matings and offspring of individuals with blond or brown hair and blue or brown eye color are observed ( Fig. 1-6 ). If two blond-haired, blue-eyed individuals mate, all their offspring will have blond hair and blue eyes, because these individuals must be homozygous, and the only genes available to the offspring are those for blue eyes and blond hair. If a blond-haired, blue-eyed individual mates with a brown-haired, brown-eyed individual who is homozygous for brown hair genes and brown eye genes, all the offspring from this mating will have brown hair and brown eyes because the brown genes are dominant. However, all these offspring will be heterozygous for genes at these loci, because they must have inherited recessive blue eye and blond hair genes.
The law of independent assortment becomes evident when the offspring of two individuals who are heterozygous for eye and hair color are examined. Among the offspring of this mating, 25% will have blue eyes, and 75% will have brown eyes (50% will be heterozygous
Figure 1-4 The mitotic cell cycle. During mitosis, the DNA of a diploid cell is replicated, which results in the formation of a tetraploid cell that divides to form two identical diploid daughter cells.
for eye color, and 25% will be homozygous for brown eye color). Similarly, 25% of the offspring will have blond hair, and 75% will have brown hair (again, 50% will be heterozygous for hair color, and 25% will be homozygous for brown hair color). However, the 25% of offspring with blue eyes will not necessarily have blond hair. Some offspring will have blond hair and blue eyes, and some offspring will have brown hair and blue eyes. This is because the eye color and hair color genes are located at distinct loci that segregate independently of each other. Independent segregation, or assortment, occurs because maternal and paternal chromosomes segregate randomly into gametes during meiosis, and because of the random recombination that occurs between homologous chromosomes when they pair during meiosis.
At the same time that Mendel observed that most traits segregate independently, according to the law of independent assortment, he unexpectedly found that some traits frequently segregate together. The physical arrangement of genes in a linear array along a chromosome is the explanation for this surprising observation. On average, a recombination event occurs once or twice between two paired homologous chromosomes during meiosis
Figure 1-5 The meiotic cell cycle. During meiosis, the DNA of a diploid cell is replicated, which results in the formation of a tetraploid cell that divides twice to form four haploid cells (gametes). As a consequence of the crossing over and recombination events that occur during the pairing of homologous chromosomes prior to the first division, the four haploid cells may contain different segments of the original parental chromosomes. For brevity, prophase II and telophase II are not shown.
( Fig. 1-7 ). Most observable traits, by chance, are located far away from one another on a chromosome, such that recombination is likely to occur between them, or they are located on entirely different chromosomes. If two traits are on separate chromosomes, or a recombination event is likely to occur between them on the same chromosome, the resultant gamete formed during meiosis has a 50% chance of inheriting different alleles from each loci, and the two traits respect the law of independent assortment. If, however, the loci for these two traits are close together on a chromosome, with the result that a recombination event occurs between them only rarely, the alleles at each loci are passed to descendant gametes “in phase.” This means that the particular alleles present at each loci in the offspring reflect the orientation in the parent, and the traits appear to be “linked.” For example, in Mendel’s study of pea plants, curly leaves were always found with pink flowers, even though the genes for curly leaves and pink flowers are located at distinct loci. These traits are linked, because the curly-leaf gene and the pink-flower gene are located close to each other on a chromosome, and a recombination event only rarely occurs between them.
Figure 1-6 Independent assortment of mendelian traits. Shown are the results of a mating between a blond-haired, blue-eyed father and a blond-haired, blue-eyed mother; a mating between a blond-haired, blue-eyed father and a brown-haired, brown-eyed mother; and a mating between a couple heterozygous for blond and brown hair and for blue and brown eyes.
Recombination and linkage are the fundamental concepts behind genetic linkage analysis. The search for a gene responsible for a phenotypic trait (or disease) depends on the ability to observe linkage between the trait and mapped genetic markers. The identification of a marker that segregates with the trait (i.e., is linked genetically to the trait) defines the location of the gene for that trait, because the lack of recombination between the marker and the trait means that the gene responsible for the trait is located physically near the linked marker. The chromosomal locations of genetic markers are readily available to the public as a result of the successful efforts of the nationally funded Human Genome Project. Once an approximate location of a gene responsible for a trait has been determined, analysis of rare recombination events between markers in the region and that trait can help further define the precise physical location of the gene on the chromosome. In this way, “positional cloning” of genes may be accomplished. 
Mutations are changes in the gene DNA sequence that result in a biologically significant change in the function of the encoded
Figure 1-7 Genetic recombination by crossing over. Two copies of a chromosome are copied by DNA replication. During meiosis, pairing of homologous chromosomes occurs, which enables a crossover between chromosomes to take place. During cell division, the recombined chromosomes separate into individual daughter cells.
Figure 1-8 Expanded trinucleotide repeat and anticipation in myotonic dystrophy. Results of a study to determine the size of the trinucleotide repeat in three individuals affected by myotonic dystrophy. The results from a normal individual are shown at the far left. The size of the repeat element increases with the severity of the disease in the affected individuals.
protein. If a particular gene is mutated, the protein product might not be made, or it might be produced but work poorly. In some cases, mutations create proteins that have an adverse effect on the cell (dominant negative effect). Point mutations (the substitution of a single base pair) are the most common mutations encountered in human genetics. Missense mutations are point mutations that cause a change in the amino acid sequence of the polypeptide chain. The severity of the missense mutation is dependent on the chemical properties of the switched amino acids and on the importance of a particular amino acid in the function of the mature protein. Point mutations also may decrease the level of polypeptide production because they interrupt the promoter sequence, splice site sequences, or create a premature stop codon.
Gene expression can be affected by the insertion or deletion of large blocks of DNA sequence. These types of mutations are less common than point mutations but may result in a more severe change in the activity of the protein product. A specific category of insertion mutations is the expansion of trinucleotide repeats found in patients affected by certain neurodegenerative disorders. An interesting clinical phenomenon, “anticipation,” was understood on a molecular level with the discovery of trinucleotide repeats as the cause of myotonic dystrophy.  Frequently, offspring with myotonic dystrophy were affected more severely and at an earlier age than their affected parents and grandparents. Examination of the disease-causing trinucleotide
Figure 1-9 Reciprocal translocation between two chromosomes. The Philadelphia chromosome (responsible for chronic myelogenous leukemia) is shown as an example of a reciprocal chromosomal translocation that results in an abnormal gene product responsible for a clinical disorder. In this case, an exchange occurs between the long arm of chromosome 9 and the long arm of chromosome 22.
repeat in affected pedigrees demonstrated that the severity of the disease correlated with the number of repeats found in the myotonic dystrophy gene in affected individuals. This phenomenon has been observed in a number of other diseases, including Huntington’s disease ( Fig. 1-8 ).
Chromosomal rearrangements may result in breaks in specific genes that cause an interruption in the DNA sequence. Usually, the break in DNA sequence results in a truncated, unstable, dysfunctional protein product; occasionally, the broken gene fuses with another gene to cause a “fusion polypeptide product,” which may have a novel activity in the cell. Often such a novel activity results in an abnormality in the function of the cell. An example of such a fusion protein is the product of the chromosome 9;22 translocation that is associated with many cases of leukemia ( Fig. 1-9 ).
The use of molecular tools to demonstrate causative DNA mutations and identify individuals at risk for an inherited condition is called DNA-based diagnosis.  The goal of genetic diagnosis is early recognition of a disease so that intervention can be undertaken to prevent or reverse the disease process. This was one of the goals of the Human Genome Project. Two general approaches have been used to detect mutations in genes. The indirect approach uses genetic linkage analysis, and the direct approach identifies specific changes in DNA sequence.
Linkage analysis can be used to diagnose any genetically mapped disorder. Segregation of genetic markers known to be linked to a gene responsible for a condition is used to determine whether an individual has inherited a chromosome that carries the abnormal gene. This method does not require physical isolation and sequencing of the gene. Linkage analysis is useful when large genes with many possible mutations are responsible for a disease ( Fig. 1-10 ). Several important disadvantages of this approach must be recognized. First, analysis of DNA from multiple family members is required to identify the markers that segregate with the abnormal chromosome in each affected pedigree. Second, not all genetic markers provide useful information for this analysis. Some individuals may not be “informative” at a particular marker, and a definitive demonstration of the abnormal chromosome may not be possible. Third, recombination may occur between the genetic markers used for testing and the disease-causing mutation. Although the markers selected for the analysis are physically close to the disease gene, a rare recombination event may occur and result in a misdiagnosis because
Figure 1-10 DNA diagnosis using genetic linkage analysis. This pedigree shows a mother and two daughters affected by a condition inherited as an autosomal dominant trait. Analysis carried out using a marker closely linked to the disease gene shows that allele 1 segregates with the condition. The daughter in the third generation has inherited this allele from her affected mother, which suggests that she has also inherited the disease gene and is therefore at risk for development of the condition.
of an apparent separation between the genetic markers that define the normal and abnormal chromosomes.
Direct mutation analysis uses a variety of techniques based on the DNA sequence of a gene to identify the specific base-pair change that is responsible for the disease. Because this method does not rely on the segregation of genetic markers to identify the abnormal chromosome, multiple family members are not usually required. Also, potential errors caused by rare recombination events between the markers and the disease gene do not occur with this method, but there are several drawbacks to direct mutation analysis. The gene responsible for the disease must first be isolated and sequenced. Some genes are very large (e.g., the gene for retinoblastoma spans more than 200,000 kilobases of DNA sequence) and are difficult and time-consuming to sequence. Multiple mutations and novel mutations present in a single gene may require complete sequencing of the DNA for each diagnostic test.
In some disorders, the majority of stricken individuals are affected by the same mutation. For example, 70% of individuals affected by cystic fibrosis have the delta 508 mutation. For disorders of this type, a simple screening test based on the particular mutation may be developed. This technique involves the synthesis of an oligonucleotide probe that hybridizes only to the mutated sequence. Such a probe, called an allele-specific oligonucleotide, is very useful when the DNA sequence that causes the genetic disease is known and the number of disease-causing mutations is limited ( Fig. 1-11 ). Those patients whose DNA hybridizes with the normal sequence do not have the mutation, and those patients whose DNA hybridizes with the mutant sequence do have the mutation.
Genetic counseling has become an important part of any clinical medicine practice. In 1975, the American Society of Human Genetics adopted the following descriptive definition of genetic counseling :
Genetic counseling is a communication process which deals with the human problems associated with the occurrence or risk of occurrence of a genetic disorder in a family. This process involves an attempt by one or more appropriately trained persons to help the individual or family to: (1) comprehend the medical facts including the diagnosis, probable course of the disorder, and the available management, (2) appreciate the way heredity contributes to the disorder and the risk of recurrence in specified relatives, (3) understand the alternatives for dealing with the
Figure 1-11 DNA diagnosis using an allele-specific oligonucleotide. Oligonucleotides specific for mutations are synthesized, as well as oligonucleotides that correspond to the normal sequence. DNA purified from individuals to be tested is placed on a small “dot” on a piece of filter paper and allowed to hybridize (base pair) with the specific oligonucleotides. Individuals A and B are normal, as their DNA hybridizes with the normal sequence only and not with the mutant sequence. Individual C’s DNA hybridizes with both the normal and the mutant sequences; hence, this individual has one normal gene and one mutant gene. Individual C is a carrier of the disease if it is a recessive condition or is affected by the disease if it is a dominant condition.
risk of recurrence, (4) choose a course of action which seems to them appropriate in their view of their risk, their family goals, and their ethical and religious standards and act in accordance with that decision, and (5) to make the best possible adjustment to the disorder in an affected family member and/or to the risk of recurrence of that disorder.
An accurate diagnosis is the first step in productive genetic counseling. The patient-physician discussion of the natural history of the disease and of its prognosis and management is entirely dependent on the correct identification of the disorder that affects the patient. Risk assessment for other family members and options for prenatal diagnosis also depend on an accurate diagnosis. In some cases, appropriate genetic testing may help establish the diagnosis.
A complete family history of the incidence of the disorder is necessary to determine the pattern of inheritance of the condition. The mode of inheritance (i.e., autosomal dominant, autosomal recessive, X-linked, or maternal) must be known to calculate the recurrence risk to additional family members, and it helps confirm the original diagnosis ( Fig. 1-12 ). A family history is recorded most easily as a pedigree using universally recognized nomenclature ( Fig. 1-13 ). For the record of family information, the gender and birth date of each individual and his or her relationship to other family members are indicated using the standard pedigree symbols. It is also helpful to record the age of onset of the disorder in question (as accurately as this can be determined). The pedigree diagram must include as many family members as possible. Miscarriages, stillbirths, and consanguineous parents are indicated.
Occasionally, a patient may appear to be affected by a condition that is known to be inherited, but the patient is unable to provide a family history of the disease. Several important explanations for a negative family history must be considered before the conclusion is made that the patient does not have a heritable condition. First, the patient may not be aware that other family members are affected by the disease. Individuals frequently are reluctant to share information about medical problems, even with close family members. Second, many disorders exhibit variable expressivity or reduced penetrance, which means that other family members may carry a defective gene that is not expressed or results in only a mild form of the disease that is not readily observed. Third, false paternity may produce an individual affected by a disease that is not found in anyone else belonging to the acknowledged pedigree. Genetic testing can easily determine the paternity (and maternity) of any individual if blood samples are obtained from relevant family members. Fourth, a new mutation may arise that affects an individual and may be passed to offspring, even though existing family members show no evidence of the disease.
Once the diagnosis and family history of the disorder are established, risk prediction in other family members (existing and unborn) may be calculated. The chance that an individual known to be affected by an autosomal dominant disorder will transmit the disease to his or her offspring is 50%. This figure may be modified, depending on the penetrance of the condition. For example, retinoblastoma is inherited as an autosomal dominant trait, and 50% of the children of an affected parent should be affected. However, usually only 40–45% of the children at risk are affected, because the penetrance of the retinoblastoma trait is only 80–90%, which means that 5–10% of children who have inherited an abnormal copy of the retinoblastoma gene do not develop ocular tumors.
An individual affected by an autosomal recessive trait will have unaffected children unless he or she partners with another individual affected by the disease or with an individual who is a carrier of the disease. Two individuals affected by an autosomal recessive disease produce only affected offspring. (There are some rare exceptions to this rule. If the disease is the result of mutations in two different genes, it is possible for two individuals affected by an autosomal recessive trait to produce normal children. Also, in rare cases, different mutations in the same gene may compensate for each other, and the resultant offspring will be normal.) If an individual affected by an autosomal recessive disease partners with a heterozygous carrier of a gene defect responsible for that disorder, the chance of producing an affected child is 50%. Among the offspring of an individual affected by an autosomal recessive disease, 50% will be carriers of the disorder. If one of these offspring partners with another carrier of the disease, the chance of producing an affected child is 25%.
X-linked disorders are always passed from a female carrier who has inherited a copy of an abnormal gene on the X chromosome received from either her mother (who was a carrier) or her father (who was affected by the disease). Man-to-man transmission is not seen in diseases caused by defects in genes located on the X chromosome. Among sons born to female carriers of X-linked disorders, 50% are affected by the disease, and 50% of daughters born to female carriers of X-linked disorders are carriers of the disease. All the daughters of men affected by X-linked disorders are carriers of the disease.
Mitochondrial disorders are inherited by sons and daughters from the mother. The frequency of affected offspring and the severity of the disease in affected offspring depend on the number of abnormal mitochondria present in the egg that gives rise to the affected child. Diseased and normal mitochondria are distributed randomly in all cells of the body, including the female gametes. As a result, not all the eggs present in a woman affected by a mitochondrial disorder have the same number of affected mitochondria (heteroplasmy). Men affected by mitochondrial disorders only rarely have affected children, because very few mitochondria in the developing embryo are derived from the sperm used to fertilize the egg.
With careful diagnosis and family history assessment, even sporadic cases of heritable disorders are identifiable. In such cases, an estimate of recurrence risk can be calculated using the available pedigree and clinical information and the statistical principle called Bayes’ theorem. These individuals should be referred to clinical genetics services, such as those commonly found in hospital settings ( Box 1-1 ).
Figure 1-12 Patterns of inheritance. For pedigrees with an autosomal dominant trait, panel 1 shows inheritance that originates from a previous generation, panel 2 shows segregation that originates in the second generation of this pedigree, and panel 3 shows an apparent “sporadic” case, which is actually a new mutation that arises in the most recent generation. This mutation has a 50% chance of being passed to offspring of the affected individual. For pedigrees with an autosomal recessive trait, panel 1 shows an isolated affected individual in the most recent generation (whose parents are obligatory carriers of the mutant gene responsible for the condition), panel 2 shows a pair of affected siblings whose father is also affected (for the siblings to be affected, the mother must be an obligate carrier of the mutant gene), and panel 3 shows an isolated affected individual in the most recent generation who is a product of a consanguineous marriage between two obligate carriers of the mutant gene. For pedigrees with an X-chromosomal trait, panel 1 shows an isolated affected individual whose disease is caused by a new mutation in the gene responsible for this condition, panel 2 shows an isolated individual who inherited a mutant copy of the gene from the mother (who is an obligate carrier), and panel 3 shows segregation of an X-linked trait through a multigeneration pedigree (50% of the male offspring are affected, and their mothers are obligate carriers of the disease). For pedigrees with a mitochondrial trait, the panel shows a large, multigeneration pedigree—men and women are affected, but only women have affected offspring.
Mutations in the DNA sequence of a particular gene can result in a protein product that is not produced, works poorly, or has adopted a novel function that is detrimental to the cell. Gene therapy involves the delivery of a normal gene to the tissue that contains the flawed gene. Theoretically, a normal copy of the gene can physically take the place of the flawed gene and restore the gene function of the cell. In practice, however, actually replacing the flawed gene with a normal gene is a difficult task. Currently, the aim of gene therapy is to add a useful gene to the cell or tissue that suffers the consequences of the flawed gene. In some cases, the new gene may code for an entirely different protein whose function compensates for the protein encoded by the flawed gene. Useful genes may be delivered to specific tissues that require treatment using modified viruses as vectors. Normally, certain types of viruses invade a host cell, are incorporated into
Figure 1-13 Basic pedigree notation. Typical symbols used in pedigree construction are defined.
Types of Clinical Genetics Services and Programs
CENTER-BASED GENETICS CLINIC
• metabolic clinic
• spina bifida clinic
• hemophilia clinic
• craniofacial clinic
• other single-disorder clinics (e.g., neurofibromatosis 1 clinic)
PRENATAL DIAGNOSIS PROGRAM: PERINATAL GENETICS
• amniocentesis/chorionic villus sampling clinics
• ultrasound program
• maternal serum a-fetoprotein program
• newborn screening program/follow-up clinic
• other population-screening programs (e.g., for Tay–Sachs disease)
• health-care professional
• general public
• school system
• teratology information services
the host genome, and express the viral genes required for replication of the virus. The mature virus eventually takes over the cell, with the result that the cell dies and releases new, infectious viral products that can infect adjacent cells. A general approach to gene therapy is to use an altered (recombinant) virus to carry the gene of interest to the desired tissue. Using genetic engineering techniques, the viral DNA is modified so that the viral genes required for virus proliferation are removed and the therapeutic gene is put in their place. Such a virus may invade the diseased tissue, become incorporated into the host DNA, and express the desired gene. Because the modified virus does not have the viral genes required for viral replication, the virus cannot proliferate, and the host cell does not die ( Fig. 1-14 ). A successful example of this approach has recently been demonstrated by the restoration of vision in a canine model of Leber congenital amaurosis using a recombinant adeno-associated virus carrying the normal gene (RPE65).
Figure 1-14 Gene therapy using a retrovirus vector. A therapeutic gene is engineered genetically into the retrovirus DNA and replaces most of the viral DNA sequences. The “recombinant virus” that carries the therapeutic gene is allowed to replicate in a special “packaging cell,” which also contains normal virus that carries the genes required for viral replication. The replicated recombinant virus is allowed to infect the human diseased tissue, or “target cell.” The recombinant virus may invade the diseased tissue but cannot replicate or destroy the cell. The recombinant virus inserts copies of the normal therapeutic gene into the host genome and produces the normal protein product.
Diseases caused by mutations that create a gene product destructive to the cell need to be treated using a different approach. In these cases, genes or oligonucleotides that may inactivate the mutated gene are introduced into the cell. This is called “antisense therapy,” and it is proving to be a useful approach for diseases caused by the “gain of function mutations.” A number of different viral vectors likely to be useful for gene therapy are currently under investigation. In addition, evaluation of nonviral mechanisms for the introduction of therapeutic genes into diseased tissue is ongoing.
In general, most of the current approaches to gene therapy are aimed at repairing the somatic cells of the particular tissue affected by the disease gene. Gene delivery may be tailored to the somatic cells affected by the disorder. Gene therapy of ocular disorders benefits from the accessibility of the eye, the ability to visualize the diseased tissue, and the large number of specific
gene defects known to be responsible for many inherited eye disorders.
Specific treatment of the diseased cells does not affect the other cells of the body, which include the germline cells. Because the germline cells continue to carry flawed copies of the gene, the disease may still be passed to offspring of the affected patient. Gene therapy targeted to germline cells as well as the diseased somatic cells results in successful treatment of the disease in the affected individual and prevents transfer of the disease to any offspring.
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