Medical Genetics and Cancer

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Unformatted text preview: Medical Genetics and Cancer Genetic Analysis of Human Diseases A genetic basis for a human disease may be suggested from a variety of observations. Human traits are determined both by genetics and the environment. The following suggest a genetic basis for a disease: When an individual exhibits a disease, this disorder is more likely to occur in genetic relatives than in the general population. Identical twins (monozygotic twins) share the disease more often than non-identical (dizygotic) twins. autosomal dominant disorder parent 1 - Aa parent 2 - aa .5 are going to be affected and .5 are not going to be affected if monozygotic (genetically identical) if dizygotic(50% chance both will have, .25 neither, .25 only one has) Concordance is the degree to which a disease is inherited. It is figured by calculating the percentage of twin pairs in which both twins exhibit the disorder relative to pairs where only one twin has the disorder. Diseases caused by a single gene should have 100% concordance. For dizygotic twins, concordance of dominant disorders is expected to be 50%. For recessive diseases, the concordance among dizygotic twins would be 25%. Frequently these are lower due to the penetrance of the disease or if one twin acquires the disease due to a mutation after fertilization. The disease does not spread to individuals sharing similar environmental situations. doesn't have to do with t he environment Different populations tend to have different frequencies of the disease. The disease tends to develop at a characteristic age (the age of onset). The human disorder may resemble a disorder that is already known to have a genetic basis in an animal (e.g., the albino phenotype – see Figure 22.1). A correlation is observed between a disease and a mutant human gene or chromosomal aberration. Inheritance patterns of human diseases may be determined via pedigree analysis. Human diseases that are caused by a mutation in a single gene may be studied using pedigree analysis. don't worry about knowing what chromosome these things are located on all disorders work the same. how can we fi gure out what the basis of this is? what is the genetic basis? 1 Autosomal recessive inheritance (e.g., Tay-Sachs Disease – Figure 22.2) has the following characteristics: if recessive, offspring from t wo unaffected parents who are going to be heterozyg aa x aa ( all children affected) The affected offspring usually has two unaffected parents. When two unaffected heterozygotes have children, the average percentage of affected children is 25%. Two affected individuals will have 100% affected children. The trait occurs with the same frequency in both sexes. Table 22.1 lists some human recessive disorders. if autosomal - equal f requencies in both sexes Autosomal dominant inheritance (e.g., Huntington Disease – Figure 22.3) has the following characteristics: The affected offspring usually has one or both affected parents. An affected individual, with only one affected parent, is expected on average to produce 50% affected children. Two affected, heterozygous individuals will have on average 25% unaffected offspring. autosomal - same freq in both sexes The trait occurs in the same frequency in both sexes. For most dominant, disease-causing alleles, the homozygote is more severely affected than the heterozygote. In some cases, a dominant allele may be lethal in the homozygous condition. A list of autosomal dominant human disorders is provided in Table 22.2. look at entire family free, not just the offspring from O each parent separately.... ther dominant disorders include: may see the 50/50 or whatever the ratio may be Haploinsufficiency, meaning that (normal gene/mutant gene) -->affected phenotype a single functional copy is insufficient to produce a wild type phenotype (e.g., aniridia). For haploinsufficient genes, an inactive allele behaves dominantly. Gain of function mutations, where a gene takes on a new or abnormal function (e.g., achondroplasia). Such mutant genes behave dominantly. Dominant negative alleles interfere with a wild type allele’s ability to function, and therefore also behave dominantly (e.g., Marfan Syndrome). really a dominant thing because it's interfering with the enzyme even though you're making enough of the thing, and making the abnormal thing, just isn't enough to get the job done even if at molecular level, it's really recessive disrupting normal pathway...end up with s hortened bones here 2 long been suggested that abe lincoln may have had Marfan Syndrome X-linked recessive inheritance of human disorders (Table 22.3) has the following characteristics: f emales have to be homozygous recessive to have it Males are much more likely to exhibit the trait because they are hemizygous for Xlinked genes. The mothers of affected males often have brothers or fathers who are affected with the same trait. The daughters of affected males will produce, on average, 50% affected sons. An example using a Royal Family from Europe is shown in Figure 22.4. Many genetic disorders exhibit locus heterogeneity. A disease that is caused by mutations in different genes is said to have heterogeneity. An example is hemophilia, which is usually caused by a defect in one of three different clotting factors. In hemophilia A, called classic hemophilia, a protein called Factor VIII is missing. Hemophilia B is caused by a deficiency in a different clotting factor, Factor IX. Both Factor VIII and Factor IX are encoded by different genes on the X chromosome and, therefore, exhibit an X-linked recessive pattern of inheritance. Hemophilia C is caused by a Factor XI deficiency. The gene encoding Factor XI is found on chromosome 4; therefore, this form of hemophilia follows an autosomal recessive pattern of inheritance. Heterogeneity of this type may greatly complicate pedigree analysis. Another type of locus heterogeneity occurs when proteins are composed of more than one subunit with each subunit encoded by a different gene. An example is thalassemia, a potentially life-threatening disease that involved defects in the ability of red blood cells to transport oxygen. In adult humans, hemoglobin is a tetrameric protein composed of two !-globin and two "-globin subunits. Two main types of thalassemia have been discovered in the human population: ! thalassemia, in which the ! subunit of hemoglobin is defective, and " thalassemia, in which the " subunit is defective. have to be homozygous recessive to have it equal opportunity male and f emale whether the defect is the alpha globin gene or the beta globin gene but overall s ymptoms are basically the s ame but two different ways t o get to it 3 Huntington - autosomal dominant transmission pattern of a molec marker for H disease. there are Disease-causing mutant genes are identified by mapping and DNA sequencing. four f orms of the G8 marker an individual carries ( a b c By comparing the transmission patterns of many molecular markers (e.g., d). because there are pairs microsatellites – refer back to Figure 20.12) with the occurrence of an inherited disease, of chromosomes an individua will have 2 of researchers can pinpoint particular markers that are closely linked to the disease-causing m t hem. Affected individuals utant gene (an example with Huntington Disease is shown in Figure 22.5) always carry C version. t here are differences in the odern mapping can localize a marker or gene to a chromosomal region that is M number of restriction sitesypically about 1Mb (1,000,000 bp) in length. Because the entire human genome has t t hat make up this gene. been sequenced and most genes identified, researchers can analyze the 1 Mb region to 50/50 chance the kid got responsible for the disease. t he A from mom or dad if one is affected. See if kid G if he has C marker and see enetic testing can identify many has it. which a marker has been mapped to determine if a mutant gene in that region is inherited human diseases. Genetic testing refers to the use of testing methods to discover if an individual carries a genetic abnormality. Genetic screening refers to population-wide genetic testing. Genetic testing strategies are listed in Table 22.4. Biochemical assays can be used to measure the activity of a defective enzyme (e.g., hexosaminidase A – hexA – the defective enzyme that causes Tay-Sachs Disease). The assay for hexA will show zero activity for affected individuals (homozygous recessives who do not produce the hexA enzyme), 100% activity for completely normal individuals (homozygous dominants who produce the full amount of the hexA enzyme), and 50% activity for carriers (heterozygotes who produce intermediate amounts of the hexA enzyme). Testing may also involve detection of single-gene mutations at the DNA level. Researchers must have previously identified the mutant gene using molecular techniques (e.g., the genes for Duchenne Muscular Dystrophy, cystic fibrosis, and Huntington Disease). Techniques for identifying carriers include RFLP analysis, DNA sequencing, and in situ hybridization). Some genetic disorders are due to changes in chromosome structure or number. ex. Trisomy 21 Such changes can be identified by karyotyping the chromosomes. looking for enzymatic is activity because homozygous dominant has 100% activity, heterozygous has 50%, and homo recessive has 0% activitity Genetic screening can be conducted on specific populations in which a genetic disease common. For example, Tay-Sachs disease is most common among members of the Ashkenazi (western European) Jewish population. Screening for heterozygous individuals in 1971 led to a reduction of TSD births by 90% over the course of one generation. 4 if you do amniotic fl uid y ou have to do a culture but with chorionic villus y ou can do karotyping directly after Genetic testing can be performed prior to birth (Figure 22.6). Amniocentesis involves the removal of amniotic fluid that contains fetal cells. Chorionic villus sampling involves the removal of a small piece of the chorion (fetal placenta), which is analyzed. prions are infectious Prions proteins are proteinaceous infectious particles that alter protein function post-translationally. The gene encoding the prion protein (PrP) is found in humans and other mammals and is expressed at low levels in certain cell types (e.g., nerve cells). Prions can cause several types of neurodegenerative diseases in humans and livestock (Table 22.5). Prions exist in two conformation states (Figure 22.7), normal and abnormal. annormal form of the protein changes the normal protein to the abnormal form. post t ranslational modifi cation The abnormal form may be obtained by being infected by an affected individual, or by eating infected meat. Alternatively, some people carry alleles of the PrP gene that cause their normal protein to spontaneously convert to the abnormal form. The presence of abnormal prion proteins acts as a catalyst to convert normal proteins to the abnormal configuration (Figure 22.7). As the prion disease progresses, the abnormal PrPSc protein forms dense aggregates in the cells of the brain and peripheral nervous tissues. This deposition is correlated with the disease symptoms affecting the nervous system. Some of the abnormal prion protein is also excreted from the infected cells and can travel through the nervous system to infect other cells. Genetic Basis of Cancer General information. Cancer is a disease characterized by uncontrolled cell division; it is a genetic disease at the cellular level. Cancers have the following characteristics: Most cancers originate in a single cell. The cancerous growth is considered to be clonal in origin. Cancer is a usually a multistep process that begins with a precancerous genetic change (benign growth) and progresses to cancerous cell growth (Figure 22.8). 5 malignant means it invades other parts of t he body and divides metastasis can have benign tumor t hat gets so big that it becomes harmful When cells have become cancerous, their growth is described as malignant. Cancer cells are invasive and metastatic (able to move to other parts of the body). An environmental agent that causes cancer is called a carcinogen. Certain viruses can cause cancer by carrying viral oncogenes into the cell. not all viruses are acute, but some of them are Some viruses are known to cause cancers in plants, animals, and humans. The process of converting a normal cell to a malignant cell is called transformation. Most cancer-causing viruses are not potent at inducing cancer. A few, called acutely transforming viruses (ACVs), are effective in transformation. A gene that promotes cancer is called an oncogene. Viruses that cause cancer carry copies of genes that occur naturally in the genomes of species they infect. As an example, we will take a look at the Rous sarcoma virus (RSV), which carries the v-src gene and causes sarcomas (malignant tumors of bone, cartilage, fat, muscle, blood vessels, or other connective or supportive tissues) in chickens. The normal copy of the src gene, termed c-src for cellular src, is found in the chromosomal DNA of the host. It does not cause cancer. Once incorporated into a viral genome, the gene can become a viral oncogene that does promote cancer. There are three possibilities for this: The many copies of the virus made during viral replication may lead to overexpression of the src gene. Incorporation of the src gene next to viral regulatory sequences may cause it to be overexpressed. The v-src gene may accumulate mutations that convert it to an oncogene. onco genes that are in v iruses come from the genomes of the species t hey are infecting retrovirsues carry reverse transcriptase in t hem and carry an RNA genome... transcribed into DNA and incorporated into the host genome. t hs is how a virus can pick up a gene from a host RSV has acquired the src gene by capturing it from a host cell’s chromosome. RSV is a retrovirus that uses reverse transcriptase to make a DNA copy of its RNA genome. This is integrated as a provirus into the host cell genome. If integration were to occur near the c-src gene, during transcription of the proviral DNA the src gene could be included in the RNA transcript that becomes the retroviral genome. Table 22.6 lists several types of viruses that are known to cause cancer. 6 Experiment 22A. DNA isolated from malignant mouse cells can transform normal mouse cells into malignant cells. Most cancers are not caused by viruses, but by environmental mutagens that alter the expression of normal cellular genes. In 1971, Hill and Hillova demonstrated that purified DNA from RSV-infected cells could be taken up by chicken fibroblasts and would transform them into malignant cells. Experiments by Weinberg and colleagues in the late 1970s were performed to determine if purified DNA from cells that have become malignant due to exposure to mutagens could transform normal cells into malignant cells. Malignant cells can be identified as a distinct focus that grows over a monolayer of cells on a culture dish (Figure 22.9). Weinberg et al. began with several malignant cell lines that had been previously characterized, along with normal cell lines. The hypothesis. Cellular DNA isolated from malignant cells will be taken up by normal cells and will transform them into malignant cells. Testing the hypothesis (Figure 22.10) Extract the chromosomal DNA from normal or malignant cell lines. Mix the DNA from the normal cells or from the malignant cells with normal mouse fibroblast cells that are growing on a tissue culture plate. Make the cells permeable to DNA. Incubate for 14-20 days. Examine the plates for cells growing as transformed foci (normal cells grow as a monolayer). The data and interpreting the data (Figure 22.10). DNA isolated from some (but not all) malignant cell lines could transform normal mouse cells. These results are consistent with the hypothesis. Two years later, the first cellular oncogene in humans was identified. It was found that a mutation in the normal gene caused it to become an oncogene that caused bladder carcinoma in humans. 7 a v irus can pick up a gene from one species and affect a different s pecies with that oncogene. not always the c ase that the virus picks Many oncogenes have abnormalities that affect proteins involved in cell division pathways. up the gene form the host and affects the host In eukaryotic organisms, the normal cell cycle is regulated by polypeptide hormones as well. known as growth factors, which promote cell division. Many oncogenes encode proteins that function in cell growth signaling pathways (Table 22.7). Exposure to EGF. binds t o two EGF receptors c ausing them to dimerize and phosphorylate each other. This leads to activation of an intracellular signlaing pathway. GTPase. Protein Kinase. Transcript factors activated. This leads to t ranscirpt of genes involved in promoting of c ell division. Oncogenes may keep the cell growth-signaling pathway in a permanent on position. This may be due to overexpression of the oncogene, or the production of a functionally aberrant protein. One example of a growth factor that activates cell proliferation is epidermal growth factor, EGF (Figure 22.11). A normal nonmutated gene that has the potential to become an oncogene is called a proto-oncogene. To become an oncogene, a proto-oncogene must incur a mutation that causes its expression to become abnormally active. The mutation typically has one of three possible effects: 1) could be mkaing too The amount of the encoded protein is greatly increased. much EGF 2) has a different type of The protein structure is changed and the protein becomes overactive. t ertiary structure that makes it overactive..could only The protein is expressed in a cell type where it is not normally expressed. get that structure if an effector is attached Genetic changes in proto-oncogenes convert them to oncogenes. 3) somehow expressing a protein where it is not There are four major pathways that can convert proto-oncogenes into oncogenes normally turned on, s omehow that is affecting c ell division A missense mutation can cause a change in the amino acid sequence of a proto- (Table 22.8). oncogene protein that causes it to function in an abnormal way. The proto-oncogene ras can become oncogenic through a missense mutation (Figure 22.12). Another event that may occur in cancer cells is gene amplification; the copy number of a proto-oncogene may be increased by gene duplication. A chromosomal translocation may affect the expression of genes at the breakpoint site. A translocation that fuses the genes bcr and abl is oncogenic (Figure 22.13). This is known as the Philadelphia chromosome, and caused chronic myelogenous leukemia (CML). Viral integration (when a virus integrates into a chromosome) may enhance the expression of a nearby proto-oncogene. chronic myelogeneous leukemia 8 Tumor-suppressor genes play a role in preventing the proliferation of cancer cells. Tumor-suppressor genes prevent cancerous growth. If inactivated by mutation, the chances of cancer increase. The first identification of a tumor-suppressor gene involved studies of retinoblastoma. don't alway just need one activated oncogene to develop cancer, s ometimes you need two activated oncogenes to develop cancer Knudson (1971) proposed a “two-hit” model for retinoblastoma. People with the inherited form already have one of the mutations and need only one more mutation in the gene to develop the disease. People with the noninherited form must have two mutations in the same retinal cell to cause the disease. The gene in which the mutation occurs is designated rb (for retinoblastoma). Most people have two normal copies of the rb gene. Persons with hereditary retinoblastoma have one normal and one defective copy. In retinal tumor cells, the normal rb gene has suffered a second mutation, which renders it defective. Inactive complex + Phosphorylation => active complex + E2F f alls off Without the tumor suppressor ability, cells are allowed to grow and divide in an unregulated manner, which ultimately leads to cancer. Rb regulates the transcription factor E2F, keeping it from promoting entrance to S from G1 phase in the cell cycle. Loss of Rb leads to unregulated E2F activity and uncontrolled cell proliferation (Figure 22.14). The vertebrate p53 gene is a master tumor-suppressor gene that senses DNA damage (Figure 22.15). p53 is the most commonly altered gene in human cancers. About 50% of all cancers are associated with defects in this gene. p53 is a tumor-suppressor gene. Its primary role is to determine if a cell has incurred DNA damage. p53 promotes three types of cellular pathways that are aimed at stopping the proliferation of cells with damaged DNA. These are 1) repair of the DNA, 2) arresting the cell cycle, and 3) initiation of apoptosis, or programmed cell death. Apoptosis involves proteases called caspases. These act as the executioners of the cells. They digest cellular components, such as microtubules. Tumor suppressor genes can promote cancer when their function is lost. Table 22.9 lists tumor-suppressor genes that can promote cancer when their function is lost. 9 I f not enough of G1 c yclin, you don't make t he G1 cyclin/Cdk c omplex, you don't get past G1 complex. If not enough mitotic Cyclin, y ou don't make the activated mitotic cyclin/ Cdk complex ... you don't pass the G2 c heckpoint Other tumor-suppressor genes play a role in the proper maintenance of the genome. Maintenance of the genome is accomplished by both checkpoint proteins and DNA repair proteins. Checkpoint proteins prevent cell division when damaged DNA is detected. Checkpoint proteins can prevent the accumulation of cyclin-CdK complexes that promote cell division (Figure 22.16). DNA repair proteins are often inactivated in cancer cells (see Chapter 16). end Tumor suppressor genes can be silenced in a variety of ways. A gene mutation can occur within a tumor-suppressor gene and inactivate its function. Aberrant CpG island methylation near the promoters of tumor-suppressor genes appears to play a role in the formation and/or progression of malignancy (CpG island methylation usually inhibits transcription). Aneuploidy can lead to the loss of a tumor suppressor gene. Most forms of cancer involve multiple genetic changes leading to malignancy. Many cancers begin with a benign genetic alteration. When additional mutations occur, the alteration may become malignant. An example, using colorectal cancer, is shown in Figure 22.17. Approximately 300 genes may play a role in preventing or causing cancer. A common genetic change associated with cancer is abnormalities in chromosome number and structure (Figure 22.18). Karyotyping cancer cells often shows aberrant copy numbers for several chromosomes and multiple chromosomal translocations. DNA microarrays are used to classify tumors. Molecular profiling allows us to increase our understanding of molecular changes that occur in diseases such as cancer. DNA microarrays allow for unique expression profiles to be examined (Figure 22.19). This can help us to distinguish different types of cancers and provide information on likely clinical outcomes for specific cancers. 10 Inherited forms of cancers may be caused by defects in tumor-suppressor genes and DNA repair genes. About 5–10% of cancers involve inherited (germ-line) mutations. These individuals have a predisposition to develop cancer. Many, but not all, of these involve a defect in a tumor-suppressor gene (Table 22.10). Some inherited cancers are due to the activation of an oncogene. Please see the Conceptual and Experimental Summaries for Chapter 22 on pages 625-626. This lecture outline was prepared from Genetics: Analysis and Principles, by Brooker, 2009 (3rd edition). It contains phrases and entire sentences taken verbatim from that source, and is in no way meant to represent original work by Mark Bierner. 11 ...
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