Ch 22: Medical Genetics and Cancer

Observations for Genetic Disease

Observations for genetic basis of human disease We must relay on analyzing the occurrence of a disease in families that already exist (600) When the occurrence of a disease correlates with several of the following observations, a geneticist will become increasingly confident that it has a genetic basis (600-601) Disorder is more likely to occur in genetic relatives than in general population: Example is cystic fibrosis. Identical twins share disease more often than nonidentical twins: This is used to evaluate a disease’s concordance, which is the degree to which it is inherited. Geneticists will calculate the percentage of twin pairs that both exhibit the disorder relative to pairs where only 1 twin shows the disorder. Actual concordance values for single-gene disorders are usually less than theoretical values due to a variety of reasons. Does not spread to individuals sharing similar environmental conditions: Meaning that the disorder can not be spread from person to person like a disease. Different populations have different frequencies of the disease: This is due to evolutionary forces Disease tends to develop at a characteristic age: There is a characteristic age of onset. May resemble a disorder that is already known to have a genetic basis in an animal: An example is the albino phenotype. Correlation between disease and a mutant human gene or chromosomal alteration: Particularly convincing piece of evidence. We expect 1 person who has it and 1 person who doesn’t to have a difference in their genetic material.

Pedigree Analysis For Recessive Traits

Pedigree analysis showing inheritance patterns When a human disorder is caused by a mutation in a single gene, the pattern of inheritance can be deduced by analyzing human pedigrees (601) The pedigree for Tay-Sachs disease (601): Individuals appear healthy at birth, but exhibit neurodegenerative symptoms at 4-6 months old. Cerebral degeneration, blindness, loss of motor function. Die in 3rd to 4th year of life. Particularly prevalent in Ashkenazi (eastern European) Jews. The mutation that causes it is in a gene that encodes the enzyme hexosaminidase A (hexA), which is responsible for breakdown of GM2-gangliosides. GM2-gangliosides are a lipid that is prevalent in the CNS; when it can’t be broken down, it builds up in nerve cells and causes neurodegenerative symptoms. Tay-Sachs is autosomal recessive. Common features of autosomal recessive inheritance are (601-602): The affected offspring will frequently have unaffected parents: This is always the case in TSD b/c it causes early death. When 2 unaffected heterozygotes have children, there is a 25% chance of children being affected. 2 affected individuals will have 100% affected children: This cannot be the case in TSD due to early death. Trait occurs with same frequency in both sexes.' Human recessive allels are are often caused by loss of function mutations in the encoded enzyme (602) In Tay-Sachs, heterozygous carriers have approximately 50% of functional enzyme, which is sufficient for a normal phenotype.

Pedigree Analysis for Dominant Traits

Huntington's is a dominant trait that occurs during middle age. The major symptoms are due to degeneration of certain types of neurons in the brain (602) A mutation has added a polyglutamine tract to the amino acid sequence of the huntingtin protein, causing an aggregation of proteins in the neurons. There are 5 common features of autosomal dominant inheritance (602-603): Affected offspring usually has 1 or both affected parents: The exceptions are with incomplete penetrance and the mutation occurring during gametogenesis.  An affected individual w/1 affected parent expected to produce 50% affected offspring. 2 heterozygous, affected individuals will have 25% unaffected offspring. Trait occurs with same frequency in both sexes In most cases, homozygote is more severely infected. In autosomal dominant disorder, 50% of the normal protein is not sufficient to produce a normal phenotype (603). 3 common explanations for dominant disorders (603): Haploinsufficiency: A person has only a single functional copy of a gene, and that single functional copy does not produce a normal phenotype. An example is aniridia, a rare disorder that results on absence of the iris of the eye.  Gain of function mutations: Mutations change gene product so that it "gains" a new or abnormal function.  An example is achondroplasia, which is characterized by abnormal bone growth that results in a relatively short stature. The disorder is caused by a point mutation in the fibroblast growth factor receptor-3 gene; the mutant form of the receptor is overactive, disrupting normal signaling pathway.  Dominant-negative mutations: Altered gene product acts antagonistically to the normal gene product. Marfan syndrome is an example. It's due to a mutation in the fibrillin-1 gene, which encode a glycoprotein that is a structural component of the extracellular matrix that provides structure and elasticity to tissues. The mutant encodes a fibrillin-1 that antagonizes the effects of the normal protein weakening the elasticity of certain body parts.

X-Linked Recessive Inheritance

Poses a problem for males b/c they're hemizygous for these genes (604) A female heterozygote will pass trait on to 50% of sons.  Example is hemophilia. This is a disorder in which the blood cannot clot properly. A minor cut may bleed for a very long time, small injuries can lead to large bruises. Common injuries pose a threat of severe internal and external bleeding (604). Hemophilia A is caused by a defect in an X-linked gene encoding for Factor VIII.  X-linked traits show this pattern: Males are much more likely to be affected. The mothers of affected males often have brothers or fathers with disorder Daughters of affected males will produce 50% affected sons

Locus Heterogeneity

Hemophilia can be used to illustrate locus heterogeneity, which refers to the phenomenon in which a particular type of disease may be caused by mutations in 2 or more different genes (604). Blood clotting involves participation of several different proteins that take place in a cellular cascade. Hemophilia is usually caused by a defect in 1 of 3 different clotting factors. Locus heterogeneity arises from participation of several proteins in a common cellular process (604). Hemophilia A is caused by missing Factor VIII, whereas hemophilia B is a deficiency in Factor IX. Both are encoded by different genes on the X chromosome, and both disorders show X-linked recessive pattern of inheritance.  Hemophilia C is due to factor IX deficiency, which is found on chromosome 4. This form shows an autosomal recessive pattern.  Another way locus heterogeneity arises is when proteins are composed of 2 or more subunits that are encoded by different genes (604).  Thalassemia is an example. This potentially life-threatening disease involves defects in the ability of RBCs  transport oxygen due to an alteration in hemoglobin. Hemoglobin is composed of 2 alpha-globin and 2 beta-globin subunits, which are encoded by separate genes.  Locus heterogeneity can greatly confound pedigree analysis, especially for rare diseases that are poorly understood at the molecular level (604).

Genetic Testing

Genetic testing refers to the use of testing methods to determine if an individual carries a genetic abnormality. Genetic screening refers to population-wide genetic testing (606). In many cases, single-gene mutations that affect cellular protein function can be examined at the protein level (606-607). Biochemical assays can measure enzyme's activity if the gene encodes an enzyme. Enzymatic assays for hexosaminidase A (hexA), the protein that causes Tay-Sachs, involves the use of 4-methylumbrlliferone (MU) covalently linked to N-acetylglucosamine (GlcNAc). HexA cleaves this covalent bond and releases MU, which is fluorescent.  A more common approach to detecting single-gene mutations is to detect them at the DNA level (607). Researches must have previously identified the mutant human gene. Most genetic abnormalities that involve changes in chromosome number result in spontaneous abortion. But when they do arise, they can be detected by karyotyping (607) Genetic screening is commonly done in pregnant women over 35 years old. Testing for phenylketonuria (PKU) is another common genetic test (607).  Genetic testing before birth involves obtaining genetic material from the fetus and using it for aminocentesis or chorionic villus sampling (607-608) Aminocentesis involves the doctor removing amniotic fluid containing fetal cells using a needle that is passed through abdominal wall. Fetal cells are cultured then karyotyped to determine chromosome number per cell and changes in chromosome . Chorionic villus sampling involves a small piece of the chorion being removed and being karyotyped. It can be performed earlier in the pregnancy, but it poses a slightly greater risk of causing a miscarriage

Prions

Prions are particles that alter protein function A prion is a disease-causing agent composed entirely of protein (608).  These diseases arise from the ability of the prion protein to exist in a normal form and an abnormal form (609).  The gene encoding the prion protein is expressed at low levels in certain types of cells (such as nerve cells), but the abnormal conformation comes about 2 different ways (609): Abnormal protein is taken into the body (such as by consumption). Some people carry alleles of the abnormal genes that cause their prion protein to convert spontaneously to the abnormal conformation at a very low rate.  An example is familial fatal insomnia Abnormal conformation  of prion protein acts as a catalyst to convert normal prion proteins within the cell to the misfolded conformation (609).  As disease progresses, the abnormal protein forms dense aggregates in the cells of the brain and peripheral nervous system (609).  Some abnormal prion protein is also excreted from infected cells and can travel through nervous system (609).

Genetics and Cancer

   Characteristics that are common to all cancers (610): Originate in a single cell. This cell and its line of daughters undergoes a series of genetic changes that accumulate during cell division.  Cancer growths can be considered clonal in origin. Usually a multistep process that begins with a precancerous genetic change (benign growth) and is followed by additional genetic changes that lead to a cancerous growth. Cancerous cell growth is described as malignant. These cells are metastatic and can migrate to other parts of the body. In 5-10% of all cases, a predisposition to develop the cancer is inherited (610)

Viruses and Cancer

Certain viruses cause cancer with viral oncogenes process of converting a normal cell to a malignant cell is called transformation (611). Most cancer-causing viruses are not very potent at inducing cancer and are inefficient at transforming. Individual must be infected for awhile for a tumor to grow (611). A few types of viruses do rapidly induce tumors and efficiently transform cells in culture. These are called acutely transforming viruses (611).  An oncogene is a gene that promotes cancer (611). If you want more information about a focus, go to page 612 of textbook. 

Oncogenes and Cell Division Pathways

Many oncogenes affect proteins involved in cell division pathways A normal, nomutated gene that has the potential to be an oncogene is called a proto-oncogene (614). The become an oncogene, a proto-oncogene must incur a mutation causing it to be abnormally active. This mutation typically has 1 of 3 possible effects (614): Amount of encoded protein is greatly increased. A change occurs in structure of encoded protein that causes it to be overly active. Encoded protein is expressed in a cell type where it is not normally expressed. Oncogenes commonly encode proteins that function in cell growth signaling pathways (614). Genetic changes cause oncogenes 4 main ways that proto-oncogenes become oncogenes: Missense mutation, gene amplification, chromosomal translocation, viral integration (616). Missense mutation: This occurs in the Ras protein, which becomes an oncogene when changes in its structure can cause it to be permanently activated. The structural changes are caused by a missense mutation in one of the 4 Ras genes (616). Gene amplification: This is an abnormal increase in the copy number of a proto-oncogene. An increase in copy number also increases the amount of encoded protein (617).  Many human cancers are associated with amplification of particular oncogenes. In these cases, the extent of oncogenic amplification may be correlated with progression of tumors to increasing malignancy.  Chromosomal translocation: This occurs in chronic myelogenous leukemia, which is associated with a shortened version of chromosome 22 resulting from a reciprocal translocation between chromosomes 9 and 22. This is now called the Philadelphia chromosome (617-618).  Translocation activates the proto-oncogene abl. Reciprocal translocation involves breakpoints within abl and bcr genes; the coding sequence of abl fuses with promoter and coding sequence of bcr, yielding an oncogene that encodes an abnormal fusion protein containing the polypeptide sequences encoded from both genes. Abl encodes a tyrosine kinase enzyme and is generally highly regulated. But in the Philadelphia chromosome, it's under the control of bcr's promoter, which is highly active in white blood cells. This leads to an overexpression of tyrosine kinase function in white blood cells.  Another example of a chromosomal translocation causing cancer is Burkitt's lymphoma. A region of chromosome 8 is translocated to either 2, 14, or 22. The break occurs near c-myc gene, and the sites on the other chromosomes correspond to different immunoglobulin genes. The translocation of c-myc to near the immunoglobulin gene leads to overexpression of c-myc. Viral integration: As part of reproductive cycle, certain viruses integrate their genomes into the chromosomal DNA of host cell. If integration occurs next to a proto-oncogene, a viral promoter or enhancer may cause it to be overexpressed (618).

Tumor Suppressor Genes

Tumor suppressor genes play a role in preventing proliferation of cancer As the name suggests, tumor-suppressor genes prevent cancerous growth. When it becomes inactivated, it becomes more likely that cancer will arise (618), Retinoblastoma is caused by a "two-hit" model of tumor-suppressor gene deactivation. People with the hereditary form only have 1 functional gene from parents; they only need 1 additional mutation in the other copy of the tumor suppressor gene to develop the disease. Since the retina contains more than 1 million cells, it is relatively likely that a mutation may occur at an early age (618).  The mutated gene is called rb; it's located on long arm of chromosome 13.  The rb protein inhibits E2F, which prevents the cell cycle from progressing. When rb goes offline, E2F is not turned off (618-619). The p53 gene Most commonly altered in all human cancers (619). Role of p53 is to determine if a cell has incurred DNA damage; if damage is detected, p53 can promote 3 types of pathways aimed at preventing the proliferation of the damaged cell (619): Repair the DNA, which may prevent the accumulation of mutations that activate oncogenes or inactivate tumor-suppressor genes. Arrest the cell cycle. This gives the cell more time to repair its DNA and prevents the production of mutated daughter cells. Stimulates apoptosis. Expression of p53 caused by formation of damaged DNA. Specifically, the inducing signal for p53 is a double-stranded DNA break (619). Functions as a transcription factor; it can activate genes that promote DNA repair, arrest the cell cycle, and promote apoptosis.  It also appears to act a negative regulator by interacting with general transcription factors, which may prevent the cell from dividing.  Tumor suppressor genes can no longer inhibit cancer when function is lost The general function of tumor-suppressor genes are either to negatively regulate cell division or negatively regulate genes that maintain genome integrity (620).  Some tumor-suppressor genes encode proteins that have direct effects on the regulation of cell division, such as the rb gene. When these go offline, it has a direct impact on the abnormal cell division rates seen in cancer cells (620). Some tumor-suppressor genes play a role in genome maintenance, which refers to the cellular mechanisms that either prevent mutations from happening and/or prevent mutant cells from surviving. These can either be checkpoint proteins and DNA repair enzymes (620). Checkpoint proteins detect abnormalities, such as DNA breaks and improperly segregated chromosomes.  Cyclins and cyclin-dependent protein kinases are responsible for advancing a cell through the 4 phases of the cell cycle.  G1 and G2 checkpoints involve sensing of the DNA has incurred damage. If it has, checkpoint proteins such as p53 can  prevent the formation of the cyclin/cyclin-dependent protein kinase complexes from forming, which stops the progression of the cell cycle.  If checkpoint proteins are lost, it becomes more likely that undesireable genetic changes will occur and cause cancerous growth.  DNA repair enzymes. The loss of these enzymes makes it more likely for a cell to accumulate mutations that create an oncogene and/or eliminate the function of a tumor-suppressor gene.  For info on how tumor-suppressor genes are deactivated, visit page 621. For info on DNA microarrays and tumor classification, visit page 623-624, For info on inherited forms of cancer, visit page 624-625