Lecture 03 Genet Disease PDF

Genetically Based Enzyme Deficiencies in Humans Chapter 4 slide 1 Genetically Based Enzyme Deficiencies in Humans Single gene mutations are responsible for many human genetic diseases. Some mutations create a simple phenotype, while others are pleiotropi...

Genetically Based Enzyme Deficiencies in Humans Chapter 4 slide 1 Genetically Based Enzyme Deficiencies in Humans Single gene mutations are responsible for many human genetic diseases. Some mutations create a simple phenotype, while others are pleiotropic (Table 4.2). ENZYME XDA PUN KIRA PHENOTYPE Chapter 4 slide 2 AUTOSOME IS NON-SEX GENE  Phenylketonuria http://www.genetic-counseling.com.tw/datebase/PKU_10.21.htm Phenylketonuria (PKU) is commonly caused by a mutation on chromosome 12 in the phenylalanine hydrolase gene, preventing the conversion of phenylalanine into tyrosine (Figure 4.1). Phenylalanine is an essential amino acid, but excess is harmful, and so is normally converted to tyrosine. *make more than 1 effect PKU’s effect is pleiotropic. Some symptoms result from excess phenylalanine. Others result from inability to make tyrosine. Diet is used to manage PKU by providing just enough phenylalanine for protein synthesis, but not enough that it accumulates. NutraSweet is aspartame, which breaks down to aspartic acid and phenylalanine, with serious consequences for a phenylketonuric. Chapter 4 slide 3 Fig. 4.1 Phenylalanine-tyrosine metabolic pathways Dihydroxyphenylalanine Chapter 4 slide 4 Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings. Albinism Classic albinism results from an autosomal recessive mutation in the gene for tyrosinase. Tyrosinase is used to convert tyrosine to DOPA in the melanin pathway. Without melanin, individuals have white skin and hair, and red eyes due to lack of pigmentation in the iris. Two other forms of albinism are known, resulting from defects in other genes in the melanin pathway. A cross between parents with different forms of albinism can produce normal children. Chapter 4 slide 5  Lesch-Nyhan Syndrome Results from a recessive mutation on the X chromosome, in the gene for hypoxanthine-guanine phosphoribosyl transferase (HGPRT). The fatal disease is found in males, while heterozygous (carrier) females may show symptoms when lyonization (X-inactivation) of the normal X chromosome leaves the X chromosome with the defective HGPRT gene in control of cells. HGPRT is an enzyme essential to purine utilization. In Lesch- Nyhan syndrome this pathway is highly impaired. Purines accumulate and are converted to uric acid. Symptoms of Lesch-Nyhan syndrome?? A defect in a single enzyme, HGPRT, has very pleiotropic effects, giving rise to uremia, kidney failure, mental deficiency and self-mutilation. Chapter 4 slide 6  Tay-Sachs Disease http://www.genetic-counseling.com.tw/datebase/Tay-Sachs%20disease-11.20.htm Tay-Sachs is one of a group of diseases called lysosomal-storage diseases. Generally caused by recessive mutations, in genes encoding lysosomal enzymes. Tay-Sachs disease results from a recessive mutation in the gene hexA, which encodes the enzyme N-acetylhexosaminidase A. The HexA enzyme cleaves a terminal N-acetylgalactosamine group from a brain ganglioside.(Fig. 4.7) Infants homozygous recessive for this gene will have nonfunctional HexA enzyme. Unprocessed ganglioside accumulates in brain cells, and causes various clinical symptoms. The disease is incurable. Carriers and affected individuals can be detected by genetic testing. Chapter 4 slide 7 Fig. 4.7 The biochemical step for the conversion of the brain ganglioside GM2 to the ganglioside GM3, catalyzed by the enzyme N-acetylhexosaminidase A (hex A) Chapter 4 slide 8 Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings. Gene Control of Protein Structure Genes also make proteins that are not enzymes. Structural proteins, such as hemoglobin, are often abundant, making them easier to isolate and purify. Chapter 4 slide 9  Sickle-Cell Anemia J. HERRICK (1910) first described sickle-cell anemia, finding that red blood cells (RBCs) change shape (form a sickle) under low O2 tension. a. Sickled RBCs are fragile, hence the anemia. b. They are less flexible than normal RBCs, and form blocks in capillaries, resulting in tissue damage downstream. c. Effects are pleiotropic, including damage to extremities, heart, lungs, brain, kidneys, GI tract, muscles and joints. Results include heart failure, pneumonia, paralysis, kidney failure, abdominal pain and rheumatism. d. Heterozygous individuals have sickle-cell trait, a much milder form of the disease. E.A. BEET and J.V. NEEL independently proposed (1949) that sickle-cell trait and disease were the result of a single mutant allele. Chapter 4 slide 10 Animation: Gene Control of Protein Structure and Function Linus PAULING and coworkers (1949) used electrophoresis (Figure 4.9) and showed: a. Hemoglobin from individuals with sickle-cell anemia (Hb-S) has altered mobility compared with normal hemoglobin (Hb-A). b. Hemoglobin from individuals with the sickle-cell trait shows equal amounts of Hb-A and Hb-S, indicating that heterozygotes make both forms of hemoglobin. c. Therefore, the sickle-cell mutation changes the form of its corresponding protein, and protein structure is controlled by genes. Chapter 4 slide 11 Fig. 4.9 Electrophoresis of hemoglobin variants Chapter 4 slide 12 Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.  Hemoglobin is formed by four polypeptide chains, two molecules of the α polypeptide and 2 of the β polypeptide, each associated with a heme group (Figure 4.10). Chapter 4 slide 13 Fig. 4.10 The hemoglobin molecule Chapter 4 slide 14 Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.  V.M. INGRAM (1956) found that the 6th amino acid of the β-chain in sickle-cell hemoglobin is valine (no electrical charge) rather than the negatively charged glutamic acid in the β chain of normal hemoglobin (Figure 4.11). Fig. 4.11 The first seven N-terminal amino acids in normal and sickled hemoglobin  polypeptides Chapter 4 slide 15  Outline of the genetics and gene products involved in sickle- cell anemia and trait: a. Wild-type β chain allele is βA, which is codominant with βS. b. Hemoglobin of βA/βA individuals has normal β subunits, while hemoglobin of those with the genotype βS/βS has β subunits that sickle at low O2 tension. c. Hemoglobin of βA/βS individuals is 1⁄2 normal, and 1⁄2 sickling form. (The two β chains of an individual hemoglobin molecule will be of the same type, rather than mixed.) These heterozygotes may experience sickle-cell symptoms after a sharp drop in the oxygen content of their environment. Chapter 4 slide 16 Other Hemoglobin Mutants Screening of hemoglobin for altered electrophoretic mobility has identified over 200 hemoglobin mutants, showing a variety of amino acid substitutions in both the α and the β chains. Each appears to derive from a single amino acid change (Figure 4.12). Most effects are not as severe as those seen in sickle-cell anemia. Chapter 4 slide 17 Fig. 4.12a Examples of amino acid substitutions found in  polypeptides of various human hemoglobin variants Chapter 4 slide 18 Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings. Fig. 4.12b Examples of amino acid substitutions found in  polypeptides of various human hemoglobin variants Chapter 4 slide 19 Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings. Cystic Fibrosis Cystic fibrosis (CF) affects the pancreas, lungs and digestive system, and sometimes the vas deferens in males. The disease is characterized by abnormally viscous secreted mucus, and lung complications are managed by percussion and antibiotics to treat infections. Life expectancy with current treatments is about 40 years. *long arm is q The affected gene is on the long arm of chromosome 7, and encodes a protein called cystic fibrosis transmembrane conductance regulator (CFTR). Comparing DNA sequences of cloned gene from normal and CF individuals shows that the CF mutation commonly is the deletion of a specific 3-bp region, removing one amino acid from the protein product. Chapter 4 slide 20 Cystic Fibrosis The structure of the protein has been deduced from its sequence (Figure 4.13). CFTR has homology with a large family of active transport membrane proteins. Functional analysis shows that CFTR normally forms a chloride channel in the cell membrane. The mutated gene results in an abnormal CFTR protein, preventing chlorine ion transport and resulting in CF symptoms. Chapter 4 slide 21 Fig. 4.13 Proposed structure for cystic fibrosis transmembrane conductance regulator (CFTR) Chapter 4 slide 22 Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings. GENETIC COUNSELING Chapter 4 slide 23  Genetic Counseling Genetic testing can detect many inherited enzyme and protein defects, yielding information about whether an individual has a disease or is a carrier. Chromosomal abnormalities can also be detected. Genetic counseling is advice based on genetic analysis, focusing either on the probability that an individual has a genetic defect, or the probability that prospective parents will produce a child with a genetic defect Some aspects of human heredity are well understood, others not yet so well. Effective genetic counseling requires up-to-the minute knowledge of genetic research, and the ability to offer clients unbiased and nonprescriptive information from two main sources: a. Pedigree analysis is an important tool of genetic counseling, considering phenotypes found in both families over several generations. Useful for identifying suspected carriers. b. Fetal analysis includes assays for enzyme activity or protein level, or detection of changes in the DNA itself. For most defective alleles, there is currently no way to change the resulting phenotype, and so genetic counseling focuses primarily on informing clients of risks and probabilities. Chapter 4 slide 24 Carrier Detection A carrier is heterozygous for a recessive gene mutation. In a cross between two carrier parents, 1⁄4 of the offspring are expected to develop the disease, and 1⁄2 to also be carriers. The carrier’s phenotype is normal, but if levels of the affected protein are determined, they may be well below those of a normal individual. Chapter 4 slide 25 Fetal Analysis Genetic counseling is also concerned with whether a fetus is normal. A sample of fetal cells is needed for the analysis. There are currently two methods of obtaining the necessary sample: a. Amniocentesis: removal of a sample of amniotic fluid using a syringe needle inserted through the uterine wall b. Chorionic villus (lining on placenta) sampling Chapter 4 slide 26 Fetal Analysis Once fetal cells are obtained they are usually cultured in the laboratory, although chorionic villus sampling may provide enough tissue to assay directly. They are examined for protein or enzyme alterations or deficiencies, DNA changes and chromosomal abnormalities. Amniocentesis is costly and cannot be performed until the second trimester, removing early abortion as an option in cases of severe genetic defects. Chorionic villus sampling can be done earlier, but carries a higher risk of fetal death and inaccurate diagnosis due to the presence of maternal cells. Chapter 4 slide 27  End of lecture Chapter 4 slide 28