1. Genetic Diseases.pdf

Istanbul Okan University Genetic Diseases Dr. Hilal Eren Gözel Genetic Disorders ❖ A genetic disorder is a disease caused in whole or in part by a change in the DNA sequence away from the normal sequence. ❖ Genetic disorders can be caused by ❖ a mutation in one gene (monogenic/single-gene disorder),...

Istanbul Okan University Genetic Diseases Dr. Hilal Eren Gözel Genetic Disorders ❖ A genetic disorder is a disease caused in whole or in part by a change in the DNA sequence away from the normal sequence. ❖ Genetic disorders can be caused by ❖ a mutation in one gene (monogenic/single-gene disorder), ❖ mutations in multiple genes (multigenic inheritance disorder), ❖ damage to chromosomes (chromosomal abnormalities), or ❖ a combination of gene mutations and environmental factors (Multifactorial disorders). Monogenic Disorders ❖ There are more than 7000 inherited diseases that result from defects in individual genes. These are called monogenic disorders. (OMIM statistics, 2022) ❖ The commonest include the lung disease cystic fibrosis, Huntington’s disease (a neurological disorder with uncoordinated body movements), and Marfan syndrome (affects connective tissue and leads to various problems including heart disease). ❖ Other diseases are much rarer, with just a few cases being reported worldwide, possibly affecting just a few families. ❖ *Online Mendelian Inheritance in Man (OMIM) is an online catalog of mendelian traits and disorders, which is updated everyday! (http://www.ncbi.nlm.nih.gov/omim) The Commonest Monogenic Disorders Genetic Basis of Inherited Disease Loss of Function ❖ The vast majority of human monogenic disorders are due to a loss-offunction mutation that inactivates a particular gene. ❖ The protein coded by that gene is absent, or modified in some way that prevents it from functioning correctly, and the resulting defect is manifested as the inherited disease. Loss of Function ❖ The loss of function could be caused by a point mutation, such as a nonsense mutation that creates an internal termination codon within a gene. ❖ It could also be caused by a small deletion or insertion within the gene, or by a large deletion that removes the entire gene. ❖ The mutation might also be in a regulatory sequence upstream of the gene so that no transcription initiation occurs, or within one of the internal sequences that control splicing. ❖ A promoter mutation can decrease the affinity of RNA polymerase for a promoter site, often resulting in reduced production of mRNA and thus decreased production of a protein. Mutations of transcription factor genes or enhancer sequences can have similar effects. Loss of Function ❖ A single disorder is often caused by more than one mutation, with different individuals carrying different mutations but still having the same disease. ❖ Cystic fibrosis disease affects approximately 8000 people in the UK and 30,000 in the USA, can be caused by any one of 1400 different mutations. Cystic Fibrosis ❖ The cystic fibrosis is an autosomal recessive disorder of exocrine gland. ❖ People who have cystic fibrosis have a faulty protein that affects the body’s cells, its tissues, and the glands that make mucus and sweat. ❖ This protein is the cystic fibrosis transmembrane regulator (CFTR), which is involved in the transport of chloride ions into and out of the cell. ❖ Normal mucus is slippery and protects the airways, digestive tract, and other organs and tissues. Cystic fibrosis causes mucus to become thick and sticky. As mucus builds up, it can cause blockages, damage, or infections in affected organs. ❖ Cystic fibrosis used to cause death in childhood. Survival has improved because of medical discoveries and advances in newborn screening, medicines, nutrition, and lung transplants. Cystic Fibrosis Cystic Fibrosis ❖ The commonest of these mutations is: ∆F508 [The notation indicates that the mutation results in deletion (∆) of a phenylalanine (F) from position 508 of the protein coded by the cystic fibrosis gene]. ❖ The ∆F508 mutation prevents the protein from attaching to the cell membrane, and this loss of function leads to inflammation and the accumulation of mucus in the lungs, the primary symptoms of cystic fibrosis. Cystic Fibrosis ❖ The next most common cystic fibrosis mutation: G542X ❖ This is a point mutation that converts the codon for a glycine (G) normally present at position 542 in the protein into a termination codon (X). ❖ The presence of this internal termination codon in the mRNA transcribed from the mutant CFTR gene switches on the surveillance system that degrades mutated mRNAs. This means that the CFTR protein is not synthesized, again resulting in the loss of function. Cystic Fibrosis ❖ The third most common mutation: G551D ❖ Replacing glycine 551 with aspartic acid does not prevent the protein from being synthesized or from folding correctly, but changes its kinetic properties so it now transports chloride ions at only 4% of the rate of the unmutated protein. ❖ Again, this results in a loss of function. Although the protein is still able to transport chloride ions, it does so at such a slow rate that the physiological function of the protein is lost. Gain of Function ❖ Gain of function is much less common among inherited diseases, simply because there are relatively few types of underlying mutation that can cause a gain of function. ❖ One possibility is overexpression of a gene, so that its protein product accumulates in the cell in greater than normal quantities. ❖ Overexpression could be due to ❖ a mutation in one of the regulatory sequences that control transcription initiation, ❖ but more frequently it arises from gene duplication. Charcot–Marie–Tooth (CMT) ❖ Charcot–Marie–Tooth (CMT) is one of the commonest inherited neurological disorders, affecting the peripheral nerves and muscle strength so that actions such as walking are affected. ❖ The increase from two to three copies of the myelin protein gene PMP22 is sufficient, in some unknown way, to cause the symptoms of this disease. ❖ Duplication of PMP22 arises by recombination between repeat sequences on each side of the chromosome 17 that contains this gene. Gain of Function ❖ A second way of acquiring a gain of function is by the mutation of a protein that acts as a cell surface receptor and that relays a message into the cell when it binds an external signaling compound, such as a hormone. ❖ The signal passed on by the surface receptor might have various effects inside the cell, including changes in gene expression patterns. ❖ A normal cell will respond to the external stimulus in an appropriate way, but some mutations can affect a cell surface receptor so that it remains active even when its external signaling compound is absent. ❖ The cell therefore acts as though it were continuously being affected by the signaling compound, resulting in the gain in function. Gain of Function ❖ The very rare Jansen’s disease, with only 20 known cases in the entire world, is due to gain of mutation in the parathyroid hormone receptor, leading to growth defects. Trinucleotide Repeat Expansions ❖ Trinucleotide repeat expansion, is a relatively short series of trinucleotide repeats becomes elongated to two or more times than its normal length. ❖ These expansions are associated with several neurodegenerative diseases, including Huntington’s disease (HD), one of the commonest inherited disorders. Huntington’s disease (HD) ❖ The normal HD gene contains the sequence 5’-CAG-3’ repeated between 6 and 35 times in tandem, coding for a series of glutamines in the protein product. ❖ In Huntington’s disease this repeat expands to a copy number of 36–121, increasing the length of the polyglutamine tract and resulting in a loss of function of the HD protein. ❖ The exact biochemical role of this protein has not yet been discovered but it is thought to act within the nerve cells of the brain. Dominantly Inherited Diseases ❖ The mutated version of a gene that leads to an inherited disease is an allele. ❖ Those inherited diseases that are dominant are displayed by individuals who either are homozygous for the disease allele or are heterozygotes with one disease allele and one normal allele. Dominantly Inherited Diseases ❖ At least one of the parents of an affected child must also have the disease, which in turn tells us that the disease, however unfortunate its symptoms, is not so severe as to prevent at least some afflicted individuals from reaching reproductive age and having children. ❖ It is therefore not surprising that many dominant inherited diseases either have incomplete penetrance, so not all individuals with a disease genotype actually develop the symptoms, or have a delayed onset and so do not affect the person until later in their life. ❖ Examples are Huntington’s disease and Marfan syndrome. Recessively Inherited Diseases ❖ If the disease allele is recessive, as is the case for cystic fibrosis, then only homozygotes will have the disease. ❖ With recessive diseases it is therefore quite possible for an affected child to have two nonaffected parents. ❖ Both parents would be heterozygotes, carriers who have one normal and one disease allele. (carrier) Sex-linked Inherited Diseases ❖ The inheritance pattern will be different if the gene is on one of the sex chromosomes. ❖ If the gene is on the X chromosome then it will always be expressed by males who possess the defective allele. ❖ This is because males have just one copy of the X chromosome, making it immaterial whether the disease allele is dominant or recessive. Sex-linked Inherited Diseases Multigenic Disorders ❖ There are some conditions that require the combined activity of alleles at a few loci to produce a phenotype. ❖ To date, the best examples are various two-locus (digenic) systems: Bardet–Biedl syndrome (BBS) ❖ Bardet–Biedl syndrome (BBS) is a rare non-motile ciliopathy. ❖ BBS is a heterogeneous recessive disorder with at least eight identified loci (BBS1 to BBS8). ❖ The BBS clinical features include retinal dystrophy, obesity, polydactyly, kidney defects, underdeveloped testes, and cognitive impairment. ❖ In some cases, a total of three mutant alleles at the BBS2 and BBS6 loci were required to give rise to the BBS phenotype. Bardet–Biedl syndrome (BBS) Multifactorial Diseases ❖ Traits in which variation is thought to be caused by the combined effects of multiple genes are called polygenic (“many genes”). ❖ When environmental factors are also believed to cause variation in the trait, which is usually the case, the term multifactorial is used. ❖ Many quantitative traits (those, such as blood pressure, that are measured on a continuous numerical scale) are multifactorial. Multifactorial Diseases ❖ Heart diseases ❖ Heart disease is the leading cause of death worldwide, and it accounts for approximately 25% of all deaths in the United States. ❖ The most common underlying cause of heart disease is coronary artery disease (CAD), which is caused by atherosclerosis (a narrowing of the coronary arteries resulting from the formation of lipid-laden lesions). Multifactorial Diseases ❖ Heart diseases ❖ This narrowing impedes blood flow to the heart and can eventually result in a myocardial infarction (death of heart tissue caused by an inadequate supply of oxygen). When atherosclerosis occurs in arteries that supply blood to the brain, a stroke can result. ❖ A number of risk factors for CAD have been identified, including obesity, cigarette smoking, hypertension, elevated cholesterol level, and positive family history (usually defined as having one or more affected first- degree relatives). Multifactorial Diseases ❖ Twin concordance studies Multifactorial Diseases ❖ Twin concordance studies ❖ Twins may be identical (monozygotic) or non-identical (dizygotic). ❖ Monozygotic twins (MZ) are genetically identical as they arise from a single zygote that divides into two embryos. ❖ Dizygotic twins (DZ) result from two ova fertilized by two spermatozoa and so have, on average, one-half of their genes in common and are genetically equivalent to brothers and sisters (siblings). Multifactorial Diseases Multifactorial Diseases ❖ Twin concordance studies ❖ Because MZ twins are genetically identical, any differences between them should be due only to environmental effects. MZ twins should thus resemble each other very closely for traits that are strongly influenced by genes. ❖ DZ twins provide a convenient comparison: their environmental differences should be similar to those of MZ twins, but their genetic differences are as great as those between siblings. ❖ Twin studies thus usually consist of comparisons between MZ and DZ twins. Mitochondrial Diseases ❖ Cellular ATP is generated by oxidative phosphorylation (OXPHOS) in cytoplasmic organelles called mitochondria. ❖ The cells of the brain, skeletal muscle, heart, kidney, and liver have high energy demands and, consequently, contain thousands of mitochondria, whereas cells with low energy requirements have only between 10 and 100 mitochondria. ❖ In humans, the mitochondria of the zygote come from the oocyte, that is, from the mother and almost never from the sperm, that is, from the father. This form of transmission is called maternal inheritance. Mitochondrial Diseases Healthy male Affected female From: An introduction to human molecular genetics Mitochondrial Diseases ❖ A human oocyte has approximately 100,000 mitochondria, but, as it matures, vast numbers of mitochondria are lost. ❖ The process of reducing the number of mitochondria from 100,000 to less than 100 has been called a genetic bottleneck. ❖ If one of the mitochondria that survives the bottleneck happens to carry a mutated gene, then this genome will be well represented in the ensuing mitochondrial population and, after development is completed, will populate the tissues of an individual. ❖ The ratio of mutated to normal mitochondrial DNA is called the mitochondrial mutation load. Mitochondrial Diseases Mitochondrial Diseases ❖ The term homoplasmy describes the situation in which all the mitochondria of a cell or tissue have the same genome, which may be either the wild-type sequence or one with a gene mutation. ❖ Heteroplasmy denotes a cell or tissue containing both mutant and wildtype mitochondrial genomes. ❖ If a mitochondrial gene mutation reduces the production of ATP, then cells with a high energy demand that are homoplasmic for the mutant mitochondrial DNA will be seriously damaged. ❖ On the other hand, homoplasmy for mutant mitochondrial DNA would have little impact on cells with low energy requirements. Mitochondrial Diseases ❖ In other words, for mitochondrial disorders, there is a threshold for phenotypic expression. ❖ The threshold at which the deleterious effects of a mitochondrial gene mutation become apparent depends on the energy needs of a particular cell or tissue. ❖ Thus, the brain, skeletal muscles, heart, and liver, all of which have considerable energy requirements, are highly susceptible to mitochondrial gene mutations. ❖ Some of the biological consequences of mitochondrial mutations are myopathy, cardiomyopathy, dementia, sudden uncontrolled muscle contractions (myoclonus epilepsy), deafness, blindness, anemia, diabetes, and loss of cerebral blood supply (stroke). ❖ The overall incidence of mitochondrial disorders is about 1 in 10,000 live births. Mitochondrial Diseases Beyond Genetics… ❖ One of the tenets of Mendelian genetics is that reciprocal crosses with autosomal loci produce the same ratios and phenotypes among the offspring. ❖ For example, parents who are each homozygous for different autosomal alleles have only heterozygous offspring with the same phenotype regardless of the type of dominance. Parent of Origin ❖ An indication of a non-Mendelian process is the consistent lack of phenotypic equivalence from reciprocal matings. ❖ Such a situation may arise when an allele from one parent always determines the phenotype and the other allele, although present and not mutated, is not expressed. Epigenetics ❖ The process that leads to the expression of an allele that is inherited from one parent and the inactivation of its counterpart in the other parent is an epigenetic phenomenon called genomic imprinting. ❖ Epigenetics is the study of heritable traits from parent to offspring that are not the result of DNA mutation. Epigenetics ❖ Epigenetics involves genetic control by factors other than an individual's DNA sequence. Epigenetic changes can switch genes on or off and determine which proteins are transcribed. ❖ Epigenetics is involved in many normal cellular processes. Consider the fact that our cells all have the same DNA, but our bodies contain many different types of cells: neurons, liver cells, pancreatic cells, inflammatory cells, and others. Epigenetics ❖ Cells, tissues, and organs differ because they have certain sets of genes that are "turned on" or expressed, as well as other sets that are "turned off" or inhibited. ❖ Epigenetic silencing is one way to turn genes off, and it can contribute to differential expression. ❖ Silencing might also explain, in part, why genetic twins are not phenotypically identical! ❖ Within cells, there are three systems that can interact with each other to silence genes: DNA methylation, histone modifications, and RNAassociated silencing Epigenetics ❖ DNA methylation is a chemical process that adds a methyl group to DNA. It is highly specific and always happens in a region in which a cytosine nucleotide is located next to a guanine nucleotide that is linked by a phosphate; this is called a CpG site. ❖ CpG sites are methylated by DNA methyltransferases (DNMTs). Inserting methyl groups changes the appearance and structure of DNA, modifying a gene's interactions with the machinery that is needed for transcription. ❖ The regions of CpG repeats (CpG islands) that precede commonly expressed genes (housekeeping genes) are not methylated. On the other hand, genes that are transcriptionally inactive are frequently heavily methylated (hypermethylated). Epigenetics Epigenetics ❖ During early mammalian development, the level of methylation of genomic DNA undergoes dramatic changes. ❖ After the zygote is formed, almost all of the chromosomal DNA is demethylated. As development proceeds, both general and gene-specific DNA methylation is restored. ❖ By the time somatic cell differentiation is complete, the overall patterns of DNA methylation are established and, subsequently, maintained through ensuing cell division cycles. ❖ It is not known how the extent of methylation of different genes is determined. Epigenetics ❖ DNA methylation can affect gene transcription either directly or indirectly. ❖ Briefly, a methylated promoter region may block transcription by preventing the binding of transcription factors. ❖ Or the DNA methyl groups may bind a protein that, in turn, binds other proteins that change the conformation of the chromosome and make the promoter region inaccessible to transcription. Genomic Imprinting ❖ In practice, genomic imprinting creates a hemizygous condition. ❖ Consequently, when a chromosome with either a mutated or a deleted imprintable gene is inherited from the parent that ensures expression of this allele, no functional gene product is produced because the active allele is mutated and the one from the other parent is silenced (Figure A and B), whereas a mutated allele that is silenced has no effect (Figure A1 and B1). ❖ A disease phenotype is likely to occur when no functional gene product is synthesized. Genomic Imprinting expressed inactive Reduced activity Genomic Imprinting ❖ The pattern of inheritance of a mutated imprinted gene in a multigeneration pedigree is determined by the parent that generates the active gene. ❖ With a paternally expressed, maternally silenced locus, the offspring are only affected when the mutated or deleted imprinted gene is transmitted through the male germ line (Figure A). ❖ Alternatively, for a maternally expressed, paternally silenced locus, the disorder is passed on exclusively by mothers (Figure B). Genomic Imprinting ❖ With a paternally expressed, maternally silenced locus Affected male Healthy female From: An introduction to human molecular genetics Genomic Imprinting ❖ With a maternally expressed, paternally silenced locus Affected female Healthy male Genomic Imprinting ❖ At least 80 human genes are known to be imprinted. ❖ Evidence of genomic imprinting has been observed in two pairs of wellknown dysmorphic syndromes: Prader-Willi and Angelman syndromes (chromosome 15q), and Beckwith-Wiedemann and Russell-Silver syndromes (chromosome 11p). Chromosome 15q deletion Angelman Syndrome Prader-Willi Syndrome Maternal Deletion Uncontrollable Laughter Paternal Deletion Obesity Jerky movements Mental Retardation Motor/Mental symptoms Short Stature Mosaicism ❖ An individual, or a particular tissue of the body, can consist of more than one cell type or line, through an error occurring during mitosis at any stage after conception. This is known as mosaicism. ❖ Mosaicism of either somatic tissues or germ cells can account for some instances of unusual patterns of inheritance or phenotypic features in an affected individual. ❖ Mosaicism is also when a person has 2 or more genetically different sets of cells in their body. A person with mosaicism may have some cells in their body with 46 chromosomes. But other cells may have a different number of chromosomes (Mosaic Down syndrome). Mosaicism ❖ If the DNA of a cell in your finger mutates to the Huntington disease genotype, or a cell in your ear picks up a cystic fibrosis mutation, there are absolutely no consequences for you or your family. Somatic Mosaicism ❖ The possibility of somatic mosaicism is suggested by the features of a single-gene disorder being less severe in an individual than is usual, or by being confined to a particular part of the body in a segmental distribution; for example, as occurs occasionally in neurofibromatosis type I (disease features are limited to the affected area). ❖ The timing of the mutation event in early development may determine whether it is transmitted to the next generation with full expression— this will depend on the mutation being present in all or some of the gonadal tissue, and hence germline cells. Gonodal Mosaicism ❖ There have been many reports of families with autosomal dominant disorders, such as achondroplasia and osteogenesis imperfecta, and Xlinked recessive disorders, such as Duchenne muscular dystrophy and hemophilia, in which the parents are phenotypically normal, and the results of genetic tests also normal, but in which more than one of their children has been affected. ❖ The most favored explanation for these observations is gonadal, or germline, mosaicism in one of the parents, i.e., the mutation is present in a proportion of the gonadal or germline cells. Chimerism ❖ In Greek mythology, a chimera was a fire-breathing creature with physical traits of a lion, goat, and dragon. Chimerism ❖ Mosaics start life as a single fertilized egg. ❖ Chimeras, in contrast, are the result of fusion of two zygotes to form a single embryo (the reverse of twinning), or alternatively of limited colonization of one twin by cells from a non-identical co-twin. ❖ Chimerism is proved by the presence of too many parental alleles at several loci. ❖ If just one locus were involved, one would suspect mosaicism for a single mutation, rather than the much rarer phenomenon of chimerism. ❖ Blood-grouping centers occasionally discover chimeras among normal donors, and some intersex patients turn out to be XX/XY chimeras. Chimera vs Mosaic Chimerism ❖ A fascinating example was described by Strain et al. ❖ Karyotyping of peripheral-blood lymphocytes revealed two cell lines, one 46,XX and the other 46,XY. ❖ They showed that a 46,XY/46,XX boy was the result of two embryos amalgamating (each derived from an independent, separately fertilized ovum) after an in vitro fertilization in which three embryos had been transferred into the mother’s uterus. ❖ Further Reading: Strain L, Dean JC, Hamilton MP & Bonthron DT (1998). A true hermaphrodite chimera resulting from embryo amalgamation after in vitro fertilization. N. Engl. J. Med. 338, 166–169. Chimerism THANK YOU! References