Medical Genetics Wk 9 Flashcards
Spontaneous and Induced Mutations
Mutations can be classified as either spontaneous or induced, although these two categories overlap to some degree.
Spontaneous mutations are changes in the nucleotide sequence of genes that appear to occur naturally. Many of these mutations arise as a result of normal biological or chemical processes in the organism that alter the structure of nitrogenous bases.
Induced mutations are result from the influence of exogenous factors. Induced mutations may be the result of either natural or artificial agents. For example, radiation from cosmic and mineral sources and ultraviolet (UV) radiation from the sun are energy sources to which most organisms are exposed and, as such, may be factors that cause induced mutations.
It is estimated that somatic cell mutation rates are between 4 and 25 times higher than those in germ-
Line cells. It is well accepted that somatic mutations are responsible for the development of most cancers. Cancer cells exhibit a wide range of types and numbers of somatic mutations—from a few to dozens of single nucleotide substitutions, as well as large chromosomal rearrangements.
Spontaneous Mutations Arise from Replication Errors and Base Modifications
There are some of the processes that lead to spontaneous mutations. Many of the DNA changes that occur during spontaneous mutagenesis also occur, at a higher rate, during induced mutagenesis.
DNA Replication Errors and Slippage
Tautomeric Shifts
Depurination and Deamination
Oxidative Damage
Transposable Elements
DNA Replication Errors and Slippage
The process of DNA replication is imperfect. Occasionally, DNA polymerases insert incorrect nucleotides during replication of a strand of DNA. If these errors are not detected and corrected by DNA repair mechanisms, they may lead to mutations. Replication errors due to mispairing predominantly lead to point mutations. The fact that bases can take several forms, known as tautomers, increases the chance of mispairing during DNA replication.
In addition to mispairing and point mutations, DNA replication can lead to the introduction of small insertions or deletions. These mutations can occur when one strand of the DNA template loops out and becomes displaced during replication, or when DNA polymerase slips or stutters during replication—events termed replication slippage. If a loop occurs in the template strand during replication, DNA polymerase may miss the looped-out nucleotides, and a small deletion in the new strand will be introduced. Replication slippage can occur anywhere in the DNA but seems distinctly more common in regions containing tandemly repeated sequences. Repeat sequences are hot spots for DNA mutation and in some cases contribute to hereditary diseases, such as fragile-X syndrome and Huntington disease. In eukaryotes, at least four specialized DNA polymerases, known as translesion DNA polymerases, replicate DNA in regions of the genome that contain DNA damage.
Spontaneous Mutations Arise from Replication Errors and Base Modifications
Tautomeric Shifts
Replication errors due to mispairing predominantly lead to point mutations. The fact that bases can take several forms, known as tautomers, increases the chance of mispairing during DNA replication. Purines and pyrimidines can exist in tautomeric forms— that is, in alternate chemical forms that differ by the shift of a single proton in the molecule. Tautomeric shifts change the covalent structure of the molecule, allowing hydrogen bonding with noncomplementary bases, and hence, may lead to permanent base-pair changes and mutations. Figure compares normal base-pairing relationships with rare unorthodox pairings. Anomalous T-G and C-A pairs, among others, may be formed.
Examples of standard base-pairing relationships (a) compared with examples of the anomalous base pairing that occurs as a result of tautomeric shifts (b). The long triangles indicate the point at which each base bonds to a backbone sugar.
Tautomeric Shifts
A mutation occurs during DNA replication when a transiently formed tautomer in the template strand pairs with a noncomplementary base. In the next round of replication, the “mismatched” members of the base pair are separated, and each becomes the template for its normal complementary base. The end result is a point mutation (Figure).
Formation of an A-T to G-C transition mutation
as a result of a transient tautomeric shift in adenine.
Spontaneous Mutations Arise from Replication Errors and Base Modifications Depurination and Deamination
Some of the most common causes of spontaneous mutations are two forms of DNA base damage: depurination and deamination.
Depurination is the loss of one of the nitrogenous bases in an intact double-helical DNA molecule.
• Most frequently, the base is either guanine or adenine-in other words, a purine /apurinic site/. If apurinic sites are not repaired, there will be no base at that position to act as a template during DNA replication. As a result, DNA polymerase may introduce a nucleotide at random at that site.
Deamination, an amino group in cytosine or adenine is converted to a keto group. In these cases, cytosine is converted to uracil, and adenine is changed to the guanine-resembling compound hypoxanthine
• When adenine is deaminated the original A-T pair is ultimately converted to a G-C pair because hypoxanthine pairs naturally with cytosine, which then pairs with guanine in the next replication. Deamination may occur spontaneously or as a result of treatment with chemical mutagens.
Spontaneous Mutations Arise from Replication Errors and Base Modifications Depurination and Deamination
Deamination of cytosine and adenine, leading to new base pairing and mutation. Cytosine is converted to uracil, which base-pairs with adenine. Adenine is converted to hypoxanthine, which base-pairs with cytosine.
Spontaneous Mutations Arise from Replication Errors and Base Modifications
Oxidative Damage
Reactive oxidants, created during cellular metabolism and also generated by exposure to high-energy radiation, can produce more than 100 different types of chemical modifications in DNA, including modifications to bases, loss of bases, and single- stranded breaks.
Transposable Elements
Transposable elements are DNA sequences that can move within genomes.
➢Present in the genomes of all organisms, from bacteria to humans;
➢Can act as naturally occurring mutagens;
➢Into the coding region of a gene, they can alter the reading frame or introduce stop codons;
➢Into the regulatory region of a gene, they can disrupt proper expression of the gene;
➢Can create chromosomal damage, including double-stranded breaks, inversions, and translocations.
Possible effects of movement of a transposable element in the function and expression of the target gene. The transposable element is shown as a red rectangle, and the target gene (X) is composed of multiple exons. Protein coding regions of exons are green and untranslated regions are gold. The angled arrow indicates the start site for transcription.
Induced Mutations Arise from DNA Damage Caused by Chemicals and Radiation
All cells on Earth are exposed to a abundance of agents called mutagens, which have the potential to damage DNA and cause induced mutations.
Mutagens may be of physical, chemical or biological origin.
Physical mutagens - Ionizing radiations such as X-rays, gamma rays and alpha particles cause DNA breakage and other damages. Ultraviolet radiation.
DNA reactive chemicals - mutagenic metabolite of benzo[α]pyrene from tobacco smoke.
Biological agents - Transposon; Virus; Bacteria
Base Analogs
One category of mutagenic chemicals is base analogs, compounds that can substitute for purines or pyrimidines during nucleic acid biosynthesis. For example, 5-bromouracil (5-BU), a derivative of uracil, behaves as a thymine analog but with a bromine atom substituted at the number 5 position of the pyrimidine ring.
Figure compares the structure of 5-BU with that of thymine. The presence of the bromine atom in place of the methyl group increases the probability that a tautomeric shift will occur. The presence of 5-BU within DNA increases the sensitivity of the molecule to UV light, which itself is mutagenic.
Similarity of the chemical structure of 5-bromouracil (5-BU) and thymine. In the common keto form, 5-BU base-pairs normally with adenine, behaving as a thymine analog. In the rare enol form, it pairs anomalously with guanine.
Ultraviolet Light
UV radiation can induce thousands of DNA lesions per hour in any cell exposed to this radiation. One major effect of UV radiation on DNA is the creation of pyrimidine dimers— chemical species consisting of two identical pyrimidines— particularly ones consisting of two thymidine residues (Figure). The dimers distort the DNA conformation and inhibit normal replication. As a result, errors can be introduced in the base sequence of DNA during replication through the actions of error-prone DNA polymerases. When UV-induced dimerization is extensive, it is responsible (at least in part) for the killing effects of UV radiation on cells.
The covalent crosslinks (shown in red) occur between carbon atoms of the pyrimidine rings.
Ionizing Radiation
The energy of radiation varies inversely with wavelength. Therefore, X rays, gamma rays, and cosmic rays are more energetic than UV radiation (Figure).
As a result, they penetrate deeply into tissues, causing ionization of the molecules encountered along the way.
Hence, this type of radiation is called ionizing radiation. As ionizing radiation penetrates cells, stable molecules and atoms are transformed into free radicals—chemical species containing one or more unpaired electrons.
Free radicals can directly or indirectly affect the genetic material, altering purines and pyrimidines in DNA, breaking phosphodiester bonds, disrupting the integrity of chromosomes, and producing a variety of chromosomal aberrations, such as deletions, translocations, and chromosomal fragmentation.
The regions of the electromagnetic spectrum and their associated wavelengths.
Single-Gene Mutations Cause a Wide Range of Human Diseases
Although most human genetic diseases are polygenic—that is, caused by variations in several genes—even a single base-pair change in one of the approximately 20,000 human genes can lead to a serious inherited disorder. These monogenic diseases can be caused by many different types of single-gene mutations. Table lists some examples of the types of single-gene mutations that can lead to serious genetic diseases. Geneticists estimate that approximately 30 percent of mutations that cause human diseases are single base-pair changes that create nonsense mutations. These mutations not only code for a prematurely terminated protein product, but also trigger rapid decay of the mRNA.
ACHONDROPLASIA (FGFR3 Mutation) Autosomal Dominant
Achondroplasia, the most common cause of human dwarfism, is an autosomal dominant disorder caused by specific mutations in FGFR3 (chromosome 4); two mutations, G>A (≈98%) and G>C (1% to 2%), account for more than 99% of cases of achondroplasia, and both result in the Gly380Arg substitution. Achondroplasia has an incidence of 1 in 15,000 to 1 in 40,000 live births and affects all ethnic groups.
MAJOR PHENOTYPIC FEATURES
• Age at onset: Prenatal
• Rhizomelic short stature
• Megalencephaly ( a condition in which an infant or child has an abnormally large, heavy, and usually malfunctioning brain).
• Spinal cord compression
Prenatal diagnosis before 20 weeks of gestation is available only by molecular testing of fetal DNA, although the diagnosis can be made late in pregnancy by analysis of a fetal skeletal radiograph .
Marfan syndrome
Marfan syndrome is an autosomal dominant genetic disorder of the connective tissue. Caused by a mutation in a gene found on the chromosome 15, that determines the structure of fibrillin. Fibrillin is a protein that is an important part of connective tissue and elastic fibers which affect multiple parts of the body such as bones, joints, eyes, blood vessels, and heart. Named after Antoine Marfan in 1899.
