Mutagenesis


Mutagenesis

Mutagenesis (pronounced /mjuːtəˈdʒɛnɪsɪs/) is a process by which the genetic information of an organism is changed in a stable manner, resulting in a mutation. It may occur spontaneously in nature, or as a result of exposure to mutagens. It can also be achieved experimentally using laboratory procedures. In nature mutagenesis can lead to cancer and various heritable diseases, but it is also the driving force of evolution. Mutagenesis as a science was developed based on work done by Hermann Muller, Charlotte Auerbach and J. M. Robson in the first half of the 20th century.[1]

Contents

Background

DNA may be modified, either naturally or artificially, by a number of physical, chemical and biological agents, resulting in mutations. In 1927, Hermann Muller first demonstrated mutation with observable changes in the chromosomes can be caused by irradiating fruit flies with X-ray,[2] and lent support to the idea of mutation as the cause of cancer.[3] His contemporary Lewis Stadler also showed the mutational effect of X-ray on barley in 1928,[4] and ultraviolet (UV) radiation on maize in 1936.[5] In 1940s, Charlotte Auerbach and J. M. Robson, found that mustard gas can also cause mutations in fruit flies.[6]

While changes to the chromosome caused by X-ray and mustard gas were readily observable to the early researchers, other changes to the DNA induced by other mutagens were not so easily observable, and the mechanism may be complex and takes longer to unravel. For example, soot was suggested to be a cause of cancer as early as 1775,[7] and coal tar was demonstrated to cause cancer in 1915.[8] The chemicals involved in both were later shown to be polycyclic aromatic hydrocarbons (PAH).[9] PAHs by themselves are not carcinogenic, and it was proposed in 1950 that the carcinogenic forms of PAHs are the oxides produced as metabolites from cellular processes.[10] The metabolic process was identified in 1960s as catalysis by cytochrome P450 which produces reactive species that can interact with the DNA to form adducts,[11][12] the mechanism by which the PAH adducts give rise to mutation however is still under investigation.

DNA may sustain more than 50,000 damages per cell per day,[13] and some estimates put the number of oxidative adducts per cell generated through reactive reactive oxidative species at 150,000.[14] Left uncorrected these adducts can give rise to mutation. In nature, the mutations that arise may be beneficial or deleterious - it is the driving force of evolution, an organism may acquire new traits through genetic mutation, but mutation may also result in impaired function of the genes, and in severe cases, causing the death of the organism. In the laboratory, however, mutagenesis is a useful technique for generating mutations that allows the functions of genes and gene products to be examined in detail, producing proteins with improved characteristics or novel function, as well as mutant strains with useful properties. Initially the ability of radiation and chemical mutagens to cause mutation was exploited to generate random mutations, later techniques were developed to introduce specific mutations.

Mechanisms

Mutagenesis may occur endogenously, for example through spontaneous hydrolysis, or through normal cellular processes that can generate reactive oxygen species and DNA adducts, or through error in replication and repair.[15] Mutagenesis may also arise as a result of the presence of environmental mutagens that induces changes to the DNA. The mechanism by which mutation arises varies according to the causative agent, the mutagen, involved. Most mutagens act either directly, or indirectly via mutagenic metabolites, on the DNA producing lesions. Some however may affect the replication or chromosomal partition mechanism, and other cellular processes.

Many chemical mutagens require biological activation to become mutagenic. An important group of enzymes involved in the generation of mutagenic metabolites is cytochrome P450.[16] Other enzymes that may also produce mutagenic metabolites include glutathione S-transferase and microsomal epoxide hydrolase. Mutagens that are not mutagenic by themselves but require biological activation are called promutagens.

Many mutations arise as a result of problems caused by the DNA lesions during replication. In bacteria, extensive damage to the DNA due to mutagens produces single-stranded DNA gaps during replication which induces the SOS response, an emergency repair process that is also error-prone, thereby generating mutations. In mammalian cells, stalling of replication at a damaged sites induces a number of rescue mechanisms that help bypass DNA lesions but which also may result in errors. The Y family of DNA polymerases specialisze in DNA lesion bypass in a process termed translesion synthesis (TLS) whereby these lesion-bypass polymerases replace the stalled high-fidelity replicative DNA polymrase, transits the lesion and extend the DNA until the lesion has been passed so that normal replication can resume.

Spontaneous hydrolysis

DNA is not entirely stable in aqueous solution. Under physiological conditions the glycosidic bond may be hydrolyzed spontaneously and 10,000 purine sites in DNA are estimated to be depurinated each day in a cell.[15]. Numerous DNA repair pathway exist for the DNA, however, if the apurinic site failed to be repaired, misincorporation of nucleotide may occur during replication. Adenine is preferentially incorporated by DNA polymerases in an apurinic site. Cytidine may also become deaminated to uridine at one five-hundredth of the rate of depurination and can result in G to A transition. Eukaryotic cells also contains 5-methylcytosine, thought to be involved in the control of gene transcription, which can become deaminated into thymine.

Modification of bases

Bases may be modified endogenously by normal cellular molecules. For example DNA may be methylated by S-adenosylmethionine, and glycosylated by reducing sugars.

Many compounds, such as PAHs, aromatic amines, aflatoxin and pyrrolizidine alkaloids may form reactive oxygen species catalyzed by cytochrome P450. These metabolites form adducts with the DNA, which can cause errors in replication, and the bulky aromatic adducts may form stable intercalation between bases and block replication. The adducts may also induce conformational changes in the DNA. Some adducts may also result in the depurination of the DNA,[17] it is however uncertain how significant the depurination as caused by the adducts is in generating mutation.[18]

Some alkylating agents such as N-Nitrosamines may also require the catalytic reaction of cytochrome-P450 for the formation of a reactive alkyl cation. Alkylation and arylation of bases can cause errors in replication. N7 and O6 of guanine and the N3 and N7 of adenine are most susceptible to attack; while N7-guanine adducts, which form the bulk of DNA adducts, appear to be non-mutagenic, alkylation at O6 of guanine is harmful because excision repair of O6-adduct of guanine may be poor in some tissues.[19] The O6 methylation of guanine can result in G to A transition, while O4-methylthymine can be mispaired with guanine. The type of the mutation generated however may be dependent on the size and type of the adduct as well as the DNA sequence.[20]

Ionizing radiations and reactive oxygen species often oxidize guanine to produce 8-oxoguanine.

Crosslinking

Some alkylating agents may produce crosslinking of DNA. Some natural occurring chemicals may also promotes crosslinking, such as psoralens after activation by UV radiation, and nitrous acid. Interstrand cross-linking blocks replication and transcription and can cause chromosomal breakages and rearrangements. Some crosslinkers such as cyclophosphamide, mitomycin C and cisplatin are used as anticancer chemotherapeutic because their high degree of toxicity to proliferating cells.

Dimerization

UV radiation promotes the formation of a cyclobutyl ring between adjacent thymines, resulting in the formation of pyrimidine dimers.[21] In human skin cells, thousands of dimers may be formed in a day due to normal exposure to sunlight. DNA polymerase η may help bypass these lesions in an error-free manner;[22] however, individuals with defective DNA repair function, such as sufferers of Xeroderma pigmentosum, are sensitive to sunlight and may be prone to skin cancer.

Ethidium intercalated between two adenine-thymine base pairs.

Intercalation between bases

The planar structure of chemicals such as ethidium bromide and proflavine allows them to insert between bases in DNA, and cause frameshift mutation. The intercalation into DNA of anthracyclines such as daunorubicin and doxorubicin interferes with the functioning of the enzyme topoisomerase II, blocking replication as well as causing mitotic homologous recombination.

Arrows indicates chromosomal breakages due to DNA damage

Backbone damage

Ionizing radiations may produce highly reactive free radicals that can break the bonds in the DNA. Double-stranded breakages are especially damaging and hard to repair, producing translocation and deletion of part of a chromosomes. Alkylating agents like mustard gas may also cause breakages in the DNA backbone. Oxidative stress may also generate highly reactive oxygen species that can damage the DNA. Incorrect repair of other damages induced by the highly reactive species can also lead to mutations.

Insertional mutagenesis

Transposon and virus may insert DNA sequence into coding region or functional elements of a gene and result in inactivation of the gene.

Error in replication

While most mutagens produce effects that ultimately result in error in replication, some mutagens may affect directly the replication process. Base analog such as 5-bromouracil may substitute for thymine in replication. Some metals such as cadmium, chromium, and nickel may alter the fidelity of DNA replication.

Mutagenesis as a laboratory technique

Mutagenesis in the laboratory is an important technique whereby DNA mutations are deliberately engineered to produce mutant genes, proteins, or strains of organism. Various constituents of a gene, such as its control elements and its gene product, may be mutated so that the functioning of a gene or protein can be examined in detail. The mutation may also produce mutant proteins with interesting properties, or enhanced or novel functions that may be of commercial use. Mutants strains may also be produced that have practical application or allow the molecular basis of particular cell function to be investigated.

Random mutagenesis

Early approaches to mutagenesis rely on methods which are entirely random in the mutations produced. Cells may be exposed to UV radiation or mutagenic chemicals, and mutants with desired characteristic are then selected. For example, Escherichia coli may be exposed to UV radition, then plated onto agar medium. The colonies formed are then replica-plated, one in rich medium, another in minimal medium, and mutants that have specific nutritional requirement can then be identified by its inability to grow in minimal medium and isolated.

A number of methods for generating random mutation in specific protein were later developed to screen for mutants with interesting or improved properties. This may be done by using doped nucleotides in oligonucleotides synthesis, conducting a PCR reaction in conditions that enhance misincorporation of nucleotide thereby generating mutants.

Site-directed mutagenesis

It is desirable that specific changes can be introduced to the DNA. Analogs of nucleotides and other chemicals were first used to generate localized point mutations.[23] Such chemicals may be aminopurine which induces AT to GC transition,[24] while nitrosoguanidine,[25], bisulfite,[26], and N4-hydroxycytidine may induce GC to AT transition.[27][28] These technique allows specific mutations to be engineered into a protein, however, they are not flexible in the kinds of mutants generated.

Current techniques for site-specific mutation commonly involve using mutagenic oligonucleotides in a primer extension reaction with DNA polymerase. This methods allows for point mutation, or deletion or insertion of small stretches of DNA to be introduced at specific sites. Advances in methodology have made such mutagenesis now a relatively simple and efficient process.

The site-directed approach may be done systematically in such technique as alanine scanning mutagenesis whereby residues are systematically mutated to alanine in order to identify residues important to the structure or function of a protein.

Combinatorial mutagenesis

Combinatorial mutagenesis is a technique whereby large number of mutants may be screened for a particular characteristic. In this technique, a few selected positions or a short stretch of DNA may be exhaustively modified to obtain a comprehensive library of mutant proteins. One approach of this technique is to excise a portion of DNA and replaced with a library of sequences containing all possible combinations at the desired mutation sites. The segment may be at an enzyme active site, or sequences that have structural significance or immunogenic property. A segment however may also be inserted randomly into the gene in order to assess the structural or functional significance of particular part of protein.

More recently, techniques such as phage display has been used to discover short peptide sequences of up to 12 residues with binding affinity for other proteins.

Insertional mutagenesis

In cancer research engineered mutations also provide mechanistic insights into the development of the disease. Insertional mutagenesis using transposons, retrovirus such as mouse mammary tumor virus and murine leukemia virus may be used to identify genes involved in carcinogenesis and to understand the biological pathways of specific cancer. Various insertional mutagenesis techniques may also be used to study the function of particular gene.

Other types of mutagenesis

See also

References

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Look at other dictionaries:

  • mutagénesis — acto de inducir una mutacion Diccionario ilustrado de Términos Médicos.. Alvaro Galiano. 2010. mutagénesis Inducción o aparición de una mutación genética. Diccionario Mosby Medici …   Diccionario médico

  • Mutagenesis — (f), Mutagenese (f) eng mutagenesis …   Arbeitssicherheit und Gesundheitsschutz Glossar

  • mutagenesis — mu ta*gen e*sis (m[=u] t[.a]*j[e^]n [ e]*s[i^]s), n. the causing of a mutation or the occurrence of a mutation[3]. [PJC] …   The Collaborative International Dictionary of English

  • mutagenesis — mutagenesis. См. мутагенез. (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) …   Молекулярная биология и генетика. Толковый словарь.

  • mutagénesis — (De mutar y génesis). f. Producción de mutaciones …   Diccionario de la lengua española

  • mutagenesis — [myo͞ot΄ə jen′ə sis] n. [ MUTA(TION) + GENESIS] the occurrence or production of mutation …   English World dictionary

  • Mutagénesis — En genética se denomina mutagénesis a la producción de mutaciones sobre DNA, clonado o no. De realizarse in vitro, dicha alteración puede realizarse al azar (mutagénesis al azar), sobre cualquier secuencia, o bien de forma dirigida (mutagénesis… …   Wikipedia Español

  • mutagénesis — ► sustantivo femenino BIOLOGÍA Cambio brusco que se produce en un gen por alterarse las moléculas de ADN, que se traduce en la aparición de individuos con un carácter nuevo que transmiten a sus descendientes. IRREG. plural mutagénesis SINÓNIMO… …   Enciclopedia Universal

  • mutagénesis — {{#}}{{LM M44052}}{{〓}} {{[}}mutagénesis{{]}} ‹mu·ta·gé·ne·sis› {{◆}}(pl. mutagénesis){{◇}} {{《}}▍ s.f.{{》}} {{♂}}En biología,{{♀}} producción o creación de mutaciones …   Diccionario de uso del español actual con sinónimos y antónimos

  • Mutagenesis — mutagenezė statusas T sritis ekologija ir aplinkotyra apibrėžtis Organizmo paveldimų pakitimų – mutacijų – atsiradimo procesas, lemiantis organizmo vieno ar keleto požymių kitimą. atitikmenys: angl. mutagenesis vok. Mutagenesis, f rus. мутагенез …   Ekologijos terminų aiškinamasis žodynas


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