[Presented by: Andrew Howlett, Cruise Slater, Mahmud Hasan, Greg Dale]

Transcription

1 Mutational Dissection [Presented by: Andrew Howlett, Cruise Slater, Mahmud Hasan, Greg Dale] Introduction What is the point of Mutational Dissection? It allows understanding of normal biological functions How? By genetic disruption of normal gene activity one can analyze the resulting phenotype of the mutant organism Forward vs Reverse Genetics: Forward Genetics: The classical approach to genetic analysis, in which genes are first identifed by mutant alles and mutant phenotypes and later cloned and subjected to molecular analysis. Reverse Genetics: An experimental procedure that begins with a cloned segment of DNA or a protien sequence and uses it (through directed mutagenesis) to introduce programmed mutations back into the genome to investigate function. Mutagens Choice of mutagen is important What needs to be achieved? What phenotype will result? Will it be relevant? Gene gives rise to phenotype Depending on mutagen being used... Results vary giving rise to gene target size The idea of saturating the system... To identify every component in the biological process being studied so that the mutagen affects the system with the best result

2 Is the mutagen random or specific? General Mutagens: -mutate at a constant frequency -produce a broad array of different mutational events -mutation target size hard to calculate -sometime don t produce the sought after mutant phenotype To use a general mutagen: -must be taken up in sufficient quantity to cause mutation -it must not be easily metabolized -must use a high dosage to provide a high mutation frequency while not being cytotoxic General mutagens and how they work: 1) Base-substitution mutagens 2) Indel Mutagens 3) Insertional Mutagens 4) Chromosomal Rearrangers Directed Mutations: -usually very specific -can inactivate a gene by changing DNA -useful for bacteria, yeast, and mice -can leave the gene intact but block activity of a gene product (mrna or Protein) 1) Targeted gene knockout 2) Site directed mutagenesis Related Techniques (the last resort) Inactivation of the Gene Product: Phenocopying: -mimicking a mutant phenotype without altering the structural gene by altering the environment of the cell Antisense RNA Double Stranded RNA interference (dsrnai): Modes of introduction, proposed mechanism 3) Chemical-library screening The Mutational Assay System Must detect mutations in a way that is appropriate to the mutagen. Somatic mutations are not passed to progeny. Germline mutations are passed to the next generation. Dominant and recessive germline mutations

3 In haploid, or sex chromosomes in diploid organisms, both dominant and recessive can be identified in F1 For autosomal genes of diploid organisms: Only dominant mutations visible in F1 (unless homozygous). Recessive mutations visible in F2 or F3, depending on: mutagenesis monoecious (self-fertilizes) or dioecious (has 2 cross-fertilizing sexes). Detecting autosomal recessive mutations In self-fertilizing species, F2 can be analyzed. In cross-fertilizing species, F3 must be analyzed. F3 screens more efficient with tracking of mutagenized chromosome, i.e. Drosophila balancer chromosomes. Recessive mutations of a specific locus can be recovered in F2. Accelerating the identification of autosomal recessive mutants Zebrafish Somatic recombination: Identification of mutations in clones of somatic cells Genetic Selections vs. Genetic Screens Genetic Selections: Involve killing off all individuals that lack the desired mutation. Every offspring that survives is a desired mutant Since mutations are rare, ~ % of offspring die. Therefore, this technique allows identification of a mutant that occurs on % of the time. Genetic Screens: Both mutated and non-mutated individuals are recovered, and the mutants are identified by their phenotype. Can be used for any phenotype, only limited by researcher s ingenuity. Labour Intensive. Genetic Selections Genetic selections are the most useful when dealing with microbes that can grow on defined media, due to the fact that microbes have many genes devoted to metabolic functioning. Genetic Screens General Phenotypes: Genetic screens require the researcher to come up with a phenotype that will reveal mutations of interest. Some examples of common phenotypes used are: Biochemical (auxotrophic) mutations Morphological mutations Lethal mutations Conditional mutations Behavioural mutations

4 Secondary Screens Not all genes can be identified by direct screens. Reasons? Some mutations may only affect homozygotes, but may also be recessive lethal. Some genes have redundancy, where one will cover the mutation of another. Modifier mutation: A mutation in one gene that suppresses or enhances the phenotype caused by a mutation in another gene. Somatic mosaics: Drosophila eye development Gene expression Analysis of the Recovered Mutations Once mutations have been detected and isolated, it is important to evaluate them and draw conclusions about their properties. One way for scientists to get the inside view of a process is by 1. Disrupting it in various ways including mutagenesis 2. Observing the consequences for each case 3. Using the information to understand each of the steps of the process Thus, understanding what has gone awry in different mutations always facilitates analysis. Mutations: A mini review Classification Systems for Mutations: The key point in any mutation is classifying the kind of alteration to gene function that occurs. An "Axle-Gear Mechanism" can be used to represent a cellular pathway. Modifying the way the gears are and by changing the speed of motion, a different result will be seen for each. The main difference is that in terms of the gear analogy, one can look inside the gearbox covering the gears to understand the process, but in terms of mutational analysis, it is seen that loss- versus gain-of-function mutations can be inferred by phenotypic analysis of different dosages of mutant and wild type genes. Counting genes in a biological process: Genetic Transmissional and Complementation Analysis are used to locate the genes represented by a mutant collection. Diagnostics for loss-of-function versus gain-of-function: Both loss-of-function and gain-of-function could be dominant or recessive. But how can we detect?

5 Let us consider dominant mutations. Knowing where any mutations map on a genome, it could be determined if chromosomal deletions or duplications of the gene exist. If they do, they can indicate definitively whether a dominant mutation is a loss-of-function or gain-of-function. For recessive mutations, gene dosage can be used to make similar distinctions between lose-of-function and gain-of-function alterations. Further Aspects of Mutational Analysis: What are the next steps once the basic genetic and phenotypic analysis of a set of mutations is accomplished? Certain procedures are taken including recombination mapping for positional cloning and insertional mutagenesis for molecular tagging, enable the molecular identification of genes, their mrna and protein products. With all these figured out, next question is where they are expressed, other products they might interact with, and how their expression is regulated.