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Gene knockout

A gene knockout (abbreviation: KO) is a sequenced, but which has an unknown or incompletely known function. Researchers draw inferences from the difference between the knockout organism and normal individuals.

The term also refers to the process of creating such an organism, as in "knocking out" a gene. The technique is essentially the opposite of a gene knockin. Knocking out two genes simultaneously in an organism is known as a double knockout (DKO). Similarly the terms triple knockout (TKO) and quadruple knockouts (QKO) are used to describe three or four knocked out genes, respectively.

Contents

  • Method 1
  • Use 2
  • See also 3
  • References 4
  • External links 5

Method

A laboratory mouse in which a gene affecting hair growth has been knocked out (left), is shown next to a normal lab mouse.

Knockout is accomplished through a combination of techniques, beginning in the test tube with a transgenic animal that has the altered gene. If so, embryonic stem cells are genetically transformed and inserted into early embryos. Resulting animals with the genetic change in their germline cells can then often pass the gene knockout to future generations.

To create knockout moss, transfection of protoplasts is the preferred method. Such transformed Physcomitrella-protoplasts directly regenerate into fertile moss plants. Eight weeks after transfection, the plants can be screened for gene targeting via PCR.[1]

Wild-type Physcomitrella and knockout mosses: Deviating phenotypes induced in gene-disruption library transformants. Physcomitrella wild-type and transformed plants were grown on minimal Knop medium to induce differentiation and development of gametophores. For each plant, an overview (upper row; scale bar corresponds to 1 mm) and a close-up (bottom row; scale bar equals 0.5 mm) are shown. A: Haploid wild-type moss plant completely covered with leafy gametophores and close-up of wild-type leaf. B–D: Different mutants.[2]

The construct is engineered to recombine with the target gene, which is accomplished by incorporating sequences from the gene itself into the construct. Recombination then occurs in the region of that sequence within the gene, resulting in the insertion of a foreign sequence to disrupt the gene. With its sequence interrupted, the altered gene in most cases will be translated into a nonfunctional protein, if it is translated at all.

A knockout mouse (left) that is a model of obesity, compared with a normal mouse.

A conditional knockout allows gene deletion in a tissue or time specific manner. This is done by introducing short sequences called loxP sites around the gene. These sequences will be introduced into the germ-line via the same mechanism as a knock-out. This germ-line can then be crossed to another germline containing Cre-recombinase which is a viral enzyme that can recognize these sequences, recombines them and deletes the gene flanked by these sites.

Because the desired type of DNA recombination is a rare event in the case of most cells and most constructs, the foreign sequence chosen for insertion usually includes a reporter. This enables easy selection of cells or individuals in which knockout was successful. Sometimes the DNA construct inserts into a chromosome without the desired homologous recombination with the target gene. To eliminate such cells, the DNA construct often contains a second region of DNA that allows such cells to be identified and discarded.

In alleles for most genes, and may as well contain several related genes that collaborate in the same role, additional rounds of transformation and selection are performed until every targeted gene is knocked out. Selective breeding may be required to produce homozygous knockout animals.

Gene knockin is similar to gene knockout, but it replaces a gene with another instead of deleting it.

Use

Knockouts are primarily used to understand the role of a specific wildtype with a similar genetic background.

Knockouts genome, such as in Saccharomyces cerevisiae.[3]

See also

References

  1. ^ Ralf Reski (1998): Physcomitrella and Arabidopsis: the David and Goliath of reverse genetics. Trends Plant in Science 3, 209-210
  2. ^ Egener et al. BMC Plant Biology 2002 2:6 doi:10.1186/1471-2229-2-6
  3. ^ http://www-sequence.stanford.edu/group/yeast_deletion_project/deletions3.html

External links

  • Diagram of targeted gene replacement
  • Frontiers in Bioscience Gene Knockout Database (available on archive only)
  • International Knockout Mouse Consortium
  • KOMP Repository

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