8. To generate a knock-in or a knock-out mouse, it is essential to use antibiotic selection to select the stem cells that have putatively integrated the construct accurately. Each construct will have 5 homology arm and 3′ homology arm to help in homologous recombination. A positive antibiotic selection cassette and a negative antibiotic selection cassette are also used. Negative selection cassette helps in de-selection of non- homologous recombinant stem cells.
Choose one statement indicated below that reflects the correct position of the negative selection cassette.
(1) Negative selection cassette is supplied in a separate construct.
(2) Negative selection cav,ette i’. placed between the two homology arms,
(3) Negative selection cassette is placed 3′ to the 3′- homology arm.
(4) Negative selection cassette is inserted between the promoter Exon I of positive selection cassette.
To generate a knock-in or knock-out mouse, the negative selection cassette is placed outside the homology arms, usually adjacent to one of the homology arms but not between them. This placement ensures that during homologous recombination, only cells with proper integration lose the negative selection marker. Cells with random, non-homologous integration retain the negative selection cassette and are deselected. Thus, the correct statement is: (3) Negative selection cassette is placed 3′ to the 3′ homology arm (or generally outside the homology arms).
Explanation of options:
(1) Negative selection cassette is supplied in a separate construct — This is incorrect because the negative selection cassette is part of the targeting vector construct, not a separate construct. It must be present in the same construct but outside the homology arms.
(2) Negative selection cassette is placed between the two homology arms — This is incorrect. The negative selection cassette is placed outside the homology arms to ensure only proper homologous recombination results in loss of the cassette.
(3) Negative selection cassette is placed 3′ to the 3′ homology arm — This is correct. Usually, the negative selection cassette (e.g., HSV thymidine kinase gene) is cloned just outside the 3′ homology arm, so only non-homologous recombinants retain it and are negatively selected against.
(4) Negative selection cassette is inserted between the promoter Exon I of positive selection cassette — This is incorrect as the negative cassette is separate and outside both homology arms and positive selection cassette, not inside the promoter-exon region of the positive cassette.
Introduction:
Generating knock-in and knock-out mouse models requires precise targeting of embryonic stem cells via homologous recombination. Key to this process is the use of both positive and negative antibiotic selection cassettes. The negative selection cassette plays a crucial role in eliminating cells with random or non-homologous integration of the targeting construct. Understanding its correct positioning relative to the homology arms is essential for successful gene targeting.
