Gene drives can spread desirable genes or suppress harmful species such as disease carrying mosquitoes by inhibiting their reproduction. Gene drives could help endangered species by spreading advantageous traits faster than natural selection. Gene drives could be used to control invasive species by spreading a sterility trait. Organisms that have become resistant to herbicides and insecticides could be re-sensitized. Agricultural pests are another application of gene drive.
Several types of gene drives have been engineered including homing-based drives, sex-linked meiotic drivers, Medea and underdominance systems. Gene drive applications have focused mostly on disease carrying mosquitoes and the evolution of resistance to these gene drives has become a challenge.
Gene drives that use homing to copy the desirable version of the gene onto the other chromosome of the pair resulting in two identical copies of the desired gene. The gene will be transmitted to almost all of the offspring, along with the ability to copy itself onto the other chromosome in a pair. Homing type gene drives have used endonuclease genes (HEGs) and CRISPR-Cas9. Scientists at UC San Diego and UC Irvine constructed a CRISPR gene drive for the mosquito Anopheles stephensi, which carries malaria. The gene drive spreads resistance to the malaria parasite and changes the eye color of the mosquitoes as a way to check which ones have the gene drive.
In this type of system the transmission of certain gene versions or alleles is biased during meiosis, when gametes like sperm and egg or spores form. Sex-linked meiotic drives can prevent maturation of gametes in which the meiotic driver is lacking. Target Malaria is an international non-profit research consortium which aims to use gene drive to reduce the numbers of malaria-transmitting mosquitoes. One way is by skewing sex ratios to produce more male mosquitoes, which don’t bite, and leading to a mosquito population crash. Drives that suppress the population are called suppression drives.
Unintended consequences of gene drive could potentially include extinctions, unanticipated effects, off-target effects, spreading outside geographical area and spreading to unintended species. Concerns have been raised about the potential uses of gene drive in bioterrorism. Once homing-based drives are released the population cannot be reverted to wild-type but second-generation drives can be designed to target and replace the original system. Defense Advanced Research Projects Agency (DARPA) is working on the Safe Genes program which aims to prepare control measures for gene drive and other gene editing technologies.
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CRISPR-Cas9: Gene Drives
17 July 2014