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Reading Jennifer Doudna's fascinating book on CRISPR. So she describes rare cases where a mutation in a single cell removes the gene responsible for a genetic disease. The cell proliferates and the disease is cured. What I don't understand is--aren't all the aberrant cells also still dividing and passing on the harmful mutation to their progeny? How does the single healthy cell edge out all the other cells to "cure" the patient.

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There are some diseases in which a minority population of normal cells can rescue the organism, even in the presence of a majority of mutant cells. Sort of like a group vacation to Kazakhstan with one friend who can speak Kazakh. Very different experience from a vacation where none of you speak Kazakh.

Take for example hemophilia A. Cells have a mutant gene for factor 8. Without factor 8 blood does not clot properly and people have bleeding trouble. You do not need much factor 8. Just a little bit can prevent spontaneous bleeds. People who are heterozygotes for hemophilia A (carriers) are fine at 50% normal. So if you edited mutant liver cells such that they could produce factor 8, even if they were only 10% of the total cell population of the liver, having some factor 8 would sidestep the full hemophilia phenotype.

Likewise metabolic mutants - some children are born unable to handle certain substances. Phenylketonuria is the one you read about every time you drink a diet coke. If a minority population of cells could be produced that were able to handle phenylalanine, those would rescue these kids even if most of the cells still could not.

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Are you talking about people?

Because in bacteria, a mutated bacterium with a beneficial mutation will definitely edge out the competition, or if it is in a "good mood" (cell humor lol) it will share its DNA with the other bacteria.

You're absolutely right about people, though. One cured cell won't cure the patient.

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