« on: 22/01/2018 20:46:26 »
Quote from: tkadm30
is there any ways we could eradicate (Down Syndrome) permanently from the human genome with genetic engineering technology like CRISPR?I'm afraid not.
Down syndrome is is caused by a problem in development of the egg and sperm. It is not inherited from the parents.
So even if you eliminated it from one generation, it would reappear in the next generation.
Down Syndrome, or "Trisomy 21" is not related to "one bad gene", but an extra copy of a whole chromosome.
Chromosome 21 is the smallest human gene, and is estimated to have around 230 protein-coding genes and 300 genes that code for RNAs. With a third chromosome, some of these genes can be expressed 50% more than normal; but it's not clear which genes are causing the problems. Down syndrome is very variable in presentation, so the genes causing the problems may differ significantly between individuals.
It is this "small" number of genes that makes the third copy less lethal than a third copy of non-sex chromosomes (mammals have some established ways of dealing with extra copies of sex chromosomes).
While CRISPR is undoubtedly a huge advance over previous genetic engineering techniques, it is still only effective at making a single change in in 80-90% of isolated cells (with current techniques).
If you now try to make 500 coordinated changes in a single cell, your chance of success drops to around 0.85500, or around 5x10-36.
Worse than that, these genes are not "faulty" - the 2 other copies of Chromosome 21 have near-identical copies of the same genes. With 85% success rate, the chance that you will wipe out all 3 copies of a particular gene is 0.853 = 61%. Having zero active copies of a gene is far more lethal than having 3 active copies. Now multiply that by 500 genes, and the therapy would be fatal.
In reality, CRISPR is mostly suitable for single cells exposed in a dish (or perhaps exposed in the retina of the eye or the walls of blood vessels), rather than cells already assembled in an organ, a baby or an adult. It is just too hard to get the CRISPR to the cells you want (and not the cells you want untouched), and then separate the numerous failures from the successes.
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