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A Medical First: CRISPR Gene-Editing Used To Treat A Genetic Disorder—Sickle Cell Anemia

Discussion in 'Hematology' started by Dr.Scorpiowoman, Aug 3, 2019.

  1. Dr.Scorpiowoman

    Dr.Scorpiowoman Golden Member

    May 23, 2016
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    This medical intervention, which may in the end prove unsuccessful, nevertheless represents a first, at least according to the NPR article below. I’m not quite sure this is a first, though, so see the discussion below. But the “precedence” aspect isn’t that important. What’s important is that if this treatment works, it opens up a whole new panorama on curing genetic disease.


    CRISPR, as I described in a 2017 book review in the Washington Post, is a method of precise gene editing that grew out of pure research: the observation of strange repeated sequences in bacterial DNA. Those repeated sequences turned out to be stored viral DNA that the bacterium used to precisely cut invading viruses whose DNA matched the stored sequences.

    From that observation, a whole group of people, as described in my article, developed CRISPR as a system to edit DNA in many organisms, and now it’s being used in humans. CRISPR is a remarkable technology which, when it works properly—and that’s not always the case—can be used to snip a defective gene—or part of a defective gene—out of an organism’s DNA and replace it with a normally functioning gene. It can also be used to change the DNA any way you want: put wanted genes into crops or food animals, or change the DNA of your offspring in a frozen embryo, say giving it genes for brown eyes instead of blue. Theoretically it’s possible to change both the DNA in the gametes (sperm and eggs), so that a change can be permanently inherited by one’s descendants, or just change the DNA in the body itself in a way that isn’t inherited.

    The latter method is what’s being used in an attempt to cure a devastating disease, sickle cell anemia. The subject is an African-American woman, Victoria Gray (34) of Mississippi, who’s being treated in Nashville. Eventually 45 other patients from 18 to 35 will be treated. There aren’t any results yet, but if it works it will be the first step in using CRISPR to cure a variety of genetic diseases. (The diseases have to be based on changes in the DNA because CRISPR works by replacing one gene with another.)

    In this case the disease is caused by a lesion in hemoglobin (see below): a replacement of one amino acid (glutamic acid in normal hemoglobin, Hb-A) with another (valine in sickle-cell hemoglobin Hb-S), and this is caused by a change in a single nucleotide in the DNA coding for the β chain of hemoglobin. (A single hemoglobin molecule has two α and two β chains.) That change is a mutation from the triplet GAG, which codes for glutamic acid, to GTG, which codes for valine. The β-hemoglobin gene is on chromosome 11 (we have 46 chromosomes, present in 23 pairs).

    This one nucleotide change out of the 3.2 billion in our genome produces the devastating disease, as the mutant hemoglobin causes the red blood cells containing it to assume a sickle shape, producing a whole syndrome of debilitating and painful traits: joint damage, strokes (the sickled cells clog the circulatory system), spleen damage, anemia, and infections. Most of the afflicted die by their mid-forties.

    Below is a picture of sickled red-blood cells from the New York Times story below:


    The vast preponderance of African-American sufferers in the U.S. is important because sickle-cell anemia is found almost entirely in people of West African origin.

    This is a famous evolutionary story: the sickle-cell gene, when present in one copy along with a “normal” gene, doesn’t cause the disease, as enough normal red blood cells are produced to prevent symptoms. It’s only when you inherit two copies of the “Hb-S” gene that you’re afflicted. In other words, the genes producing the disease act as “recessives”. Yet many in sub-Saharan Africa and India have the disease: it’s estimated that about 4 million people worldwide are afflicted, and 43 million people are “carriers,” asymptomatic people who have but a single copy of the mutant β-chain in their DNA.

    Why does this debilitating gene persist? Why hasn’t natural selection reduced its frequency to almost zero? It’s because if you’re a carrier of one copy and asymptomatic, you have a lower chance of dying from malaria than people with two “normal” genes. It’s not clear why this is true, but it surely has something to do with an altered hemoglobin environment in the blood that reduces growth of the malaria-producing protozoan. (Mosquitoes, acting as flying syringes, carry the protozoan from person to person.) So the reproductive fitness (judged by survival) of the three types is like this: HbS/HbA (heterozygote) is fittest because it doesn’t have the disease and is protected from malaria (which is often fatal), HbA/HbA (normal homozygotes) are of high but a bit lower fitness, as they can get malaria) and the HbS/HbS type is the genotype with sickle-cell anemia, and most don’t have children as they’re very sick.

    It turns out that if the “heterozygote” has the highest fitness, as in this case, then both forms of the gene are maintained in the population (this is called “overdominance” or “heterosis”), regardless of the fact that every generation a number of HbS/HbS people are born, fated to suffer and die. This shows that genetics and biology doesn’t produce the optimum situation, which would be true if everyone was a heterozygote. But this is impossible, because the nature of genetics dictates that heterozygotes segregate the two alleles every generation. The evolutionary dynamics of heterozygous advantage maintains the bad gene in the population: many carriers are protected from malaria, but the mating of two such carriers produces sickle-cell disease in 25% of their offspring.

    The Hb-S gene, then, is found largely where fatal malaria is endemic: west Africa and India (and parts of the Middle East).

    The coincidence of the distribution of endemic malaria (first figure) with the distribution of the Hb-S allele (second figure) is striking, and is further evidence that the presence of malaria is responsible for the evolutionary maintenance of the deleterious Hb-S allele:


    Frequency of the sickle cell allele (pink and purple):


    Because American black people derive largely from West Africans brought over as slaves, the gene has persisted in this population, and that’s why African-Americans are almost the only people in the U.S. who get sickle-cell anemia. (The frequency of the gene here is decreasing, though, as malaria isn’t endemic in the U.S.—eliminating the evolutionary advantage of carriers of the gene—and intermarriage with whites has also reduced its frequency among blacks.)

    Enough of that story, which is one that we all tell in evolution class, for it’s one of the very few cases we know of in which a heterozygote for a single gene is superior to either homozygote.

    Sickle-cell anemia is almost incurable, though in some people a bone-marrow transplant can get rid of it. But that’s a painful and often unsuccessful process. What researchers are doing now is taking a sickle-cell patient’s stem cells, which can differentiate into the bone marrow that produces red blood cells, treating those cells’ DNA with CRISPR, and injecting them back into the patient, hoping that they’ll take up residence in the bones and produce normal red blood cells, eliminating the disease. The NPR story below (click on screenshot) describes the treatment received by Ms. Gray.

    Now you might think that they’re editing the β-hemoglobin gene itself to turn that nasty GAG triplet to GTG, but there’s apparently another tactic they’re using. What they’re doing is first killing off the patient’s bone marrow with chemotherapy, getting rid of the cells that make sickle-cell blood cells, and then replacing them with genetically altered stem cells that allow fetal hemoglobin (the first hemoglobin we produce, which is usually turned off by four months of age) to persist. The hope is that this fetal hemoglobin will act like Hb-A and cure the disease. I’m not sure why they do this rather than inject stem cells with an edited GTG codon in the gene for β-hemoglobin, but it must be easier to use the modified fetal hemoglobin than to edit the sickle-cell gene.


    If this works, and of course it may not (the bone-marrow killing can kill the patient, the injected, altered stem cells may not take hold, etc.), it will be a revolution, and the use of this promising new technology to relieve considerable suffering. I have my fingers crossed.

    The article below, which appeared in January, explains why Victoria Gray’s treatment may not be a real first. It may be the first use of CRISPR-altered cells from a patient to cure their own sickle-cell anemia, but it’s not the first time that fetal hemoglobin-producing cells have been used to treat the disease. Also, CRISPR-altered cells have been used to treat human patients with a different genetic disease—one causing blindness,—apparently with some success.

    So there’s some confusion about whether Victoria Gray is really a first, but never mind. It’s likely she’s the first sickle-cell patient to receive CRISPR-modified cells in an attempt to replace her defective hemoglobin with a form of hemoglobin (fetal) that may work. I fervently hope it does work, as she’ll be free of the disease that has debilitated her since her childhood. And then thousands of other patients could then receive the treatment, though of course it won’t work for all of them.

    CRISPR, as I said in my book review, offers enormous promise for curing genetic diseases, and even eliminating “disease-genes” from the germline (sperm and eggs). It also has promise for genetic engineering of plants and animals, though it’s considered unethical to use the method to do eugenic manipulation in humans, like making kids taller or smarter. And even then it wouldn’t work well, since things like height and intelligence, insofar as they are genetic, are based on many genes, and you can’t edit all of them. Nor would the treatment be useful for “polygenic” diseases like familial hypertension, which is also based on several to many genes. But for single-gene diseases like sickle-cell anemia or Huntington’s chorea, CRISPR is the direction to go.

    It’s certain that a Nobel Prize will be awarded for this new form of genetic manipulation. The question is who will get it, as many are in contention. My guesses are given the Washington Post article above.

    One final lesson that I’ll drive home again: CRISPR, with all its benefits and promises, derived from pure research: seemingly mundane sequencing of bacterial DNA. The consequences of that work were completely unforeseeable.

    One never knows what benefits to humanity can come from such pure research. That in itself is a reason to fund pure research not directly aimed at improving human welfare. But of course there are other reasons, too—the biggest being that pure knowledge about the universe is a benefit in itself, regardless of whether it makes our species taller, smarter, or healthier.


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