The incredibly efficient and accurate gene-editing technology, CRISPR (clustered regularly interspaced short palindromic repeats), is regarded as unsafe for use in humans due to its off-target effects.
However, scientists in China seem to have pushed the boundary, by recently using CRISPR to edit out the CCR5 gene from a type of bone marrow cell called HSPCs, which were then transplanted into a patient with HIV and acute lymphoblastic leukemia.
The resulting donor cells persisted for over 19 months, and the leukemia went into complete remission. However, HIV-affected counts of CD4+ cells were only marginally increased.
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All healthy human beings contain a variety of white blood cells, or leukocytes, which are derived from a line of progenitor or parent cells in the bone marrow. These are called the hematopoietic stem and progenitor cells (HSPCs).
In patients with leukemia, one or more of these cells develops malignant properties, giving rise to immature and inefficient leukocytes and crowding out the remaining normal HSPCs. To treat this, the bone marrow is usually treated with intensive doses of chemotherapeutic agents to wipe out all the HSPCs, whether malignant or normal.
Subsequently, a stem cell or bone marrow transplant is used to treat these patients by giving rise to a new healthy population of blood cells. In the current study, the donor stem cells were CCR5-ablated HSPCs.
Two years later, all indications are that the transplant was successful. In fact, the leukemic cells were no longer detectable, and the patient went into complete remission. The bone marrow is supplying both red cells and white cells in sufficient amounts. All seems to be well.
Not so with the HIV infection, however.
Patients with HIV infection show a life-threatening decline in the number of immune cells of the T-lymphocyte lineage, which are responsible for attacking and destroying infected cells and tumor cells. This reduces immunity so significantly that most of these patients die because of infections by insignificant germs, the kind that most normal healthy bodies wouldn’t stop to look at.
The HIV virus itself gains entry into the cell via the CCR5 receptor, a molecule which is expressed on the cell surface by the CCR5 gene. If a blood cell lacks this receptor, the entry of the virus is almost completely blocked.
Racing to cure HIV
The Chinese scientists were attempting to make use of this immunological loophole to cut down the chances of HIV infection, by introducing a new strain of CCR5-null HSPCs into the body. They removed the CCR5 gene using CRISPR technology.
When the HSPCs were replaced with the CCR5-null type cells, it was expected that the new CD4+ cells being churned out would lack this gene.
However, despite the donor cells (that were CCR5-null) persisting in the body for more than a year and a half, without obvious harm, there was only a 5% increase in the total number of CCR5-null CD4+ cells over a short period when the antiretroviral drugs were stopped. This was not enough to cure the patient of HIV, and as a result he is still on therapy with these drugs.
The grafted stem cells appear to have been accepted by the recipient, without any immune response or toxic effects. The scientists claim this proves the safety of CRISPR use in humans, while other experts appear to agree. In an editorial published alongside the report, cancer scientist Carl June says, “The safety profile appears to be acceptable.”
The race to cure HIV through genetic editing began with a German patient who received bone marrow cells which had the CCR5 Δ32 mutation, that meant these cells lacked the receptor necessary for HIV entry. The successful cure focused attention on this means of disabling the virus and empowering the body to deal with the infection itself.
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Win or lose?
In the present study, the Chinese version of CRISPR editing used a limited-time activation of the enzyme to minimize off-target edits. This would mean the enzyme would not cut out DNA bits it’s not supposed to.
But on the other hand, it also meant a much higher chance that it wouldn’t be able to complete the job that it’s meant to do. And this seems to be what happened, judging by the 5% to 8% of CCR5-null cells that were found in the bone marrow post-transplant. That means that at least one copy of the CCR5 gene was missing in only this percentage of cells.
When it comes to peripheral blood, which is defined as the actual blood in circulation, only about 2% of T lymphocytes actually show the edit. This is not enough to leave a mark on the virus entry, according to other biologists.
This has been emphasized in another study which used a less rigorous protocol to clean out the bone marrow and edited the stem cells using zinc-finger nucleases. Here again, only 2% to 4% of cells show the mutation.
The experiment has sparked ethical controversy
The greatest challenge in curing HIV with CRISPR is achieving efficient editing without harming the DNA strand otherwise. In the present study, the researchers have the advantage that they edited only adult cells, leaving other cells in the body non-mutated and therefore incapable of passing on the mutation to future generations.
This is important because a complete absence of the CCR5 gene function increases the individual’s vulnerability to influenza, West Nile virus, and perhaps could shorten the lifespan.
This underlines the more controversial side of this experiment: how did the Chinese scientists progress from CRISPR-mediated HIV treatments in mice to human trials in just under two years, when it takes more than double the time to prove the safety of a new cancer treatment involving genetic modification in human trials in other countries?
For instance, June himself had to spend five years taking the revolutionary T-cell immunotherapy for cancer from animals to human trials. Is it the new technology, or is it the more lenient regulatory regime in China?
After all, just last year a researcher carried out the first gene-editing on unborn fetuses, and another fused human and animal embryos to make the first-ever hybrid embryo in the world.
Other countries are now following suit: Russia will soon have its own gene-edited babies and head-transplants, while human-animal hybrid embryos can now be made in Japan.
It appears to be up to the government and the type of regulatory facilities that have already been put in place to decide how strictly the emerging field of CRISPR-editing in human patients will be supervised.
If so, the world may soon see a double-faced system, of strict controlled development on one side, and on the other, a system where scientists can get away with a lot of doubtful research in their quest to be the “first” to achieve their goals. As is already happening with medical procedures that are not always allowed in one’s homeland: just go abroad and get them done elsewhere.
The cost is obvious and very heavy: unregulated research can at best deliver unpredictable results, and at worst let loose a flood of quack industries claiming to accomplish the impossible, discrediting sound research and making it more difficult to prove real results.
Xu, L., et al. (2019). CRISPR-edited stem cells in a patient with HIV and acute lymphocytic leukemia. New England Journal of Medicine. DOI: 10.1056/NEJMoa1817426.