Tag Archives: sickle cell disease

Even a ‘good’ gene edit can go wrong

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An October 24, 2022 news item on ScienceDaily highlights research into better understanding problems with ‘good’ CRISPR (clustered regularly interspaced short palindromic repeats) gene editing,

A Rice University lab is leading the effort to reveal potential threats to the efficacy and safety of therapies based on CRISPR-Cas9, the Nobel Prize-winning gene editing technique, even when it appears to be working as planned.

Bioengineer Gang Bao of Rice’s George R. Brown School of Engineering and his team point out in a paper published in Science Advances that while off-target edits to DNA have long been a cause for concern, unseen changes that accompany on-target edits also need to be recognized — and quantified.

Bao noted a 2018 Nature Biotechnology paper indicated the presence of large deletions. “That’s when we started looking into what we can do to quantify them, due to CRISPR-Cas9 systems designed for treating sickle cell disease,” he said.

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An October 24, 2022 Rice University news release (also on EurekAlert), which originated the news item, details the concerns (Note: Links have been removed),

Bao has been a strong proponent of CRISPR-Cas9 as a tool to treat sickle cell disease, a quest that has brought him and his colleagues ever closer to a cure. Now the researchers fear that large deletions or other undetected changes due to gene editing could persist in stem cells as they divide and differentiate, thus have long-term implications for health.

“We do not have a good understanding of why a few thousand bases of DNA at the Cas9 cut site can go missing and the DNA double-strand breaks can still be rejoined efficiently,” Bao said. “That’s the first question, and we have some hypotheses. The second is, what are the biological consequences? Large deletions (LDs) can reach to nearby genes and disrupt the expression of both the target gene and the nearby genes. It is unclear if LDs could result in the expression of truncated proteins. 

“You could also have proteins that misfold, or proteins with an extra domain because of large insertions,” he said. “All kinds of things could happen, and the cells could die or have abnormal functions.”

His lab developed a procedure that uses single-molecule, real-time (SMRT) sequencing with dual unique molecular identifiers (UMI) to find and quantify unintended LDs along with large insertions and local chromosomal rearrangements that accompany small insertions/deletions (INDELs) at a Cas9 on-target cut site. 

“To quantify large gene modifications, we need to perform long-range PCR, but that could induce artifacts during DNA amplification,” Bao said. “So we used UMIs of 18 bases as a kind of barcode.”

“We add them to the DNA molecules we want to amplify to identify specific DNA molecules as a way to reduce or eliminate artifacts due to long-range PCR,” he said. “We also developed a bioinformatics pipeline to analyze SMRT sequencing data and quantified the LDs and large insertions.”

The Bao lab’s tool, called LongAmp-seq (for long-amplicon sequencing), accurately quantifies both small INDELs and large LDs. Unlike SMRT-seq, which requires the use of a long-read sequencer often only available at a core facility, LongAmp-seq can be performed using a short-read sequencer.

To test the strategy, the lab team led by Rice alumna Julie Park, now an assistant research professor of bioengineering, used Streptococcus pyogenes Cas9 to edit beta-globin (HBB), gamma-globin (HBG) and B-cell lymphoma/leukemia 11A (BCL11A) enhancers in hematopoietic stem and progenitor cells (HSPC) from patients with sickle cell disease, and the PD-1 gene in primary T-cells.  

They found large deletions of up to several thousand bases occurred at high frequency in HSPCs: up to 35.4% in HBB, 14.3% in HBG and 15.2% in BCL11A genes, as well as on the PD-1 (15.2%) gene in T-cells. 

Since two of the specific CRISPR guide RNAs tested by the Bao lab are being used in clinical trials to treat sickle cell disease, he said it’s important to determine the biological consequences of large gene modifications due to Cas9-induced double-strand breaks. 

Bao said the Rice team is currently looking downstream to analyze the consequences of long deletions on messenger RNA, the mediator that carries code for ribosomes to make proteins. “Then we’ll move on to the protein level,” Bao said. “We want to know if these large deletions and insertions persist after the gene-edited HSPCs are transplantation into mice and patients.”  

Co-authors of the study from Rice are graduate students Mingming Cao and Yilei Fu, alumni Yidan Pan and Timothy Davis, research specialist Lavanya Saxena, microscopist/bioinstrumentation specialist Harshavardhan Deshmukh and Todd Treangen, an assistant professor of computer science, and Emory University’s Vivien Sheehan, an associate professor of pediatrics. 

Bao is the department chair and Foyt Family Professor of Bioengineering, a professor of chemistry, materials science and nanoengineering, and mechanical engineering, and a CPRIT Scholar in Cancer Research.

The National Institutes of Health (R01HL152314, OT2HL154977) supported the research.

Here’s a link to and a citation for the latest paper,

Comprehensive analysis and accurate quantification of unintended large gene modifications induced by CRISPR-Cas9 gene editing by So Hyun Park, Mingming Cao, Yidan Pan, Timothy H. Davis, Lavanya Saxena, Harshavardhan Deshmukh, Yilei Fu, Todd Treangen, Vivien A. Sheehan, and Gang Bao. Science Advances Vol 8, Issue 42 DOI: 10.1126/sciadv.abo7676 First published online: 21 Oct 2022 Published in print: March 3, 2023

This paper is behind a paywall.

Gold nanoparticle loaded with CRISPR used to edit genes

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CRISPR (clustered regularly interspaced short palindromic repeats) gene editing is usually paired with a virus (9, 12a, etc.) but this time scientists are using a gold nanoparticle. From a May 27, 2019 news item on Nanowerk (Note: Links have been removed),

Scientists at Fred Hutchinson Cancer Research Center took a step toward making gene therapy more practical by simplifying the way gene-editing instructions are delivered to cells. Using a gold nanoparticle instead of an inactivated virus, they safely delivered gene-editing tools in lab models of HIV and inherited blood disorders, as reported in Nature Materials (“Targeted homology-directed repair in blood stem and progenitor cells with CRISPR nanoformulations”).

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A May 27, 2019 Fred Hutchinson Cancer Research Center news release (also on EurekAlert) by Jake Siegel, which originated the news item, expands on the theme, provides more detail,

It’s the first time that a gold nanoparticle loaded with CRISPR has been used to edit genes in a rare but powerful subset of blood stem cells, the source of all blood cells. The CRISPR-carrying gold nanoparticle led to successful gene editing in blood stem cells with no toxic effects.

“As gene therapies make their way through clinical trials and become available to patients, we need a more practical approach,” said senior author Dr. Jennifer Adair, an assistant member of the Clinical Research Division at Fred Hutch, adding that current methods of performing gene therapy are inaccessible to millions of people around the world. “I wanted to find something simpler, something that would passively deliver gene editing to blood stem cells.”

While CRISPR has made it faster and easier to precisely deliver genetic modifications to the genome, it still has challenges. Getting cells to accept CRISPR gene-editing tools involves a small electric shock that can damage and even kill the cells. And if precise gene edits are required, then additional molecules must be engineered to deliver them –adding cost and time.

Gold nanoparticles are a promising alternative because the surface of these tiny spheres (around 1 billionth the size of a grain of table salt) allows other molecules to easily stick to them and stay adhered.

“We engineered the gold nanoparticles to quickly cross the cell membrane, dodge cell organelles that seek to destroy them and go right to the cell nucleus to edit genes,” said Dr. Reza Shahbazi, a Fred Hutch postdoctoral researcher who has worked with gold nanoparticles for drug and gene delivery for seven years.

Shahbazi made the gold particles from laboratory-grade gold that is purified and comes as a liquid in a small lab bottle. He mixed the purified gold into a solution that causes the individual gold ions to form tiny particles, which the researchers then measured for size.

They found that a particular size – 19 nanometers wide – was the best for being big and sticky enough to add gene-editing materials to the surface of the particles, while still being small enough for cells to absorb them.

Packed onto the gold particles, the Fred Hutch team added these gene-editing components (diagram available [see below]):

A type of molecular guide called crRNA acts as a genetic GPS to show the CRISPR complex where in the genome to make the cut.

CRISPR nuclease protein, often called “genetic scissors,” makes the cut in the DNA. The CRISPR nuclease protein most often used is Cas9. But the Fred Hutch researchers also studied Cas12a (formerly called Cpf1) because Cas12a makes a staggered cut in DNA. The researchers hoped this would allow the cells to more efficiently repair the cut and while so doing embed the new genetic instructions into the cell. Another advantage of Cas12a over Cas9 is that it only requires one molecular guide, which is important because of space constraints on the nanoparticles. Cas9 requires two molecular guides.

Instructions for what genetic changes to make (“ssDNA”). The Fred Hutch team chose two inherited genetic changes that bestow protection from disease: CCR5, which protects against HIV, and gamma hemoglobin, which protects against blood disorders such as sickle cell disease and thalassemia.

A coating of a polyethylenimine swarms the surface of the particles to give them a more positive charge, which enables them to more readily be absorbed into cells. This is an improvement over another method of getting cells to take up gene editing tools, called electroporation, which involves lightly shocking the cells to get them to open and allow the genetic instructions to enter.

Then the researchers isolated blood stem cells with a protein marker on their surface called CD34. These CD34-positive cells contain the blood-making progenitor cells that give rise to the entire blood and immune system.

“These cells replenish blood in the body every day, making them a good candidate for one-time gene therapy because it will last a lifetime as the cells replace themselves,” Adair said.

Observing human blood stem cells in a lab dish, the researchers found that their fully loaded gold nanoparticles were taken up naturally by cells within six hours of being added and within 24 to 48 hours they could see gene editing happening. They observed that the Cas12a CRISPR protein partner was better at delivering very precise genetic edits to the cells than the more commonly used cas9 protein partner.

The gene-editing effect reached a peak eight weeks after the researchers injected the cells into mouse models; 22 weeks after injection the edited cells were still there. The Fred Hutch researchers also found edited cells in the bone marrow, spleen and thymus of the mouse models, a sign that the dividing blood cells in those organs could carry on the treatment without the mice having to be treated again.

“We believe we have a good candidate for two diseases — HIV and hemoglobinopathies — though we are also evaluating other disease targets where small genetic changes can have a big impact, as well as ways to make bigger genetic changes,” Adair said. “The next step is to increase how much gene editing happens in each cell, which is definitely doable. That will make it closer to being an effective therapy.”

In the study, the researchers report 10 to 20 percent of cells took on the gene edits, which is a promising start, but the researchers would like to aim for 50% or more of the cells being edited, which they believe will have a good chance of combatting these diseases.

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Adair and Shahbazi are looking for commercial partners to develop the technology into therapies for people. They hope to begin clinical trials within a few years.

Here’s the diagram of a gold nanoparticle loaded with CRISPR,

Caption: Graphic of a fully loaded gold nanoparticle with CRISPR and other gene editing tools. Credit: Image courtesy of the Adair lab at Fred Hutch.

Here’s a link to and a citation for the paper,

Targeted homology-directed repair in blood stem and progenitor cells with CRISPR nanoformulations by Reza Shahbazi, Gabriella Sghia-Hughes, Jack L. Reid, Sara Kubek, Kevin G. Haworth, Olivier Humbert, Hans-Peter Kiem & Jennifer E. Adair. Nature Materials (2019) DOI https://doi.org/10.1038/s41563-019-0385-5Published 27 May 2019

This paper is behind a paywall.

The CRISPR ((clustered regularly interspaced short palindromic repeats)-CAS9 gene-editing technique may cause new genetic damage kerfuffle

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Setting the stage

Not unexpectedly, CRISPR-Cas9  or clustered regularly interspaced short palindromic repeats-CRISPR-associated protein 9 can be dangerous as these scientists note in a July 16, 2018 news item on phys.org,

Scientists at the Wellcome Sanger Institute have discovered that CRISPR/Cas9 gene editing can cause greater genetic damage in cells than was previously thought. These results create safety implications for gene therapies using CRISPR/Cas9 in the future as the unexpected damage could lead to dangerous changes in some cells.

Reported today (16 July 2018) in the journal Nature Biotechnology, the study also revealed that standard tests for detecting DNA changes miss finding this genetic damage, and that caution and specific testing will be required for any potential gene therapies.

This CRISPR-Cas9 image reminds me of popcorn,

CRISPR-associated protein Cas9 (white) from Staphylococcus aureus based on Protein Database ID 5AXW. Credit: Thomas Splettstoesser (Wikipedia, CC BY-SA 4.0)[ downloaded from https://phys.org/news/2018-07-genome-crisprcas9-gene-higher-thought.html#jCp]

A July 16, 2018 Wellcome Sanger Institute press release (also on EurekAlert), which originated the news item, offers a little more explanation,

CRISPR/Cas9 is one of the newest genome editing tools. It can alter sections of DNA in cells by cutting at specific points and introducing changes at that location. Already extensively used in scientific research, CRISPR/Cas9 has also been seen as a promising way to create potential genome editing treatments for diseases such as HIV, cancer or sickle cell disease. Such therapeutics could inactivate a disease-causing gene, or correct a genetic mutation. However, any potential treatments would have to prove that they were safe.

Previous research had not shown many unforeseen mutations from CRISPR/Cas9 in the DNA at the genome editing target site. To investigate this further the researchers carried out a full systematic study in both mouse and human cells and discovered that CRISPR/Cas9 frequently caused extensive mutations, but at a greater distance from the target site.

The researchers found many of the cells had large genetic rearrangements such as DNA deletions and insertions. These could lead to important genes being switched on or off, which could have major implications for CRISPR/Cas9 use in therapies. In addition, some of these changes were too far away from the target site to be seen with standard genotyping methods.

Prof Allan Bradley, corresponding author on the study from the Wellcome Sanger Institute, said: “This is the first systematic assessment of unexpected events resulting from CRISPR/Cas9 editing in therapeutically relevant cells, and we found that changes in the DNA have been seriously underestimated before now. It is important that anyone thinking of using this technology for gene therapy proceeds with caution, and looks very carefully to check for possible harmful effects.”

Michael Kosicki, the first author from the Wellcome Sanger Institute, said: “My initial experiment used CRISPR/Cas9 as a tool to study gene activity, however it became clear that something unexpected was happening. Once we realised the extent of the genetic rearrangements we studied it systematically, looking at different genes and different therapeutically relevant cell lines, and showed that the CRISPR/Cas9 effects held true.”

The work has implications for how CRISPR/Cas9 is used therapeutically and is likely to re-spark researchers’ interest in finding alternatives to the standard CRISPR/Cas9 method for gene editing.

Prof Maria Jasin, an independent researcher from Memorial Slone Kettering Cancer Centre, New York, who was not involved in the study said: “This study is the first to assess the repertoire of genomic damage arising at a CRISPR/Cas9 cleavage site. While it is not known if genomic sites in other cell lines will be affected in the same way, this study shows that further research and specific testing is needed before CRISPR/Cas9 is used clinically.”

For anyone who’d like to better understand the terms gene editing and CRISPR-Cas9, the Wellcome Sanger Institute provides these explanatory webpages, What is genome editing? and What is CRISPR-Cas9?

For the more advanced, here’s a link and a citation for the paper,

Repair of double-strand breaks induced by CRISPR–Cas9 leads to large deletions and complex rearrangements by Michael Kosicki, Kärt Tomberg, & Allan Bradley. Nature Biotechnology DOI: https://doi.org/10.1038/nbt.4192 Published 16 July 2018

This paper appears to be open access.

The kerfuffle

It seems this news has affected the CRISPR market. From a July 16, 2018 article by Cale Guthrie Weissman for Fast Company,

… CRISPR could unknowingly delete or alter non-targeted genes, which could lead to myriad unintended consequences. This is especially frightening, since the technology is going to be used in human clinical trials.

Meanwhile, other scientists working with CRISPR are trying to downplay the findings, telling STAT [a life sciences and business journalism website] that there have been no reported adverse effects similar to what the study describes. The news, however, has brought about a market reaction–at least three publicly traded companies that focus on CRISPR-based therapies are in stock nosedive. Crispr Therapeutics is down by over 6%; Editas fell by over 3%; and Intellia Therapeutics dropped by over 5%. [emphasis mine]

Damage control

Gaetan Burgio (geneticist, Australian National University)  in a July 16, 2018 essay on phys.org (originating from The Conversation) suggests some calm (Note: Links have been removed),

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But a new study has called into question the precision of the technique [CRISPR gene editing technology].

The hope for gene editing is that it will be able to cure and correct diseases. To date, many successes have been reported, including curing deafness in mice, and in altering cells to cure cancer.

Some 17 clinical trials in human patients are registered [emphasis mine] testing gene editing on leukaemias, brain cancers and sickle cell anaemia (where red blood cells are misshaped, causing them to die). Before implementing CRISPR technology in clinics to treat cancer or congenital disorders, we must address whether the technique is safe and accurate.

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There are a few options for getting around this problem. One option is to isolate the cells we wish to edit from the body and reinject only the ones we know have been correctly edited.

For example, lymphocytes (white blood cells) that are crucial to killing cancer cells could be taken out of the body, then modified using CRISPR to heighten their cancer-killing properties. The DNA of these cells could be sequenced in detail, and only the cells accurately and specifically gene-modified would be selected and delivered back into the body to kill the cancer cells.

While this strategy is valid for cells we can isolate from the body, some cells, such as neurons and muscles, cannot be removed from the body. These types of cells might not be suitable for gene editing using Cas9 scissors.

Fortunately, researchers have discovered other forms of CRISPR systems that don’t require the DNA to be cut. Some CRISPR systems only cut the RNA, not the DNA (DNA contains genetic instructions, RNA convey the instructions on how to synthesise proteins).

As RNA [ribonucleic acid] remains in our cells only for a specific period of time before being degraded, this would allow us to control the timing and duration of the CRISPR system delivery and reverse it (so the scissors are only functional for a short period of time).

This was found to be successful for dementia in mice. Similarly, some CRISPR systems simply change the letters of the DNA, rather than cutting them. This was successful for specific mutations causing diseases such as hereditary deafness in mice.

I agree with Burgio’s conclusion (not included here) that we have a lot more to learn and I can’t help wondering why there are 17 registered human clinical trials at this point.