Emerging Methods for Battling Cancer - TALEN Gene Editing

"You can't lose hope when it's hopeless. You got to hope more, then put your fingers in your ears and go, ‘Blah! Blah! Blah! Blah!...'"
- Fry, Futurama, 20th Century Fox.
As we've discussed multiple times before, cancer is a major public health issue and continues to be one of the leading causes of death in the United States and the world. Cancers always seem to find a way to recur after resection, evade our immunosurveillance, or become resistant to our therapies. To combat this, the scientific community is always looking for new and inventive methods for killing cancer cells while leaving healthy cells intact. In this blog, we're going to discuss a recent cancer fighting method that seems to have saved a young girl’s life.

Family Guy, 20th Century Fox.
A couple of weeks ago, the scientific world learned about a little girl who was dying of leukemia and had already exhausted all current treatment options. A one-year old was living with an aggressive leukemia when most doctors thought it was best to just make the baby as comfortable as possible. Instead, her parents decided to have the girl undergo a risky procedure in which T cells from an outside donor were injected back into her bloodstream.

South Park, Comedy Central Productions.
Typically, this type of adoptive transfer may occur with a patient's own immune cells, where the patient's cells are extracted, stimulated in vitro and trained to target the specific tumor type. However, the patient in this instance did not have enough of her own immune cells for transfer since they were all killed off during chemotherapy. To get around this, an outside donor's immune cells were used instead, but with some modifications. The donor's cells underwent gene-editing - the use of specific enzymes designed to introduce or remove segments of DNA from the unrelated healthy donor's cells.

Taken, 20th Century Fox.
We've talked about other types of gene editing in the past, as in the case of CRISPR. However, this trial was the first successful use of gene-editing in cancer, and the second successful trial in humans (the first being an HIV trial from last year). The gene-editing enzymes in this case were TALENs, transcription activator-like effector nucleases, which are synthetic restriction enzymes that can be honed to specific DNA sequences and introduce double-strand breaks (DSBs). TALENs are based on TALE, a type of protein secreted by Xanthomonas bacteria as a method to aid in their infection of plants by inducing specific expression of certain genes. By altering the DNA-binding domain of TALE, specific sequences of DNA can be targeted.
TALEN binds to DNA, just like a bird’s talons grab on to prey!
TALENs edit a genome through two major methods:
  1. Introducing a set of DSBs that excises a portion of DNA, which, after non-homologous end joining DNA repair, causes the resultant gene product protein to lose function.
  2. Introducing DSBs that excise a portion of the DNA in the presence of double-stranded DNA fragments that can incorporate in the genome at the location of the break.

Workflow of TALEN genome editing.
For this leukemia patient, a healthy donor's T cells were genetically altered in such a way so that they would not be detectable by the chemotherapy treatment, and also so that they would target the patient's leukemia cells. Indeed, the patient survived the therapy and two months later, the leukemia was no longer detectable. The procedure is now being planned for a larger trial over the next few years. Doctors are hesitant to say the girl is cured until they are certain the cancer is in full remission. Even so, they're quite excited to see such dramatic results from this experimental therapy. Of course, some caution about the use of healthy donors for transfer therapies is required, as this could lead to GVHD in a patient, as we've discussed before. Additionally, certain details have yet to be revealed about this experiment, such as what the exact modifications were that they performed on the donor’s T cells. The company and the team that worked on the project are presenting more details on their findings at this year's American Society of Hematology Meeting (ASH 2015), so if you're there, you can learn more about this trial. BioLegend will also be at ASH this year, so make sure you stop by our booth, too!
References:
  1. Gene editing saves girl
  2. Designer immune cells clear leukemia

House, Universal Television.

Contributed by Ed Chen, PhD.
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