Sickle cell disease gene therapy study set back by the wrong mouse

Sickle cell disease (SCD) is a debilitating illness affecting up to 40% of the population in some African countries. It’s caused by mutations in the gene that makes haemoglobin – the protein that carries oxygen in red blood cells.

It might one day be possible to treat this disease using gene editing – by switching back on the production of a healthy form of haemoglobin called foetal haemoglobin, which is usually only produced by the body when we’re in the womb.

But a new study testing this promising new treatment in mice has found that scientists still have a long way to go before it can be attempted in humans. The research has been published in Disease Models & Mechanisms.

Healthy red blood cells (RBCs) are shaped similar to a donut – but with an indentation instead of a hole.

In sickle cell disease the abnormal haemoglobin distorts the RBC’s shape when they aren’t carrying oxygen. Instead, sickled RBCs are C-shaped, like the farm tool called a “sickle”, and they become hard and sticky, and die earlier.

Scd sickledcells
Human red blood cells with sickle cell disease. Credit: Image courtesy of the Weiss lab

Because of their shape, sickled RBCs can become stuck and stop blood flow when travelling through small blood vessels. This causes patients to suffer from episodes of excruciating pain, organ damage and a reduced life-expectancy.

Although current treatments have reduced complications and extended the life expectancies of affected children, most still die prematurely.

Red blood cells are made from haematopoietic stem cells in our bone marrow. These stem cells are able to develop into more than one cell type, in a process called haematopoiesis.

Researchers hope to edit the genes of these stem cells so that they produce RBCs with foetal haemoglobin instead of the abnormal protein and can be reintroduced into the body to alleviate the symptoms of SCD.

Unfortunately, they found that although two types of lab mice had the symptoms of sickle cell disease, their foetal haemoglobin gene and surrounding DNA were not properly configured, making the stem-cell treatment ineffective or even harmful.

These mice – called “Berkley” and “Townes” mice – were genetically engineered in different ways to carry several human haemoglobin genes (replacing the mice genes) so scientists could study sickle cell disease in an animal model.

The researchers removed stem cells from the mice and used CRISPIR-Cas9 to try to turn on the healthy foetal haemoglobin gene. They then put the reprogrammed stem cells back into the mice and monitored the animals for 18 weeks to find out how the treatment affected them.

Surprisingly, 70% of Berkley mice died from the therapy and production of foetal haemoglobin was activated in only 3.1% of the stem cells. On the other hand, treatment did not affect the survival of Townes mice and even activated the foetal haemoglobin gene in 57% of RBCs.

Image of the nucleus of a stem cell from berkley mice, showing where the dna has been edited.
This shows the nucleus holding the DNA in a stem cell from the Berkeley mice. The green and red spots show the DNA that has been edited by this new stem cell-based gene therapy for sickle cell disease. These coloured dots illustrate abnormal gain or loss of DNA segments after editing. Credit: Image courtesy of the Weiss lab

Even then, the levels of foetal haemoglobin produced were seven to 10 times lower than seen when this approach was used in human cells grown in the laboratory and were not high enough to reduce clinical signs of sickle cell disease.

“We realised that we did not know enough about the genetic configurations of these mice,” says senior author Dr Mitchell Weiss, chair of the haematology department at St Jude Children’s Research Hospital, US.

The researchers sequenced the mice’s haemoglobin genes and surrounding DNA, and discovered that Berkley mice – instead of having a single copy of the mutated human gene – had 22 randomly arranged, broken-up copies of the mutated human sickle cell disease gene and 27 copies of the human foetal haemoglobin.

This caused the fatal effects seen and meant that the mice cannot be used to test this treatment in the future.

“Our findings will help scientists using the Berkeley and Townes mice decide which to use to address their specific research question relating to sickle cell disease or haemoglobin,” concludes Weiss.

“Additionally, this work provides a reminder for scientists to carefully consider the genetics of the mice that they are using to study human diseases and find the right mouse for the job.”

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