Building bones using stem cells and gene therapy

Dr Pascale V. Guillot is based at the University College London Elizabeth Garrett Anderson Institute for Women’s Health. She is leading a team of cell biologists who are using human stem cells to grow both healthy and brittle bone tissue in the lab. Why? So they can investigate the causes of a brittle bone disease called osteogenesis imperfecta and develop innovative treatments to make bones stronger

TALK LIKE A CELL BIOLOGIST

COLLAGEN – a major structural protein in the body, found in skin, bones and connective tissues. Type I collagen is the main component of bones

CRISPR – a genetic engineering tool that involves cutting and inserting genetic material in the DNA

EPITHELIAL CELLS – a specialised type of cell that lines the surface of organs, typically acting as a protective barrier

GENE THERAPY – modifying a person’s genes to treat or cure disease

IN VITRO – a process taking place in the lab (e.g. in a test tube); the opposite of in vivo (in a living organism)

OSTEOBLAST – a specialist type of cell that produces a bone matrix. Bone is not uniformly solid but consists of a flexible matrix (about 30%) and bound minerals (about 70%) that are intricately woven and endlessly remodelled by osteoblasts and osteoclasts. To read more about this, visit: https://en.m.wikipedia.org/wiki/Bone

OSTEOCLAST – a specialist type of cell that resorbs the old bone matrix

OSTEOGENESIS IMPERFECTA – also known as brittle bone disease, osteogenesis imperfecta is a rare genetic disease that starts in the womb and leads to deformities of the skeleton and breaking bones (fractures)

PLURIPOTENT – used to describe stem cells that can create all the different cell types present in the body, but are not totipotent (i.e. cannot create the placenta or umbilical cord)

STEM CELLS – unspecialised cells that are used as the ‘raw material’ to produce specialised cells

THERAPEUTIC – a therapy or medical treatment

“Our bones are extremely important, not just for supporting our body and protecting our internal organs, but also as a reservoir of minerals and other substances involved in metabolism,” says Dr Pascale V Guillot. “It is therefore very important to maintain bone health.” Pascale and her team are part of the Elizabeth Garrett Anderson Institute for Women’s Health at University College London in the UK. They are performing research involving some truly cutting-edge techniques: growing human bone tissue in the lab, developing stem cell-based therapeutics, reprogramming cells to pluripotency and editing the genetic makeup of stem cells to correct genetic defects.

Bone fragility affects many people for different reasons. Menopause, inactivity or immobility, and even lack of gravity (such as astronauts experience) can all weaken bones. Fragile bones can also be genetic: osteogenesis imperfecta, or brittle bone disease, is a rare inheritable disease that leads to fractures, short stature and skeletal deformities. Pascale and her team are focusing on this disease, but their findings will also help many people with bone fragility.

BONE BIOLOGY

Bones may seem like relatively simple parts of the body, but there is plenty to them that does not initially meet the eye. “Bones are complicated structures, and are constantly remodelling,” says Pascale. “Cells called osteoblasts build new bone tissue, while cells called osteoclasts destroy old bone tissue – ideally at the same rate.” The osteoblasts produce fibres of a protein called type I collagen, which is a major building block within our bodies, contributing to skin, muscles and tendons, as well as bones. Once the osteoblasts have secreted the collagen, they then control the deposition of calcium and other minerals on these fibres, which add strength and structure to our skeleton.

Osteogenesis imperfecta is the result of errors (mutations) in genes that lead to insufficient or abnormal production of collagen fibres in bones. “We originally thought that the faulty fibres were directly responsible for weak bones, but it turns out there is more to it than that,” says Pascale. “Faulty fibres do lead to a weak bone structure – imagine a house made of cardboard bricks. However, these fibres also prevent osteoblasts from maturing properly, meaning they aren’t able to mineralise the fibres. So, as well as the house being made from cardboard, it also has fewer bricks than it should.” Added to this, reduced osteoblast activity also leads to increased osteoclast activity, meaning that bone destruction is happening faster than bone construction.

Now, Pascale’s team has an understanding of the pathway that leads from genetic mutations to bone fractures, they can move on to tackling osteogenesis imperfecta.

BUILDING BONES IN THE LAB

Pascale’s team is extracting epithelial cells found naturally in samples of people’s urine. Though these cells are already fully specialised, the team use a ground-breaking technique to return them to a pluripotent stem cell state. This cell rejuvenating technique, known

as cellular reprogramming to pluripotency (induced pluripotent stem cells, iPSCs), is revolutionising cell biology. Previously, nonpluripotent stem cells (called multipotent stem cells) were mainly isolated from various tissues of healthy donors and grown in the laboratory. However, as they grow, they also start to age and progressively lose their ability to specialise into bone-forming cells (osteoblasts) and repair tissues. Now, cell biologists can derive osteoblasts directly from iPSCs. “The iPSCs we create have the potential to become any type of specialist cell in the body,” says Pascale. “For our research, we cultivate them to become osteoblasts.”

Using these osteoblasts, Pascale’s team is working to build three-dimensional structures made up of human bone in vitro. “This technique means we don’t have to experiment using animals, and increases the physiological relevance of our work,” says Pascale. “However, we have several challenges to overcome, namely engineering a system where human cells can thrive and produce a bone matrix.” Bones are complex tissues requiring precise processes to build them correctly, so this is no easy task, but the team is making strong progress.

FROM EXPERIMENTS TO TREATMENTS

“Using urine from healthy patients and osteogenesis imperfecta patients, we’re able to build healthy bone structures and fragile bone structures,” says Pascale. “We can then test the effects of different treatments on the fragile bone structures, seeing if any stimulate the osteoblasts to mature properly and improve bone strength.” Excitingly, the team is also involved in a clinical trial that will determine the safety and efficacy of prenatal cell therapy for osteogenesis imperfecta. Since osteogenesis imperfecta is a genetic disease, there is another possible solution: editing the genes themselves to repair the genetic mutation.

“Using patients’ cells, we can correct the mutation using CRISPR, and then transplant these corrected cells back into the patients,” says Pascale. This technique has even greater potential than stem cell transplants, since the human body is much more likely to accept its own cells than those from a different person.

When will we see the team’s research in action? As Pascale explains, “The next step is to move from discovery to commercialisation, so these techniques can help individuals with osteogenesis imperfecta – and potentially people with other fragile bone conditions – across the world!”

Download the article

Reference
https://doi.org/10.33424/FUTURUM144

Link to the activity sheet

The tissue culture room at the Zayed Centre for Research into Rare Disease in Children, where Pascale’s team works. Pascale belongs to the Elizabeth Garrett Anderson Institute but works in the Zayed Centre, which is part of University College London (UCL).

Entrance to the Zayed Centre for Research into Rare Disease in Children.

The laboratory where experiments are performed.

Bone remodelling showing new bone formation by osteoblasts and old bone resorption by osteoclasts.