Making mice that could save children’s lives

“Over the past 20 years, we’ve begun to realise that paediatric brain tumours are not the same as adult brain tumours,” says neurosurgeon, Michael DeCuypere, director of translational research and clinical trials in neurosurgery at the Ann & Robert H. Lurie Children’s Hospital of Chicago, Illinois.

Brain tumours are the top cause of cancer-related deaths in children¹. Although survival rates are improving for children with brain cancer², the outcome of some forms of this disease are still bleak. The median survival rate for a child diagnosed with diffuse intrinsic pontine glioma (DIPG), for example, is only nine months³. “Those tumours cannot be removed by surgery because of their location within the brainstem and the central structures of the brain,” DeCuypere says. For DIPG and other paediatric brain cancers, Lurie Children’s scientists apply a powerful tool in the search for better treatments.

The tool’s name is a mouthful: patient-derived orthotopic xenograft (PDOX) mouse models of childhood brain cancers. Broken down, it means that Lurie Children’s scientists insert cells from a child’s tumour into the matching location in the brain of a mouse. For simplicity, shorthand for this procedure is known as making a PDOX model.

“Paediatric tumours are not the same tumours that adults get,” DeCuypere explains. “That can be from a histology standpoint, from a molecular standpoint — kind of across the board.” So, treatments that work in adults won’t necessarily work in children.

By learning more about the biology of these malignancies in children, scientists can unearth new targets for better therapies. PDOX models help scientists find those targets and learn how to treat them.

Making the models

Xiao-Nan Li, programme director of precision medicine PDOX modelling of paediatric tumours at Lurie Children’s, started making these models in 2002. The process begins with a sample, which comes from surgery to remove a child’s brain tumour.

Li injects the human brain cancer cells into a specific location in a mouse’s brain. He studies the anatomy of the mouse brain and makes careful measurements — and has even developed a custom device to control the depth of the implantation — in order to ensure that the cells are injected into the desired location. With Li’s skills and years of practice, “Injecting the cells takes about 30 seconds per mouse, and we can implant up to 250 mice in four hours.”

Originally, Li used immune-deficient mice. Without an immune system to attack the foreign cells, the cancer cells grow unchecked in the mouse’s brain. The mouse can then be used to test how a drug impacts the cancer.

“A downside of the classic patient-derived xenograft model is the use of an immune-compromised mouse,” DeCuypere explains, which prevents the study of the immune system’s role in cancer. Nonetheless, Li and his colleagues can induce immunodeficiency, essentially killing a mouse’s immune system, then give it a bone-marrow transplant of human cells, and that creates a humanized mouse — one with a human-like immune system. These mice can be used to test the effect of a drug on a human tumour in conjunction with a human immune system. This gives a more accurate demonstration of how the drug might work in a child.

For the near future, Li and his team, in collaboration with neurosurgeon–scientists like DeCuypere, will make humanized mice by using bone-marrow and cancer cells from the same patient. As a result, a drug could be tested against a specific child’s tumour and immune system. The complexity of getting both cell types from the same patient makes it more difficult to create these PDOX models. “We still start with immune-deficient mice to confirm that the tumour will grow,” Li explains. “Then, we transfer cells from the tumour in the immune-deficient mouse’s brain to grow it in a humanized mouse.”

Lurie Children’s scientists also compare cancer in a child’s brain to one grown in a mouse. These comparisons include microscopy to confirm the tumour’s location in the mouse brain and to see if the cells look, grow, and spread in the same ways. Also, genetic tools, such as next-generation sequencing, confirm that the DNA from the cells is the same as well.

Making the most of PDOX models, however, requires a collection of them. “Li has close to 170 PDOX models for paediatric brain tumours, which is an astronomical amount,” says Alicia Lenzen, director of the Pediatric Neuro-Oncology Fellowship. “They cover the vast majority of paediatric tumours, and come from cells at diagnosis, after recurrence, and based on autopsies.” According to Li, this is one of the world’s largest collections of patient brain tumour models.

Applying the models

With such a vast collection of PDOX models, scientists at Lurie Children’s can address many questions. “A core goal is truly understanding the biology of how a tumour works,” says Lenzen. “These models allow scientists at Lurie Children’s to do much more detailed work in the lab in mice before moving forward with clinical trials and kids.” Although that can’t promise success in every clinical trial, it improves the odds.

Much of this work aims to develop new therapies for paediatric brain cancers, including immunotherapies. “This is the future of paediatric neuro-oncology — very specific, tailored immunotherapies,” DeCuypere says. As part of developing such therapies, DeCuypere and his colleagues analyse the immune cells in a tumour. “I do immune profiling on any paediatric brain tumour that I get my hands on,” he says.

By analysing a variety of PDOX models of paediatric brain tumours, scientists look for ways to attack the disease. For example, Li and his colleagues studied PDOX models of paediatric glioblastoma multiforme — also known as high-grade glioma — an aggressive brain cancer that only 5–15% of children survive for five years — and showed higher levels of specific forms of micro-RNA in more invasive cells⁴. Based on that discovery, Li and his colleagues treated the PDOX models with a drug that silences the driver gene of these micro-RNAs, and that blocked the invasive cells and significantly extended how long the mice lived.

Such results, however, are only the beginning of using Lurie Children’s PDOX models. “We’ve demonstrated that we can replicate human cancers very well, but we want to do even more,” Li says.

Part of that will come from collaborating with scientists, clinician–scientists and surgeon–scientists around the world. “There are many other investigators who are super good at something of their choosing,” Li says. “And we’re happy to share our models with them.”

“Every patient and family that we take care of can participate in and contribute to research,” says Sandi Lam, division chief of paediatric neurosurgery at Lurie. “We hope to advance knowledge and make care of paediatric brain tumours better for the present and the future.”