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Medulloblastomas originate much earlier in pregnancy than previously thought

An aggressive type of pediatric brain tumor, medulloblastoma, exists in a pre-malignant form at birth after initially developing during the first or second trimester of pregnancy, according to a new international study. As medulloblastomas typically present during childhood, around seven years of age, the team's discovery is the first indication there could be a window of several years during which a medulloblastoma could be prevented from ever occurring.

Four leading research groups partnered to study the highly aggressive brain tumor, including groups at the Max Rady College of Medicine at the University of Manitoba, the National Cancer Center in Tokyo, Japan, Seattle Children's Research Institute, and The Hospital for Sick Children (SickKids). The team's findings were published today in Nature.

Brain tumors are the deadliest form of childhood cancer and account for 20 per cent of all new pediatric cancer cases. Treatment for medulloblastoma currently consists of aggressive surgery, high doses of chemotherapy, and/or radiation of the whole brain and spinal cord. Up to 40 per cent of patients still succumb to the disease, while survivors must deal with the long-term toxicities associated with chemotherapy and radiation.

Medulloblastoma is comprised of multiple subgroups and the study focused on the most common and least well-understood subgroup, called Group 4. The study team's goal was to identify how and where Group 4 tumors arise during embryonic development in the hopes of accelerating discoveries of more effective treatments.

The researchers collected medulloblastoma samples from children's hospitals around the world and used several sequencing technologies to successfully identify genetic variations that can cause Group 4 medulloblastomas. They found these genetic variations were effectively "stalling" normal cellular differentiation in a specific cell type only present during early fetal development of the human cerebellum. This "stalling" of normal cellular differentiation results in a pre-malignant form of the tumor that then resides in the brain after birth, a finding which may be generalizable to the other forms of medulloblastoma.

The researchers note that the cell type they identified is far more common and is found in a region of the developing cerebellum that is more structurally complex in humans compared to other mammals. They say this could mean medulloblastoma is a direct consequence of the evolved complexity of the human cerebellum.

Crucially, the team's findings show medulloblastomas originate much earlier in pregnancy than previously thought and that they could even be preventable. They say clinicians may have a window of opportunity between birth and symptom onset to detect medulloblastomas before they develop, and possibly to prevent them from ever turning into deadly brain tumors. This is thought to be the first time that medulloblastoma has been identified as potentially preventable.

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