Scientists have created a new and efficient way to study mutations found in schizophrenia, autism, and brain cancer.

About 23 million people worldwide live with schizophrenia. Another 50 million live with dementia, and 300 million with depression. Yet the roots of these diseases and many other brain disorders elude scientists. The brain’s complexity makes answering questions about neuropsychiatric conditions and diseases difficult, says Howard Hughes Medical Institute Investigator Frederick Alt, of Boston Children’s Hospital.

Now, he and his colleagues have developed a technique that could help. They have invented a new way to grow engineered brain cells inside living mice. “Our system allows us to use stem cells and basically make a new brain,” says Bjoern Schwer of the University of California, San Francisco. Schwer, Alt, and a team of scientists published the work on October 10, 2018, in the journal, Nature.

Research in mice has already provided clues about how the larger and more complex human brain works. The mouse brain’s small size and the plethora of genetic tools available make it useful for gleaning insight into neurological diseases, Alt says.

The new mouse brains his team are able to grow can carry mutations that contribute to autism, schizophrenia, Alzheimer’s disease, or various cancers – giving the researchers a way to study specific genetic glitches. The team’s system could speed brain research and even be a step toward growing other types of organs for transplant patients. “The approach is versatile,” Alt says. “It’s a powerful thing.”

Long road

Alt and Schwer’s approach arises from work Alt and his colleagues began nearly three decades ago. In the 1990s, they developed a mouse with a mutation in a gene called RAG2. Disabling this gene crippled the mouse’s ability to form lymphocytes, a type of white blood cell that fights infections. A simple injection of stem cells could replace the missing blood cells­­ – the stem cells respond to cues in the developing embryo and fill in the empty niche.

What’s more, the researchers could add mutations to those stem cells. Usually, to study such mutations, scientists have to breed mice. But for disorders involving multiple mutations, adding each one to a mouse lineage can take three to four years of crossbreeding. The new approach can add multiple mutations to all of the lymphocytes in mice in just three to four months, Alt says.

The resulting mice are called chimeras. Just as the mythical monster combines parts of different animals, scientific chimeras contain a mix of cells with distinct genetic lineages. “Chimeras give you options to study certain genes in a particular tissue,” he explains. “In some cases, you couldn’t do it with mouse breeding alone.”

The chimera approach took off, and other research groups used it to eliminate, then replace, other organs and tissues in mice – including the pancreas, thymus, kidney, and heart. The Alt lab wanted to develop a similar chimera approach for the mouse brain. They wanted to study genes they recently discovered to be highly susceptible to breakage in brain progenitor cells. But, there was no single gene his team could mutate to create a mouse with a missing brain. Too many other systems would be affected, he says.

Gene scissors

The gene-editing tool CRISPR finally helped Alt and his colleagues clear that hurdle.

CRISPR can snip out a specific DNA location and paste in a gene of the researchers’ choosing. Working in mice, Alt and his team added a gene for a toxin from diphtheria bacteria. The toxin was active only in cells that give rise to the forebrain. During development, the toxin wiped out those cells, eliminating the mouse’s forebrain. Then, the team injected stem cells into the mouse embryo to replace the missing structure.

The result is a chimeric mouse that researchers can use to study mutations affecting the forebrain.

To test their new mouse, Alt and Schwer’s team introduced stem cells carrying a well-studied mutation they knew would affect the brain. The mutation, in an X-chromosome gene called Doublecortin, makes it hard for neural stem cells to migrate to their correct locations during development. The problem is visible – mice have disrupted layers in the hippocampus, a structure that plays an important role in memory formation.

The team’s chimeric mice with a Doublecortin mutation also had disrupted hippocampi, but otherwise, their brains were normal. That indicates the approach works as expected, Alt says.

CRISPR opens up the possibility of creating even more types of mouse chimeras, including those that enable the study of other parts of the brain, such as regions affected by Parkinson’s disease. “It is very easy to test the system and know that it’s working,” Alt says. “It should be an approach that many neuroscientists may wish to use.”

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