The Promise of Stem-Cell Biology: Treating People at High Risk for Psychiatric Illness Before They Become Patients
Kristen Brennand, Ph.D.
Icahn School of Medicine at Mount Sinai
Yale University School of Medicine
BBRF Scientific Council
2018 Maltz Prize winner for Innovative & Promising Schizophrenia Research
2016 BBRF Independent Investigator Grant
2012 BBRF Young Investigator Grant
"Why would you treat schizophrenia after the first psychotic episode—if you could intervene when someone was 10 years old, before the first symptoms appear? Why would you treat Alzheimer’s disease in 60-year-olds who already have neurons in their brain that are dying—if you could prevent those cell-deaths in 30-year-olds? Our ultimate goal is to treat patients before they become patients.”
This is the vision that drives the research of Kristen Brennand, Ph.D., whose early-career successes, which grew out of preliminary studies she performed with BBRF grants in 2012 and 2016, have led to multiple career-supporting NIH grants. In 2018 Dr. Brennand was honored with BBRF’s Maltz Prize for Innovative and Promising Schizophrenia Research and in 2019 her leadership and expertise were recognized when she was asked to join BBRF’s Scientific Council.
She has come a long way in just a few years, reflecting great strides she and her colleagues have made in one of the most promising areas of biological research: applying what we know about stem cells to the problems of psychiatric and neurodevelopmental illness.
Stem cells are the “mothers” of all cells and come in several varieties. Dr. Brennand works with those that are “pluripotent”—stem cells that have the power to develop into the many different types of cells that make up the organs of the human body, including the brain.
The focus of Dr. Brennand’s research is something that 20 years ago might have sounded like science fiction: taking skin or blood cells sampled from psychiatric patients, reprogramming them to a pluripotent stem-cell state, and then directing them to redevelop as brain cells.
The technology as it stands today is particularly useful in studying pathology in illnesses like schizophrenia and autism that are rooted in developmental processes at the very beginning of life, when the brain is forming.
BACK IN TIME
How can a mature cell be forced to go “back in time”? In 2006, Dr. Shinya Yamanaka, a scientist in Japan, demonstrated this was possible by taking single cells and activating the genes of four transcription factors (regulators of gene activity). Once they reverted to the pluripotent state, the cells could then be brought forward, by application of specific chemical factors, so that they matured into different kinds of somatic cells—specialized cells that comprise the organs. For these breakthroughs, Dr. Yamanaka in 2012 was awarded the Nobel Prize.
Dr. Brennand’s career has advanced along with this revolutionary technology and has contributed significantly to its maturation. In her lab, she takes skin or blood cells donated by a patient with illness and uses a variety of methods to induce them to differentiate into, say, dopamine neurons, or neural support cells called astrocytes, or glial cells which are immune cells unique to the brain. Each of these reborn cells is grown in a petri dish, and will bear the genetic code of the patient who supplied the original cells. Importantly, the cells reprogrammed from the pluripotent state—they can be generated as many times as desired and grow into functional, interconnected groupings—do not resemble mature neurons, astrocytes and glia. Rather, they are almost identical to immature versions of these cells that are found in the fetal brain.
This fact makes the new technology, called “human induced pluripotent stem cell” (hiPSC) technology, uniquely valuable for psychiatry. It provides a chance to observe, from inception, pathology in brain illnesses with genetic roots that are thought to begin at the dawn of life. It’s a chance to see what goes awry in cells of a specific patient; and to compare those results with similar experiments in other patients, and in comparative experiments with healthy controls.
An important premise of hiPSC research and its future applications is that it will be easier to prevent or lessen in intensity a pathological process when it has just begun, or even before it begins, than it is to treat its system-wide effects once an illness has fully manifested.
Making that vision a reality will depend on knowing in advance who tomorrow’s “patients” are most likely to be—before symptoms emerge. And the key to this, in disorders like schizophrenia and autism that are strongly rooted in genetic variations, is the ability to obtain an individual’s genetic sequence at birth or shortly after, and based on that read-out, apply interventions that are most likely to minimize or prevent early pathologies from developing.
“Assume you have my DNA sequence,” Dr. Brennand postulates. “What we want to know is which genes are differentially expressed in which cell types—and what does that mean for my disease risk and for my drug responses? If someone knew you carried risk factors that changed gene expression in your neurons, increasing the risk of psychiatric illness, then your disease risk could be predicted when you were born. And then you have a whole lifetime for preventive or therapeutic intervention.”
A longstanding obstacle to progress in treatments, she explains, is that schizophrenia is highly heterogeneous, meaning that different patients experience different combinations of symptoms. This clinical heterogeneity is thought to strongly reflect schizophrenia’s genetic heterogeneity. Studies that have scanned the genomes of several hundred thousand patients and healthy controls over the last 20 years have identified over 200 locations in the genome (“risk loci”) where the DNA sequence is different in people who have schizophrenia (or are at high risk for it). Most of these “risk variations” in the genome are small stretches of DNA, and do not, by themselves, disable vital genes, but instead mostly affect parts of the genome that regulate genes. These risk variations for schizophrenia are common—nearly every human being has one or several of them. What remains to be explained is how, in a bit less than 1 percent of the human population, different numbers of small risk variations, perhaps in combination with environmental and other factors, result in schizophrenia.
How then can we untangle the complexity of this illness? Stem cell technologies, to begin with, solve a problem that always has limited brain research: difficulty accessing human brain tissue and the functioning brain in living people. Useful studies have been made of postmortem brains of individuals who have lived with schizophrenia and other psychiatric illnesses. But these brains reflect a lifetime of disease impact and can’t tell researchers enough about how pathology emerges.
Because of this difficulty, researchers have turned to animal models. But again, there are limitations. When a rodent is repeatedly exposed to stress and begins to show behaviors that are analogous in certain respects to human behavior in depression, it is possible to look at brain cells and circuits and make observations about changes that are occurring. But it is impossible to generate a realistic rodent version of schizophrenia or autism; these are uniquely human illnesses, with effects that alter thinking, speaking, and perception.
‘HOW BIG CAN WE GO?’
Stem-cell technology enables researchers to generate virtually limitless quantities of live human neurons, every cell perfectly representing a patient’s genetics in all its complexity. But this fidelity can be both a blessing and a curse.
In 2011 Dr. Brennand, at the time a postdoctoral researcher, and her mentor at the Salk Institute for Biological Studies in San Diego, Fred Gage, Ph.D., a BBRF Scientific Council member and 2013 Distinguished Investigator, published in Nature the first study in which a living model of schizophrenia was created using stem cells grown from skin cells donated by schizophrenia patients.
After reprogramming the derived stem cells to become neurons and observing their function as they grew in a lab dish, the team was able to document diminished neuronal connectivity (compared with neurons from healthy people), as well as decreases in the function of glutamate receptors, and changes in gene expression, among other things. They exposed the newly created neurons to five antipsychotic medicines, and found that one of the five reversed a number of the changes seen in the patient-derived cells.
Based on samples from only four schizophrenia patients, this study was only a hint of what soon would be accomplished. A key moment for Dr. Brennand came after her first BBRF grant, a 2012 Young Investigator award, supported a project that generated data needed to secure her first large NIH grant. “What I did with BBRF support and that first NIH grant was ask, ‘How big can we go with this?’ And the most ambitious thing I could imagine at the time was a study with 13 patients and 13 controls. We made two to three lines of stem cells from each.”
What she and her team came to appreciate was the inherent difficulty of determining how the genomes of these 26 individuals were (or were not) related to irregularities seen in the neurons grown from their donated cells. Since every human has DNA variations, and only some of these are relevant to disease (most in fact are not) how can specific variations be matched reliably with specific effects on how cells behave?
Using the gene-editing technology called CRISPR to make cells “isogenic”— genetically identical except for one or more variations under study—“you can ask, for example, what the same exact mutation in 10 different people leads to, and how much the effects vary between the cells we generate from each person,” Dr. Brennand says. Or, “you can use CRISPR to introduce one variation, then two, then three, keeping everything else the same, and ask: what is the effect? Or, a patient may have two risk variants; if I use CRIPSR to fix one, is that sufficient to change the pathology we saw in the original cells?”
Isogenic brain cells grown from stem cells also make possible an experiment impossible to conduct in mice—to study the interplay of hundreds of DNA “risk variants” at once, across a multitude of cells types. They can be studied cell type by cell type, or in combinations with one another—either in petri dishes or in 3-dimensional assemblages of cells called organoids.
“No cell exists in isolation,” Dr. Brennand notes. “In ALS (Lou Gehrig’s Disease), pathology is defined by motor neuron death. But a lot of the risk variants that have been identified in ALS are expressed not in neurons but in astrocytes.” In that case, growing them together makes great sense.
Making isogenic lines of patient-derived cells also opens a new vista on drug discovery. In 2018 Dr. Brennand and colleagues published a pioneering study in which hiPSC technology was used to generate neural cells from 12 people with schizophrenia and 12 controls. A drug screen was performed: the cells were exposed systematically to 135 drugs, generating 4,320 unique drug-response signatures based on changes in gene activity within each cell line. This enabled the team to identify 18 existing drugs that were able to reverse gene-expression changes that had been seen in the brains of schizophrenia patients that were examined in a postmortem study. There will be many more stem cell-enabled drug screens to come in the years ahead.
FINDING THE SECRET OF RESILIENCE
Stem-cell technology also enables Dr. Brennand’s team to approach the important question of why some people have severe illness and others have milder illness. “I think it’s really important to know, for example, why some people with autism are high-functioning and others are low-functioning,” she says. “Even if we never cured autism—but we were able to turn low-functioning patients into high-functioning ones, we would be doing an incredible amount of good.”
The same applies to resilience. “I love the idea that there are biological cures walking around that we just haven’t been able to recognize,” she says. The secret may be in learning more about the genetic basis of resilience. “For example, we know that a person with two mutated copies of a gene called APOE4 has a 90% chance of developing Alzheimer’s. But this still means that 10% of people with 2 bad copies of APOE4 aren’t going to get sick. We have to find the pathways that confer this resilience, and find drugs that target them.”
The prospects in psychiatric illness are equally exciting. “Wouldn’t it be great,” Dr. Brennand concludes, “if you could have a high-risk patient come into your clinic when they were 15, or 12, or 5, and be able to tell them: ‘You’re at extremely high risk for schizophrenia, or for bipolar disorder. Let’s start treating you now.’ That’s our ultimate goal.”
Written By Peter Tarr, Ph.D.